AMERICAN JOURNAL OF PHYSIOLOGY Vol. 229, No. 5, Novembef 1975. Printed
Hypothalamic baroreceptor
in U.S.A.
modulation afferent
unit
of activity
J. RANDLE ADAIR AND JOHN W. MANNING Department of Physiology, Emory University, Atlanta, Georgia 3032 2
ADAIR, J. RANDLE, AND JOHN W. MANNING. Hypothalamic modulation of baroreceptor afferent unit activity. Am. J. Physiol. 229(5) : 1357- 1364. 1975.-Unit responses to sinus nerve stimulation were recorded in the medulla. A conditioning stimulus to the posterior hypothalamus produced inhibition of 65yC of unit responses tosinus nerve stimulation as early as 7 ms and extending as long as 790 ms after conditioning; 507, recovered after 300 ms. Unit responses to hypothalamic stimulation alone were also recorded in the medulla, some in the same loci as other unit responses to sinus nerve stimulation. They could be activated by contralateral as well as ipsilateral hypothalamic stimulation and showed recurrent bursts of firing over a 1,OOO-ms poststimulus interval. Evoked potentials and unit responses were recorded in the posterior hypothalamus, some occurring within lo-20 ms poststimulation of the sinus nerve, indicating that baroreceptor information is ascending in a time sufficiently short to involve the hypothalamus in reflex regulation of blood pressure as well as more generalized homeostatic responses which include the cardiovascular system.
carotid sinus reflex; baroreceptor inhibition; hypothalamusmedulla interaction; unit responses; evoked potentials; blood pressure control
MODULATION of baroreceptor-evoked cardiac and vasomotor activity has been observed by different investigators for years, but the neural substrate underlying thin modulation is only beginning to be understood (1, 3, 15-16, 19, 21-23). Ablation studies by Manning and others (10, 1 I) have confirmed that the integrity of supramedullary structures is necessary for the complete range of reflexly evoked cardiovascular changes that can be observed in the intact animal. Early work by Uvnas (24) on the sympathetic cholinergic vasodilator pathway, a path distinct from classical vasomotor areas, outlines hypothalamic areas which might be important in supramedullary cardiovascular control. Work by Hilton et al. (1, 5, 6) on the hypothalamic defense reaction and by Takeuchi and Manning (18, 19) on carotid sinus baroreceptor-evoked sympathetic cholinergic vasodilation, for which the integrity of the posterior hypothalamus is necessary, further underline the critical role of the hypothalamus. The evidence of Weiss and Criil (25), demonstrating primary afferent depolarization of the carotid sinus nerve with diencephalic stimulation, indicates potential for an intermediate feedback system between the hypothalamus and primary afferent information entering the medulla. SUPRAMEDULLARY
The present investigation is concerned with the interaction of the primary receiving areas for carotid sinus baroreceptor afferents, the nucleus and tractus solitarius and adjacent reticular formation, and the hypothalamus. The possibility of an input from the carotid sinus nerve to the posterior hypothalamus, as has been suggested by the work of Thomas and Calaresu (20) and Takeuchi and Manning ( 18, 19), is also investigated. METHODS
AND
MATERIALS
Twenty-five cats of both sexes were utilized in this study. Anesthesia was induced with ether and sustained with 35-40 mg/kg of ar-chloralose. Routinely, the animals were placed in a David Kopf Instruments stereotaxic frame, vagotomy and tracheotomy were performed, and both femoral vein and artery on one side were cannulated to permit recording of arterial blood pressure and intravenous administration of fluids. The carotid sinus nerve was exposed on the right side, placed on a platinum bipolar stimulating electrode, and identified by observation of a systemic depressor reponse to a 10-s train of stimuli, parameters being 1.0-5.0 V, 0.8 ms duration, 70 stim/s. The skull overlying the cerebrum and cerebellum was removed above the stereotaxic points where recording and stimulating electrodes were to be placed; electrode localization was confirmed histologically. Hypothalamic stimulating electrodes were two strands of 30-gauge insulated, interwined stainless steel wire exposed at the tips, with stimulating points 0.5-1.0 mm apart. Recording electrodes in both the medulla oblongata and hypothalamus were tungsten microelectrodes of 0.5- to l.O-pm tip diam. Recording of unit activity and evoked potentials was done at stimulation frequency and strengths of 0.7 stim/s and 1.0-2.0 times threshold voltage for systemic depressor response. The stimulation frequency during recording was sufficiently low that no reflex cardiovascular compensatory responses were noted to occur. RESULTS
Interaction between hypothalamic stimulation and medullary unit responses to carotid sinus nerve stimulation. Sixty units were recorded in the medulla oblongata which were responsive to stimulation of the carotid sinus nerve. Two-thirds of the units responded with multiple spikes, and only 20 % followed stimulation frequencies greater than 5 stim/s. One such unit is shown in Fig. 1. Since units recorded in
1357
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1358
J. R. ADAIR
G
2 8
1OO”VL 5 msec
-
;u” w Od
FIG.
nerve
1 10
I 20
I 30
1 40
l-l
50 msec 1. Histograms of medullary unit response to carotid sinus stimulation. Ten trials. Photo inset, 2 sweeps superimposed. 151 Primary response 0
inhibited
q
noninhibited
Units
Peak response 0
10
inhibited
Units 5 I 5
lb
15
35
30
35
msec FIG. 2. First spike (primary) and peak latencies of 60 medullary unit responses to carotid sinus nerve stimulation. Units not inhibited by hypothalamus stimulation indicated by stippling.
the reticular formation in response to sinus nerve stimulation generally fire multiply in response to a single stimulus, latencies are expressed in two ways: first-spike latency refers to the earliest spike recorded in response to a single stimulus; the peak latency was calculated from repetitive trials of the unit to determine the time at which a multiplefiring unit showed the greatest probability of an evoked response. For the unit in Fig. 1, the first-spike latency and peak latency are both 15 ms. For some units, the peak latency occurs later than the first spike. The latencies and range of all unit responses are shown in Fig. 2. The greatest number of first-spike responses occurs with a latency of 6 ms, the greatest number of peak responses at 13 ms. For 27 units surveyed, the average period of the multiple discharge was 17.7 ms. These first-spike and peak responses to sinus nerve stimulation are consistent with latencies recorded by other investigators (8, 14) for units postsynaptic to the primary afferents of the tractus solitarius. The frequency following of first-spike and multiple-response characteristics summarized in Table 1 are also in agreement with Humphrey’s work describing characteristics of units postsynaptic to primary afferents responding to baroreceptor nerve stimulation. Later responses in the multiple discharge failed first as frequency of stimulation was raised.
AND
J. W. MANNING
The anatomical locations of all unit responses are shown in Fig. 3. In addition to recording responses in the nucleus solitarius, units were also noted in the nuclei parvo and gigantocellularis, subjacent to the nucleus. This distribution is consistent with the anatomical studies of Cottle (2) and Morest (12). Recording techniques employed in this study did not permit analysis of unit spike wave form, such as that done by Hubel (7), to distinguish between axonal and somal responses. The latencies, frequency following, and multiplefiring characteristics of the units do, however, indicate that-the units are postsynaptic to the afferents stimulated. The activity of 39 of 60 units evoked by a test stimulus to the sinus nerve could be inhibited when preceded by a conditioning stimulus to the same region of the hypothalamus shown by Takeuchi and Manning (19) to evoke sympathetic cholinergic vasodilation and a strong systemic pressor response. The distributions of first-spike and peak latencies and anatomical loci for both inhibited and noninhibited units are shown in Figs. 2 and 3. There are no apparent distinctions between inhibited and noninhibited units: both types are distributed over the entire range of anatomical loci and ranges of first-spike and peak latencies. The time course of the inhibited behavior of the medullary units is illustrated in Fig. 4, a histogram of 32 units responding to sinus nerve stimulation plotted against the conditioning-testing interval observed when the units cease responding to sinus nerve stimulation and later commence firing. The first units cease firing within a conditioning-testing interval of 7 ms, 50 % within 9 ms, and all within 30 ms; firing commences again after an interval of 120 ms, 50 % after 300 ms, all after 790 ms. The time course of inhibition refers to the interval between stimulation in which both single and multiple discharges of units were inhibited. It was not determined if there was any difference between the period of inhibition for firstspike and for peak responses. Medullary unit res-onses to hypothalamic stimulation. During stimulation of the hypothalamus to produce inhibition of medullary unit responses to carotid sinus nerve stimulation, other units were observed firing synchronously with the hypothalamic conditioning stimulus. In 17 experiments, 167 units were noted in the medulla responsive to hypothala1. Units recorded in medulla sinus nerve stimulation _____~
TABLE
A) Frequency following : Stimulus Frequency, Hz 0.5 1 .o 1.5 2.0 5.0 7.0 10.0 B) No. of spikes
per stimulus Spikes 1 2 3 4 5
responsive to
No. of Units 1 10 1 2 6 2 5
No. of Units 18 21 14 3 1
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HYPOTHALAMIC-MEDULLARY-SINUS
NERVE
1359
INTERACTIONS
noted previously. The average period of the multiple discharge was 28.5 ms for 91 units studied. Units recorded in the same loci as units responsive to carotid sinus nerve stimulation showed no characteristics different from other loci. The long-term ( 1,000 ms) poststimulus response characteristics of units in the medulla responsive to hypothalamic stimulation were also considered. Figure 6 shows the poststimulus response behavior of several units in the photo
M
30 Units
15
10
0
i0
20 Conditioninq
FIG.
responsive pothalamic
4. Conditioning-testing to carotid sinus stimulation.
3i
ml
400
600
tDMH1 - Testing tCSNJ interwl
nerve
al0
msec
intervals for stimulation
32 and
IPSIlNERAL
- Prifnory
medullary inhibited
by
units hy-
PI& I3
3. Anatomical cordings of responses (A) and not inhibited FIG.
locations of 39 multipleand single-unit reto carotid sinus nerve stimulation inhibited (a) by hypothalamic stimulation.
mic stimulation; 3 1 were found in the same loci from which could also be recorded units responsive to sinus nerve stimulation, and only one unit was recorded which appeared to be driven by both stimuli. The first-spike latencies for unit responses to hypothalamic stimulation are shown in Fig. 5. All of the units tested with the exception of one could also be driven by stimulation of the contralateral hypothalamus. Figure 5 shows the latencies for both sites of stimulation. Frequency following of the first-spike repetitive firing characteristics for some of these units are shown in Table 2. These data on the relatively low following frequencies noted for units recorded in the medulla responsive to hypothalamic stimulation imply that the units are being orthodromically activated and that the response does not represent an artifactual observation of a long ascending projection to the hypothalamus being antidromically invaded. The repetitive firing characteristics of medullary units in response to hypothalamic stimulation have not been
20
msa
CCWRAtATERAl
FIG.
ipsilateral
5. First-spike latencies of 167 and contralateral hypothalamic
25
30
35
- Pt tmofy
medullary unit stimulation.
responses
to
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1360
J. R. ADAIR
TABLE: 2. Units recorded in medulla responsive to hypothalamic stimulation -I____-. -~~~ ______~--. - --A)
Frequency
following
Stimulus
Frequency,
(to ipsilateral
hypothalamic
Hz
No.
0.5 1 .o 1.8 2.0 4.0 5.0 6.0 B) No.
of spikes
per
stimulation)
:
of Units
7 3 3 5 2 2 1 stimulus
Spikes
No.
of Units
Ipsilateral
Con trala
46 74 28 3 0 1
teral
31 60 21 2 0 0
AND
J.
W. MANNING
taneous firing frequency and variability of interspike intervals around that mean were not computed, distinctions such as possible differences in firing patterns and their functional significance must remain speculative. Evoked boten tials ar2d unit responses in hy~othlnmus elicited by carotid sinus nerve stimulation. Figure 8 demonstrates the potential evoked in the hypothalamus by stimulation of the carotid sinus nerve. The wave of depolarization (negativity downward) has two prominent peaks: an earlier, sharper peak which can be subdivided into two distinct peaks with depth of penetration and a longer latency single peak. Unit activity associated with the shortest latency early peak is shown in the photo inset. Unit activity was noted in association with the early peaks at their points of maximum deflection, with latencies correspondent; in contrast, activity in the long-latency peak was difficult to analyze,
1.0 MM
5ouv
L5 msec
5ouv L 50 msec
30 25 h i?esponses m 5
0 800 200 400 1000 600 6. Medullary unit responses to hypothalamic stimulation. Histogram of 20 trials of largest unit in photo insets (for details see text; photo insets, 2 sweeps superimposed). FIG.
insets, one photo a 50-ms time base, the other 500 ms; the poststimulus response histogram of 20 consecutive trials of the largest unit is below. After the initial burst of firing over 15-30 ms, there is a period of quiet followed by a recurrence of firing extending over 100-300 ms. Firing after this second period of activity is also noted, but the grouping is least synchronized. The anatomical locations of unit responses to hypothalamic stimulation are shown in Fig. 7. The interpretation of these long-term poststimulus response characteristics is limited by a lack of information on spontaneous activity over such a long period of observation. No attempt was made to quantitate levels of spontaneous activity in these units, but the recurrent burst pattern of activity was not observed during recording except following hypothalamic stimulation. Since differences in mean spon-
-. . . . . . a: .-
FIG.
recordings lation.
7. Anatomical of medullary
locations responses
of 131 singleand multiple-unit to ipsilaterial hypothalamic stimu-
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HYPOTHALAMIC-MEDULLARY-SINUS
NERVE
INTERACTIONS
as units observed did not follow faithfully or synchronously, regardless of frequency of stimulation. The region from which the earliest peak is recorded corresponds to the area shown by Takeuchi and Manning (19) to produce cholinergic vasodilation and a systemic pressor response and was shown in this study to produce inhibition of medullary unit activity responding to carotid sinus nerve stimulation. The latency of maximum deflection of this earliest peak is 25 ms (indicated at arrow). The maximum deflection of single long-latency peak occurs approximately 0.3 mm more ventral and is approximately 1.0 mm more ventral in the electrode track and has a latency of 45 ms. That the two early peaks are distinct can be noted at a point 1.05 mm deep in the track where the first earlier peak diminishes and the second begins to appear. Figure 9 shows 14 experiments with recording of evoked potentials in five cross-sectional areas of the hypothalamus. The numbers along the tracks refer to the latencies (in ms) of the peak negative waves of the short-latency evoked potentials recorded at those locations. It is clear that the organization of the sinus nerve input to the hypothalamus is not as neatly organized as Fig. 8 might depict. In some electrode tracks, the organization of Fig. 8 can be noted;
1361
. *
iI k ‘.,,-. .;i.+ . !\ I I .* I -w..i.: r.I...... I L-....., *:r;:.. . . . . . .....’ ( -.; ‘..f .. .._j . . . . . a=-* !;” .-:i
B+q
r-T. I }l*.J;,~ ‘J *s-j 129 I
20 -
+
MSEC
: \..
’ ..,I.
C A
Q&
FIG. 9. Summary of latencies of early peak negative deflections of evoked potentials recorded in hypothalamus in response to carotid sinus nerve stimulation. Numbers indicate latency (in Ins) of earliest response recorded at each location. Arrows indicate locations of multiunit potentials shown in photo insets.
8 25 msec 8 lb mra 8 0 i45
mrec
i
I I mm
unilory
raponsc;
2 sweeps
40 uv
4olJv I IO msac
A 13 0
0
50
IW
150
200
250
msec FIG. 8. Evoked potentials recorded in hypothalamus in response to carotid sinus nerve stimulation. Arrow (+) indicates location of unitary response shown in photo inset. Numbers by locations indicate latency of greatest negative voltage deflection recorded for each peak.
in others, the three components are variously missing or that stimulation of the rearranged. It is clear, however, sinus nerve can evoke responses over a wide area of the medial and lateral hypothalamus in as short a time as lo-20 ms poststimulus, almost parallel in time to the response of some areas of the medulla. In one experiment, at several points in the electrode track, stimulus strength was varied to determine if the early and late negative waves No difference in stimulus were differentially activated. threshold was noted, indicating that the response involves both simple and complex pathways and most likely does not represent activation of two distinct systems, such as chemoreceptor versus baroreceptor. Unit activity associated with the negative waves is shown for three areas in the photo insets. Additional information on hypothalamic unit activity is confined to the experiments displayed here in Fig. 9 and experiments on four units during normal and high blood pressure. Units were hard to isolate and hold; when units were isolated in response to stimulation of the sinus nerve, it was found that they tended to fire in bursts in response to single stimuli, display low levels of spontaneous activity (1-2 spikes/s), and would not follow stimulation frequencies greater than 1.5 stim/s. Four units noted to fire spontaneously were monitored during normal and elevated (by intravenous injection of norepinephrine) systemic blood pressure. Three of these units decreased their rate of firing during elevated pressure; one showed no change.
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1362 DISCUSSION
The purpose of this investigation was to obtain electrophysiological evidence for the functional description of hypothalamic involvement in baroreceptor reflex regulation and medullary-hypothalamic interrelationships necessary for such integration (1, 3, 6, 10, 15-16, 19-26). The results of the investigation rest on the assumption that unit activity recorded in the medulla and the hypothalamus and the phenomena observed with interaction of the hypothalamus and the medulla represent activity of neural elements subserving cardiovascular function. We are content to make this assumption for the following reasons. First, all the units recorded from both the medulla and the hypothalamus in response to sinus nerve stimulation were located in areas of the brain shown by others to modulate baroreceptor function when stimulated and/or eliminated. The distribution of medullary recording sites overlaps those shown by Henry and Calaresu (4) for inhibition and excitation of sympathetic cardioacceleratory neurons in the intermediolateral nucleus of the thoracic cord. Humphrey (8) has shown that destruction of the areas of the nucleus solitarius and reticular formation recorded from eliminates the reflex. Takeuchi and Manning (18, 19) have shown that similar ablations in the hypothalamus where the hypothalamic response to sinus nerve stimulation was recorded result in the elimination of the hypothalamic component of the sinus reflex. Second, while it is true that both baroreceptor and chemoreceptor afferents, of both A and C fiber types, course in the carotid sinus nerve, the cardiovascular response to the stimulation parameters used in this study is that characteristic of the baroreceptor, not chemoreceptor reflex. Therefore, the neural activity is recorded under conditions associated with the functional description of the baroreceptor depressor reflex. Third, investigators ( 13, 17) have shown repeatedly that very few neural units show activity directly correspondent to either the cardiac rhythm or arterial pulse. The information is transferred in a much more subtle and complex manner. An analysis to determine such patterns of information transfer was neither the aim nor result of this investigation. The results of this investigation are: I) single-shock stimulation of the carotid sinus nerve evoked responses in the posterior hypothalamus within lo-20 ms. Thus, information from the baroreceptor nerve can ascend to the hypothalamus with sufficiently short latency to permit its participation in reflex cardiovascular regulation. The area of the hypothalamus which is involved is that shown to be necessary for the total reflex depressor response to high carotid sinus pressure (19). 2) Single-shock stimulation of this same area of the hypothalamus will produce an inhibition of medullary responses to baroreceptor nerve stimulation in an interval between conditioning of the hypothalamus and testing of the sinus nerve as short as 7 ms and extending as late as 790 ms. Further, only 65 % of units recorded were inhibited, implying that the descending projection does not act upon all cells receiving an input from the carotid sinus nerve. 3) Single-shock stimulation of this same hypothalamic area, as well as the contralateral hypothalamus, produces unit responses in the medulla, some in the same area as unit responses to sinus nerve
J. R. ADAIR
AND
J. W.
MANNING
stimulation. It may be that these responses are involved in mechanisms which can produce the inhibition that is noted in unit responses to sinus nerve stimulation during hypothalamic conditioning, as well as being involved in overall visceromotor regulation. A functional path in the baroreceptor reflex which bypasses the medullary vasomotor areas and includes the hypothalamus is evidenced by Manning (1 l), Takeuchi and Manning (19), and Hilton and co-workers (1, 6), who have demonstrated that the baroreceptor reflex is not abolished by loss of the classical vasomotor area as long as supramedullary structures are left intact and that the full range of the autonomic response to baroreceptor activation is dependent on systems that engage hypothalamic neural mechanisms. Direct electrophysiological evidence for such a projection has been scarce, confined to the report of Hilton and that spontaneous unit Spyer m which demonstrated activity in the anterior hypothalamus could be affected by
stimulation. Hilton and Spyer suggest from their observations that the whole brainstem, from hypothalamus through medulla, is the functional unit for integration of baroreceptor afferent information. Thomas and Calaresu have shown hypothalamic units that are time locked in response to sinus nerve stimulation. The latencies of the increase of activity was 29 ms, with a range of 17-40 ms. The results presented in Figs. 8 and 9 of our investigation stand in confirmation of their observation in that the first early negative deflection of the evoked potentials and accompanying units occurs over a range of lo-50 ms, with the earliest responses occurring within lo-20 ms. Our results, however, indicate that areas more rostra1 having a greater lateral distribution than those noted receive excitatory input from the sinus nerve. Thus, a description of the excitatory input from sinus nerve to the posteriomedial hypothalamus as discrete may be too narrow an interpretation. The significance of units observed during the period of time corresponding with the second, long-latency, negative deflection of the evoked potential is difficult to analyze. The probability that unit activity was synchronous with the potential change was not determined; hence, it is not possible to say if unit firing was truly synchronous or simply coincident spontaneous activity. It may be that the long-latency response is the result of activation of complex reticulodiencephalic paths much less direct than that producing the early response. Hilton (5) has suggested that the long-latency response recorded in the hypothalamus to sensory stimulation is mediated by such complex paths. The experiment which determined that both negative peaks of the evoked potential were excited at the same stimulus strength only suggests that the two potentials recorded in the hypothal amus are not due to excitation of different svs terns having different thresholds of activation. An alternative explanation for the late negative potential is that it might represent recurrent hypothalamic activity in response to sinus nerve stimulation. The data presented in this paper do not resolve the question.
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HYPOTHALAMIC-MEDULLARY-SINUS
NERVE
1363
INTERACTIONS
The observations of units responsive to sinus nerve stimulation in the medulla in the regions of the nucleus solitarius and the medullary reticular formation are not new, nor is the demonstration of inhibition of baroreccptor affcrent information during stimulation of the hypothalamus. What is interesting is that units postsynaptic to primary afferents are shown in this study also to be inhibited during hypothalamic stimulation. Weiss and Crill (25) showed that primary afferent depolarization occurred over a time period of 5-120 ms, whereas our results, including observations of higher neurons, indicate the unit inhibition does begin as early as 7 ms, but extends to as late as 790 111s. It might be possible to conclude that higher-order neurons show apparent inhibition because the afferents from elements postsynaptic to the primary a ffkren ts are not being driven due to primary aflerent inhibition. While this is reasonable in accounting for a period of inhibition up to from primary I 20 ms plus delay time for conduction synapse to the higher-order neuron, it clearly cannot account for inhibition up to 300 ms for 50 ‘,‘: of the units studied. The observation of units in the medulla responding to hypothalamic single-shock stimulation has functional implications for several reasons. Stimulation of the carotid sinus nerve will produce unit responses in the medulla which can then be inhibited by stimulation of the same area of the hypothalamus. Some medullary units responding to hypothalamic stimulation are found in areas of the medial reticular formation and the nucleus solitarius where unit responses to baroreceptor nerve stimulation can also be found, and many (65 7;) but not all of the unit responses to nerve stimulation arc inhibited during hypothalamic stimulation. Additionally, repetitive stimulation of this posterior hypothalamic area can produce sympathetic choline1 gic vasodilation in cardiovascular studies ( 18) and a strong defense reaction in behavioral studies (1) supposedly involving a functional resetting of baroreceptors
w. I-\
The work of Weiss and Crill (25) did not include observations of medullary responses during hypothalamic stimulation. Keene and Casey (9) have demonstrated responses in the nucleus gigantocellularis in response to stimulation with latencies of 2-4 lateral hypothalamic ms, and they associated these with facilitation of responses to sensory stimulation in the nucleus. If their interpretation is correct, then the descending projection we have demonstrated is part of a much more complicated mechanism for modulating visceral and somatic sensory information at primary levels. Smith and Nathan (16) have noted that dorsal olivary stimulation has an inhibitory effect on the carotid sinus reflex. Unit responses to hypothalamic stimulation were noted (Fig. 7) in the area of the dorsal olive as well as the nucleus ambiguus, which may be significant
to Thomas and Calaresu’s (22) observations on the role of the nucleus ambiguus in vagal bradycardia and that hypothalamic stimulation can inhibit chemoreceptor-induced vagal bradycardia (2 1). Unit responses in the medulla to hypothalamic stimulation, both ipsilateral and contralateral, occur with a wide range of first-spike latencies, with some occurring very early. Smith’s ( 15) observations of a direct descending projection from the hypothalamus to the medulla, based on anatomical degeneration techniques, has been questioned (26). The shortest latency (2-4 ms) responses recorded in the medulla to hypothalamic stimulation in this study were in the region of the medial longitudinal fasciculus where Smith observed terminal degeneration. Given the information presented in this study and based on conclusions made from the data, an organism is provided with a cardiovascular regulatory reflex center with flexibility than previously imagined and much greater involving the hypothalamus in basic reflex activity. This expanded reflex center, in addition to receiving information and generating an appropriate autonomic response, also includes intrinsic feedback in that the area of the hypothalamus impinged upon by baroreceptor aflerents can generate a response in medullary areas limiting the amount of infor mation which will be accepted : 65 % of medullary units to sinus nerve stimulation were inhibited by responsive hypothalamic stimulation. Certainly, this same descending projection producing the inhibition can possibly be driven by higher centers in the motor cortical and limbic areas. It is important to note that under conditions of hypothalarnic stimulation, 35 % of medullary responses to sinus nerve stimulation remained noninhibited and presumably could continue to relay afferent information, however modified. This descending hypothalamic-medullary projection may be further involved in modulation of other reflex autonomic systems, such as the sympathetic choliner pgic vasodilator system. One then can beg in to envision how a threatened a nimal c an si multa .neously elevate heart rate and blood pressure, shift priorities of blood flow, temperature, and pain response, all the while retaining the characteristics of reflex cardiovascular regulation, only now reset to new levels. This descending projection could possibly reset baroreceptors to hypertensive levels by simply altering the percentage of processed information from the baroreceptors. The
technical assistance of Mrs. is gratefully acknowledged. This investigation was supported NS-05669, NS-02645, and HL- 16648. Present address of J. R. Adair : John Hopkins University School of
Lucy
McElrath
and
Mrs.
Bessie
Lane
Received
for
publication
14 August
by
Public
Health
Dept. of Biomedical Medicine, Baltimore,
Service
Grants
Engineering, Md. 21205.
1974.
REFERENCES 1. ABRAHAMS, V. C., S. M HILTON, AND ,4. ZBROZYNA. Active muscle vasodilation produced by stimulation of the brain stem: its significance in the defense reaction. J. Physiol., London 154: 491-513, 1960. 2. COTTLE, M. A. Degeneration studies of primary afferents of IX and X cranial nerves. J. Camp. Neural. 122 : 329-345, 1964. 3. GEBBER, G. L., AND D. W. SNYDER. Hypothalamic control of baroreceptor reflexes. Am. J. Physzol. 218: 124-131, 1969.
4.
HENRY, J. L., AND F. R. CALARESU. Excitatory and inhibitory inputs from medullary nuclei projecting to spinal cardioacceleratory neurons in the cat. Exptl. Brain Res. 20: 485-504, 1974. 5. HILTON, S. Al. Hypothalamic regulation of the cardiovascular system. Brit. Med. Bull. 22 : 243-248, 1966. 6. HILTON, S. hi., AND K. RI. SPYER. Participation of the anterior hypothalamus in the baroreceptor reflex. J. Physiol., London 218: 271-293,1971.
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D. H. Single unit activity in lateral geniculate body and optic tract of unrestrained cats. J. Physiol., London 150 : 91-104, 1960. HUMPHREY, D. R. Neuronal activity in the medulla oblongata of cat evoked by stimulation of the carotid sinus nerve. In: Baroreceptors and Hypertension, edited by P. Kezid. New York : Pergamon, 1967, p. 131. KEENE, J. J., AND K. L. CASEY. Excitatory connections from lateral hypothalamic self-stimulation sites to escape sites in medullary reticular formation. Exptl. Neurol. 28 : 155-I 66, 1970. KENT, B., J. DRANE, AND J. W. MANNING. Suprapontine contributions to the carotid sinus reflex in the cat. Circulation Res. 29: 534-541, 1971. MANNING, J. W. Cardiovascular reflexes following lesions in medullary reticular formation. Am. J. Physiol. 208: 282-288, 1965. MOREST, D. K. Experimental study of the projections of the nucleus of the tractus solitarius and the area postrema in the cat. J. Camp. Neural. 130: 277-300, 1967. SALMOIRAGHI, G. C. ‘Cardiovascular’ neurons in brain stem of cat. J. Neurophysiol. 25 : 182-197, 1962. SELLER, H., AND RI. ILLERT. The localization of the first synapse in the carotid sinus baroreceptor reflex pathway and its alteration of the afferent input. P’uegers Arch. 306: I-19, 1969. SMITH, 0. A. Anatomy of central neural pathways mediating cardiovascular functions. In : Nervous Control of the Heart, edited by W. C. Randall. Baltimore: Williams & Wilkins, p. 34-53. SMITH, 0. A., AND hl. A. NATHAN. Inhibition of the carotid sinus HUBEL,
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by stimulation
of the inferior
AND olive.
J. W. Science
MANNING 154 : 674-675,
17. SMITH, R. E., AND J. W. PEARCE. Microelectrode recordings from the region of the nucleus solitarius in the cat. Canad. J. Biochem. Physiol. 39 : 933-39, 1961. 18. TAKEUCHI, T., AND J. W. MA,NNING. Muscle cholinergic dilators in the sinus baroreceptor response in cats. Circulation Res. 29: 350-357, 1971. 19. TAKEUCHI, T., AND J. W. MANNING. Hypothalamic mediation of sinus baroreceptor-evoked muscle cholinergic dilator response Am. J. Fhysiol. 224: 1280-1278, 1973. 20. THOMAS, R., AND R. CALARESU. Responses to single units in the medial hypothalamus to electrical stimulation of the carotid sinus nerve in the cat. Brain Res. 44: 49-62, 1972. 21. THOMAS, R., AND R. CALARESU. Hypothalamic inhibition of chemoreceptor-induced bradycardia in the cat. Am. J. Physiol. 225 : 201-208, 1973. 22. THOMAS, M. R., AND F. R. CALARESU. Localization and function of medullary sites mediating vagal bradycardia in the cat. Am. J. Physiol. 226: 1344-1349, 1974. 23 . TUTTLE, R. S., AND M. MCCLEARY. Central inhibition of the defense reaction. Am. J. Physiol. 2 19: 23-29, 1970. 24. UVNAS, B. Sympathetic vasodilator outflow. Physiol. Rev. 34: 608-618, 1954. 25. WEISS, G. K., AND W. E. CRILL. Carotid sinus nerve: primary afferent depolarization evoked by hypothalamic stimulation Brain Res. 16 : 269-272, 1969. 26. WOLF, G., AND J. SUTIN. Fiber degeneration after lateral hypothalamic lesions in the rat. J. Camp. Neural. 127 : 137-156, 1966.
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Hypothalamic baroreceptor
in U.S.A.
modulation afferent
unit
of activity
J. RANDLE ADAIR AND JOHN W. MANNING Department of Physiology, Emory University, Atlanta, Georgia 3032 2
ADAIR, J. RANDLE, AND JOHN W. MANNING. Hypothalamic modulation of baroreceptor afferent unit activity. Am. J. Physiol. 229(5) : 1357- 1364. 1975.-Unit responses to sinus nerve stimulation were recorded in the medulla. A conditioning stimulus to the posterior hypothalamus produced inhibition of 65yC of unit responses tosinus nerve stimulation as early as 7 ms and extending as long as 790 ms after conditioning; 507, recovered after 300 ms. Unit responses to hypothalamic stimulation alone were also recorded in the medulla, some in the same loci as other unit responses to sinus nerve stimulation. They could be activated by contralateral as well as ipsilateral hypothalamic stimulation and showed recurrent bursts of firing over a 1,OOO-ms poststimulus interval. Evoked potentials and unit responses were recorded in the posterior hypothalamus, some occurring within lo-20 ms poststimulation of the sinus nerve, indicating that baroreceptor information is ascending in a time sufficiently short to involve the hypothalamus in reflex regulation of blood pressure as well as more generalized homeostatic responses which include the cardiovascular system.
carotid sinus reflex; baroreceptor inhibition; hypothalamusmedulla interaction; unit responses; evoked potentials; blood pressure control
MODULATION of baroreceptor-evoked cardiac and vasomotor activity has been observed by different investigators for years, but the neural substrate underlying thin modulation is only beginning to be understood (1, 3, 15-16, 19, 21-23). Ablation studies by Manning and others (10, 1 I) have confirmed that the integrity of supramedullary structures is necessary for the complete range of reflexly evoked cardiovascular changes that can be observed in the intact animal. Early work by Uvnas (24) on the sympathetic cholinergic vasodilator pathway, a path distinct from classical vasomotor areas, outlines hypothalamic areas which might be important in supramedullary cardiovascular control. Work by Hilton et al. (1, 5, 6) on the hypothalamic defense reaction and by Takeuchi and Manning (18, 19) on carotid sinus baroreceptor-evoked sympathetic cholinergic vasodilation, for which the integrity of the posterior hypothalamus is necessary, further underline the critical role of the hypothalamus. The evidence of Weiss and Criil (25), demonstrating primary afferent depolarization of the carotid sinus nerve with diencephalic stimulation, indicates potential for an intermediate feedback system between the hypothalamus and primary afferent information entering the medulla. SUPRAMEDULLARY
The present investigation is concerned with the interaction of the primary receiving areas for carotid sinus baroreceptor afferents, the nucleus and tractus solitarius and adjacent reticular formation, and the hypothalamus. The possibility of an input from the carotid sinus nerve to the posterior hypothalamus, as has been suggested by the work of Thomas and Calaresu (20) and Takeuchi and Manning ( 18, 19), is also investigated. METHODS
AND
MATERIALS
Twenty-five cats of both sexes were utilized in this study. Anesthesia was induced with ether and sustained with 35-40 mg/kg of ar-chloralose. Routinely, the animals were placed in a David Kopf Instruments stereotaxic frame, vagotomy and tracheotomy were performed, and both femoral vein and artery on one side were cannulated to permit recording of arterial blood pressure and intravenous administration of fluids. The carotid sinus nerve was exposed on the right side, placed on a platinum bipolar stimulating electrode, and identified by observation of a systemic depressor reponse to a 10-s train of stimuli, parameters being 1.0-5.0 V, 0.8 ms duration, 70 stim/s. The skull overlying the cerebrum and cerebellum was removed above the stereotaxic points where recording and stimulating electrodes were to be placed; electrode localization was confirmed histologically. Hypothalamic stimulating electrodes were two strands of 30-gauge insulated, interwined stainless steel wire exposed at the tips, with stimulating points 0.5-1.0 mm apart. Recording electrodes in both the medulla oblongata and hypothalamus were tungsten microelectrodes of 0.5- to l.O-pm tip diam. Recording of unit activity and evoked potentials was done at stimulation frequency and strengths of 0.7 stim/s and 1.0-2.0 times threshold voltage for systemic depressor response. The stimulation frequency during recording was sufficiently low that no reflex cardiovascular compensatory responses were noted to occur. RESULTS
Interaction between hypothalamic stimulation and medullary unit responses to carotid sinus nerve stimulation. Sixty units were recorded in the medulla oblongata which were responsive to stimulation of the carotid sinus nerve. Two-thirds of the units responded with multiple spikes, and only 20 % followed stimulation frequencies greater than 5 stim/s. One such unit is shown in Fig. 1. Since units recorded in
1357
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1358
J. R. ADAIR
G
2 8
1OO”VL 5 msec
-
;u” w Od
FIG.
nerve
1 10
I 20
I 30
1 40
l-l
50 msec 1. Histograms of medullary unit response to carotid sinus stimulation. Ten trials. Photo inset, 2 sweeps superimposed. 151 Primary response 0
inhibited
q
noninhibited
Units
Peak response 0
10
inhibited
Units 5 I 5
lb
15
35
30
35
msec FIG. 2. First spike (primary) and peak latencies of 60 medullary unit responses to carotid sinus nerve stimulation. Units not inhibited by hypothalamus stimulation indicated by stippling.
the reticular formation in response to sinus nerve stimulation generally fire multiply in response to a single stimulus, latencies are expressed in two ways: first-spike latency refers to the earliest spike recorded in response to a single stimulus; the peak latency was calculated from repetitive trials of the unit to determine the time at which a multiplefiring unit showed the greatest probability of an evoked response. For the unit in Fig. 1, the first-spike latency and peak latency are both 15 ms. For some units, the peak latency occurs later than the first spike. The latencies and range of all unit responses are shown in Fig. 2. The greatest number of first-spike responses occurs with a latency of 6 ms, the greatest number of peak responses at 13 ms. For 27 units surveyed, the average period of the multiple discharge was 17.7 ms. These first-spike and peak responses to sinus nerve stimulation are consistent with latencies recorded by other investigators (8, 14) for units postsynaptic to the primary afferents of the tractus solitarius. The frequency following of first-spike and multiple-response characteristics summarized in Table 1 are also in agreement with Humphrey’s work describing characteristics of units postsynaptic to primary afferents responding to baroreceptor nerve stimulation. Later responses in the multiple discharge failed first as frequency of stimulation was raised.
AND
J. W. MANNING
The anatomical locations of all unit responses are shown in Fig. 3. In addition to recording responses in the nucleus solitarius, units were also noted in the nuclei parvo and gigantocellularis, subjacent to the nucleus. This distribution is consistent with the anatomical studies of Cottle (2) and Morest (12). Recording techniques employed in this study did not permit analysis of unit spike wave form, such as that done by Hubel (7), to distinguish between axonal and somal responses. The latencies, frequency following, and multiplefiring characteristics of the units do, however, indicate that-the units are postsynaptic to the afferents stimulated. The activity of 39 of 60 units evoked by a test stimulus to the sinus nerve could be inhibited when preceded by a conditioning stimulus to the same region of the hypothalamus shown by Takeuchi and Manning (19) to evoke sympathetic cholinergic vasodilation and a strong systemic pressor response. The distributions of first-spike and peak latencies and anatomical loci for both inhibited and noninhibited units are shown in Figs. 2 and 3. There are no apparent distinctions between inhibited and noninhibited units: both types are distributed over the entire range of anatomical loci and ranges of first-spike and peak latencies. The time course of the inhibited behavior of the medullary units is illustrated in Fig. 4, a histogram of 32 units responding to sinus nerve stimulation plotted against the conditioning-testing interval observed when the units cease responding to sinus nerve stimulation and later commence firing. The first units cease firing within a conditioning-testing interval of 7 ms, 50 % within 9 ms, and all within 30 ms; firing commences again after an interval of 120 ms, 50 % after 300 ms, all after 790 ms. The time course of inhibition refers to the interval between stimulation in which both single and multiple discharges of units were inhibited. It was not determined if there was any difference between the period of inhibition for firstspike and for peak responses. Medullary unit res-onses to hypothalamic stimulation. During stimulation of the hypothalamus to produce inhibition of medullary unit responses to carotid sinus nerve stimulation, other units were observed firing synchronously with the hypothalamic conditioning stimulus. In 17 experiments, 167 units were noted in the medulla responsive to hypothala1. Units recorded in medulla sinus nerve stimulation _____~
TABLE
A) Frequency following : Stimulus Frequency, Hz 0.5 1 .o 1.5 2.0 5.0 7.0 10.0 B) No. of spikes
per stimulus Spikes 1 2 3 4 5
responsive to
No. of Units 1 10 1 2 6 2 5
No. of Units 18 21 14 3 1
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HYPOTHALAMIC-MEDULLARY-SINUS
NERVE
1359
INTERACTIONS
noted previously. The average period of the multiple discharge was 28.5 ms for 91 units studied. Units recorded in the same loci as units responsive to carotid sinus nerve stimulation showed no characteristics different from other loci. The long-term ( 1,000 ms) poststimulus response characteristics of units in the medulla responsive to hypothalamic stimulation were also considered. Figure 6 shows the poststimulus response behavior of several units in the photo
M
30 Units
15
10
0
i0
20 Conditioninq
FIG.
responsive pothalamic
4. Conditioning-testing to carotid sinus stimulation.
3i
ml
400
600
tDMH1 - Testing tCSNJ interwl
nerve
al0
msec
intervals for stimulation
32 and
IPSIlNERAL
- Prifnory
medullary inhibited
by
units hy-
PI& I3
3. Anatomical cordings of responses (A) and not inhibited FIG.
locations of 39 multipleand single-unit reto carotid sinus nerve stimulation inhibited (a) by hypothalamic stimulation.
mic stimulation; 3 1 were found in the same loci from which could also be recorded units responsive to sinus nerve stimulation, and only one unit was recorded which appeared to be driven by both stimuli. The first-spike latencies for unit responses to hypothalamic stimulation are shown in Fig. 5. All of the units tested with the exception of one could also be driven by stimulation of the contralateral hypothalamus. Figure 5 shows the latencies for both sites of stimulation. Frequency following of the first-spike repetitive firing characteristics for some of these units are shown in Table 2. These data on the relatively low following frequencies noted for units recorded in the medulla responsive to hypothalamic stimulation imply that the units are being orthodromically activated and that the response does not represent an artifactual observation of a long ascending projection to the hypothalamus being antidromically invaded. The repetitive firing characteristics of medullary units in response to hypothalamic stimulation have not been
20
msa
CCWRAtATERAl
FIG.
ipsilateral
5. First-spike latencies of 167 and contralateral hypothalamic
25
30
35
- Pt tmofy
medullary unit stimulation.
responses
to
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1360
J. R. ADAIR
TABLE: 2. Units recorded in medulla responsive to hypothalamic stimulation -I____-. -~~~ ______~--. - --A)
Frequency
following
Stimulus
Frequency,
(to ipsilateral
hypothalamic
Hz
No.
0.5 1 .o 1.8 2.0 4.0 5.0 6.0 B) No.
of spikes
per
stimulation)
:
of Units
7 3 3 5 2 2 1 stimulus
Spikes
No.
of Units
Ipsilateral
Con trala
46 74 28 3 0 1
teral
31 60 21 2 0 0
AND
J.
W. MANNING
taneous firing frequency and variability of interspike intervals around that mean were not computed, distinctions such as possible differences in firing patterns and their functional significance must remain speculative. Evoked boten tials ar2d unit responses in hy~othlnmus elicited by carotid sinus nerve stimulation. Figure 8 demonstrates the potential evoked in the hypothalamus by stimulation of the carotid sinus nerve. The wave of depolarization (negativity downward) has two prominent peaks: an earlier, sharper peak which can be subdivided into two distinct peaks with depth of penetration and a longer latency single peak. Unit activity associated with the shortest latency early peak is shown in the photo inset. Unit activity was noted in association with the early peaks at their points of maximum deflection, with latencies correspondent; in contrast, activity in the long-latency peak was difficult to analyze,
1.0 MM
5ouv
L5 msec
5ouv L 50 msec
30 25 h i?esponses m 5
0 800 200 400 1000 600 6. Medullary unit responses to hypothalamic stimulation. Histogram of 20 trials of largest unit in photo insets (for details see text; photo insets, 2 sweeps superimposed). FIG.
insets, one photo a 50-ms time base, the other 500 ms; the poststimulus response histogram of 20 consecutive trials of the largest unit is below. After the initial burst of firing over 15-30 ms, there is a period of quiet followed by a recurrence of firing extending over 100-300 ms. Firing after this second period of activity is also noted, but the grouping is least synchronized. The anatomical locations of unit responses to hypothalamic stimulation are shown in Fig. 7. The interpretation of these long-term poststimulus response characteristics is limited by a lack of information on spontaneous activity over such a long period of observation. No attempt was made to quantitate levels of spontaneous activity in these units, but the recurrent burst pattern of activity was not observed during recording except following hypothalamic stimulation. Since differences in mean spon-
-. . . . . . a: .-
FIG.
recordings lation.
7. Anatomical of medullary
locations responses
of 131 singleand multiple-unit to ipsilaterial hypothalamic stimu-
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HYPOTHALAMIC-MEDULLARY-SINUS
NERVE
INTERACTIONS
as units observed did not follow faithfully or synchronously, regardless of frequency of stimulation. The region from which the earliest peak is recorded corresponds to the area shown by Takeuchi and Manning (19) to produce cholinergic vasodilation and a systemic pressor response and was shown in this study to produce inhibition of medullary unit activity responding to carotid sinus nerve stimulation. The latency of maximum deflection of this earliest peak is 25 ms (indicated at arrow). The maximum deflection of single long-latency peak occurs approximately 0.3 mm more ventral and is approximately 1.0 mm more ventral in the electrode track and has a latency of 45 ms. That the two early peaks are distinct can be noted at a point 1.05 mm deep in the track where the first earlier peak diminishes and the second begins to appear. Figure 9 shows 14 experiments with recording of evoked potentials in five cross-sectional areas of the hypothalamus. The numbers along the tracks refer to the latencies (in ms) of the peak negative waves of the short-latency evoked potentials recorded at those locations. It is clear that the organization of the sinus nerve input to the hypothalamus is not as neatly organized as Fig. 8 might depict. In some electrode tracks, the organization of Fig. 8 can be noted;
1361
. *
iI k ‘.,,-. .;i.+ . !\ I I .* I -w..i.: r.I...... I L-....., *:r;:.. . . . . . .....’ ( -.; ‘..f .. .._j . . . . . a=-* !;” .-:i
B+q
r-T. I }l*.J;,~ ‘J *s-j 129 I
20 -
+
MSEC
: \..
’ ..,I.
C A
Q&
FIG. 9. Summary of latencies of early peak negative deflections of evoked potentials recorded in hypothalamus in response to carotid sinus nerve stimulation. Numbers indicate latency (in Ins) of earliest response recorded at each location. Arrows indicate locations of multiunit potentials shown in photo insets.
8 25 msec 8 lb mra 8 0 i45
mrec
i
I I mm
unilory
raponsc;
2 sweeps
40 uv
4olJv I IO msac
A 13 0
0
50
IW
150
200
250
msec FIG. 8. Evoked potentials recorded in hypothalamus in response to carotid sinus nerve stimulation. Arrow (+) indicates location of unitary response shown in photo inset. Numbers by locations indicate latency of greatest negative voltage deflection recorded for each peak.
in others, the three components are variously missing or that stimulation of the rearranged. It is clear, however, sinus nerve can evoke responses over a wide area of the medial and lateral hypothalamus in as short a time as lo-20 ms poststimulus, almost parallel in time to the response of some areas of the medulla. In one experiment, at several points in the electrode track, stimulus strength was varied to determine if the early and late negative waves No difference in stimulus were differentially activated. threshold was noted, indicating that the response involves both simple and complex pathways and most likely does not represent activation of two distinct systems, such as chemoreceptor versus baroreceptor. Unit activity associated with the negative waves is shown for three areas in the photo insets. Additional information on hypothalamic unit activity is confined to the experiments displayed here in Fig. 9 and experiments on four units during normal and high blood pressure. Units were hard to isolate and hold; when units were isolated in response to stimulation of the sinus nerve, it was found that they tended to fire in bursts in response to single stimuli, display low levels of spontaneous activity (1-2 spikes/s), and would not follow stimulation frequencies greater than 1.5 stim/s. Four units noted to fire spontaneously were monitored during normal and elevated (by intravenous injection of norepinephrine) systemic blood pressure. Three of these units decreased their rate of firing during elevated pressure; one showed no change.
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1362 DISCUSSION
The purpose of this investigation was to obtain electrophysiological evidence for the functional description of hypothalamic involvement in baroreceptor reflex regulation and medullary-hypothalamic interrelationships necessary for such integration (1, 3, 6, 10, 15-16, 19-26). The results of the investigation rest on the assumption that unit activity recorded in the medulla and the hypothalamus and the phenomena observed with interaction of the hypothalamus and the medulla represent activity of neural elements subserving cardiovascular function. We are content to make this assumption for the following reasons. First, all the units recorded from both the medulla and the hypothalamus in response to sinus nerve stimulation were located in areas of the brain shown by others to modulate baroreceptor function when stimulated and/or eliminated. The distribution of medullary recording sites overlaps those shown by Henry and Calaresu (4) for inhibition and excitation of sympathetic cardioacceleratory neurons in the intermediolateral nucleus of the thoracic cord. Humphrey (8) has shown that destruction of the areas of the nucleus solitarius and reticular formation recorded from eliminates the reflex. Takeuchi and Manning (18, 19) have shown that similar ablations in the hypothalamus where the hypothalamic response to sinus nerve stimulation was recorded result in the elimination of the hypothalamic component of the sinus reflex. Second, while it is true that both baroreceptor and chemoreceptor afferents, of both A and C fiber types, course in the carotid sinus nerve, the cardiovascular response to the stimulation parameters used in this study is that characteristic of the baroreceptor, not chemoreceptor reflex. Therefore, the neural activity is recorded under conditions associated with the functional description of the baroreceptor depressor reflex. Third, investigators ( 13, 17) have shown repeatedly that very few neural units show activity directly correspondent to either the cardiac rhythm or arterial pulse. The information is transferred in a much more subtle and complex manner. An analysis to determine such patterns of information transfer was neither the aim nor result of this investigation. The results of this investigation are: I) single-shock stimulation of the carotid sinus nerve evoked responses in the posterior hypothalamus within lo-20 ms. Thus, information from the baroreceptor nerve can ascend to the hypothalamus with sufficiently short latency to permit its participation in reflex cardiovascular regulation. The area of the hypothalamus which is involved is that shown to be necessary for the total reflex depressor response to high carotid sinus pressure (19). 2) Single-shock stimulation of this same area of the hypothalamus will produce an inhibition of medullary responses to baroreceptor nerve stimulation in an interval between conditioning of the hypothalamus and testing of the sinus nerve as short as 7 ms and extending as late as 790 ms. Further, only 65 % of units recorded were inhibited, implying that the descending projection does not act upon all cells receiving an input from the carotid sinus nerve. 3) Single-shock stimulation of this same hypothalamic area, as well as the contralateral hypothalamus, produces unit responses in the medulla, some in the same area as unit responses to sinus nerve
J. R. ADAIR
AND
J. W.
MANNING
stimulation. It may be that these responses are involved in mechanisms which can produce the inhibition that is noted in unit responses to sinus nerve stimulation during hypothalamic conditioning, as well as being involved in overall visceromotor regulation. A functional path in the baroreceptor reflex which bypasses the medullary vasomotor areas and includes the hypothalamus is evidenced by Manning (1 l), Takeuchi and Manning (19), and Hilton and co-workers (1, 6), who have demonstrated that the baroreceptor reflex is not abolished by loss of the classical vasomotor area as long as supramedullary structures are left intact and that the full range of the autonomic response to baroreceptor activation is dependent on systems that engage hypothalamic neural mechanisms. Direct electrophysiological evidence for such a projection has been scarce, confined to the report of Hilton and that spontaneous unit Spyer m which demonstrated activity in the anterior hypothalamus could be affected by
stimulation. Hilton and Spyer suggest from their observations that the whole brainstem, from hypothalamus through medulla, is the functional unit for integration of baroreceptor afferent information. Thomas and Calaresu have shown hypothalamic units that are time locked in response to sinus nerve stimulation. The latencies of the increase of activity was 29 ms, with a range of 17-40 ms. The results presented in Figs. 8 and 9 of our investigation stand in confirmation of their observation in that the first early negative deflection of the evoked potentials and accompanying units occurs over a range of lo-50 ms, with the earliest responses occurring within lo-20 ms. Our results, however, indicate that areas more rostra1 having a greater lateral distribution than those noted receive excitatory input from the sinus nerve. Thus, a description of the excitatory input from sinus nerve to the posteriomedial hypothalamus as discrete may be too narrow an interpretation. The significance of units observed during the period of time corresponding with the second, long-latency, negative deflection of the evoked potential is difficult to analyze. The probability that unit activity was synchronous with the potential change was not determined; hence, it is not possible to say if unit firing was truly synchronous or simply coincident spontaneous activity. It may be that the long-latency response is the result of activation of complex reticulodiencephalic paths much less direct than that producing the early response. Hilton (5) has suggested that the long-latency response recorded in the hypothalamus to sensory stimulation is mediated by such complex paths. The experiment which determined that both negative peaks of the evoked potential were excited at the same stimulus strength only suggests that the two potentials recorded in the hypothal amus are not due to excitation of different svs terns having different thresholds of activation. An alternative explanation for the late negative potential is that it might represent recurrent hypothalamic activity in response to sinus nerve stimulation. The data presented in this paper do not resolve the question.
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HYPOTHALAMIC-MEDULLARY-SINUS
NERVE
1363
INTERACTIONS
The observations of units responsive to sinus nerve stimulation in the medulla in the regions of the nucleus solitarius and the medullary reticular formation are not new, nor is the demonstration of inhibition of baroreccptor affcrent information during stimulation of the hypothalamus. What is interesting is that units postsynaptic to primary afferents are shown in this study also to be inhibited during hypothalamic stimulation. Weiss and Crill (25) showed that primary afferent depolarization occurred over a time period of 5-120 ms, whereas our results, including observations of higher neurons, indicate the unit inhibition does begin as early as 7 ms, but extends to as late as 790 111s. It might be possible to conclude that higher-order neurons show apparent inhibition because the afferents from elements postsynaptic to the primary a ffkren ts are not being driven due to primary aflerent inhibition. While this is reasonable in accounting for a period of inhibition up to from primary I 20 ms plus delay time for conduction synapse to the higher-order neuron, it clearly cannot account for inhibition up to 300 ms for 50 ‘,‘: of the units studied. The observation of units in the medulla responding to hypothalamic single-shock stimulation has functional implications for several reasons. Stimulation of the carotid sinus nerve will produce unit responses in the medulla which can then be inhibited by stimulation of the same area of the hypothalamus. Some medullary units responding to hypothalamic stimulation are found in areas of the medial reticular formation and the nucleus solitarius where unit responses to baroreceptor nerve stimulation can also be found, and many (65 7;) but not all of the unit responses to nerve stimulation arc inhibited during hypothalamic stimulation. Additionally, repetitive stimulation of this posterior hypothalamic area can produce sympathetic choline1 gic vasodilation in cardiovascular studies ( 18) and a strong defense reaction in behavioral studies (1) supposedly involving a functional resetting of baroreceptors
w. I-\
The work of Weiss and Crill (25) did not include observations of medullary responses during hypothalamic stimulation. Keene and Casey (9) have demonstrated responses in the nucleus gigantocellularis in response to stimulation with latencies of 2-4 lateral hypothalamic ms, and they associated these with facilitation of responses to sensory stimulation in the nucleus. If their interpretation is correct, then the descending projection we have demonstrated is part of a much more complicated mechanism for modulating visceral and somatic sensory information at primary levels. Smith and Nathan (16) have noted that dorsal olivary stimulation has an inhibitory effect on the carotid sinus reflex. Unit responses to hypothalamic stimulation were noted (Fig. 7) in the area of the dorsal olive as well as the nucleus ambiguus, which may be significant
to Thomas and Calaresu’s (22) observations on the role of the nucleus ambiguus in vagal bradycardia and that hypothalamic stimulation can inhibit chemoreceptor-induced vagal bradycardia (2 1). Unit responses in the medulla to hypothalamic stimulation, both ipsilateral and contralateral, occur with a wide range of first-spike latencies, with some occurring very early. Smith’s ( 15) observations of a direct descending projection from the hypothalamus to the medulla, based on anatomical degeneration techniques, has been questioned (26). The shortest latency (2-4 ms) responses recorded in the medulla to hypothalamic stimulation in this study were in the region of the medial longitudinal fasciculus where Smith observed terminal degeneration. Given the information presented in this study and based on conclusions made from the data, an organism is provided with a cardiovascular regulatory reflex center with flexibility than previously imagined and much greater involving the hypothalamus in basic reflex activity. This expanded reflex center, in addition to receiving information and generating an appropriate autonomic response, also includes intrinsic feedback in that the area of the hypothalamus impinged upon by baroreceptor aflerents can generate a response in medullary areas limiting the amount of infor mation which will be accepted : 65 % of medullary units to sinus nerve stimulation were inhibited by responsive hypothalamic stimulation. Certainly, this same descending projection producing the inhibition can possibly be driven by higher centers in the motor cortical and limbic areas. It is important to note that under conditions of hypothalarnic stimulation, 35 % of medullary responses to sinus nerve stimulation remained noninhibited and presumably could continue to relay afferent information, however modified. This descending hypothalamic-medullary projection may be further involved in modulation of other reflex autonomic systems, such as the sympathetic choliner pgic vasodilator system. One then can beg in to envision how a threatened a nimal c an si multa .neously elevate heart rate and blood pressure, shift priorities of blood flow, temperature, and pain response, all the while retaining the characteristics of reflex cardiovascular regulation, only now reset to new levels. This descending projection could possibly reset baroreceptors to hypertensive levels by simply altering the percentage of processed information from the baroreceptors. The
technical assistance of Mrs. is gratefully acknowledged. This investigation was supported NS-05669, NS-02645, and HL- 16648. Present address of J. R. Adair : John Hopkins University School of
Lucy
McElrath
and
Mrs.
Bessie
Lane
Received
for
publication
14 August
by
Public
Health
Dept. of Biomedical Medicine, Baltimore,
Service
Grants
Engineering, Md. 21205.
1974.
REFERENCES 1. ABRAHAMS, V. C., S. M HILTON, AND ,4. ZBROZYNA. Active muscle vasodilation produced by stimulation of the brain stem: its significance in the defense reaction. J. Physiol., London 154: 491-513, 1960. 2. COTTLE, M. A. Degeneration studies of primary afferents of IX and X cranial nerves. J. Camp. Neural. 122 : 329-345, 1964. 3. GEBBER, G. L., AND D. W. SNYDER. Hypothalamic control of baroreceptor reflexes. Am. J. Physzol. 218: 124-131, 1969.
4.
HENRY, J. L., AND F. R. CALARESU. Excitatory and inhibitory inputs from medullary nuclei projecting to spinal cardioacceleratory neurons in the cat. Exptl. Brain Res. 20: 485-504, 1974. 5. HILTON, S. Al. Hypothalamic regulation of the cardiovascular system. Brit. Med. Bull. 22 : 243-248, 1966. 6. HILTON, S. hi., AND K. RI. SPYER. Participation of the anterior hypothalamus in the baroreceptor reflex. J. Physiol., London 218: 271-293,1971.
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1364 7. 8.
9.
10.
11.
12.
13. 14.
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