Brain Research, 549 (1991) 118-126 © 1991 Elsevier Science Publishers B.V. 0006-8993/91/$03.50 A DONIS 000689939116605 K

118

BRES 16605

Electrophysiological properties of neurons in the rostral ventrolateral medulla of normotensive and spontaneously hypertensive rats R.K.W. Chan, Y.S. Chan and T.M. Wong Department of Physiology, Faculty of Medicine, University of Hong Kong (Hong Kong) (Accepted 11 December 1990)

Key words: Wistar-Kyoto rat; Spontaneously hypertensive rat; Ventrolaterai medulla; Blood pressure; Spontaneous neuronal activity; Single discharge unit; Double discharge unit

Single unit activities were recorded from the rostral ventrolateral medulla (RVL) of pentobarbital-anesthetized normotensive Wistar Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). Throughout the recording period, arterial blood pressures of WKY (mean arterial pressure, MAP = 103.1 mm Hg) and SHR (MAP = 159.2 mm Hg) remained stable at the respective basal levels. The units recorded in this study were all spontaneously active and cardiac-locked. Two types of discharge patterns, namely single and double discharges, were identified. These single and double discharge units were found to distribute randomly in RVL. In WKY, 92.6% of RVL neurons exhibited single discharges whereas in SHR, the majority (57%) of RVL neurons exhibited double discharges. The mean firing rate of single discharge units in RVL of SHR was significantly higher than that of WKY, whereas the mean firing rate of double discharge units in WKY was similar to that of SHR. About half of the units studied were also tested for antidromic collision; all units tested could be antidromically activated from the intermediolateral column (IML) of the thoracic spinal cord and the lowest threshold sites were consistently localized within IML. In both groups of rats, the axonal conduction velocity of RVL neurons showed a bimodal distribution viz. the fast and slow conducting axons. The mean conduction velocities of each of these two groups of neurons in WKY and SHR were similar. Most of the double discharge units in WKY and SHR belonged to the fast conducting type. In comparison with WKY, the present findings in SHR demonstrated that the majority of RVL units exhibited double discharge pattern with a fast conduction velocity, and the remaining RVL units showed single discharge pattern with a higher firing rate and regularity. These properties of RVL neurons may probably contribute to the enchanced sympathetic outflow from RVL of SHR and in turn account for the higher BP observed in SHR.

INTRODUCTION T h e rostral ventrolateral medulla ( R V L ) has been well d o c u m e n t e d to play a crucial role in the regulation of cardiovascular functions. This a r e a receives information from the a m y g d a l a 22, defense area of the hypothalamus and m i d b r a i n 24, lateral tegmental field s, fastigial nucleus of the cerebellum 32, nucleus raphe magnus 29 and nucleus tractus solitarius 43, and is responsible for mediating various reflex adjustments of the circulation 32'35. Through its efferents, the R V L provides tonic drive to sympathetic preganglionic neurons in the intermediolateral column of the thoracolumbar spinal cord 1"41, thereby generating the sympathetic outflow which is necessary for the m a i n t e n a n c e of arterial b l o o d pressure (BP) 12'35'42. Chemical stimulation of R V L with excitatory amino acids 17'33"40, which selectively excite neuronal cell bodies rather than fibres of passage 23, or electrical microstimulation of the area 14"4°'42 elicits an increase in sympathetic nervous activity and a strong pressor response. Conversely, bilateral lesion 6'32, cooling TM or microinjection of

tetrodotoxin 4°'44 into the R V L reduces BP to a level equivalent to that found after cervical spinal transection, a t r e a t m e n t that removes s y m p a t h e t i c outflow from the brain. A d m i n i s t r a t i o n of neuronal hyperpolarizing agents (such as glycine), inhibitory n e u r o t r a n s m i t t e r G A B A , or G A B A agonists (such as muscimol) into the R V L eliminates p e r i p h e r a l sympathetic nervous activities and blocks the s y m p a t h o e x c i t a t o r y effect p r o d u c e d by electrical stimulation of the h y p o t h a l a m u s 53. These findings suggest that neurons in the R V L are responsible for generating v a s o m o t o r tone and m a i n t e n a n c e of BP. Sympathoexcitatory neurons in R V L of normotensive rats, cats and rabbits are tonically active 4,3s's°. T h e y fire in close t e m p o r a l synchronization with the cardiac cycle and the rhythmic discharge in the splanchnic nerve 35'42. The v a s o m o t o r sympathetic outflow is therefore generally believed to be largely d e p e n d e n t on the activity of neurons in R V L 35'42. In rat 26'51'56 and m a n s'3s, increase in sympathetic outflow has been shown to be a m a j o r contributing factor in the initiation and m a i n t e n a n c e of hypertension. In the anesthetized 26 or conscious 31'51

Correspondence: Y.S. Chan, Department of Physiology, Faculty of Medicine, University of Hong Kong, 5 Sassoon Road, Hong Kong.

119

s p o n t a n e o u s l y h y p e r t e n s i v e rat ( S H R ) , t h e s y m p a t h e t i c n e r v o u s activity is f o u n d to b e h i g h e r t h a n that in the n o r m o t e n s i v e W i s t a r K y o t o rat ( W K Y ) , t h e p a r e n t strain of S H R . Surgical 54, p h a r m a c o l o g i c a l 2° o r i m m u n o l o g i c a l 9 s y m p a t h e c t o m y l e a d s to a g r e a t e r r e d u c t i o n in b l o o d p r e s s u r e in S H R t h a n in W K Y while n e o n a t a l d e s t r u c t i o n o f t h e s y m p a t h e t i c n e r v o u s s y s t e m p r e v e n t s the d e v e l o p m e n t of h y p e r t e n s i o n in S H R 39. H i t h e r t o , t h e n e u r o n a l substrate(s)

r e s p o n s i b l e for t h e e n h a n c e d s y m p a t h e t i c

activity still r e m a i n s u n k n o w n . R V L , b e i n g a g e n e r a t i n g s o u r c e o f s y m p a t h e t i c v a s o m o t o r drive, m a y be implic a t e d in t h e genesis o f h y p e r t e n s i o n .

Enhanced

elec-

t r o p h y s i o l o g i c a l activities of R V L n e u r o n s in t h e h y p e r tensive

state

sympathetic

may

possibly

outflow

and

result

therefore

in an

increase

a higher

BP.

in In

s u b s t a n t i a t i n g t h e r o l e p l a y e d by R V L in the genesis o f h y p e r t e n s i o n , electrical m i c r o s t i m u l a t i o n of R V L in S H R has b e e n s h o w n in o u r p r e v i o u s study to p r o d u c e an e n h a n c e d p r e s s o r r e s p o n s e with a s u s t a i n e d increase in b l o o d p r e s s u r e e v e n a f t e r t e r m i n a t i o n o f stimulation 14. T h e t h r e s h o l d c u r r e n t s r e q u i r e d to elicit the cardiovascular c h a n g e s are also l o w e r in S H R as c o m p a r e d with WKY

TM, suggesting

that R V L m a y be i m p o r t a n t in the

genesis o f h y p e r t e n s i o n . In o r d e r to i n v e s t i g a t e w h e t h e r the h i g h e r B P in S H R is r e l a t e d to a l t e r e d i n h e r e n t n e u r o n a l p r o p e r t i e s , the p r e s e n t study was u n d e r t a k e n to c h a r a c t e r i z e the elect r o p h y s i o l o g i c a l p r o p e r t i e s of c a r d i o v a s c u l a r n e u r o n s in

Recording of spontaneous neuronal activities The occipital bone overlying the cerebellum was surgically removed to allow placement of the microelectrode. A tungsten microelectrode (tip diameter 1/~m, 10 MD, Frederick Haer) used for electrical microstimulation or extraceUular recording of neuronal activities was inserted vertically through the intact cerebellum in a dorsoventral fashion to the RVL on the right side 37. Throughout the period of electrophysiological recording, arterial blood pressures of WKY (mean arterial pressure, MAP = 103.1 + 1.2 mm Hg, n = 28) and SHR (MAP = 159.2 + 2.3 mm Hg, n -30) remained stable at the respective basal levels. The firing rates of the RVL neurons were monitored continuously by a window discriminator/rate counter and recorded by a strip chart recorder. The spontaneous neuronal activities and TTL pulses derived from the R-wave of the ECG were stimultaneously recorded with a 2-channel tape recorder for off-line analysis of the cardiac-locked rhythmicity using an IBM XT computer. Each RVL unit was characterized by: (1) its barosensitivity in response to an i.v. bolus injection of 2-4 /~g/kg phenylephrine, and (2) the presence of rhythmicity of spontaneous unit activity associated with the cardiac cycle. The extent of synchronization with cardiac cycle was studied using a post-R-wave interval histogram (Fig. 1). The method used was similar to that described by Morrison et alY. In brief, the TI'L pulse derived from the R-wave of the ECG was employed to trigger the averaging of the spontaneous RVL spikes by an IBM XT computer. Units that were rapidly and completely silenced by baroreceptor activation and displayed cardiac cycle synchronous rhythmicity were regarded as 'cardiovascular neurons 'l°m. Those RVL units with bursting activity linked to the respiratory cycle were not studied further. Interspike-interval (ISI) histograms were constructed for each unit recorded in RVL of WKY and SHR. For the single discharge unit, ISI statistics including coefficient of variation (CV) and skewness (SK) of the ISI histogram were also computed; CV and SK were calculated according to the following equations: CV = standard deviation/mean and SK = 3 (mean-median)/standard deviation.

R V L of W K Y a n d S H R . T h e p r o p e r t i e s b e i n g s t u d i e d include:

(1) t h e s p o n t a n e o u s

firing p a t t e r n

neurons,

which may ultimately determine the sympa-

t h e t i c o u t f l o w , (2) t e m p o r a l s y n c h r o n i z a t i o n

of R V L of R V L

n e u r o n a l d i s c h a r g e with R - w a v e o f t h e cardiac cycle, and (3) t h e c o n d u c t i o n v e l o c i t y o f a x o n s p r o j e c t i n g f r o m R V L to the i n t e r m e d i o l a t e r a l c o l u m n ( I M L ) of the thoracic spinal c o r d by the a n t i d r o m i c collision test. P a r t o f this study has b e e n p r e s e n t e d as an abstract 55.

MATERIALS AND METHODS

Animals Female WKY (n = 28) and SHR (n = 30), weighing 210-230 g (19-20 weeks of age), were used in this study. The rat was anesthetized with pentobarbital sodium at a dose of 40 mg/kg i.p. and a supplementary dose of 20 mg/kg was given i.v. when necessary. The femoral artery was cannulated with a polyethylene catheter, which was connected to a Statham pressure transducer and a Gould recorder for the continuous measurement of arterial blood pressure (BP) throughout the experiment. The femoral vein was also cannulated for the infusion of drugs. Electrocardiogram (ECG) signals were obtained from a ECG monitor via 4 platinum electrodes inserted into muscles of the limbs. The head of the animal was then mounted onto a stereotaxic apparatus (Narishige). All pressure points were infiltrated with lidocaine. The rat was allowed to breathe spontaneously and the breathing rate was not significantly disturbed by any of the manipulations employed in this study. Body temperature was maintained at 37 °C with a heating pad.

Antidromic stimulation and determination of axonal conduction velocities To antidromically identify RVL neurons that project to IML of the thoracic spinal cord and to determine the conduction velocity of axons projecting from the recordings sites in RVL to IML, the spinal cord was immobilized with a spinal clamp and a laminectomy was performed at T2-T 5 levels to permit placement of stimulating electrodes. The exposed nerve tissues were covered with warm paraffin oil. An array of two stainless steel electrodes, which were made of lacquer-insulated insect pins with exposed tips of 50-100 /~m and a diameter of 100/~m, were inserted into two different levels of the thoracic spinal cord ipsilateral to the recording sites. Another stainless steel electrode was inserted into the central sulcus of the spinal cord at a level between the two active electrodes to serve as an indifferent electrode. Monopolar cathodal pulses (0.5 ms, 0.5 Hz, single shock) were delivered to sites in the IML regions. Stimulus current (40-60/zA) was monitored on a storage oscilloscope by measuring the voltage drop across a 10-k~ resistor in series with the anode, an alligator clip attached to the back muscle. Criteria used to confirm antidromic activation of RVL neurons include: (1) constant latency in response to antidromic stimulation, and (2) the antidromic collision test 27'35. For the collision test, Iq'L pulse derived from a spontaneous RVL unitary discharge was used to trigger a stimulator (Grass $48), which then delivered the stimulating pulse via a constant current unit (Grass) to the IML of the thoracic spinal cord at a variable delay. Conduction velocities of RVL neurons were obtained by dividing the distance between the RVL and spinal stimulating electrode by the antidromic latency ~9. The distance was measured precisely by a cotton thread from the position of the spinal stimulating electrode (as identified by the scar left on the surface of the spinal cord at the point of insertion) to the position of the recording electrode in the brainstem after perfusion of fixatives and complete laminectomy.

120

Verification of stimulating and recording sites Before electrophysiological recording was performed, the position of the electrode placement in RVL was first determined by electrical microstimulation, which could elicit the characteristic pressor responses as described previouslyTM. At the end of each experiment, small electrolytic lesions (0.5 mA DC for 5 s) were made at the most ventral site of electrode penetrations to mark the locations of the recording electrode in the RVL as well as the stimulating electrodes in the spinal cord. The brain and spinal cord from T2-T5 levels were cut at 60/~m with a frozen cryostat. The stimulation and recording sites were reconstructed from Cresyl violet-stained serial frontal sections with reference to coordinates of the electrode tracks and electrolytic lesions.

(n = 30). They were distributed within RVL at coordinates 11.6-13.3 m m posterior to bregma, lateral 1.9-2.1 mm, and 6.0-6.7 m m deep from the brain surface. In this study, all R V L units recorded in both W K Y and SHR were spontaneously active and displayed a clear cardiac cycle-related rhythmicity, i.e. cardiac-locked (Fig. 1). Although these n e u r o n s did not fire at fixed times in the

Student's t-test was employed to compare the differences in the electrophysiological properties between RVL neurons of WKY and SHR except in comparing the proportion of RVL neurons exhibiting single and double discharge units in WKY and SHR when the Chi-square test was employed. All values were expressed as mean + S.E.M. A difference at the level of P < 0.05 was considered statistically significant.

cardiac cycle, their discharges occurred most frequently in a range of 40-55 ms with respect to the R-wave, approximately in late diastole. All units recorded were also rapidly and completely silenced by baroreceptor activation accomplished by an i.v. bolus injection of 2 - 4 ~g phenylephrine. In both W K Y and SHR, n e u r o n s in R V L exhibited two types of discharge patterns, namely single and double discharges. Single discharge units were characterized by the appearance of single spikes separated from each

RESULTS

other by an interspike interval (ISI) greater than 10 ms. For double discharge units, each firing event consisted of

Spontaneous activities were recorded from 231 neurons in R V L of W K Y (n = 28) and 244 neurons in SHR

a burst of two successive spikes separated by an interval of less than 2 ms, and the ISI between two consecutive double discharges was comparable to that of single

Statistical analysis

A

B

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PosI-FI wave inten~al (rnsee) Fig. 1. RVL neurons with activity phasically related to cardiac cycle. A, B: simultaneous oscilloscope traces of ECG activity (upper traces), arterial BP pulses (middle traces) and spontaneous firing of a single discharge (lower trace of A) or a double discharge (lower trace of B) unit recorded in RVL of a SHR. C: post-R-wave interval histogram of a double discharge RVL unit in a SHR for two consecutive cardiac cycles (1500 sweeps; bin width, 1 ms). • indicates the position of the two consecutive R-waves of the ECG pulses. Similar post-R-wave interval histograms were also obtained in single and double discharge units of WKY as well as in single discharge units of SHR.

121

C

A single discharge SIiR

WKY 5oo 400

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Fig. 2. A, B: interspike interval (ISI) histograms (2 ms bin width) of single and double discharge RVL units in WKY and SHR. (Abbreviations: X, mean; SD, standard deviation; CV, coefficient of variation; SK, skewness). C: oscilloscope traces of single discharge (upper) and double discharge (lower) unit in RVL of a SHR. D: diagram illustrates the location of the 4 units (shown in A and B), namely A, single discharge unit in WKY; &, single discharge unit in SHR; O, double discharge unit in WKY; and Q, double discharge unit in SHR. Abbreviations: AMB, nucleus ambiguus; IO, inferior olive; NTS, nucleus tractus solitarius; RVL, rostral ventrolaterai medulla.

discharges (Figs. 1A,B and 2C). The single and double discharge units were found to distribute randomly in RVL. In WKY, 92.6% of RVL neurons exhibited single discharges and the remaining units (7.4%) exhibited double discharges, whereas in SHR, the majority (57%) of RVL neurons exhibited double discharges and only 43% of units exhibited single discharges. The proportion of neurons exhibiting single and double discharges in SHR was statistically different from those in WKY (Table I). For single discharge RVL units, the mean spontaneous firing rate in SHR was significantly higher than that of WKY. For double discharge RVL units, the mean spontaneous firing rates in WKY and SHR were however similar (Table I). Figure 2 illustrates the ISI histograms of single and double discharge RVL units in WKY and SHR. For single discharge units, the CV and SK of the ISI histogram of SHR were significantly smaller than those of WKY, while no difference was found between the modal ISI (i.e. the most frequent ISI) of these two groups of rats (Table I). This indicates that the single discharge RVL units of SHR exhibited a relatively regular discharge pattern. For double discharge units, the

analyses of CV and SK of the ISI histogram were not applicable as the histogram showed a bimodal distribution, with the first peak representing the modal ISI between the two spikes of each double discharge and the second peak representing the modal ISI between two consecutive double discharges. Apart from a slightly shorter modal ISI of the second peak in SHR than WKY (Table I), no obvious difference was found between the shape of the ISI histogram of double discharge RVL units of WKY and SHR (Fig. 2B). A total of 124 units in WKY and 130 units in SHR were tested for antidromic activation from the IML of the thoracic spinal cord. They could all be antidromatically activated and the lowest threshold sites were consistently localized within IML (Fig. 3D). The electrophysiological properties of these antidromically activated single and double discharge RVL neurons, including the proportion of each type of firing pattern, mean firing rate, shape of the ISI histogram, ISI statistics (CV and SK values), and mean modal ISI values were similar to those units not tested for antidromic collision. Their data were therefore combined in Table I. In both groups of rats, the histogram for the axonal conduction velocity of RVL

122 neurons showed a bimodal distribution namely, the fast conducting axons and slow conducting axons (Fig. 3A,B). The conduction velocity for the slow conducting group ranged from 0.11 to 0.92 m/s, and that for the fast conducting group ranged from 1.3 to 8.8 m/s. The fast conducting group constituted 59.7% of the RVL axons in WKY and 60.8% in SHR. The mean conduction velocity for the slow group was 0.54 + 0.07 m/s in WKY and 0.58 + 0.03 rn/s in SHR while that of the fast group was 2.80 + 0.10 m/s in WKY and 3.2 + 0.6 m/s in SHR (Fig. 3). No difference was found between the mean axonal conduction velocity for the slow or fast conducting axons in both groups of rats. Most of double discharge units in WKY and SHR belonged to the fast conducting type.

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In this study, we have characterized the electrophysiological properties of cardiovascular neurons in the RVL, within which characteristic BP changes could be elicited with electrical microstimulation 14 of WKY and SHR. In these two groups of rats, the firing of both single and double discharge RVL units displayed a strong pulse synchronization with the cardiac cycle and was lowest at 80-120 ms after the occurrence of the R-wave (Fig. 1C). These observations suggest that activities of RVL cardiovascular neurons are strongly associated with baroreceptor input n'13, and substantiate earlier reports of the inhibitory influence of such input on the RVL neurons in each systolic phase 3'21"35. We have also demonstrated that both the single and double discharge RVL cardiovascular units of WKY and SHR responded to phenylephrineinduced increase in arterial BP with a depression in firing rate, but the quantitative relationship between the unitary firing rate and BP for these two types of RVL cardiovascular units in WKY and SHR may not be the same (cf. refs. 47 and 48) and remains to be investigated. Hitherto, no comparison has been made between the firing rate and discharge pattern of RVL cardiovascular neurons in normotensive and spontaneously hypertensive rats. In the present study using pentobarbital anesthesia, the mean firing rate of single discharge RVL units of SHR was found to be significantly higher than that of WKY, whereas the mean firing rate of double discharge RVL units in WKY was similar to that in SHR. As the activity of RVL neurons is found to be highly correlated with the tonic discharges of sympathetic nerves 35, the high spontaneous firing rate of single discharge RVL cardiovascular neurons in SHR may be responsible, at least in part, for the enhanced sympathetic outflow and subsequently higher BP observed in SHR. It should be noted that comparison was made between halothaneanesthetized WKY and SHR with regard to cardiovas-

123

A

C

WKY mean condudion

= 2.8 i

==

wlodly

0.1 m/s

2O m e a n c o n d u c l i o n velocity = 0.54 ± 0.07 m / s

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Conduction

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(m/s)

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m e a n conduction velocity = 3,2 ± 0.6 mls

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6

Fig. 3. A, B: histograms showing the distribution of conduction velocities of antidromically activated RVL units in WKY and SHR. Open columns depict the single discharge firing pattern; shaded columns depict the double discharge firing pattern. The percentages of single and double discharge units with slow and fast conducting spinal projecting axons are shown inside the histogram. C: collision of spontaneous and antidromic spikes of a double discharge unit in RVL of SHR (superimposition of 4 successive traces). Upper trace: antidromic spike in response to spinal cord stimulation at IML; middle trace, a spontaneous spike triggered the stimulus (with a delay of 9.2 ms) which in turn elicited an antidromic spike at RVL with a latency of 7.1 ms; lower trace, same as the middle trace but with a shorter interval (6.0 ms) between the spontaneous spike and the antidromic stimulation, no antidromic spike could be elicited. Q, spontaneous spike that triggered the oscilloscope sweep; &, stimulus artifact; *, antidromic spikes. D: histological section through the thoracic spinal cord at the level of T 3 illustrating the location of the electrode track in the spinal cord for antidromic collision test. White arrow idicates the anatomical position of the lesion site, which was in close vicinity of IML.

cular neurons of the paragigantocellularis lateralis (PGL), a region which is more medial to but overlaps partly with the medial border of R V L 47. No difference was found between the animals in the mean firing rate and discharge pattern of a relatively small population of P G L cardiovascular neurons 47. The discrepancy observed in R V L and P G L neurons may be due to: (1) an intrinsic difference in the electrophysiological properties of neurons in these two regions of the ventral medulla, and/or (2) a difference in neuronal response to different anesthetics 49. The possible influence of anesthesia on the sympathetic outflow responsible for the maintenance of arterial BP, and the existence of neural connections secondary to the ones mediated by RVL in maintaining arterial BP, have both been addressed in recent studies that demonstrate a recovery of arterial BP of anesthetized RVL-lesioned normotensive rats from the hypotensive to the normotensive state 15,16.

Two types of discharge patterns, namely single and double discharges, were identified in R V L of WKY and SHR. This group of bursting double discharge RVL cardiovascular units has never been reported before in the normotensive rat 11,35, cat 33 and rabbit 5° in vivo, probably due to its scarcity in normotensive animals (see Table I). Neurons in the ventral medulla (including the PGL, an area which may include part of RVL) of the in vitro guinea pig brainstem preparation have been found to show a repetitive multiple bursting discharge pattern 34. The occurrence of double discharge units in RVL may be a consequence of the intrinsic membrane property of RVL neurons and/or synaptic drive from neuronal circuitry. In comparison with the single discharge units, these bursting cardiovasuclar neurons in R V L may exert a more efficient influence on the effectors in eliciting the vasoconstrictor responses. For example, the vasoconstrictor responses elicited in rat mesenteric arteries in vitro

124 were larger when the stimulating impulses were delivered in bursts rather than in a regular pattern even though the total number of impulses delivered was identical 36. Intermittent train pulses at a frequency of 20 Hz were also found to favour the release of neuropeptide Y in addition to the coexisting noradrenaline from the pig splenic nerve in vitro 3°. Further studies however have to be performed to elucidate the relationship between the double discharge R V L cardiovascular neurons and the sympathetic outflow in determining the level of arterial BP. The R V L of W K Y and S H R also possess cardiovascular neurons which differ in the regularity and skewness of firing as well as the conduction velocity of spinal projecting axons. When compared with WKY, the firing of single discharge R V L units in S H R was highly regular as indicated by the small CV and SK values. The interspike interval (ISI) histogram of these R V L units in S H R assumed the shape of a Gaussian density with a very small standard deviation, a kind of ISI distribution one would expect from a pacemaker neuron in R V L 49'52. In agreement with previous studies in the normotensive rat 1°'35 and rabbit 5°, there was coexistence of slow and fast conducting reticulospinal cardiovascular neurons in R V L of W K Y and SHR. In SHR, it is interesting to find that the majority of double discharge units were fastconducting while most of the single discharge units were slow-conducting (Fig. 3B). Based on the conduction velocity of the neurons, the axons of the fast-conducting neurons were probably lightly myelinated while those of the slow-conducting axons were unmyelinated t1'35. These features of single and double discharge R V L cardiovascular neurons of S H R may contribute to a higher sympathetic outflow and hence hypertension in these animals. Apart from a difference in the discharge pattern and conduction velocity, neurons of R V L have been shown to be heterogenous in morphology 2 and colocalized neurotransmitter types 7'25'28'45. In addition, two distinct neuREFERENCES 1 Amendt, K., Czachurski, J., Dernbowsky, K. and Seller, H., Bulbospinal projections to the intermediolateral cell column: a neuroanatomical study, J. Auton. Nerv. Syst., 1 (1979) 103-117. 2 Andrezik, J.A., Chan-Palay, V. and Palay, S.L., The nucleus paragigantocellularis in the rat - - Conformation and cytology, Anat. Embryol., 161 (1981) 355-371. 3 Barman, S.M. and Gebber, G.L., Spinal intemeurons with sympathetic nerve-related activity, Am. J. Physiol., 247 (1984) R761-R767. 4 Barman, S.M. and Gebber, G.L., Axonal projection patterns of ventrolateral medullospinal sympathoexcitatory neurons, J. Neurophysiol., 53 (1985) 1551-1566. 5 Barman, S.M. and Gebber, G.L., Lateral tegmental field neurons of cat medulla: a source of basal activity of ventrolateral medullospinal sympathoexcitatory neurons, J. Neurophysiol., 57

ronal populations that bear different sensitivity to the central-acting hypotensive agent, cionidine, have also been demonstrated in the R V L of the normotensive rat 46, suggesting a possibility of differential roles of different R V L neuronal types in cardiovascular regulation. The present study also revealed two types of R V L units having different patterns (Fig. 2, Table I). How these two types of unitary discharges are related to neurons with different morphology, neurotransmitter types or sensitivity to clonidine awaits further investigations. In conclusion, the present study demonstrates for the first time the co-existence of two groups of cardiovascular neurons in R V L of W K Y and SHR. In SHR, a large proportion of units (57%) showed double discharge pattern with a faster conduction velocity than the remaining units (43%) that showed single discharge pattern. These single discharge R V L units of S H R also exhibited a higher firing rate and more regular discharge pattern than those of normotensive animals. These electrophysiological properties revealed in the present study and the enhanced responsiveness of R V L to electrical microstimulation demonstrated in our previous study TM are probably responsible, at least in part, for the genesis of hypertension in SHR. Unresolved questions include: (1) the functional significance of single and double discharge units in the possible differential control of circulation, (2) the neurotransmitter type(s) of single and double discharge unit, and (3) whether the higher neuronal activity in S H R results from a lower baroreceptor input 26 and/or neuromodulation by neuropeptides. Further investigations are therefore needed to provide better insight into the role of R V L in the genesis of hypertension. Acknowledgements. The study was supported by research grants from the Hong Kong University Research Grants Committee, Sun Yat Sen Foundation Fund for Medical Research, Lee Wing Tat Medical Research Fund and Croucher Foundation to Y.S.C. and T.M.W. We thank Mr. C.P. Mok and Mr. S.M. Chan for technical assistance.

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Electrophysiological properties of neurons in the rostral ventrolateral medulla of normotensive and spontaneously hypertensive rats.

Single unit activities were recorded from the rostral ventrolateral medulla (RVL) of pentobarbital-anesthetized normotensive Wistar Kyoto rats (WKY) a...
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