Local application of somatostatin in the rat ventrolateral brain medulla induces apnea.

Local application of somatostatin in the rat ventrolateral brain medulla induces apnea ZIBIN CHEN, THOMAS HEDNER, AND JAN HEDNER Department of Pharmacology and Clirtical Pharmacology, University and Sahlgrenska Hospital, S-400 33 Goteborg, Sweden

CHEN, ZIBIN, THOMAS HEDNER, AND JAN HEDNER. Local application of somatostutin in the rat uentrolateral brain medulla induces apnea.J. Appl. Physiol. 69(6): 2233-2238,1990.-Local injections of the tetradecapeptidesomatostatin (SOM) into the brain stem region were performed in anesthetized and decerebrate rats. SOM administration (0.648 nmol) into the nucleus paragigantocellularisand the nucleusreticularis lateralis of the ventrolateral medullaoblongata inducedventilatory depression and apnea. The occurrence of apnea was dosedependent and attributed to the anesthetic depth, and it was seenwithin 60240 s after injection. In anesthetized rats the apnea was seen asa termination or a continuous decreasein tidal volume while respiratory frequency remainedunaltered. SOM-induced apnea was causedby depressionof central inspiratory drive. SOM injections into the dorsal medulla were ineffective in eliciting apnea, although a ventilatory depressionbut no apnea was induced in the awake unanesthetized state. In addition to its effect on basalventilation, SOM administration in the ventrolateral medulla resulted in a blunted ventilatory responseto hypoxic and hypercapnic stimuli in anesthetized rats. We conclude that SOM has potent inhibitory effects on respiration that are specifically located in the nucleusparagigantocellularis and the nucleusreticularis lateralis. microinjection; nucleusparagigantocellularis

SOMATOSTATIN (SOM) is a cyclic tetradecapeptide located in nerve cell bodies and terminals in several brain areas involved in the central regulation of respiration (9). Thus a high density of SOM neurons has been demonstrated by immunohistochemical techniques in the ventral portion of the medulla oblongata (nucleus paragigantocellularis, nucleus reticularis lateralis, and nucleus ambiguus) as well as in the dorsal region of the medulla oblongata (nucleus tractus solitarius). In previous studies in the rat (5-7) and the cat (I$), SOM was shown to induce changes in respiratory rhythmicity and even to cause apnea after intracerebroventricular (5) or local (18) brain administration. The specific site(s) for this effect is not yet conclusively defined, and locations within the dorsal (5, 6) as well as the ventral (18) parts of the brain stem have been suggested.

To further study the areas responsible for the regulatory actions of SOM on central regulation of respiration, we have microinjected SOM into several areas of the medulla oblongata of halothane-anesthetized, decerebrate, and conscious rats. In addition to effects on basal ventilation,

we have also investigated the effects of local

of Goteborg

SOM application on the hypoxic and hypercapnic tilatory responses in the anesthetized rat.

ven-

MATERIALS AND METHODS

Experiments were performed on 55 male SpragueDawley rats (Anticimex, Siiderthlje, Sweden) weighing 250-350 g. Animal preparation. One to three days before experi-

ments, the animals were prepared under pentobarbital sodium (40 mg/kg ip) anesthesia. The head of the rat was placed in a stereotaxic frame. One or two 26-gauge stainless steel tubes (13.545 mm long) were lowered stereotaxically to a point 1.5 mm dorsal to the intended sites of microinjection. The coordinates for the microinjection sites were all chosen and are described according to the atlas of Paxinos and Watson (15). The guide cannula was then anchored with cranioplastic cement to metallic screws placed in the skull and was occluded with a stainless steel stylet. The animals were housed in the department with food and water ad libitum to allow full recovery from the surgical interventions. In some rats, decerebration at the level of the lower brain stem was performed under methohexital (5 mg/kg ip) anesthesia. The guide cannulas were implanted and anchored after decerebration. The animal was given 2030 min for recovery. If stable conditions were reached, the experiment proceeded without further anesthesia. Experimental procedures. The trachea was cannulated with a Venflow cannula (2.0 mm diam, Viggo, Helsingborg, Sweden) under methohexital (Brietal, 15 mg/kg) or rompun (12 mg/kg) and ketamine (60 mg/kg) intraperitoneal anesthesia. The ventral tail artery was exposed and cannulated with a polyethylene cannula (model PP50, Portex, Hythe, Kent, UK) for continuous blood pressure monitoring and arterial blood sampling for blood gas analysis. A tail vein was cannulated with a

Venflow cannula (0.4 mm diam, Viggo). A tracheostomy was performed in some rats. All animals were then placed in a closed cylinder-formed body plethysmograph (80 mm ID, 300 mm long), and further anesthesia was maintained with 0.7% halothane (Halothan, Hoechst) in 02 continuously administered via the tracheal cannula by means of a Draeger vaporizer. The body plethysmograph contained openings at both ends for connections of the arterial and the local injection cannulas to the exterior. The interior of the plethysmograph was connected to a Grass polygraph via a low-pressure transducer. Heart

0161-7567/90 $1.50 Copyright 0 1990 the American Physiological Society

2233

Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 18, 2019.

2234

SOMATOSTATIN

rate was calculated from an electrocardiogram recorded simultaneously from subcutaneously placed electrodes in the two forelimbs and the right hindlimb of the rat. Mean arterial pressure was calculated from the blood pressure recordings. One or two 32-gauge stainless steel tubes for drug administration were inserted 15.0-16.5 mm into the cannula guide. No significant effects on ventilation were seen in separate control experiments employing saline microinjection volumes 51 ~1 (2). Ten to twenty minutes were allowed to pass with the animal in the plethysmograph before the control values were obtained. Tidal volume (VT) and respiratory frequency (f) were continuously recorded by the Grass polygraph. Minute ventilation (VE) was calculated according to the formula VT X f = TE. At the end of each experiment, during anesthesia, the rats were given an intravenous injection of pancuronium bromide (0.4 mg). After cessation of respiratory movements, a stepwise calibration of VT was performed with a graded 2-ml syringe. The internal temperature of the plethysmograph was continuously registered, and rectal temperature was measured with a telethermometer (Opti-Lab Instrumentation). A stepwise hypoxic ventilatory response test was performed by changing the 02 content of inspired gas from 100 to 20, 13, and 9% O2 mixed with Nz. Each step was maintained for 30 s. The maximum VT, which was usually the value at the end of the period, and the average f during the last 15 s were used for further calculations. A steady-state hypercapnic ventilatory response was obtained by exposure of the animals to continuous inhalation of 5% COZ-95% O2 for 4-5 min. The maximum value during the last 30 s was registered. In some experiments, 0.2-0.3 ml of arterial blood was withdrawn from the ventral tail artery before and during low 02 or high CO2 inhalation. Blood samples were well isolated from the room air and immediately stored in a freezer. Arterial POT, Pco~, and pH were measured with a blood gas analyzer (model ABL 30, Radiometer). Withdrawn blood volumes were substituted with 0.9% NaCl solution injected intra-arterially. A total of 5-6 blood samples were withdrawn from each animal during a period of 2-3 h with no significant effects on the systemic blood pressure. The influence of anesthesia on the ventilatory response to SOM was studied in a separate group of animals. Different levels of anesthesia were induced according to the following protocol. Intermediate anesthesia was induced by intraperitoneal injection of rompun (12 mg/kg) and ketamine (60 mgfkg) or during 0.7% halothane inhalation. Deep anesthesia was induced by rompun and ketamine mixed with 0.7% halothane or by 1.5% halothane inhalation only. Light anesthesia was obtained by lowering the concentration of inspired halothane to 30%of i7E) but no apnea; 0, no effect. nTS, nucleus of solitary tract; nAmb, nucleus ambiguus; nLRt, nucleus reticularis lateralis; nPGi, nucleus paragigantXocellularis; n7, nucleus facialis. FIG.


SOMATOSTATIN mHg BP

1. Effects on Tr, TE, VT/TI, and TI/TT in anesthetized rats in which microinjection of 0.6-1.8 nmol of SOM into caudal VLM induced apnea TABLE

150 0I

VTlmt

2235

AND RESPIRATION

I

Time Before Apnea (after SOM)

Cbntrol (before SOM)

1 min

EGG SOMlug 20 control

Time

4b3C)

7

5 1

T(min)

SOhmJg

3

t

3

3

2. Representative original blood pressure (BP), respiratory Hnd ECG tracings after microinjections (vertical arrows) of 0.6 (1 pg) and 1.8 (3 pg) nmol of SOM into ventrolateral medulla (VLM). Injection site at interaural -3.3 mm is indicated at Wtowa. right. Two different chart speeds were used (see “Time” at bottom left). Apnea occurred almost immediately after 1.8 nmol of SOM. FIG

(VT),

1.4-

n=l4

30 s

0.18t0.005 0.173-0.007 0.20&0.014* 0.41t0.022 0.43t0.075 0.67t0.014* ml/s 6.34t0.24 4.29-+0.43t 2.04+0.44? 0.30t0.01 0.34t0.03 0.28t0.05 are means t SE of 6 rats. * P < 0.05; t P < 0.01 vs. control values.

TI, s TE, s

VT/TI, TI/TT

Values preinjection

No apnea induced p 1 Cmin after SCM ! ...*................~...*.~~..~.~.*..~.*~~~..~.~.~..~~..~...~..... 10 c A

n l a

18-

z

0

03

0.4

6-

53

0.2

-

0

s

4O-

contr

1 150

I 320

I 90

1 60

i 30

I 0

The toApnea (set) FIG. 3. Ti$al volume (VT), respiratory frequency (f), and minute ventilation (VE) in animals in which apnea was seen after microinjection of 0.6-1.8 nmol of SOM. Values shown are during control conditions and during 150 s before apnea (0 s) in 14 intact halothaneanesthetized animals. Mean time point for SOM injection is indicated by vertical arrow. * P < 0.05, *** P < 0.001 vs. preinjection control values.

laris (nPGi) and nucleus reticularis lateralis (nLRt). Apnea was seen more frequently in the caudal than in the rostra1 portions of these nuclei (Fig. 1 and see also Fig. 5). In general, ventilatory depression without apnea often occurred in the more rostra1 portions of nPGi and the nucleus facialis. When apnea was seen, it occurred within -150 s after the local injection (Figs. 2 and 3). The apnea was preceded by a progressive decrease in VT while f remained essentially unaltered. Inspiratory time (TI) and expiratory time (TE) were not altered until ~30 s before apnea. During this period of ventilatory instability, the respiratory time intervals were significantly decreased (Table 1). However, because

of considerable interindividual variability, f remained unaltered for the group. The respiratory duty cycle (TI/ TT) remained unchanged until apnea occurred, whereas central inspiratory drive (VT/TI) decreased in a progressive manner (Table 1). When given within the borders of nPGi or nLRt, 0.6 nmol of SOM was generally sufficient to elicit apnea. In the ventral portions of these nuclei, the response was characterized by a shorter latency and longer recovery time compared with the more rostra1 areas (see Fig. 5). Borderline injections medial or lateral to the nPGi and nLRt were less effective, because larger doses were needed or no apnea could be induced (Fig. 4).

c 3 co 2-

0 0

2

a

8

8

!

E i-

Od 0

0

I 1.0

1 3.0

Dose of SUM

i

3.0

4.0

(AJcd

FIG. 4. Occurrence of or latency to apnea after local administration of 0.6-1.8 nmol (l-3 ,ug) of SOM into or in vicinity of nucleus paragigantocellularis and nucleus reticularis lateralis. Injection sites are demonstrated in brain stem sections indicated (top right, interaural -4.3 and -3.3 mm). l , Injections within central region of nuclei; A and O, borderline injections medial or lateral to these nuclei, respectively.

Effects of anesthesia on the ventilator-y response to SOM. In addition to dose and site of injection of SOM,

the anesthetic depth was found to be an important determinant for the occurrence of apnea. When SOM (0.61.2 nmol) was injected into the caudal portion of the VLM of deeply anesthetized animals, apnea appeared rapidly (80 t 27 s) and was long lasting (1,975 t 654 s); in 10 of 12 intermediately anesthetized rats, a longer latency (192 & 36 s) and shorter recovery time (1,054 t 270 s) were observed. No apnea could be induced by local injection of SOM (0.6-1.8 nmol) in six lightly anesthe-

tized rats. Effects of SOM on ventilation in decerebrate rats (Fig. 5). Decerebration was successfully performed in eight rats. These animals consistently showed a lower and sometimes a variable respiratory timing and VT (data not shown). Microinjection of SOM (0.6-1.2 nmol) into the caudal VLM (ventral portion of nRLt and nPGi) of these animals readily induced apnea. A high individual variability of VT and f before apnea was seen. The time to apnea after local application of SOM was dose dependent, with a shorter latency after higher doses. The


2236

SOMATOSTATIN

No apnea

AND

RESPIR

{ATION

induced

Conscious -2.0 No apnea

-4.0

-3.0

induced

l.

g

4

‘.

5

A t

l.

‘. f

4

2

-5.0

2

IOf

5

*.

l*..

l “‘\**

Intact anesthetized

*-E

55

1

1

-2.0

l

e

Q*” ..**d

l */.. ****‘***-f *...---... ..I.....-.....-I l

a

-3.0

1

-4.0

t

I

Contr

-5.0

6. VT, f, and VE 0.6 nmol (1 pg) of SOM FIG.

No

1,

0

ra apne

induced 0 00 0 - ~..~.....~..~.~~..~......~~....................~..~........~......~~..........~.....~~.~..~.......

I

1

1

I

1

4

15 20 25 30 Time (mid changes within 30 min after local injection of into caudal VLM in 12 conscious rats. 5

10

areas of the nRLt during intermediate or deep anesthesia resulted in apnea within a short period of time. Repeated application of 0.6-1.2 nmol of SOM during conscious conditions did not induce any significant effects on ventilation. Effects of SOM on hypoxic and hypercupnic ventilatory

Decerebrate ”

-2.0

-4.0

-3.0 Brain

level

: Interaural

-5.0

(mm)

FIG. 5. Relationshin of time to apnea (vertical axis) vs. iniection site at different levels-of medulla (hkizontal axis) in conscious (top), intact anesthetized (middle), and unanesthetized decerebrate rats (bottom). Filled symbols, sites where apnea occurred after local administration of 0.6-1.8 nmol of SOM; open symbols, sites where no apnea occurred. Injections of 0.6, 1.2, and 1.8 nmol are presented by circles, triangles, and squares, respectively.

latency to apnea and site of injection showed a curvilinear relationship. The injection sites associated with the shortest latency were similar to those in intact anesthetized rats (Fig. 5). When 0.6-1.8 nmol of SOM were injected into the rostra1 nPGi or into the nucleus facialis, only slight decreases in f and/or VT were seen. No apnea occurred. Local SOM administration in conscious ruts. Only slight decreases in f and VT but no apnea were seen (Figs. 5 and 6) when 0.6-1.2 nmol of SOM were applied in apnea-sensitive sites in 15 conscious rats. Effects of SOM on ventilation during conscious and anesthetized states were studied by shifting the anesthetic level in the same rat. In four rats, 0.6 nmol of SOM injected into the

responses. Local injections of 0.6-1.8 nmol of SOM into the VLM significantly blunted the ventilatory response to graded hypoxia (Fig. 7). The VT response was reduced by ~60%, and no increase in f was observed. Ventilatory responses to increases in inspired CO2 were depressed after local administration of 0.6-1.8 nmol of SOM into the VLM. After SOM,the expected increase in VT during increasing arterial PCO~ was blunted, and there was no significant difference in the f response between normal and high arterial PCO~ (Fig. 8). DISCUSSION

Endogenous SOM in the brain seems to have a general inhibitory function in various systems. Recent reports also suggest that SOM has important inhibitory actions on respiration (5-7, 18). Some of these studies (5-7) involved intracisternal injections of SOM, and the finding of SOM nerve cell bodies and terminals in the ventral and ventrolateral subnuclei of the nucleus of the tractus solitarius suggests that the ventilatory effects were induced from this site. However, as in Yamamoto and coworkers’ (18) experiments in the cat, we found that microinjections of SOM into the rat VLM readily induced apnea, whereas administration into the dorsal medulla (DM) was ineffective. The VLM, especially in the region of nPGi, has been suggested as an important site for the integration of ventilatory drive inputs (3). The DM, especially the neural population of the nucleus


SOMATOSTATIN

2237

AND RESPIRATION

180

.

80 110 Is 2E 100 z (D ii

90

*

80

1.8 p w

1.6 1.4

5 1.2 1.0 I

I

I

1

I

I

I

4

5

6

7

8

9

IO

wiu2

0

(kPa)

FIG. 8. Ventilatory responses to local injections of SOM into VLM during normal and increased arterial CO2 tensions (Pace,). Values are means t SE of VE, f, and VT for 6 rats during control conditions (A) and for 9 rats after injection of 0.6-1.8 nmol of SOM (A). Statistical comparison by t test, * P C 0.05.

~ 20

50

Inspired

100

0’2 ( % 1

FIG. 7. Ventilatory response to local injections (0.6-1.8 nmol) of SOM (m) into VLM during hyperoxia, normoxia, and graded hypoxia. Values are means t SE of iTE, f, and VT for 20 rats. * P < 0.05, ** P < 0.01, and *** P < 0.001 vs. corresponding preinjection ventilation curve w.

of the solitary tract, seems, in addition to affecting somewhat the depth of inspiration (l7), to be involved in different respiratory reflexes and control mechanisms that influence the pattern of breathing (3). Thus experiments in the cat that apply focal cooling in the regions of nPGi and nucleus preolivaris have resulted in apnea, whereas reversible cooling or chemical lesions in the nucleus of the solitary tract of the DM mainly influenced the timing of the respiratory cycle (1, 14). Interestingly, we found that, in intact anesthetized rats, SOM-induced apnea in the VLM was caused by a depression of inspiratory drive, while the respiratory timing remained constant. Moreover, previous experiments by Schlaefke (16) demonstrated that graded cooling of the intermediate chemoceptive area in the cat causes a marked reduction of the ventilatory amplitude, whereas the respiratory frequency remains unaltered. Evidently these effects on ventilation could be mimicked by the local SOM injections into the VLM, as was shown in our experiments. Furthermore, our data are in agreement with the recent results of Yamamoto et al. (18), who reported that the ventilatory depression induced by injection of SOM into the nPGi was associated with a decrease in integrated Phrenic nerve activitv.

To further evaluate the influence of anesthesia on the apneic response, decerebration was performed at the level of the lower pons on the border to the medulla oblongata. Apnea elicited in the decerebrate and in the intact anesthetized animals had almost the same characteristics in all aspects. These findings indicate that the caudal VLM is indeed essential for the apnea-inducing effect. Moreover, because apnea could not be elicited in conscious intact animals, our data imply that the ascending activating system in the brain stem may have a role in antagonizing the apnea-inducing effect of SOM. Thus during certain physiological situations associated with lower activity in this system, e.g., sleep or drug-induced anesthesia, the apnea-inducing effect of endogenous SOM may be enhanced. Speculatively, such conditions may be present in association with occurrence of sleepapneic events or irregular breathing during sleep in the neonate. The unstable respiratory pattern seen in the decerebrate animals may result from damage of several neuronal pools associated with normal breathing rhythmicity by transection of the brain stem. Our results also demonstrate that the most caudal portions of the nPGi and nLRt were the most sensitive to SOM microinjections. Within these nuclei, apnea appeared to be dose dependent, with a threshold dose of 4.6 nmol. In addition, apnea could also be induced by microinjections into the nucleus ambiguus. Microinjections medial and lateral to these nuclei could also induce aDnea, but higher doses were generally needed.


2238

SOMATOSTATIN

First, these findings indicate that the anatomic distribution of locally injected SOM is very limited. Second, the apnea-inducing effect of SOM seems to be anatomically well defined. Interestingly, these locations have been shown to contain high levels of SOM in previous immunohistochemical studies (9). In addition to its vital role in integrating various drive inputs to the medullary rhythm-generating and patterncontrolling neuronal structures, the VLM also has an important integrative role for the function of the central chemoreceptors located within defined areas near the surface of the VLM (10, 13). The chemosensitive areas that also have been reported in the rat (8), although this finding was not confirmed by Malcolm and co-workers (ll), respond to local changes in Pm2 and/or [H+] in the medullary extracellular fluid (16). The present fkding that SOM administration into the VLM blunted the hypercapnic as well as the hypoxic responses offers further support for a chemosensitive region in the ventral medulla of the rat. No effects on these responses were seen after injections into the DM. Although the DM is a more likely region for the synaptic conversion of peripheral chemoreceptor inputs (16), this probability is not unanimously agreed on (12). Previous research from our laboratory has also indicated that other neuropeptides, such as substance P, microinjetted into the VLM have profound effects not only on the hypercapnic but also on the hypoxic ventilatory responses (2). Interestingly, the basal ventilatory response to local VLM injection of SOM was opposite to that previously reported for substance P microinjections in this region (2). In fact, our data indicate that there is a physiological antagonism between these two peptides at the level of the VLM (Chen et al., unpublished results). In conclusion, the present study shows that SOM, when applied locally in the VLM but not in the DM, depressed basal ventilation. The finding suggests that SOM may modulate the drive input to the pattern- and rhythm-generating structures. Although our experiments suggest that the ventilatory inhibitory effects of SOM are functional during normal conditions as well as during hypoxia and hypercapnia, the present lack of a SOM antagonist makes it impossible to determine how important this mechanism is during physiological conditions. However, our results indicate that endogenous SOM in the VLM has an important rule in the central respiratory integration of central and peripheral drive inputs. Moreover, during physiological states associated with decreased awareness, such as sleep or anesthesia, increased SOM activity may result in irregular breathing or apneic events by blunting central chemosensitivity. This project was supported by Swedish Medical Research Council Grants 2464, 2862, and 8642. 2. Chen was supported by the Swedish Institute and the Swedish Foundation for Clinical Pharmacolom.

AND

RESPIRATION

2. Chen was on leave from the Dept. of Physiology, Shanghai Medical University, Shanghai 20032, People’s Republic of China. Address for reprint requests: 2. Chen, Dept. of Pharmacology, University of Gijteborg, Box 33031, S-400 33 Gijteborg, Sweden. Received 7 February 1989; accepted in final form 27 July 1990. REFERENCES

F. F. KAO, T. PANTALEO, AND Y. Effect of graded focal cold block in rostra1 areas of the medulla. Acta Physiol. Stand. 124: 329-340, 1985. CHEN, Z., J. HEDNER, AND T. HEDNER. Substance P in the ventrolateral medulla oblongata regulates ventilatory responses. J. Appl. Physiol. 68: 2631-2639, 1990. EULER, C. VON. Brain stem mechanisms for generation and control of the breathing pattern. In: Handbook of Physiology. The Respiratory System. ControZ of Breathing. Bethesda, MD: Am. Physiol. Sot., 1986, sect. 3, vol. II, pt. 1, chapt. 1, p. l-67. FLEMING, P. J., M. R. LEVINE, A. L. GONCALVES, AND S. WOLLARD. Barometric plethysmograph: advantages and limitations in recording infant respiration. J. Appl. Physiol. 55: 1924-1931,1983. FUXE, K., L. F. AGNATI, A. HARFSTRAND, V. MUTT, K. ANDERSSON, T. HOKFELT, W. VALE, M. BROWN, AND J. RIVIER. Cardiovascular and respiratory actions of somatostatin peptides following intracisternal injections into the a-chloralose anaesthetized rat, Neurosci. Lett. Suppl. 10: 189, 1982. HARFSTRAND, A., K. FUXE, M. KALIA, AND L. F. AGNATI. Somatostatin induced apnea: prevention by central and peripheral administration of the opiate receptor blocking agent naloxone. Acta Physiol. Stand. 125: 91-95, 1985. KALIA, M., K. FUXE, L. F. AGNATI, T. HOKFELT, AND A. H~~RFSTRAND. Somatostatin produces apnea and is localized in medullary respiratory nucleus: a possible role in apneic syndromes. Brain Res. 236: 339-344, 1984. LAHA, P. K., U. NAYAR, G. S. CHHINA, AND B. SINGH. Carbon

I. BUDZINSKA, YAMAMOTO. 2.

3.

4.

5.

6

7.

8.

dioxide

K., C. VON EULER,

sensitivity

of the central

chemosensitive

mechanisms.

An

explanation by direct stimulation in rats. pfluegers Arch. 367: 241247,1977. 9. LEIBSTEIN, A. G., R. DERMITZEL, I. M. WILLENBERG, AND R. PAUSCHERT. Mapping of different neuropeptides in the lower brainstem of the rat: with special reference to the ventral surface. J. A&on. Nerv. Syst. 14: 299-313, 1985. 10. LOESCHCKE, H. H., J. DELATTRE, M. E. SCHLAEFKE, AND C. 0. TROUTH. Effect on respiration and circulation of electrically stimulating the ventral surface of the medulla oblongata. Respir. Physiol. 10: 184-197, 1970. 11. MALCOLM, J. L., I. H. SARELIUS, AND J. D. SINCLAIR. The respi-

ratory role of the ventral surface of the medulla studied in the anaesthetized rat. J. Physiol. Lond. 307: 503-515, 1980. 12. MILLHORN, D. E., AND F. L. ELDRIDGE. Role of ventrolateral medulla in regulation of respiratory and cardiovascular systems. J. AppE. Physiol. 61: 1249-1263, 1986. 13. MITCHELL, R. A., H. H. LOESCHCKE, J. W. SEVERINGHAUS, B. WI RICHARDSON, AND W. H. MASSIUN. Region of respiratory chemosensitivity on the surface of the medulla. Ann. NY Acad. Sci. 109: 661-681,1963. 14. MORIN-SURUN, M. P., J. CHAMPAGNAT, E. BOUDINOT, AND M. DENAVIT-SAUBIE. Differentiation of two respiratory areas in the cat medulla using kainic acid. Respir. Physiol. 58: 323-334, 1984. 15. PAXINOS, G., AND C. WATSON. The Rat Bruin in Stereotaxic Coordinates. New York: Academic, 1982. M. E. Central chemosensitivity: a respirat,ory drive. 16. SCHLAEFKE, Rev. Physiol. Biochem. Phurmacol. 90: 171-244, 1981. 17. SPECK, D. F., AND J. C. FELDMAN. The effects of microstimulation

and microlesions in the ventral and dorsal respiratory groups in medulla of cat. J. Neurosci. 2: 744-757, 1982. 18. YAMAMOTO, Y., M. RUNOLD, N, PRABHAKAR, T. PANTALEO, AND H. LAGERCRANTZ. Somatostatin in the control of respiration. Acta Ph.ysiol.

Scmd.

134: 529-533,

1988.


Somatostatin in the rat rostral ventrolateral medulla: Origins and mechanism of action.

Role of the rostral ventrolateral medulla in the generation of synchronized sympathetic rhythmicities in the rat.

Intracellular analysis in vivo of different barosensitive bulbospinal neurons in the rat rostral ventrolateral medulla.

Cardiovascular responses to electrical stimulation of the ventrolateral medulla of the spontaneously hypertensive rat.

Projections from the rostral ventrolateral medulla to brainstem monoamine neurons in the rat.

Responses of single units in the midline medulla to stimulation of the rostral ventrolateral medulla.

Axonal projections from respiratory centres towards the rostral ventrolateral medulla in the rat.

Characteristics of caudal ventrolateral medullary neurons antidromically activated from rostral ventrolateral medulla in the rabbit.

The ventrolateral medulla of the cat mediates vestibulosympathetic reflexes.

Renal afferent input to the ventrolateral medulla of the cat.

Changes in the tail surface temperature of the rat following injection of 5-hydroxytryptamine into the ventrolateral medulla.

The regional distribution of somatostatin in the rat brain.

Neurons in the caudal ventrolateral medulla mediate the somato-sympathetic inhibitory reflex response via GABA receptors in the rostral ventrolateral medulla.

Mapping and Analysis of the Connectome of Sympathetic Premotor Neurons in the Rostral Ventrolateral Medulla of the Rat Using a Volumetric Brain Atlas.

Interaction between pressor and depressor areas in cat ventrolateral medulla.

Rostral ventrolateral medulla and sympathorespiratory integration in rats.

Preferential release of neuroactive amino acids from the ventrolateral medulla of the rat in vivo as measured by microdialysis.

Vasopressin V1a receptors mediate the hypertensive effects of [Pyr1 ]apelin-13 in the rat rostral ventrolateral medulla.

Evidence for a cholinergic projection from the pedunculopontine tegmental nucleus to the rostral ventrolateral medulla in the rat.

Anatomical evidence for multiple pathways leading from the rostral ventrolateral medulla (nucleus paragigantocellularis) to the locus coeruleus in rat.

Interactions of endogenous opioid and excitatory amino acid inputs to the caudal ventrolateral medulla of the rat.

Development of antibodies against the rat brain somatostatin receptor.

Involvement of endogenous hydrogen sulfide (H2S) in the rostral ventrolateral medulla (RVLM) in hypoxia-induced hypothermia.

Immunohistochemical evidence of tissue hypoxia and astrogliosis in the rostral ventrolateral medulla of spontaneously hypertensive rats.