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Suppression of arterial pressure, heart rate and cardiac contractility following microinjection of kainic acid into the nucleus ambiguus of the rat
Neuroscience Letters, 47 (1984) 57-62 Elsevier Scientific Publishers Ireland Ltd.
57
NSL 02717 SUPPRESSION OF ARTERIAL PRESSURE, HEART RATE AND CARDIAC CONTRACTILITY FOLLOWING MICROINJECTION OF KAINIC ACID INTO THE NUCLEUS AMBIGUUS OF THE RAT
SAMUEL H.H. CHAN 1'*, B.T. ONG 1 and PETER T.-H. WONG 2 Departments of 1physiology and 2pharmacology, Faculty of Medicine, National University of Singapore, Kent Ridge, Singapore 0511 (Republic of Singapore) (Received January 21st, 1984; Revised version received March 6th, 1984; Accepted March 9th, 1984)
Key words: hypotension - bradycardia - heart contractility reduction - kainic acid - nucleus ambiguus rat -
In pentobarbital anesthetized rats, unilateral microinjection (500 ng) of the selective perikaryal excitotoxin, kainic acid, into the nucleus ambiguus promoted a significant suppression of arterial pressure, heart rate and cardiac contractility. The degree of such cardiovascular depression followed the order of heart contractility reduction (50.707o) > hypotension (28.907o) > bradycardia (20.8°7o) at the end of a 60-min postinjection period. We concluded that the nucleus ambiguus may exert its control on cardiac activities by both negative chronotropic and inotropic effects on the heart.
It is g e n e r a l l y a g r e e d t h a t an i m p o r t a n t c a r d i o v a s c u l a r r e g u l a t o r y m a c h i n e r y exists in the m e d u l l a o b l o n g a t a [1-3]. T h e exact f u n c t i o n a l o r g a n i z a t i o n o f this c o n t r o l m e c h a n i s m , h o w e v e r , has n o t been fully e s t a b l i s h e d . F o r e x a m p l e , a l t h o u g h the m o s t likely c a n d i d a t e s for the origin o f v a g a l c a r d i a c p r e g a n g l i o n i c n e u r o n s are the nucleus a m b i g u u s ( N A ) a n d d o r s a l m o t o r nucleus o f vagus ( D M N ) [2,4,5,8,12,15], the relative roles o f these two m e d u l l a r y a r e a s in the r e g u l a t i o n o f h e a r t r h y t h m a n d c o n t r a c t i l i t y r e m a i n to be settled. T h e present i n v e s t i g a t i o n was c a r r i e d o u t to i d e n t i f y t h e c a r d i o v a s c u l a r consequences o f specific a c t i v a t i o n o f N A n e u r o n s , using as o u r t o o l [7] t h e e x c i t o t o x i c g l u t a m a t e a n a l o g u e , k a i n i c acid, which is p u r p o r t e d to be selective in exciting o n l y cell p e r i k a r y a b u t n o t fibers o f p a s s a g e o r a f f e r e n t nerve t e r m i n a l s [9,13]. E x p e r i m e n t s were c o n d u c t e d on a d u l t m a l e S p r a g u e - D a w l e y rats (147-185 g) t h a t were a n e s t h e t i z e d with s o d i u m p e n t o b a r b i t a l (40 m g / k g , i.p.). A f t e r r o u t i n e l y int u b a t i n g the t r a c h e a a n d c a n n u l a t i n g t h e right f e m o r a l a r t e r y a n d vein, t h e a n i m a l was fixed to a s t e r e o t a x i c h e a d h o l d e r ( K o p f ) a n d p l a c e d o n a h e a t i n g - p a d . *Author for correspondence at: Department of Physiology, Faculty of Medicine, National University of Singapore, Kent Ridge, Singapore 0511, Republic of Singapore. 0304-3940/84/$ 03.00 © 1984 Elsevier Scientific Publishers Ireland Ltd.
58 Pulsatile and mean arterial pressures were measured from the cannulated femoral artery, via a Statham pressure transducer (P23ID) and through two separate preamplifiers (Grass 7P1) with different bandpass filter settings. An indirect index for cardiac contractility was provided by the firs derivative of the arterial pressure signals over time (dP/dt) [14], using a Grass 7P20 preamplifier. Heart rate was counted by a tachometer (Grass 7P4) triggered by the arterial pressure pulses. All cardiovascular parameters were displayed simultaneously on a Grass 7D polygraph. Unilateral microinjection of KA (dissolved in saline) into the NA on the righthand side was delivered by a stereotaxically positioned 27-gauge stainless-steel cannula, connected to a 1-#I Hamilton micro-syringe. The stereotaxic coordinates used were: 6-8 mm posterior to the lambda, 2 mm lateral to the midline and 8 mm from the cerebellar surface. As a routine, 500 nl of solution was administered over at least 2 min to minimize tissue damage. Verifications of microinjection sites were aided by the addition of 5°7o Evans blue into the medium, and were performed on frozen 30-/zm brainstem sections stained with cresyl violet. A supplementary dose of pentobarbital sodium (20 mg/kg, i.v.) was regularly administered 5-10 min prior to the recording session. This often elicited a transient decrease in arterial pressure a n d / o r heart rate. Microinjection of KA would only commence after these cardiovascular parameters returned to their original baselines. During the experiment, the animal was allowed to breathe spontaneously and the rectal temperature was maintained at 38°C. Results presented in this communication were based only on experiments with histologically confirmed KA microinjection into the NA and its immediate dorsal, medial or dorsomedial border (Fig. 1). Twelve experiments were performed with 500 ng (2.34 nmol) in 500 nl, which was found to be an optimal dose in producing considerable changes in the cardiovascular parameters but was not high enough to cause mortality. When treated with a higher dose of 1000 ng, death resulting from pulmonary congestion was c o m m o n within 3-5 min postinjection [6]. Hence, our limited data thus far obtained from successful application of such a dose (n = 3) did not allow us to elaborate beyond a mention that there were some indications of
Fig. 1. Schematic representations of the medulla oblongata showing the KA microinjection sites in the vicinity of the NA. Abbreviations: m, medial vestibular nucleus; mlf, medial longitudinal fasciculus; na, nucleus ambiguus; nrgc, nucleus reticularis gigantocellularis; nrl, nucleus reticularis lateralis; nrpc, nucleus reticularis parvocellularis; p, pyramid; r, raph6 nucleus; s, spinal vestibular nucleus; sol, nucleus of and tractus solitarius; V, spinal nucleus and tract of trigeminal nerve.
59
a dose-dependency o f KA effects on cardiovascular activities. Unilateral microinjection o f KA (500 ng, n = 12) to the N A invariably promoted a significant (P < 0.001) decrease in arterial pressure, heart contractility and rate (Fig. 2). The respective onset latencies for these events were: 31.2 ___ 6.7 s, 9.6-62.5 s; 44.8 + 10.4 s, 5.5-88.5 s; and 60.8 + 15.8 s, 9.2-131.0 s (mean + S.E., range). There was a gradual progression in such circulatory suppressive effects, albeit differential degrees amongst the three cardiovascular parameters. Intuitively, we anticipated a drastic reduction in heart rhythm because o f the proposed linkage o f N A with the vagal negative chronotropic mechanisms [2,4,5,8,12]. To our surprise, the Aca "I"
30
E
20 z
/
Q
400
300
200
100
J /
0
i
J
l
l
l
80
60
40
20
~ 1'0 1'5 2'0 2'5 3'0 3'5 4'0 4'5 ~'0 5'5 6'0 l K A (500 ng)
P O S T I N J E C T I O N T I M E (min)
Fig. 2. Effects of unilateral microinjection of kainic acid (KA) (500 ng) into the right nucleus ambiguus
on mean arterial pressure (MAP), indirect cardiac contractility index (dP/dt) and heart rate (HR). Values presented are mean + S.E. (n = 12) and were significantly different from the preinjection controls (Student's t-test) at P < 0.001 level.
60 bradycardiac effects of NA amounted to only 20.8% of control at the end of a 60-min postinjection period, as opposed to a respective 28.9% and 50.7% decrease in arterial pressure and cardiac contractility. Several experimental procedures were included to ascertain that our observed results were promoted specifically by the vagal mechanism. Pretreatment with bilateral cervical vagotomy or atropine sulfate (1 mg/kg, i.v.) significantly antagonized the cardiovascular suppressive effects of KA on the NA. Such an antagonism, on the other hand, was not noted after cervical (C4-C5) spinalization. In one experiment, bilateral vagotomy not only completely reversed the reduction in arterial pressure, heart rate and d P / d t following microinjection of KA into the NA, but in fact promoted hypertension ( + 50 mmHg), tachycardia ( + 150 beats/min) and increase in heart contractility ( + 280 mmHg/s). The latter events appeared to be sympathetic in origin, since subsequent cold-blockage of the cervical spinal cord returned the cardiovascular parameters to control levels. The negative chronotropic and inotropic effects following micro-injection of KA into the NA were apparently important causative factors for the observed hypotension. Significant correlations existed between both vasodepression and cardioinhibition (r -- 0.9681, P < 0.001), as well as the reduction in arterial pressure and heart contractility (r = 0.8773, P < 0.001). Control microinjection of the vehicle (saline and 5°70 Evans blue, 500 nl) into the NA did not produce appreciable changes in the cardiovascular parameters. These observations were taken to indicate that the physical action of localized injection was not a confounding factor in our results. Based on its proposed property of specific cell perikaryal activation [9,13], we observed that localized excitation of NA neurons by KA promoted a significant suppression of arterial pressure, heart rate and ventricular contractility in the present study. At a dose of 500 ng, the degree of cardiovascular depression followed the order: heart contractility reduction > hypotension > bradycardia. The origin of vagal cardiac preganglionic neurons in the medulla has undergone a pendular change for almost a century. At this point in time, the best educated conclusion is that they are located in the NA and DMN [2,4,5,8,12,15]. However, conflicting views exist on the functional roles of these two medullary nuclei in the regulation of cardiac activities. Geis et al. [5] suggested that the NA exerts negative chronotropic effects while the DMN promotes negative inotropic actions. This notion, however, is at variance with findings by other investigators. For example, Nosaka and co-workers showed that electrical stimulation of both NA and DMN [10], as well as B- and C-fibers in the vagus nerve [11], promotes bradycardia. They further demonstrated that these fibers are axons originating respectively from the NA and DMN [12]. Moreover, injection of horseradish peroxidase into the right myocardium, at either the sinoatrial or mid-ventricular regions, labels cells primarily in and around the NA [15]. Our observed suppression of heart rhythm and contractility by selective activation of NA perikarya is therefore more in line with the
61 views of the latter authors. The amount of negative inotropic effects produced, in our hands, was in fact more than double that of the chronotropic actions. Such differential actions may be the result of a simultaneous, but more intensified reflex cardioaccelerative mechanism activated by the reduction in arterial pressure. Whether the DMN may participate in similar cardiac controls is at present under investigation. Inherent in the technique of microinjection is the possibility that the chemical may affect adjacent areas by diffusion. Thus, it is feasible that our results may be caused by a KA excitation of cell somata outside the immediate vicinity of NA. Superficially, the rather long onset latencies for the circulatory suppressive effects in some experiments might also point to such a possibility. However, we have obtained preliminary data which indicated that microinjection of KA into the reticular formation neighboring the N A produced somewhat different patterns of cardiovascular changes. Chemical activation of the nucleus reticularis parvocellularis, for example, elicited positive chrono- and inotropic effects and hypotension. On the other hand, selective excitation of the nucleus reticularis gigantocellularis resulted in an elevation in arterial pressure and heart rate, but a reduction in cardiac contractility. Furthermore, based on our previous experience with KA microinjection [6] and the location of the injection sites in the present study, the chance for this excitotoxic agent to reach the depressor area in the ventrolateral medulla oblongata [7] is considered to be small. Thus, although we are unable to absolutely preclude the abovementioned experimental constraints, our results strongly indicated that selective activation of the N A by KA produced simultaneous though differential suppression of arterial pressure, heart rhythm and cardiac contractility. The implied presence of glutamate receptors at this medullary site is presently being confirmed. Suffice it to say, microinjection of L-glutamate into the N A also promoted significant, though transient, hypotension and bradycardia [16]. This study was supported in part by research grants from the National University of Singapore (RP 133/82 to S.H.H.C. and RP 46/83 to P . T . H . W . ) and Singapore National Heart Association (S.H.H.C.). We thank Professor P.Y. Tan for her continuous support.
1 Bard, P., Anatomical organization of the central nervous system in relationship to control of the heart and blood vessels, Physiol. Rev., 40, Suppl. 4 (1960) 3-26. 2 Calaresu, F.R., Faiers, A.A. and Mogenson, G.J., Central neural regulation of heart and blood vessels in mammals, Prog. Neurobiol., 5 (1975) 1-35. 3 Chan, S.H.H., Kuo, J.S., Chen, Y.H. and Hwa, J.Y., Modulatory actions of the gigantocellular reticular nucleus on baroreceptor reflexes in the cat, Brain Res., 196 (1980) 1-9. 4 Ciriello, J. and Calaresu, F.R., Medullary origin of vagal preganglionicaxons to the heart of the cat, J. auton, nerv. Syst., 5 (1982) 9-22. 5 Geis, G.S., Kozelka, J.W. and Wurster, R.D., Organization and reflex control of vagal cardiomotor neurons, J. Auton. Nerv. Syst., 3 (1981) 437-450.
62 6 Lai, Y.Y. and Chan, S.H.H., Antagonization of clonidine- and morphine-promoted antinociception by kainic acid lesion of nucleus reticularis gigantocellularis in the rat, Exp. Neurol., 78 (1982) 38-45. 7 McAllen, R.M., Neil, J.J. and Loewy, A.D., Effects of kainic acid applied to the ventral surface of the medulla oblongata on vasomotor tone, the baroreceptor reflex and hypothalamic autonomic responses, Brain Res., 238 (1982) 65-76. 8 McAllen, R.M. and Spyer, K.M., The location of cardiac vagal preganglionic motoneurons in the medulla of the cat, J. Physiol. (Lond.), 258 (1976) 187-204. 9 McGeer, P.L., McGeer, E.G. and Hattori, T., Kainic acid as a tool in neurobiology. In E.G. McGeer, J.W. Olney and P.L. McGeer (Eds.), Kainic Acid as a Tool in Neurobiology, Raven Press, New York, 1978, pp. 123-138. 10 Nosaka, S., Yamamoto, T. and Yasunaga, K., Localization of vagal cardioinhibitory preganglionic neurons within rat brain stem, J. comp. Neurol., 186 (1979) 79-92. 11 Nosaka, S., Yasunaga, K. and Kawano, M., Vagus cardioinhibitory fibers in rats, PfliJger's Arch., 379 (1979) 281-285. 12 Nosaka, S., Yasunaga, K. and Tamai, S., Vagal cardiac preganglionic neurons: distribution, cell types and reflex discharges, Amer. J. Physiol., 243 (1982) R92-R98. 13 Olney, J.W., Neurotoxicity of excitatory amino acids. In E.G. McGeer, J.W. Olney and P.L. McGeer (Eds.), Kainic Acid as a Tool in Neurobiology, Raven Press, New York, 1978, pp. 95-121. 14 Randall, W.C., Sympathetic control of the heart. In W.C. Randall (Ed.), Neural Regulation of the Heart, Oxford University Press, New York, 1977, pp. 43-94. 15 Stuesse, S.L., Origins of cardiac vagal preganglionic fibers: a retrograde transport study, Brain Res., 236 (1982) 15-25. 16 Willette, R.N., Barcas, P.P., Krieger, A.J. and Sapru, H.N., Vasopressor and depressor areas in the rat medulla, Neuropharmacology, 22 (1983) 1071-1079.
57
NSL 02717 SUPPRESSION OF ARTERIAL PRESSURE, HEART RATE AND CARDIAC CONTRACTILITY FOLLOWING MICROINJECTION OF KAINIC ACID INTO THE NUCLEUS AMBIGUUS OF THE RAT
SAMUEL H.H. CHAN 1'*, B.T. ONG 1 and PETER T.-H. WONG 2 Departments of 1physiology and 2pharmacology, Faculty of Medicine, National University of Singapore, Kent Ridge, Singapore 0511 (Republic of Singapore) (Received January 21st, 1984; Revised version received March 6th, 1984; Accepted March 9th, 1984)
Key words: hypotension - bradycardia - heart contractility reduction - kainic acid - nucleus ambiguus rat -
In pentobarbital anesthetized rats, unilateral microinjection (500 ng) of the selective perikaryal excitotoxin, kainic acid, into the nucleus ambiguus promoted a significant suppression of arterial pressure, heart rate and cardiac contractility. The degree of such cardiovascular depression followed the order of heart contractility reduction (50.707o) > hypotension (28.907o) > bradycardia (20.8°7o) at the end of a 60-min postinjection period. We concluded that the nucleus ambiguus may exert its control on cardiac activities by both negative chronotropic and inotropic effects on the heart.
It is g e n e r a l l y a g r e e d t h a t an i m p o r t a n t c a r d i o v a s c u l a r r e g u l a t o r y m a c h i n e r y exists in the m e d u l l a o b l o n g a t a [1-3]. T h e exact f u n c t i o n a l o r g a n i z a t i o n o f this c o n t r o l m e c h a n i s m , h o w e v e r , has n o t been fully e s t a b l i s h e d . F o r e x a m p l e , a l t h o u g h the m o s t likely c a n d i d a t e s for the origin o f v a g a l c a r d i a c p r e g a n g l i o n i c n e u r o n s are the nucleus a m b i g u u s ( N A ) a n d d o r s a l m o t o r nucleus o f vagus ( D M N ) [2,4,5,8,12,15], the relative roles o f these two m e d u l l a r y a r e a s in the r e g u l a t i o n o f h e a r t r h y t h m a n d c o n t r a c t i l i t y r e m a i n to be settled. T h e present i n v e s t i g a t i o n was c a r r i e d o u t to i d e n t i f y t h e c a r d i o v a s c u l a r consequences o f specific a c t i v a t i o n o f N A n e u r o n s , using as o u r t o o l [7] t h e e x c i t o t o x i c g l u t a m a t e a n a l o g u e , k a i n i c acid, which is p u r p o r t e d to be selective in exciting o n l y cell p e r i k a r y a b u t n o t fibers o f p a s s a g e o r a f f e r e n t nerve t e r m i n a l s [9,13]. E x p e r i m e n t s were c o n d u c t e d on a d u l t m a l e S p r a g u e - D a w l e y rats (147-185 g) t h a t were a n e s t h e t i z e d with s o d i u m p e n t o b a r b i t a l (40 m g / k g , i.p.). A f t e r r o u t i n e l y int u b a t i n g the t r a c h e a a n d c a n n u l a t i n g t h e right f e m o r a l a r t e r y a n d vein, t h e a n i m a l was fixed to a s t e r e o t a x i c h e a d h o l d e r ( K o p f ) a n d p l a c e d o n a h e a t i n g - p a d . *Author for correspondence at: Department of Physiology, Faculty of Medicine, National University of Singapore, Kent Ridge, Singapore 0511, Republic of Singapore. 0304-3940/84/$ 03.00 © 1984 Elsevier Scientific Publishers Ireland Ltd.
58 Pulsatile and mean arterial pressures were measured from the cannulated femoral artery, via a Statham pressure transducer (P23ID) and through two separate preamplifiers (Grass 7P1) with different bandpass filter settings. An indirect index for cardiac contractility was provided by the firs derivative of the arterial pressure signals over time (dP/dt) [14], using a Grass 7P20 preamplifier. Heart rate was counted by a tachometer (Grass 7P4) triggered by the arterial pressure pulses. All cardiovascular parameters were displayed simultaneously on a Grass 7D polygraph. Unilateral microinjection of KA (dissolved in saline) into the NA on the righthand side was delivered by a stereotaxically positioned 27-gauge stainless-steel cannula, connected to a 1-#I Hamilton micro-syringe. The stereotaxic coordinates used were: 6-8 mm posterior to the lambda, 2 mm lateral to the midline and 8 mm from the cerebellar surface. As a routine, 500 nl of solution was administered over at least 2 min to minimize tissue damage. Verifications of microinjection sites were aided by the addition of 5°7o Evans blue into the medium, and were performed on frozen 30-/zm brainstem sections stained with cresyl violet. A supplementary dose of pentobarbital sodium (20 mg/kg, i.v.) was regularly administered 5-10 min prior to the recording session. This often elicited a transient decrease in arterial pressure a n d / o r heart rate. Microinjection of KA would only commence after these cardiovascular parameters returned to their original baselines. During the experiment, the animal was allowed to breathe spontaneously and the rectal temperature was maintained at 38°C. Results presented in this communication were based only on experiments with histologically confirmed KA microinjection into the NA and its immediate dorsal, medial or dorsomedial border (Fig. 1). Twelve experiments were performed with 500 ng (2.34 nmol) in 500 nl, which was found to be an optimal dose in producing considerable changes in the cardiovascular parameters but was not high enough to cause mortality. When treated with a higher dose of 1000 ng, death resulting from pulmonary congestion was c o m m o n within 3-5 min postinjection [6]. Hence, our limited data thus far obtained from successful application of such a dose (n = 3) did not allow us to elaborate beyond a mention that there were some indications of
Fig. 1. Schematic representations of the medulla oblongata showing the KA microinjection sites in the vicinity of the NA. Abbreviations: m, medial vestibular nucleus; mlf, medial longitudinal fasciculus; na, nucleus ambiguus; nrgc, nucleus reticularis gigantocellularis; nrl, nucleus reticularis lateralis; nrpc, nucleus reticularis parvocellularis; p, pyramid; r, raph6 nucleus; s, spinal vestibular nucleus; sol, nucleus of and tractus solitarius; V, spinal nucleus and tract of trigeminal nerve.
59
a dose-dependency o f KA effects on cardiovascular activities. Unilateral microinjection o f KA (500 ng, n = 12) to the N A invariably promoted a significant (P < 0.001) decrease in arterial pressure, heart contractility and rate (Fig. 2). The respective onset latencies for these events were: 31.2 ___ 6.7 s, 9.6-62.5 s; 44.8 + 10.4 s, 5.5-88.5 s; and 60.8 + 15.8 s, 9.2-131.0 s (mean + S.E., range). There was a gradual progression in such circulatory suppressive effects, albeit differential degrees amongst the three cardiovascular parameters. Intuitively, we anticipated a drastic reduction in heart rhythm because o f the proposed linkage o f N A with the vagal negative chronotropic mechanisms [2,4,5,8,12]. To our surprise, the Aca "I"
30
E
20 z
/
Q
400
300
200
100
J /
0
i
J
l
l
l
80
60
40
20
~ 1'0 1'5 2'0 2'5 3'0 3'5 4'0 4'5 ~'0 5'5 6'0 l K A (500 ng)
P O S T I N J E C T I O N T I M E (min)
Fig. 2. Effects of unilateral microinjection of kainic acid (KA) (500 ng) into the right nucleus ambiguus
on mean arterial pressure (MAP), indirect cardiac contractility index (dP/dt) and heart rate (HR). Values presented are mean + S.E. (n = 12) and were significantly different from the preinjection controls (Student's t-test) at P < 0.001 level.
60 bradycardiac effects of NA amounted to only 20.8% of control at the end of a 60-min postinjection period, as opposed to a respective 28.9% and 50.7% decrease in arterial pressure and cardiac contractility. Several experimental procedures were included to ascertain that our observed results were promoted specifically by the vagal mechanism. Pretreatment with bilateral cervical vagotomy or atropine sulfate (1 mg/kg, i.v.) significantly antagonized the cardiovascular suppressive effects of KA on the NA. Such an antagonism, on the other hand, was not noted after cervical (C4-C5) spinalization. In one experiment, bilateral vagotomy not only completely reversed the reduction in arterial pressure, heart rate and d P / d t following microinjection of KA into the NA, but in fact promoted hypertension ( + 50 mmHg), tachycardia ( + 150 beats/min) and increase in heart contractility ( + 280 mmHg/s). The latter events appeared to be sympathetic in origin, since subsequent cold-blockage of the cervical spinal cord returned the cardiovascular parameters to control levels. The negative chronotropic and inotropic effects following micro-injection of KA into the NA were apparently important causative factors for the observed hypotension. Significant correlations existed between both vasodepression and cardioinhibition (r -- 0.9681, P < 0.001), as well as the reduction in arterial pressure and heart contractility (r = 0.8773, P < 0.001). Control microinjection of the vehicle (saline and 5°70 Evans blue, 500 nl) into the NA did not produce appreciable changes in the cardiovascular parameters. These observations were taken to indicate that the physical action of localized injection was not a confounding factor in our results. Based on its proposed property of specific cell perikaryal activation [9,13], we observed that localized excitation of NA neurons by KA promoted a significant suppression of arterial pressure, heart rate and ventricular contractility in the present study. At a dose of 500 ng, the degree of cardiovascular depression followed the order: heart contractility reduction > hypotension > bradycardia. The origin of vagal cardiac preganglionic neurons in the medulla has undergone a pendular change for almost a century. At this point in time, the best educated conclusion is that they are located in the NA and DMN [2,4,5,8,12,15]. However, conflicting views exist on the functional roles of these two medullary nuclei in the regulation of cardiac activities. Geis et al. [5] suggested that the NA exerts negative chronotropic effects while the DMN promotes negative inotropic actions. This notion, however, is at variance with findings by other investigators. For example, Nosaka and co-workers showed that electrical stimulation of both NA and DMN [10], as well as B- and C-fibers in the vagus nerve [11], promotes bradycardia. They further demonstrated that these fibers are axons originating respectively from the NA and DMN [12]. Moreover, injection of horseradish peroxidase into the right myocardium, at either the sinoatrial or mid-ventricular regions, labels cells primarily in and around the NA [15]. Our observed suppression of heart rhythm and contractility by selective activation of NA perikarya is therefore more in line with the
61 views of the latter authors. The amount of negative inotropic effects produced, in our hands, was in fact more than double that of the chronotropic actions. Such differential actions may be the result of a simultaneous, but more intensified reflex cardioaccelerative mechanism activated by the reduction in arterial pressure. Whether the DMN may participate in similar cardiac controls is at present under investigation. Inherent in the technique of microinjection is the possibility that the chemical may affect adjacent areas by diffusion. Thus, it is feasible that our results may be caused by a KA excitation of cell somata outside the immediate vicinity of NA. Superficially, the rather long onset latencies for the circulatory suppressive effects in some experiments might also point to such a possibility. However, we have obtained preliminary data which indicated that microinjection of KA into the reticular formation neighboring the N A produced somewhat different patterns of cardiovascular changes. Chemical activation of the nucleus reticularis parvocellularis, for example, elicited positive chrono- and inotropic effects and hypotension. On the other hand, selective excitation of the nucleus reticularis gigantocellularis resulted in an elevation in arterial pressure and heart rate, but a reduction in cardiac contractility. Furthermore, based on our previous experience with KA microinjection [6] and the location of the injection sites in the present study, the chance for this excitotoxic agent to reach the depressor area in the ventrolateral medulla oblongata [7] is considered to be small. Thus, although we are unable to absolutely preclude the abovementioned experimental constraints, our results strongly indicated that selective activation of the N A by KA produced simultaneous though differential suppression of arterial pressure, heart rhythm and cardiac contractility. The implied presence of glutamate receptors at this medullary site is presently being confirmed. Suffice it to say, microinjection of L-glutamate into the N A also promoted significant, though transient, hypotension and bradycardia [16]. This study was supported in part by research grants from the National University of Singapore (RP 133/82 to S.H.H.C. and RP 46/83 to P . T . H . W . ) and Singapore National Heart Association (S.H.H.C.). We thank Professor P.Y. Tan for her continuous support.
1 Bard, P., Anatomical organization of the central nervous system in relationship to control of the heart and blood vessels, Physiol. Rev., 40, Suppl. 4 (1960) 3-26. 2 Calaresu, F.R., Faiers, A.A. and Mogenson, G.J., Central neural regulation of heart and blood vessels in mammals, Prog. Neurobiol., 5 (1975) 1-35. 3 Chan, S.H.H., Kuo, J.S., Chen, Y.H. and Hwa, J.Y., Modulatory actions of the gigantocellular reticular nucleus on baroreceptor reflexes in the cat, Brain Res., 196 (1980) 1-9. 4 Ciriello, J. and Calaresu, F.R., Medullary origin of vagal preganglionicaxons to the heart of the cat, J. auton, nerv. Syst., 5 (1982) 9-22. 5 Geis, G.S., Kozelka, J.W. and Wurster, R.D., Organization and reflex control of vagal cardiomotor neurons, J. Auton. Nerv. Syst., 3 (1981) 437-450.
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