Reciprocal connection between nucleus ambiguus and caudal ventrolateral medulla.

To investigate interactions between parasympathetic preganglionic neurons in the nucleus ambiguus (NA) and sympathoinhibitory neurons in the caudal ventrolateral medulla (CVLM) the activity of CVLM neurons was recorded during glutamate stimulation of cardioinhibitory sites in the NA of urethane-anesthetized rats. Neurons in the CVLM were identified as cardiovascular neurons if they increased their activity after i.v. phenylephrine and displayed cardiac cycle-related rhythmicity. Of 23 cardiovascular neurons studied, 10 decreased activity after glutamate (GLU) microinjection in the NA, five neurons were excited and eight did not respond. In another series of experiments, the nature of the influence of the CVLM on unit activity in the NA was investigated. Sites in the CVLM from which decreases in arterial pressure and heart rate were elicited after GLU microinjection were identified and the activity of cardiovascular neurons in the NA was recorded. Of 22 NA cardiovascular neurons studied, eight decreased activity after microinjection of GLU in the CVLM and 14 did not change firing frequency. These results demonstrate the existence of a reciprocal pathway between the NA and CVLM and provide evidence for functional interactions between medullary sites implicated in the control of the parasympathetic and sympathetic nervous systems.

Nucleus ambiguus inhibits activity of cardiovascular units in RVLM.

To investigate interactions between sites in the medulla which control the sympathetic (SNS) and the parasympathetic (PNS) nervous system, the activity of cardiovascular neurons in the rostral ventrolateral medulla (RVLM) was recorded during chemical stimulation of cardioinhibitory neurons in the nucleus ambiguus (NA). Cardioinhibitory sites in the NA were identified in urethane anesthetized rats. The spontaneous activity of single units in the RVLM was recorded and cardiovascular units were identified as units which displayed a cardiac cycle-related rhythmicity (CR) and decreased activity after i.v. phenylephrine. Of 55 units studied, 36 units were identified as cardiovascular and of these, 24 decreased activity after glutamate (GLU) microinjection in the NA, two units were excited and ten did not respond. Decreases in RVLM unit activity were positively correlated with the magnitude and duration of heart rate decreases elicited by GLU. Seven of the 24 units were tested for their response to microinjection of glycine (GLY) in the NA. GLY did not change firing frequency but did accentuate CR. These results demonstrate an inhibitory connection between the NA and RVLM which influences the rhythmicity of RVLM unit activity and provide clear evidence for functional interactions between the SNS and PNS within the central nervous system.

Insulin microinjection into the nucleus tractus solitarii of the rat attenuates the baroreceptor reflex.

The presence of insulin and insulin binding sites in the central nervous system and the demonstration that central insulin has an effect on cardiovascular function has led to the hypothesis that insulin may play a role in mediating the baroreceptor reflex. To investigate this possibility, insulin was microinjected into depressor sites in the nucleus tractus solitarius (NTS), the first central synapse of the baroreceptor reflex, of urethane anesthetized rats. Microinjection of insulin into the NTS (110 nL of 1, 10 and 100 IU/mL) did not change mean arterial pressure (MAP) or heart rate (HR). However, insulin microinjection attenuated phenylephrine-elicited reflex bradycardia and depressor responses elicited by glutamate (GLU). The attenuation of GLU-elicited depressor responses was time-dependent for MAP changes and time and concentration-dependent for HR changes (p < 0.05). Insulin-like growth factor-1 microinjection into the NTS also attenuated GLU-elicited decreases in MAP (p < 0.05) but not HR. The effect of insulin on GLU-elicited responses was inhibited after peripheral adrenergic blockade by nadolol (1.0 mg/kg i.v.) but not after cholinergic blockade by methyl-atropine (2.0 mg/kg i.v.). These results demonstrate that insulin inhibits baroreceptor reflex responses in the NTS likely through an influence on the effects of excitatory amino acid neurotransmitters on the activity of NTS neurons involved in sympathetic control of the cardiovascular system.

Influence of GABAergic mechanisms on baroreceptor inputs to nucleus tractus solitarii of rats.

The firing frequency of baroreceptive neurons in the nucleus tractus solitarii (NTS) during microiontophoretic application of muscimol, a gamma-aminobutyric acid (GABA)A agonist, or baclofen, a GABAB agonist, was monitored in anesthetized rats. Muscimol decreased the spontaneous discharge of 69 of 73 (94.5%) NTS baroreceptive neurons without affecting the remaining four neurons (5.5%). The statistical comparison on a bin-by-bin basis of the peri-R wave interval histograms of the discharge of each NTS neuron showed that the inhibitory action of muscimol was always exerted on the whole neuronal discharge independently of its correlation to the cardiac cycle. Baclofen inhibited 60 of 73 (82.2%) NTS neurons without affecting the remaining 13 neurons (17.8%). In 31 of 60 (51.7%) neurons inhibited by baclofen, this substance significantly affected only pulse-synchronous peaks of neuronal discharge without significant inhibition of the neuronal firing between cardiac cycle-related peaks. Fifty-eight of 73 (79.5%) NTS neurons studied were inhibited by both muscimol and baclofen, 11 neurons (15%) only by muscimol, 2 neurons (2.7%) only by baclofen, and 2 neurons (2.7%) were unaffected by both substances. These results demonstrate that both GABAA and GABAB receptors mediate inhibition of the spontaneous discharge in the great majority of the NTS baroreceptive neurons studied and suggest different functions of the two types of GABA receptors in influencing baroreceptor inputs to the NTS.

Role of ventrolateral medulla in reflex cardiovascular responses to activation of skin and muscle nerves.

Central neuronal circuits mediating reflex cardiovascular responses to skin and muscle nerve stimulation were studied in rats under urethan anesthesia. Responses of right rostral ventrolateral medulla (RVLM) and caudal ventrolateral medulla (CVLM) cardiovascular neurons to stimulation of contralateral skin and muscle afferent fibers were investigated. Stimulation of the tibial (muscle) nerve excited 19 (86%) of 22 CVLM neurons and inhibited 18 (82%) of 22 RVLM neurons. Stimulation of the sural (skin) nerve excited 20 (91%) of the 22 RVLM neurons but did not affect the firing rate of any of the 22 CVLM neurons. Electrolytic lesions of the CVLM abolished the depressor responses induced by stimulation of the tibial nerve without affecting the pressor response caused by sural nerve stimulation. Similarly, reversible blockade of the CVLM by microinjection of gamma-amino-butyric acid or CoCl2 abolished the depressor response to stimulation of the tibial nerve without affecting the pressor response induced by sural nerve stimulation. These results suggest that vasodepressor responses to muscle nerve activation are mediated by a neuronal inhibitory pathway to the RVLM relayed through the CVLM.

Cardiovascular effects of microinjection of atrial natriuretic factor (ANF) in the nucleus tractus solitarii of spontaneously hypertensive rats.

The effects on mean arterial pressure (MAP) and heart rate (HR) of unilateral microinjections of atrial natriuretic factor (ANF) into discrete sites of the nucleus tractus solitarii (NTS) of spontaneously hypertensive rats (SHR) were compared with those observed in normotensive Wistar-Kyoto rats (WKY). NTS sites were identified to be involved in cardiovascular control on the basis of the bradycardia and hypotension elicited by microinjections of 20 nl of 0.1 M L-glutamate. Microinjection of 20 nl of 10(-7) M ANF into 38 NTS 'cardiovascular sites' in rats of the SHR strain decreased MAP (-8.7 +/- 1.8 mmHg) and HR (-7.8 +/- 1.9 bpm) in 9 sites (24%), but caused no changes in the remaining 29 sites (76%). In WKY rats 35 cardiovascular sites within the NTS were studied. In 18 sites (51%) ANF microinjections induced a decrease in MAP (-15.1 +/- 1.9 mmHg) and in HR (-18.1 +/- 3.9 bpm), whereas the remaining 17 sites (49%) were unaffected. The decreased responsiveness of the NTS to ANF in the SHR animals could play a role in the development and/or maintenance of the elevated arterial blood pressure in genetically hypertensive rats.

Cardiovascular and single unit responses elicited by stimulation of the islands of Calleja and by changes in arterial pressure.

Recent experiments in this laboratory have investigated the distribution of the nuclear protein Fos in the rat brain after unilateral electrical stimulation of the aortic depressor nerve and have revealed intense ipsilateral straining in the islands of Calleja. To test the hypothesis that the islands of Calleja may have a role in cardiovascular control, we tested the effect on arterial pressure and heart rate of microinjection of L-glutamate into the islands of Calleja of the artificially ventilated, urethane anesthetized rat. Microinjection of glutamate into the islands of Calleja resulted in a decrease in arterial pressure (24.5 +/- 1.5 mmHg) and heart rate (16.6 +/- 1.6 bpm). These responses were eliminated by i.v. injection of propranolol or by transection of the spinal cord at the C1 level. These results are the first demonstration that the islands of Calleja are involved in mediating cardiovascular reflexes primarily through an influence on sympathetic outflow to the heart and vessels. To determine the effects of increases or decreases of arterial pressure on single unit activity in the islands of Calleja, responses of single units in the islands to i.v. injection of phenylephrine or sodium nitroprusside were recorded. Of 87 units recorded from in the islands of Calleja, 40 (46%) responded to phenylephrine and 27 (68%) decreased their firing frequency. Of these 27, 16 (59.3%) increased their firing frequency to administration of nitroprusside. In addition, in 11 (31%) of 35 units recorded from in the islands, glutamate microinjection in the nucleus tractus solitarius decreased the firing frequency in 9 (82%) units.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiovascular changes elicited by microinjection of glycine or GABA into the spinal intermediolateral nucleus in urethane-anesthetized rats.

Sympathetic preganglionic neurons (SPNs), located in the intermediolateral cell column (IML) of the thoracolumbar spinal cord, contribute to the maintenance of arterial pressure (AP) and heart rate (HR) within normal limits under different physiological conditions. The purpose of this study was to examine the effects of microinjecting the putative inhibitory transmitters glycine (GLY) or gamma-amino-butyric acid (GABA) into functionally identified cardioacceleratory and vasoconstrictor IML sites at T1-T3. Rats were anesthetized (1.4 g/kg urethane i.p.), paralysed with decamethonium bromide (3.3 mg/kg i.v.) and artificially ventilated. Glutamate (GLU) microinjection (10-20 nl, pH = 7.4, 0.15 M in phosphate-buffered saline (PBS)) was used to identify 29 vasoconstrictor sites, of which 23 were also cardioacceleratory, in the right side of the T2 segment. GLY microinjection (10-20 nl, pH = 7.4, 0.5 M in PBS) into these sites resulted in significant decreases in both AP (n = 18) and HR (n = 14). The AP and HR responses to GLY were brief in duration and were attenuated by the specific GLY antagonist strychnine (STR). Microinjection of GABA (10-20 nl, pH = 7.4, 0.15-0.84 M in PBS, n = 6) and its agonist muscimol (10-20 nl, pH = 7.4, 0.9 mM in PBS, n = 6) into GLU-identified sites in the IML caused no changes in AP or HR. However, after the application of either GABA or muscimol, the changes in AP or HR elicited by GLU were eliminated, suggesting that GABA and muscimol decrease the excitability of SPNs.(ABSTRACT TRUNCATED AT 250 WORDS)

Pontine reticular neurons provide tonic excitation to neurons in rostral ventrolateral medulla in rats.

To determine whether the pontine reticular formation (PRF) is a source of tonic activity for cardiovascular neurons in the rostral ventrolateral medulla (RVLM), the discharge of PRF neurons was inhibited by unilateral microinjections of glycine (1.0 M; 60 nl) while recording the discharge of single neurons in the RVLM in 14 Saffan-anesthetized rats. RVLM units were characterized as cardiovascular if their spontaneous activity was changed by baroreceptor activation and was synchronized to the cardiac cycle. Glycine injection into the ipsilateral PRF eliminated the ongoing activity of six cardiovascular units and reduced the activity of four (mean decrease -91 +/- 4%). Inhibition of these units lasted 20-115 s (mean 59 +/- 9 s). Glycine injection into the PRF had no effect on the discharge of five cardiovascular units. Activity of six noncardiovascular units did not respond to PRF blockade. Glycine injection into the PRF caused decreases in arterial pressure (-28 +/- 5 mmHg), heart rate (-23 +/- 3 beats/min), and renal nerve activity (-42 +/- 7%) that also returned to control values between 25 and 120 s (mean 55 +/- 5 s). These results indicate that PRF neurons provide tonic excitatory drive to some cardiovascular neurons located in the RVLM.

Expression of Fos in rat central nervous system elicited by afferent stimulation of the femoral nerve.

To investigate the distribution of Fos-like immunoreactivity (FLI) in the central nervous system of urethane anesthesized rats after activation of a somatosympathetic reflex pathway, the cut central end of the right femoral nerve of 17 male Wistar rats was stimulated electrically for 1 h at parameters such that increases in heart rate and arterial pressure were elicited. Sections of brain and spinal cord were incubated in anti-Fos antibody and the presence of FLI was detected using the ABC immunoperoxidase method. In the spinal cord FLI was present in the ipsilateral lumbar spinal cord (laminae 1 and 2, 4-6 and 10) and contralateral intermediolateral nucleus in the thoracic spinal cord. In the hindbrain, FLI was present in the contralateral rostral ventrolateral medulla and bilaterally in the cochlear nucleus, external cuneate nucleus, locus coeruleus and lateral parabrachial nucleus. In the midbrain, label appeared in the Edinger-Westphal nucleus and peripeduncular nucleus on both sides. In the forebrain, FLI appeared bilaterally in the central nucleus of the amygdala, para- and periventricular hypothalamus, supraoptic nucleus, paraventricular thalamus, reuniens nucleus, subfornical organ and bed nucleus of the stria terminalis. These results define the central nervous system pathways of somatosympathetic reflexes and demonstrate that areas in the forebrain not previously known to be activated by somatosympathetic reflexes, but previously implicated in mediating the defense reaction, are activated by these reflexes.

Supramedullary inputs to cardiovascular neurons of rostral ventrolateral medulla in rats.

Experiments were done to test the hypothesis that selective activation of cell bodies in different nuclei known to be involved in central cardiovascular control could excite or inhibit the discharge of neurons in the rostral ventrolateral medulla (RVLM). It is known that chemical stimulation of the lateral parabrachial nucleus (LPBN), locus ceruleus (LC), and lateral hypothalamic area (LHA) in anesthetized animals elicits increases (LPBN) or decreases (LC and LHA) in arterial pressure. We therefore recorded extracellularly spontaneous activity from RVLM units in urethan-anesthetized rats and monitored the changes in firing frequency of these neurons during chemical stimulation of one of LPBN, LC, and LHA. Thirty-two units were classified as cardiovascular neurons because their activity was inhibited by baroreceptor activation (1-3 micrograms phenylephrine iv) and displayed a cardiac cycle-related rhythmicity. Chemical stimulation with sodium glutamate of arterial pressor sites in the ipsilateral LPBN increased the firing frequency (40.3 +/- 1.3%) of 11 cardiovascular neurons. Activation of cell bodies in arterial depressor sites in the ipsilateral LC inhibited the firing rate (59.1 +/- 7.1%) of 10 cardiovascular neurons and excited 1 unit. Activation of cell bodies in arterial depressor sites in the ipsilateral LHA inhibited the discharge rate (25.4 +/- 4.7%) of six cardiovascular neurons, excited one unit, and did not alter the rate of the remaining three units. These results provide direct evidence for the existence of excitatory and inhibitory pathways from neurons located in the LPBN, LC, and LHA to cardiovascular neurons in the RVLM.

Expression of c-fos protein in rat brain after electrical stimulation of the aortic depressor nerve.

To reveal central nervous system (CNS) structures involved in the baroreceptor reflex we studied the distribution of Fos protein-like immunoreactivity in the rat brain after one hour of electrical stimulation of the aortic depressor nerve (ADN). In 13 male Wistar rats under urethane the ADN was cut on both sides and the central ends were placed on stimulating electrodes. Intermittent (11 s on, 6 s off) electrical stimulation at parameters set to elicit a drop in mean arterial pressure of 15-30 mmHg was applied to one, both or neither ADNs for 1 h. CNS sections were incubated for 48 h in anti-Fos antibody and prepared for visualization of the reaction product using the ABC immunoperoxidase technique. Label was found in several discrete brain nuclei primarily on the side ipsilateral to the side of stimulation. In the medulla labelled nuclei were found in the nucleus tractus solitarius, area postrema, rostral and caudal ventrolateral medulla, nucleus ambiguus and medullary reticular formation. In the pons labelled neurons were found in the lateral and ventrolateral parabrachial nucleus, locus coeruleus, pontine reticular field and A5 region. In the forebrain labelled nuclei were observed in the peri- and paraventricular hypothalamus, supraoptic nucleus, subfornical organ, preoptic area, central nucleus of the amygdala, median preoptic area, horizontal limb of the diagonal band, bed nucleus of the stria terminalis and islands of Calleja. In control animals moderate amounts of label were present in the supraoptic nucleus and periventricular hypothalamus bilaterally. These results define central pathways involved in mediating the baroreceptor reflex.

Additive effects of dopamine and 8-OH-DPAT microinjected into the nucleus ambiguus in eliciting vagal bradycardia in rats.

The effects of combined microinjection into the nucleus ambiguus (NA) of dopamine (DA) and the 5-hydroxytryptamine-1A (5HT-1A) receptor agonist 8-hydroxy-2-[di-n-propylamino]tetralin (8-OH-DPAT) on arterial pressure (AP) and heart rate (HR) were studied in 24 urethane-anaesthetized, artificially ventilated spinal (C1) rats. Sites from which bradycardia was elicited by microinjection of L-glutamate (GLU) were selected for microinjection of DA and 8-OH-DPAT into the NA. Microinjections of 8-OH-DPAT (60-300 pmol in 10 nl) elicited a dose-dependent bradycardia; the HR responses elicited by microinjection of 300 pmol of 8-OH-DPAT were significantly larger (-34.0 +/- 3.0) than responses elicited by the threshold dose of 60 pmol (-3.1 +/- 0.1 bpm). The onset latency of the HR responses elicited by the 300 pmol of 8-OH-DPAT was 6.5 +/- 0.5 s and the peak was reached in 40.0 +/- 8.0 s. The duration of these responses was 615 +/- 3.5 s. Microinjection of DA (1 nmol in 10 nl) within 3-5 s from the time of 8-OH-DPAT microinjection (60 pmol in 2 nl) into the NA at sites previously shown to elicit decreases in HR following microinjection of GLU, produced significant additive effects in eliciting bradycardia (20.4 +/- 2.9 bpm) when compared with decreases in HR elicited by microinjection of DA (11.1 +/- 1.8 bpm) or of 8-OH-DPAT (3.1 +/- 0.1 bpm) alone. There were no changes in AP after microinjections of DA, 8-OH-DPAT or of these two substances combined.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine microinjected into the nucleus ambiguus elicits vagal bradycardia in spinal rats.

To investigate the effects of dopamine (DA) on vagal efferent activity, DA was microinjected into the right nucleus ambiguus (NA) in rats. Experiments were done in 19 urethane anaesthetized, artificially ventilated spinal (C1) rats. Sites in the right NA containing cardioinhibitory neurons were identified by observing a marked and reproducible decrease in heart rate (HR; 64.9 + 2.8 bpm; n = 36) elicited by microinjecting L-glutamate (GLU; 1.5. nmol in 10 nl). No decreases in arterial pressure (AP) were obtained at these sites. Microinjection of DA (1-15 nmol in 10 nl) into 24 of these 36 sites caused a dose-dependent decrease in HR. The responses to 1 nmol and 3 nmol DA were blocked by (+/-)-sulpiride, a specific D2 receptor antagonist (0.1 nmol in 10 nl). A higher dose of (+/-)-sulpiride (1 nmol in 10 nl) was required to block the responses to 15 nmol of DA. Bradycardia elicited by even the lowest amount of DA (1 nmol) was not blocked by SCH-23390, a specific D1 receptor antagonist. These experiments demonstrate that the bradycardia caused by microinjection of DA into the NA is due to the excitation of dopamine D2 receptors present on vagal preganglionic cardioinhibitory neurons controlling HR.

Interaction of putative neurotransmitters in rostral ventrolateral medullary cardiovascular neurons.

As recent immunohistochemical evidence has shown the coexistence of putative neurotransmitters in the rostral ventrolateral medulla (RVLM), we have investigated the possibility that there may be an interaction of putative transmitters on the firing frequency of cardiovascular neurons in the RVLM. Extracellular activity was recorded from 37 spontaneously firing units in the right RVLM of urethane anaesthetized and artificially ventilated rats. Nine of these units were classified as cardiovascular neurons because: (i) they were silenced by baroreceptor activation (1-3 micrograms phenylephrine i.v.); and (ii) they showed rhythmicity of their spontaneous activity in synchrony with the cardiac cycle. Microiontophoresis of combinations of near threshold amounts of L-glutamate (GLU; 10 nA), acetylcholine (Ach; 30 nA) and substance-P (SP; 60 nA) showed a synergistic interaction of these substances with one another in eliciting changes in firing frequency of cardiovascular neurons. These results show that GLU and Ach, GLU and SP and Ach and SP interact synergistically to influence the firing frequency of cardiovascular neurons in the RVLM and suggest that these substances play a physiological role in the neural control of the circulation.

Electrical stimulation of nucleus tractus solitarius excites vagal preganglionic cardiomotor neurons of the nucleus ambiguus in rats.

Recent evidence indicates that the cell bodies of vagal cardioinhibitory neurons are located principally in the external formation of the nucleus ambiguus (NA). As activation of baroreceptor afferent fibers projecting to the nucleus tractus solitarius (NTS) elicits a decrease in heart rate it is likely that there is a connection between the NTS and NA. To test the hypothesis that stimulation of the NTS can excite vagal preganglionic cardiomotor neurons (VPCN) in the NA, activity from 78 neurons in the NA was recorded extracellularly before and during stimulation of a depressor site in the NTS (1 Hz, 0.1 ms) in urethan anesthetized and artificially ventilated male Wistar rats. Sixteen neurons were characterized as vagal preganglionic cardiomotor neurons (VPCN) because they were excited by baroreceptor activation (1-3 micrograms phenylephrine i.v.) and showed rhythmicity of their spontaneous activity in synchrony with the cardiac cycle. Stimulation of the NTS increased the firing rate of all these VPCN. The remaining 62 neurons could not be considered as VPCN because they either had respiratory rhythmicity or were not sensitive to baroreceptor activation, or they were sensitive to baroreceptor activation but did not display cardiac cycle related rhythmicity. These results provide evidence for the existence of an excitatory pathway from NTS to vagal preganglionic cardiomotor neurons in the NA.

Microinjection of glycine into the nucleus ambiguus elicits tachycardia in spinal rats.

In 30 male Wistar spinal (C1) rats, anaesthetized with urethane and artificially ventilated, experiments were done to study the effect on heart rate (HR) and arterial pressure (AP) of microinjection of the inhibitory amino acid glycine (Gly) into the nucleus ambiguus (NA). L-Glutamate (Glu; 1.5 nmol) was microinjected into the region of the right NA to search for sites from which decreases in AP and HR could be elicited. The decreases in HR were found to be 73.1 +/- 7.0 bpm (n = 30). No changes in AP were observed. Microinjection of Gly (1 M; 2-20 nmol in 2-20 nl; n = 12) elicited a dose dependent increase in HR with no changes in AP. Microinjection of Gly 1-2 min before microinjection of Glu in 7 sites reduced significantly (P less than 0.05) the decrease in HR elicited by Glu from 87.0 +/- 27.3 bpm to 17.7 +/- 7.2 bpm. Increases in HR elicited by Gly in the right NA of another 12 rats were not affected significantly by prior microinjection of the Gly antagonist strychnine hydrochloride (30-90 pmol in 10-30 nl in one group of animals, n = 6; and 2.5 nmol in 50 nl in another group, n = 6). In addition, to determine whether the effects of Gly were caused by actions on N-methyl-D-aspartate (NMDA) receptors, kynurenic acid (KYN; 4.5 nmol in 30 nl) was microinjected into the right NA of 6 rats prior to microinjection of Gly. KYN failed to block the response to Gly microinjection and instead potentiated the HR increase elicited by Gly.(ABSTRACT TRUNCATED AT 250 WORDS)

Enkephalins, substance P and acetylcholine microinjected into the nucleus ambiguus elicit vagal bradycardia in rats.

Little is known about putative transmitters in the nucleus ambiguus (NA) mediating parasympathetic control of the heart, although Met-enkephalin (m-ENK), Leu-enkephalin (l-ENK), substance P (SP) and acetylcholine (Ach) have been detected in the cell bodies and fibers of this nucleus. The effects of these substances on arterial pressure (AP) and heart rate (HR) were studied by microinjecting them (4-20 nl) into the NA. Experiments were done in 26 spinal (high cervical) rats that were anesthetized with urethane and artificially ventilated. L-Glutamate (GLU) was microinjected into the right NA to identify the location of cell bodies from which decreases in HR and AP could be elicited. m-ENK, l-ENK, SP or Ach was then microinjected into these sites. Microinjection of 1 nmol of GLU elicited significant decreases in HR (-72.2 +/- 9.7 bpm, n = 15) which were not accompanied by significant decreases in mean AP. Microinjection of m-ENK (15-200 pmol; n = 7), l-ENK (15-200 pmol; n = 6), SP (0.9-15 pmol; n = 7) and Ach (2.0-20 pmol; n = 7) into the NA decreased HR in a dose-dependent manner but did not affect AP. The magnitudes of HR responses to m-ENK, l-ENK, SP and Ach were smaller but of longer duration than the changes in HR to microinjection of GLU. These results suggest a physiological role for GLU, enkephalins, SP and Ach in the vagal control of HR mediated by the NA.

Interaction of putative transmitters in central nervous pathways involved in the control of heart rate and arterial pressure.

As many putative transmitter substances have been shown to be co-localized in areas of the central nervous system involved in cardiovascular control, we have investigated the possibility that some of these substances may interact in eliciting changes in heart rate and arterial pressure in anesthetized rats. In a first set of experiments, interactions between atrial natriuretic factor and glutamate were investigated by microinjection into the nucleus of the tractus solitarius, the site of termination of baroreceptor fibers of the aortic depressor nerve. In addition, interactions between the transmitter released in the nucleus tractus solitarius by electrical stimulation of the aortic depressor nerve and atrial natriuretic factor microinjected into the nucleus tractus solitarius were investigated. Combined microinjection of atrial natriuretic factor and glutamate into the nucleus tractus solitarius, or stimulation of the aortic depressor nerve combined with atrial natriuretic factor in the nucleus tractus solitarius, elicited decreases in heart rate and arterial pressure which were greater than the responses to either substance or stimulation alone or their algebraic sum. In a second set of experiments, interactions between substance P and acetylcholine were investigated in the intermediolateral nucleus of the spinal cord, the location of sympathetic preganglionic neurons. Furthermore, we investigated the possibility that the cardiovascular responses to microinjection of substance P and acetylcholine into the intermediolateral nucleus could be potentiated by the transmitter released in the intermediolateral nucleus by microinjection of glutamate into the rostral ventrolateral medulla, a region with known sympatho-excitatory function.(ABSTRACT TRUNCATED AT 250 WORDS)

Monosynaptic connection from caudal to rostral ventrolateral medulla in the baroreceptor reflex pathway.

Experiments were done to test the hypothesis that caudal ventrolateral medulla (CVLM) neurons excited by activation of arterial baroreceptors and by stimulation of depressor sites in the nucleus tractus solitarii (NTS) project monosynaptically to the rostral ventrolateral medulla (RVLM). In urethan anaesthetized and artificially ventilated rats we recorded extracellular activity from 46 spontaneously firing units in the CVLM. Twenty of these units were excited by baroreceptor activation (1-3 micrograms phenylephrine i.v.) and of these 6 were excited (mean latency of 9.8 +/- 2.3 ms) by single pulses (0.1 ms, 30 +/- 8.3 microA) delivered once per second to a depressor site in the ipsilateral NTS. These 6 units were also antidromically activated with a latency of 4.1 +/- 0.12 ms by stimulation of a pressor region in the ipsilateral RVLM. These results provide evidence for the existence of an excitatory projection from the NTS to the CVLM which, in turn, projects monosynaptically to sympathoexcitatory neurons in the RVLM.