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The caudal ventrolateral medulla (CVLM) contains vasodepressor neurons which, when activated, decrease vasomotor tone. To investigate whether excitatory amino acid receptors in the CVLM of the rat are involved in mediation of the aortic baroreceptor reflex, we microinjected amino acid antagonists unilaterally into the CVLM and examined their effects on the depressor response to electrical stimulation of the aortic nerve which contains mainly baroreceptor afferent fibers in rats. Male Wistar rats were anaesthetized with urethane, paralyzed and artificially ventilated. To block reflex vagal effects, methylatropine (1 mg/kg) was given intravenously. Kynurenate (227 ng), an excitatory amino acid antagonist, injected ipsilaterally but not contralaterally into the CVLM markedly inhibited the depressor response to aortic nerve stimulation, while both injections produced a similar small increase in basal blood pressure. Muscimol (1 ng), a GABA receptor agonist, injected ipsilaterally into the CVLM partly inhibited the baroreflex response, while it produced a moderate increase in basal blood pressure. 2-Amino-5-phosphonovalerate (APV) (10 ng), a N-methyl-D-aspartate (NMDA) receptor antagonist, and MK-801 (30 ng), a NMDA receptor channel blocker, partly inhibited the baroreflex response. MK-801 (30 ng) injected into the CVLM reduced the depressor response to the NMDA receptor agonist NMDA (0.3 ng) but not to the quisqualate receptor agonist quisqualate (0.1 ng) and the kainate receptor agonist kainate (0.1 ng), while kynurenate (227 ng) inhibited the depressor response to all three excitatory amino acid receptor agonists. These findings provide further evidence for the presence of excitatory amino acid receptors involved in mediating the aortic baroreceptor reflex in the rat CVLM.(ABSTRACT TRUNCATED AT 250 WORDS)

Using antisera raised against excitatory neurotransmitter glutamate fixed to carrier proteins and purified by affinity chromatography, localization of glutamate in the ventrolateral medulla oblongata and the nucleus tractus solitarius of the rat was studied. Within the rostral portion of the ventrolateral medulla, glutamate-like immunoreactive neurons were found in the lateral wing of the raphe magnus and in the region of the paragigantocellular reticular nucleus. In the caudal portion of the ventrolateral medulla, glutamate-positive neurons were clustered within the nucleus reticularis lateralis and a lesser number of stained neurons were scattered in the region around this nucleus. Glutamate-immunoreactive neurons were found in the nucleus tractus solitarius at the level of the area postrema. They were densely distributed especially in the medial subdivision of the nucleus whereas other subdivisions contained considerable glutamate positive cells. These results provide immunohistochemical evidence for the presence of glutamate-containing neurons in the ventrolateral medulla oblongata and the nucleus tractus solitarius of the rat.

The effects of aminooxyacetic acid (AOAA), an aspartate aminotransferase (AAT) inhibitor, L-canaline, an ornithine aminotransferase inhibitor, and gamma-acetylenic GABA and gabaculine, both gamma-aminobutyric acid transaminase (GABA-T) inhibitors, on the release of aspartate from slices of rat medulla oblongata and hippocampus were studied. The slices were superfused and electrically stimulated. There was a Ca2(+)-dependent stimulus-evoked release of endogenous aspartate. AOAA (10(-4) and 10(-3) M) decreased the evoked release of aspartate in the medulla oblongata but not in the hippocampus. In addition, AOAA produced a decrease in the spontaneous efflux and tissue content of aspartate in the medulla oblongata. L-Canaline (5 x 10(-5) M), gamma-acetylenic GABA (10(-4) M) and gabaculine (10(-5) M) did not affect the evoked release of aspartate in the medulla oblongata, while these agents produced a decrease in spontaneous efflux and tissue content of aspartate. These findings suggest that AAT participates in the synthesis of transmitter aspartate in the medulla oblongata of the rat. It appears that there are the pools of transmitter aspartate and non-transmitter aspartate in the rat medulla oblongata.

Microinjections of beta-alanine and glycine into the nucleus tractus solitarii (NTS) of the rat, led to an increase in blood pressure and heart rate. The responses to glycine but not to beta-alanine could be blocked by strychnine. Both amino acids reduced the responses to aortic nerve stimulation. High K+ stimulation caused a calcium-dependent release of beta-alanine from tissues in the area of the NTS. These results suggest that beta-alanine and glycine may modulate the cardiovascular control within the NTS.

Cardiovascular effects of chemical stimulation of the rostral ventrolateral medulla were studied in pentobarbital-anesthetized, paralyzed rats to examine the medullary GABAergic mechanisms responsible for cardiovascular regulation. Bilateral microinjections of 3-mercaptopropionic acid (10 micrograms), an inhibitor of gamma-aminobutyric acid (GABA) synthesis, into the ventrolateral medulla produced an increase in blood pressure and heart rate. Unilateral microinjection of nipecotic acid (3 micrograms), a reuptake inhibitor of GABA, elicited a fall in arterial pressure and heart rate. Bilateral microinjections of bicuculline (100 pmol), a GABA antagonist, into the ventrolateral medulla produced a sustained increase in arterial pressure and heart rate and, in addition, inhibited the hypotensive action induced by veratrine (0.3 mg/kg, i.v.). Lesioning of the rat nucleus tractus solitarii did not alter GABA content in the rostral ventrolateral medulla. These results support the hypothesis that in the rostral ventrolateral medulla, there exist GABAergic mechanisms involved in cardiovascular regulation. It seems unlikely that the GABAergic mechanisms originate mainly from neurons in the nucleus tractus solitarii.

Concentrations of glutamate, gamma-aminobutyric acid (GABA), glycine, beta-alanine and taurine were measured in microdissected areas of the rostral ventrolateral medulla (RVL), nucleus tractus solitarii (NTS) and caudal ventrolateral medulla (CVL) of the rat, and the effects of hydralazine-induced hypotension on the concentrations of amino acids were examined. All five amino acids were ubiquitously present in these brainstem regions. Hydralazine-induced hypotension increased glutamate levels in the NTS and CVL regions, but decreased GABA levels in all the regions studied. The level of glycine was decreased in the RVL region after hydralazine. Hydralazine decreased beta-alanine levels in the NTS and CVL regions. Electrical stimulation of slices containing the RVL region decreased glutamate levels but increased GABA levels in tissues. These results are compatible with the hypothesis that glutamate and GABA systems in these brainstem areas are related to blood pressure regulation.

The purpose of this study was to examine, using pentobarbital-anesthetized rats, whether vasopressin contributes to hypertension caused by denervation of baroreceptors, in comparison to the contribution of vasopressin to hypertension caused by nucleus tractus solitarius (NTS) lesions. Bilateral baroreceptor denervation caused acute hypertension which was inhibited by intravenous injection of an antagonist of the vasoconstrictor action of vasopressin. Lesions of the NTS also produced an increased in blood pressure which was reduced by the vasopressin antagonist. The magnitude of the antagonist-induced hypotension was significantly greater in NTS hypertension than that in baroreceptor denervation hypertension. It is concluded, that in rats, vasopressin contributes to the neurogenic hypertension caused by baroreceptor denervation and NTS lesions. Thus, it appears that interruption of baroreceptor afferents at the NTS level is responsible for the vasopressin-mediated hypertension caused by NTS lesions but that it is not the only factor involved.

Slices of the rat medulla oblongata were superfused and electrically stimulated. The amount of [3H]- and endogenous GABA was determined. There was a stimulus-evoked release of [3H]- and endogenous GABA, which was Ca2+-dependent and tetrodotoxin-sensitive. 3-Mercaptopropionic acid (3-MP) reduced the release of endogenous GABA, whereas the release of [3H]-GABA was not affected by this agent. Nipecotic acid enhanced the spontaneous and stimulus-evoked release of both [3H]- and endogenous GABA. 3-MP, nipecotic acid and diaminobutyric acid modified the stimulus-evoked release of endogenous GABA from slices containing the rostral ventrolateral medulla, the caudal ventrolateral medulla or the nucleus tractus solitarii, regions responsible for blood pressure regulation. These results suggest that 3-MP inhibits the stimulus-evoked release of GABA without affecting its release process, whereas nipecotic acid enhances the release via its uptake inhibition action.

Substance P in a dose of 0.1-10 ng injected into the nucleus tractus solitarii (NTS) of the rat caused hypotension, bradycardia and apnea whereas a dose of 100 ng led to no response. A substance P antagonist injected into the NTS abolished the cardiovascular responses to substance P. The antagonist alone increased blood pressure and heart rate. The data suggest a role for substance P in the cardiovascular regulation by the NTS.

Evidence was obtained for the release of amino acids by electrical stimulation of slices of regions of the rat medulla oblongata: rostral ventrolateral, caudal ventrolateral and caudal dorsomedial. There was a Ca2+-dependent, tetrodotoxin-sensitive increase in the efflux of aspartate, glutamate, gamma-aminobutyric acid (GABA), glycine, and beta-alanine in all regions examined. There were distinct regional differences in the relative amounts of amino acids released. These results provide evidence for the possible neurotransmitter role of aspartate, glutamate, GABA, glycine, and beta-alanine in these regions of the rat medulla oblongata.

The effects of aminooxyacetic acid (AOAA), a transaminase inhibitor, and 2-oxoglutarate, a precursor to glutamate by the activity of aspartate aminotransferase (AAT), on slices of rat medulla oblongata, cerebellum, cerebral cortex, and hippocampus were studied. The slices were superfused and electrically stimulated. There was a Ca2+-dependent stimulus-evoked release of endogenous glutamate, gamma-aminobutyric acid (GABA), and beta-alanine in all regions examined. AOAA (10(-4) and 10(-3) M) decreased the release of glutamate in the medulla oblongata and cerebellum but not in the hippocampus. L-Canaline, a specific inhibitor of ornithine aminotransferase, did not affect the glutamate release in the medulla. 2-Oxoglutarate (10(-3) M) increased the release of glutamate in the medulla oblongata and cerebellum but not in the cerebral cortex and hippocampus. Treatment with AOAA (10(-4) M) almost abolished the activities of AAT in all regions studied. AOAA (10(-4) and 10(-3) M) increased the stimulus-evoked release of GABA in the cerebellum, cerebral cortex, and hippocampus, whereas the stimulus-evoked release of beta-alanine was decreased by this agent in all regions studied. These results suggest the participation of AAT in the synthesis of the transmitter glutamate in the medulla oblongata and cerebellum of the rat.

Microinjections of N-methyl-D-aspartate (NMDA) into the caudal ventrolateral medulla of the rat led to a decrease in arterial pressure whereas 2-amino-5-phosphonovalerate injected bilaterally into this region produced hypertension and blocked the depressor effect of aortic nerve stimulation. High K+ stimulation via push-pull cannula caused a calcium-dependent release of endogenous glutamate from the region. These results provide evidence for the NMDA receptor system involved in tonic vasodepressor control in the rat caudal ventrolateral medulla.

Microinjections of N-methyl-D-aspartate (NMDA) into the medial area of the nucleus tractus solitarii (NTS) of the rat led to a decrease in arterial pressure and heart rate. The NMDA receptor antagonist 2-amino-5-phosphonovalerate (AP5) reduced the cardiovascular responses to NMDA. Depressor and bradycardic responses to aortic nerve stimulation were reduced by AP5 but not by a substance P antagonist, injected into the NTS. High K+ stimulation caused a calcium-dependent release of glutamate and aspartate from tissues in the area of the NTS. These results provide evidence of NMDA receptor-mediated modulation of the aortic baroreceptor reflex in the rat NTS.

Drugs activating GABAergic systems were microinjected into the caudal ventrolateral medulla of the rat and effects on blood pressure and heart rate were investigated. In urethane-anesthetized rats, a dose-dependent increase in blood pressure and heart rate was induced by unilateral injections of GABA (30-300 ng) into the caudal ventrolateral medulla. Unilateral injections of the GABA receptor agonist muscimol (1-10 ng) and baclofen (0.1-10 ng) into the brainstem area also increased both the cardiovascular parameters. The GABA uptake inhibitors nipecotic acid (1-3 micrograms) and L-2,4-diaminobutyric acid (1 microgram), and the GABA-T inhibitor gamma-acetylenic GABA (1-3 micrograms) injected unilaterally into the brainstem area also produced hypertension and tachycardia. Bilateral microinjections of nipecotic acid (0.3 microgram) into the area potentiated the pressor and tachycardiac responses to carotid artery occlusion in pentobarbital-anesthetized rats. Lesioning of the rat nucleus tractus solitarii did not alter the GABA content in the lesioned side of the caudal ventrolateral medulla. The data provide further evidence that endogenous GABAergic mechanisms in the rat caudal ventrolateral medulla are involved in the regulation of blood pressure. It seems unlikely that the GABAergic mechanisms originate mainly from neurons in the nucleus tractus solitarii.

Slices of the rat medulla oblongata were superfused and electrically stimulated. The amount of endogenous GABA, beta-alanine and glutamate release from the slices was determined by high performance liquid chromatography with fluorometric detection. Inhibitors of GABA-transaminase (GABA-T), aminooxyacetic acid (10(-5) M), gamma-acetylenic GABA (10(-4) and 10(-3) M) and gabaculine (10(-5) M), enhanced the stimulus-evoked release of GABA and reduced that of beta-alanine, while no change was observed in the release of glutamate. These changes in amino acid release from the slices were accompanied by an increase in the content of GABA and a decrease in that of beta-alanine. The stimulus-evoked release of these amino acids was abolished by Ca2+-deprivation, in either the presence or absence of GABA-T inhibitors. These results suggest a modulatory role of GABA-T for synaptically releasable GABA and involvement of this enzyme in the synthesis of releasable beta-alanine.

Release of endogenous glutamate, gamma-aminobutyric acid (GABA) and glycine was investigated in the caudal dorsomedial (CDM) region of the rat medulla oblongata. High potassium stimulation (50 mM) produced an increase in release of glutamate, GABA and glycine by 579.1, 303.4 and 774.5 (pmol/mg protein), respectively. The release was abolished by Ca2+ deprivation. These results indicate a possible neurotransmitter role for glutamate, GABA and glycine in the CDM medulla of the rat.

Microinjections of gamma-aminobutyric acid (GABA) and glycine, into the medial area of the nucleus tractus solitarii (NTS) of the rat, led to an increase in arterial pressure and heart rate. The GABA receptor antagonist bicuculline and the glycine receptor antagonist strychnine decreased both of these cardiovascular parameters whereas the GABA uptake inhibitor nipecotic acid produced hypertension. High K+ stimulation caused a calcium-dependent release of GABA and glycine from tissues in the area of the NTS. Our results suggest that GABA and glycine may modulate the cardiovascular control within the NTS.

Localization of gamma-aminobutyric acid (GABA) in the ventrolateral medulla oblongata of the rat was studied, using antisera directed against GABA molecule fixed to bovine serum albumin. Within the rostral portion of the ventrolateral medulla, GABA-like immunoreactive neurons were found in the lateral wing of the raphe magnus and in the region of the paragigantocellular reticular nucleus. In the caudal portion of the ventrolateral medulla, a lesser number of GABA-stained neurons were found in the region around the nucleus reticularis lateralis. GABA-like immunoreactive punctate structures were also found throughout the ventrolateral medulla. These results provide further evidence for the existence of GABAergic neurons in the ventrolateral medulla oblongata of the rat.

Microinjections of the gamma-aminobutyric acid (GABA) receptor antagonist bicuculline into the medial area of the nucleus tractus solitarii of the rat enhanced the depressor and bradycardiac responses to aortic nerve stimulation whereas the glycine receptor antagonist strychnine did not affect them. The GABA receptor agonist muscimol and the GABA uptake inhibitor nipecotic acid reduced the responses to aortic nerve stimulation. These results provide evidence suggesting GABA receptor-mediated modulation of the aortic baroreceptor reflex in the nucleus tractus solitarii of the rat.