Release by electrical stimulation of endogenous glutamate, gamma-aminobutyric acid, and other amino acids from slices of the rat medulla oblongata

Thinking outside the cleft to understand synaptic activity: contribution of the cystine-glutamate antiporter (System xc-) to normal and pathological glutamatergic signaling

System x(c)(-) represents an intriguing target in attempts to understand the pathological states of the central nervous system. Also called a cystine-glutamate antiporter, system x(c)(-) typically functions by exchanging one molecule of extracellular cystine for one molecule of intracellular glutamate. Nonvesicular glutamate released during cystine-glutamate exchange activates extrasynaptic glutamate receptors in a manner that shapes synaptic activity and plasticity. These findings contribute to the intriguing possibility that extracellular glutamate is regulated by a complex network of release and reuptake mechanisms, many of which are unique to glutamate and rarely depicted in models of excitatory signaling. Because system x(c)(-) is often expressed on non-neuronal cells, the study of cystine-glutamate exchange may advance the emerging viewpoint that glia are active contributors to information processing in the brain. It is noteworthy that system x(c)(-) is at the interface between excitatory signaling and oxidative stress, because the uptake of cystine that results from cystine-glutamate exchange is critical in maintaining the levels of glutathione, a critical antioxidant. As a result of these dual functions, system x(c)(-) has been implicated in a wide array of central nervous system diseases ranging from addiction to neurodegenerative disorders to schizophrenia. In the current review, we briefly discuss the major cellular components that regulate glutamate homeostasis, including glutamate release by system x(c)(-). This is followed by an in-depth discussion of system x(c)(-) as it relates to glutamate release, cystine transport, and glutathione synthesis. Finally, the role of system x(c)(-) is surveyed across a number of psychiatric and neurodegenerative disorders.

Inhibitory actions of serotonin on glutamate release in dorsal medulla suppress systemic arterial pressure of cats

The role of serotonin and glutamate release in dorsal medulla (DM) for regulation of systemic arterial pressure (SAP) was examined with microdialysis and high performance liquid chromatograph in anesthetized cats. KCl-perfusion in DM increased serotonin and glutamate concentrations in DM. Perfusion of serotonin resulted in decreases in glutamate concentration and SAP. Perfusion of alaproclate, a serotonin reuptake inhibitor that produced an increase in serotonin concentration in DM, had the same results as perfusion of serotonin. In conclusion, serotonin and glutamate appeared to be tonically and endogenously released from nerve terminals in DM, and the decrease in SAP could be attributed to the decreased glutamate release resulting from inhibitory action of serotonin in DM. The putative roles of serotonin and glutamate in DM may be important in SAP regulation.

Glutamatergic and serotonergic mechanisms in the dorsal facial area for common carotid artery blood flow control in the cat

This study explored which subtyes of glutamate receptors in the dorsal facial area are involved in the interaction between glutamatergic and serotonergic actions in controlling common carotid arterial blood flow. Microinjection of glutamate (25-100 nmol), N-methyl-D-aspartate (NMDA; 1-4 nmol), or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA; 0.5-2 nmol) into the dorsal facial area dose-dependently increased common carotid arterial blood flow. The potency order was AMPA>NMDA>glutamate. The glutamate-induced increase in common carotid arterial blood flow was reduced by pretreatment with either D-2-amino-5-phosphonopentanoate (D-AP5; 2.5-5.0 nmol), or glutamate diethylester (25-50 nmol). The common carotid arterial blood flow was increased by ketanserin (1.0 nmol) and decreased by (+/-)-1-(2, 5-dimethoxy-4-iodophenyl)-2-aminopropane (1.0 nmol). Both effects were attenuated by pretreatment with either D-AP5 or glutamate diethylester. We conclude that activation of both NMDA and AMPA receptors in dorsal facial area is responsible for the increase in common carotid arterial blood flow, and AMPA receptor may play a greater role. Such response may be suppressed by 5-HT2 action.

Rostral ventrolateral medulla suppresses reflex bradycardia by the release of gamma-aminobutyric acid in nucleus tractus solitarii of the rat

We investigated the role of gamma-aminobutyric acid (GABA) in the nucleus tractus solitarii (NTS), the principal recipient of baroreceptor afferent fibers in the medulla oblongata, in the suppression of cardiac baroreceptor reflex (BRR) response by the rostral ventrolateral medulla (RVLM). Direct microinfusion via reverse microdialysis of L-glutamate (50 microM) into the RVLM promoted an inhibition of the BRR response, alongside an increase in the concentration of GABA in the dialysate collected from the ipsilateral NTS. Such an increase in GABA concentration in the NTS to RVLM activation was site-specific, as microinfusion of L-glutamate into areas outside the confines of RVLM resulted in no discernible change in GABA concentration in the dialysate of the NTS and minimal effect on the cardiac BRR response. The RVLM-induced BRR suppression of cardiac BRR response to microinjection into the bilateral RVLM of L-glutamate (1 nmol) was antagonized by administration into the bilateral NTS of the GABA(A) receptor antagonist, bicuculline methiodide (1 or 5 pmol), or the GABA(B) receptor antagonist, 2-hydroxy-saclofen (100 or 500 pmol). These results suggest that GABA released in the NTS may participate in cardiac BRR suppression induced by glutamatergic activation of the RVLM, via an action on both GABA(A) and GABA(B) receptor subtypes.

Parabrachial nucleus induces suppression of baroreflex bradycardia by the release of glutamate in the rostral ventrolateral medulla of the rat

The involvement of glutamatergic neurotransmission in the rostral ventrolateral medulla (RVLM) in the suppression of baroreflex bradycardia by the parabrachial nucleus (PBN) was investigated. Repeated electrical activation of the PBN increased the concentration of glutamate in the dialysate collected from the RVLM. The same stimulation also suppressed baroreflex bradycardia in response to transient hypertension evoked by phenylephrine (5 microg/kg, intravenously). Microinfusion of L-glutamate (10, 50 or 100 microM) via the microdialysis probe into the RVLM dose-dependently elicited a significant inhibition of baroreflex bradycardia that paralleled the concentration and time course of the PBN-elicited elevation in extracellular glutamate in the RVLM. The suppression of baroreflex bradycardia elicited by microinjection of L-glutamate (1 nmol) into the RVLM was appreciably reversed by coinjection of the NMDA receptor antagonist, dizocilpine (500 pmol), or the non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2, 3-dione (50 pmol). These results suggest that an increase in the extracellular concentration of glutamate and activation of both NMDA and non-NMDA receptors in the RVLM may mediate the suppression of baroreflex bradycardia by activation of the PBN.

Dorsal facial area of cat medulla; 5-HT2 action on glutamate release in regulating common carotid blood flow

Serotonin (5-HT) may inhibit glutamate release in the dorsal facial area (DFA) of the medulla and decrease common carotid arterial (CCA) blood flow. We attempted to clarify which subtype(s) of 5-HT receptor was involved. A microdialysis probe was inserted in DFA. The concentration of glutamate in dialysates were determined by chromatography. Glutamate concentration was dose-dependently decreased by perfusion of 5-HT or DOI, a 5-HT2 agonist, but not by 5-CT, a 5-HT1 agonist. The 5-HT-induced decrease in glutamate was reversed by co-perfusion of ketanserin, a 5-HT2 antagonist, but not by propranolol, a 5-HT1 antagonist. CCA blood flow was decreased when 5-HT or DOI was perfused, and was reversed by co-perfusing ketanserin. In conclusion, 5-HT may inhibit glutamate release via 5-HT2 receptor in DFA, resulting in the reduction of CCA blood flow.

Beta-alanine release from the adult and developing hippocampus is enhanced by ionotropic glutamate receptor agonists and cell-damaging conditions

The release of the inhibitory amino acid beta-alanine was investigated in hippocampal slices from adult (3-month-old) and developing (7-day-old) mice, using a superfusion system. The release was enhanced by beta-alanine itself and the structural analogs taurine and y-aminobutyrate. It was dependent on Na+, but independent of Ca2+ in both mature and immature hippocampus, being thus mostly mediated by uptake carriers operating in an outward direction. The release was potentiated in the developing mice, but not affected in the adults, by the ionotropic glutamate receptor agonists N-methyl-D-aspartate, kainate, 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate and tetrazolylglycine in a receptor-mediated manner. Cell-damaging conditions, including hypoxia, hypoglycemia, ischemia, oxidative stress and the presence of free radicals, greatly enhanced beta-alanine release at both ages, but more markedly in the adults. The great amounts of beta-alanine, together with the inhibitory amino acids taurine and gamma-aminobutyrate, released simultaneously with the excitatory amino acids in the hippocampus may constitute an important protective mechanism against excitotoxicity, which leads to neuronal death.

Type I and II metabotropic glutamate receptors regulate the outflow of [3H]D-aspartate and [14C]gamma-aminobutyric acid in rat solitary nucleus

Metabotropic glutamate (mGlu) receptors modulating amino acid outflow were examined in a model system in order to further characterize the pharmacological nature of the mGlu receptors involved in viscerosensory processing in the nucleus tractus solitarii. The actions of a number of subtype-selective mGlu receptor agonists and antagonists were monitored on the K+-evoked outflow of [3H]D-aspartate and [14C]gamma-aminobutyric acid (GABA) from superfused slices of rat nucleus tractus solitarii. (+/-)1S,3R-1-Amino-cyclopentane-1,3-dicarboxylate (10-300 microM), produced a concentration-dependent increase in outflow, which was attenuated by a number of phenylglycine antagonists. (2S,3S,4S)-alpha-(Carboxycyclopropyl)-glycine (30-300 microM) had mixed effects on outflow. The type I-selective agonist (RS)-3,5-dihydroxyphenylglycine (300 microM) also increased outflow and these effects were reversed by the type I antagonist (RS)-1-aminoindan-1,5-dicarboxylate (100 microM). Activation of type II mGlu receptors with (2R,4R)-aminopyrrolidine-2,4-dicarboxylate (300 microM), however, decreased outflow, and this effect was antagonized by the type II antagonist LY307452 (200 microM). Interestingly, LY307452 (200 microM) alone, enhanced outflow of [3H]D-aspartate, but not [14C]GABA. Type III mGlu receptors may not be involved in outflow of [3H]D-aspartate and [14C]GABA in the nucleus tractus solitarii, as L-2-amino-4-phosphonobutyrate (30-300 microM) had no effect under the present experimental conditions. These in vitro studies provide new evidence for roles for Type I and II mGlu receptors in viscerosensory processing in nucleus tractus solitarii.

Evidence for the involvement of endogenous aspartate in the mediation of carotid chemoreceptor reflexes in the rostral ventrolateral medulla of the rat

2-Amino-5-phosphonovalerate (AP5; 153 pmol) injected into the rostral ventrolateral medulla (RVLM) inhibited pressor responses induced by carotid chemoreceptor stimulation. AP5 also inhibited pressor responses to aspartate (0.75 nmol) but not to glutamate (0.53 nmol) similarly injected. High K+ (50 mM) released endogenous aspartate and glutamate in a Ca2+-dependent manner from the RVLM. Chemoreceptor stimulation caused a release of aspartate but not of glutamate in the RVLM, and sinus nerve denervation abolished the release of aspartate. Increases in blood pressure induced by intravenous phenylephrine did not release aspartate. These results support the hypothesis that endogenous aspartate in the rat RVLM is involved in the mediation of chemoreceptor reflexes.

Autonomic processing of the cardiovascular reflexes in the nucleus tractus solitarii

The nucleus tractus solitarii (NTS) receives afferent projections from the arterial baroreceptors, carotid chemoreceptors and cardiopulmonary receptors and as a function of this information produces autonomic adjustments in order to maintain arterial blood pressure within a narrow range of variation. The activation of each of these cardiovascular afferents produces a specific autonomic response by the excitation of neuronal projections from the NTS to the ventrolateral areas of the medulla (nucleus ambiguous, caudal and rostal ventrolateral medulla). The neurotransmitters at the NTS level as well as the excitatory amino acid (EAA) receptors involved in the processing of the autonomic responses in the NTS, although extensively studied, remain to be completely elucidated. In the present review we discuss the role of the EAA L-glutamate and its different receptor subtypes in the processing of the cardiovascular reflexes in the NTS. The data presented in this review related to the neurotransmission in the NTS are based on experimental evidence obtained in our laboratory in unanesthetized rats. The two major conclusions of the present review are that a) the excitation of the cardiovagal component by cardiovascular reflex activation (chemo- and Bezold-Jarisch reflexes) or by L-glutamate microinjection into the NTS is mediated by N-methyl-D-aspartate (NMDA) receptors, and b) the sympatho-excitatory component of the chemoreflex and the pressor response to L-glutamate microinjected into the NTS are not affected by the NMDA receptors antagonist, suggesting that the sympatho-excitatory component of these responses is mediated by non-NMDA receptors.

Release of glutamate in the nucleus tractus solitarii in response to baroreflex activation in rats

Release of endogenous aspartate and glutamate from the region of the nucleus tractus solitarii was measured in vitro by perfusion methods and in vivo by microdialysis. Stimulation of the nucleus tractus solitarii with 35 mM potassium in vitro significantly increased extracellular concentrations of aspartate and glutamate. Glutamate and aspartate concentrations also increased with dialysis of 100 mM KCl into the nucleus tractus solitarii in vivo, but only changes in glutamate were significant. Experiments in vivo revealed that activation of the baroreflex by intravenous infusion of phenylephrine significantly increased glutamate in dialysates, while hypoventilation that accompanies baroreceptor activation and may activate chemoreceptors tended to increase aspartate but not glutamate. The demonstration that glutamate, but not aspartate, is released with activation of the baroreflex further supports the hypothesis that glutamate is a neurotransmitter of baroreceptor afferents terminating in the nucleus tractus solitarii.

Neurobiology of L-DOPAergic systems

L-DOPA is proposed to be a neurotransmitter and/or neuromodulator in CNS. It is released probably from neurons, which may contain L-DOPA as an end-product, and/or from some compartment other than catecholamine-containing vesicles. The L-DOPA itself produces presynaptic and postsynaptic responses. All are stereoselective and most are antagonized by competitive antagonist. In striatum, L-DOPA is neuromodulator, mother of catecholamines, not only a precursor for dopamine but also a potentiator of children for presynaptic beta-adrenoceptors to facilitate dopamine release and postsynaptic D2 receptors, and ACh release inhibitor. All may cooperate for Parkinson's disease. Meanwhile, supersensitization of increase in L-glutamate release to nanomolar levodopa was seen in Parkinson's model rats, which may relate to dyskinesia or "on-off" during chronic therapy. In lower brainstem, L-DOPA tonically activates postsynaptic depressor sites of NTS and CVLM and pressor sites of RVLM. L-DOPA is probably a neurotransmitter of primary baroreceptor afferents terminating in NTS. GABA, the inhibitory neuromodulator for baroreflex in NTS, tonically functions to inhibit, via GABAA receptors, L-DOPA release and depressor responses to levodopa. Levodopa inversely releases GABA. L-DOPAergic monosynaptic relay from NTS to CVLM and from PHN to RVLM is suggested. Tonic L-DOPAergic baroreceptor-aortic nerve-NTS-CVLM relay seems to carry baroreflex information. Disturbance of neuronal activity to release L-DOPA in NTS, loss of the activity in CVLM, enhancement of the activity with decreased decarboxylation and increase in sensitivity to levodopa in RVLM may be involved in maintenance of hypertension in SHR. This is a story of "L-DOPAergic receptors" with extremely high affinity and low density.

The dorsal facial area of the medulla in cats: inhibitory action of serotonin on glutamate release in regulating common carotid blood flow

Whether glutamate and serotonin would release and interact in the dorsal facial area (DFA) of cat medulla to regulate common carotid arterial (CCA) blood flow was explored by placing a microdialysis probe in DFA and employing high performance liquid chromatographic technique. Glutamate concentration was dose-dependently decreased by perfusion with serotonin, or alaproclate, a serotonin reuptake inhibitor. Serotonin and glutamate concentrations were increased by perfusion with KCl, a depolarizing agent. Furthermore, CCA blood flow was decreased when glutamate concentration was reduced by serotonin or alaproclate perfusion, and conversely increased when glutamate concentration was increased by KCl perfusion. In conclusion, glutamate and serotonin releases in DFA that involve regulation of CCA blood flow are tonically mediated by nerve terminals. The glutamate release is depressed by the serotonin release.

Neurochemical modulation of cardiovascular control in the nucleus tractus solitarius

The central control of cardiovascular function has been keenly studied for a number of decades. Of particular interest are the homeostatic control mechanisms, such as the baroreceptor heart-rate reflex, the chemoreceptor reflex, the Bezold-Jarisch reflex and the Breuer-Hering reflex. These neurally-mediated reflexes share a common termination point for their respective centrally-projecting sensory afferents, namely the nucleus tractus solitarius (NTS). Thus, the NTS clearly plays a critical role in the integration of peripherally initiated sensory information regarding the status of blood pressure, heart rate and respiratory function. Many endogenous neurochemicals, from simple amino acids through biogenic amines to complex peptides have the ability to modulate blood pressure and heart rate at the level of the NTS. This review will attempt to collate the current knowledge regarding the roles of neuromodulators in the NTS, the receptor types involved in mediating observed responses and the degree of importance of such neurochemicals in the tonic regulation of the cardiovascular system. The neural pathway that controls the baroreceptor heart-rate reflex will be the main focus of attention, including discussion of the identity of the neurotransmitter(s) thought to act at baroafferent terminals within the NTS. In addition, this review will provide a timely update on the use of recently developed molecular biological techniques that have been employed in the study of the NTS, complementing more classical research.

L-DOPA systems for blood pressure regulation in the lower brainstem

We have explored probable neurotransmitter roles of L-3,4-dihydroxyphenylalanine (L-DOPA) in baroreceptor reflex and blood pressure regulation in depressor sites of the nucleus tractus solitarii (NTS) and the caudal ventrolateral medulla (CVLM), and in pressor sites of the rostral ventrolateral medulla (RVLM) in anesthetized rats. During microdialysis of these three areas, the basal L-DOPA release is in part tetrodotoxin (TTX)-sensitive and Ca2(+)-dependent, high K+ Ca2(+)-dependently releases dL-DOPA. L-DOPA microinjected (10-300 ng) dose-dependently produces postsynaptic depressor responses in the NTS and CVLM and pressor responses in the RVLM, and a recognition site for L-DOPA functions tonically to activate depressor neurons in the NTS and CVLM and pressor neurons in the RVLM. It is highly probable that L-DOPA is a neurotransmitter of the baroreceptor afferents terminating in the NTS, which is based on further findings such as (1) antagonism by a competitive L-DOPA antagonist against depressor responses to aortic nerve stimulation, (2) TTX-sensitive L-DOPA release by aortic nerve stimulation, (3) abolition of baroreceptor-stimulated L-DOPA release by bilateral sino-aortic denervation and (4) decreases in tyrosine hydroxylase (TH)- and L-DOPA-immunoreactivities without modifications of dopamine- and DBH-immunoreactivities in the left NTS and ganglion nodosum 7 days after ipsilateral aortic nerve denervation peripheral to the ganglion. In the NTS, GABA tonically functions to inhibit via GABAA receptors L-DOPA release and depressor responses to L-DOPA, whereas L-DOPA induces GABA release. Impaired TTX-sensitive neuronal activity to release L-DOPA in the NTS and enhanced TTX-sensitive neuronal activity including a decrease in decarboxylation of L-DOPA to dopamine and an increase in sensitivity of the recognition site to L-DOPA in the RVLM are relevant to the maintenance of hypertension in spontaneously hypertensive rats. Decreases in the contents of L-DOPA in the right CVLM 10 days after electrical lesion of the ipsilateral NTS suggest a 'L-DOPAergic' and monosynaptic relay from the NTS to the CVLM. L-DOPA seems to play major roles as a neurotransmitter for baroreceptor reflex and blood pressure regulation in the lower brainstem of rats.

Characterization of sodium-independent beta-alanine binding to cerebral cortical membranes from 7-day-old and adult mice

The sodium-independent binding of beta-alanine to cerebral cortical membranes from adult (3- and 12-month-old) and developing (7-day-old) mice was characterized for the first time. The binding was saturable in each age group, consisting of only one component. The affinity for beta-alanine was highest and the number of available binding sites greatest in young animals. The binding was not affected by strychnine, but inhibited by beta-alanine itself, glycine, L-alanine and L-serine, the IC50 values being lower in immature mice. Glycine was shown to be a competitive inhibitor. The binding was also inhibited, albeit only in adults, by N-methyl-D-aspartate receptor antagonists acting at the glycine modulatory site and by some GABAergic substances. It is concluded that even though beta-alanine may possess binding sites of its own, particularly in the immature cerebral cortex, beta-alanine could at least partly bind to strychnine-insensitive glycine sites in the N-methyl-D-aspartate receptor complex.

Excitatory amino acid receptors in the rostral ventrolateral medulla mediate hypertension induced by carotid body chemoreceptor stimulation

The rostral ventrolateral medulla (RVLM) is involved in the mediation of cardiovascular responses to peripheral chemoreceptor stimulation. To investigate whether excitatory amino acid inputs in the RVLM are related to the responses to chemoreceptor stimulation, we microinjected kynurenate, an amino acid antagonist, unilaterally into the RVLM and examined its effects on the pressor response to stimulation of carotid body chemoreceptors. Male Wistar rats were anesthetized with urethane, paralyzed and artificially ventilated. The carotid chemoreceptors were stimulated with isotonic solutions of inorganic phosphate solution. Stimulation of carotid body chemoreceptors produced increases in blood pressure. Kynurenate injected ipsilaterally but not contralaterally into the RVLM markedly inhibited the pressor response to chemoreceptor stimulation. In rats with spinal transection, stimulation of carotid body chemoreceptors also produced increases in blood pressure. The pressor response in rats with spinal transection was inhibited by intravenous injection of a vasopressin antagonist or by kynurenate injected ipsilaterally into the RVLM. Kynurenate injected into the RVLM inhibited the pressor response to NMDA, AMPA and kainate but not to acetylcholine in intact rats. These findings indicate that excitatory amino acid receptors are involved in mediating the pressor response to carotid body chemoreceptor stimulation in the rat RVLM. It appears that the chemoreceptor stimulation produces an increase in vasopressin release and the enhancement of vasopressin release is also mediated by an increase in excitatory amino acid inputs in the RVLM.

Prevention by NMDA receptor antagonists of the centrally-evoked increases of cardiac inotropic responses in rabbits

1. The purpose of this study was to investigate further the role of the excitatory amino acid (EAA) system of neurotransmission, particularly of the NMDA receptor, in the central regulation of cardiac function. 2. Electrical stimulation of the paraventricular nucleus of the hypothalamus (PVN) in pentobarbitone anaesthetized rabbits induced a cardiovascular response mainly characterized by a positive inotropic effect, hypertension and a marked increase in the myocardial oxygen demand index. 3. The intracerebroventricular (i.c.v.) or intravenous (i.v.) injection of different EAA antagonists acting on different sites of the NMDA receptor/channel complex dose-dependently blunted the excitatory cardiovascular effects of PVN stimulation. 4. 5,7 Dichlorokynurenic acid was used as a specific glycine site antagonist and 2-amino-5-phosphonovaleric acid was used to block the agonist recognition site; ketamine was used as a channel blocker site antagonist and ifenprodil as a blocker of the polyamine binding site. 5. 5,7 Dichlorokynurenic acid (125 and 250 micrograms kg-1, i.c.v.) virtually abolished the cardiovascular responses, inducing only haemodynamic depression at the highest dose used. 2-Amino-5-phosphonovaleric acid (0.1 to 1.0 mg kg-1, i.c.v.) elicited a reduction of the peak values observed during PVN stimulation which was accompanied by a decrease of the basal cardiovascular parameters. Ketamine (2.5 and 10 mg kg-1) and ifenprodil (1 mg kg-1), injected intravenously, blocked the haemodynamic response induced by PVN stimulation without marked reduction of the basal haemodynamics. 6. It is concluded that glutamate neurotransmission is not only involved in vasomotor tone control but also in the central control of cardiac function and can therefore modulate the myocardial oxygen demand.

L-glutamate as a neurotransmitter at baroreceptor afferents: evidence from in vivo microdialysis

In vivo microdialysis was employed to measure release of endogenous L-glutamate and L-aspartate in the region of the dorsomedial medulla oblongata including the medial nucleus tractus solitarius of the anaesthetized rat. Basal extracellular levels of these amino acids were stable and increased over two-fold when the perfusate was changed to a high KCl (80 mM) artificial cerebrospinal fluid. This high K(+)-evoked release was calcium-dependent, while basal levels were insensitive to removal of calcium ions from the perfusate. An intravenous infusion of phenylephrine, which elevated blood pressure, caused a marked increase of both spontaneous and evoked release of glutamate. In contrast aspartate efflux was not significantly altered. The present data provide evidence that the excitatory amino acids, glutamate and aspartate, serve a neurotransmitter function in the nucleus tractus solitarius of the rat. Furthermore, the increase in glutamate release following baroreceptor activation with phenylephrine suggests that glutamate may be a neurotransmitter at baroreceptor afferent nerve terminals within the nucleus tractus solitarius. On the other hand, aspartate appears to be possibly an inter-neuronal transmitter in this brain region.

N-methyl-D-aspartate receptors but not non-N-methyl-D-aspartate receptors mediate hypertension induced by carotid body chemoreceptor stimulation in the rostral ventrolateral medulla of the rat

In urethane-anesthetized rats, excitatory amino acid antagonists were microinjected into the rostral ventrolateral medulla (RVLM) and their effects on the pressor response and tachycardia evoked by carotid chemoreceptor stimulation were examined. Microinjections of the N-methyl-D-aspartate (NMDA) receptor antagonists 2-amino-5-phosphonovalerate (AP5) and MK-801 into the RVLM inhibited these chemoreceptor reflex responses whereas these responses were not affected by injection of the non-NMDA receptor antagonist CNQX. AP5 and MK-801 but not CNQX abolished the pressor response evoked by NMDA whereas CNQX but not AP5 and MK-801 abolished that evoked by AMPA or kainate. These results provide evidence that NMDA receptors in the RVLM of the rat are involved in the carotid chemoreceptor reflex.

Amino acid neurotransmitters in nucleus tractus solitarius: an in vivo microdialysis study

Amino acid neurotransmitters in the nucleus tractus solitarius (NTS) are thought to play a key role in the mediation of visceral reflexes and glutamate has been proposed as the neurotransmitter of visceral afferent nerves projecting to this region. The present studies sought to characterize the use of in vivo microdialysis to examine extracellular fluid levels of amino acids in the NTS of anesthetized rats. Using a microdialysis probe that was 450 microns in length and a sensitive HPLC assay for amino acids, amino acids could be measured in dialysate samples collected from the NTS. Perfusion of the microdialysis probe with 60 mM K+, to elicit depolarization of nerve terminals in the vicinity of the probe, resulted in increased dialysate fluid levels of aspartate, glutamate, glycine, taurine, and GABA. In contrast, glutamine and tyrosine were decreased and other amino acids were not significantly affected. Prior removal of the ipsilateral nodose ganglion did not alter the K(+)-evoked changes in dialysate levels of any of these amino acids. Electrical stimulation of the vagus nerves, using a variety of stimulus parameters, did not significantly alter dialysate levels of glutamate or any of the other amino acids that were measured. Blockade of glutamate uptake with dihydrokainate increased dialysate levels of glutamate, aspartate, and GABA, but in the presence of dihydrokainate vagal stimulation did not alter dialysate levels of these amino acids. The results show that in vivo microdialysis can be used to examine amino acid efflux in the rat NTS and provide further evidence for amino acidergic neural transmission in the NTS. However, these studies fail to support the hypothesis that vagal afferents release glutamate or aspartate.

Dual activation of GABAA and glycine receptors by beta-alanine: inverse modulation by progesterone and 5 alpha-pregnan-3 alpha-ol-20-one

The differential sensitivity of the glycine and GABAA receptors to modulation by progesterone and 5 alpha-pregnan-3 alpha-ol-20-one (5 alpha 3 alpha) was used to determine whether beta-alanine acts through its own receptor, or through the glycine and/or GABAA receptor(s). The response to beta-alanine resembles the glycine response as it is inhibited by strychnine (a competitive glycine antagonist) or progesterone (a negative modulator of the glycine response). Significantly, the response to beta-alanine also resembles the GABA response in that it is inhibited by 2-(carboxy-3'-propyl)-3-amino-6-paramethoxy-phenylpyridazinium+ ++ bromide (SR-95531; a competitive GABA antagonist) and potentiated by 5 alpha 3 alpha (a positive modulator of the GABA response). The efficacy of beta-alanine at the GABAA receptor is comparable to that of GABA. Similarly, the efficacy of beta-alanine at the glycine receptor is comparable to that of glycine. The greater potency of beta-alanine at the glycine receptor indicates that, if beta-alanine is a neurotransmitter, its effects are more likely to be mediated by glycine receptors than by GABAA receptors. However, activation of the GABAA receptor by beta-alanine may become important in the presence of steroid modulators such as progesterone or 5 alpha 3 alpha.

L-dopa induces Ca(2+)-dependent and tetrodotoxin-sensitive release of endogenous glutamate from rat striatal slices

L-DOPA (10-1000 microM) concentration-dependently released glutamate (Glu) from superfused rat striatal slices. D-DOPA and dopamine (300 microM) produced no effects. The L-DOPA-induced release of Glu was not affected by 3-hydroxybenzylhydrazine (20 microM), an L-aromatic amino acid decarboxylase inhibitor. L-DOPA methyl ester (200 microM), a selective L-DOPA antagonist, antagonized the effect of L-DOPA in a competitive manner. Ca2+ deprivation and tetrodotoxin decreased L-DOPA (300 microM)-induced release of Glu. These findings indicate that L-DOPA induces a transmitter-like release of Glu via activation of a recognition site for itself.

Nitric oxide increases interstitial excitatory amino acid release in the rat dorsomedial medulla oblongata

In vivo microdialysis was employed to measure release of the neuroactive amino acids L-glutamate (Glu) and L-aspartate (Asp) in the dorsomedial medulla oblongata of the anaesthetised rat. Basal levels of endogenous extracellular Glu and Asp were increased over 5-fold and 3-fold, respectively, following perfusion with a depolarising stimulus of KCl. Intracerebral administration of the nitric oxide (NO) donor S-nitroso-N-acetylpenicillamine (SNAP, 30 microM) caused a 5-fold increase in extracellular Asp and a 2-fold increase in extracellular Glu, which was blocked by Methylene blue. These data suggest that NO, acting through guanylate cyclase, can affect excitatory amino acid neurotransmission in the dorsomedial medulla oblongata.

Uptake and release of beta-alanine in cerebellar granule cells in primary culture: regulation of release by glutamatergic and GABAergic receptors

The uptake and release of beta-[3H]alanine were studied in cultured glutamatergic cerebellar granule cells of the rat. The uptake of beta-alanine was saturable and sodium-dependent, comprising one high-affinity transport component. It was inhibited by hypotaurine, taurine, GABA and homotaurine but not by glycine or glutamate. The release was enhanced by homoexchange, veratridine and high K+ concentrations (50 mM). The K(+)-stimulated release was at least partially Ca(2+)-dependent. The release was shown to be subject to regulation by GABAA receptors and glutamate receptors of the kainate type. The results signify that beta-alanine may have a functional role in cerebellar granule cells.

Release of endogenous amino acids, including homocysteic acid and cysteine sulphinic acid, from rat hippocampal slices evoked by electrical stimulation of Schaffer collateral-commissural fibres

This study examined the release of endogenous amino acids from acute hippocampal slices, upon stimulation of the Schaffer collateral-commissural fibres. One-minute samples of superfusate were collected via a cannula placed over the CA1 stratum radiatum, and were analysed by reversed-phase high performance liquid chromatography. Evoked potentials were recorded to ascertain stimulation efficacy. Four minutes of continuous 50 Hz stimulation produced a tetrodotoxin-sensitive release of aspartate and glycine in the second minute of stimulation, as well as a tetrodotoxin-sensitive release of cysteine sulphinic acid, during stimulation and of homocysteic acid, following stimulation. Such 50 Hz stimulation also produced a tetrodotoxin-insensitive decrease in methionine levels, but no significant changes in any of the other 15 amino acids measured. Four minutes of continuous 1 Hz stimulation produced no changes in the levels of any of the amino acids measured, but four 600-ms trains of 100 Hz stimulation, which, unlike the 1 Hz stimulation, produced long-term potentiation, resulted in significant increases in levels of cysteine sulphinic acid and homocysteic acid, but not of any of the other amino acids measured. These results suggest that aspartate, glycine, homocysteic acid, and cysteine sulphinic acid play a role in synaptic transmission in the Schaffer collateral-commissural fibres, and that cysteine sulphinic acid and homocysteic acid may be released specifically by high-frequency stimulation.

Evidence for L-dopa systems responsible for cardiovascular control in the nucleus tractus solitarii of the rat

Microinjections of L-DOPA (10-100 ng) into the medial area of the nucleus tractus solitarii (NTS) led to dose-dependent decreases in arterial blood pressure and heart rate in rats treated with i.p. 3-hydroxybenzylhydrazine, a central inhibitor of DOPA decarboxylase, or similarly with intraventricular 6-hydroxydopamine. D-DOPA, dopamine or noradrenaline (100 ng) produced no effect. L-DOPA methyl ester (1 microgram), a competitive antagonist for L-DOPA, microinjected into NTS, blocked the depressor and bradycardic responses to L-DOPA. High K+ (40 mM) released endogenous DOPA in a Ca(2+)-dependent manner from slices of the rat dorsomedial medulla including NTS. These results support the hypothesis that there exist systems of L-DOPA itself responsible for cardiovascular regulation in NTS of rats. This regulatory action of L-DOPA seems to be postsynaptic in nature.

Modulation of extracellular gamma-aminobutyric acid in the ventral pallidum using in vivo microdialysis

Intracranial microdialysis was used to investigate the origin of extracellular gamma-aminobutyric acid (GABA) in the ventral pallidum. Changes in basal GABA levels in response to membrane depolarizers, ion-channel blockers, and receptor agonists were determined. Antagonism of Ca2+ fluxes with high Mg2+ in a Ca(2+)-free perfusion buffer decreased GABA levels by up to 30%. Inhibition of voltage-dependent Na+ channels by the addition of tetrodotoxin also significantly decreased basal extracellular GABA concentrations by up to 45%, and blockade of Ca2+ and Na+ channels with verapamil reduced extracellular GABA by as much as 30%. The addition of either the GABAA agonist, muscimol, or the GABAB agonist, baclofen, produced a 40% reduction in extracellular GABA. GABA release was stimulated by high K+ and the addition of veratridine to increase Na+ influx. High K(+)-induced release was predominantly Ca(2+)-dependent, whereas the effect of veratridine was potentiated in the absence of extracellular Ca2+. Both high K(+)- and veratridine-induced elevations in extracellular GABA were inhibited by baclofen, whereas only veratridine-induced release was antagonized by muscimol. These results demonstrate that at least 50% of basal extracellular GABA in the ventral pallidum is derived from Ca(2+)- or Na(+)-dependent mechanisms. They also suggest that Na(+)-dependent release of GABA via reversal of the uptake carrier can be shown in vivo.

Glutamate-induced energetic stress in hippocampal slices: evidence against NMDA and glutamate uptake as mediators

The introduction of exogenous glutamate to normally respiring hippocampal slices produced substantial reductions in ATP, phosphocreatine (PCr) and intracellular pH (pHi) when the concentration exceeded 1 mM. These changes were not prevented by addition of MK-801 (an NMDA receptor antagonist), nor were they mimicked by NMDA or high potassium. In addition, the glutamate-induced metabolic alterations were not prevented by addition of aspartate-b-hydroxymate or sodium substitution by choline, both of which should inhibit high-affinity sodium-dependent glutamate uptake. These results suggest that glutamate alone can produce marked energetic stress in neural tissue, even when glucose and oxygen are maintained at control levels; and that the energetic stress does not appear to be specifically mediated by NMDA-induced depolarization, or by high-affinity uptake of glutamate.

Release of GABA and taurine from brain slices

In brain slices the mechanisms of release of GABA have been extensively studied, but those of taurine markedly less. The knowledge acquired from studies on GABA is, nevertheless, still fragmentary, not to speak of that obtained from the few studies on taurine, and firm conclusions are difficult, even impossible, to draw. This is mainly due to methodological matters, such as the diversity and pitfalls of the techniques applied. Brain slices are relatively easy to prepare and they represent a preparation that may most closely reflect relations prevailing in vivo, since the tissue structure and cellular integrity are largely preserved. In our opinion the most recommendable method at present is to superfuse freely floating agitated slices in continuously oxygenated medium. Taurine is metabolically rather inert in the brain, whereas the metabolism of GABA must be taken into account in all release studies. The use of inhibitors of GABA catabolism is discouraged, however, since a block in GABA metabolism may distort relations between different releasable pools of GABA in tissue. It is not known for sure how well, and homogeneously, incubation of slices with radioactive taurine labels the releasable pools but at least in the case of GABA there may prevail differences in the behavior of labeled and endogenous GABA. It is suggested therefore that the results obtained with radioactive GABA or taurine should be frequently checked and confirmed by analyzing the release of respective endogenous compounds. The spontaneous efflux of both GABA and taurine from brain slices is very slow. The magnitude of stimulation of GABA release by homoexchange is greater than that of taurine under the same experimental conditions. However, the release of both amino acids is generally enhanced by a great number of structural analogs, the most potent being those which are simultaneously the most potent inhibitors of uptake. This may result in part from inhibition of reuptake of amino acid molecules released from slices but the findings may also signify that the efflux of GABA and taurine is at least partially mediated by the membrane carriers operating in an outward direction. It is thus advisable not to interpret that stimulation of release in the presence of uptake inhibitors solely results from the block of reuptake of exocytotically released molecules, since changes in the carrier-mediated transport are also likely to occur upon stimulation. The electrical and K+ stimulation evoke the release of both GABA and taurine. The evoked release of GABA is several-fold greater than that of taurine in slices from the adult brain.(ABSTRACT TRUNCATED AT 400 WORDS)

[The nucleus tractus solitarius: neuroanatomic, neurochemical and functional aspects]

The nucleus tractus solitarii (NTS) has long been considered as the first central relay for gustatory and visceral afferent informations only. However, data obtained during the past ten years, with neuroanatomical, biochemical and electrophysiological techniques, clearly demonstrate that the NTS is a structure with a high degree of complexity, which plays, at the medullary level, a key role in several integrative processes. The NTS, located in the dorsomedial medulla, is a structure of small size containing a limited number of neurons scattered in a more or less dense fibrillar plexus. The distribution and the organization of both the cells and the fibrillar network are not homogeneous within the nucleus and the NTS has been divided cytoarchitectonically into various subnuclei, which are partly correlated with the areas of projection of peripheral afferent endings. At the ultrastructural level, the NTS shows several complex synaptic arrangements in form of glomeruli. These arrangements provide morphological substrates for complex mechanisms of intercellular communication within the NTS. The NTS is not only the site of vagal and glossopharyngeal afferent projections, it receives also endings from facial and trigeminal nerves as well as from some renal afferents. Gustatory and somatic afferents from the oropharyngeal region project with a crude somatotopy within the rostral part of the NTS and visceral afferents from cardiovascular, digestive, respiratory and renal systems terminate viscero-topically within its caudal part. Moreover the NTS is extensively connected with several central structures. It projects directly to multiple brain regions by means of short connections to bulbo-ponto-mesencephalic structures (parabrachial nucleus, motor nuclei of several cranial nerves, ventro-lateral reticular formation, raphe nuclei...) and long connections to the spinal cord and diencephalic and telencephalic structures, in particular the hypothalamus and some limbic structures. The NTS is also the recipient of several central afferent inputs. It is worth to note that most of the structures that receive a direct projection from the NTS project back to the nucleus. Direct projections from the cerebral cortex to the NTS have also been identified. These extensive connections indicate that the NTS is a key structure for autonomic and neuroendocrine functions as well as for integration of somatic and autonomic responses in certain behaviors. The NTS contains a great diversity of neuroactive substances. Indeed, most of the substances identified within the central nervous system have also been detected in the NTS and may act, at this level, as classical transmitters and/or neuromodulators.(ABSTRACT TRUNCATED AT 400 WORDS)

Release of segmental amino acid neurotransmitters in response to peripheral afferent and motor cortex stimulation: a pilot study

The role of amino acid (AA) neurotransmitters in the spinal cord has been primarily studied using in vitro preparations and histochemical methods. The technology necessary to estimate AA levels in an intact animal has only recently become available. Such an investigation could yield valuable information regarding the segmental neurochemical environment. We measured the release of AAs into the rabbit lumbar spinal cord in response to sciatic nerve and transcranial stimulation with stereotaxically placed microdialysis catheters. Samples were obtained periodically during 90 minutes of continuous stimulation of either the left or right sciatic nerve, or motor cortex. Quantification of gamma-amino butyric acid (GABA), aspartate, glutamate, glycine, and taurine was performed using high pressure liquid chromatography (HPLC). Adequate neural excitation was verified by recording somatosensory evoked potentials (SSEPs) or corticomotor evoked potentials (CMEPs). Sensory activation at intensities sufficient to activate small and large diameter peripheral fibers of the ipsilateral (to the microdialysis probe) sciatic nerve produced a significant change only in segmental glycine levels. Contralateral sciatic nerve stimulation failed to evoke a significant elevation of AAs. In addition, a significant increase in the release of glycine and taurine was measured after 90 minutes of transcranial stimulation. SSEP and CMEP components repeatedly showed adequate activation of primary afferent, descending motor fiber pathways, and segmental interneuron pools during dialysis sampling. Our data are consistent with the hypothesis that suprasegmental influence over peripheral afferent and motor activity may be, in part, through these amino acid neurotransmitters in the rabbit lumbar spinal cord.

Segmental release of amino acid neurotransmitters from transcranial stimulation

The present study used microdialysis techniques in an intact rabbit model to measure the release of amino acids within the lumbar spinal cord in response to transcranial electrical stimulation. Dialysis samples from the extracellular space were obtained over a stimulation period of 90 minutes and were examined using high pressure liquid chromatography. Neuronal excitation was verified by recording corticomotor evoked potentials (CMEPs) from the spinal cord. A significant increase in the release of glycine and taurine compared to sham animals was measured after 90 minutes of transcranial stimulation. Glutamate and aspartate release was not significantly elevated. GABA concentrations were consistently low. CMEP components repeatedly showed adequate activation of descending fiber pathways and segmental interneuron pools during dialysis sampling. Since glycine, and to a lesser extent taurine, have been shown to inhibit motor neuron activity and are closely associated with segmental interneuron pools, suprasegmental modulation of motor activity may be, in part, through these inhibitory amino acid neurotransmitters in the rabbit lumbar spinal cord.

Electrical stimulation-evoked release of endogenous aspartate from rat medulla oblongata slices. Effects of inhibitors of aspartate aminotransferase and GABA transaminase

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.

Beta-alanine, like glycine, microinjected into the rat nucleus tractus solitarii increases blood pressure

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.

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

The basal overflow of extracellular endogenous amino acids was measured from the ventrolateral medulla of urethane anaesthetized rats in vivo by microdialysis. Inclusion of a mercury salt, p-chloromercuriphenylsulphonic acid, in the dialysate (Krebs' solution), results in a preferential increase in the overflow of aspartate, glutamate, glycine and GABA. A smaller increase in the overflow of the glutamate precursor and metabolite, glutamine, was also found. There was no significant change in the basal extracellular levels of taurine, asparagine, alanine, serine, ornithine or lysine. Inclusion of a specific GABA uptake inhibitor, nipecotic acid, in the dialysate results in an immediate, dose dependent increase in the overflow of GABA, and to a lesser extent, taurine. Since it is likely that mercury salts increase neurotransmitter release by increasing free intracellular calcium ion concentrations, it is suggested that these results provide further evidence for a physiologically relevant neurotransmitter role for aspartate, glutamate, glycine and GABA in the ventrolateral medulla.

Genetically altered brain amino acid metabolism in spontaneously hypertensive rats: a study by using young spontaneously hypertensive rats and renal hypertensive rats

Previously we demonstrated altered amino acid levels in brainstem regions of adult spontaneously hypertensive rats (SHR). For comparison, in this study, we determined amino acid concentrations in discrete brainstem regions in young prehypertensive SHR and renal hypertensive rats. In prehypertensive SHR, the content of glutamate was increased in the rostral ventrolateral medulla and the caudal ventrolateral medulla, and the content of beta-alanine was decreased in the nucleus tractus solitarii. In renal hypertensive rats, there was no change in glutamate and beta-alanine contents in all the regions. The profiles of contents of glutamate and beta-alanine in the brainstem regions in young SHR but not in renal hypertensive rats are the same as those found previously in adult SHR. Thus, the results of the present study suggest that the altered amino acid metabolism in the brainstem of SHR may be genetically inherent.

Hypotension decreases GABA levels in brainstem regions of the rat

Concentrations of gamma-aminobutyric acid (GABA) and other amino acids were measured in microdissected areas of the rostral ventro-lateral medulla (RVL), nucleus tractus solitarii (NTS) and caudal ventrolateral medulla (CVL) of the rat, and effects on the amino acid levels, of hypotension induced by hexamethonium and nifedipine were examined. Hexamethonium and nifedipine decreased concentrations of GABA in all the regions studied, while concentrations of glycine, beta-alanine and taurine were not affected by these agents. Both agents caused a prolonged fall in blood pressure. These results strongly suggest that a decrease in blood pressure produces a decrease in GABA levels in the brainstem regions of the rat.

A high performance liquid chromatography/electrochemical assay for glutamatergic neurotransmitters in the rat brain

The release and content of the excitatory amino acid neurotransmitters glutamate and aspartate in rat striatum were determined by liquid chromatography/electrochemistry. This determination was based on precolumn off-line derivatization of the amino acids with o-phthaldialdehyde and 2-mercaptoethanol (OPT/2-MCE), and the adducts formed were separated under isocratic conditions and oxidized on a glassy carbon electrode at moderate potential (+0.6 V). The standard and the extracted glutamate when derivatized with OPT/2-MCE produced similar electrochemical and chromatographic characteristics. The detection limit of glutamate was 0.5 pmol. Depolarization induced by the high potassium medium (40 mmol/L) enhanced the release of glutamate and aspartate from superfused rat striatum, whereas the efflux of glutamine remained unchanged. Perfusion (for 60-70 min) removed 50-80% of the free amino acid content of striatal tissue. The method described here is useful in neurochemical investigations of the brain amino acid neurotransmitters.

Immunocytochemical localization of GABA containing neurons in the ventrolateral medulla oblongata of the rat

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.