GABA receptor subtypes involved in the neuronal mechanisms of baroreceptor reflex in the nucleus tractus solitarii of rabbits

Neurodegenerative diseases and blood pressure variability: A comprehensive review from HOPE Asia

Asia has an enormous number of older people and is the primary contributor to the rise in neurodegenerative diseases such as Alzheimer's and Parkinson's disease. The therapy of many neurodegenerative diseases has not yet progressed to the point where it is possible to alter the course of the disease. Mid-life hypertension is an important predictor of later-life cognitive impairment and brain neurodegenerative conditions. These findings highlight the pivotal role of preventing and managing hypertension as a risk factor for neurodegenerative disease. Autonomic dysfunction, neuropsychiatric and sleep disturbances can arise in neurodegenerative diseases, resulting in blood pressure variability (BPV). The BPV itself can worsen the progression of the disease. In older people with neurodegenerative disease and hypertension, it is critical to consider 24-h blood pressure monitoring and personalized blood pressure therapy.

A Potential Interface between the Kynurenine Pathway and Autonomic Imbalance in Schizophrenia

Schizophrenia is a neuropsychiatric disorder characterized by various symptoms including autonomic imbalance. These disturbances involve almost all autonomic functions and might contribute to poor medication compliance, worsened quality of life and increased mortality. Therefore, it has a great importance to find a potential therapeutic solution to improve the autonomic disturbances. The altered level of kynurenines (e.g., kynurenic acid), as tryptophan metabolites, is almost the most consistently found biochemical abnormality in schizophrenia. Kynurenic acid influences different types of receptors, most of them involved in the pathophysiology of schizophrenia. Only few data suggest that kynurenines might have effects on multiple autonomic functions. Publications so far have discussed the implication of kynurenines and the alteration of the autonomic nervous system in schizophrenia independently from each other. Thus, the coupling between them has not yet been addressed in schizophrenia, although their direct common points, potential interfaces indicate the consideration of their interaction. The present review gathers autonomic disturbances, the impaired kynurenine pathway in schizophrenia, and the effects of kynurenine pathway on autonomic functions. In the last part of the review, the potential interaction between the two systems in schizophrenia, and the possible therapeutic options are discussed.

Angiotensin II increases GABAB receptor expression in nucleus tractus solitarii of rats

Increasing evidence indicates that both the angiotensin II (ANG II) and gamma-aminobutyric acid (GABA) systems play a very important role in the regulation of blood pressure (BP). However, there is little information concerning the interactions between these two systems in the nucleus tractus solitarii (NTS). In the present study, we examined the effects of ANG II on GABAA and GABAB receptor (GAR and GBR) expression in the NTS of Sprague-Dawley rats. The direct effect of ANG II on GBR expression was determined in neurons cultured from NTS. Treatment of neuronal cultures with ANG II (100 nM, 5 h) induced a twofold increase in GBR1 expression, as detected with real-time RT-PCR and Western blots, but had no effect on GBR2 or GAR expression. In electrophysiological experiments, perfusion of neuronal cultures with the GBR agonist baclofen decreased neuronal firing rate by 39% and 63% in neurons treated with either PBS (control) or ANG II, respectively, indicating that chronic ANG II treatment significantly enhanced the neuronal response to GBR activation. In contrast, ANG II had no significant effect on the inhibitory action of the GAR agonist muscimol. In whole animal studies, intracerebroventricular infusion of ANG II induced a sustained increase in mean BP and an elevation of GBR1 mRNA and protein levels in the NTS. These results indicate that ANG II stimulates GBR expression in NTS neurons, and this could contribute to the central nervous system actions of ANG II that result in dampening of baroreflexes and elevated BP in the central actions of ANG II.

Effect of lithium hydroxybutyrate on hemodynamics and respiration in rats with different resistance to hypoxia

Acute experiments on narcotized rats showed that intravenous infusion of GABA derivative lithium hydroxybutyrate induced different changes in hemodynamic and respiratory parameters in animals with high and low resistance to hypoxia. Rats highly resistant to hypoxia better tolerated lithium hydroxybutyrate treatment compared to low resistant animals.

ROLE OF THE CENTRAL NUCLEUS OF THE AMYGDALA IN THE CONTROL OF BLOOD PRESSURE: DESCENDING PATHWAYS TO MEDULLARY CARDIOVASCULAR NUCLEI

1. One of the key areas that links psychologically induced stress with the blood pressure-regulatory system is the central nucleus of the amygdala (CeA). This is an integratory forebrain nucleus that receives input from higher centres in the forebrain and has extensive connections with the hypothalamus and the medulla oblongata, areas involved in the regulation of the cardiovascular reflexes. 2. Based on studies using electrical or chemical stimulation or electrolytic lesions of the CeA, it has become clear that the CeA plays an important role in the regulation of blood pressure in response to stressful or fearful stimuli. 3. Two important medullary areas known to receive projections from the CeA are the nucleus tractus solitarius (NTS) and the rostral ventrolateral medulla (RVLM). The NTS is the site of the first synapse for afferent fibres originating from baroreceptors, chemoreceptors and the heart, whereas the RVLM contains neurons that maintain resting blood pressure and sympathetic nerve activity via projections to sympathetic preganglionic neurons in the intermediolateral cell column of the thoracolumbar spinal cord. 4. Electron microscopic studies using combined anterograde tracing and pre- and post-embedding immunogold labelling have shown that the pathways originating from the CeA to the NTS are inhibitory and may use GABA as a neurotransmitter. The results of these studies suggest that blood pressure changes produced by activation of the CeA may be mediated by attenuation of baroreceptor reflexes through a GABAergic mechanism at the level of the NTS. 5. Neuronal tract tracing combined with neurofunctional studies using the Fos protein as a marker of activated neurons indicate that the CeA projects directly to baroreceptive neurons in the NTS and RVLM that are activated by changes in blood pressure. 6. In conclusion, studies that have examined the efferent pathways of the CeA suggest that CeA neurons with projections to medullary baroreceptive neurons may play a vital role in the reflex changes in sympathetic nerve activity that are involved in blood pressure regulation in response to stress or anxiety.

Benzodiazepine-sensitive GABA(A) receptors in the commissural subnucleus of the NTS are involved in the carotid chemoreceptor reflex in rats

We studied the role of benzodiazepine (BDZ) receptors in the commissural subnucleus of the nucleus tractus solitarius (commNTS) in chemoreceptor reflex in urethane-anesthetized, pancronium-immobilized, artificially ventilated and bilaterally vagotomized rats. A BDZ agonist, diazepam (1-4 micromol/kg), administered intravenously reduced resting phrenic nerve activity (PNA) and blood pressure (BP). Stimulation of carotid chemoreceptors induced an increase in PNA and an increase in BP. Diazepam inhibited this chemoreceptor reflex. The effects of intravenous injection of diazepam (4 micromol/kg) on the chemoreceptor reflex were antagonized by microinjection of the BDZ antagonist flumazenil (100 pmol) into the commNTS. Microinjection of flumazenil (100 pmol) alone had no effect on the basal PNA and BP, and the chemoreceptor reflex. These results suggest that BDZ receptors are present in the carotid chemoreceptor reflex pathway in the commNTS and potentiate GABA(A) transmission.

The gamma-aminobutyric acidergic effects of valerian and valerenic acid on rat brainstem neuronal activity

Valerian is a medicinal herb that produces anxiolytic and sedative effects. It was suggested that valerian acts via gamma-aminobutyric acid (GABA)ergic mechanisms. Previous studies showed binding of valerian extract to GABA receptors, but the functional effect of the binding has not been demonstrated. In this study we evaluated the GABAergic effect of valerian extract and one of its major constituents, valerenic acid, on brainstem neuronal activity in an in vitro neonatal rat brainstem preparation. We first observed that muscimol, a GABA(A) receptor agonist, decreased the firing rate in most brainstem neurons in a concentration-related fashion; 30 micro M produced a 38.9% +/- 3.0% (mean +/- SE) inhibition compared with control values (P < 0.01; 50% inhibitory concentration [IC(50)], 2.0 +/- 0.1 microM). This effect was antagonized by bicuculline (10 microM), a GABA(A) antagonist. Then we showed that valerian extract 3 mg/mL induced a 29.6% +/- 5.1% inhibition with an IC(50) of 240 +/- 18.7 microg/mL, whereas 100 microM valerenic acid induced a 22.2% +/- 3.4% inhibition with an IC(50) of 23 +/- 2.6 microM (both P < 0.01). Bicuculline antagonized the inhibitory effects of both the valerian extract and valerenic acid. In addition, pretreatment with valerian extract or valerenic acid decreased the brainstem inhibitory effects produced by muscimol (both P < 0.05), suggesting that these compounds play an important role in the regulation of GABAergic activity. Data from this study suggest that the pharmacological effects of valerian extract and valerenic acid are mediated through modulation of GABA(A) receptor function. Thus, valerian may potentiate the sedative effects of anesthetics and other medications that act on GABA receptors, and presurgical valerian use may cause a valerian-anesthetic interaction.

IMPLICATIONS

Valerian is an herb used in treating anxiety and insomnia. We observed that the valerian effects are mediated through brain gamma-aminobutyric acid (GABA) receptors in a rat brainstem preparation. Thus, valerian may potentiate the effects of anesthetics that act on GABA receptors, and presurgical valerian use may cause a valerian-anesthetic interaction.

17beta-estradiol inhibits excitatory amino acid-induced activity of neurons of the nucleus tractus solitarius

The effects of 17beta-estradiol (17betaE2) on spontaneous and excitatory amino acid (EAA) induced nucleus tractus solitarius (NTS) neuronal activity were investigated by electrophysiological and immunohistochemical experiments in ovariectomized female Sprague-Dawley rats. Out of 62 NTS neurons tested, 42 were inhibited (68%) following iontophoretic application of 17betaE2 in a current-dependent manner. The averaged firing rate decreased from 3.06+/-0.40 to 0.78+/-0.17 Hz. The inhibitory responses were rapid in onset (within 1 min) and variable in duration (2-4 min). The inhibitory effects of 17betaE2 were blocked by simultaneously applied 17betaE2 antagonist ICI182,780, but not by GABA antagonist, bicuculline and phaclofen. L-Glutamate, AMPA or NMDA enhanced the activity of 71, 73 or 69% of NTS cells tested, respectively. The excitatory effects of EAA were significantly inhibited in the presence of 17betaE2. Fluorescent immunohistochemistry revealed that all subnuclei of the NTS contained high levels of estrogen receptors (ERs) immunoreactivity. These results suggest that 17betaE2 inhibits spontaneous and EAA-induced NTS neuronal activity through 17betaE2 activation of ERs.

Baroreceptor reflex pathways and neurotransmitters: 10 years on

The central nervous system plays a critical role in the management of blood flow to the tissues and its return to the heart and lungs. This is achieved by a complex interplay of neural efferent pathways, humoral mechanisms and afferent pathways. In this review, we focus on recent progress (within the past 10 years) that has been made in the sympathetic control of arterial blood pressure with a special emphasis on the role of baroreceptor mechanisms and central neurotransmitters. In particular, we focus on new features since 1991, such as neurotransmission in the nucleus tractus solitarius, the role of neurons in the most caudal part of the ventrolateral medulla oblongata and the increasing understanding of the exquisite control of different sympathetic pathways by different neurotransmitter systems.

What does the brain know about blood pressure?

The integration of baroreceptor inputs within the central nervous system is modulated by a variety of inhibitory processes. It is proposed that, in hypertension, brain stem neurons adapt to increased excitatory baroreceptor inputs by increasing the efficacy of these inhibitory processes. Enhanced inhibition maintains some degree of reflex function in hypertension.

GABA(A) receptor epsilon-subunit may confer benzodiazepine insensitivity to the caudal aspect of the nucleus tractus solitarii of the rat

1. Benzodiazepines (BZ) and barbiturates both potentiate chloride currents through GABA(A) receptors to enhance inhibition. However, unlike barbiturates BZ do not impair autonomic control of heart rate. We hypothesised that BZ might not significantly potentiate GABAergic transmission in the caudal nucleus of the solitary tract (cNTS), which is critically important for mediating the baroreceptor reflex. 2. In rat brain slices the BZ agonists chlordiazepoxide and midazolam (2 and 50 microM) did not significantly enhance currents evoked by GABA in voltage-clamped cNTS neurones. Chlordiazepoxide (50 microM) reversibly increased electrically evoked IPSPs in 5/10 rostral NTS (rNTS) neurones but only in 2/10 cNTS neurones. Pentobarbitone (50-100 microM) was effective in enhancing GABA(A)-mediated responses in all NTS neurones. An inverse BZ agonist, methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM; 1 or 10 microM), failed to depress GABA-induced currents in the cNTS. 3. Microinjections of midazolam (10 and 100 microM solutions) into the cNTS did not affect the baroreceptor reflex (P > 0.2) while pentobarbitone (100 microM) significantly and reversibly depressed it (gain decrease to 53 +/- 11 % of control, P < 0.01). 4. Reverse transcriptase polymerase chain reaction revealed the presence of alpha(1), alpha(2), beta(2), beta(3) and gamma(2) GABA(A) receptor subunit mRNA in the cNTS. No alternatively spliced variants of the alpha(1)- and gamma(2)-subunits were revealed. Moreover, GABA(A) epsilon-unit mRNA was found in both the cNTS and rNTS as two alternatively spliced transcripts. 5. Immunocytochemical analysis revealed numerous GABA(A) epsilon-subunit-positive neurones within the cNTS with significantly fewer epsilon-subunit-positive cells in the rNTS. 6. As incorporation of the epsilon-subunit in recombinant GABA(A) receptors may confer BZ insensitivity we propose that the paucity of BZ actions in the cNTS is due to a high level of epsilon-subunit expression. This is the first demonstration of a possible physiological impact of the epsilon-subunit on native GABA(A) receptors.

GABAergic neurotransmission at the nucleus tractus solitarii in the suppression of reflex bradycardia by parabrachial nucleus

We investigated the role of GABAergic neurotransmission at the nucleus tractus solitarii (NTS) in the suppression of cardiac baroreceptor reflex (BRR) response induced by parabrachial nucleus (PBN) complex in adult Sprague-Dawley rats maintained under pentobarbital anesthesia. Based on in vivo microdialysis coupled with high-performance liquid chromatography-fluorescence detection for gamma-aminobutyric acid (GABA), we found that electrical stimulation of the ventrolateral regions and Koelliker-Fuse (KF) subnucleus of PBN complex resulted in a site-specific increase in GABA concentration in the dialysate collected from the NTS. The temporal increase in extracellular GABA concentration in the NTS coincided with the time course of PBN-induced cardiac BRR inhibition. In addition, the PBN-induced cardiac BRR suppression was reversed by microinjection bilaterally into the NTS of a GABA(A) receptor antagonist, bicuculline methiodide (5 pmol), or a GABA(B) receptor antagonist, 2-OH saclofen (500 pmol). Blockade of neuronal activity in the ventrolateral regions and KF subnucleus of PBN complex with lidocaine (5%) elicited an enhancement of the same reflex response. The time course of this facilitatory effect of lidocaine correlated positively with the temporal decrease in extracellular GABA concentration in the NTS. Anatomically, Fast Blue-labeled neurons were identified in the same subnuclei of the PBN complex after microinjection of the retrograde transport tracer into the NTS. Some of these Fast Blue-labeled neurons were also immunoreactive to glutamic acid decarboxylase. These results suggest that a direct GABAergic descending projection from the KF subnucleus and surrounding areas of the PBN complex to the NTS may inhibit cardiac BRR response by activating GABA(A) and GABA(B) receptors at the NTS.

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.

Responses of aortic depressor nerve-evoked neurones in rat nucleus of the solitary tract to changes in blood pressure

Using electrophysiological techniques, the discharge of neurones in the nucleus of the solitary tract (NTS) receiving aortic depressor nerve (ADN) inputs was examined during blood pressure changes induced by I.V. phenylephrine or nitroprusside in anaesthetized, paralysed and artificially ventilated rats. Various changes in discharge rate were observed during phenylephrine-induced blood pressure elevations: an increase (n = 38), a decrease (n = 5), an increase followed by a decrease (n = 4) and no response (n = 11). In cells receiving a monosynaptic ADN input (MSNs), the peak discharge frequency response was correlated to the rate of increase in mean arterial pressure (P < 0.01) but was not correlated to the absolute increase in blood pressure. The peak discharge frequency response of cells receiving a polysynaptic ADN input (PSNs) was correlated to neither the absolute increase in blood pressure nor the rate of increase in mean arterial pressure. Diverse changes in discharge rate were observed during nitroprusside-induced reductions in blood pressure: an increase (n = 3), a decrease (n = 10), an increase followed by a decrease (n = 3) and no response (n = 6). Reductions in pressure of 64 +/- 2 mmHg produced weak reductions in spontaneous discharge of 1.3 +/- 0.9 Hz and only totally abolished spontaneous discharge in one neurone. These response patterns of NTS neurones during changes in arterial pressure suggest that baroreceptor inputs are integrated differently in MSNs compared to PSNs. The sensitivity of MSNs to the rate of change of pressure provides a mechanism for the rapid regulation of cardiovascular function. The lack of sensitivity to the mean level of a pressure increase in both MSNs and PSNs suggests that steady-state changes in pressure are encoded by the number of active neurones and not graded changes in the discharge of individual neurones. Both MSNs and PSNs receive tonic excitatory inputs from the arterial baroreceptors; however, these tonic inputs appear to be insufficient to totally account for their spontaneous discharge.

Activation of GABA receptors in the NTS of awake rats reduces the gain of baroreflex bradycardia

In the present study we evaluated the effects of bilateral microinjection of muscimol (a GABA(A) receptor agonist) and baclofen (a GABA(B) receptor agonist) into the lateral commissural nucleus tractus solitarii (NTS) of awake rats on the gain of the baroreflex (BG) activated by a short duration (10-15 s) infusion of phenylephrine (Phe, 2.5 microg/0.05 ml, i.v.). Microinjection of muscimol (50 pmol/50 nl, n=8) into the NTS produced a significant increase in baseline mean arterial pressure ((MAP) 122+/-6 vs. 101+/-2 mmHg), no changes in baseline heart rate (HR) and a reduction in BG (-1.59+/-0. 1 vs. -0.69+/-0.1 beats/mmHg). Microinjection of baclofen (6.25 pmol/50 nl, n=6) into the NTS also produced a significant increase in baseline MAP (138+/-5 vs. 103+/-2 mmHg), no changes in baseline HR and a reduction in BG (-1.54+/-0.3 vs. -0.53+/-0.2 beats/mmHg). Considering that the reduction in BG could be secondary to the increase in MAP in response to microinjection of muscimol (n=6) or baclofen (n=7) into the NTS, in these two groups of rats we brought the MAP back to baseline by infusion of sodium nitroprusside (NP, 3.0 microg/0.05 ml, i.v.). Under these conditions, we verified that the BG remained significantly reduced after muscimol (-1.49+/-0.2 vs. -0.35+/-0.2 beats/mmHg) and after baclofen (-1.72+/-0.2 vs. -0.33+/-0.2 beats/mmHg) when compared to control. Reflex tachycardia was observed during the normalization of MAP by NP infusion and, in order to prevent the autonomic imbalance from affecting BG, we used another group of rats treated with atenolol (5 mg/kg, i.v.), a beta1 receptor antagonist. In rats previously treated with atenolol and submitted to NP infusion, we verified that BG remained reduced after microinjection of muscimol or baclofen into the NTS. The data show that activation of GABA(A) and GABA(B) receptors, independently of the changes in the baseline MAP or HR, inhibited the neurons of the NTS involved in the parasympathetic component of the baroreflex.

Differential effects of angiotensin II in the nucleus tractus solitarii of the rat--plausible neuronal mechanism

1. Cellular mechanisms of the actions of angiotensin II (ANGII) within the nucleus of the solitary tract (NTS) were studied using rat brain slices in 78 neurones recorded in the whole-cell configuration. Twenty-nine per cent of cells had an on-going activity and with only one exception these cells responded to tractus solitarii (TS) stimulation with a monophasic excitatory postsynaptic potential (EPSP). In approximately half of the silent cells, TS stimulation evoked an EPSP-inhibitory postsynaptic potential (IPSP) complex. 2. The ANGII (200 or 1000 nM) effect on TS-evoked EPSPs depended on the cell subpopulation. In cells with on-going activity, ANGII (1000 nM) increased evoked EPSP amplitude by +70 +/- 13 % (means +/- s.e.m., n = 5) but reduced it (200 and 1000 nM) in silent cells where both evoked EPSPs and IPSPs were present. ANGII either increased TS-evoked IPSP conductances in cells where they were detectable or revealed an evoked IPSP (200 nM ANGII: IPSP conductance increased from 70 +/- 29 to 241 +/- 34 pS; n = 11). All ANGII effects were prevented by the ANGII type 1 (AT1) receptor blocker losartan. Since 200 nM ANGII did not increase responses to iontophoretically applied GABA, the effect of ANGII on TS-evoked IPSPs may occur presynaptically. 3. The neurokinin type 1 (NK1) receptor antagonist CP-99,994 (5 microM) blocked the ANGII-induced increase in EPSPs but had no effect on TS-evoked IPSP potentiation by ANGII. 4. Thus, ANGII can potentiate both inhibitory and excitatory synaptic transmission within different subpopulations of NTS neurones. Potentiation of evoked EPSPs, but not of IPSPs, involves activation of NK1 receptors. The balance of these actions of ANGII could be reflex specific: for the baroreflex circuitry the inhibitory action might predominate while the peripheral chemoreceptor reflex may be facilitated due to enhanced excitatory transmission.

Activation of GABAA but not GABAB receptors in the NTSblocked bradycardia of chemoreflex in awake rats

In the present study we analyzed effects of bilateral microinjections of muscimol (a GABAA agonist) and baclofen (a GABAB agonist) into the nucleus tractus solitarius (NTS) on bradycardic and pressor responses to chemoreflex activation (potassium cyanide, 40 micrograms/rat iv) in awake rats. Bilateral microinjections of muscimol (25 and 50 pmol/50 nl) into the NTS increased baseline mean arterial pressure (MAP): 119 +/- 8 vs. 107 +/- 2 mmHg (n = 6) and 121 +/- 8 vs. 103 +/- 3 mmHg (n = 6), respectively. Muscimol at 25 pmol/50 nl reduced the bradycardic response to chemoreflex activation 5 min after microinjection; with 50 pmol/50 nl the bradycardic response to chemoreflex activation was reduced 5, 15, 30, and 60 min after microinjection. Neither muscimol dose produced an effect on the pressor response of the chemoreflex. Effects of muscimol (50 pmol/50 nl) on basal MAP and on the bradycardic response of the chemoreflex were prevented by prior microinjection of bicuculline (a GABAA antagonist, 40 pmol/50 nl) into the NTS. Bilateral microinjections of baclofen (12.5 and 25 pmol/50 nl) into the NTS produced an increase in baseline MAP [137 +/- 9 vs. 108 +/- 4 (n = 7) and 145 +/- 5 vs. 105 +/- 2 mmHg (n = 7), respectively], no changes in basal heart rate, and no effects on the bradycardic response; 25 pmol/50 nl only attenuated the pressor response to chemoreflex activation. The data show that activation of GABAA receptors in the NTS produces a significant reduction in the bradycardic response, whereas activation of GABAB receptors produces a significant reduction in the pressor response of the chemoreflex. We conclude that 1) GABAA but not GABAB plays an inhibitory role in neurons of the lateral commissural NTS involved in the parasympathetic component of the chemoreflex and 2) attenuation of the pressor response of the chemoreflex by activation of GABAB receptors may be due to inhibition of sympathoexcitatory neurons in the NTS or may be secondary to the large increase in baseline MAP produced by baclofen.

GABA(B) receptors in the nucleus tractus solitarii modulate the carotid chemoreceptor reflex in rats

In urethane-chloralose anesthetized rats, the role of GABA(B) receptor in the commissural subnucleus of the nucleus tractus solitarii (commNTS) on the carotid chemoreceptor reflex was investigated. Microinjection of a GABA(B) agonist baclofen into the commNTS did not have any effects on arterial blood pressure (BP) or respiration (RP), while it attenuated the increases in BP and RP elicited by carotid chemoreceptor stimulation. These effects were blocked by microinjection of a GABA(B) antagonist 2-OH-saclofen into the same site. Prior microinjection of 2-OH-saclofen did not have any effects on the chemoreflex or on resting BP or RP, while the effects of baclofen on the chemoreflex were completely blocked. These results suggest that GABAB receptors are present in the carotid chemoreceptor reflex pathway in commNTS and modulate the chemoreceptor reflex.

Receptor subtype specific effects of GABA agonists on neurons receiving aortic depressor nerve inputs within the nucleus of the solitary tract

The inhibitory amino acid gamma amino butyrate (GABA) has been shown to profoundly alter the integration of arterial baroreceptor inputs within the nucleus of the solitary tract (NTS). However, the relative roles of the major GABA receptor subtypes, the GABA(A) and the GABA(B) receptors, in the modulation of monosynaptic compared to polysynaptic afferent transmission within the NTS remain uncharacterized. In anesthetized rats, three types of NTS neuron were identified by their responses to aortic depressor nerve (ADN) stimulation; monosynaptic neurons (MSNs), polysynaptic neurons (PSNs) and ADN non-evoked neurons (NENs). Selective GABA(A) and GABA(B) agonists were applied to these neurons using iontophoretic techniques. The endogenous ligand GABA (2 mM), the selective GABA(A) agonist muscimol (0.04 and 0.02 mM) and the GABA(B) agonist baclofen (10 mM) all inhibited the spontaneous discharge of MSNs, PSNs and NENs (P < 0.01 for each group). In addition, GABA, muscimol and baclofen also inhibited ADN evoked discharge in both MSNs and PSNs (P < 0.05 for each group). Both GABA and baclofen significantly inhibited ADN evoked discharge in PSNs to a greater extent than in MSNs (P < 0.05 for each comparison). Muscimol at both doses, however, similarly inhibited ADN evoked discharge in both MSNs and PSNs. Examination of action potential amplitude and co-iontophoretic application of glutamate and GABA agonists suggested that GABA and muscimol induced inhibition were likely to be post-synaptic in origin, while baclofen produced both pre-synaptic and post-synaptic inhibition, depending upon the cell. In conclusion, GABA can influence baroreceptor afferent integration through both pre-synaptic and post-synaptic mechanisms. Furthermore, the effects of GABA(B) agonists are variable depending upon the level of afferent integration, with MSNs being generally less sensitive than PSNs.

Modulation of American ginseng on brainstem GABAergic effects in rats

Single neurons in the region of the medial nucleus tractus solitarius (NTS), responding or not responding to gastric vagal branch stimulation, were recorded in an in vitro neonatal rat brainstem-gastric preparation. The spontaneous activity of the majority of these two types of NTS units was inhibited by GABA(A) receptor agonist, muscimol (30 microM), and this inhibition (approximately 52% compared to 100% of the control level) could be antagonized by selective GABA(A) receptor antagonist, bicuculline (10 microM). Application of Panax quinquefolium L. extracts (3.0 microg/ml) into the brainstem compartment of the preparation also significantly reduced the discharge rate of these NTS neurons (approximately 27% compared to the control level), but this reduction could not be reversed by bicuculline (10 microM). Pretreatment with Panax quinquefolium L. (3.0 microg/ml) significantly decreased the NTS inhibitory effects induced by muscimol (30 microM), approximately from 51 to 33%. Our results demonstrated the interactions of Panax quinquefolium L. with ligand-bindings of GABA(A) receptors, and the modulation of the brainstem GABAergic mechanism by Panax quinquefolium L. Our data suggest that the regulation of GABAergic neurotransmission may be an important action of Panax quinquefolium L.

Functional GABAA receptors on rat vagal afferent neurones

1. In the present study, in vitro electrophysiology and receptor autoradiography were used to determine whether rat vagal afferent neurones possess gamma-aminobutyric acid (GABA)A receptors. 2. GABA (1-100 microM) and isoguvacine (3-100 microM) caused a concentration-dependent depolarization of the rat isolated nodose ganglion preparation at room temperature. When applied to the tissue 20 min before the agonist, SR95531 (3 microM) and bicuculline (3 microM) caused a parallel shift to the right of the GABA and isoguvacine concentration-response curves, yielding shifts of 81 fold and 117 fold for SR95531 and 4 fold and 12 fold for bicuculline, respectively. 3. Baclofen (10 nM-100 microM) was unable to elicit a depolarization of the rat isolated nodose ganglion preparation at either room temperature or at 36 degrees C, whilst 5-aminovaleric acid (10 microM), a GABAB receptor antagonist, was unable to antagonize significantly the GABA-induced depolarization at either room temperature or at 36 degrees C. 4. [3H]-SR95531 (7.2 nM), a GABAA receptor-selective antagonist, bound topographically to sections of rat brainstem. Specific binding was highest in the medial nucleus tractus solitarius (NTS) and dorsal motor nucleus of the vagus nerve (DMVN). Binding was also observed in certain medullary reticular nuclei, in particular the parvocellular reticular nucleus. 5. Unilateral nodose ganglionectomy caused a reduction in GABAA binding site density in the medial NTS from 93 +/- 7 to 68 +/- 6 d.p.m./mm2. This procedure also caused a reduction in GABAA binding site density in the side of the NTS contralateral to the lesion, from 151 +/- 12 to 93 +/- 7 d.p.m./mm2. Sham surgery had no effect on the binding of [3H]-SR95531 in rat brainstem. 6. The present data provide evidence for the presence of GABAA receptors located on the soma and central terminals of rat vagal afferent neurones. Additionally, a population of GABAA receptors is evidenced postsynaptically in the rat NTS with respect to vagal afferent terminals. These data are discussed in relation to the functional pharmacology of GABA in this region of the NTS.

GABAergic neurons in rat nuclei of solitary tracts receive inhibitory-type synapses from amygdaloid efferents lacking detectable GABA-immunoreactivity

Gamma-aminobutyric acid (GABA) is a prominent inhibitory transmitter in both the central nucleus of the amygdala (Ce) and the medial nuclei of the solitary tracts (mNTS). These regions are reciprocally connected by anatomical pathways mediating the coordinated visceral responses to emotional stress. To further determine whether GABA is present in the amygdaloid efferents or their targets in the mNTS, we combined peroxidase labeling of Phaseolus vulgaris leucoagglutinin (PHA-L) or biotinylated dextran amine (BDA) anterogradely transported from the Ce with immunogold-silver detection of antibodies against GABA in the rat mNTS. By light microscopy, peroxidase labeling for either PHA-L or BDA was seen in varicose processes, whereas immunogold-silver labeling for GABA was detected in perikarya and processes throughout the rostrocaudal mNTS. The intermediate mNTS at the level of the area postrema, a region receiving mainly cardiorespiratory and gastric visceral afferents, were examined by electron microscopy. In this region, anterograde labeling was observed exclusively in unmyelinated axons and axon terminals. These terminals lacked detectable GABA-immunoreactivity, but formed symmetric synapses that are associated with inhibition. The targets of the anterogradely labeled terminals were medium-sized dendrites both with and without GABA-labeling. These dendrites often also received convergent input from terminals that were intensely GABA-immunoreactive. We conclude that visceral activation accompanying emotional response to stress is likely to involve inhibition of GABAergic neurons in the mNTS by non-GABA-containing amygdaloid efferents. Furthermore, our results indicate that the inhibition of these GABAergic neurons may be further augmented by release of GABA from other converging terminals in the mNTS.

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.