GABA--an inhibitory neurotransmitter that is involved in cardiovascular control

Characterisation of GABA(A) receptors in fetal, neonatal and adult ovine brain: region and age related changes and the effects of allopregnanolone

Progesterone metabolites acting via GABA(A) receptors suppress central nervous system (CNS) activity. The aim of the present study was to examine binding characteristics of GABA(A) receptors in fetal, newborn and adult sheep brains using [(35)S]TBPS, and to determine the effects of allopregnanolone on this binding. Receptor affinity (K(D)) and density (B(MAX)) in the brainstem were not different in fetal, newborn (1-2 days old) and adult brains. In the hypothalamus K(D) and B(MAX) increased significantly in the fetus between 85 and 128 days gestation, and were then similar to postnatal and adult values. In the frontal cortex K(D) and B(MAX) increased progressively between 85 days and term ( approximately 147 days gestation), and were then not different from postnatal and adult values. The K(i) values for the GABA(A) receptor antagonist picrotoxin was similar at all ages. Allopregnanolone inhibited [(35)S]TBPS binding in the presence of 5 microM GABA, but enhanced binding in the absence of GABA. These results show that (i), functional GABA(A) receptors are present in the fetal brain from at least 85 days gestation; (ii), 3alpha-pregnane steroids modify receptor affinity in the late gestation fetal brain; and (iii) there are region-specific changes in GABA(A) receptor binding parameters. Steroid modulation of the GABA(A) receptor in the fetal brain is likely to influence fetal CNS activity in late gestation.

Pro- and anticonvulsant effects of stress: the role of neuroactive steroids

The present review deals with findings related to the contribution of pro- and anticonvulsant effects of "neuroactive" steroids and the role of the gamma-aminobutyric acid (GABA) receptor as a physiological target for naturally occurring steroids. Ways are discussed via which GABAergic neurotransmission can be enhanced or reduced following maneuvers that inflict stress. The duality of stress effects is emphasized in conjunction with different types of epileptogenesis (e.g., grand mal vs petit mal) that undergo dissimilar evolution. Among the issues covered are steroid-induced sedation and epileptogenicity, excitatory steroids, stress and epilepsy, GABA and respiratory functions, asymmetric brain injury, and psychopathology and stress.

Ontogeny and distribution of GABAA receptors in rat brainstem and rostral brain regions

Previous studies from our laboratory and others have shown that there are major age-related differences in brainstem neuronal function. Since GABAA receptors are major targets for GABA-mediated inhibitory modulation and play a key role in regulating cardiorespiratory function, especially during O2 deprivation, we examined differences in GABAA receptor density and distribution during postnatal development. Using quantitative receptor autoradiography, the present study was performed to examine the postnatal expression of GABAA receptors in the rat brainstem and rostral brain areas at five ages, i.e. postnatal day 1 (P1), P5, P10, P21 and P120. Ten-micrometer brain sections at different brain levels were labelled with [3H]muscimol in Tris-citrate buffer. We found that (i) GABAA receptors appeared very early in almost all the brainstem as well as rostral areas; (ii) at P1, the brainstem had a higher GABAA receptor binding density than rostral areas and its density peaked at P5 or P10; and (iii) receptor densities of the cerebellum and rostral brain areas such as cortex, thalamus and dentate gyrus increased with age, especially between P10 and P21, but most other subcortical areas like caudate-putamen and hippocampal CA1 area did not increase remarkably after birth. We conclude that: (i) GABAA receptors exist in most brain areas at birth; (ii) there are several patterns of postnatal development of GABAA receptors in the CNS with dramatic differences between the brainstem and cortex; (iii) brainstem functions rely more on GABAA receptors in early postnatal life than at more mature stages. We speculate that GABAA receptors develop earlier in phylogenetically older structures (such as brainstem) than in newer brain regions (such as cortex).

Central neurotoxic effects of guanethidine: altered serotonin and enkephalin neurons within the area postrema

This study reveals that the 5-HT and enkephalin cell body population of the area postrema (AP) is dramatically reduced via exposure to the peripherally circulating neurotoxin guanethidine. Efforts to restore cell body numbers to control levels, with colchicine and monoamine oxidase inhibitors, subsequent to guanethidine treatment are ineffective. However, 5-HT and enkephalin immunostaining of surrounding bulbar nuclei appears normal, implying that the neurotoxic effect of guanethidine is restricted to the AP. In addition, gamma-aminobutyric acid and neurotensin immunostaining within the AP appears normal, subsequent to guanethidine treatment, suggesting that the neurotoxic effect is restricted to specific AP cell populations.

A comparison of GABA- and GAD-like immunoreactivity within the area postrema of the rat and cat

The immunohistochemical localization of gamma-aminobutyric acid (GABA) was compared to that of glutamate decarboxylase (GAD) in the rat and cat area postrema with the aid of a polyclonal antibody produced in rabbits directed against GABA. In both the rat and cat, dense and very dense accumulations of GABA-like immunoreactive (GABA-LI) varicosities were present throughout the area postrema. GABA-LI cell bodies were present in both species and were evenly distributed throughout the area postrema's extent. However, the rat area postrema contained more GABA-LI cell bodies and varicosities than the cat area postrema. In the cat area postrema, a range of cell sizes were immunostained with the GABA antibody. The GAD antibody, however, failed to reveal cell bodies in the area postrema of the cat, thus indicating that the GABA polyclonal antibody may be a better indicator of GABA-containing somata. Although the mechanism of action of GABA in the area postrema is not understood, it is possible that GABA may play a role in the different functions of the area postrema in emetic and nonemetic species.

Intravenous GABA administration is anxiogenic in man

Seven persons (three normal volunteers and four euthymic bipolar patients) received one to four doses of gamma-aminobutyric acid (GABA) intravenously. All subjects reported dysphoria, and their mood disturbance scores on the Profile of Mood States were significantly increased in a dose-related fashion. Placebo infusions did not produce similar responses. Mood disturbance was accompanied by a dose-related increase in pulse and blood pressure.

Cardiovascular effects of tetanus toxin after systemic and intraventricular administration in rats

A single intraperitoneal injection of tetanus toxin (50,100 and 200 minimum lethal dose per rat) produced a significant and dose-dependent increase in systemic blood pressure and heart rate in conscious rats. On the contrary, tetanus toxin, given directly into the third cerebral ventricle (20 and 40 minimum lethal dose), produced a gradual and dose-dependent decrease in blood pressure accompanied by bradycardia. In conclusion, the present findings show that tetanus toxin is able to affect cardiovascular activity and it is suggested that this may be due to an interference with central GABAergic mechanisms.

Immunohistochemistry of gamma-aminobutyric acid in the cat nucleus tractus solitarius

Using a new antibody directed solely against the gamma-aminobutyric acid (GABA) molecule, distribution of GABA was studied in the nucleus tractus solitarii of the cat. Both immunoreactive puncta and cell bodies had a homogenous distribution within the nucleus. The one exception was in the parvocellular subdivision where very little immunoreactive puncta, but numerous immunoreactive cell bodies, were found. Results of this investigation provide immunohistochemical evidence of GABA's localization in an autonomic nucleus involved in cardiovascular regulation.

Pharmacological analysis of the central cardiovascular effects of four GABA analogues

The central cardiovascular effects of 4 structural analogues of GABA were investigated. The drugs were injected intracerebroventricularly (i.c.v.) in cumulative doses into pentobarbital-anaesthetized normotensive rats. Muscimol (0.01-10 micrograms/kg), THIP (0.01-100 micrograms/kg), kojic amine (0.1-100 micrograms/kg) and isoguvacine (0.1-100 micrograms/kg) produced dose-dependent hypotension and bradycardia. The maximal fall in the mean blood pressure was of about 35% of the initial values. These effects appears to be of central origin since the intravenous (i.v.) injection of the same doses of the drugs did not produce any similar cardiovascular modifications. The hypotensive effects of muscimol and kojic amine were antagonized partly by i.c.v. bicuculline. The combination of bicuculline and kainic acid almost completely prevented the blood pressure lowering effects of muscimol, kojic amine and isoguvacine. THIP however was only slightly antagonized by bicuculline and kainic acid. Atropine i.v. also prevented partly the cardiovascular effects of all these drugs. Thus, the mechanisms of the central cardiovascular actions of GABA analogues appear to be more complex than expected and variable from one drug to another. The involvement of GABA receptors of the A and B types and of cholinergic mechanisms in the hypotensive effect of the drugs is discussed.

Benzodiazepine-GABA receptor-ionophore complex. Current concepts

The benzodiazepine--gamma-aminobutyric acid (GABA) receptor--ionophore system is an oligomeric complex, composed of at least three interacting components. These three components have been well characterized in vitro by radioreceptor binding assays. A variety of centrally acting anxiolytic, depressant, anticonvulsant and convulsant drugs, which affect GABAergic transmission, bind to one of the sites and modulate the binding of ligands at the other sites. Thus, depressant barbiturates, nonbarbiturate hypnotics (like etomidate) and pyrazolopyridines (like etazolate), while inhibiting the binding of alpha-dihydropicrotoxinin (DHP), enhance the binding of GABA and benzodiazepines. These enhancing effects are blocked by convulsant drugs that inhibit the binding of dihydropicrotoxinin and also by bicuculline. These interactions involving barbiturates and other modulatory drugs, exhibit stereoselectivity, anion dependence and brain regional selectivity. Several classes of drugs which facilitate GABAergic transmission appear to interact with the sites for GABA and benzodiazepines allosterically via the dihydropicrotoxinin site of the oligomeric complex. The GABA system has also been implicated in a variety of pathological conditions, including anxiety, seizure activity, movement disorders, cardiovascular control, pain and in drug dependence. Since most of the GABA agonists do not pass the blood-brain barrier, future trends in the pharmacology of GABA may be the development of drugs that will activate the GABA receptor system via picrotoxinin or benzodiazepine sites.

Muscimol inhibits ADH release induced by hypertonic sodium chloride in rats

The effect of the GABA-agonist muscimol on ADH release induced in rats by administration of hypertonic sodium chloride solutions was studied by means of intracerebroventricular and intraperitoneal injections of the drug. Injected by the intracerebroventricular route, muscimol produced a significant reduction of plasma ADH concentration not only in animals treated with hypertonic sodium chloride, but also in unstimulated animals. Following intraperitoneal administration larger doses were required to produce such an effect, thus suggesting a central site of action for the effect of muscimol on ADH release. Bicuculline, given intraperitoneally before muscimol injection, completely blocked ADH inhibition induced by muscimol, thus suggesting a specific involvement of GABAergic receptors. These findings indicate that GABAergic mechanisms may be involved in the regulation of body fluids in the rat by affecting ADH release.