GABA and “neuro-cardiovascular” mechanisms

Reciprocal synaptic relations between enkephalinergic and GABAergic neurons in the area postrema of the rat

A preembedding, double immunostaining technique was used to study synaptic relations between enkephalinergic and GABAergic neurons in the area postrema of the rat. As a main result, the enkephalinergic dendrites received many synapses from GABAergic axon terminals, and most of the synapses were symmetrical. Enkephalinergic neuronal perikarya received a few synapses from GABAergic axon terminals, and a few enkephalinergic axon terminals were found presynaptic to GABAergic neurons. Synapses between enkephalinergic profiles were frequent, but no axo-axonic synapses were seen. These findings suggest that GABAergic innervation of enkephalinergic neurons is the main relation between the two kinds of neurons in the area postrema. The synapses between the enkephalinergic axon terminals and GABAergic neurons might be explained as being part of the local servo system of the area postrema.

Supraventricular tachyarrhythmia associated with baclofen overdose

Baclofen overdose occasionally results in cardiac effects, but serious tachyarrhythmias have not been reported. We present the case of a 21-year-old man with an acute baclofen overdose who developed increased cardiac conduction times and a rapid supraventricular tachyarrhythmia which required medical intervention. Acute baclofen toxicity can result in the development of tachyarrhythmias and cardiac monitoring is warranted in acute overdose states.

Resolution of central sites involved in picrotoxin-induced vagal activation and vasopressin release

This investigation tested the hypotheses that picrotoxin, a drug which blocks the inhibitory effect of gamma-aminobutyric acid (GABA), would, in spinal cord-transected rats, (1) suppress the cardiac vagus when localized to the forebrain and stimulate the cardiac vagus by acting in the brainstem and (2) stimulate the release of vasopressin into the systemic circulation through separate forebrain and brainstem GABAergic mechanisms. An intra-arterial infusion technique allowed for delivery of picrotoxin selectively to either forebrain or brainstem areas. Administration of picrotoxin via the vertebral artery decreased sinus rate and increased circulating levels of vasopressin. Infusion of picrotoxin into the internal carotid artery caused increases in sinus rate, blood pressure and plasma vasopressin. These data support the hypothesis that GABAergic mechanisms at different levels of the neuraxis exert opposite effects on cardiac vagal activity, and that GABAergic mechanisms in both the brainstem and forebrain inhibit the release of AVP into the systemic circulation.

Effect of agonist concentration on the lifetime of GABA-activated membrane channels in spinal cord neurons

The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) hyperpolarizes spinal neurons by activating bicuculline sensitive GABAA receptors coupled to chloride permeable ionic channels in the cell membrane. Single channel measurements using outside-out membrane patches from cultured mouse spinal neurons showed that there is a large excess of brief openings of these channels, compared to the number predicted from a simple, two-state model of channel function. The cumulative open time distributions for channel openings were well fit by the sum of two exponential terms, a fast component with amplitude Nf and time constant TAUf, and a slow term with amplitude Ns and time constant TAUs. These kinetics could arise if monoliganded GABAA receptors triggered an open state less stable than that induced by the biliganded receptor. A prediction of this hypothesis is that the ratio Nf/Ns should decline in an approximately linear fashion with increasing doses of GABA. This was found to be the case, as Nf/Ns declined by a factor of 4.8 on increasing the GABA dose from 0.5 microM to 5 microM. Application of 1.25 microM GABA produced an intermediate value of Nf/Ns, as predicted. In contrast the parameters TAUf and TAUs were not significantly influenced by GABA concentration. Spontaneous, bicuculline sensitive currents were seen in some patches. These events resembled currents triggered by low doses of exogenous GABA, with regard to the values of Nf/Ns, TAUf and TAUs. These results suggest that the spontaneous currents are triggered by endogenous GABA molecules, present in the culture environment at a concentration of 0.5 microM or less.

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.

The GABAA receptor: new insights from single-channel recording

The GABAA receptor of mammalian neurons exists as a macromolecular complex incorporating several interacting components. These include a chloride-permeable ion channel and a recognition site for GABA. The binding of GABA molecules at the latter site triggers the transient opening of the chloride ion channel, resulting in a flow of charge which inhibits the generation of action potentials in the cell. The precise amount of charge passed during this event is modulated by ligand binding at at least three regulatory sites. These sites act as receptors for barbiturates, benzodiazepines, and certain convulsants. The extracellular patch clamp method has now been used to study the gating of chloride channels by GABA and the modulation of this process by drugs. Even in the absence of drugs, GABAA channels exhibit complex gating behavior, indicating the presence of multiple open and closed states and of substate conductance levels. GABAA channels from different preparations show considerable diversity in their detailed gating characteristics. In the presence of the barbiturate pentobarbital, GABAA channels open in prolonged bursts, consistent with the potentiating effect of this drug on macroscopic GABA responses. In contrast, the convulsant penicillin decreases charge transfer through open GABAA channels by shortening the average duration of the open state. Benzodiazepine receptor ligands exert complex effects on the GABAA channel. A general model of barbiturate and benzodiazepine potentiation of GABAA receptor responses is proposed.

Baclofen overdose with cardiac conduction abnormalities: case report and review of the literature

Cardiac conduction abnormalities and hypertension developed in a patient who ingested approximately 500 mg of baclofen (Lioresal). The patient also exhibited the more common features of baclofen overdose including coma, respiratory depression, hypotonia, and hyporeflexia. A review of the literature and a discussion of the interesting manifestations and treatment of baclofen overdose are included.

Protective effect of GABA and sodium valproate on stress-induced gastric lesions in guinea-pigs

gamma-Aminobutyric acid (GABA) and sodium valproate (VPA) inhibit the formation of stress-induced gastric ulcers in guinea-pigs. The present study was conducted to evaluate the effect of these drugs on the development of cold- and restraint-induced gastric ulcers in guinea-pigs. In control saline-pretreated animals, a 3 h exposure to cold and restriction resulted in the production of gastric ulcers in 9 out of 10 animals. GABA (200 mg kg-1 oral, i.p.) completely prevented the development of gastric ulcers. VPA (100 and 200 mg kg-1 i.p.) exerted no significant effects on the development of gastric ulcers. GABA (100 mg kg-1 oral and i.p.) and VPA (oral) also exhibited partially protective activity. It is suggested that GABA may participate in a physiological modulation of the gastric mucosal barrier, by increasing its resistance to stress-induced lesions.

The effect of peripherally administered GABA on noradrenaline-induced reflex vagal bradycardia in urethane anaesthetized rats

Intravenously administered GABA significantly reduced the size of noradrenaline-induced reflex vagal bradycardia without affecting blood pressure increase. Intravenously administered GABA reduced bradycardia elicited by stimulation of preganglionic fibres in the vagus nerve. These findings provide evidence suggesting that, in addition to its well known effects on structures regulation cardiovascular function at CNS level GABA could modulate baroreceptor reflex(es) by acting at a peripheral site.

The GABA/benzodiazepine receptor complex in the nervous system of a hypertensive strain of rat

gamma-Aminobutyric acid (GABA) has been implicated in the development of hypertension and in the regulation of blood pressure. The spontaneously hypertensive rat (SHR) offers an opportunity to explore the role of central GABA and other neurotransmitters in the genesis of high blood pressure. The receptor binding of [3H]GABA, [3H]flunitrazepam, and [3H]glutamate to synaptic membranes from the cerebral cortex and cerebellum of SHR rats were measured in animals of various ages. No significant differences between the SHR and a normotensive control strain of rats were found for any of the assays. The results indicate that in this model of hypertension, neither GABA nor glutamate function are involved, at least not in the cerebral cortex or cerebellum.

Blockade of GABA receptors in periventricular forebrain of anesthetized cats: effects on heart rate, arterial pressure, and hindlimb vascular resistance

Pharmacologic antagonism of GABAergic inhibition in the periventricular forebrain of anesthetized cats caused dose related sympathetically mediated increases in heart rate and arterial pressure in vagotomized cats, and suppression of reflex vagal activation in vagus-intact preparations where sympathetic effects are prevented by cervical spinal cord transection. In the present study performed in cats with intact autonomic pathways, similar administration of the GABA antagonist bicuculline methiodide (BMI) produced dose-related increases in arterial pressure, heart rate, and, when perfusion pressure in the autoperfused hindlimb was measured, vascular resistance. Furthermore, the relationship between heart rate and hindlimb vascular resistance changes suggested that the effects on heart rate reflected the combined sympathetic and vagal baroreflex effects of BMI. Intraventricular administration of muscimol, a potent GABA agonist, elicited abrupt and parallel reversal of BMI-induced effects on systemic arterial pressure and vascular resistance while attempts to acutely denervate the hindlimb at the height of the BMI response by cutting the femoral and sciatic nerves sharply reversed the increase in hindlimb vascular resistance in most experiments. The results suggest that the cardiovascular changes observed represent an integrated pattern consisting of excitation of sympathetic nerves innervating the heart and vasculature along with suppression of reflex evoked vagal excitability.

Ligand-binding studies on GABA receptors--relation to physiology and behavior

Some studies that have been conducted with ligand-binding methods support the contention that activation of cerebral GABA-receptors is involved in the control of certain behaviors. Further study of the changes in GABA-receptors that are associated with altered physiology and behavior in experimental animals and in man might lead to the development of new therapies for certain neuropsychiatric, cardiovascular, and endocrinological disorders.

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.

GABA and benzodiazepine binding sites in spontaneously hypertensive rat

The binding of GABA to its central receptor sites and of benzodiazepines to central and peripheral types of binding sites was compared in age-matched spontaneously hypertensive rat (SHR) and normotensive (WKY) rat. The KD and Bmax values of GABA agonist binding were not significantly different between SHR and WKY rat in the forebrain, cerebellum or pons-medulla. The binding of [3H]flunitrazepam to central types of binding sites was also not different in 12 week-old SHR and WKY rat. However, the binding of [3H]flunitrazepam and [3H]RO5-4864 was significantly lower in the SHR kidney, as compared to WKY rat kidney. This decrease was due to a lower Bmax in SHR kidney. In contrast, the binding of these radioligands in the hearts of the two strains was not different. These results indicate that the SHR has lower peripheral benzodiazepine binding sites in the kidney as compared to age-matched normotensive WKY rat kidney.