Elsevier

Brain Research

Volume 773, Issues 1–2, 31 October 1997, Pages 53-60
Brain Research

Research report
Diazepam-sensitive GABAA receptors in the NTS participate in cardiovascular control

https://doi.org/10.1016/S0006-8993(97)00882-2Get rights and content

Abstract

The present study employed neuropharmacological and receptor binding protocols to determine if diazepam-sensitive (DS) γ-aminobutyric acid-A (GABAA) receptors in the nucleus tractus solitarius (NTS) participate in autonomic regulation of cardiovascular function. The first set of protocols was designed to determine if GABAA receptors in the NTS were functionally modulated by the benzodiazepine agonist, diazepam. Mean arterial pressure and heart rate responses to microinjection of GABAergic substances into the NTS were examined in urethane-anesthetized rats. Microinjection of the GABAA agonist isoguvacine into the NTS increased mean arterial pressure and heart rate, and these effects were blocked by the GABAA receptor antagonist, bicuculline. Preadministration of diazepam into the NTS potentiated the pressor actions of isoguvacine and had variable effects on heart rate changes. Flumazenil, a benzodiazepine antagonist, blocked the diazepam-induced potentiation of the pressor response to isoguvacine. The second protocol employed receptor autoradiography to examine the presence of DS and diazepam-insensitive (DI) GABAA receptors in the NTS. Autoradiography confirmed that DS GABAA receptors were present in the NTS; however, no measurable levels of DI GABAA receptors were detected. We conclude that GABAA-mediated integration of central autonomic control in the NTS is mediated solely by DS GABAA receptors.

Introduction

The nucleus of the solitary tract (NTS) receives primary afferent input from a wide variety of peripheral organs and tissues and is essential in integration of autonomic nervous system functions 12, 22, 33. γ-Aminobutyric acid (GABA) is the predominant inhibitory neurotransmitter in the CNS 3, 6and is present in the NTS 14, 20, 26. Numerous studies have clearly established that in the NTS GABA plays an important role in central autonomic regulation of the peripheral circulation 7, 24, 30, 39, 40, 46.

GABAergic neurotransmission in the NTS involves two types of receptors, GABAA and GABAB. This study focused on the GABAA receptor for the following reasons. First, it is well established that administration of GABAA agonists into the NTS elevates arterial pressure, while administration of GABAA receptor antagonists reduce arterial pressure 24, 30, 39, 40, 46. Second, the GABAA receptor has a unique characteristic which allows it to be modulated by CNS depressants, such as benzodiazepines, barbiturates and neurosteroids 25, 27, 36. Binding of these agents to their recognition sites on the GABAA receptor allosterically modulates GABA-induced chloride conductance 8, 25, 36.

While modulation of NTS GABAA receptors by benzodiazepines, barbiturates, or neurosteroids has the potential for significant influences on arterial pressure control, there is little specific information about the physiological effects of modulation of GABAA receptors in the NTS. In the present study, we have examined benzodiazepine modulation of GABAA receptors. Benzodiazepines were chosen due to the clinical application of these agents and because of the existence of the specific benzodiazepine antagonist flumazenil which is not available with either barbiturates or neurosteroids. Since a population of GABAA receptors is insensitive to benzodiazepines [43], experiments were was also performed to determine the presence of the diazepam-sensitive (DS) and diazepam-insensitive (DI) forms of the GABAA receptor in the NTS.

The purpose of this study was two-fold. The first goal was to determine if GABAA receptors in the NTS can be modulated by the benzodiazepine agonist diazepam. This goal was addressed first by examining the resting arterial pressure and heart rate responses to microinjection of diazepam into the NTS, and second by determining if localized administration of diazepam into the NTS augments cardiovascular responses to microinjection of a GABAA agonist. The second goal of this study was to anatomically determine the presence of DS and DI forms of the GABAA receptor in the NTS. This was performed through application of autoradiographic binding techniques.

Section snippets

Materials and methods

Measures were taken to minimize discomfort to all animals. All protocols in this study were approved by the Institutional Animal Care and Use Committee.

Dose response to isoguvacine administration into the NTS

The objective of the first experiment in Protocol 1 was to determine the dose–response relationship of isoguvacine administration into the NTS. In this group, resting mean arterial pressure was 107±7 mmHg, and resting heart rate was 387±5 beats/min (n=7). Fig. 1 shows the arterial pressure and heart rate responses to microinjection of isoguvacine into the NTS. Each dose increased mean arterial pressure and heart rate. The 0.10 mM dose of isoguvacine elicited a greater increase in mean arterial

Discussion

The results of this study show that isoguvacine is a selective GABAA receptor agonist in the NTS. Furthermore, our data demonstrate that diazepam augments the pressor response to microinjection of isoguvacine in the NTS, and suggest that diazepam can augment the effects of tonic release of GABA in the NTS. The effects of diazepam are selective for the benzodiazepine binding site on the GABAA receptor since these effects are blocked by the benzodiazepine antagonist flumazenil. Receptor binding

Acknowledgements

This work was supported by the Presbyterian Health Foundation, Oklahoma City, and the American Heart Association – Oklahoma Affiliate. The authors also wish to thank Ciba-Geigy for the generous gift of CGP 35348.

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