Research reportDiazepam-sensitive GABAA receptors in the NTS participate in cardiovascular control
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.
References (49)
- et al.
Region specific expression of GABAA receptor α3 and α4 subunits mRNAs in the rat brain
Mol. Brain Res.
(1992) Neurotransmitters in the CNS control of breathing
Resp. Physiol.
(1995)Electrophysiology of GABAA and GABAB receptor subtypes
Trends Neurosci.
(1988)- et al.
The central projections of the trigeminal, facial, glossopharyngeal and vagus nerves: an autoradiographic study in the rat
J. Auton. Nerv. Syst.
(1982) Evidence for control of cardiac vagal tone by benzodiazepine receptors
Neuropharmacology
(1987)- et al.
γ-Aminobutyric acid immunoreactive structures in the nucleus tractus solitarius: a light and electron microscopic study
Brain Res.
(1992) - et al.
Evidence for the presence of GABAergic and glycine-like systems responsible for cardiovascular control in the nucleus solitarii of the rat
Neurosci. Lett.
(1987) - et al.
Immunohistochemistry of γ-aminobutyric acid in the cat nucleus tractus solitarius
Brain Res.
(1985) GABAA receptors: ligand-gated Cl− ion channels modulated by multiple drug-binding sites
Trends Pharmacol. Sci.
(1992)- et al.
Involvement of biogenic amines and amino acids in the central regulation of cardiovascular homeostasis
Trends Pharmacol. Sci.
(1996)
Cardiovascular responses elicited by γ-aminobutyric acid in the nucleus tractus solitarius: evidence for action at the GABAB receptor
Neuropharmacology
Tonic stimulation of GABAB receptors in the nucleus tractus solitarius modulates the baroreceptor reflex
Brain Res.
Excitatory amino acid receptors in the nucleus tractus solitarius mediate the responses to the stimulation of cardio-pulmonary vagal afferent C-fiber endings
Brain Res.
Inhibition of baroreflex bradycardia by ethanol involves both GABAA and GABAB receptors in the brainstem of the rat
Eur. J. Pharmacol.
Influence of GABA on neurons of the gustatory zone of the rat nucleus tractus solitarius
Brain Res.
A single histidine in GABAA receptors is essential for benzodiazepine agonist binding
J. Biol. Chem.
Cloning and pharmacological characteristics and expression pattern of the rat GABAA receptor α4-subunit
FEBS Lett.
Blockade of GABAA receptors in the dorsal vagal complex inhibits intestinal motility in rats
FASEB J.
Localizing 3H-GABA in nerve terminals of rat cerebral cortex by electron microscopic autoradiography
Nature
Possible involvement of GABA mechanisms in central cardiovascular and respiratory control. Studies on the interaction between diazepam, picrotoxin and clonidine
Med. Biol.
Evidence for a neuromodulatory role of GABA at the first synapse of the baroreceptor reflex pathway – effects of GABA derivatives injected into the NTS
Naunyn Schmiedeberg's Arch. Pharmacol.
GABAA receptor subtypes: from pharmacology to molecular biology
FASEB J.
Effects of phenobarbital and diazepam on imipramine-induced changes in blood pressure, heart rate and rectal temperature of rats
Res. Commun. Chem. Pathol. Pharmacol.
Pentobarbital-enhanced [3H]flunitrazepam binding throughout the rat brain: an autoradiographic study
J. Pharmacol. Exp. Ther.
Cited by (21)
Cardiovascular responses to intravenous injection of a novel isoindolin-1-one derivate in conscious rats
2009, Brain ResearchCitation Excerpt :The mechanism of cardiovascular action of JM-1232(−) may be different from that of other benzodiazepines. Diazepam-sensitive GABAA receptors in the NTS participate in the autonomic regulation of cardiovascular function, and flumazenil blocks diazepam-induced effects mediated through the NTS (Barron et al., 1997). The NTS neurons send an excitatory projection to the caudal ventrolateral medullary depressor area (CVLM), which, in turn, sends an inhibitory GABAergic projection to the RVLM, which is of critical importance in the regulation of cardiovascular function (Andresen, 2004).
Adenosine suppresses GABA<inf>A</inf> receptor-mediated responses in rat sacral dorsal commissural neurons
2004, Autonomic Neuroscience: Basic and ClinicalDiazepam reduces both arterial blood pressure and muscle sympathetic nerve activity in human
2004, Neuroscience Lettersγ-Aminobutyric acid-induced responses in acutely dissociated neurons from the rat sacral dorsal commissural nucleus
1999, Journal of the Autonomic Nervous SystemTonic GABA(A) receptor-mediated neurotransmission in the dorsal vagal complex regulates intestinal motility in rats
1998, European Journal of Pharmacology