Elsevier

Brain Research

Volume 859, Issue 2, 24 March 2000, Pages 341-351
Brain Research

Research report
Characterization of strychnine-sensitive glycine receptors in acutely isolated adult rat basolateral amygdala neurons

https://doi.org/10.1016/S0006-8993(00)02026-6Get rights and content

Abstract

Large concentrations of the β-amino acid, taurine, can be found in many forebrain areas such as the basolateral amygdala, a portion of the limbic forebrain intimately associated with the regulation of fear/anxiety-like behaviors. In addition to its cytoprotective and osmoregulatory roles, taurine may also serve as an agonist at GABAA- and strychnine-sensitive glycine receptors. In this latter context, the present study demonstrates that application of taurine to acutely isolated neurons from the basolateral amygdala of adult rats causes significant alterations in resting membrane current, as measured by whole-cell patch clamp electrophysiology. Using standard pharmacological approaches, we find that currents gated by concentrations of taurine ≤3 mM are predominantly mediated by strychnine-sensitive receptors. Furthermore, these strychnine-sensitive receptors are shown to be pharmacologically and biophysically similar to `classic' strychnine-sensitive, chloride-conducting glycine receptors expressed in brainstem and spinal cord. While amygdala glycine receptors can be distinguished from GABAA receptors expressed by the same neurons, these two chloride channels are functionally expressed at comparable levels. Given that a number of clinically relevant compounds are associated with the regulation of GABAA receptors in this brain region, the presence of both strychnine-sensitive glycine receptors and their agonist, taurine, in the basolateral amygdala may suggest an important role for these receptors in the limbic forebrain of adult rats.

Introduction

As part of the limbic system, the amygdala plays a highly integrative role in behavioral regulation. Its afferent and efferent systems connect cognitive, sensory, and `central autonomic control' brain regions, thus occupying a pivotal position in the sense/memory–integration–response pathway. In humans and non-human primates (reviewed in Refs. 13, 45), the amygdala appears to regulate behaviors like social interaction, aggression, and fear/anxiety. Such findings have also been extended to more experimental models like the rat where many of the behavioral responses to fear/apprehension-inducing stimuli are very similar to those in humans [8]. Of particular relevance for the studies outlined below, rat models of fear/anxiety have implicated one particular amygdala subdivision, the basolateral nucleus (BLA), as being centrally important in both the acquisition and expression of fear/apprehension-related behaviors (reviewed in Ref. [9]). Because of this central, integrative niche played by the BLA, an intimate understanding of the neurotransmitter systems governing neuronal excitability will provide important insight into the neurophysiological basis of many sensory-related, amygdala-dependent behaviors, particularly fear and anxiety.

While GABAA receptor activation in the BLA has been shown to mediate the anxiolytic activity of benzodiazepines 12, 38, 39, relatively little is known about other neurotransmitter systems involved in the regulation of fear/anxiety-related behaviors (but see Refs. 12, 23, 37). Recently, it has been demonstrated in rats that acute administration of ethanol, an intoxicant whose anxiolytic properties are well-established [3], can cause significant increases in extracellular concentrations of taurine in the basolateral amygdala 31, 32. The physiologic role of taurine is multifaceted (reviewed in Ref. [17]), mirrored by the multitude of conditions, such as experimentally induced status epilepticus [48], iso-osmotic elevated extracellular K+10, 27, hypoglycemia [43], ischemia [2], and direct ionotropic glutamate receptor activation [27], that are known to instigate its release from intracellular storage sites. While such findings are often interpreted in the context of taurine's activity as a cellular osmolyte [17], this β-amino acid is also an agonist for neurotransmitter receptors, namely glycine- and GABA-gated chloride channels 15, 50. Importantly, the neurons of the basolateral amygdala not only express GABAA receptors [25]but appear to concentrate taurine [28], suggesting that taurine may play a neurotransmitter role in this brain area. However, the neuromodulatory potential of taurine release in the basolateral amygdala has not been directly examined.

As an initial step towards understanding any neurotransmitter role for taurine in the basolateral amygdala, we have used whole-cell patch-clamp electrophysiology to investigate the effects of this amino acid on resting membrane properties in acutely isolated basolateral amygdala neurons from adult rats. We find that taurine not only interacts with GABAA receptors but also defines a neurotransmitter receptor not yet described in this limbic forebrain area. Our investigations into the characteristics of this receptor provide some insights into the neurophysiology of both taurine- and chloride-conducting neurotransmitter receptors in the basolateral amygdala.

Section snippets

Preparation of coronal brain slices

Adult male rats (198±13 g, n=45) were anesthetized with isoflurane and decapitated in accordance with the NIH Guide for the Care and Use of Laboratory Animals using a protocol approved by the Texas A&M University Laboratory Animal Care Committee. The brain was rapidly removed and chilled with oxygenated, ice-cold `high Mg2+/low Ca2+ artificial cerebrospinal fluid (aCSF)' containing (in mM): 125 NaCl, 5 KCl, 25 NaHCO3, 1.25 NaH2PO4, 1 MgSO4, 1.5 MgCl2, 0.5 CaCl2, and 20 d-glucose. Coronal

Taurine-gated currents in acutely isolated basolateral amygdala neurons

To determine whether the β-amino acid, taurine, could act as a neurotransmitter receptor agonist, relatively high concentrations (3 and 10 mM) were applied to acutely isolated basolateral amygdala neurons during whole-cell recordings. From a holding potential of −60 mV and using high intracellular Cl concentrations, taurine elicited inward currents (Fig. 2A) with a rapid onset, a pronounced decay in the continued presence of the agonist, and a rapid offset upon washoff of the amino acid. The

Discussion

In this study, we have demonstrated that isolated adult rat basolateral amygdala neurons respond to extracellular application of glycine and the β-amino acids, taurine and β-alanine, with pronounced changes in resting membrane current. Like GABAA and ionotropic glutamate receptors, relatively high concentrations of `native' agonists are required to produce maximal responses from amygdala glycine receptors. For these `amino acid'-gated receptors, it is likely that such high neurotransmitter

Acknowledgements

This work was supported, in part, by a Research Starter Grant from the Pharmaceutical Research and Manufacturers of America Foundation. We thank Drs. G.D. Frye and J.T. Trezciakowski for their helpful comments.

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