Ethanol inhibition of kainate receptor-mediated excitatory neurotransmission in the rat basolateral nucleus of the amygdala

Abstract

The neurobiological mechanisms governing alcohol-induced alterations in anxiety-like behaviors are not fully understood. Given that the amygdala is a major emotional center in the brain and regulates the expression of both learned fear and anxiety, neurotransmitter systems within the basolateral amygdala represent likely mechanisms governing the anxiety-related effects of acute ethanol exposure. It is well established that, within the glutamatergic system, N-methyl-d-aspartate (NMDA)-type receptors are particularly sensitive to intoxicating concentrations of ethanol. However, recent evidence suggests that kainate-type glutamate receptors are sensitive to ethanol as well. Therefore, we examined the effect of acute ethanol on kainate receptor (KA-R)-mediated synaptic transmission in the basolateral amygdala (BLA) of Sprague–Dawley rats. Acute ethanol decreased KA-R-mediated excitatory postsynaptic currents (EPSCs) in the BLA in a concentration-dependent manner. Ethanol also inhibited currents evoked by focal application of the kainate receptor agonist (R,S)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl) propanoic acid (ATPA), and ethanol inhibition of kainate EPSCs was not associated with a change in paired-pulse ratio, suggesting a postsynaptic mechanism of ethanol action. The neurophysiological consequences of this acute sensitivity were tested by measuring ethanol's effects on KA-R-dependent modulation of synaptic plasticity. Acute ethanol, like the GluR5-specific antagonist (R,S)-3-(2-carboxybenzyl)willardiine (UBP 296), robustly diminished ATPA-induced increases in synaptic efficacy. Lastly, to better understand the relationship between KA-R activity and anxiety-like behavior, we bilaterally microinjected ATPA directly into the BLA. We observed an increase in measures of anxiety-like behavior, assessed in the light/dark box, with no change in locomotor activity. This evidence suggests that kainate receptors in the BLA are inhibited by pharmacologically relevant concentrations of ethanol and may contribute to some of the acute anxiolytic effects of this drug.

Introduction

Animal and human studies have identified an important relationship between anxiety and alcohol-related disorders. For example, clinical and epidemiological studies have shown a significant degree of co-morbidity between anxiety disorders and alcoholism (Boyd et al., 1984). Moreover, acute alcohol exposure is known to decrease anxiety-related behaviors, while withdrawal from chronic alcohol abuse markedly increases anxiety (Costall et al., 1988, Hershon, 1973). Despite compelling evidence linking anxiety and alcoholism, the physiological substrates underlying this interaction are not well understood.

The amygdala serves as the center for regulation of specific aspects of fear and anxiety behaviors. Within the amygdala, the lateral/basolateral subdivision (BLA) receives sensory and cognitive information from thalamic and cortical inputs (McDonald, 1998, Pitkanen, 2000) and integrates these environmentally-driven stimuli in a poorly-understood process that ultimately results in the expression of anxiety-like or fearful behavioral responses (Campeau and Davis, 1995a, Campeau and Davis, 1995b). Suppression of glutamatergic receptors in the BLA has been shown to block bicuculline-induced anxiety (Sajdyk and Shekhar, 1997) as well as to prevent predator stress-induced increases in anxiety-like behavior (Adamec et al., 1999).

Alcohol is thought to act by a summation of interactions with a number of neurotransmitter systems that mediate fast excitatory and inhibitory synaptic transmission in the CNS. There are three major subtypes of ionotropic glutamate receptors: NMDA, AMPA and kainate-type (KA-R). While the physiological role of AMPA and NMDA receptors in the amygdala with respect to alcohol have been fairly well characterized, the role of KA-Rs in mediating the effects of ethanol has only begun to emerge with the development of pharmacological tools that have allowed the separation of AMPA- and KA-receptor mediated responses (reviewed in Pinheiro and Mulle, 2006). For example, KA-Rs contribute to postsynaptic glutamatergic excitatory responses in the BLA (Li and Rogawski, 1998) and also mediate a form of long-lasting heterosynaptic facilitation in this brain region (Li et al., 2001). We have previously demonstrated that KA-Rs in the rat hippocampus are potently inhibited by acute ethanol (Carta et al., 2003, Weiner et al., 1999). In fact, the potency of these effects was, in some cases, four-fold greater than that of the well-characterized ethanol inhibition of NMDA receptors. Our goal in this study was to determine what role the actions of acute ethanol may have on KA-Rs for both amygdala-dependent neurophysiology and behavioral anxiety.