Different disruptive effects on the acquisition and expression of conditioned taste aversion by blockades of amygdalar ionotropic and metabotropic glutamatergic receptor subtypes in rats

Abstract

Conditioned taste aversion (CTA) is based on the gustatory long-term memory established after association of the taste of food (conditioned stimulus, CS) with visceral signals of poisoning (unconditioned stimulus, US). After the acquisition of CTA, hedonics of the taste CS changes from positive to negative as indicated by reduced ingestive and increased aversive taste reactivities in response to re-exposures to the CS. We examined the effects of reversible and selective blockades of the amygdalar glutamate receptor subtypes, AMPA, NMDA and metabotropic glutamate receptors, on the formation of CTA. Blockades of each of the three receptor subtypes between ingestion of saccharin (CS) and malaise-inducing LiCl (US) disrupted the acquisition of CTA. After the acquisition of CTA, however, blockades of only AMPA receptors, but not NMDA or metabotropic receptors, impaired the expression of CTA. This effect was seen only during the period when the antagonistic action to AMPA receptors lasted. These results indicate that both ionotropic and metabotropic glutamate receptor subtypes in the amygdala are indispensable for the acquisition of CTA, but that the expression of acquired CTA is mediated only by AMPA receptors. The present results also suggest that the amygdalar glutamatergic neural transmission is involved in the formation and storage of long-term gustatory memory associated with the altered hedonics from positive to negative.

Introduction

It is known that the amygdala plays important roles in the formation of aversive association learnings including fear conditioning, inhibitory avoidance task, fear-potentiated startle. For example, the amygdala, especially its lateral subnuclei group including the lateral nucleus of the amygdala, is pointed out as the locus for the association of an auditory stimulus with an electrical shock in the fear conditioning paradigm [21], [26]. In fear conditioning, both modified excitatory unitary activities and long-term enhancement of the responses to the auditory CS or CS-like stimulus have been reported in neurons of the amygdala [36], [37], [39]. Recent anatomical, physiological and behavioral studies have indicated that the amygdalar excitatory glutamatergic transmission via α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and N-methyl-d-aspartate (NMDA) receptors is important in these learnings [15], [18], [38]. Actually, blockades of NMDA receptors in the amygdala impaired fear conditioning, inhibitory avoidance task and fear-potentiated startle [7], [9], [15], [16], [19], [23], [24], [30].

When ingestion of a food induces malaise, animals reject eating the food on the basis of the gustatory information of the food. This phenomenon is called as conditioned taste aversion (CTA) where gustatory CS becomes hedonically aversive after the CS has been paired with visceral malaise (US) [5], [47]. The CTA is an acquired adaptive reaction preventing animals from repeated ingestion of a novel tastant which has induced illness. Although numerous reports indicate that the amygdala is essential for CTA, other studies opposed this viewpoint. Thus, the involvement of amygdala in CTA still remains to be solved. Excitotoxic [31], [46] and electrolytic [1], [17], [33], [43] lesions of the amygdala impaired the acquisition and retention of CTA. In addition, electrolytic lesions of the amygdala reduced c-Fos-like immunoreactivity (c-FLI) in the nucleus of the solitary tract (NST) induced by re-exposure to the CS [41]. These irreversible lesion techniques, however, have limitations in determining the timing of the amygdala’s involvement or cellular mechanisms of CS–US association in the formation of CTA. Some compensatory plasticities may take place after the lesions have formed and make it difficult to elucidate the actual function of the amygdala. These issues might lead to a contradictory result such that the amygdala contributes little to the CTA acquisition [2], [8]. Schafe et al. [42] have also shown that the role of the amygdala is dependent on the conditioning method, showing no effect of excitotoxic amygdala lesion in rats conditioned using bottle presentation of taste. Reversible lesion experiments with tetrodotoxin [40] and protein kinase C inhibitors [49] have shown the importance of the amygdala in the acquisition and/or retention of CTA. Electrophysiological recording experiments have revealed an enhanced responsiveness to the gustatory CS after the formation of CTA in some amygdalar neurons [45], [48] as found in other fear conditioning paradigms [36]. These changes in neuronal responsiveness may reflect a long-term synaptic plasticity occurring in the amygdala.

Roles of glutamate in the amygdala in CTA formation are not fully understood. The present experiment was designed to examine whether the glutamatergic transmission in the amygdala inputs plays a critical role in CTA formation. We also intended to elucidate the functional roles of the three glutamatergic receptors, AMPA, NMDA and metabotropic glutamate receptors, in taste aversion learning. Additionally, it is necessary to re-examine whether the amygdala’s involvement is indispensable to acquisition, retrieval or expression of CTA in the bottle-trained method. To this end, we compared the effects of intra-amygdalar injections of selective antagonists for the three subtypes of glutamatergic receptors on the CTA formation in conscious rats.