Extinction of conditioned taste aversion is related to the aversion strength and associated with c-fos expression in the insular cortex

Highlights

• CTA learning and its extinction is characterized by individual differences.

• Extinction of CTA depends on the strength of the initially learned CS–US association.

• Extinction of CTA is accompanied with neuronal activation in the insular cortex.

Abstract

Taste aversion learning is a type of conditioning where animals learn to associate a novel taste (conditioned stimulus; CS) with a stimulus inducing symptoms of poisoning or illness (unconditioned stimulus; US). As a consequence animals later avoid this taste, a reaction known as conditioned taste aversion (CTA). An established CTA extinguishes over time when the CS is repeatedly presented in the absence of the US. However, inter-individual differences in CTA extinction do exist. Using a model of behavioral conditioning with saccharin as CS and the immunosuppressant cyclosporine A as US, the present study aimed at further elucidating the factors underlying individual differences in extinction learning by investigating whether extinction of an established CTA is related to the strength of the initially acquired CS–US association. In addition, we analyzed the expression of the neuronal activation marker c-fos in brain structures relevant for acquisition and retrieval of the CTA, such as the insular cortex and the amygdala. We here show that animals, displaying a strong CS–US association during acquisition, maintained a strong CTA during unreinforced CS re-exposures, in contrast to animals with moderate CS–US association. Moreover, the latter animals showed increased c-fos mRNA expression in the insular cortex. Our data indicate that CTA extinction apparently depends on the strength of the initially learned CS–US association. In addition, these findings provide further evidence that the memory for the initial excitatory conditioning and its subsequent extinction is probably stored in those structures that participate in the processing of the CS and the US.

Introduction

Conditioned taste aversion (CTA) is a established learning paradigm where rats learn to associate a novel taste (conditioned stimulus; CS) with a drug or other treatment (unconditioned stimulus; US), that induces symptoms of poisoning or illness (Garcia et al., 1955, Ader, 1985, Bures et al., 1998). During the learning phase (acquisition), the CS is presented once or multiple times together with the US, leading to the establishment of a CS–US engram. Re-exposure to the CS alone leads to retrieval of the newly acquired memory (Bermudez-Rattoni and McGaugh, 1991, Bermudez-Rattoni, 2004). On the behavioral level, the conditioned response is characterized by avoidance of the CS, evident by a reduced consumption of the gustatory stimulus (Garcia et al., 1955, Bermudez-Rattoni, 2004, Wirth et al., 2011). A number of anatomical and pharmacological studies have shown that the neural network involved in taste-visceral associative learning is mediated via a basic neural circuit comprising sensory and hedonic pathways (Yamamoto, 1993, Sewards, 2004). These include the nucleus tractus solitarius, the parabrachial nucleus, the medial thalamus, the amygdala, and the insular cortex (Yamamoto et al., 1994, Sewards, 2004). Specifically, the insular cortex is essential for acquisition and retention of associative learning (Bermudez-Rattoni and McGaugh, 1991, Cubero et al., 1999), and has been shown to be involved in integration of gustatory and visceral stimuli (Sewards and Sewards, 2001, Pacheco-Lopez et al., 2005).

The initially strong avoidance of the CS is gradually reduced and almost completely eliminated upon subsequent non-reinforced re-exposure to the gustatory CS, a process called extinction (Pavlov, 1927, Berman and Dudai, 2001, Dudai, 2002, Bermudez-Rattoni, 2004). Extinction is defined as a form of learning in which associations between cues (CS) and the events (US) they predict are weakened by exposure to the cues in the absence of those events (Myers and Carlezon, 2010). Berman and Dudai (2001) found that extinction of CTA memory depends on protein synthesis and β-adrenoceptors in the insular cortex. Moreover, it has been shown that extinction of CTA leads to pronounced changes in c-fos expression in the medial prefrontal- and the gustatory cortex (Mickley et al., 2004, Mickley et al., 2005).

Pre-exposure to the CS, reduced CS intensity, and longer CS–US delays are well-known factors influencing the strength of CTA learning (Bernstein and Koh, 2007). In addition, inter-individual differences in CTA extinction do exist (Exton et al., 1998b). Against this background, the present study investigated in rats whether extinction of a CTA is related to the strength of the initially acquired CS–US association, using a established model of behavioral conditioning with saccharin as CS and the immunosuppressive drug cyclosporine A (CsA) as US (Exton et al., 2001, Wirth et al., 2011) (Wirth et al., 2011). CsA inhibits the protein phosphatase calcineurin and leads to a suppression of essential T cell functions mainly through inhibition of interleukin (IL)-2 and interferon (IFN)-γ production (Halloran et al., 1999). In experiments of short and long retrieval, animals were divided into groups with either a strong (STA) or a moderate taste aversion (MTA) based on their overall CTA performance. To analyze whether the observed differences in taste aversion learning were related to differences in neuronal activation, we then measured expression of the immediate early gene c-fos as a marker of neuronal activity (Sagar et al., 1988, Herbert et al., 1990, Morgan and Curran, 1991) in brain structures that previously have been shown to be relevant for acquisition and retrieval of the CTA (Yamamoto, 1993, Yamamoto et al., 1994, Pacheco-Lopez et al., 2005).