Abstract
Taste memory depends on motivational and post-ingestional consequences; thus, it can be aversive (e.g., conditioned taste aversion, CTA) if a novel, palatable taste is paired with visceral malaise, or it can be appetitive if no intoxication appears after novel taste consumption, and a taste preference is developed.The nucleus accumbens (NAc) plays a role in hedonic reactivity to taste stimuli, and recent findings suggest that reward and aversion are differentially encoded by the activity of NAc neurons. The present study examined whether the requirement for N-methyl-D-aspartate (NMDA) receptors in the NAc core during rewarding appetitive taste learning differs from that during aversive taste conditioning, as well as during retrieval of appetitive vs. aversive taste memory, using the taste preference or CTA model, respectively. Bilateral infusions of NMDA (1 μg/μl, 0.5 μl) into the NAc core were performed before acquisition or before retrieval of taste preference or CTA. Activation of NMDA receptors before taste preference training or CTA acquisition did not alter memory formation. Furthermore, NMDA injections before aversive taste retrieval had no effect on taste memory; however, 24 h later, CTA extinction was significantly delayed. Also, NMDA injections, made before familiar appetitive memory retrieval, interrupted the development of taste preference and produced a preference delay 24 h later. These results suggest that memory formation for a novel taste produces neurochemical changes in the NAc core that have differential requirements for NMDA receptors during retrieval of appetitive or aversive memory.
Highlights
The nucleus accumbens (NAc) is a brain region located within the ventral striatum, and its critical role in processing rewardrelated stimuli and instrumental responses is well documented (Salamone, 1996; Corbit et al, 2001; Hall et al, 2001; Holland and Gallagher, 2003; Kelley, 2004a; Pothuizen et al, 2005)
The present results demonstrated that NMDA receptor activation in the NAc core has differential effects on taste memory formation and retrieval, depending on whether an appetitive or aversive memory is formed
NMDA receptor activation before acquisition of aversive taste memory had no effect on memory formation
Summary
The nucleus accumbens (NAc) is a brain region located within the ventral striatum, and its critical role in processing rewardrelated stimuli and instrumental responses is well documented (Salamone, 1996; Corbit et al, 2001; Hall et al, 2001; Holland and Gallagher, 2003; Kelley, 2004a; Pothuizen et al, 2005). The evidence suggests that NMDA receptors in the NAc have an important function during reward and/or aversive taste stimulus processing, possibly by modulating NAc dopaminergic activity (Howland et al, 2002) and interacting with other brain structures (Bermudez-Rattoni, 2004; Núñez-Jaramillo et al, 2010) Despite such evidence, there is little information that directly compares the function of NMDA receptors in the NAc core during appetitive taste learning vs during aversive taste conditioning, i.e., comparing their role in the different taste emotional/hedonic reactivity induced during and after learning. The present study, using the same taste stimulus in a preference protocol or CTA model, examined the requirement for NMDA receptors in the NAc core during appetitive taste learning or during CTA acquisition, and the role of these receptors during retrieval of appetitive or aversive taste memory
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