The Neuroscience of Pleasure. Focus on “Ventral Pallidum Firing Codes Hedonic Reward: When a Bad Taste Turns Good”

Abstract

For more than 2,000 years, philosophers have struggled to define, let alone explain, pleasure or hedonics. With contemporary animal models and techniques, scientists now have the ability to provide a comprehensive description of this affective state by examining its neural underpinnings. In this issue of the Journal of Neurophysiology (p. 2399 –2409), Tindell and colleagues use an elegant, reductionist approach to take a profound step in defining these neural foundations (Tindell 2006). The authors control for various elements of reward that historically have confounded researchers ambitious enough to attempt to study such an intangible topic. Specifically, Tindell and colleagues establish a role for ventral pallidum (VP) neurons in encoding hedonics. The authors infused an unpalatable concentration of saline into the oral cavities of rats while recording the extracellular electrophysiological activity of single VP neurons. Aversive orofacial reactions to the unpalatable taste were simultaneously measured. Three days later, rats were made sodium deplete and retested. In a sodium-depleted state, these rats now exhibited a behavioral switch from aversive taste reactivity to appetitive taste reactivity coincident with increased VP activity to the infusion of saline (similar to responses for a palatable sucrose solution). The clever implicit controls indicate that the altered electrophysiological activity encodes a hedonic shift rather than alterations in motivated behavior or sensory coding.