Gustatory reward and the nucleus accumbens
Section snippets
The problem of reward
Reward is a psychological construct still in search of neural mechanisms. The problems associated with this pursuit arise because the construct itself has several definitions. Natural rewards are produced by sensory stimuli but they are not synonymous with them. Physiological conditions and experience influence, even reverse, the reward value of stimuli. Non-natural rewards are produced by drugs and electrical stimulation acting directly on the brain. This provides strong evidence that reward
Taste as reward
The research reviewed here attempts to reduce this remoteness by tracing a well characterized sensory system, taste, to the neurons of the nucleus accumbens whose activity appears to track reward value. The first task was to determine if a normally rewarding gustatory signal, sucrose, released DA in the accumbens. Mark et al. [21] demonstrated that saccharin intake increased dopamine flux in the NAc. More importantly, when they rendered the saccharin aversive by associating it with a toxin,
The central gustatory system
In order for the forebrain to participate in assigning hedonic value to afferent activity, the sensory systems involved must interact with the neural mechanisms that elaborate reward and aversion. This interaction might occur in forebrain reward areas, such as the mesolimbic dopamine system, or in the sensory nuclei themselves. For taste, there is evidence for both, but only the former is addressed here in any detail. The first central synapse for the gustatory system is in the rostral half of
The logic of taste reward
In the present context, the bifurcated central taste projection provides two potential routes by which gustatory afferent activity could interact with central reward systems. Our research has focused on which of these routes supports the release of dopamine from nucleus accumbens when rats are licking sucrose. We chose this index because the evidence reviewed above indicates that NAc dopamine tracts the reward value of substances that taste sweet to humans. To test the route of afferent
Discussion
Taken with the dialysis data, these results add weight to the inference that the sensory activity produced by sapid sucrose reaches the nucleus accumbens via the parabrachial limbic projections rather than the thalamocortical route. The fact that all the rats were sham feeding confirms that the differential activation elicited by sucrose was due to oral sensory stimulation. The group with lateral, visceral afferent PBN lesions, which had little if any effect on c-fos labeling, adds emphasis to
Acknowledgments
This paper is based on a presentation at a workshop on Peripheral–Central Interactions in the Control of Food Intake and Energy Balance held at the Centro Stefano Franscini in Ascona, Switzerland in August 2005. The authors' research was supported by grants DC00240, DC05435 and DK065709.
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