Contrasting roles of mesostriatal dopamine and opioid systems in food 'wanting' and 'liking'.

Abstract

What role do brain dopamine and opioid systems play in the mediation of reward? Do these neurotransmitter systems mediate pleasure? Are reward and pleasure identical? What neural substrates mediate the 'liking' component of reward? The 'taste reactivity paradigm' of affective reactions to tastes was used to investigate the role of dopamine and opioid agents in taste pleasure and to identify neural substrates involved in its regulation. The taste reactivity technique is the only paradigm that allows assessment of the 'liking' for a taste incentive independently from 'wanting' components of reward, and thus the most direct measure available for the study of hedonic regulation. Results indicate that while dopamine and opioid systems are implicated in the mediation of reward, only the brain opioid systems are involved in the regulation of hedonic processes. Brain dopaminergic systems are not. In addition, the boundaries of the specific subregion within the nucleus accumbens (a forebrain structure related to motivation and reward) that mediates opioid-induced feeding were defined. A c-fos based technique was used to identify plumes of neuronal activation triggered by intra-accumbens microinjections of morphine (0.5 $\mu$g) that elicit feeding. Based on the determined size and shape of morphine-induced fos plumes, a mapping procedure identified the extent and boundary of the accumbens 'opioid-eating' site. The site was found to be contained primarily within a medial caudal subregion of the nucleus accumbens shell, and did not substantially penetrate the accumbens core. However, portions of several other structures, immediately medial and adjacent to the shell, appeared to be included in the functional site (such as rostral ventral pallidum and vertical limb of the diagonal band). Finally, it was shown that hedonic taste reactivity patterns were also enhanced by microinjections of morphine into the accumbens 'opioid eating site'. This suggests that accumbens opioid-mediated increases in food intake involve an enhancement in the impact of taste pleasure. Increases in both 'wanting' and 'liking' for food after accumbens administration of opioid agonists thus appear to be mediated by opioid receptors localized primarily in the medial and caudal shell of the nucleus accumbens.