Overlapping striatal sites mediate scopolamine-induced feeding suppression and mu-opioid-mediated hyperphagia in the rat

Intra-striatal infusions of the muscarinic antagonist, scopolamine, markedly suppress feeding; however, the underlying mechanisms are unclear. Recent findings suggest that scopolamine influences opioid-dependent mechanisms of feeding modulation. Robust mu-opioid-mediated feeding responses are obtained in anterior, ventral sectors of the striatum with progressively weaker effects posteriorly and dorsally. One might therefore expect the effects of scopolamine to conform to similar boundaries, but a systematic mapping of scopolamine-induced feeding suppression has not yet been undertaken. This study aimed to assess the overlap between the striatal sites mediating scopolamine-induced feeding suppression and mu-opioid-induced hyperphagia. Dose-effect functions for scopolamine (0, 1, 5, and 10 mu g) were obtained in the nucleus accumbens (Acb), anterior dorsal striatum (ADS), and posterior dorsal striatum (PDS) in three different groups of rats. In the same subjects, the mu-opioid receptor agonist (d-Ala2-N-MePhe4, Glyol)-enkephalin (DAMGO; 0.25 mu g) was infused on a separate test day. The dependent variables were food and water intake, ambulation, and rearing. The greatest dose sensitivity for scopolamine-induced feeding suppression was observed in the Acb. Only the highest dose was effective in the ADS, and no effects were seen in the PDS. Water intake and general motor activity were not altered by scopolamine in any site. DAMGO infusions produced hyperphagia only in the Acb. These results support a model in which the behavioral effects of muscarinic blockade are limited by the same anatomical constraints that govern mu-opioid receptor-mediated control of feeding. These constraints are likely imposed by the topographic arrangement of feeding-related afferent inputs and efferent projections of the striatum.