Suppression of third ventricular NPY-elicited feeding following medullary reticular formation infusions of muscimol.

The appetitive component of feeding is controlled by forebrain substrates, but the consummatory behaviors of licking, mastication, and swallowing are organized in the brainstem. The target of forebrain appetitive signals is unclear but likely includes regions of the medullary reticular formation (RF). This study was undertaken to determine the necessity of different RF regions for mastication induced by a descending appetitive signal. We measured solid food intake in response to third ventricular (3V) infusions of the orexigenic peptide neuropeptide Y 3-36 in awake, freely moving rats and determined whether focal RF infusions of the GABAA agonist muscimol suppressed eating. RF infusions were centered in either the lateral tegmental field, comprising the intermediate (IRt) and parvocellular (PCRt) RF, or in the nucleus gigantocellularis (Gi). Infusions of NPY 3-36 (5 microg/5 microl) into 3V significantly increased feeding of solid food over a 90-min period compared with the noninfused condition (4.3 g +/- 0.56 vs. 0.57 g +/- 0.57, p < .001). NPY 3-36-induced food intake was suppressed (1.7 g +/- 0.48) by simultaneous infusions of muscimol (0.6 mM/100 nl) into the IRt/PCRt (p < .01). Coincident with the decrease in feeding was a decrease in the amplitude of anterior digastric muscle contractions in response to intraoral sucrose infusions. In contrast, infusions of muscimol into Gi had no discernible effect on food intake or EMG amplitude. These data suggest that the IRt/PCRt is essential for forebrain-initiated mastication, but that the Gi is not a necessary link in this pathway.