Muscimol infusions in the brain stem reticular formation reversibly block ingestion in the awake rat

Previous studies have localized a central pattern generator for mastication to the midline pontomedullary reticular formation (RF) based on cortically induced ororhythmic movements. The present study determined whether this same substrate mediated licking responses evoked by more natural stimuli. Licking in the awake rat was initiated either through an appetitive response to sucrose presented in a bottle or by intraoral (IO) infusions. Oral rejection responses also were obtained by IO infusions of quinine hydrochloride. Small volumes of the GABA(A) agonist muscimol bilaterally infused into the lateral medullary RF significantly reduced licking and oral rejection responses measured electromyographically from the anterior digastric and geniohyoid muscles. Other than the decrement or absence of ororhythmic activity, rats appeared normal and actively approached and probed the water bottle. The suppression was reversible and returned to baseline within 3 h. In contrast, midline infusions of muscimol did not affect licking or rejection responses. We postulate that the lateral medullary RF is an essential final common path for ingestive consummatory responses.