Medullary reticular formation activity during ingestion and rejection in the awake rat

The consummatory components of ingestion and rejection, organized in the caudal brainstem, include licking, swallowing, and the oral phase of rejection (gaping). Studies employing electrical-stimulation induced motor activity have localized interneurons controlling these complex motor patterns to the medullary reticular formation (RF), but the characteristics of these neurons during more naturally induced behavior are unknown. The purpose of the present study was to record the activity profiles of RF neurons during licking, swallowing, and oral rejection in response to gustatory stimulation. Two-hundred and two neurons recorded from awake, freely moving rats were broadly classified as orally related (67%) or non-orally related (33%). Orally related neurons included a large number that were rhythmically active during licking (n = 76; 38%). These "lick-rhythmic" neurons were widely distributed in the RF, but were concentrated in the caudal medullary reticular formation adjacent to the hypoglossal nucleus (Probst's region) and further rostral in the intermediate zone (IRt) of the RF. An analysis of autocorrelations determined that lick-rhythmic neurons in these regions were more closely coupled to licking than to lick-rhythmic neurons more lateral in the parvocellular RF (PCRt). In addition to neurons with weak lick-rhythmic activity, the PCRt also contained a disproportionate number of neurons with orosensory or mixed oro-sensorimotor properties. These data provide evidence for functional specialization within different regions of the medullary RF. A high proportion of lick-rhythmic neurons also showed differential activity associated with swallowing (41%) and/or gaping (75%), further suggesting that the different components of ingestion and rejection share brainstem substrates instead of being produced by unique subsets of interneurons.