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

The consummatory components of ingestion and rejection, organized in the ca udal brainstem, include licking, swallowing, and the oral phase of rejectio n (gaping). Studies employing electrical-stimulation induced motor activity have localized interneurons controlling these complex motor patterns to th e medullary reticular formation (RF), but the characteristics of these neur ons during more naturally induced behavior are unknown. The purpose of the present study was to record the activity profiles of RF neurons during lick ing, swallowing, and oral rejection in response to gustatory stimulation. T wo-hundred and two neurons recorded from awake, freely moving rats were bro adly classified as orally related (67%) or non-orally related (33%). Orally related neurons included a large number that were rhythmically active duri ng licking (n=76; 38%). These "lick-rhythmic" neurons were widely distribut ed in the RF, but were concentrated in the caudal medullary reticular forma tion adjacent to the hypoglossal nucleus (Probst's region) and further rost ral in the intermediate zone (IRt) of the RF. An analysis of autocorrelatio ns determined that lick-rhythmic neurons in these regions were mon closely coupled to licking than to lick-rhythmic neurons more lateral in the parvoc ellular RF (PCRt). In addition to neurons with weak lick-rhythmic activity, the PCRt also contained a disproportionate number of neurons with orosenso ry or mixed oro-sensorimotor properties. These data provide evidence for fu nctional specialization within different regions of the medullary RE A high proportion of lick-rhythmic neurons also showed differential activity asso ciated with swallowing (41%) and/or gaping (75%), further suggesting that t he different components of ingestion and rejection share brainstem substrat es instead of being produced by unique subsets of interneurons.