Electromyographic analysis of oral habituation in rat pups

Rat pups show decreases in mouthing activity in response to a series of repeated oral infusions of a diet. This decrease in mouthing activity has been termed "oral habituation" and these changes have been readily recorded with simple behavioral observations. Oral habituation appears to be a component of satiety in young rats. In the present study, to more specifically characterize changes in motor response topography during habituation in muscle groups used for mastication, mouthing activity was recorded by implanting fine wire electromyographic electrodes in the superficial masseter, anterior digastric, sternohyoideus, and genioglossus muscles of 12-day-old rat pups. During testing, pups received a series of brief oral infusions of a 10% sucrose diet delivered through an oral cannula. The results demonstrated that mouthing activity as observed and scored behaviorally was highly correlated with mouthing behavior recorded by EMG, with oral habituation distinctly emerging in both measures. In addition, the pattern of motor activity in the four masticatory muscles changed during the course of oral habituation. Within the minute following a single infusion, the cycle frequency, duration of activity, and relative onset time of activity in the four muscles changed. In addition, across the course of habituation, both cycle frequency and relative onset times of muscle activity changed. These results demonstrate the general reliability of behavioral observations of masticatory motor activity in young rats and provide further information on how the pattern of activity of muscles involved in the mouthing motor pattern is altered during the course of oral habituation.