Habituation of oromotor responding to oral infusions in rat pups.

The influence of oromotor experience on the pattern of ingestion in rat pups and the relation of this influence to age and pups' physiological state were investigated using a procedure designed to mimic the sham-feeding preparation in adult rats. Six-, 12-, and 18-day-old pups received brief intra-oral infusions of sucrose solutions once every minute. Small infusion volumes minimized postoral effects. Pups' oromotor responsiveness was assessed by recording the pattern of mouthing behavior continuously during the test. Pups were tested after 24, 6 or 0 h deprivation. During testing, the mouthing behavior of all pups except 24-hr deprived 6-day olds showed a marked decline. The specificity of this decrement was demonstrated in a second experiment in which the decremented response was restored by a switch in solution flavor. Finally, the influence of postoral signals on the decline in responsiveness was evaluated by comparing the oral responsiveness of 18-day-old pups following intragastric, oral, or no infusions. Oral infusions suppressed subsequent oral responding, but intragastric infusions did not. These results provide evidence for a habituation-like role of oromotor experience in determining patterns of ingestive behavior within a feeding test. Here, major determinants of the pattern of decline were pups' physiological state and developmental age.