Habituation in the rat fetus.

Rat fetuses exhibit motor and cardiac responses to chemosensory stimulation on Days 20 and 21 of gestation. The first experiment demonstrated that fetuses exhibit an increase in overall motor activity and decrease in heartrate in response to an initial intraoral infusion of a lemon solution. After a series of nine exposures, however, fetuses no longer exhibit motor or cardiac responses to lemon infusion, suggesting the existence of a habituation-like process. Responsiveness recovers spontaneously following a 3- to 9-min period without stimulation. In a second experiment, a dishabituation treatment was administered to distinguish habituation, which is a centrally mediated decrement in response, from effector fatigue, sensory adaptation, and other peripheral mechanisms that can result in reduced responsiveness. A single infusion of mint following a series of nine lemon exposures was effective in reinstating fetal motor responses to lemon on both Days 20 and 21, but reinstated cardiac responses only on Day 21. Rat fetuses habituate to repeated chemosensory stimulation, suggesting the utility of the habituation paradigm in measuring CNS development during the perinatal period.