Intracranial action of corticosterone facilitates the development of behavioral inhibition in the adrenalectomized preweanling rat.

Abstract

We tested the hypothesis that in preweanling rats central administration of exogenous corticosterone (CORT) is sufficient to facilitate the development of behavioral inhibition. 28-gauge cannulae containing varying concentrations of CORT (0, 25, 50 and 100%) were implanted unilaterally into the lateral ventricles of 9-day-old rat pups. After a 24-h postoperative recovery period, pups were adrenalectomized. At 14 days of age, pups were tested for behavioral inhibition which consisted of removing the pup from the nest and exposing it to an unfamiliar adult male rat. Pups implanted with cannulae containing 0, 25 and 50% concentrations of CORT spent significantly less time in freezing postures than pups implanted with cannulae containing 100% CORT. These freezing pups also tended to emit fewer ultrasonic vocalizations than pups in the other three implant conditions, albeit the level obtained was not statistically significant. RIAs indicated that, in general, hormone-filled cannulae produced no detectable concentrations of plasma CORT on the day of the test or on days preceding testing. Results suggest that in the early postnatal period endogenous CORT acts centrally to facilitate the development of neural pathways involved in the ontogenetic expression of behavioral inhibition.