Abstract
Termination of ongoing behavior and assumption of defensive postures when threatened are adaptive characteristics of vertebrates. Altricial rat pups develop these characteristics by 14 days of age. At this time, pups inhibit their ultrasonic vocalizations and freeze when threatened. This emergence of behavioral inhibition is impaired when rats are adrenalectomized (ADX) at 10 days of age. That is, 14-day-old ADX pups exhibit deficits in freezing and continue to emit ultrasounds when confronted by an adult male rat. Studies also showed that removal of adrenal hormones does not potentiate vocalizations or render pups incapable of reducing their ultrasounds. More important, 3.0 mg/kg of corticosterone (CORT), but not lower doses, administered daily to ADX pups restored freezing, with lesser effects on ultrasound inhibition. Disrupting the developmental action of endogenous CORT appears to impair the ontogenetic expression of behavioral inhibition.
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