Organizing action of corticosterone on the development of behavioral inhibition in the preweanling rat

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Altricial rat pups develop the ability to freeze and to terminate their emission of ultrasonic vocalizations when exposed to an unfamiliar adult male rat. This developmental competence in expressing behavioral inhibition is impaired when rat pups are adrenalectomized (ADX) on postnatal day 10, a period prior to the emergence of behavioral inhibition. Adrenalectomy, however, fails to induce similar behavioral deficits when performed after behavioral inhibition has developed. Results suggest that adrenal steroids are involved in promoting the development but not the activation of behavioral inhibition. To critically test this hypothesis, four groups of rats were adrenalectomized on day 10 and tested for behavioral inhibition on day 18. Prior to testing, one group of rats received daily s.c. injections of vehicle whereas another group was treated with daily injections of 3.0 mg/kg of corticosterone (B). The other two groups of rats received daily B injections on only days 10–13 or days 14–17. Results indicated that ADX rats treated with B only on days 10–13 as well as throughout exhibited significantly higher levels of freezing than the other two treatment groups. In order to evaluate whether the behavioral inhibitory deficits produced by ADX at 10 days of age are due to a delayed insensitivity to the 3.0 mg/kg dose of B, day 10 pups were ADX and injected on days 14–17 with doses of B as high as 12 mg/kg. When tested for behavioral inhibition on day 18, these high doses of B were found to be ineffective in potentiating freezing above the level of vehicle-treated rats. Together, results suggest that prior to the end of the second postnatal week endogenous adrenal steroids and in particular B, are critically involved in developmental changes necessary for the eventual species typical expression of behavioral inhibition.