The effects of opioid and benzodiazepine antagonists on dam-induced reductions in rat pup isolation distress.

In 10-day-old pups isolated in a novel environment, the presence of the dam reduced the rate of ultrasonic vocalization by 93%. The opiate antagonist naltrexone caused a dose-related increase in the rate of vocalization that was significant only at 5.0 mg/kg. This suggests that the dam exerts comforting effects through endogenous opioid mechanisms. Although the benzodiazepine agonist chlordiazepoxide is as effective as morphine in the quieting of isolation distress, its antagonist, Ro 15-1788, at doses up to 20 mg/kg did not alter the lowered rate of vocalization associated with the presence of the dam. Instead, it facilitated quiet contact with the companion. This raises the possibility that the presence of a social companion precludes the release of an endogenous anxiogen that binds to the benzodiazepine receptor.