Isolation-induced vocalization in Wistar rat pups is not increased by naltrexone.

Rat pups, when removed from dam and littermates and isolated in an unfamiliar milieu, emit a characteristic ultrasonic vocalization that can be quieted by the administration of a nonsedating dose of morphine. Wistar pups, aged 7, 10, 12, 14, or 16 days, were tested after receiving intraperitoneal injections of the opiate antagonist naltrexone (0.0, 0.5, 1.0, or 5.0 mg/kg). The rate of isolation-induced ultrasonic vocalizations was unaffected by naltrexone at any dose, and there were no significant naltrexone-related changes on other behavioral measures. The complexity of the opioid system is discussed, as it may be involved in the vocal reaction to isolation.