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During early ontogeny infant rats show specific responses to a variety of age-dependent threatening situations. When isolated from nest and dam, they emit ultrasonic vocalizations and show decreased reactivity to noxious stimulation, or analgesia. When exposed to an unfamiliar adult male, they become immobile and analgesic. The midbrain periaqueductal gray (PAG) is an important area within the circuitry that controls responses to threatening stimuli in the adult. Little is known about the functions of the PAG in early life. It was hypothesized that the PAG mediates the responses to the age-specific threats social isolation and male exposure in the infant rat. Rat pups were lesioned electrolytically either in the lateral or the ventrolateral PAG on postnatal day 7, tested in social isolation on day 10, and exposed to a male on day 14. On day 10 during isolation, ultrasonic vocalizations and isolation-induced analgesia were decreased in both lesion groups. On day 14, male-induced immobility and analgesia were decreased in ventrally lesioned animals. In conclusion, the PAG seems to play a developmentally continuous role in age-specific responses to threat such as ultrasonic vocalization, analgesia, and immobility. (C) 2000 Elsevier Science B.V. All rights reserved

Adult. --- Analgesia. --- Animal. --- Animals. --- Area. --- Behavior. --- Control. --- Defensive responses. --- Exposure. --- Expression. --- Fear. --- Function. --- Group. --- Immobility. --- Infant rats. --- Infant. --- Intrathecal injection. --- Isolation. --- Lesion. --- Lesions. --- Life. --- Male. --- Midbrain periaqueductal gray. --- Midbrain. --- Nest. --- Ontogeny. --- Periaqueductal gray. --- Play. --- Preweanling rats. --- Pups. --- Rat. --- Rats. --- Reactivity. --- Response. --- Responses. --- Situations. --- Social isolation. --- Social-isolation. --- Social. --- Stimulation. --- Stimuli. --- Stress. --- Systems. --- Ultrasonic vocalization. --- Vocalization.