The effect of periaqueductal gray lesions on responses to age-specific threats in infant rats

During early ontogeny infant rats show specific responses to a variety of a ge-dependent threatening situations. When isolated from nest and dam, they emit ultrasonic vocalizations and show decreased reactivity to noxious stim ulation, or analgesia. When exposed to an unfamiliar adult male, they becom e immobile and analgesic. The midbrain periaqueductal gray (PAG) is an impo rtant area within the circuitry that controls responses to threatening stim uli in the adult. Little is known about the functions of the PAG in early l ife. It was hypothesized that the PAG mediates the responses to the age-spe cific threats social isolation and male exposure in the infant rat. Rat pup s 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 a nd isolation-induced analgesia were decreased in both lesion groups. On day 14, male-induced immobility and analgesia were decreased in ventrally lesi oned animals. In conclusion, the PAG seems to play a developmentally contin uous role in age-specific responses to threat such as ultrasonic vocalizati on, analgesia, and immobility. (C) 2000 Elsevier Science B.V. All rights re served.