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Aversive situations may reduce nociception. The mechanism underlying such analgesia has been suggested to involve the interaction between the two separate but interconnected motivational systems "defense" and "pain." To determine the developmental course of defense and nociception, these processes were analyzed during early ontogeny in rats. To elicit a defensive reaction, a huddle of preweanling rat pups was exposed to an unfamiliar, unrelated adult male, or, for comparison, to the mother. On postnatal Day 7 the pups did not show a behavioral reaction to the presence of the mother or the male, and no reduction in nociceptive threshold in a thermal paw withdrawal test. On Day 14, pups in the presence of the male stopped ongoing behaviors and became immobile, and showed reduced paw withdrawal after the exposure. At Day 21, 22 pups of 32 became immobile when exposed to the male, whereas 10 pups explored the partition separating them from the male. Neither group showed reduced paw withdrawal. Immobility was considered a defensive reaction because it reduces auditory and visual cues and therefore the probability of being detected. The developmental course of immobility seems to reflect both the changes in threat imposed on the pups by a potentially infanticidal male and the ability of pups to react to that threat. The reduction in paw withdrawal that followed male exposure indicates an inhibitory mechanism. It is discussed whether the activation of the defense system results in an inhibition of nociception. (C) 1998 Elsevier Science Inc

Ability. --- Activation. --- Adult. --- Analgesia. --- Auditory. --- Behavior. --- Cues. --- Defense. --- Defensive behavior. --- Defensive. --- Exposure. --- Expression. --- Group. --- Immobility. --- Inhibition. --- Interaction. --- Intrathecal injection. --- Male rat. --- Male. --- Mechanisms. --- Mice. --- Mother. --- Nociception. --- Ontogeny. --- Organization. --- Partition. --- Periaqueductal gray. --- Preweanling rat. --- Preweanling rats. --- Pups. --- Rat. --- Rats. --- Reduction. --- Responses. --- Sex-differences. --- Situations. --- System. --- Systems. --- Test. --- Thermal.

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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.