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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.