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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, stimuli that pose a threat to an animal change. Unrelated adult male rats may kill young rats, bur infanticide ends around weaning. Predation, on the other hand may increase during early ontogeny when mts begin to extend their activity range. We investigated the developmental course of two defensive responses, immobility and analgesia, in young rats exposed to an adult male rat or to predator cues. Preweaning 14-day-old mts became immobile and analgesic when exposed to the male and showed immobility but not analgesia when exposed to cat odor On Day 26, around weaning, the presence of the male rat no longer induced immobility and analgesia whereas cat odor produced higher levels of immobility and analgesia compared to control and male-exposed animals. This developmental change in responsivity may reflect the differences in the risk of being harmed by a male or a cat during different periods of ontogeny. (C) 2001 John Wiley & Sons, Inc

Activity. --- Adult. --- Analgesia. --- Animal. --- Animals. --- Cat odor. --- Cat. --- Control. --- Cues. --- Defensive behavior. --- Defensive immobility. --- Defensive responses. --- Defensive. --- Dentate gyrus. --- Developmental-changes. --- Emotional motor system. --- Fear. --- Immobility. --- Increase. --- Infanticide. --- Level. --- Male conspecifics. --- Male rat. --- Male-rats. --- Male. --- Neurobiological basis. --- Odor. --- Ontogeny. --- Periods. --- Predation. --- Predator odor. --- Predator. --- Rat. --- Rats. --- Rattus-norvegicus. --- Response. --- Responses. --- Risk. --- Stimuli. --- Stress-induced analgesia. --- Stress. --- Weaning. --- Young-rats. --- Young.

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We examined how the experience of a threatening stimulus alters subsequent behavior in a situation where the immediate threat is absent. A small huddle of 12-day-old rats was exposed to a potentially infanticidal adult male rat for 5 min. During male exposure, pups were significantly more immobile than control pups. Thirty, 60, and 180 min after male exposure, the pups were isolated for 5 min from litter and dam in an unfamiliar environment. When isolated, pups that had been previously exposed to the male emitted significantly fewer ultrasonic vocalizations than controls, but did not differ in immobility. Low levels of vocalization were apparent 30 and 60 min after male exposure and were not evident at 180 min. The pups seemed to have adjusted their behavior to a potential male threat in a different context for a limited period of time. (C) 2003 Wiley Periodicals, Inc

2-week-old rats. --- Adult. --- Alters. --- Anxiety-like behavior. --- Behavior. --- Behavioral-inhibition. --- Cat odor. --- Control. --- Defensive behaviors. --- Environment. --- Experience. --- Exposure. --- Immobility. --- Infant rats. --- Infanticide. --- Level. --- Male rat. --- Male. --- Neural plasticity. --- Ontogeny. --- Periaqueductal gray. --- Preweanling rats. --- Pups. --- Rat. --- Rats. --- Responses. --- Stimulus. --- Time. --- Ultrasonic vocalization. --- Vocalization.

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Young animals respond to threatening stimuli in an age-specific way. Their endocrine and behavioral responses reflect the potential threat of the situation at a given age. The aim of the present study was to determine whether corticotropin-releasing factor (CRF) is involved in the endocrine and behavioral responses to threat and their developmental changes in young rats. Preweaning 14-day-old and postweaning 26-day-old rats were exposed to two age-specific threats, cat odor and an adult male rat. The acute behavioral response was determined during exposure. After exposure, the time courses of the corticosterone response and of CRF expression in the paraventricular nucleus of the hypothalamus (PVN) and in extrahypothalamic areas were assessed. Preweaning rats became immobile when exposed to cat odor or the male rat, whereas postweaning rats became immobile to cat odor only. Male exposure increased serum corticosterone levels in 14-day-old rats, but cat odor failed to increase levels at either age. Exposure induced elevation of CRF mRNA levels in the PVN that paralleled changes in corticosterone levels. CRF may thus play a role in endocrine regulation and its developmental changes during early life. Neither cat odor nor the adult male altered CRF mRNA levels in the bed nucleus of the stria terminalis (BNST) or the amygdala, but both stimuli increased levels in the hippocampus. Hippocampal CRF mRNA expression levels did not parallel cat odor or male-induced immobility, indicating that CRF is not involved in this response in young rats but may be involved in aspects of learning and memory. (C) 2004 Elsevier Inc. All rights reserved

Adult. --- Age. --- Amygdala. --- Animal. --- Animals. --- Area. --- Bed nucleus. --- Behavioral-responses. --- Cat odor. --- Cat. --- Corticosterone. --- Corticotropin-releasing factor. --- Corticotropin-releasing hormone. --- Corticotropin-releasing-factor. --- Crf. --- Defensive-withdrawal. --- Dentate gyrus. --- Developmental-changes. --- Endocrine. --- Exposure. --- Expression. --- Fear. --- Gene-expression. --- Glucocorticoid. --- Hippocampal. --- Hippocampus. --- Hpa axis. --- Hypothalamus. --- Immature rat. --- Immobility. --- Immobilization stress. --- Increase. --- Learning. --- Level. --- Life. --- Male rat. --- Male. --- Memory. --- Messenger-rna expression. --- Nucleus. --- Odor. --- Paraventricular nucleus. --- Pituitary-adrenal axis. --- Play. --- Predation. --- Rat. --- Rats. --- Regulation. --- Response. --- Responses. --- Serum. --- Stimuli. --- Stria terminalis. --- Time-course. --- Time. --- Ultrasonic vocalization. --- Young-rats. --- Young.

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Stress activates endogenous opioids that modulate nociceptive transmission. Exposure to a potentially infanticidal adult male rat suppresses pain-related behaviors in pre-weaning but not in older rats. This male-induced analgesia is mediated by I opioid receptors in the periaqueductal gray, a midbrain structure that is innervated by amygdala projections. To determine whether enkephalin, a l and d opioid receptor agonist, is activated by male exposure, mRNA levels of its precursor, preproenkephalin, were measured in subdivisions of the amygdala and the periaqueductal gray. In 14-day-old but not in 21-day-old rats, 5 min of male exposure induced analgesia to heat and increased preproenkephalin mRNA levels in the central nucleus of the amygdala but not in the periaqueductal gray. The change in the activation of enkephalinergic neurons in the central amygdala may contribute to the change in stress-induced analgesia during early ontogeny. (C) 2002 IBRO. Published by Elsevier Science Ltd. All rights reserved

Activation. --- Adult. --- Amygdala. --- Analgesia. --- Behavior. --- Brain. --- Central amygdala. --- Endogenous. --- Enkephalin. --- Exposure. --- Expression. --- Heat. --- Hypothalamus. --- Level. --- Male rat. --- Male. --- Midbrain. --- Neurons. --- Neurotensin. --- Nucleus. --- Ontogeny. --- Opioid receptors. --- Opioid. --- Opioids. --- Paraventricular nucleus. --- Periaqueductal gray. --- Proenkephalin messenger-rna. --- Projections. --- Rat. --- Rats. --- Receptor. --- Receptors. --- Stress-induced analgesia. --- Stress. --- Transmission.