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Rat pups become immobile and analgesic when exposed to an adult male rat. The aim of this study was to determine whether these reactions are under the control of endogenous opioids and to determine the role of the midbrain periaqueductal gray (PAG), which mediates stress-induced immobility and analgesia in adult animals. In Experiment 1, 14-day-old rats were injected systemically with the general opioid receptor antagonist naltrexone (1 mg/kg), which blocked male-induced analgesia to thermal stimulation but did not affect immobility. In Experiment 2, the selective mu opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP; 50 or 100 ng/200 nl) was microinjected into the ventrolateral and lateral PAG. CTOP suppressed male-induced analgesia when injected into the ventrolateral PAG. Male-induced immobility was not affected by CTOP. Male proximity therefore seems to induce analgesia in rat pups by releasing endogenous opioids that bind to mu opioid receptors in the ventrolateral PAG

Adult. --- Analgesia. --- Animal. --- Animals. --- Behavioral-inhibition. --- Beta-endorphin. --- Columnar organization. --- Conditional hypoalgesia. --- Control. --- Endogenous. --- Endorphin-like immunoreactivity. --- Experiment. --- Immobility. --- Intrathecal injection. --- Male rat. --- Male. --- Midbrain periaqueductal gray. --- Midbrain. --- Morphine-induced analgesia. --- Naltrexone. --- Neurons in-vitro. --- Opioid receptors. --- Opioid. --- Opioids. --- Periaqueductal gray. --- Preweanling rats. --- Proximity. --- Pups. --- Rat. --- Rats. --- Receptor antagonist. --- Receptor. --- Receptors. --- Rostral ventromedial medulla. --- Stimulation. --- Stress-induced analgesia. --- Thermal.

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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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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.