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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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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.