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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.