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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.

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Roman high- and low-avoidance (RHA/Verh and RLA/Verh) rats are selected and bred for extreme divergence in two-way active avoidance acquisition. In addition, compared to RLA/Verh rats, RHA/Verh rats are (behaviorally and physiologically) less anxious or reactive to stressors, show increased novelty (sensation)-seeking behavior as well as a higher preference for rewarding substances, and are usually less efficient in learning tasks not involving shock administration. The present article reviews evidence showing that neonatal handling and/or environmental enrichment leads to enduring effects (their magnitude frequently depending upon the rat line) on those behaviors. For example, it has been found that neonatal handling reduces most of the (behavioral and physiological) signs of emotionality/anxiety in RLA/Verh rats, while environmental enrichment increases their novelty seeking (also the case with RHA/Verh rats), saccharin and ethanol intake, and sensitivity to amphetamine. Finally, initial results (currently being further elaborated upon) support a preventive action of both environmental treatments on age-related impairments in learning a spatial, water maze task as well as on hippocampal neuronal atrophy

Acquisition. --- Age-related impairments. --- Amphetamine. --- Avoidance. --- Behavior. --- Central amygdala. --- Early stimulation. --- Elevated plus-maze. --- Emotionality. --- Enrichment. --- Environmental enrichment. --- Ethanol. --- Female rats. --- Handling. --- Hippocampal. --- Impairments. --- Increase. --- Increases. --- Learning. --- Locomotor-activity. --- Low-avoidance rats. --- Neonatal handling. --- Neonatal. --- Neuroendocrine traits. --- Neuronal. --- Novelty-seeking. --- Novelty. --- Perinatal flumazenil. --- Physiological. --- Preference. --- Protein-kinase-c. --- Rat. --- Rats. --- Review. --- Roman high-and low-avoidance rats,reactivity to stress,anxiety,novelty/reward seeking,neonatal handling,enriched environment,age-related deficit,spatial learning,hippocampus. --- Seeking. --- Sensation-seeking. --- Sensitivity. --- Shock. --- Sign. --- Spatial. --- Stressor. --- Stressors. --- Task. --- Tasks. --- Treatment.