The role of endogenous opioid systems (endogenous opioids and opioid receptors) in neuronal development was examined in 10‐ and 21‐day‐old rats by utilizing an opioid antagonist (naltrexone) paradigm. Throughout the first 3 weeks of life, Sprague‐Dawley rats were given daily subcutaneous injections of either 50 mg/kg naltrexone, a dosage that invoked a complete (24 hours/day) receptor blockade, or 1 mg/kg naltrexone, a dosage which intermittently blocked (4–6 hours/day) opioid receptors and exacerbated opioid action; animals injected with sterile water served as controls. Pyramidal cells from the frontoparietal cortex (layer III) and hippocampal field CA1, and cerebellar Purkinje cells, were impregnated by using the Golgi‐Kopsch method; total and mean dendrite segment length, branch frequency, and spine concentration were analyzed morphometrically. Perturbations of endogenous opioid systems caused region‐dependent alterations in dendrite complexity and/or spine concentration in all brain areas. Continuous opioid receptor blockade resulted in dramatic increases in dendrite and/or spine elaboration compared to controls at 10 days in all brain regions; however, these increases were only evident in the hippocampus at 21 days. With intermittent blockade, dendrite and/or spine growth were often subnormal, being predominant at day 21. Our results indicate that endogenous opioid systems are critical regulators of neuronal differentiation, and they control growth through an inhibitory mechanism. Considering previous findings demonstrating that neurobehavioral ontogeny is dependent on endogenous opioid‐opioid receptor interactions, the present results suggest an opioid‐dependent, structure‐function relationship between neuronal and behavioral maturation.
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