Opioid antagonist-induced modulation of cerebral and hippocampal development: histological and morphometric studies

The role of endogenous opioid systems in preweaning cerebral and hippocampal development was explored in rats utilizing naltrexone, a potent opioid antagonist. Sprague-Dawley rats were given daily injections (s.c.) of either 1 or 50 mg/kg naltrexone to invoke a temporary or complete blockade, respectively, of opioid receptors throughout the first 3 weeks of postnatal life; animals injected with sterile water served as controls. At weaning (Day 21), macroscopic, morphometric, and histological assessments were undertaken. In general, 50 mg/kg naltrexone had a stimulatory action on brain development, whereas 1 mg/kg naltrexone had an inhibitory influence. In most cases, both males and females were affected comparably. Opioid antagonist action was especially directed at cellular and tissue differentiation, with marked changes in macroscopic and areal dimensions and histotypic organization observed in the cerebrum. A prominent effect on the cerebrum of the 1 mg/kg naltrexone group was a substantial increase in packing density of the neural cells, reflecting a reduced area for accommodating neural elements. Changes in the hippocampus were largely restricted to the 1 mg/kg group. However, the number of granule cells was increased in the dentate gyrus of the 50 mg/kg group, suggesting that opioid receptor blockade affects cell types undergoing postnatal proliferation. Cellular elements derived prior to naltrexone treatment (e.g., pyramidal neurons) were capable of being influenced in only differentiative capacity. Our results show that endogenous opioids are natural trophic factors in brain development and provide evidence for the crucial role of endogenous opioid-opioid receptor interaction in neuro-ontogeny.