Improved learning in microencephalic rats.

ABSTRACT Environmental enrichment (EE) facilitates recovery from behavioral abnormalities and spatial memory disabilities in several neurological disease models. Exposure to EE improves spatial memory acquisition and enhances the survival of newly generated cells in the dentate gyri of adult rodents. However, the effects of EE on spatial learning and neurogenesis in the methylazoxymethanol acetate-induced microencephalic rat have not been investigated. Depletion of serotonin in the rat hippocampus is known to influence spatial memory and adult neurogenesis, suggesting a role for serotonin in these processes. To confirm this hypothesis, male methylazoxymethanol acetate-induced microencephalic rats were exposed to EE or conventional housing after weaning; half of these rats further received intracisternal 5,7-dihydroxytryptamine on postnatal day 3, to induce long-lasting depletion of serotonin. As adults, these microencephalic rats were observed using the Morris water maze test and examined for hippocampal neurogenesis. EE alleviated the impairment of spatial memory acquisition and enhanced neurogenesis in the dentate gyri of adult microencephalic rats. Injection of 5,7-dihydroxytryptamine during the neonatal period caused pronounced reductions in hippocampal serotonin levels in these rats. Long-lasting depletion of serotonin eliminated the EE-induced alleviation of spatial memory acquisition and neurogenesis impairment in microencephalic rats. The present results suggest that EE alleviates spatial memory performance deficits in microencephalic rats and further indicate that serotonin might be involved in the underlying mechanisms through increased hippocampal neurogenesis. These data provide new insights into therapeutic interventions for individuals with human migration disorders associated with learning disabilities.