Regeneration of 5-HT fibers in hippocampal heterotopia of methylazoxymethanol-induced micrencephalic rats after neonatal 5,7-DHT injection

Effects of methylazoxymethanol-induced micrencephaly on parvalbumin-positive GABAergic interneurons in the rat rostral basolateral amygdala

The amygdala plays a crucial role in anxiety-related behavior and various neuropsychiatric disorders. The offspring of dams, administered methylazoxymethanol acetate (MAM) intraperitoneally at gestational day 15, exhibit micrencephaly and anxiety-related behavior, such as hyperactivity in rearing and crossing behavior, alongside a distinct Fos expression profile in the basolateral (BLA) and central amygdala. However, the histochemical underpinnings of these changes remain to be elucidated. To determine the histochemical alterations in MAM-induced model rats, we performed Nissl staining, immunohistochemistry for parvalbumin (PV) or calbindin (Calb), and immunohistochemistry for PV in conjunction with in situ hybridization for glutamate decarboxylase (GAD). We compared immunoreactivity in the BLA between normal and MAM-induced model rats and observed a significant decrease in the number of PV-positive neurons in MAM-induced model rats; however, no significant differences in the number of Nissl- and Calb-positive neurons were observed. We did not detect any significant between-group differences with regards to the effects of environmental enrichment on the number of PV-positive neurons in the BLA. Double-labeling for GAD and PV revealed that many PV-positive neurons colocalized with digoxigenin-GAD65/67 signals. In addition, GAD/PV double-positive neurons and the total number of GAD-positive neurons in the BLA were lower in the MAM-induced model rats. These results indicate that histochemical alterations observed in the BLA of the MAM-induced model rats may attribute to an aberrant GABAergic inhibitory system.

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.