Activity-dependent changes in "transplanted" cerebellar cultures.

Organotypic cerebellar cultures were used to assess the effects of increasing or blocking neuronal activity on circuit reconstruction in an in vitro transplantation model. Granule cells and oligodendrocytes were destroyed and astrocytes were functionally compromised by exposing newborn mouse-derived cerebellar explants to cytosine arabinoside for the first 5 days in vitro. Such cultures were "transplanted" at 9 days in vitro with granule cells and glia and maintained in standard nutrient medium; in medium with the GABA antagonist, picrotoxin, to increase neuronal activity; or with tetrodotoxin and elevated levels of magnesium to block neuronal activity. Transplanted cultures exposed to picrotoxin were not significantly different from control transplanted cultures. Transplanted cultures deprived of neuronal activity had reduced inhibitory synaptogenesis, greater persistence of heterotypical axospinous synapses, and hyperactive cortical spontaneous discharges after recovery from the blockade. Transplantation-induced changes that were not affected included myelination, reduction of sprouted Purkinje recurrent axon collaterals, astrocytic ensheathment of Purkinje cells, reduction of excess Purkinje cell axosomatic synapses, and formation of excitatory parallel fiber-Purkinje cell dendritic spine synapses. The results were consistent with previous studies indicating the necessity of neuronal activity for the full development of inhibitory circuitry, and suggested that neuronal activity is also necessary for the reconstruction of inhibitory circuitry after transplantation.