Granule cells of the internal granule layer have increased expression of GABA(A) receptor beta 2/beta 3 subunits

We have examined the expression of GABA(A) receptor beta 2/beta 3 subunits from postnatal day 4 (P4) to P23. beta 2/beta 3 subunits are not detected in premigratory granule cells. Expression of beta 2/beta 3 was also low in granule cells found in the internal granule layers (IGLs) of P7 and P10 cerebella. However, between P10 and P16, the levels of beta 2/beta 3 increase substantially and reach high levels at P16-23. Because granule cells continue to migrate from the external granule layer (EGL) into the IGL after P10, this increase in the number of cells and the intensity of beta 2/beta 3 expression could be the result of a second wave of granule cells expressing beta 2/beta 3 that have migrated from the EGL. To test this hypothesis, migrating granule cells after P10 were eliminated by gamma-irradiation. Despite elimination of migrating granule cells, beta2/3 expression remained high in the IGLs of P16 irradiated animals similar to that observed in non-irradiated controls, suggesting that the increase of beta 2/beta 3 is not due to the arrival of a new population of granule cells expressing GABA(A) receptors. To explore the possibility that the increase of beta2/beta 3 is triggered by synaptic activity, we cultured P10 cerebellar sections, free of mossy fiber inputs, for 6 days in vitro. As observed in vivo, beta 2/3 expression in the IGL of cultured slices continued to increase, suggesting that beta 2/3 expression could be triggered without synaptic inputs from mossy fibers.