Mouse Cerebellar Granule Cell Differentiation: Electrical Activity Regulates the GABAAReceptor α6 Subunit Gene

GABAAreceptor α6 subunit gene expression marks cerebellar granule cell maturation. To study this process, we used the Δα6lacZ mouse line, which has a lacZ reporter inserted into the α6 gene. At early stages of postnatal cerebellar development, α6-lacZ expression is mosaic; expression starts at postnatal day 5 in lobules 9 and 10, and α6-lacZ is switched on inside-out, appearing first in the deepest postmigratory granule cells. We looked for factors regulating this expression in cell culture. Membrane depolarization correlates inversely with α6-lacZ expression: granule cells grown in 25 mm[K+]ofor 11–15 d do not express the α6 gene, whereas cultures grown for the same period in 5 mm[K+]odo. This is influenced by a critical early period: culturing for ≥3 d in 25 mm[K+]ocurtails the ability to induce the α6 gene on transfer to 5 mm[K+]o. If the cells start in 5 mm[K+]o, however, they still express the α6-lacZ gene in 25 mm[K+]o. In contrast to granule cells grown in 5 mm[K+]o, cells cultured in 25 mm[K+]oexhibit no action potentials, mEPSCs, or mIPSCs. In chronic 5 mm[K+]o, factors may therefore be released that induce α6. Blockade of ionotropic and metabotropic GABA and glutamate receptors or L-, N-, and P/Q-type Ca2+channels did not prevent α6-lacZ expression, but inhibition of action potentials with tetrodotoxin blocked expression in a subpopulation of cells.