Abstract

The levels of several GABA_A receptor subunit mRNAs increase as cerebellar granule neurons migrate to their adult positions and receive excitatory mossy fiber input. Despite the temporal similarity of these increases in transcript expression in vivo, studies in cultured granule neurons demonstrated that the subunit mRNAs are differentially regulated. To address the possibility that neuronal activity regulates transcript expression, GABA_A receptor subunit mRNA levels were assessed in cultured granule neurons grown in chemically defined, serum-free medium containing either nondepolarizing (5 mM) or depolarizing (25 mm) KCl concentrations. Whereas the δ subunit mRNA was almost undetectable in cultures maintained in nondepolarizing medium, an eightfold increase occurred between days 2 and 4 in cultures grown in depolarizing medium. Furthermore, δ subunit transcript expression was reduced by 76 ± 6% when neurons in depolarizing medium were switched into nondepolarizing medium. The importance of depolarization in the initiation and maintenance of subunit transcript expression in granule neurons was selective for the GABA_A receptor δ subunit. These changes in transcript expression involved calcium entry through L-type calcium channels. Nifedipine treatment (1 μm) both reduced intracellular calcium and decreased δ subunit mRNA expression by 79 ± 4%. Furthermore, inhibition of Ca²⁺/calmodulin-dependent protein kinases (CaM kinases) by KN-62 (1 μm) also reduced δ subunit transcript expression. These studies demonstrate that KCl-induced depolarization, a condition that mimics the effects of neuronal activity, selectively modulates GABA_A receptor δ subunit mRNA expression through a pathway involving calcium entry and activation of a CaM kinase.