Granule neurons in cerebellum express distinct splice variants of the inositol trisphosphate receptor that are modulated by calcium

Primary cultures of granule cells ( GC) from rat cerebellar cortex were used to determine whether bioelectric activity, via a Ca2+/calmodulin-dependent kinase (CaMK) signaling cascade, modulates expression and exon selection in the inositol trisphosphate receptor type 1 (IP(3)R1). IP(3)R1 contains or lacks three exons (S1, S2, and S3) that are regulated in a regionally and temporally specific manner. The neuronal, or long, form of IP(3)R1 is distinguished from peripheral tissues by inclusion of the S2 exon. Although previous studies indicated that IP(3)R1 are undetectable in the cerebellar granular layer in vivo, receptor protein and mRNA are induced in cultured GC grown in medium supplemented with 25 mM KCl or NMDA, two trophic agents that promote long-term survival, compared with GC grown in 5 mM KCl. IP(3)R1 induction in response to 25 mM KCl or NMDA is attenuated by coaddition of voltage-sensitive calcium channel or NMDA receptor antagonists, respectively. Actinomycin D, CaMK, and calcineurin antagonists likewise suppress induction. Unlike the major variants of IP(3)R1 in Purkinje neurons, which lack S1 and S3, GC grown with trophic agents express mRNA containing these exons. Both neuronal types contain S2. Evidence obtained using mutant mice with Purkinje cell lesions, laser-microdissected GC neurons from slices, and explant cultures indicates that GC predominantly express the S1-containing variant of IP(3)R1 in vivo.