Functional expression of metabotropic glutamate receptor type 5 in rat pinealocytes

Mammalian pinealocytes, endocrine cells for melatonin, express class II metabotropic glutamate receptors (mGluRs), which are involved in negative regulation of melatonin synthesis through an inhibitory cAMP cascade. We investigated whether mGluRs other than class II receptors are expressed in rat pinealocytes. RT-PCR analysis and Northern blotting indicated that the mRNA of mGluR5, a class I receptor, was present in pineal glands. Quisqualate and 1-aminocyclopentane-1,3-dicarboxylate (1S,3R-ACPD), class I receptor agonists, increased the intracellular [Ca²⁺] of fura-2 loaded cultured pinealocytes in the absence of extracellular Ca²⁺ which is blocked by methylcarboxyphenylglycine, a class I receptor antagonist. These results suggest that mGluR5 is functionally expressed in pinealocytes and triggers Ca²⁺ efflux from intracellular stores.