The messenger RNAs encoding metabotropic glutamate receptor subtypes are expressed in different neuronal subpopulations of the rat suprachiasmatic nucleus.

Glutamate is the principal transmitter of retinal projections to the rodent suprachiasmatic nucleus, a circadian clock synchronized with the light-dark cycle through the activation of glutamate receptors of the ionotropic type. In vitro, an intracellular mobilization of calcium can be induced by glutamate within cells of the suprachiasmatic nucleus maintained in a calcium-free medium, suggesting a participation of metabotropic glutamate receptors coupled to phospholipase C. Using in situ hybridization histochemistry, we examined the expression of messenger RNAs encoding the mGluR1 and mGluR5 subtypes of metabotropic glutamate receptors in the suprachiasmatic nucleus of the adult rat and during postnatal development. In the adult, mGluR1 was expressed in a small subset of neurons segregated caudally within the ventrolateral subdivision of the nucleus, while mGluR5 was mainly expressed in ventrolateral neurons within the middle third of the nucleus. Both subtypes were expressed in morphologically similar small cells, but mGluR5 was also solely expressed in a small population of larger neurons located at the dorsalmost aspect of the ventrolateral subdivision. In addition, with mGluR1 probe silver grain clusters exhibiting a grain density close but below the significant level were observed throughout the ventrolateral subdivision of the nucleus. At birth, mGluR1 and mGluR5 were similarly expressed throughout the caudal half of the nucleus. The expression of mGluR1 increased during early postnatal development and exhibited an adult pattern at postnatal day 21. The expression of mGluR5 increased from postnatal day 7 and reached the adult pattern at postnatal day 45. These observations suggest that each subtype of metabotropic glutamate receptor coupled to phospholipase C underlies specific roles within the rat suprachiasmatic nucleus during postnatal development and in the adult. In the adult, ionotropic and metabotropic receptors likely co-expressed within neuronal subsets located in the retinal terminal field may have interactive and/or additive effects on intracellular calcium concentration. Metabotropic receptors may thus participate in the mediation of photic information conveyed to a subset of neurons. During postnatal development, metabotropic receptors may play a role in the maturation of glutamatergic synapses associated with the retinal input.