Group I metabotropic glutamate receptors mGluR1 alpha and mGluR5a: Localization in both synaptic layers of the rat retina

We examined the distribution of the group I metabotropic glutamate receptors, mGluR1 alpha and mGluR5a, in the adult rat retina and during postnatal development using receptor-specific antisera. In contrast to the restricted localization of group II and group III mGluRs to either the outer plexiform layer (OPL) or the inner plexiform layer (IPL), group I mGluRs are present in both synaptic layers in the rat retina. Double-labeling experiments and electron microscopy showed that in the OPL the two receptors are localized on the dendritic tips of bipolar cells postsynaptic to photoreceptor terminals. In the IPL the two mGluRs are localized on amacrine cell processes postsynaptic to bipolar cell terminals. These results suggest that group I mGluRs are involved in synaptic processing in both plexiform layers and in both the scotopic and photopic pathways in the rat retina. We propose that mGluR1 alpha and mGluR5a play an important modulatory role in the responses of retinal neurons to inhibitory and excitatory neurotransmitters.