Pre- and Postsynaptic Sites of Action of mGluR8a in the mammalian retina

The distribution of the most recently identified metabotropic glutamate receptor type 8a (mGluR8a) in the mammalian retina is unknown, but it is known to function as a presynaptic autoreceptor in rod photoreceptors. In this study, the localization of mGluR8a in the adult rat, mouse, and rabbit retina and during postnatal development of the rat retina was analyzed.

mGluR8a immunoreactivity was detected using immunocytochemistry and light and electron microscopy.

mGluR8a expression was found in both synaptic layers and in extrasynaptic locations, predominantly on the somata of ganglion, amacrine, and horizontal cells. This distribution pattern is different from the localization patterns of the other members of group III mGluRs in the mammalian retina, which are restricted to either the outer plexiform layer (OPL) or the inner plexiform layer (IPL). Analysis of the expression of mGluR8a at the ultrastructural level in rat retina showed that the receptor is localized pre- and postsynaptically in the OPL and postsynaptically in the IPL. During postnatal development, mGluR8a was expressed at synapses in parallel with synapse formation, but appeared earlier at extrasynaptic sites.

These results suggest that mGluR8a is involved in synaptic processing in both plexiform layers and in both the scotopic and photopic pathways in the mammalian retina. The authors propose that, depending on its localization-pre- versus postsynaptic-mGluR8a modulates the release of L-glutamate by photoreceptors and the responses of retinal neurons to inhibitory and excitatory neurotransmitters, respectively. Furthermore, mGluR8a may have regulatory functions during neuronal development of the mammalian retina.