We have examined the distribution of metabotropic glutamate receptors
(mGluRs) 1 and 5 in the adult chicken retina using preembedding
immuno-electronmicroscopy. Immunoreactivity for mGluRs 1 and 5 was found
in both the outer plexiform layer (OPL) and the inner plexiform layer
(IPL). For mGluR1, OPL labeling was observed at cone pedicles and
horizontal and bipolar cell processes. In the IPL, mGluR1 labeling could
be found on bipolar cell terminals, as well as postsynaptic processes,
including amacrine cell processes. Neither presynaptic nor postsynaptic
elements were labeled at rod synapses. For mGluR5, OPL labeling was
associated with cone pedicles as well as bipolar and horizontal cell
processes. As for mGluR1, rod synapses were unlabeled. In the IPL,
labeling for mGluR5 was found on bipolar cell terminals and amacrine cell
processes. The presynaptic expression of these receptors in the OPL was
confirmed at the light level by double-labeling experiments with SV2. The
distributions of mGluRs 1 and 5 indicate that they have the potential to
regulate function in both synaptic layers. Furthermore, the similar
expression patterns for these two receptors indicate that they might be
co-expressed and thus have the potential to interact functionally.