Immunoreactivity for the group III receptor subtype mGluR4a in the visual layers of the rat superior colliculus

Several studies indicate that metabotropic glutamate receptors (mGluRs) participate in the transmission of visual stimuli in optic layers of the superior colliculus (SC). We examined the cellular and subcellular distribution of the group III mGluR4a in superficial layers of the rat SC by means of a specific antiserum and a preembedding immunogold method for electron microscopy. Deposits of mGluR4a immunoparticles were mostly observed on presynaptic membranes of large synaptic terminals, which made asymmetrical synapses and contained abundant spherical, clear synaptic vesicles and numerous electron translucent mitochondria. These characteristic ultrastructural features correspond to retinocollicular synaptic terminals. Also, chains of synaptic retinal terminals along dendrites were labeled for mGluR4a. About 70% of morphologically identified retinal terminals were mGluR4a immunopositive. Furthermore, mGluR4a immunoreactivity in SC greatly disappeared following retinal ablation. About 28% of cortical terminals identified by anterograde tracing showed mGluR4a labeling, whereas only 2% of collicular GABAergic profiles were labeled for mGluR4a. These results reveal that retinal terminals are the major contributors to the mGluR4a immunoreactivity observed in the superior collicular circuitry.