The NMDAR1 subunit of the N-methyl-D-aspartate receptor is localized at postsynaptic sites opposite both retinal and cortical terminals in the cat superior colliculus

The N-methyl-D-aspartate receptor (NMDAR) is an ionotropic glutamate recept or that is important in neurotransmission as well as in processes of synapt ic plasticity in the mammalian superior colliculus (SC). Despite the import ance of this receptor in synaptic transmission, there is as yet no evidence that demonstrates directly the synaptic localization of the NMDAR receptor in SC. We have used electron-microscope (EM) immunocytochemistry to locali ze the NMDAR1 subunit of this receptor protein and its association with sen sory afferents in the cat SC. Retinal synaptic terminals were identified by normal morphology and cortical synaptic terminals by degeneration after le sions of areas 17-18 of the visual cortex. At the light-microscope level, l abel was densest within the superficial gray and upper optic layers, but al so present in all other layers. Label was contained within cell bodies, den drites, and a few putative axons. At the EM level, antibody labeling was fo und along postsynaptic densifications and internalized within the cytoplasm of a variety of dendrites and some cell bodies. Postsynaptic profiles labe led by NMDAR1 included conventional dendrites and presynaptic dendrites whi ch contained pleomorphic synaptic vesicles and are known to be GABAergic. M any of the labeled postsynaptic densifications of both of these profile typ es received synaptic inputs from retinal or cortical terminals. Virtually n o NMDAR1 immunoreactivity was found on thin dendritic thorns or putative sp ines, even when these were postsynaptic to retinal or cortical terminals. I n summary, these results show that the NMDAR1 subunit is postsynaptic to bo th retinal and cortical afferents, which are known to be glutamatergic, and are consistent with physiological evidence showing that stimulation of eit her pathway can activate the NMDA receptor.