Differential postsynaptic distribution of GluRs 1-4 on cartwheel and octopus cell somata in the gerbil cochlear nucleus.

Differences were demonstrated in the distribution of glutamate receptors (GluR) 1, 2, 2/3 and 4 postsynaptic immunoreactivity (PSIR) on the somata of cartwheel and octopus cells in the adult gerbil cochlear nucleus (CN). Montages of electron micrographs of cartwheel and octopus cells immunoreacted with antibodies to GluR 1, 2, 2/3 and 4 were prepared. The number of synaptic terminals with PSIR were counted on all cells for each antibody, normalized to the total length of somatic surface analyzed. The density of terminals apposed to PSIR on octopus cells was similar for the antibodies GluR1, 2/3 and 4, but significantly less for GluR2. On cartwheel somata the numbers of terminals apposed to immunoreactive postsynaptic specializations with GluR1, 2, 2/3 or 4 were not significantly different from each other. The density of terminals apposed to GluR2/3 and 4 positive postsynaptic specializations was significantly less on cartwheel cells than on octopus somata. The data suggest that the decreased presence of the GluR2 subunit, which confers calcium impermeability to the assembled receptor and slower gating kinetics to receptors with a high GluR4 content, is the major difference in the AMPA receptors on the somata of these cell types. The presence on cartwheel cells of a majority of AMPA receptors which contain GluR2 may account for the fact that cartwheel cells respond to shocks to the auditory nerve with 100 ms excitatory postsynaptic potentials (EPSPs), while octopus cells, most of whose AMPA receptors lack GluR2, respond with 1 ms EPSPs.