Time course and permeation of synaptic AMPA receptors in cochlear nuclear neurons correlate with input

AMPA receptors mediate rapid glutamatergic synaptic transmission. In the ma mmalian cochlear nuclei, neurons receive excitatory input from either audit ory nerve fibers, parallel fibers, or both fiber systems. The functional co rrelates of differences in the source of input were examined by recording A MPA receptor-mediated, miniature EPSCs (mEPSCs) in whole-cell voltage-clamp mode from identified neurons. Bushy, octopus, and T-stellate cells of the ventral cochlear nucleus (VCN) and tuberculoventral cells of the dorsal coc hlear nucleus (DCN) receive most of their excitatory input from the auditor y nerve; fusiform cells receive excitatory inputs from both the auditory ne rve and parallel fibers; cartwheel cells receive excitatory input from para llel fibers alone. mEPSCs from bushy, octopus, T-stellate, and tuberculoven tral cells had significantly faster decay time constants (0.35-0.40 msec) t han did those from fusiform and cartwheel cells (1.32-1.79 msec). Some fusi form cells had two populations of mEPSCs with distinct time courses. mEPSCs in cells with auditory nerve input alone were inhibited by philanthotoxin, a blocker of calcium-permeable AMPA receptors, whereas mEPSCs in cells wit h parallel fiber input were not. Thus AMPA receptors postsynaptic to the au ditory nerve differ from those postsynaptic to parallel fibers both in chan nel-gating kinetics and in their permeability to calcium. These results con firm the conclusion that synaptic AMPA receptors are specialized according to the source of input (Hunter et al., 1993; Rubio and Wenthold, 1997; Wang et al., 1998).