AMPA and NMDA receptors regulate responses of neurons in the rat's inferior colliculus

The contribution of N-methyl-D-aspartate (NMDA) and AMPA receptors to audit ory responses in the rat's inferior colliculus was examined by recording si ngle-unit activity before, during, and after local iontophoretic applicatio n of receptor-specific antagonists. Tone bursts and sinusoidal amplitude mo dulated sounds were presented to one ear, and recordings were made from the contralateral central nucleus of inferior colliculus (ICC). The receptor s pecific antagonists, (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) for NMDA receptors and 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benz o[f]quinoxaline-7-sulfonamide (NBQX) for AMPA receptors, were released at t he recording site through a multi-barreled pipette. For most neurons, eithe r CPP or NBQX alone resulted in a reversible reduction in the number of act ion potentials evoked by tonal stimulation. For neurons with an onset respo nse pattern, NBQX either completely eliminated or greatly reduced the numbe r of action potentials. CPP also reduced the number of action potentials bu t had a less pronounced effect than NBQX. For neurons with a sustained firi ng pattern, NBQX reduced the total number of action potentials, but had a p referential effect on the early part (first 10-20 ms) of the response. CPP also resulted in a reduction in the total number of action potentials, but had a more pronounced effect on the later part (>20 ms) of the response. Th ese results indicate that both AMPA and NMDA receptors contribute to sound evoked excitatory responses in the ICC. They have a selective influence on early and late components of tone-evoked responses. Both receptor types are involved in generating excitatory responses across a wide range of sound p ressure levels as indicated by rate level functions obtained before and dur ing drug application. In addition, both CPP and NBQX reduced responses to s inusoidal amplitude modulated sounds. The synchrony of firing to the modula tion envelope as measured by vector strength at different rates of modulati on was not greatly affected by either CPP or NBQX in spite of the decrease in firing rate.