GluN2D-containing NMDA receptors enhance temporal integration in VIP neurons in the inferior colliculus

Along the ascending auditory pathway, there is a broad shift from temporal coding, which is common in the lower auditory brainstem, to rate coding, which predominates in auditory cortex. This temporal-to-rate transition is particularly prominent in the inferior colliculus (IC), the midbrain hub of the auditory system, but the mechanisms that govern how individual IC neurons integrate information across time remain largely unknown. Here, we report the widespread expression of GluN2C and GluN2D mRNA in IC neurons. GluN2C/D-containing NMDA receptors are relatively insensitive to voltage-dependent Mg(2+) block, and thus can activate at resting membrane potential. Using in situ hybridization and pharmacology, we show that VIP neurons in the IC express GluN2D-containing NMDA receptors that are activatable by ascending input from T-stellate cells in the anteroventral cochlear nucleus and commissural inputs from the contralateral IC. In addition, GluN2D-containing receptors have much slower kinetics than other NMDA receptors, and we found that GluN2D-containing receptors facilitate temporal summation in VIP neurons by prolonging the time window for synaptic integration. These results suggest that GluN2C/D-containing NMDA receptors support the shift from temporal to rate coding in the auditory system by facilitating the integration of ascending inputs.