Spectral integration by type II interneurons in dorsal cochlear nucleus

The type II unit is a prominent inhibitory interneuron in the dorsal cochle ar nucleus (DCN), most likely recorded from vertical cells. Type II units a re characterized by low rates of spontaneous activity, weak responses to br oadband noise, and vigorous, narrowly tuned responses to tones. The weak re sponses of type II units to broadband stimuli are unusual for neurons in th e lower auditory system and suggest that these units receive strong inhibit ory inputs, most likely from onset-C neurons of the ventral cochlear nucleu s. The question of the definition of type II units is considered here; the characteristics Listed in the preceding text define a homogeneous type II g roup, but the boundary between this group and other low spontaneous rate ne urons in DCN (type VIII units) is not yet clear. Type II units in decerebra te cats were studied using a two-tone paradigm to map inhibitory responses to tones and using noisebands of varying width to study the inhibitory proc esses evoked by broadband stimuli. Iontophoresis of bicuculline and strychn ine and comparisons of two-tone responses between type II units and auditor y nerve fibers were used to differentiate inhibitory processes occurring ne ar the cell from two-tone suppression in the cochlea. For type II units, a significant inhibitory region is always seen with two-tone stimuli; the ban dwidth of this region corresponds roughly to the previously reported excita tory bandwidth of onset-C neurons. Bandwidth widening experiments with nois ebands show a monotonic decline in response as the bandwidth increases; the se data are interpreted as revealing strong inhibitory inputs with properti es more like onset-C neurons than any other response type in the lower audi tory system. Consistent with these properties, iontophoresis of inhibitory antagonists produces a large increase in discharge rate to broadband noise, making tone and noise responses nearly equal.