Spectral and temporal coding in the ventral cochlear nucleus and the role of inhibition

Two major classes of projecting neurons in the ventral cochlear nucleus (VCN)—the bushy cell and the stellate cell—exhibit distinct spectral and temporal coding characteristics. While some of these distinctions can be attributed to auditory nerve fiber synapses or membrane properties, others are likely caused by differences in inhibitory input. This work examines the role of inhibitory neurotransmitters in shaping the response properties of VCN primarylike and chopper units—the physiological correlates of the bushy and stellate cells, respectively.;Spectral and temporal coding properties were assessed under pharmacologically normal conditions and during the presence of an antagonist to inhibitory neurotransmitter receptors. During disruption of glycinergic inhibition, chopper units displayed increased excitatory gain only at frequencies that were excitatory in the normal condition, suggesting that tuning of excitation and glycinergic inhibition are matched. In contrast, disruption of GABAergic inhibition does not affect spectral tuning, although a fixed rate increase was observed during stimulus presentations. The absence of tuning changes suggests GABA broadly modulates baseline firing rates, perhaps as a means of preventing saturation and increasing the range of levels over which units remains sensitive to spectral changes. During disruption of glycinergic inhibition, primarylike units typically exhibited only modest rate changes, if any. Low rate changes coupled with sparse sampling prohibited any meaningful analysis of inhibitory effects on spectral coding in primarylike units.;Temporal envelope coding properties of chopper units were similarly assessed under functional and disrupted inhibitory conditions. Under functional conditions inhibitory onset occurred 3-4 ms after excitatory onset and patterns of oscillation with periods of 7-8 ms were apparent. These temporal characteristics are reflected in temporal modulation transfer functions (TMTFs) with band-pass shapes centered near 150 Hz. During disruption of glycinergic inhibition, TMTFs were flattened and inhibitory onset was delayed and reduced. The temporal oscillation patterns were abolished. No clear temporal changes were observed during disruption of GABAergic inhibition. Band-pass TMTFs in chopper units appear to be a consequence of delayed inhibitory onset, which occurs with one or two synaptic delays. Patterns of oscillation suggest this inhibition may be from a feedback rather than feedforward source. A single primarylike unit was assessed during disruption of glycinergic inhibition. No notable change in temporal envelope coding was observed. A small inhibitory component occurring immediately after excitatory onset is likely attributable to the unit's refractory period.