Two Distinct Types of Inhibition Mediated by Cartwheel Cells in the Dorsal

35 Individual neurons have been shown to exhibit target cell specific synaptic 36 function in several brain areas. The time course of the post synaptic conductances (PSCs) 37 strongly influences the dynamics of local neural networks. Cartwheel cells (CWCs) are 38 the most numerous inhibitory interneurons in the dorsal cochlear nucleus (DCN). They 39 are excited by parallel fiber synapses, which carry polysensory information, and in turn 40 inhibit other CWCs and the main projection neurons of the DCN, pyramidal cells (PCs). 41 CWCs have been implicated in “context-dependent” inhibition, producing either 42 depolarizing (other CWCs) or hyperpolarizing (PCs) post synaptic potentials. In the 43 present study, we used paired whole cell recordings to examine target-dependent 44 inhibition from CWCs in neonatal rat DCN slices. We found that CWC IPSPs onto PCs 45 are large (1.3 mV) and brief (half-width=11.8 ms) while CWC IPSPs onto other CWCs 46 are small (0.2 mV) and slow (half-width= 36.8 ms). Evoked IPSPs between CWCs 47 exhibit paired-pulse facilitation, while CWC IPSPs onto PCs exhibit paired-pulse 48 depression. Perforated patch recordings showed that spontaneous IPSPs in CWCs are 49 hyperpolarizing at rest, with a mean estimated reversal potential of -67 mV. Spontaneous 50 IPSCs were smaller and lasted longer in CWCs than in PCs, suggesting that the kinetics 51 of the receptors are different in the two cell types. These results reveal that CWCs play a 52 dual role in the DCN. The CWC-CWC network interactions are slow and sensitive to the 53 average rate of CWC firing, whereas the CWC-PC network is fast and sensitive to 54 transient changes in CWC firing. 55