The response properties of single neurons in the dorsal cochlear nucleus (DCN) of unanesthetized decerebrate cats can be divided into 2 general classes: type IV cells have broad response areas that are predominantly inhibitory; type II/III cells have V-shaped central excitatory areas that are usually flanked by inhibitory side bands. Previous results had suggested that those type II/III cells with low rates of spontaneous activity are inhibitory interneurons terminating on type IV cells. In this study, pairs of neurons in the DCN were recorded simultaneously with a single microelectrode. The spike trains of the 2 neurons of a pair were separated and examined for signs of interaction using cross-correlation techniques. Cross-correlograms were obtained for 17 pairs consisting of a type II/III unit and a type IV unit. In 12 of these cases there was an inhibitory trough in the cross-correlogram that corresponded to a decrease in the type IV's discharge rate immediately following spikes in the type II/III cells. Four additional pairs had no significant feature in their cross-correlogram and one pair had a complex cross-correlogram with no inhibitory trough. In type II/III type IV pairs with inhibitory troughs, the excitatory area of the type II/III unit's response map was largely within the inhibitory region of the type IV unit's response map; the maximum overlap of excitatory areas in this group was 6 dB. On the other hand, 4 of the 5 pairs in which no inhibitory trough was observed had greater overlap of excitatory areas. In 11 of the 12 pairs with an inhibitory trough, the type II/III unit had little or no spontaneous activity (less than 2.5 spikes/s). In 1 case the response map of a type IV cell was observed before and after the disappearance due to injury of a simultaneously recorded type II/III neuron. The portion of the inhibitory region of the type IV6s response map originally overlapped by the type II/III cell's excitatory area was partially transformed into an excitatory region following loss of the type II/III unit. Cross-correlograms were obtained for 20 pairs consisting of 2 type IV units. In half the cases there were no significant features in the cross-correlograms. In 7 cases the cross-correlograms had a pair of excitatory peaks in the vicinity of the origin, which are most likely due to the effects of shared input. The remaining 3 cases had complex cross-correlograms. The response maps of the 2 units in all type IV-type IV pairs overlapped extensively. These results are consistent with the idea that the type II/III responses are recorded from interneurons in the DCN that contribute inhibitory terminals to cells with type IV response properties.
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