Large GABAergic neurons form a distinct subclass within the mouse dorsal cortex of the inferior colliculus with respect to intrinsic properties, synaptic inputs, sound responses, and projections

Neurons in the inferior colliculus (IC) show a remarkable diversity in their responses to sound, but it has been difficult to relate these responses to their morphology. Large cells, which are found in all subdivisions of the IC, may form an exception. We found that large neurons of the mouse dorsal cortex of the IC were GABAergic and were contacted by vesicular glutamate transporter 2-containing somatic terminals, as previously observed for the rat IC. Large cells, which were targeted under two-photon guidance, typically had a low input resistance in comparison with the other cells in the dorsal cortex of the IC. Large cells received short-latency excitatory inputs and had short first-spike latencies. These excitatory inputs were often followed by long-latency inhibitory postsynaptic potentials. In four cells, it was possible to reconstruct the ascending axon following labeling with biocytin. We found evidence that they projected to both the ventral and the dorsal divisions of the medial geniculate body of the thalamus, but they also branched off large collaterals while passing through the brachium of the IC. Our data indicate that, owing to their somatic glutamatergic inputs, large GABAergic tectothalamic projection neurons can generate short-latency, well-timed, feed-forward inhibition, which affects not only the thalamus, but also other ascending nuclei. Their remarkably homogeneous properties, which generally differed from those of the other cells in the dorsal cortex of the IC, suggest that large neurons form a distinct subclass within the dorsal cortex of the IC.

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