The inferior colliculus (IC) is a large midbrain nucleus that integrates inputs from many auditory brainstem and cortical structures. Despite its prominent role in auditory processing, the various cell types and their connections within the IC are not well characterized. To further separate GABAergic and non-GABAergic neuron types according to their physiological properties, we used a mouse model that expresses channelrhodopsin and EYFP in all GABAergic neurons and allows identification of

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... rgic cells by light stimulation. Neuron types were classified upon electrophysiological measurements of the hyperpolarizing-activated current (Ih) in acute brain slices of young adult mice. All GABAergic neurons from our sample displayed slow activating Ih with moderate amplitudes, whereas a subset of excitatory neurons showed fast activating Ih with large amplitudes. This is in agreement with our finding that immunoreactivity against the fast gating HCN1 channel was present around excitatory neurons, whereas the slow gating HCN4 channel was found perisomatically around most inhibitory neurons. Ih properties and neurotransmitter types were correlated with firing patterns to depolarizing current pulses. All GABAergic neurons displayed adapting firing patterns very similar to the majority of glutamatergic neurons. About 15% of the glutamatergic neurons showed an onset spiking pattern, always in combination with large and fast Ih. We conclude that HCN channel subtypes are differentially distributed in IC neuron types and correlate with neurotransmitter type and firing pattern. In contrast to many other brain regions, membrane properties and firing patterns were similar in GABAergic neurons and about one third of the excitatory neurons.