Membrane Properties and Firing Patterns of Inferior Colliculus Neurons: An In Vivo Patch-Clamp Study in Rodents

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Abstract
The inferior colliculus (IC) is a large auditory nucleus in the midbrain, which is a nearly obligatory relay center for ascending auditory projections. We made in vivo whole cell patch-clamp recordings of IC cells in young-adult anesthetized C57/Bl6 mice and Wistar rats to characterize their membrane properties and spontaneous inputs. We observed spikelets in both rat (18%) and mouse (13%) IC neurons, suggesting that IC neurons may be connected by electrical synapses. In many cells, spontaneous postsynaptic potentials were sufficiently large to contribute to spike irregularity. Cells differed considerably in the number of simultaneous spontaneous postsynaptic potentials that would be needed to trigger an action potential. Depolarizing and hyperpolarizing current injections showed six different types of firing patterns: buildup, accelerating, burst-onset, burst-sustained, sustained, and accommodating. Their relative frequencies were similar in both species. In mice, about half of the cells showed a clear depolarizing sag, suggesting that they have the hyperpolarization-activated current I h. This sag was observed more often in burst and in accommodating cells than in buildup, accelerating, or sustained neurons. Cells with I h had a significantly more depolarized resting membrane potential. They were more likely to fire rebound spikes and generally showed long-lasting afterhyperpolarizations following long depolarizations. We therefore suggest a separate functional role for I h.