Modification of membrane excitability of neurons in the rat's dorsal cortex of the inferior colliculus by preceding hyperpolarization

Dec 25, 2007Neuroscience

How Previous Inhibition Changes Neuron Excitability in the Rat’s Sound Processing Cortex

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Abstract

Ninety-one percent of inferior colliculus dorsal cortex neurons displayed sustained firing during current injection.

Three types of neurons were identified: sustained-regular, sustained-adapting, and buildup.
Rebound depolarization and spikes after hyperpolarization occurred in 51.7% of sustained neurons but were absent in buildup neurons.
Preceding hyperpolarization altered membrane excitability in various ways, with some neurons showing suppressed or unchanged excitability.
Changes in first spike latency varied depending on the firing pattern: lengthened for buildup, shortened for pauser, and unchanged for sustained.
Rebound neurons experienced different firing rate changes depending on the transition of their firing patterns, with a consistent shortening of first spike latency.

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The inferior colliculus (IC) is among the largest nuclei in the central auditory system and is considered to be a major integration center in the auditory pathway. To understand how IC contributes to auditory processing, we investigated the effects of preceding hyperpolarization on membrane excitability and firing behavior of neurons located in the dorsal cortex of the inferior colliculus (ICD). We made whole-cell patch clamp recordings from ICD neurons (n=96) in rat brain slices. We classified ICD neurons into three types, i.e. sustained-regular, sustained-adapting and buildup, according to their responses to depolarizing current injection. Nearly 91% of the neurons had sustained firing throughout the period of current injection, showing either regular or adapting pattern. About 9% of the neurons exhibited a buildup pattern, in which sustained firing started after a long delay. Rebound depolarization and spikes after hyperpolarization were seen in 51.7% of the sustained neurons, but were not seen in buildup neurons. When depolarizing current was preceded by a hyperpolarizing current, various forms of the modification on membrane excitability were observed. For non-rebound neurons, the membrane excitability was either suppressed or unchanged after pre-hyperpolarization. The first spike latency lengthened in neurons whose firing changed to a buildup pattern, shortened in those whose firing changed to a pauser pattern, and remained unchanged in those whose discharge pattern remained sustained. For rebound neurons, the firing rate decreased in neurons whose firing pattern was changed to onset or pauser, increased in neurons whose firing was changed to adapting, or remained unchanged in neurons whose firing became irregular. The first spike latency was shortened in all the rebound cells. The results suggest that intrinsic membrane properties can play an important role in integration of excitatory and inhibitory inputs and thereby in determination of the output of ICD neurons.

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Modification of membrane excitability of neurons in the rat's dorsal cortex of the inferior colliculus by preceding hyperpolarization

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