Mechanisms of vestibular compensation

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To clarify which types of vestibular nucleus neurons change their activity during the period of the vestibular compensation, we must know electrophysiological membrane properties of vestibular neurons in intact rats. In our previous study using the whole-cell patch clamp technique in rat brainstem slices, we characterized three membrane properties of neurons in the medial vestibular nucleus (MVN) : afterhyperpolarization (AHP) profiles, firing patterns, and response patterns to hyperpolarizing current pulses. However, the membrane properties have not been ascertained in vivo and their functional significance has not been clarified. To address these issues, we applied the whole-cell patch clamp recording method to in vivo preparations of young adult rats and investigated the membrane and spontaneous discharge properties. We found three AHP profiles, three firing patterns, and three response patterns to hyperpolarizing current pulses in MVN neurons in vivo, which were characterized in our previous in vitro study. This indicates that the three membrane properties are applicable in vivo. The analysis of discharge properties of MVN neurons both in vivo and in vitro revealed that the coefficient of variation of interspike intervals was smaller in neurons exhibiting AHP with a slow component [AHP(s+)] than in those exhibiting AHP without a slow component [AHP(s-)] and AUP with afterdepolarization followed by a slow component [AHP(s+) with ADP]. The result indicates that neurons exhibiting AHP(s+) are regular discharging neurons, whereas neurons exhibiting AHP(s-) and AHP(s+) with ADP are irregular discharging neurons.