Abstract
To determine whether neurosteroids suggested to be present in the brain act on MVN neurons, we electrophysiologically examined the effects of pregnenolone sulfate (PS), a neurosteroid, on the neuronal activity of the medial vestibular nucleus (MVN) in cats anesthetized with α-chloralose. Single neuronal activity in the MVN was extracellularly recorded using a glass-insulated silver wire microelectrode attached to a sevenbarreled micropipette. Each micropipette was filled with PS (3 mM), monosodium glutamate (1 M), glutamic acid diethyl ester (GDEE 50 mM), or NaCI (3 mM). These chemicals were microiontophoretically applied to the immediate vicinity of the target neuron being recorded. The MVN neurons were classified as type I or II neurons according to their responses to horizontal and sinusoidal rotation. We examined the effects of the drugs on type I neurons. Microiontophoretically applied PS in doses of 50-200 nA dose-dependently increased spontaneous firing in 20 of 26 neurons examined; 6 neurons were not affected by drug doses up to 200 nA. Spontaneous firing did not decrease in any neurons. The PS-induced (100 nA) increase in spontaneous firing of MVN type I neurons was dose-dependently suppressed by the iontophoretic application of GDEE (50-200 nA), which also inhibited glutamate- and rotation-induced firing. These findings suggest that PS excites type I neurons through the glutamate receptor in MVN.
