Serotonergic facilitation of facial motoneuron excitation.

The effect of serotonin (5-HT) on motoneurons located in the facial nucleus of the rat was investigated in the present study. Microiontophoretic application of 10--200 nA pulses of 5-HT lasting from 1 to 10 min failed to excite facial motoneurons. However, small amounts of 5-HT facilitated the subthreshold and threshold excitatory effects of iontophoretically applied glutamate on these cells. Typically, the current of glutamate required to produce an activation of facial motoneurons was reduced by at least 50% in the presence of 5-HT. In addition, 5-HT markedly shifted to the left the cumulative dose-response curve of glutamate-induced excitation of motoneurons. The 5-HT releasing agent p-chloroamphetamine (PCA) facilitated the excitatory effects of glutamate on montoneurons in control animals, but not in those pretreated with the 5-HT also facilitated the subthreshold and threshold excitation of motoneurons produced by stimulation of the motor cortex and the red nucleus. The facilitating effect of 5-HT was blocked by methysergide. Norepinephrine also facilitated facial motoneuron excitation but this effect was not blocked by methysergide. It is concluded that 5-HT in the facial nucleus functions in a manner that is not analagous to direct excitation, but rather acts as a gain setter to enhance the effects of excitatory afferent inputs.