Activation of brainstem serotoninergic pathways decreases homosynaptic depression of monosynaptic responses of frog spinal motoneurons.

In the isolated neuraxis of the frog, low frequency stimulation (0.5-2 Hz) of the lateral columns produces monosynaptic responses in the ventral roots which are depressed with an exponential time course. Serotonin (10 mumol/liter) added to the bath, or stimulation of the brain-stem midline raphe nuclei, but not of the lateral reticular formation, reduced the magnitude of the low frequency depression of the responses. The above actions were abolished by methysergide (1 mumol/liter), a specific antagonist of serotonin. These observations show that the magnitude of the homosynaptic depression of monosynaptic responses of motoneurons can be controlled by descending serotonergic mechanisms. This action is considered to be an important component of the arousal behavior mediated by the brain-stem raphe nuclei.