Serotonergic modulation of synaptic transmission and action potential firing in frog motoneurons.

Frog spinal neurons receive a prominent innervation from the bulbar serotonergic nuclear complex. We used an isolated spinal cord preparation to examine the effect of serotonin (5-hydroxytryptamine, 5-HT) receptor activation on segmental and descending monosynaptic excitatory inputs to frog lumbar motoneurons. Bath-application of 5-HT (0.05 mM) caused a significant reduction in the peak amplitude of segmental EPSP elicited by dorsal root (DR) stimulation (P < 0.05). Contrasting to DR evoked responses 5-HT did not affect the descending monosynaptic EPSP conditioned by ventrolateral column (VLC) stimulation. Recording of the VLC induced EPSP-spike (E-S) field response within the ventral horn motor nucleus disclosed a substantial enhancement in the population discharge of motoneurons upon 5-HT application (P < 0.05). These data suggest the potential importance of serotonergic receptors in motor integration and gaining of motor output in the frog spinal cord.