The effect of serotonin precursors on alpha- and gamma-motoneuron activity.

The effects of the 5-hydroxytryptamine (5-HT) precursors, 5-hydroxytryptophan (5-HTP) and tryptophan on alpha- and gamma-motoneuron discharges were examined in the spinal cat with a deafferented cord. The injection of 75 mg/kg of dl-5-HTP resulted in a doubling of the spontaneous discharge rate of gamma-motoneurons and the induction of spontaneous alpha-motoneuron activity. These effects of 5-HTP were reversed by the 5-HT antagonists, cinanserin (4 mg/kg) and methysergide (2 mg/kg). Tryptophan alone (100 mg/kg) exhibited minimal effects of motoneuron activity, but in animals pretreated with pargyline it significantly excited alpha- and gamma-motoneurons. Recordings of alpha- and gamma-motoneuron activity in gastrocnemius and semitendinosus nerves revealed that 5-HTP increased alpha- and gamma-motoneuron activity in both flexor and extensor nerves. Reversal of the effects of 5-HTP by 5-HT antagonists suggests that these effects were mediated by 5-HT. The fact that the 5-HTP effects were observed in preparations with an open gamma-loop indicates that the effects on the alpha-motoneurons are not mediated via the gamma-motoneuron facilitation, but results from a central activation.