The modification of lumbar motoneurone excitability by stimulation of a putative 5-hydroxytryptamine pathway.

1 Changes in lumbar motoneurone excitability were monitored by recording spinal reflex activity from the ventral roots of rats anaesthetized with fluothane.2 Electrical stimulation of nucleus raphes medianus increased the amplitude of the monosynaptic reflex via a pathway having a slow conduction velocity. Stimulation elsewhere in the lower brain stem was less effective. This increase in motoneurone excitability was potentiated by the intravenous injection of L-tryptophan and reduced by intravenous injections of lysergic acid diethylamide (LSD), methysergide or Cinanserin.3 Extracellular field potential responses to stimulation of dorsal or ventral roots were recorded with six barrelled microiontophoresis electrodes. Stimulation of nucleus raphes medianus and iontophoretic application of 5-hydroxytryptamine (5-HT) both increased the excitability of lumbar motoneurones as reflected by an increase in field potential amplitude.4 Responses to both stimulation of raphe nuclei and iontophoretic application of 5-HT were reduced by iontophoretic application of Cinanserin and methysergide.5 The similarities of the responses of lumbar motoneurones to applied 5-HT and activity within the raphe-spinal pathway are discussed. It is suggested that activity within the raphe-spinal pathway can increase lumbar motoneurone excitability via the release of 5-HT in the ventral horn of the spinal cord.