Serotonin and co-localized peptides: effects on spinal motoneuron excitability

Iontophoretically applied serotonin (5HT), substance P (SP), thyrotropin releasing hormone (TRH) and proctolin all enhanced spinal motoneuron activity evoked by glutamate in urethane-anesthetized rats. 5HT and SP produced more consistent facilitation of excitability both within and between motoneurons than did TRH and proctolin which failed to facilitate several motoneurons. It is concluded that 5TH, SP, TRH and proctolin, which appear to co-exist in various combinations in ventral horn terminals near spinal motoneurons, have complimentary effects on motoneuron excitability.