The blockade of bulbospinal inhibition by 5-hydroxytryptamine antagonists.

Abstract

Histochemical evidence suggests that neurons in the brain stem raphe nuclei which contain serotonin (5-HT) project to the spinal cord. Stimulation of the raphe nuclei facilitated and inhibited spinal reflexes: the conduction velocity calculated for the bulbospinal inhibitory pathway was 10 m/sec. Antagonists of 5-HT were without effect on facilitation, but readily blocked raphe-evoked inhibition of the monosynaptic reflex. The antagonists also blocked the inhibition evoked from the ventral medial reticular formation. Blockade of bulbospinal inhibition was observed after LSD (0.25 mg/kg), methysergide (0.5 mg/kg), BOL (1–1.5 mg/kg) and cinanserin (4 mg/kg) but not cyproheptadine (5 mg/kg). The 5-HT antagonists were more effective when injected into the arterial supply of the spinal cord than when injected into the arterial supply of the brain stem. Thus, the antagonists appear to act at the spinal level, presumably blocking the effect of a 5-HT-releasing interneuron intercalated in the bulbospinal inhibitory pathway.