The effects of serotonin antagonists on the inhibition of primate spinothalamic tract cells produced by stimulation in nucleus raphe magnus or periaqueductal gray.

The effects of serotonin antagonists were examined on the inhibition and excitation of nociceptive spinothalamic tract cells produced by brainstem stimulation with short (200 msec) or long (2 sec) stimulus trains. The inhibitory effects resulting from stimulation in either nucleus raphe magnus (NRM) or in the periaqueductal gray with short stimulus trains were significantly reduced after the administration of serotonin receptor blockers. Reductions in periaqueductal gray inhibition were also observed on inhibition produced by long duration trains, whereas the effects of NRM inhibition were dependent on stimulus duration, current strength and dose of antagonist. The rare excitatory effect of NRM stimulation was also found to be relatively reduced after the administration of a serotonin antagonist. The relatively weak effects of serotonin antagonists on NRM inhibition are discussed in relation to three hypotheses: 1) parallel pathways; 2) multiple receptors; or 3) corelease of serotonin and another transmitter from raphe-spinal neurons.