Inhibition of phrenic and sympathetic vasomotor neurons in cats by the serotonin analog 5-methoxy-N,N-dimethyltryptamine.

5-Methoxy-N,N-dimethyltryptamine (20-200 microgram/kg) inhibits phrenic and sympathetic neural discharges and depresses blood pressure. The effects occur within seconds after injection. Complete recovery occurs, on the average, after 20 min. The inhibition of phrenic nerve discharges is prevented during hypercapnia and hyperthermia and reversed during 50 Hz electrical stimulation of the carotid sinus nerve. Cinanserin (2-4 mg/kg) antagonize the inhibition of phrenic nerve activity. Inhibition of sympathetic activity by 5MeODMT is also reversed during 50 Hz carotid sinus nerve stimulation, but is unaltered by hypercapnia, hyperthermia or cinanserin. High doses of 5MeODMT (200-700 microgram/kg i.v.) produce an initial apneustic discharge which is followed by inhibition of phrenic discharges. The inhibitory effects of 5MeODMT are attributed to a direct agonist action on central nervous serotonin receptors. The inhibition of phrenic and sympathetic neurons appears to be mediated by different types of serotonin receptors.