Prolonged stimulation of respiration by endogenous central serotonin.

We have recently reported a new neural brainstem mechanism which is uniquely activated by stimulation of carotid body afferent input to the brain and which facilitates respiration for hours after the immediate affects of the stimulation have dissipated (Millhorn, Eldridge and Waldrop, 1980). In the present study respiratory responses to carotid body or carotid sinus nerve stimulation were measured in vagotomized, anesthetized, and paralyzed cats whose end-tidal PCO2 and temperature were servo-controlled and kept constant. The responses of animals pretreated with various serotonin antagonists and a dopamine-norepinephrine antagonist were compared to the responses of untreated control animals. All three differently acting serotonin antagonists (methysergide, parachlorophenylalanine, and 5, 7-dihydroxytryptamine) either prevented or significantly reduced the magnitude of the long-lasting respiratory response whereas the dopamine-norepinephrine antagonist (alpha-methyltyrosine) failed to alter it. We conclude that the long-lasting increase of respiratory activity following stimulation of carotid body afferents is due to activation of an endogenous central serotoninergic mechanism which facilitates respiration.