Serotonin-mediated excitation of recurrent laryngeal and phrenic motoneurons evoked by stimulation of the raphe obscurus.

Short-latency averaged responses in the recurrent laryngeal nerve (RLN) and C5 phrenic nerve to electrical stimulation (2.5-80 microA; 2.5-160 Hz; 150 microseconds pulse duration) of the medullary nucleus raphe obscurus (RO) were investigated in anesthetized, paralyzed and artificially ventilated cats. The response evoked in RLN by stimulation within RO was excitatory and consisted of a single peak. Characteristics of this response in RLN were compared with those of the delayed excitatory response in C5 phrenic nerve, which we previously showed to be elicited by stimulation within RO. Mean latency to onset for the excitatory response in RLN was 5.7 +/- 0.3 ms, while the delayed excitatory response in C5 phrenic nerve occurred at 7.0 +/- 0.3 ms. The excitatory response in both nerves could be evoked when stimulation was applied during inspiration as well as during expiration. The stimulus threshold varied between 2.5 and 5 microA for evoking the inspiratory-phase response in each nerve. The magnitude of this response in RLN and in C5 phrenic nerve was directly related to current intensity and was dependent upon stimulus frequency. Intravenous administration of the serotonin receptor antagonist, methysergide (0.1-2.4 mg/kg) caused significant dose-related reductions in the response in each nerve. In summary, characteristics of the evoked response in RLN and phrenic nerve are similar in several ways. Both responses are: (1) excitatory in nature, (2) elicited at small stimulus currents, (3) affected similarly by increasing stimulus current and frequency, (4) elicited by stimulation during inspiration and expiration, and (5) mediated at least in part by activation of pathways using serotonin as a neurotransmitter.