Controlled bradycardia induced by nasal stimulation in the muskrat, Ondatra zibethicus.

Respiration was disrupted and bradycardia induced in anesthetized muskrats by stimulating the nasal cavity with a stream of either water or various concentrations of ammonia vapors. When responses induced by either ammonia or water were compared, ammonia vapors were considered preferable because the responses could be maintained reliably through relatively rapid periods of stimulation, and the post-stimulus recovery of heart rate and respiration was more predictable. Moreover, the bradycardia induced in the first 5 s of stimulation by dilutions of ammonia vapors was graded. After injections of lidocaine were made into the nucleus tractus solitarius a profound bradycardia to ammonia stimulation persisted despite disruption of normal respiratory rhythms and an inhibition of the baroreceptor reflex induced by phenylephrine administration. These results show that ammonia vapors stimulating the nasal chambers effectively elicit cardio-respiratory adjustments in anesthetized muskrats and that the bradycardia may be controlled by varying the intensity of the peripheral stimulus. The trigeminal contribution for this is emphasized since the bradycardia persists after reversible blockade of the solitary complex. These data suggest that the trigeminal input to cardiac motorneurons is via relatively few synapses and is over circuits which run parallel to those modulating cardiac activity in response to chemoreceptors, baroreceptors and pulmonary afferent fibers.