Response of the cricothyroid and thyroarytenoid muscles to stereotactic injection of substance P into the region of the nucleus tractus solitarius indeveloping dogs

Substance P (SP), a putative sensory neurotransmitter, mediates reflex lary ngeal adductor activity in developing dogs. Such reflex activity includes l ife-threatening laryngospasm induced by stimulation of distal esophageal af ferent nerves. The site of SP's activity is unknown. This research was unde rtaken to determine whether injection of SP into the nucleus tractus solita rius (NTS) of developing beagles alters laryngeal adductor motor activity. Six animals, 57 to 78 days of age, underwent stereotactic injection of 5 to 10 muL of SP into the region of the NTS, identified by electrical stimulat ion of the ipsilateral superior laryngeal nerve. In 8 additional studies, S P was injected into the cerebellum (2) or brain stem (6) distant from the N TS. Cardiovascular and electromyographic (EMG) responses of the diaphragm a nd the cricothyroid (CT) and/or thyroarytenoid (TA) muscles were recorded i n all 6 animals. Injection of SP into the region of the NTS induced a decre ase in blood pressure in all animals and an increase in either ipsilateral CT or TA activity. Three of these animals experienced mixed apnea character ized by sustained EMG activity (spasm) of the ipsilateral CT or TA muscles and an absence of diaphragm EMG activity. The apnea event was fatal in 1 of these animals. In the 6 animals who underwent injections in the brain stem but outside the region of the NTS, diaphragm and laryngeal EMG activity ge nerally did not change after injection of SP, with the exception of 1 anima l who experienced a mild, short-lived increase in ipsilateral CT activity. A brief phasic increase in ipsilateral CT activity was seen in both animals who underwent injection of SP into the cerebellum. A putative sensory neur otransmitter, SP evokes ipsilateral CT and/or TA EMG activity when injected into the region of the NTS in developing beagle dogs. This research sugges ts that SP in the NTS may play a role in mediating life-threatening larynge al adductor reflexes in developing mammals and may provide important inform ation regarding therapeutic intervention.