Naturalistic activation of barosensitive afferents release substance P in the nucleus tractus solitarius of the cat

The role for substance P (SP) in baroreceptor transmission in the nucleus t ractus solitarius (NTS) remains an area of active research. The purpose of the present study was to determine whether naturalistic activation of baros ensitive afferent fibers in the glossopharygneal and vagus nerves release S P in the caudal NTS. Experiments were performed on chloralose anesthetized, artificially ventilated and paralyzed cats. A microdialysis probe was ster eotaxically positioned unilaterally in the NTS. Dialysate samples were coll ected and SP-like immunoreactivity (SP-LI) was measured by radioimmunoassay . Barosensitive afferents were mechanically activated by inflation of a bal loon catheter positioned in the thoracic aorta at heart level. Graded ballo on inflation produced increases in mean arterial pressure (MAP) of 33+/-5 m mHg and 60+/-3 mmHg (P<0.05) and evoked proportional baroreflex decreases i n heart rate of 8+/-3 b.p.m. and 19+/-3 b.p.m. (P<0.05). This was accompani ed by increases in SP-LI of 16+/-3% and 39+/-8%, respectively (P<0.05). A p ositive linear relationship was found between changes in MAP and SP-LI (slo pe=1.73 fmol/<mu>l/mmHg. r(2)=0.62) that was completely abolished following barodenervation. These findings provide evidence that naturalistic activat ion of pressure-sensitive afferents in the glossopharygneal and vagus nerve s release SP in a region of the NTS that receives primary afferent projecti ons from aortic, carotid sinus and cardiac receptors in the cat. (C) 2001 E lsevier Science B.V. All rights reserved.