Evidence suggesting a transmitter or neuromodulatory role for substance P at the first synapse of the baroreceptor reflex.

There is evidence that the undecapeptide substance P is the transmitter released from pain fibres in the dorsal horn of the spinal cord. This, in turn, suggested to use the possibility of a similar role for substance P in another type of primary sensory structure, namely the baroreceptor neurones which terminate in the bulbar nucleus of the solitary tract (NTS). Substance P, injected into a lateral brain ventricle of urethane-anaesthetized rats, caused dose-dependent increases in blood pressure and heart rate. By contrast, local application of substance P to the region of the NTS, using small pieces of filter paper soaked in substance P-containing solution, resulted in hypotension and bradycardia. In cats anaesthetized with urethane, the same procedure also decreased blood pressure, heart rate and spontaneous sympathetic nervous activity. Release of substance P from nerve endings, through local application of capsaicin to the NTS, produced cardiovascular effects which were indistinguishable from those of substance P. A combined cannula-electrode with an uninsulated tip enabled identification of sites in the NTS of rats and cats, where electrical stimulation elicited decreases in blood pressure, heart rate and spontaneous sympathetic nervous activity. These sites were considered to contain the first synapse of the baroreceptor reflex. Subsequent microinjection of substance P through the cannula into these defined areas resulted in effects which were similar to those of the preceding electrical stimulation. The results suggest a transmitter or neuromodulatory role for substance P at the first synapse of the baroreceptor reflex in the NTS.