Reflex fall in blood pressure mediated by capsaicin-sensitive afferent fibers of the rat splanchnic nerve.

1. Capsaicin treated rats, in which the function of substance P-containing primary sensory neurons was impaired, were used to investigate the function of afferent fibers within the splanchnic nerve. The effects of electrical stimulation of the splanchnic nerve either distal or proximal to the site of its transsection on blood pressure and heart rate were investigated. 2. Distal splanchnic nerve stimulation evoked an equal rise in blood pressure in capsaicin treated rats and in their controls. Distal splanchnic nerve stimulation did not cause plasma extravasation in the adrenal medulla, an effect which is produced by antidromic stimulation of cutaneous sensory nerves. Peripheral effects of stimulation of primary afferent fibers within the splanchnic nerve cannot be assumed from these experiments. 3. Proximal stimulation of the splanchnic nerve evoked a reflex fall in blood pressure but no bradycardia. The fall in blood pressure was absent in capsaicin treated rats, which indicates that this effect is mediated by primary afferent fibers. Since the reflex fall in blood pressure was abolished by adrenergic blockade with guanethidine, it can be explained by vasodilatation resulting from reduction of sympathetic vasoconstrictor tone.