The role of sensory fibres in the rat splanchnic nerve in the regulation of adrenal medullary secretion during stress.

We have studied the involvement of sensory nerves containing substance P (SP) in the modulation of stress-induced catecholamine (CA) secretion from the sympathetic nervous system and adrenal medulla. Adrenaline and noradrenaline (NA) levels were measured in blood samples withdrawn from the inferior vena cava (i.v.c.) at 5 or 15 min intervals for periods of up to 60 min, in adult rats during stress induced by insulin or cold. Insulin stress caused a biphasic elevation of plasma CA. Previous studies from our laboratory have shown that the first phase lasting 30 min is neurogenic, and the second phase from 30 to 60 min is non-neurogenic in mechanism. In control adult rats (with normal levels of SP in their splanchnic nerve), insulin stress caused a slow and progressive secretion of adrenaline into the circulation for the first 30 min (neurogenic phase). In the period 30-60 min (non-neurogenic phase) plasma adrenaline and NA levels rose at a much higher rate. In capsaicin-pre-treated rats (in which SP levels in the splanchnic nerve were depleted by 68%) insulin stress produced a steady increase in plasma adrenaline levels for up to 5 min similar to that in insulin-stressed control animals; however, by 10 min the plasma adrenaline levels had fallen to basal and remained low up to 30 min. From 30 to 60 min, plasma adrenaline and NA levels rose steeply as seen with control animals. We conclude that capsaicin pre-treatment affected the neurogenic phase but did not affect the non-neurogenic phase. Cold stress increased the plasma adrenaline levels by a neurogenic mechanism over 30 min in control rats. In contrast, in capsaicin-pre-treated, cold-stressed rats, plasma adrenaline did not increase significantly. Plasma NA levels were also significantly lowered in capsaicin-pre-treated, cold-stressed rats during the neurogenic phase but NA increases were not dependent on an intact adrenal innervation. The results using both insulin stress and cold stress suggest that capsaicin-sensitive (sensory) nerve fibres in the adrenal medulla and in sympathetic ganglia are capable of modifying the secretory responses of these tissues to stress. Results from our previous in vitro work are compatible with the view that SP may be the neuromodulator released from such sensory nerves to produce these effects. This suggests that the previously reported ability of SP to modulate nicotinic receptor function in vitro by either inhibiting the nicotinic response or protecting against nicotinic desensitization may be more than a mere pharmacological curiosity.(ABSTRACT TRUNCATED AT 400 WORDS)