Pharmacological characterization of adrenal paraneurons: substance P and somatostatin as inhibitory modulators of the nicotinic response

Abstract A pharmacological study was made of the effects of various muscarinic and nicotinic agonists and their antagonists on the release of [ 3 H]noradrenaline ([ 3 H]NA) from cultures of isolated bovine adrenal medullary cells. A study was also made of the effects of substance P and somatostatin on the release of [ 3 H]NA evoked by nicotinic agonists. By 2 days in culture these adrenal ‘paraneurons’ had developed long varicose processes with growth cones and generally resembled noradrenergic neurons in culture. In the present study, adrenal paraneurons were incubated with [ 3 H]NA which was taken up and stored in reserpine-sensitive sites. Exposure of the cultures to acetylcholine (ACh) resulted in release of [ 3 H]NA into the external medium. High concentrations of K + (56 mM) also evoked release of [ 3 H]NA. The release of [ 3 H]NA induced by ACh or K + (56 mM) was Ca 2+ -dependent. Pharmacological studies with nicotinic (ACh, nicotine) and muscarinic (methacholine, pilocarpine) agonists and their antagonists (mecamylamine, d-tubocurarine, hexamethonium; and atropine, scopolamine, respectively) showed that the adrenal paraneurons contained only nicotinic receptors. Substance P produced a dose-dependent inhibition of ACh ( 5 × 10 −5 M ) stimulated [ 3 H]NA release in the range of 10 −8 to 5 × 10 −5 M with an ID 50 of 10 −6 M. A similar inhibition of NA release by substance P was obtained when nicotine ( 5 × 10 −6 M ) was used as the agonist, but not when K + (56 mM) was used to depolarize the cells. Substance P (10 −10 to 5 × 10 −5 M ) by itself did not have a significant effect on the basal release rate of [ 3 H]NA from these cells. Somatostatin at relatively high concentrations ( 10 −6 -10 −3 M;ID 50 2 × 10 −5 M ) inhibited the release induced by ACh, but not by K + (56 mM). The present results provide the first directevidence at a cellular level that substance P and somatostatin act as inhibitory modulators of the nicotinic ACh response, and support a role for these peptides as inhibitory neuromodulators at nicotinic receptor sites in the nervous system.