EVIDENCE FOR THE INVOLVEMENT OF CHROMAFFIN CELLS IN THE STIMULATORY EFFECT OF TACHYKININS ON CORTICOSTEROID SECRETION BY THE FROG ADRENAL-GLAND

The adrenal gland of the frog is innervated by a network of fibers containing two tachykinins (ranakinin and [Leu(3),Ile(7)]neurokinin A), which both stimulate corticosteroid secretion from frog adrenal tissue. The aim of the present study was to determine the mode of action of tachykinins on the frog adrenal gland. Double immunolabeling of tissue sections with a monoclonal antibody to tyrosine hydroxylase and an antiserum to substance P showed that tachykinin-containing fibers are preferentially apposed onto chromaffin cells. Immunocytochemical labeling at the electron microscope level revealed that tachykinin-immunoreactive fibers establish close contacts only with adrenochromaffin cells. Ranakinin stimulated corticosterone and aldosterone secretion from perifused adrenal slices, but had no stimulative effect on dispersed adrenal cells. Cytoautoradiographic labeling of frog adrenal cells in primary culture with [H-3]substance P revealed the existence of specific binding sites located exclusively on chromaffin cells. Microfluorimetric measurement of cytosolic calcium concentrations ([Ca2+](i)) in cultured adrenal cells showed that ranakinin induced a dose-dependent increase in [Ca2+](i) in chromaffin cells (ED(50) = 2 x 10(-7) M). In contrast, ranakinin did not affect [Ca2+](i) in adrenocortical cells. The present results indicate that in the frog adrenal gland, tachykinin-containing fibers make preferential contacts with chromaffin cells, and tachykinins directly activate chromaffin cells. These data suggest that the stimulative effect of tachykinins on corticosteroid secretion is mediated via presynaptic activation of adrenochromaffin cells.