PARACRINE CONTROL OF STEROID-HORMONE SECRETION BY CHROMAFFIN CELLS INTHE ADRENAL-GLAND OF LOWER-VERTEBRATES

The adrenal glands of lower vertebrates display a notable interminglin g between steroidogenic and chromaffin tissues, which increases from P isces to Aves. As in mammals, adrenal chromaffin cells contain and rel ease, in addition to catecholamines, serotonin and several peptides, w hich may affect the secretory activity of steroidogenic cells in a par acrine manner. Stimulatory molecules include serotonin, arginine-vasot ocin, tachykinins, vasoactive intestinal peptide, pituitary adenylate cyclase-activating peptide and calcitonin gene-related peptide; inhibi tory molecules are dopamine, somatotropic hormone-release inhibiting h ormone and galanin. Epinephrine and norepinephrine appear to stimulate steroid secretion in Aves and to inhibit it in Pisces, while their ac tion in Amphibia is controversial. Likewise, atrial natriuretic peptid e exerts an anti-secretagogue action in Amphibia and a marked secretag ogue effect in Pisces and Aves. The effects of opioids (enkephalins an d endorphins) have scarcely been investigated and the findings obtaine d are highly questionable. Compared with the amazing mass of investiga tions carried out in mammals, studies in lower vertebrates are few, an d in large. part performed in Amphibia and Aves. It appears that much further work has to be done by comparative endocrinologists to fully c larify the physiological relevance of the functional interactions betw een chromaffin and steroidogenic cells in the adrenal glands of lower vertebrates.