IDENTIFICATION OF beta-LIKE ADRENOCEPTORS ASSOCIATED WITH BIOLUMINESCENCE IN THE SEA PANSY RENILLA KOELLIKERI

Abstract

Previous studies have reported pharmacological and biochemical evidence for the involvement of adrenergic substances in the regulation of neuroeffector activities in the bioluminescent cnidarian Renilla koellikeri (Cnidaria, Anthozoa). Therefore, direct radiobinding assays were developed to identify and characterize beta-adrenergic binding in membrane preparations from this species, using the two beta-antagonists [3H]dihydroalprenolol and [3H]CGP12177 as tracers. In addition, the effect of various beta-adrenergic agents on luminescence was examined. Binding of the radioligands at 25°C was rapid, reversible, saturable and specific. Saturation studies revealed the presence of two different and independent classes of binding site, site1 and site2, in the body of the colony (rachis). In contrast, homogeneous populations of binding sites corresponding to site1 were detected in autozooid polyps and to site2 in the peduncle. The pharmacological profile of beta-adrenergic binding in R. koellikeri membrane preparations displayed properties consistent with the presence of two sites and followed a pattern similar to beta2- and beta1-adrenergic receptor subtypes for site1 and site2, respectively. Bioluminescence in polyps was induced by beta-agonists as well as by one beta1-selective antagonist, atenolol, and was blocked by several beta-blockers including (+/−)CGP12177. The specificity pattern of the physiological effect of beta-adrenergic drugs on luminescence mirrors that of the radioligand interaction with site1. This suggests that radioligand binding to site1 represents binding to the receptor that mediates luminescence excitation in R. koellikeri. Blockade of the luminescent responses to site1 agonists by isotonic MgCl2 indicates that this beta-adrenergic mechanism must rely on interneuronal transmission. Collectively, these results suggest the evolutionary conservation of beta-adrenoceptors and of their dual character from coelenterates to higher vertebrates.