Biogenic amine receptors in the sea pansy: activity, molecular structure, and physiological significance

Abstract

Although cnidarian nervous systems appear to make heavy use of neuropeptides, evidence of the presence of biogenic amine transmitter systems has also surfaced over the past two decades. Our and other laboratories have used several analytical methods to reveal the presence of catecholamines and indoleamines in cnidarians, some of which are traceable to specific subsets of neurons. Evidence was also provided of amine biosynthesis, and of neuronal catecholamine release. Adrenergic-like and serotonin-like receptors were pharmacologically characterized by bioassays and by radiobinding assays in the sea pansy Renilla koellikeri, and these receptors appear to mediate activities such as bioluminescence and peristaltic contractions. The search for their structural identity has led to the molecular cloning and characterization in the sea pansy of two new G protein-coupled receptors, Ren1 and Ren2, exhibiting homologies with aminergic receptors. In contrast to the receptors characterized in sea pansy membranes, none of the two cloned receptors responded to conventional amines when expressed in mammalian cell lines. Ren1 displayed constitutive cyclic AMP-generating activity when expressed in mammalian cells. These findings illustrate the challenges posed by attempting to reconcile physiological and molecular data in the quest for functional relevance of cnidarian transmitter systems.