OCTOPAMINE RECEPTOR SUBTYPES AND THEIR MODES OF ACTION

Octopamine receptor subclasses were first proposed to explain differen ces in the pharmacological profiles of a range of physiological respon ses to octopamine obtained in the extensor-tibiae neuromuscular prepar ation of the locust. Thus, OCTOPAMINE1 receptors which inhibit an endo genous myogenic rhythm, increase intracellular calcium levels. Also OC TOPAMINE2 receptors which modulate neuromuscular transmission in this preparation, increase the level of adenylate cyclase activity. The cur rent status of this classification is reviewed by examining the pharma cology of responses to octopamine in a range of preparations. It is co ncluded that the distinction between OCTOPAMINE1 and OCTOPAMINE2 recep tor types is still valid, but that OCTOPAMINE2 receptors exhibit some tissue specific variations. Studies on a cloned Drosophila octopamine/ tyramine (phenolamine) receptor are discussed and illustrate many of t he difficulties presently encountered in making a definitive classific ation of octopamine receptors. These include the possibilities that si ngle receptors may activate multiple second messenger systems and that different agonists may differentially couple the same receptor to dif ferent second messenger systems.