Octopamine--after a decade as a putative neuroregulator

From the above, it doesn't seem too rash now to elevate octopamine to the status of a neuroregulator in the invertebrates. To encapsulate, octopamine occurs in high concentration in many higher invertebrates and is found in specific octopaminergic neurons from which it is released upon stimulation. It then interacts with specific receptors, some of which at least, are linked to adenylate cyclase. The action of octopamine is then terminated by re-uptake or by N-acetylation (or a combination of these functions). It is fair to say we know as much about octopamine and its role as a neuromodulator in vertebrates as we do about noradrenaline in vertebrates-only our ignorance isn't as well documented. But where does octopamine fit into the scheme of Barchas et al. (1978)? We can agree that it is a neuroregulator but is it a neurotransmitter or a neuroregulator? Turning to the three well characterized octopaminergic systems, these all seem to be neuromodulatory in nature. Clearly the octopaminergic neurons in the lobster which release octopamine into the haemohymph fall into the neuromodulator class. In the case of the octopaminergic neurons in the adult firefly lantern, octopamine released from these neurons appears to interact with a specific adenylase cyclase-linked receptor and this leads to a response, the flash of the lantern. This therefore appears to be a situation where octopamine is a neurotransmitter, not dependent on other transmitters for actions (Nathanson, 1979). In the system of octopaminergic neurons which originate in the DUM neurons and innervate skeletal muscle, octopamine again appears to be a neuromodulator, altering the response of the muscle to another neurotransmitter (O'Shea and Evans, 1979). However, these peripheral octopaminergic system probably form only a small portion of the octopaminergic neurons in arthropods. The role of octopamine in the central nervous system must remain conjectural for the present.