DISTRIBUTION AND DEVELOPMENTAL EXPRESSION OF OCTOPAMINE-IMMUNOREACTIVE NEURONS IN THE CENTRAL-NERVOUS-SYSTEM OF THE LEECH

Octopamine, a biogenic amine analogous to norepinephrine, plays an imp ortant role in the orchestration and modulation of invertebrate behavi or. In the leech, the behavioral actions of octopamine have been demon strated; however, identification of octopaminergic neurons had not bee n determined by using immunohistochemical techniques. Thus, we used an antibody highly specific to octopamine to examine the distribution of octopamine-immunoreactive neurons in the segmental ganglia of America n and European medicinal leeches (Macrobdella decora and Hirudo medici nalis). One pair of octopamine-immunoreactive neurons was located in t he dorsolateral ganglionic region of anterior ganglia 1-6 and posterio r ganglia 15-21. No corresponding octopamine-immunoreactive neurons we re found in midbody ganglia 7-14. Using Neutral Red staining in combin ation with intracellular Neurobiotin injections and octopamine immunos taining, we determined the identity of the dorsolateral octopamine-imm unoreactive cells. The dorsolateral octopamine-immunoreactive neuron ( the DLO) was not cell 21, the only previously reported Neutral Red sta ining neuron in the dorsolateral position. We also determined that the Leydig neuron was not octopamine immunoreactive in either of the two medicinal leech species. Octopamine immunostaining in the sex ganglia revealed hundreds of immunoreactive neurons in sexually mature leeches . Such neurons were not observed in juvenile leeches. The developmenta l time course of octopamine immunoreactivity in the dorsolateral octop amine-immunoreactive neurons was also investigated by staining embryon ic Hirudo medicinalis. Octopamine expression occurred relatively late as compared with the detectable onset of serotonin expression. Octopam ine expression in the dorsolateral octopamine-immunoreactive cells was not detectable at early to mid-embryonic stages, and must commence du ring late embryonic to early juvenile stages. The identification of oc topamine-immunoreactive cells now sets the stage for further investiga tions into the functional role of octopamine in leech behavior and the development of behavior. (C) 1995 Wiley-Liss, Inc.