Cytoarchitecture of histamine-, dopamine-, serotonin- and octopamine-containing neurons in the cricket ventral nerve cord

The present article provides a comparative neuroanatomical description of t he cellular localization of the biogenic amines histamine, dopamine, seroto nin and octopamine in the ventral nerve cord of an insect, namely the crick et, Gryllus bimaculatus. Generally, different immunocytochemical staining t echniques reveal a small number of segmentally distributed immunoreactive ( -IR) amine-containing neurons allowing single cell reconstruction of promin ent elements. Aminergic neurons share common morphological features in that they innervate large portions of neurophil and often connect different neu romeres by intersegmental 'wide-field' projections of varicose appearance. In many cases aminergic terminals are also found on the surface of peripher al nerves suggesting additional neurohemal release sites. Despite such morphological similarities histological analysis demonstrates for any given amine functionally distinct neuron types with specific innerv ation patterns establishing discrete pathways. Histamine-IR interneurons ar e characterized by both ascending and descending projections forming centra l and peripheral terminals. The descending branches from dopamine-IR cells mainly converge within the terminal ganglion, whereas serotonin-IR interneu rons with ascending projections often terminate within the brain. Serotonin is also present in sensory and motor neurons. In contrast to other aminerg ic neurons, most octopamine-LR cells represent unpaired neurons projecting through motor nerves of the soma-containing neuromere. Octopamine-IR cells with intersegmental branches are only rarely found. Based on these findings , a colocalization of different amines within the same neuron seems to be u nlikely to occur in the cricket ventral nerve cord. With respect to the neuroanatomical description of amine-containing neurons known physiological effects of biogenic amines and their possible neuromod ulatory functions in insects are discussed. Microsc. Res. Tech. 44:137-165, 1999. (C) 1999 Wiley-Liss, Inc.