Neurons coexpressing FRU^M and serotonin in the abdominal ganglion. These cells, dubbed s-Abg, were revealed in the posterior tip of the male VNC by anti-5-HT single-labeling and double-labeling application of that antibody along with anti-FRU^M. Twelve 4- to 7-d-old wild-type males were used for anti-5-HT immunohistochemistry alone and 10 separate such males for double-labeling. A, Confocal image showing a dorsal view of serotonergic neurons in the thoracic and abdominal ganglia. Two different sizes of anti-5-HT-labeled neurons appear in the abdominal ganglion; the boxed area shows a cluster of eight s-Abg neurons; the black arrowhead points to the proximal portion of axons projecting posteriorly from these cells into the median trunk nerve. B, C, Neurons expressing 5-HT- and FRU^M neurons in an adult-male abdominal ganglion. These sagittal views of the s-Abg neurons show the cell bodies to be located dorsally (toward the right of each panel). D, Combined image of B andC, depicting coexpression of the two antigens. 5-HT immunoreactivity was observed mostly within (and throughout) the cell bodies (red), and FRU^M immunoreactivity was detected only in nuclei (green). E, F, Higher-magnification dorsal view of s-Abg neurons labeled by anti-5-HT and anti-FRU^M, respectively. Six consecutive 1.2 μm focal planes were combined to show all s-Abg neurons; thedotted line in E was drawn to indicate the symmetrically paired structure of the s-Abg neuronal clusters.G, Combined image of E andF (5-HT in red, FRU^Min green). Scale bars: A–D, 100 μm;E–F, 25 μm.