Serotonin immunoreactivity in the abdominal ganglion resulting from exogenous application of 5-HT. Immunohistochemistry with anti-5-HT was performed with dissected VNCs of fru³ adult males that were incubated in 5-HT solutions. Immunofluorescently labeled tissues were examined by confocal microscopy, and stacked images of abdominal ganglia were obtained from specimens exposed to the range of concentrations indicated. A–D, Representative immunostaining after application of 5-HT at 5 μm(n = 4) (A), 10 μm (n = 5) (B), 100 μm (n = 2) (C), and 500 μm(n = 5) (D). No s-Abg-like 5-HT-immunoreactive neurons or their projections were observed to result from the lowest two concentrations of 5-HT used (A, B). Thus, the thin neurite signal inA, which runs down the center of the median trunk, was traced back to certain cell bodies in the posterior tip of the abdominal ganglion, but they were too small to be s-Abg cell bodies. Another feature of A is a cell body and fiber (white arrowhead) that appears to be outside of the main abdominal nerve. (In association with these signals are varicosities that suggest the structures are neurohemal fibers that could not be projections from s-Abg cells.) B, None of the stained fibers (white arrowheads) in the main nerve could be traced back to s-Abg-like cell bodies. D, At the highest concentration, a few cells that putatively took up exogenous 5-HT appeared to be similar to s-Abg neurons in their size and shape (black arrowhead within the box; image of the box is based on one focal plane); no such cells exhibit intrinsically derived immunostaining in these cells in this mutant (Fig. 7E). The putative s-Abg cell bodies, to which 5-HT was supplied in thefru³ abdominal ganglion shown (D), are in a dorsomedial region of the posterior CNS tip. This is the location of such neurons in more definitively identifiable circumstances (Fig.7A,C,D). Scale bars, 100 μm.