Pigment‐dispersing hormone‐like peptide in the nervous system of the flies Phormia and Drosophila: Immunocytochemistry and partial characterization

Abstract

beta-pigment-dispersing hormone (beta-PDH) isolated from the fiddler crab (Rao et al., '85) is a member of an octadecapeptide family of neuropeptides common to arthropods. Whereas earlier studies of these peptides in insects were limited to orthopterans, this investigation focuses on dipteran flies. Extracts of heads from the blowfly Phormia terraenovae were assessed in a fiddler crab bioassay for PDH activity. Immunocytochemistry, dose-response curves, gel filtration chromatography and reversed-phase HPLC, combined with bioassay and enzyme-linked immunosorbent assay (ELISA), indicate the presence of PDH-like peptide in the blowfly. Immunocytochemical mapping of PDH-like immunoreactive (PDHLI) neurons was performed for the entire nervous systems of Phormia and the fruitfly Drosophila with a beta-PDH antiserum. In the cephalic ganglion (brain, optic lobe and subesophageal ganglion) PDHLI cell bodies could be detected (34 in Phormia and 16 in Drosophila). In both species, each hemisphere contains 8 PDHLI cell bodies in the optic lobes. These innervate the optic lobe neuropils bilaterally. In Phormia, another set of 8 cell bodies are located in each of the lateral neurosecretory cell groups in the superior protocerebrum. These neurons send axons to the corpora cardiaca-hypocerebral ganglion complex and to portions of the foregut. In contrast, only the optic lobe neurons display immunoreactivity in Drosophila. Except for the optic lobes, PDHLI processes are distributed only in nonglomerular neurophils of the brain of both species. In the fused thoracico-abdominal ganglia of Phormia, 28 PDHLI cell bodies were found (only six were found in Drosophila). In both species, six abdominal PDHLI neurons are efferents with axons innervating the hindgut. We also found that some of the PDHLI neurons in the Phormia brain and abdominal ganglion contain colocalized FMRFamide-like immunoreactivity. Since the flies studied here do not display hormonally controlled, fast pigment migrations, the PDH-like peptide may have a role as neurotransmitter or neuromodulator in the central nervous system, especially in the visual system, and a regulatory role in the stomatogastric system and the hind-gut.