The nervous system and gut of worker, soldier and alate castes of the eastern subterranean termite, Reticulitermes flavipes Kollar (Isoptera: Rhinotermitidae) were examined for immunoreactivity to an antiserum to Helicoverpa zea (Boddie) (Leipidoptera: Noctuidae) MP-I (QAARPRF-NH2), a truncated form of neuropeptide F. More than 145 immunostained axons and cell bodies were seen in the brain and all ganglia of the ventral nerve cord. Immunoreactive axons exiting the brain projected anteriorly to the frontal ganglion and posteriorly to the corpora cardiaca and corpora allata. In the stomatogastric nervous system, immunoreactive axons were observed over the surface of the foregut, salivary glands, midgut and rectum. These axons originated in the brain and from 15–25 neurosecretory cells on the foregut. Staining patterns were consistent between castes, with the exception of immunostaining observed in the optic lobes of alates. At least 600 immunoreactive endocrine cells were evenly distributed in the midguts of all castes with higher numbers present in the worker caste. Immunostaining of cells in the nervous system and midgut was blocked by preabsorption of the antiserum with Hez MP-I but not by a peptide having only the RF-NH2 in common. This distribution suggests NPF-like peptides coordinate feeding and digestion in all castes of this termite species.
The midgut of R. flavipes is uniformly tube-like with no gastric cecae () (, ). The midgut surface is composed of circular nodes (52.2 ± 11.3 µm in diameter [n = 21]) () that contain regenerative nidi and the surrounding mature columnar cells. Regenerative cell nidi were observed at the center of these nodes as revealed by transmission electron microscopy (TEM) (). Endocrine cells were identified by the presence of secretory granules near the basal lamina ( inset). NPF-like endocrine cells were visualized with fluorescence microscopy using the Hez MP-I antibody (, ). These cells were clearly differentiated from axons by their characteristic bottle shape with a wide base at the basal lamina and an apical extension to the lumen of the gut ( inset) (). Two immunoreactive endocrine cells were observed on opposite sides of each node (), but the number was not always consistent and as many as five were observed in one node.
Immunocytochemistry has been an important tool for describing NPY-related peptides in insects. Detailed immunocytochemical studies with NPF have so far only been reported for holometabolous insects, in particular D. melanogaster (), Ae. aegypti () and H. zea (Huang 1995). The brains of Manduca sexta also showed PP-like immunoreactivity in cells and in axons that led to the corpora cardiaca and the aorta (). Patterns of immunostaining in central nervous system and gut among these insects and R. flavipeswere similar. Paired, immunoreactive, neurosecretory cells were observed in the brain of all of these insects. In H. zea and R. flavipes, immunoreactivity was observed in cells on the ventral nerve cord ganglia and axons on the surface of the corpora cardiaca. Also, the gut tract of H. zea, Ae. aegypti, and R. flavipes each exhibited immunoreactive axons at various points.