Myotropic peptide hormones and serotonin in the regulation of feeding in the adult blow fly Phormia regina, and the adult horse fly Tabanus nigrovittatus
The act of feeding and all subsequent post-ingestive physiological processes requires the strict control and coordination of many body systems, often by neuropeptides and biogenic amines. In this study, the roles of two classes of neuropeptides, the FMRFamide related peptides (FaRPs) and the tachykinin related peptides (TRPs), and the biogenic amine serotonin were examined as they relate to alimentary function and feeding behavior in both the adult, female blow fly, P. regina, and the adult, female horse fly, Tabanus nigrovittatus.
FMRFamide-like immunoreactivity (FLI) and locustatachykinin-like immunoreactivity (LTKLI) was observed throughout nervous and alimentary tissues in both flies using whole mount immunofluorescence and paraffin section immunohistochemical techniques. The observed immunolocalization suggests that both peptide classes are brain gut peptides in these two flies. Immunoreactivity was fairly well conserved between these two flies despite their divergent anatomy.
The TRP callitachykinin-I (Cav TK-I) was found to have myotropic effects on in vitro P. regina crop preparations. Exogenous Cav TK-1 simultaneously relaxed the distal lobes of the crop sac while increasing contractions in the crop “pump” region of the crop.
Serotonin like immunoreactivity (SLI) was observed throughout the nervous system and alimentary tract in T. nigrovittatus. A dense network of SLI processes covered the surface of the midgut, suggesting an important role for serotonin in midgut activity. T. nigrovittatus injected with 50 μg of the serotonin depleter α-methyltryptophan (AMTP) were more likely to blood-feed than saline injected flies.
To investigate the role of serotonin in feeding in P. regina , flies were injected with serotonin and allowed to feed on liver homogenate. Flies injected with 50 μg of serotonin ingested less liver than control flies at 1, 3, 6, and 24 h post-injection and also exhibited increased body weight loss at the 3, 6, and 24 h post-injection time intervals.