The neuropeptide SIFamide in the brain of three cockroach species.

The sequence as well as the distribution pattern of SIFamide in the brain of different insects is highly conserved. As a general rule, at least four prominent SIFamide-immunoreactive somata occur in the pars intercerebralis. They arborize throughout the brain and the ventral nerve cord. Whereas SIFamide is implicated in mating and sleep regulation in Drosophila, other functions of this peptide remain largely unknown. To determine whether SIFamide plays a role in the circadian system of cockroaches, we studied SIFamide in Rhyparobia (= Leucophaea) maderae (Blaberidae), Periplaneta americana (Blattidae), and Therea petiveriana (Polyphagidae). Matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry revealed identical SIFamide sequences (TYRKPPFNGSIFamide) in the three species. In addition to four large immunoreactive cells in the pars intercerebralis (group 1), smaller SIFamide-immunoreactive somata were detected in the pars intercerebralis (group 2), in the superior median protocerebrum (group 3), and in the lateral protocerebrum (group 4). Additional cells in the optic lobe (group 5) and posterior protocerebrum (group 6) were stained only in P. americana. Almost the entire protocerebrum was filled with a beaded network of SIFamide-immunoreactive processes that especially strongly invaded the upper unit of the central body. Double-label experiments did not confirm colocalizations with γ-aminobutyric acid (GABA) or the circadian coupling peptide pigment-dispersing factor (PDF). In contrast to locusts, colocalization of SIFamide and histamine immunoreactivity occurred not in group 1, but in group 4 cells. Because the accessory medulla displayed SIFamide immunoreactivity and injections of SIFamide delayed locomotor activity rhythms circadian time-dependently, SIFamide plays a role in the circadian system of cockroaches. J. Comp. Neurol. 524:1337-1360, 2016. © 2015 Wiley Periodicals, Inc.