FMRFamide-Related Peptides Signaling Is Involved in the Regulation of Muscle Contractions in Two Tenebrionid Beetles

Abstract

Peptidergic signaling regulates various physiological processes in insects. Neuropeptides are important messenger molecules that act as neurotransmitters, neuromodulators or hormones. Neuropeptides with myotropic properties in insects are known as FMRFamide-like peptides (FaLPs). Here, we describe the myotropic effects of the endogenous FaLPs in the regulation of contractile activity of the heart, ejaculatory duct, oviduct and the hindgut in two beetle species, Tenebrio molitor and Zophobas atratus. A putative receptor was identified in silico in both species. Using RT-PCR these putative FaLPs receptors were found in the various tissues of both beetles, including visceral organs. Analysis of the amino acid sequence of the receptor indicated that it is similar to other insect FaLPs receptors and belongs to G-protein coupled receptors. A synthetic FaLP (NSNFLRFa) found as the bioanalogue of both species demonstrated concentration-dependent and organ-specific myoactive properties. The peptide had species-specific cardioactivity, in that it stimulated Z. atratus heart contractions, while slightly inhibiting that of T. molitor and had mainly myostimulatory effect on the examined visceral organs of both beetle species, with the lowest activity in the ejaculatory duct of these beetles. The peptide was the most active in the hindgut of both species, but only at high concentration of 10^-5 M. The results suggest that FaLPs are potent modulators of endogenous contractile activity of the visceral muscles in beetles and may indirectly affect various physiological processes.

Keywords: FMRFamide-like peptides; G-protein coupled receptor; beetles (Coleoptera); heart; neuropeptides; visceral organs.

FIGURE 1

Sequence alignment of FaLPs precursors from T. molitor (T. molitor_FaLPs – this study), Z. atratus (Z. atratus_FaLPs – this study) and T. castaneum (T. castaneum_FaLPs, EFA02863), all from Tenebrionidae. Peptides present in the precursor are marked in red boxes.

FIGURE 2

Alignment of the FMRFRs sequences from T. molitor (Tenmo-FMRFR), Z. atratus (Zopat-FMRFR), and T. castaneum (Trica-FMRFR), all from the Tenebrionidae family. Predicted seven transmembrane domains are highlighted in black.

FIGURE 3

Distribution of FMRFR transcript in different tissues of T. molitor (A) and Z. atratus (B). 1, whole body; 2, brain; 3, ventral nerve cord; 4, retrocerebral complex; 5, heart; 6, hindgut; 7, ejaculatory duct; 8, oviduct; C, control H₂0; M, marker.

FIGURE 4

Changes compared to control in the contractions frequency of heart (A), hindgut (B), ejaculatory duct (C), and oviduct (D) of adult T. molitor beetle after application of FMRFa. Mean ± SEM are given from at least eight determinations. Statistically significant differences (p ≤ 0.05) in the contractions frequency from control (saline application) are indicated by asterisks (t-Student test) n = 10–15.

FIGURE 5

Typical responses in the contraction frequency of Z. atratus heart after application of FMRF6 at the concentration 10^–5 M (A); T. molitor hindgut at the concentration 10^–6 M (B); Z. atratus oviduct at the concentration 10^–6 M (C). Peptide application is indicated by an arrow.

FIGURE 6

Changes compared to control in the contractions frequency of heart (A), hindgut (B), ejaculatory duct (C), and oviduct (D) of adult Z. atratus after application of FMRF6. Mean ± SEM are given from at least eight determinations. Statistically significant differences (p ≤ 0.05) in the contractions frequency from control (saline application) are indicated by asterisks (t-Student test).