Potent pheromonotropic/myotropic activity of a carboranyl pseudotetrapeptide analogue of the insect pyrokinin/PBAN neuropeptide family administered via injection or topical application

A pseudotetrapeptide analogue of the pyrokinin/PBAN or FXPRLamide family (Cbe-Thr-Pro-Arg-Leu-NH2; Cbe = 2-o-carboranylethanoyl-), in which the phenyl ring of the Phe side chain is replaced with the hydrophobic cage-like o-carborane moiety, was synthesized and found to be 10-fold more potent than cockroach leucopyrokinin on an isolated cockroach hindgut bioassay system. In contrast with the naturally occurring peptide, the myostimulatory activity could not be immediately reversed following a saline rinse, providing evidence that the pseudopeptide analogue binds very strongly to the receptor. Once the analogue reaches the receptor, strong receptor binding characteristics may allow it to avoid inactivation by hemolymph peptidases. Although it has an eightfold smaller sequence than the endogenous 33-membered pheromone biosynthesis activating neuropeptide (PBAN), the carboranyl analogue is 10-fold more potent in an in vivo pheromonotropic bioassay of the female tobacco budworm moth Heliothis virescens, demonstrating that the small, C-terminal pentapeptide pyrokinin core analogue contains all the structural information necessary to fully activate pyrokinin receptors. In contrast with PBAN, the amphiphylic carboranyl analogue elicits pheromone production following topical application in aqueous solution to the lateral abdominal surface of H. virescens. providing a noninvasive means of inducing pheromone production in moths. The analogue can potentially serve as a useful tool to insect researchers studying, and/or attempting to disrupt, physiological processes regulated by pyrokinin-like neuropeptides in insects. A possible role for this and related pyrokinin analogues in future pest insect management strategies is briefly discussed.