Proctolin is an endogenous pentapeptide that acts as an excitatory neuromodulator.
Proctolin can increase the frequency of action potentials, increase the amplitude of muscle contraction, and initiate activity in quiescent systems[1]. In the arthropods, Proctolin acts as a neuromodulator and possibly as a neurohormone. It does not appear to function as a conventional neurotransmitter[2]. Proctolin is a pentapeptide with the mature peptide of RYLPT, and it is the first insect neuropeptide to be sequenced and chemically characterized. The first identification of a Proctolin precursor gene is CG7105 in D. melanogaster. Although a previous study showed that Proctolin is absent in B. mori, this pentapeptide is recently identified in a proteomic analysis of B. mori wings. However, the Bombyx Proctolin gene does not produce a mature peptide because cleavage sites are not present at the N-terminal and C-terminal of the RYLPT sequence, and a similar gene is observed in C. suppressalis. Therefore, a true Proctolin has been considered to be not observed in B. mori and C. suppressalis[3].
[1]. McGrath LL, et al. De novo transcriptome assembly for the lobster Homarus americanus and characterization of differential gene expression across nervous system tissues. BMC Genomics. 2016 Jan 16;17:63. [2]. Orchard I, et al. Proctolin: a review with emphasis on insects. J Neurobiol. 1989 Jul;20(5):470-96. [3]. Xu G, et al. Identification and expression profiles of neuropeptides and their G protein-coupled receptors in the rice stem borer Chilo suppressalis. Sci Rep. 2016 Jun 29;6:28976.