Orcokinin Peptides

Introduction

Orcokinin is a novel myotropic peptide from the nervous system of the crayfish, Orconectes limosus. A myotropic peptide, named orcokinin, was isolated from approximately 1200 abdominal nerve cords of the crayfish, Orconectes limosus. Its amino acid sequence was determined as follows: Asn-Phe-Asp-Glu-Ile-Asp-Arg-Ser-Gly-Phe-Gly-Phe-Asn. This structure was confirmed by syn-thesis. There is no sequence similarity to any known neuropeptide. An interesting feature of or-cokinin is the lack of both N- and C-terminal blocked residues. This is quite uncommon among neuropeptides. Orcokinin exhibits high potency on the crayfish hindgut, enhancing both frequency and amplitude of spontaneous contractions. Orcokinin proved to be a highly potent stimulator of hindgut contractions, particularly with regard to force. Orcokinins are likely to function as mod-ulators of the crustacean stomatogastric ganglion.

Mechanism of action

The orcokinins can modulate the excitability of a central neural circuit. Interestingly, most of the neuropeptides found in descending projections to the STG, including the extended FLRFamides that colocalize with histamine and orcokinin in the PS neurons, excite the LP neuron. In contrast, in the presence of orcokinin, the number of LP neuron spikes/burst decreased. Further work will be needed to determine the direct cellular targets of the orcokinins and the current(s) that they evoke or modulate. Nonetheless, the bias towards PY neuron activity seen in orcokinin makes it somewhat unusual among STG modulators. Of course, PS neuron activation is likely to deliver a complex mixture of orcokinin, histamine, and extended FLRF-amides, so a great deal more remains to be understood about the potential roles each of these plays in the complex physiological changes evoked by PS neuron activation.

Application of Orcokinin Peptides

The widespread distribution of the orcokinins in the crustacean nervous system allows us to add them to the complement of neuromodulatory peptides, such as proctolin, the extended FLRFamides, and crustacean cardioactive peptide (CCAP), that all have profound physiological actions at many sites within the animal. Therefore, we anticipate that the orcokinins will play numerous and as yet poorly understood roles in many aspects of the control of complex behaviors.

References

1. Li, L., Pulver, S. R., Kelley, W. P., Thirumalai, V., Sweedler, J. V., & Marder, E. (2002). Orcokinin peptides in developing and adult crustacean stomatogastric nervous systems and pericardial organs. The Journal of Comparative Neurology, 444(3), 227–244.
2. Lin, M., Egertová, M., Zampronio, C. G., Jones, A. M., & Elphick, M. R. (2018). Functional characterization of a second pedal peptide/orcokinin‐type neuropeptide signaling system in the starfish Asterias rubens. Journal of Comparative Neurology, 526(5), 858-876.
3. Stangier, J., Hilbich, C., Burdzik, S., & Keller, R. (1992). Orcokinin: A novel myotropic peptide from the nervous system of the crayfish, Orconectes limosus. Peptides, 13(5), 859–864.