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Definition
Leucopyrokinin (LPK) is a myotropic substance originally isolated from head extracts of the Madeira cockroach, Leucophaea maderae.

Discovery
The first investigations into the role of neuropeptide hormones in insect physiology was by Kopec in 1917 and the first report of the isolation and sequence analysis of an insect neuropeptide was in 1975. Since then a large number of insect neuropeptides that have been identified . Evaluation of analogs of the blocked insect myotropic neuropeptide leucopyrokinin (LPK) has demonstrated its relative insensitivity to amino acid substitution in the N-terminal in contrast to the C-terminal region. Truncated analogs of LPK without the first, second, and third N-terminal amino acids retain a significant 144%, 59% and 30% of the activity of the parent octapeptide, respectively. The [2-8] LPK analog is the first fragment of an insect neuropeptide to exhibit greater activity than the parent hormone. In contrast, truncated analogs of the insect myotropic, proctolin, exhibit little or no activity. The pentapeptide fragment Phe-Thr-Pro-Arg-Leu-NH2 has been identified as the active core of LPK 1.

Structural Characteristics
The active conformation of a cyclic pyrokinin analog was determined by both experimental and computational techniques. The tight constraints on the active core structure of this cyclized peptide, which maintains biological activity despite its relative rigidity, suggest that this is the conformation recognized by the myotropic receptor. Members of this insect neuropeptide family share the common C-terminal pentapeptide sequence Phe-Xaa-Pro-Arg- Leu-NH2 (Xaa = Ser, Thr, or Val). Circular dichroic, nuclear magnetic resonance, and molecular dynamics analyses of the conformationally restricted cyclic pyrokinin analog cydo(-Asn- Thr-Ser-Phe-Thr-Pro-Arg-Leu-) indicated the presence of a ß-turn in the active core region encompassing residues Thr- Pro-Arg-Leu. The rigid cyclic analog retains biological activity, suggesting that its C-terminal ß-turn is the active pyrokinin conformation recognized by the myotropic receptor. As individual pyrokinins and pyrokinin-like neuropeptides demonstrate both oviduct-contractile and pheromone-biosynthesis
activities in various insects, the biologically active ß-turn structure holds broad significance for many biological processes 2.

Mode of Action
The pyrokinin insect neuropeptides stimulate contractions of cockroach proctodeum (hindgut) and oviduct, and their myotropic activity can be readily tested with the isolated hindgut bioassay, which is both rapid and reproducible. Intact central innervation is essential for the action of LPK on puparial contraction, whereas central neurones take no part in mediation of LPK action on tanning of the cuticle. An analysis of tensiometric recordings of muscular activity revealed that the actual time of LPK accelerated puparial contraction coincides with the beginning of the immobilisation/retraction phase. LPK accelerates the switch from wandering behaviour to immobilisation/retraction behaviour but has no effect on the onset and duration of motor patterns that normally underlie puparial contraction in controls. The morphology of an accelerated puparium is normal but its formation is temporally dissociated from normal ‘contraction patterns’ that are performed a long time after the puparium has contracted. It means that neuromuscular activity of larvae accelerated by LPK does not cease upon formation of the white puparium, but continues until the whole motor programme of pupariation behaviour is completed. Apparently the peptide acts on the integument by stimulating it to contract and shrink, and no specific patterns of muscular contractions are needed to properly shape the puparium.

Functions
LPK found in the corpora cardiaca, a neurosecretory organ analogous to the pituitary and hypothalamus glands of the vertebrate endocrine system. Three related neuropeptides from the locust are more potent stimulators of oviduct than hindgut contraction 3, 4. Insect neuropeptide leucopyrokinin and [2-8]-leucopyrokinin, a truncated analog without the first aminoacid of leucopyrokinin peptide chain exert an antinociceptive effect in rats. 

An insect neuropeptide LPK (pQTSFTPRLamide) accelerates pupariation in wandering larvae of the fleshfly Sarcophaga bullata. The period of sensitivity to the action of LPK begins approximately 4 h before pupariation. Within this period the degree of acceleration of contraction into the shape of a puparium is practically independent of the age at which the larvae are injected, while acceleration of tanning is distinctly more age dependent.

LPK elicited consistent proctolin-like responses on the hindgut, foregut, oviduct and heart of the Madeira cockroach, Leucophaea maderae, with increases in both amplitude and frequency of contraction. The brain and internal organs distribution of 125I-labeled [2-8]-leucopyrokinin ([2-8]-LPK), a truncated analog of LPK, an insect myotropic peptide injected into the lateral brain ventricle was determined in rats. A high accumulation of this analog in adrenals and in the hypothalamus and hippocampus of the brain was found. A lesser but significant [2-8]-LPK accumulation in other internal organs and parts of the brain was also observed 5.

References

1. Nachman RJ, Holman GM, Cook BJ (1986). Active fragments and analogs of the insect neuropeptide leucopyrokinin: structure-function studies. Biochem Biophys Res Commun., 137:936-942.
2. Nachman RJ, Victoria A. Robertso VA, Dyson HJ, Holman GM, Tainer JA (1991). Active conformation of an insect neuropeptide family (pyrokInin/P-turn/molecular dynamics/NMR/pheromone). PNAS., 88:4518-4522.
3. Schoofs L, Holman GM, Hayes TK, Tips A, Nachman RJ, Vandesaande F, DeLoof A (1990). Isolation, identification and synthesis of locustamyotropin (Lom-MT), a novel biologically active insect peptide. Peptide., 11(3):427-433.
4. Schoofs L, Holman GM, Hayes TK, Nachman RJ, DeLoof A (1990). Locustatachykinin I and II, two novel insect neuropeptides with homology to peptides of the vertebrate tachykinin family. FEBS Lett., 261(2):397-401.
5. Plech A, Rykaczewska-Czerwinska M, Ryszka F, Suszka-Switek A, Dolinska B, Konopinska D (2005).  Distribution of 125I-labeled [2-8]-leucopyrokinin, active analog of leucopyrokinin in rats.  Acta poloniae pharmaceutica  62(5):393-397.

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