Endokinin A/B stimulates rat gastric motility through myogenic NK1 receptors located in the fundus.

Endokinin A/B (EKA/B), the common C-terminal decapeptide in endokinins A and B, is a preferred ligand of the NK1 receptor and regulates pain and itch. The study focused on the effects of EKA/B on rat gastric motility in vivo and in vitro. Gastric emptying was measured to evaluate gastric motility in vivo. Intragastric pressure and the contraction of gastric muscle strips were measured to evaluate gastric motility in vitro. Moreover, various neural blocking agents and neurokinin receptor antagonists were applied to explore the mechanisms. TAC4 and TACR1 mRNAs were expressed throughout rat stomach. EKA/B promoted gastric emptying by intraperitoneal injection in vivo. Correspondingly, EKA/B also increased intragastric pressure in vitro. Additionally, EKA/B contracted the gastric muscle strips from the fundus but not from the corpus or antrum. Further studies revealed that the contraction induced by EKA/B on muscle strips from the fundus could be significantly reduced by NK1 receptor antagonist SR140333 but not by NK2 receptor antagonist, NK3 receptor antagonist, or the neural blocking agents used. Our results suggested that EKA/B might stimulate gastric motility mainly through the direct activation of myogenic NK1 receptors located in the fundus.

Pain regulation of endokinin A/B or endokinin C/D on chimeric peptide MCRT in mice.

The present study focused on the interactive pain regulation of endokinin A/B (EKA/B, the common C-terminal decapeptide in EKA and EKB) or endokinin C/D (EKC/D, the common C-terminal duodecapeptide in EKC and EKD) on chimeric peptide MCRT (YPFPFRTic-NH2, based on YPFP-NH2 and PFRTic-NH2) at the supraspinal level in mice. Results demonstrated that the co-injection of nanomolar EKA/B and MCRT showed moderate regulation, whereas 30 pmol EKA/B had no effect on MCRT. The combination of EKC/D and MCRT produced enhanced antinociception, which was nearly equal to the sum of the mathematical values of single EKC/D and MCRT. Mechanism studies revealed that pre-injected naloxone attenuated the combination significantly compared with the equivalent analgesic effects of EKC/D alone, suggesting that EKC/D and MCRT might act on two totally independent pathways. Moreover, based on the above results and previous reports, we made two reasonable hypotheses to explain the cocktail-induced analgesia, which may potentially pave the way to explore the respective regulatory mechanisms of EKA/B, EKC/D, and MCRT and to better understand the complicated pain regulation of NK1 and µ opioid receptors, as follows: (1) MCRT and endomorphin-1 possibly activated different µ subtypes; and (2) picomolar EKA/B might motivate the endogenous NPFF system after NK1 activation.

MCRT, a chimeric peptide based on morphiceptin and PFRTic-NH2, regulates the depressor effects induced by endokinin A/B.

The interactions of the chimeric peptide MCRT (YPFPFRTic-NH2), based on morphiceptin and neuropeptide FF derivative PFRTic-NH2, on the effects of endokinin A/B (EKA/B) on mean arterial blood pressure of the urethane-anaesthetized rat have been investigated in the absence and presence of tachykinin receptor antagonists, naloxone and NO synthase inhibitors. While MCRT produced dose dependent decreases in mean arterial pressure, in its presence only a small but statistically insignificant decreases in the magnitude and the time course of the depressor effect of EKA/B (10nmol/kg) were observed. MCRT had little influence on the depressor effect of EKA/B (1 nmol/kg), but strongly potentiated that of EKA/B (100nmol/kg). The tachykinin NK1 receptor antagonist SR140333B (1mg/kg) and the NK3 antagonist SR142891 (2.79mg/kg) both reduced the hypotensive effects of EKA/B and MCRT alone and blocked those of the two peptides in combination. The NK2 antagonist GR159897 (4mg/kg) partially blocked the depressor effects of EKA/B and MCRT alone. Naloxone (2mg/kg) completely blocked the depressor effect of MCTR, but partially blocked that of EKA/B. The NO synthase inhibitor l-NAME (50mg/kg) partially blocked the depressor effects of EKA/B, MCRT, and EKA/B + MCRT. These results could help to better understand the role of tachykinin receptors, opioid receptors and neuropeptide FF receptors in cardiovascular system.

Interaction of endokinin A/B and (Mpa(6))-γ2-MSH-6-12 in pain regulation in mice.

The present study focused on the interactive effects of (Mpa(6))-γ2-MSH-6-12 (Mpa, spinal level) and endokinin A/B (EKA/B, supraspinal level) on pain regulation in mice. EKA/B (30 pmol) only weakened 100 pmol Mpa-induced hyperalgesia at 5 min, but could enhance it during 20-30 min. However, EKA/B (100 pmol) antagonized all dose levels of Mpa significantly at 5 min and blocked them completely at 10 min. EKA/B (3 nmol) co-injected with Mpa presented marked analgesia at 5 min and enduring hyperalgesia within 20-60 min. To investigate the underlying mechanisms between Mpa and EKA/B, SR140333B and SR142801 (NK1 and NK3 receptor antagonists, respectively) were utilized. SR140333B had no influence on Mpa, while SR142801 potentiated it during 20-30 min. Whereas, SR140333B and SR142801 could block the co-administration of Mpa and EKA/B (30 pmol) separately at 5 min and 30 min. These phenomena might attribute to that these two antagonists promoted the antagonism of EKA/B (30 pmol) at the early stage, while antagonized EKA/B preferentially in the latter period. SR140333B weakened the analgesia of EKA/B (3 nmol), but produced no effect on Mpa. However, SR140333B failed to affect the co-injection of Mpa and EKA/B, which implied that EKA/B cooperated with Mpa prior to SR140333B. These results could potentially help to better understand the interaction of NK and MrgC receptors in pain regulation in mice.