Antinociceptive Effects And Opioid Side Effects Of Hybrid Peptides Based On ?-opioid/Ghrelin Receptors

Objectives:Opioid analgesics such as morphine can effectively treat pain,but long-term use will cause addiction,tolerance,constipation and other opioid side effects.Endomorphin-2?EM-2?is an endogenous ligand of?-opioid receptors.EM-2 has analgesic effect similar to morphine in the central and side effect is less.Disadvantages such as tolerance,addiction,constipation,and difficulty in entering the blood-brain barrier limit its clinical application.The endogenous brain-gut peptide ghrelin has the analgesic activity,but also has the advantages of promoting gastrointestinal transport and being in a position to penetrate the blood-brain barrier.Some studies have shown that the active fragments of ghrelin are a ghrelin receptor agonist.Ghrelin action fragments have analgesic activities and can cross the blood-brain barrier.In recent years,hybrid peptides based on EM-2 and other neuropeptides have shown good analgesic effects,and their opioid side effects have been partially reduced,but cannot be completely overcome.Based on the research ideas of hybrid peptides,this topic constructed and synthesized hybrid peptides based on?-opioid/ghrelin receptors G?1-5?-EM-2,EM-2-G?1-5?,G?1-9?-EM-2 and EM-2-G?1-9?.The analgesic activities and opioid side effects of hybrid peptides were investigated in order to provide some experimental ideas and theoretical support for the synthesis of novel analgesic molecules.Methods:The animals selected in this paper are all Kunming male 18-22 g mice,randomly grouped.The analgesic activities of hybrid peptides injected through the intracerebroventricular?i.c.v.?administration and intravenous?i.v.?administration was tested through a warm bath tail-flick experiment in mice,and its pathway of action and blood-brain barrier permeability were investigated.Through tolerance experiments,the tolerances of hybrid peptides were tested.The effects of hybrid peptides on gastrointestinal transport and constipation were explored through the gastric emptying experiments,intestinal transit experiments and bead latency experiments.Results:1.The hybrid peptides produced good analgesic effects after i.c.v.administration,the ED₅₀0 values of G?1-5?-EM-2,EM-2-G?1-5?,G?1-9?-EM-2 and EM-2-G?1-9?are31.61?26.62-37.54?nmol/kg,32.71?28.88-37.04?nmol/kg,19.56?15.45-24.77?nmol/kg and 37.06?30.05-45.04?nmol/kg.2.After various specific opioid receptor antagonists and ghrelin receptor antagonists[D-Lys³]-GHRP-6?DLS?were co-injected with these hybrid peptides,the results suggested that the analgesic pathway of the hybrid peptides were mediated by different opioid receptors and ghrelin receptors.3.G?1-5?-EM-2 produced excellent analgesic effect after i.v.administration.After naloxone and naloxone methionide were combinated with G?1-5?-EM-2,the results suggested that G?1-5?-EM-2 could reach the center through the blood-brain barrier and exert analgesic effects.4.The tolerances of the hybrid peptides were measured after i.c.v.administration.The results showed that compared with EM-2,G?1-5?-EM-2,EM-2-G?1-5?and EM-2-G?1-9?did not produce acute tolerance,tolerance rates are 1.06,1.39,1.08;the acute tolerance of G?1-9?-EM-2 was significantly reduced compared to EM-2,tolerance rate is 2.31.The chronic tolerances of the hybrid peptides were comparable to EM-2.There is only partially lower chronic cross-tolerance between hybrid peptides and EM-2.5.The gastric emptying and intestinal transit effects of hybrid peptides were measured after i.c.v.administration.The results showed that G?1-5?-EM-2 and EM-2-G?1-5?were more favorable for gastric emptying than EM-2;G?1-9?-EM-2was more conducive to intestinal transit advancement in mice than EM-2.6.The bead latency of hybrid peptides were measured after i.c.v.administration.The results showed that EM-2-G?1-5?and EM-2-G?1-9?were more conducive to shorten the bead latency than EM-2.Conclusion:The i.c.v.administration of hybrid peptides based on?-opioid/ghrelin receptors produced a good analgesic effects,and its analgesic effects mainly worked through opioid receptors and ghrelin receptors.The i.v.administration of G?1-5?-EM-2produced a good analgesic effect and could penetrate the blood-brain barrier.Compared with EM-2,the hybrid peptides greatly improved the analgesic tolerance and the partial hybrid peptides greatly improved its inhibitory effects on gastrointestinal transport.In conclusion,our results suggest that G?1-5?-EM-2 can produce better analgesic effects through central and peripheral administration,and can penetrate the blood-brain barrier and can better improve opioid side effects,and is expected to develop into a new analgesic drug.