Study On Central Analgesic Effects Of Ghrelin Active Fragments And Their Blood-brain Barrier Permeabilities

Objectives:Ghrelin is an endogenous brain-gut peptide with analgesic effect discovered in recent years.Its unique N-terminal Ser³ acylation structure enables it to pass the blood-brain barrier smoothly,but difficult synthesis,high price and short half-life greatly limit its clinical application.The active fragments of ghrelin have N-terminal Ser³ acylated structure,which have good binding abilities with ghrelin receptor GHS-R1?.They are simple in structure and easy to synthesize.In this study,the central analgesic activities and receptor mechanisms of the active fragments G?1-5?-NH₂,G?1-7?-Lys-NH₂,G?1-9?and G?1-11?were investigated,and the blood-brain barrier permeabilities were further studied.The above experiments provided a reference for clinical research and development of effective analgesics with good analgesic effect,simple structure and good permeability of blood-brain barrier.Methods:The central analgesic activities of G?1-5?-NH₂,G?1-7?-Lys-NH₂,G?1-9?and G?1-11?were explored with tail withdrawal test.The relationships between the four ghrelin active fragments and GHS-R1?and opioid receptors were investigated after intracerebroventriculary co-injection.The blood-brain barrier permeabilities of ghrelin active fragments were investigated by peripheral naloxone methiodide and central naloxone.Naloxone methiodide did not pass the blood-brain barrier.Results:1.Four ghrelin active fragments caused concentration-and time-dependent analgesic effects in the tail withdrawal test after intracerebroventriculary injection?mouse,i.c.v.?.2.The analgesic effects caused by intracerebroventriculary injection of G?1-5?-NH₂,G?1-7?-Lys-NH₂ and G?1-11?could be antagonized by[D-Lys³]-GHRP-6?DLS?which is the antagonist of GHS-R1?.This indicated that GHS-R1?was involved in the analgesic effects caused by G?1-5?-NH₂,G?1-7?-Lys-NH₂ and G?1-11?.3.The central analgesic effect of G?1-5?-NH₂?i.c.v.?was mediated by?-opioid receptor and?-opioid receptor.The central analgesic effect of G?1-7?-Lys-NH₂?i.c.v.?was mediated by?-opioid receptor and?-opioid receptor,the partly central analgesic effect was mediated by?-opioid receptor.The central analgesic effect of G?1-9??i.c.v.?was mediated by?-opioid receptor.The central analgesic effect of G?1-11??i.c.v.?was mediated by?-opioid receptor and?-opioid receptor.4.After injection through the caudal vein,G?1-5?-NH₂ and G?1-9?caused concentration-and time-dependent analgesic effects in the tail withdrawal test.5.The analgesic effects caused by caudal vein injection of G?1-5?-NH₂ or G?1-9?could not be antagonized by naloxone methiodide?i.p.?which did not pass the blood-brain barrier.The analgesic effects caused by caudal vein injection of G?1-5?-NH₂ or G?1-9?could be antagonized by naloxone?i.c.v.?.The results showed that G?1-5?-NH₂ and G?1-9?had analgesic effects after being administered through the caudal vein and reached the center through the blood-brain barrier.Conclusion:Intracerebroventricular injection of ghrelin active fragments G?1-5?-NH₂,G?1-7?-Lys-NH₂,G?1-9?and G?1-11?produced obvious time and concentration-dependent analgesic effects.G?1-9?had the best analgesic effect among them.GHS-R1?was involved in the central analgesic effects caused by G?1-5?-NH₂,G?1-7?-Lys-NH₂ and G?1-11?.The central analgesic effects of G?1-5?-NH₂,G?1-7?-Lys-NH₂,G?1-9?and G?1-11?were all mediated by different opioid receptors.The active fragments G?1-5?-NH₂ and G?1-9?can produce analgesic effects in the center through the blood-brain barrier after intravenous injection.