G(1-5)-EM2, a multi-targeted agonist to opioid and growth hormone secretagogue receptors exhibited nontolerance forming antinociceptive effects in a mouse model of burn pain

Burn induced-pain (BIP) is one of the most common pain symptoms, which seriously affects the quality of sufferer life. Researches show that multi-targeted drug therapies offer superior efficacy and fewer side effects compared to single-target drug therapies. Consequently, in this study, we developed G(1-5)-EM2, a multi-targeted peptide designed to target μ-opioid receptor and the growth hormone secretagogue receptor 1α (GHS-R1α), and explored its antinociceptive effects on burn injury pain. Calcium mobilization experiments revealed that G(1-5)-EM2 demonstrated weak multi-agonist activities to μ-opioid receptor and κ-opioid receptor as well as GHS-R1α in vitro. Near-infrared fluorescence imaging experiments demonstrated that G(1-5)-EM2 could penetrate the blood-brain barrier (BBB) and access the brain following intravenous injection. The enzymatic stability of G(1-5)-EM2 was significantly enhanced compared to EM2. Our results indicated that intrathecal administration of G(1-5)-EM2 mitigated mechanical allodynia and thermal hyperalgesia in BIP. These antinociceptive effects of G(1-5)-EM2 were partially mediated through μ-opioid receptor and GHS-R1α. Moreover, intrathecal administration of G(1-5)-EM2 significantly decreased burn-induced up-regulation of phosphorylated p38 MAPK, phosphorylated NF-κBp65 and TRPV1 in the ipsilateral spinal cord, reduced the levels of IL-1β, IL-6 and TNF-α in serum, and enhanced wound healing in burned skin. Repeated intrathecal administration of G(1-5)-EM2 produced a non-tolerance-forming antinociception in BIP. These results suggest that the multi-targeted peptide G(1-5)-EM2 exhibits a novel role in alleviating BIP with fewer side effects and may represent a promising strategy for developing new analgesic drugs.