Novel chimeric peptides based on endomorphins and ghrelin receptor antagonist produced supraspinal antinociceptive effects with reduced acute tolerance in mice

It is widely recognized that developing bi- or multifunctional opioid compounds could offer a valuable approach to pain management with fewer side effects compared to single-target compounds. In this study, we designed and characterized two novel chimeric peptides, EM-1-DLS and EM-2-DLS, incorporating endomorphins (EMs) and the ghrelin receptor antagonist [D-Lys3]-GHRP-6 (DLS). Functional assays demonstrated that EM-1-DLS and EM-2-DLS acted as κ-opioid receptor (κ-OR)-preferring agonists, weak μ-opioid receptors (μ-OR) and ghrelin receptor (GHSR) agonists. Upon intracerebroventricular (i.c.v.) administration in mice, both EM-1-DLS and EM-2-DLS exhibited dose- and time-dependent antinociceptive effects in the tail withdrawal test. EM-1-DLS demonstrated the highest antinociceptive potency among the peptides, with an ED₅₀ approximately 8-fold greater than EM-1, while EM-2-DLS showed comparable effects to EM-2. The antinociceptive actions of EM-1-DLS involved activation of GHS-R1α, μ-OR, and κ-OR, whereas EM-2-DLS acted via GHS-R1α, δ-OR, and κ-OR pathways. Additionally, acute antinociceptive tolerance was investigated, revealing that EM-1-DLS induced a tolerance ratio of 2.33-fold, significantly lower than the 5.19-fold ratio induced by EM-1. Cross-tolerance ratios between the chimeric peptides and EMs ranged from 0.92 to 1.76, indicating reduced tolerance compared to EMs alone. These findings highlight the potential of these chimeric peptides to mitigate pain with diminished tolerance development, suggesting a promising strategy for the development of new analgesic therapies with improved safety profiles.