[Tyr(6)]-gamma2-MSH(6-12) with a short effecting time of about 20 min is one of the most potent rMrgC receptor agonists. To possibly increase its potency and metabolic stability, a series of analogues were prepared by replacing the Tyr(6) residue with the non-canonical amino acids 3-(1-naphtyl)-L-alanine, 4-fluoro-L-phenylalanine, 4-methoxy-L-phenylalanine and 3-nitro-L-tyrosine. Dose-dependent nociceptive assays performed in conscious rats by intrathecal injection of the MSH peptides showed [Tyr(6)]-gamma2-MSH(6-12) hyperalgesic effects at low doses (5-20 nmol) and analgesia at high doses (100-200 nmol). This analgesic activity is fully reversed by the kyotorphin receptor-specific antagonist Leu-Arg. For the two analogues containing in position 6, 4-fluoro-L-phenylalanine and 3-nitro-L-tyrosine, a hyperalgesic activity was not observed, while the 3-(1-naphtyl)-L-alanine analogue at 10 nmol dose was found to induce hyperalgesia at a potency very similar to gamma2-MSH(6-12), but with longer duration of the effect. Finally, the 4-methoxy-L-phenylalanine analogue (0.5 nmol) showed greatly improved hyperalgesic activity and prolonged effects compared to the parent [Tyr(6)]-gamma2-MSH(6-12) compound.
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