NPYFa, A Chimeric Peptide of Met-Enkephalin, and NPFF Induces Tolerance-Free Analgesia

Methionine-enkephalin-Arg-Phe is an endogenous amphiactive analgesic peptide. Neuropeptide FF, on the other hand, is reported for its role in opioid modulation and tolerance development. Based on these reports, in the present study we designed a chimeric peptide NPYFa (YGGFMKKKPQRFamide), having the Met-enkephalin (opioid) and PQRFamide sequence of neuropeptide FF, which can then target both the opioid and neuropeptide FF receptors. We hypothesized that the chimeric peptide so designed would have both analgesic properties and further aid in understanding of the role of neuropeptide FF in the development of opiate tolerance. Our studies indicated that NPYFa induced an early onset, potent, dose-dependent and prolonged antinociception. Additionally, antagonists (MOR, KOR, and DOR) pretreatment studies determined a KOR-mediated antinociception activity of the ligand. Further, in vitro binding studies using the Eu-GTP-γS binding assay on cell lines expressing opioid and NPFF receptors showed binding to both the opioid and neuropeptide FF receptors suggesting a multiple receptor binding character of NPYFa. Moreover, chronic (6 days) treatment with NPYFa exhibited an absence of tolerance development subsequent to its analgesia. The current study proposes NPYFa as a potent, long-acting antinociceptor lacking tolerance development as well as a probe to study opioid analgesia and the associated complex mechanisms of tolerance development.