Abstract
A C-terminal analog of the hexapeptide orphanin FQ/nociceptin-(1–6), [Ala 6]-orphanin FQ/nociceptin-(1–6), and a pentapeptide orphanin FQ/nociceptin-(1–5) were tested in vivo for their analgesic/hyperalgesic activity in the hot-plate test with rats. Replacement of the C-terminal glycine by l-alanine (Phe-Gly-Gly-Phe-Thr-Ala) in orphanin FQ/nociceptin-(1–6) abolished the hyperalgesic potency of native orphanin FQ/nociceptin-(1–6) (Phe-Gly-Gly-Phe-Thr-Gly), but analgesic activity was retained and was diminished by naloxone. Removal of the C-terminal amino acid (glycine or alanine) from orphanin FQ/nociceptin-(1–6) caused a significant loss of analgesic activity. It is anticipated that glycine plays a crucial role in the biphasic activity of orphanin FQ/nociceptin-(1–6). This may suggest the existence of a mechanism for terminating the biological action of orphanin FQ/nociceptin.
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