Selective activation of peripheral κ opioid receptors (KORs) may overcome the dose-limiting adverse effects of conventional opioid analgesics. We recently developed a vicinal disulfide-stabilized class of peptides with subnanomolar potency at the KOR. The aim of this study was to assess the analgesic effects of one of these peptides, named conorphin-1, in comparison with the prototypical KOR-selective small molecule agonist U-50488, in several rodent pain models. Surprisingly, neither conorphin-1 nor U-50488 were analgesic when delivered peripherally by intraplantar injection at local concentrations expected to fully activate the KOR at cutaneous nerve endings. While U-50488 was analgesic when delivered at high local concentrations, this effect could not be reversed by coadministration with the selective KOR antagonist ML190 or the nonselective opioid antagonist naloxone. Instead, U-50488 likely mediated its peripheral analgesic effect through nonselective inhibition of voltage-gated sodium channels, including peripheral sensory neuron isoforms NaV1.8 and NaV1.7. Our study suggests that targeting the KOR in peripheral sensory nerve endings innervating the skin is not an alternative analgesic approach.

No abstract

A large body of evidence now exists for the immune cell expression, production, and the release of beta-endorphin (BE 1–31) within inflamed tissue. The inflammatory milieu is characterised by increased acidity, temperature and metabolic activity. Within these harsh conditions BE 1–31 is even more susceptible to increased enzymatic degradation over that of plasma or other non-injured tissue. To elucidate the biotransformation pathways of BE 1–31 and provide an insight to the impact of inflamed tissue environments, BE 1–31 and three of its major N-terminal fragments (BE 1–11, BE 1–13 and BE 1–17) were incubated in inflamed tissue homogenates at pH 5.5 for 2 hrs. In addition, the potency of BE 1–31 and five main N – terminal fragments (BE 1–9, BE 1–11, BE 1–13, BE 1–17, BE 1–20) was assessed at mu-opioid receptors (MOR), delta-opioid receptors (DOR), and kappa-opioid receptors (KOR). Opioid receptor potency was investigated by examining the modulation of forskolin induced cAMP accumulation. The majority of the N-terminal fragment of BE 1–31 had similar efficacy to BE 1–31 at MOR. The shortest of the major N-terminal fragments (BE 1–9), had partial agonist activity at MOR but possessed the highest potency of all tested peptides at DOR. There was limited effect for BE 1–31 and the biotransformed peptides at KOR. Major N-terminal fragments produced within inflamed tissue have increased presence within inflamed tissue over that of the parent molecule BE 1–31 and may therefore contribute to BE 1–31 efficacy within disease states that involve inflammation.

No abstract