Possible involvement of endogenous nociceptin/orphanin FQ in the pain-related behavioral responses induced by its own metabolite, nociceptin/orphanin FQ(14-17)

Nociceptin and its metabolite attenuate U0126-induced memory impairment through a nociceptin opioid peptide (NOP) receptor-independent mechanism

Nociceptin binds to nociceptin opioid peptide (NOP) receptors. We reported that although high doses of nociceptin impaired memory function and that these effects were mediated via NOP receptors, low doses of nociceptin attenuated the memory impairment, and these attenuating effects were not mediated via NOP receptors. Even very low doses of nociceptin were biologically active and suggested a certain binding site for this peptide, but the mechanism underlying this attenuating effect has not yet been elucidated. In the present study, we investigated the effect of an intrahippocampal injection (i.h.) of nociceptin on memory impairment induced by U0126, a MEK inhibitor, and Rp-cAMPS, a PKA inhibitor in a step-down type passive avoidance test. U0126 (2.63 nmol/mouse, i.h.) impaired memory formation and training-dependent phosphorylation of ERK2 in the hippocampus. Co-administration of nociceptin (10 fmol/mouse) significantly attenuated memory impairment, while it did not attenuate the inhibition of training-dependent phosphorylation of ERK2 induced by U0126. On the other hand, nociceptin did not attenuate memory impairment induced by Rp-cAMPS (0.448 nmol/mouse, i.h.). Nociceptin (1 fmol/mouse) also attenuated U0126 (5.26 nmol/mouse)-induced memory impairment in NOP receptor knockout mice. Nociceptin was reported to metabolize into fragments (1-13) and (14-17) in vivo, which showed pharmacological activities without affecting NOP receptors. Our findings showed that nociceptin (14-17) (1 fmol/mouse) also attenuated U0126-induced memory impairment, while nociceptin (1-13) (0.1-10 fmol/mouse) did not attenuate memory impairment. These results suggest a novel action site or mechanism for the attenuating effects of nociceptin and its metabolite, and the sequence of nociceptin (14-17) is a critical structure.

Distinct effects of nociceptin analogs on scopolamine-induced memory impairment in mice

Nociceptin, also known as orphanin FQ, binds to opioid receptor like-1 (NOP) receptors. Nociceptin and NOP receptor play important roles in several physiological functions in the central nervous system. We reported that although high doses of nociceptin impaired learning and memory and these effects were blocked by nocistatin, naloxone benzoylhydrazone and [NPhe(1)]nociceptin(1-13)NH(2), low doses of nociceptin improved scopolamine- or mecamylamine-induced impairment of learning and memory, and these ameliorating effects were not blocked by these antagonists. In the present study, to confirm our previous findings, the effects of [Arg(14), Lys(15)]nociceptin and [(pF)Phe(4)]nociceptin(1-13)NH(2), highly potent and long-lasting nociceptin analogs, on the memory impairment induced by scopolamine using the Y-maze and step-down type passive avoidance tests were investigated. [Arg(14), Lys(15)]Nociceptin (0.1 and/or 1 pmol/mouse, i.c.v.) significantly improved impairment of memory function. Although this analog was about 30 times more potent than nociceptin, the doses ameliorating these memory impairments were comparable to those of the natural ligand nociceptin. The ameliorating effects of [Arg(14), Lys(15)]nociceptin were not blocked by an NOP receptor antagonist, [NPhe(1)]nociceptin(1-13)NH(2). Interestingly, another potent nociceptin analog, [(pF)Phe(4)]nociceptin(1-13)NH(2) could not improve impairment of memory function. These results confirmed that there are novel mechanisms underlying these ameliorating effects and these seem not to be mediated via an NOP receptor.

Endogenous opiates and behavior: 2007

This paper is the thirtieth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2007 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.