The neuropeptide FF analogue, 1DMe, reduces in vivo dynorphin release from the rat spinal cord

Opioid-modulating properties of the neuropeptide FF system

Opioid receptors are involved in the control of pain perception in the central nervous system together with endogenous neuropeptides, termed opioid-modulating peptides, participating in a homeostatic system. Neuropeptide FF (NPFF) and related peptides possess anti-opioid properties, the cellular mechanisms of which are still unclear. The purpose of this review is to detail the phenomenon of cross-talk taking place between opioid and NPFF systems at the in vivo pharmacological level and to propose cellular and molecular models of functioning. A better knowledge of the mechanisms underlying opioid-modulating properties of NPFF has potential therapeutic interest for the control of opioid functions, notably for alleviating pain and/or for the treatment of opioid abuse.

Neuropeptide FF receptors have opposing modulatory effects on nociception

The role of neuropeptide FF (NPFF) and its analogs in pain modulation is ambiguous. Although NPFF was first characterized as an antiopioid peptide, both antinociceptive and pronociceptive effects have been reported, depending on the route of administration. Currently, two NPFF receptors, termed FF1 and FF2, have been identified and cloned, but their roles in pain modulation remain elusive because of the lack of availability of selective compounds suitable for systemic administration in in vivo models. Ligand-binding studies confirm ubiquitous expression of both subtypes in brain, whereas only FF2 receptors are expressed spinally. This disparity in localization has served as the foundation of the hypothesis that FF1 receptors mediate the pronociceptive actions of NPFF. We have identified novel small molecule NPFF receptor agonists and antagonists with varying degrees of FF2/FF1 functional selectivity. Using these pharmacological tools in vivo has allowed us to define the roles of NPFF receptor subtypes as pertains to the modulation of nociception. We demonstrate that selective FF2 agonism does not modulate acute pain but instead ameliorates inflammatory and neuropathic pains. Treatment with a nonselective FF1/FF2 agonist potentiates allodynia in neuropathic rats and increases sensitivity to noxious thermal and to non-noxious mechanical stimuli in normal rats in an FF1 antagonist-reversible manner. Treatment with FF1 antagonists reversed established mechanical allodynia, indicating the possibility of increased NPFF tone through FF1 receptors. In conclusion, we provide evidence for the opposing roles of NPFF receptors and highlight selective FF2 agonism and/or selective FF1 antagonism as potential targets warranting further investigation.

A neuropeptide FF agonist blocks the acquisition of conditioned place preference to morphine in C57Bl/6J mice

Neuropeptide FF behaves as an opioid-modulating peptide that seems to be involved in morphine tolerance and physical dependence. Nevertheless, the effects of neuropeptide FF agonists on the rewarding properties of morphine remain unknown. C57BL6 mice were conditioned in an unbiased balanced paradigm of conditioned place preference to study the effect of i.c.v. injections of 1DMe (D-Tyr1(NMe)Phe3]NPFF), a stable agonist of the neuropeptide FF system, on the acquisition of place conditioning by morphine or alcohol (ethanol). Morphine (10 mg/kg, i.p.) or ethanol (2 g/kg, i.p.) induced a significant place preference. Injection of 1DMe (1-20 nmol), given 10 min before the i.p. injection of the reinforcing drug during conditioning, inhibited the rewarding effect of morphine but had no effect on the rewarding effect of ethanol. However, a single injection of 1DMe given just before place preference testing was unable to inhibit the rewarding effects of morphine. By itself, 1DMe was inactive but an aversive effect of this agonist could be evidenced if the experimental procedure was biased. These results suggest that neuropeptide FF, injected during conditioning, should influence the development of rewarding effects of morphine and reinforce the hypothesis of strong inhibitory interactions between neuropeptide FF and opioids.

Neuropeptide FF (NPFF) analogs functionally antagonize opioid activities in NPFF2 receptor-transfected SH-SY5Y neuroblastoma cells

To elucidate the mechanism of the cellular antiopioid activity of neuropeptide FF (NPFF), we have transfected the SH-SY5Y neuroblastoma cell line, which expresses mu-and delta-opioid receptors, with the human NPFF2receptor. The selected clone, SH2-D9, expressed high-affinity NPFF2 receptors in the same range order as mu- and delta-opioid receptors (100-300 fmol/mg of protein). The NPFF analog [D-Tyr1, (NMe)Phe3]NPFF (1DMe) did not modify the binding parameters of the mu- and delta-specific agonists [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin and deltorphin-I, respectively. 1DMe dose dependently inhibited 75 to 80% of the cAMP production stimulated by forskolin. Preincubation with 1DMe halved the maximal inhibition of N-type Ca2+ channels by opioid agonists. In the presence of carbachol, acting on muscarinic receptors to release Ca2+ from the intracellular stores, deltorphin-I and 1DMe enhanced this release. Preincubation with 1DMe reduced the maximal effect of deltorphin-I by 40%, demonstrating an antiopioid effect in this experimental model for the first time. By using peptides corresponding to the carboxyl terminus of the alphai1,2, alphai3, alphao, and alphas subunits of G proteins, which specifically uncouple receptors from G proteins, we demonstrated that mu-opioid and NPFF2 receptors couple to the four subunits assayed. The Ca2+ release from the intracellular stores by 1DMe resulted from the coupling of NPFF2 receptors with Galphao and Galphai1,2, whereas the coupling with Galphas reduced the antiopioid effect of 1DMe in the modulation of N-type channels. This SH2-D9 cell line now provides the opportunity to study the interaction between both receptors.

Endogenous opiates and behavior: 2002

This paper is the twenty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2002 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 (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).