Antinociceptive effects of central administration of the endogenous cannabinoid receptor type 1 agonist VDPVNFKLLSH-OH [(m)VD-hemopressin(α)], an N-terminally extended hemopressin peptide.

The cannabinoid system has been demonstrated to modulate the acute and chronic pain of multiple origins. Mouse VD-hemopressin(α) [(m)VD-Hpα], an 11-residue α-hemoglobin-derived peptide, was recently reported to function as a selective agonist of the cannabinoid receptor type 1 (CB1) in vitro. To characterize its behavioral and physiological properties, we investigated the in vivo effects of (m)VD-Hpα in mice. In the mouse tail-flick test, (m)VD-Hpα dose-dependently induced antinociception after supraspinal (EC50 = 6.69 nmol) and spinal (EC50 = 2.88 nmol) administration. The antinociceptive effects of (m)VD-Hpα (intracerebroventricularly and intrathecally) were completely blocked by N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3- carboxamide (AM251; CB1 antagonist), but not by 6-iodo-2-methyl-1-[2-(4-morpholinyl)ethyl]-1H-indol-3-yl(4-methoxyphenyl)-methanone (AM630; CB2 antagonist) or naloxone (opioid antagonist), showing its selectivity to the CB1 receptor. Furthermore, the central nervous system (CNS) effects of (m)VD-Hpα were evaluated in body temperature, locomotor activity, tolerance development, reward, and food intake assays. At the highly antinociceptive dose (3 × EC50), (m)VD-Hpα markedly exerted hypothermia and hypoactivity after supraspinal administration. Repeated intracerebroventricular injection of (m)VD-Hpα resulted in both development of tolerance to antinociception and conditioned place aversion. In addition, central injection of (m)VD-Hpα dose-dependently stimulated food consumption. These findings demonstrate that this novel cannabinoid peptide agonist induces CB1-mediated central antinociception with some CNS effects, which further supports a CB1 agonist character of (m)VD-Hpα. Moreover, the current study will be helpful to understand the in vivo properties of the endogenous peptide agonist of the cannabinoid CB1 receptor.

Neuropeptide FF attenuates the acquisition and the expression of conditioned place aversion to endomorphin-2 in mice.

It has been demonstrated that the endogenous mu opioid (MOP) agonist endomorphin-2 (EM-2) produces conditioned place aversion (CPA) and in contrast, morphine exerts opposite action. Neuropeptide FF (NPFF) was reported to act as a functional antagonist of mu opioid receptor and to exert opioid-modulating activities. The present study examined the influence of NPFF on the rewarding action of EM-2, using the unbiased conditioned place preference (CPP) paradigm. For testing the effect of NPFF on the acquisition of EM-2-induced CPA, NPFF and EM-2 were co-injected on the conditioning days without drug treatment on the followed test day. To explore the effect of NPFF on the expression of EM-2-induced CPA, EM-2 was administered alone on the conditioning days, and NPFF was given 5 min before placement in the CPP apparatus on the test day. The results showed that NPFF (2.5, 5 and 10 nmol, i.c.v.) alone caused little place preference change. However, NPFF dose-dependently reversed the acquisition of CPA induced by 30 nmol EM-2 (i.c.v.). Similarly, the expression of EM-2-induced CPA was also reduced by NPFF. Moreover, the effects of NPFF on the acquisition and the expression of EM-2-induced CPA were completely blocked by the NPFF receptors antagonist RF9 (10 nmol, i.c.v.). However, central injection of NPFF neither changed the locomotor activity nor modified the locomotor action of EM-2. These data provide the first evidence for a functional interaction of the endogenous ligands for NPFF and MOP receptors, and further support an anti-opioid character of NPFF system.

Neuropeptide FF and related peptides attenuates warm-, but not cold-water swim stress-induced analgesia in mice.

Neuropeptide FF (NPFF) belongs to a neuropeptide family including two receptors (NPFF(1) and NPFF(2)). NPFF system has been reported to play important roles in pain transmission. The aim of the present study was to investigate the roles of NPFF related peptides and their receptors in swim stress-induced analgesia (SIA). Nociceptive test was performed in mice stressed by forced swimming in water at 15 °C (cold water swimming) or 32 °C (warm water swimming). Warm water swimming produced a naloxone-mediated antinociceptive effect. This warm water swim SIA was dose-dependently antagonized by i.c.v. injection of NPFF and two related peptides (3-30 nmol), NPVF and dNPA, which exhibited the highest selectivities for NPFF(1) and NPFF(2) receptors, respectively. Moreover, the selective NPFF receptor antagonist RF9 (30 nmol) was inactive by itself, but prevented the effects of NPFF and related peptides. Cold-water swimming produced a wilder analgesic effect that was blocked by MK-801, but not naloxone. However, NPFF system failed to modify the cold water swim stress-induced analgesia. These findings demonstrated that NPFF and related peptides attenuated opioid-mediated form of SIA via NPFF receptors in the brain, but not non-opioid swim stress-induced analgesia. These data further support an anti-opioid character of NPFF system.