Targeting MOR-mGluR5 heteromers reduces bone cancer pain by activating MOR and inhibiting mGluR5.

Pain is among the most common symptoms in cancer and approximately 90% of patients experience end-stage cancer pain. The management of cancer pain is challenging due to the significant side effects associated with opioids, and novel therapeutic approaches are needed. MMG22 is a bivalent ligand containing MOR agonist and mGluR5 antagonist pharmacophores joined by a 22-atom spacer. MMG22 exhibited extraordinary analgesia following intrathecal administration in a mouse model of bone cancer pain. Here, we assessed the effectiveness of systemic administration of MMG22 in reducing cancer pain and evaluated whether MMG22 displays side effects associated with opioids. Fibrosarcoma cells were injected into and around the calcaneus bone in C3H mice. Mechanical hyperalgesia was defined as an increase in the paw withdrawal frequencies (PWFs) evoked by application of a von Frey monofilament (3.9 mN bending force) applied to the plantar surface of the hind paw Subcutaneous (s.c.), intramuscular (i.m.), and oral (p.o.) administration of MMG22 produced robust dose-dependent antihyperalgesia, whose ED50 was orders of magnitude lower than morphine. Moreover, the ED50 for MMG22 decreased with disease progression. Importantly, s.c. administration of MMG22 did not produce acute (24 h) or long-term (9 days) tolerance, was not rewarding (conditioned place preference test), and did not produce naloxone-induced precipitated withdrawal or alter motor function. A possible mechanism of action of MMG22 is discussed in terms of inhibition of spinal NMDAR via antagonism of its co-receptor, mGluR5, and concomitant activation of neuronal MOR. We suggest that MMG22 may be a powerful alternative to traditional opioids for managing cancer pain. This article is part of the Special Issue entitled 'New Vistas in Opioid Pharmacology'.

Denatured G-protein coupled receptors as immunogens to generate highly specific antibodies.

G-protein coupled receptors (GPCRs) play a major role in a number of physiological and pathological processes. Thus, GPCRs have become the most frequent targets for development of new therapeutic drugs. In this context, the availability of highly specific antibodies may be decisive to obtain reliable findings on localization, function and medical relevance of GPCRs. However, the rapid and easy generation of highly selective anti-GPCR antibodies is still a challenge. Herein, we report that highly specific antibodies suitable for detection of GPCRs in native and unfolded forms can be elicited by immunizing animals against purified full length denatured recombinant GPCRs. Contrasting with the currently admitted postulate, our study shows that an active and well-folded GPCR is not required for the production of specific anti-GPCR antibodies. This new immunizing strategy validated with three different human GPCR (µ-opioid, κ-opioid, neuropeptide FF2 receptors) might be generalized to other members of the GPCR family.

The effect of intrathecal mu, delta, kappa, and alpha-2 agonists on thermal hyperalgesia induced by mild burn on hind paw in rats.

PURPOSE: Mild cutaneous thermal injury, leading to a first-degree burn, induces a sensation of burning pain and enhances the pain sensitivity of the skin. Opioid and α(2) receptor agonists are commonly used to reduce such hyperalgesia. We investigated conditions that induced adequate thermal hyperalgesia in rats and compared the effects of µ, δ, κ, and α(2) receptors at the level of the spinal cord in this model. METHODS: A total of 149 male Sprague-Dawley rats were submitted to this study. A first-degree burn injury was induced in the hind paw by contact with a hot plate. The nociceptive threshold was determined by measuring the time from the application of a light beam to the hind paw to the withdrawal response (paw withdrawal latency, PWL). Various hot-plate exposure times and light beam intensities were tested to determine the conditions that induced adequate hyperalgesia. We also tested the effects of intrathecal morphine (µ agonist), DPDPE ([D-Pen2, D-Pen5] enkephalin, a δ agonist), U50488H (trans(+)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl]-benzacetamide methane sulfonate salt, a κ agonist), and ST-91 (2-[2,6-diethyl-phenylamino]-2-imidazoline, an α(2) agonist) on PWL. RESULTS: A first-degree burn was induced by contact with the hot plate for 45 s. Using current of 5.0 A, PWL was reduced by 40% from baseline. Intrathecally administered morphine, DPDPE, and ST-91, but not U50488H, showed dose-dependent antinociceptive effects in both injured and normal paws. CONCLUSIONS: Based on these findings, we could find adequate conditions for thermal hyperalgesia model. In this experimental model, µ, δ, and α(2) receptor agonists produced antinociceptive effects at the level of the spinal cord, but the κ receptor agonist did not.

mu-Opioid receptors and limbic responses to aversive emotional stimuli.

Functional neuroimaging studies implicate limbic and paralimbic activity in emotional responses, but few studies have sought to understand neurochemical mechanisms which modulate these responses. We have used positron emission tomography to measure mu-opioid receptor binding, and cerebral blood flow in the same subjects, and demonstrated that the baseline binding potential and the regional cerebral blood flow in the left inferior temporal pole are functionally related. Higher baseline mu-opioid receptor binding potential was associated with lower regional cerebral blood flow in this region during presentation of emotionally salient stimuli. This is consistent with an inhibitory/anxiolytic role of the endogenous opioid system in limbic regions of the temporal lobe and basal forebrain.

Long-term voluntary access to running wheels decreases kappa-opioid antinociception.

Previous research has demonstrated that voluntary exercise is associated with a reduction in mu-opioid-induced antinociception. To determine if the effects of voluntary exercise on opioid-induced antinociception were limited to drugs that affect the mu opioid receptor or were more general, the analgesic effects of the kappa opioid agonist U50,488H were compared in active and sedentary rats. Eight adult male Long-Evans rats were housed in standard hanging cages and eight in cages with attached running wheels for 20 days prior to antinociceptive testing. Pain thresholds were determined using a tail-flick procedure, and antinociception was expressed as percent maximal possible effect (%MPE). In the first study, U50,488H was administered in a cumulative dosing procedure (5.0, 10.0, 20.0 mg/kg). Tail-flick latencies were measured immediately prior to and 30 min following each injection. In the second study, the time course of U50,488H effects was examined in animals from the first experiment. Tail-flick latencies were measured immediately prior to and 30, 60, and 90 min following 10.0 mg/kg U50,488H. In the first study, U50,488H produced significant antinociception in both groups of rats. However, antinociceptive responses were significantly reduced for rats given access to running wheels relative to inactive rats. In the second study, antinociceptive responses to U50,488H continued for 90 min. Again, antinociceptive responses were lower for rats given access to running wheels relative to inactive rats. These results indicate that long-term voluntary exercise decreases the antinociceptive properties of the kappa agonist U50,488H, as well as the mu agonist morphine.