Pharmacological Research 60 (2009) 284–290 Contents lists available at ScienceDirect Pharmacological Research journal homepage: www.elsevier.com/locate/yphrs Cooperative N-methyl-d-aspartate (NMDA) receptor antagonism and -opioid receptor agonism mediate the methadone inhibition of the spinal neuron pain-related hyperactivity in a rat model of neuropathic pain Maria Luisa Sotgiu a,∗ , Maurizio Valente a , Riccardo Storchi a,b , Giancarlo Caramenti c , Gabriele E.M. Biella a a Institute of Molecular Bioimaging and Physiology (IBFM), National Research Council (CNR), Segrate, (Milan), Italy b Dept. of Biomedical Sciences, School of Neuroscience, University of Modena, Italy c Institute of Biomedical Technology (ITB), National Research Council (CNR), Segrate, (Milan), Italy article info Article history: Received 29 January 2009 Received in revised form 6 April 2009 Accepted 6 April 2009 Keywords: Methadone Opioid receptors NMDA receptors Rat spinal WDR neurons Neuropathic pain abstract Methadone (Racemic methadone) exerts its antinociceptive effect by activation of -opioid receptors and/or blockade of NMDA receptors. The aim of this study is to determine whether the methadone analgesic effect on neuropathic pain is achieved only by the agonism of the -opioid receptors or cooperatively with the antagonism of the NMDA receptors. To this purpose, in rats with neuropathic pain model of chronic constriction of one sciatic nerve (CCI rats), we administered methadone before or after opioid receptor blockade with naloxone and checked its effects on the spinal Wide Dynamic Range (WDR) neuron dynamics in three experimental conditions: on the spontaneous and noxious evoked neuronal activities in control rats (sham operated and naïve); on iontophoretic NMDA induced neuronal hyperactivity in intact rats; on pain-related spontaneous and noxious evoked hyperactiv- ities in CCI rats. The results, as from the spike-frequency analysis, show that: (i) in control rats, methadone inhibits the noxious evoked neuronal activity and naloxone prevents or reverses about 94% of methadone inhibitory effect; (ii) in intact rats, pretreated with naloxone, methadone reduces the NMDA induced neuronal hyperactivity; (iii) in CCI rats, methadone inhibits the neuronal spontaneous and noxious evoked hyperactivities, and naloxone prevents or reverses about 60% of methadone inhibitory effect. These findings allow to conclude that methadone inhibition of the noxious evoked activity in normal rats is achieved predominantly through the agonism of the -opioid receptors, while the inhibition of the pain-related hyperactivity in rats with signs of neuropathic pain (CCI rats), involves also the NMDA receptors antagonism. © 2009 Elsevier Ltd. All rights reserved. 1. Introduction Methadone is an opioid -receptor agonist mainly used in replacement therapy in the course of heroin detoxification pro- cedures [1] and, to a lesser extent, as analgesic in chronic non-malignant pain [2] and in cancer pain [3,4]. Methadone is an atypical opioid both for the clinical behaviour and the mechanism of action. Clinical evidence show that it may induce analgesia in patients refractory to other congener drugs [5], and that, after methadone chronic infusion, a slower tolerance devel- ops than after morphine infusion [6]. As for the mechanism of action, studies in the rat central nervous system show that, ∗ Corresponding author at: IBFM-CNR, Bldg LITA, Via Fratelli Cervi 93, 20090 Segrate, (Milan), Italy. Tel.: +39 02 21717504; fax: +39 02 21717558. E-mail address: maria.luisa.sotgiu@ibfm.cnr.it (M.L. Sotgiu). other than activates the -opioid receptors, the drug also binds with low affinity NMDA receptors as non-competitive antagonist [7–9]. The commonly used laboratory form of methadone (used also in this study) is the racemic d-l-methadone mixture that recent data ranked as the most potent NMDA-receptor inhibiting opi- oid [10]. The methadone analgesic effect could thus result from the activation of -opioid receptors, reversible by naloxone, and the blockade of NMDA receptors. Experimental results show that the methadone-induced antinociception is mediated only by the -opioid receptors agonism in intact rats [11] and in rats with inflammatory pain [12], and involves also the NMDA receptors antagonism in the rat formalin test [13]. Given the crucial role played by the NMDA receptors activa- tion on neuropathic pain [14], we decided to deeper investigate the mechanisms of methadone antinociceptive action on this pathology. 1043-6618/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.phrs.2009.04.002