Neuropharmacology 47 (2004) 1157–1169 www.elsevier.com/locate/neuropharm Modulation of morphine-induced Fos-immunoreactivity by the cannabinoid receptor antagonist SR 141716 M.E. Singh a , A.N.A. Verty a , I. Price a , I.S. McGregor b , P.E. Mallet a, a School of Psychology, University of New England, Armidale, NSW 2351, Australia b School of Psychology, University of Sydney, Sydney, NSW 2006, Australia Received 16 September 2003; received in revised form 19 July 2004; accepted 17 August 2004 Abstract A growing body of evidence suggests the existence of a functional interaction between opioid and cannabinoid systems. The present study further investigated this functional interaction by examining the combined effects of morphine and the cannabinoid receptor antagonist SR 141716 on Fos-immunoreactivity (Fos-IR), a marker for neural activation. Male albino Wistar rats were treated with SR 141716 (3 mg/kg, intraperitoneally), morphine HCl (10 mg/kg, subcutaneously), vehicle, or SR 141716 and mor- phine combined (n ¼ 6 per group). Rats were injected with morphine or its vehicle 30-min after administration of SR 141716 or its vehicle and perfused 3 h later. Locomotor activity and body temperature were both increased in the morphine-treated group and SR 141716 significantly inhibited these effects. Morphine increased Fos-IR in several brain regions including the caudate- putamen (CPu), cortex (cingulate, insular and piriform), nucleus accumbens (NAS) shell, lateral septum (LS), bed nucleus of the stria terminalis (BNST), median preoptic nucleus (MnPO), medial preoptic nucleus (MPO), hypothalamus (paraventricular, dor- somedial and ventromedial), paraventricular thalamic nucleus (PV), amygdala (central and basolateral nuclei), dorsolateral peria- queductal gray, ventral tegmental area (VTA), and Edinger–Westphal nucleus. SR 141716 alone increased Fos-IR in the cortex (cingulate, insular and piriform), NAS (shell), LS, BNST, hypothalamus (paraventricular, dorsomedial and ventromedial), PV, amygdala (central, basolateral and medial nuclei), VTA, and Edinger–Westphal nucleus. SR 141716 attenuated morphine-induced Fos-IR in several regions including the CPu, cortex, NAS (shell), LS, MnPO, MPO, paraventricular and dorsomedial hypothala- mus, PV, basolateral amygdala, VTA, and Edinger–Westphal nucleus (EW). These results provide further support for functional interplay between the cannabinoid and opioid systems. Possible behavioural and physiological implications of the interactive effects of SR 141716 on morphine-induced Fos-IR are discussed. # 2004 Elsevier Ltd. All rights reserved. Keywords: Immunochemistry; Opioid; Cannabinoid; Locomotion; Thermoregulation; Rat; Morphine; SR 141716 1. Introduction The brain’s cannabinoid and opioid systems produce many of the same physiological and behavioural effects. For example, administration of opioid or cannabinoid receptor agonists both produce hypother- mia, sedation, analgesia and other similar pharmaco- logical effects (Manzanares et al., 1999). These two systems also interactively modulate appetite (Gallate et al., 1999; Verty et al., 2003), analgesia (Tulunay et al., 1981), addiction and reward (Chen et al., 1990; Gardner et al., 1988). At present, not much is known about the neural substrates underlying cannabinoid– opioid interactions and the role of endogenous canna- binoid activation in mediating acute opiate effects. Studies using the expression of the immediate early gene c-fos to map the distribution of CNS neurons activated by stimulation in vivo provide a valuable tool to study the neuroanatomical substrates of many drugs of abuse. An increase in Fos-immunoreactivity (Fos-IR) is found following administration of numer- ous drugs including cocaine, amphetamine, MDMA (ecstasy), D 9 -tetrahydrocannabinol (THC), and mor- phine. These drugs produce characteristic patterns of c-fos expression indicating distinct distribution of drug-induced neuronal activation (Allen et al., 2003;  Corresponding author. Tel.: +61-2-6773-3725; fax: +61-2-6773- 3820. E-mail address: paul.mallet@une.edu.au (P.E. Mallet). 0028-3908/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.neuropharm.2004.08.008