NALOXONE FAILS TO PRODUCE CONDITIONED PLACE AVERSION IN W-OPIOID RECEPTOR KNOCK-OUT MICE P. D. SKOUBIS, a;c H. W. MATTHES, b W. M. WALWYN, c B. L. KIEFFER b and N. T. MAIDMENT c * a Interdepartmental Neuroscience Ph.D. Program, University of California at Los Angeles, Los Angeles, CA 90024, USA b CNRS UPR 9050, Illkirch, France c Department of Psychiatry and Biobehavioral Sciences, University of California at Los Angeles, Neuropsychiatric Institute, 760 Westwood Plaza Boulevard, Los Angeles, CA 90024-1759, USA AbstractöThere is growing evidence that tonic activity of the opioid system may be important in the modulation of a¡ective state. Naloxone produces a conditioned place aversion in rodents, an e¡ect that is centrally mediated. Previous pharmacological data using antagonists with preferential actions at W-, N-, and U-opioid receptors indicate the importance of the W-opioid receptor in mediating this e¡ect. We sought to test the W-opioid receptor selectivity of naloxone aversion using W-opioid receptor knock-out mice. W-Opioid receptor knock-out and wild-type mice were tested for naloxone (10 mg/kg, s.c.) aversion using a place conditioning paradigm. As a positive control for associative learning, knock-out mice were tested for conditioned place aversion to a U agonist, U50,488H (2 mg/kg, s.c.). Naloxone produced a signi¢cant place aversion in wild-type mice, but failed to have any e¡ect in W-opioid receptor knock-out mice. On the other hand, both knock-out and wild-type mice treated with U50,488H spent signi¢cantly less time in the drug-paired chamber compared to their respective vehicle controls. We conclude that the W-opioid receptor is crucial for the acquisition of naloxone-induced conditioned place aversion. Furthermore, in a separate experiment using C57BL/6 mice, the N-selective antagonist naltrindole (10 or 30 mg/kg, s.c.) failed to produce conditioned place aversion. Taken together, these data further support the notion that naloxone produces aversion by antagonizing tonic opioid activity at the W-opioid receptor. ß 2001 IBRO. Published by Elsevier Science Ltd. All rights reserved. Key words: opioid, aversion, knock-out, antagonists, a¡ect, locomotion. The rewarding and reinforcing e¡ects of opiates such as morphine and heroin are thought to be mediated primar- ily through an agonist action at the W-opioid receptors (MOR). Concurrent administration of selective MOR antagonists reverses the actions of morphine and heroin in both self-administration and conditioned place prefer- ence (CPP) paradigms (Negus et al., 1993; Piepponen et al., 1997). Furthermore, a line of genetically engineered mice lacking the MOR fail to demonstrate morphine- induced CPP (Matthes et al., 1996). However, N-opioid receptors (DOR) may also be important mediators of reward and reinforcing behavior since DOR-selective agonists are e¡ective in both CPP (Stapleton et al., 1979; Phillips and LePiane, 1982; Phillips et al., 1983; Shippenberg et al., 1987; Bals-Kubik et al., 1990; Longoni et al., 1998) and self-administration paradigms (Glimcher et al., 1984; Goeders et al., 1984a,b; Devine and Wise, 1994). There is growing evidence that tonic activity of the opioid system may be important for maintenance of `hedonic homeostasis' (Koob and Le Moal, 1997). Opioid antagonists are known to produce dysphoria (Hollister et al., 1981; Martin del Campo et al., 1994) or tension/anxiety (Grevert and Goldstein, 1977a) in humans and to elicit aversion in animal models of a¡ect (Downs and Woods, 1976; Grevert and Goldstein, 1977b; Mucha and Iversen, 1984; Mucha et al., 1985; Mucha and Herz, 1985; Mucha and Walker, 1987). Such aversion is centrally mediated as i.c.v. administration of a general opioid antagonist, naloxone (NLX), produces conditioned place aversion (CPA) in rats (Bals-Kubik et al., 1989). Moreover, methylated analogues of NLX, which do not readily penetrate the blood^brain barrier, do not produce CPA in mice when administered periph- erally (Hand et al., 1988; Kuzmin et al., 1997). It is not clear whether endogenous ligands act solely at the MOR to maintain a stable a¡ective state or if the DOR may play a role. The preferential MOR antagonist, D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP), administered either i.c.v. (Bals-Kubik et al., 1989) or locally into the ventral tegmental area (VTA) and nucleus accumbens (NAC) (Shippenberg and Bals- Kubik, 1995), produces CPA in rats whereas the prefer- 757 *Corresponding author. Tel. : +1-310-206-7767 ; fax : +1-310-825- 7067. E-mail address : nmaidmen@ucla.edu (N. T. Maidment). Abbreviations : ANOVA, analysis of variance ; CPA, conditioned place aversion; CPP, conditioned place preference; CTA, condi- tioned taste aversion ; CTOP, D-Phe-Cys-Tyr-D-Trp-Orn-Thr- Pen-Thr-NH2 ; DOR, N-type opioid receptor; MOR, W-type opioid receptor ; MOR3/3, W-opioid receptor knock-out ; NAC, nucleus accumbens ; NLX, naloxone ; NTD, naltrindole ; U50,488H, [trans-3,4-dichloro-N-methyl-N-(2-(1-pyrrolidinyl) cyclohexyl)-benzeneacetamide] methane-sulfonate salt; VEH, vehicle ; VTA, ventral tegmental area ; WT, wild-type. NSC 5167 18-10-01 www.neuroscience-ibro.com Neuroscience Vol. 106, No. 4, pp. 757^763, 2001 ß 2001 IBRO. Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved PII:S0306-4522(01)00333-5 0306-4522 / 01 $20.00+0.00