1. ¹Department of Pharmacology and Anatomy, Rudolf Magnus Institute of Neuroscience, UMC Utrecht, Stratenum, Universiteitsweg 100, Utrecht, The Netherlands
2. ²Institute of Cytology and Genetics, SD RAS, Novosibirsk, Russia

Correspondence: Dr MAFM Gerrits, Department of Pharmacology and Anatomy, Rudolf Magnus Institute of Neuroscience, UMC Utrecht, Stratenum, Universiteitsweg 100, 3584 CG Utrecht, The Netherlands. Tel: +31 30 2538842; Fax: +31 30 2538155; E-mail: m.a.f.m.gerrits@med.uu.nl

Received 22 December 2003; Revised 19 May 2004; Accepted 18 June 2004; Published online 14 July 2004.

Abstract

Although -opioid receptors have been extensively investigated for their role in drug reinforcement, little is known about the contribution of these receptors to the acute and sensitized locomotor response to cocaine. In this study -opioid receptor involvement in acute cocaine-induced locomotor activity and in the development of cocaine-induced behavioral sensitization was evaluated using -opioid receptor knockout mice and chronic naltrexone (NTX) pretreatment as models. In addition, co-administration of the specific -opioid receptor antagonist CTOP with repeated saline or cocaine injections was used to establish the involvement of -opioid receptors in sensitization to the locomotor stimulant effects of cocaine. The acute locomotor response to cocaine (3, 10, 20, or 30 mg/kg i.p.) of -opioid receptor knockout or chronic NTX pretreated mice was not different from the cocaine response of their respective controls. With respect to cocaine-induced behavioral sensitization, induced by daily injections of 20 mg/kg cocaine for 11 subsequent days, -opioid receptor knockout mice developed behavioral sensitization to the locomotor stimulant effects of cocaine (challenge 10 mg/kg i.p.) comparable to wild-type littermates and the -opioid receptor antagonist CTOP did not affect cocaine-induced sensitization either. However, mice that were pretreated with NTX exhibited augmented cocaine-induced behavioral sensitization relative to placebo pretreated controls, which may be ascribed to increased -opioid receptor levels as has been described for chronic NTX pretreated mice. The present findings suggest that -opioid receptors are not required for the acute locomotor response to cocaine nor are they essential for the development of cocaine-induced behavioral sensitization.