Peripherally acting NMDA receptor/glycine B site receptor antagonists inhibit morphine tolerance * Wojciech Danysz a , Ewa Kozela b , Chris G. Parsons a , Meik Sladek a , Tanja Bauer a , Piotr Popik b, ) a Merz Pharmaceuticals GmbH, Eckenheimer Landstrasse 100, 60318 Frankfurt/Main, Germany b Institute of Pharmacology PAN, Smetna 12, 31-343 Krako ´w, Poland Received 17 June 2004; received in revised form 29 October 2004; accepted 27 November 2004 Abstract The present study focused on the role of peripheral ionotropic N-methyl-D-aspartate (NMDA) receptors in the development of tolerance to morphine-induced antinociception. An initial experiment revealed that NMDA channel blocker memantine, and NMDA receptor/glycine B site antagonist MRZ 2/576 inhibited maximal electroshock-induced convulsions (MES) in female NMR mice with respective potency of 5.93 and 20.8 mg/kg, while other NMDA receptor/glycine B site antagonists MRZ 2/596 and MDL 105,519 were ineffective, supporting lack of CNS activity of the latter two agents. This observation was also supported by blood– brain barrier experiments in vitro. In male Swiss mice, morphine (10 mg/kg) given for 6 days twice a day (b.i.d.) produced tolerance to its antinociceptive effects in the tail-flick test. The NMDA receptor/glycine B site antagonists, MRZ 2/576 at 0.03, 0.1, 0.3 mg/kg and MRZ 2/596 at 0.1, 0.3, 3 and 10 mg/kg attenuated the development of morphine tolerance. Similarly, in male C57/Bl mice, morphine (10 mg/kg) given for 6 days b.i.d. produced tolerance to its antinociceptive effects in the tail-flick test. Like in Swiss mice, in C57/Bl mice morphine tolerance was attenuated by both MRZ 2/576 and MRZ 2/596. Another NMDA receptor/glycine B site receptor antagonist, MDL 105,519 (that very weakly penetrates to the central nervous system) also inhibited morphine tolerance at the dose of 1 but not 0.1 mg/kg. Moreover, both naloxone hydrochloride (5 and 50 mg/kg) and centrally inactive naloxone methiodide (50 mg/kg) inhibited morphine tolerance suggesting the involvement of peripheral opioid receptors in this phenomenon. The present data suggest that blockade of NMDA receptor/glycine B sites in the periphery may attenuate tolerance to the antinociceptive effects of morphine. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Antinociception; Pain; Tolerance; Glutamate; NMDA receptor/glycine B site antagonist; Blood–brain barrier 1. Introduction Over the last decade research has provided compel- ling evidence that glutamate receptors are crucially involved in phenomena related to opioid tolerance (see Mao, 1999; Price et al., 2000 for reviews). Antagonists of the ionotropic N-methyl-D-aspartate (NMDA) receptor complex, including memantine, the moderate affinity and highly voltage-dependent clinically used NMDA channel blocker (Parsons et al., 1999) inhibit the development of morphine tolerance (Trujillo and Akil, * The referees of the present paper suggested to assess brain penetration of MRZ 2/596 using brain microdialysis. It was not possible at the time of manuscript preparation but such opportunity appeared after manuscript acceptance. The study revealed, that in rats after application of MRZ 2/596 at the dose of 30 mg/kg ip maximal concentrations in brain reach c.a. 100 nM (NZ3) concentrations corrected for in vitro recovery. Thus, it is unlikely that dose used in the present study (0.05–1 mg/kg) resulted in brain levels of MRZ 2/596 that would affect NMDA receptors assuming lack of differences in blood brain barrier between rat and mice. ) Corresponding author. Institute of Pharmacology, Polish Acad- emy of Sciences, 12 Smetna Street, 31-343 Krako´w, Poland. Tel.: C48 12 6623375; fax: C48 12 6374500. E-mail address: nfpopik@cyf-kr.edu.pl (P. Popik). 0028-3908/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.neuropharm.2004.11.005 Neuropharmacology 48 (2005) 360–371 www.elsevier.com/locate/neuropharm