Prevention of cocaine-induced convulsions and lethality in mice: effectiveness of targeting different sites on the NMDA receptor complex

Abstract

N-methyl-d-aspartate (NMDA) receptors appear to be involved in the behavioral toxic effects of cocaine. Therefore, different classes of NMDA receptor antagonists were compared for their ability to attenuate cocaine-induced convulsions and lethality in male, Swiss Webster mice. The mice were pre-treated (i.p.) with vehicle or an antagonist from one of the following classes: NMDA/glycine site antagonist (7-chlorokynurenic acid, ACEA-1021, ACEA-1031, ACEA-1328, DCQX, R(+)-HA-966), competitive antagonist (CPP, D-AP7), channel blocker (MK-801, memantine), or allosteric modulator (ifenprodil, CP-101,606, Co 101022, haloperidol). After a 15 min pre-treatment period, the mice were administered a convulsive (60 mg/kg, i.p.) or lethal (125 mg/kg, i.p.) dose of cocaine, equivalent to the calculated ED/LD97 values. Pre-treatment with competitive or NMDA/glycine site antagonists dose-dependently attenuated cocaine-induced convulsions and lethality (P<0.05). Pre-treatment with channel blockers or allosteric modulators of the NMDA receptor protected against cocaine-induced convulsions (P<0.05), but were ineffective or less effective than the competitive and glycine site antagonists in preventing death. The glutamate release inhibitor riluzole failed to prevent both the convulsions and lethality induced by cocaine. Significantly, post-treatment with NMDA/glycine site antagonists (ACEA-1021, ACEA-1031, ACEA-1328) after a cocaine overdose prevented death in a significant number of animals. The data suggest that NMDA receptors are involved in the pathophysiology of a cocaine overdose.

Introduction

Currently, no effective treatments are available for cocaine overdose and it is responsible for more serious intoxications and deaths than any other illicit drug (Benowitz, 1993). The glutamatergic system has recently been implicated in the addictive properties of cocaine (McGinty, 1995), and it may also have a role in the toxic effects of the drug (Itzhak and Stein, 1992, Rockhold et al., 1991, Witkin and Tortella, 1991). Among the different glutamate receptors, the N-methyl-d-aspartate (NMDA) receptor plays an important role in cardiovascular, respiratory, and neural systems (Berger et al., 1995, Olney et al., 1986, West and Huang, 1994), all of which are seriously compromised after an overdose of cocaine. Therefore, given the importance of NMDA receptors in end-stage events resulting from an overdose (e.g. convulsions, respiratory distress, cardiovascular collapse), the receptor complex is a logical target for pharmacological interventions, even if cocaine does not interact directly with it.

The NMDA receptor contains several antagonist binding and modulatory sites that can serve as targets for pharmacotherapies: glutamate binding sites, glycine co-agonist sites, sites within the receptor ionophore, and allosteric modulatory sites (Bigge, 1993). Previous investigations have shown that competitive antagonists, channel blockers, and glycine site antagonists of the NMDA receptor can attenuate cocaine-induced behavioral toxicity in animals (Derlet and Albertson, 1990, Itzhak and Stein, 1992, Rockhold et al., 1991, Witkin and Tortella, 1991), but most of the antagonists tested in these earlier studies had limitations, such as marginal efficacy, bioavailability problems, and unwanted side effects, that compromised their clinical potential (Auer, 1997, Bigge, 1993, Kornhuber and Weller, 1997, Witkin and Tortella, 1991). Recently, two novel quinoxaline-2,3-diones with preferential affinities for NMDA/glycine sites (Keana et al., 1995, Woodward et al., 1995) were shown to prevent the convulsions and lethality produced by a cocaine overdose in mice (Matsumoto et al., 1997). These NMDA/glycine site antagonists are known to be bioavailable and well tolerated at therapeutic doses in many animal models of neurological disorders (Kanthasamy et al., 1996, Lutfy et al., 1995, Lutfy et al., 1996, Lutfy and Weber, 1996, Matsumoto et al., 1995, Tsuchida and Bullock, 1995, Vaccarino et al., 1993, Warner et al., 1995). In addition, they do not produce the phencyclidine-like effects that have been problematic for many earlier NMDA receptor antagonists (Auer, 1997, Balster et al., 1995, Hawkinson et al., 1997), including psychotomimetic effects in humans (Albers et al., 1999).

Therefore, the goals of the present study were to rigorously examine the effectiveness of NMDA/glycine site antagonists for attenuating the behavioral toxic effects of cocaine and to compare their effectiveness relative to antagonists that target other sites on the NMDA receptor complex. In all of the studies, one well characterized compound for each target site on the NMDA receptor served as a reference for the newer ligands. Compounds that target the NMDA/glycine and allosteric moduatory sites received the most attention because they have a favorable clinical profile compared to competitive antagonists and channel blockers, which have been associated with unacceptable side effects in human clinical trials of stroke and epilepsy (cf. Kornhuber and Weller, 1997). The following NMDA/glycine site antagonists were tested: 7-chlorokynurenic acid (Kemp et al., 1988), DCQX (Ogita and Yoneda, 1990), R(+)-HA-966 (Singh et al., 1990a), ACEA-1021, ACEA-1031, and ACEA-1328 (Keana et al., 1995). CPP and D-AP7 were used as representative competitive antagonists (Bigge, 1993). The classical, high potency channel blocker MK-801 was also used (Wong et al., 1986). For comparison, memantine was tested because it is a partial channel blocker with a low incidence of side effects in both humans and animals (Bormann, 1989). In addition, the following allosteric modulators were tested: ifenprodil (Carter et al., 1988), CP-101,606 (Chenard et al., 1995), Co 101022, haloperidol and nylidrin (Whittemore et al., 1997). All of the allosteric modulators exhibit a strong preference for the NR1/NR2B subtype of NMDA receptor (Ilyin et al., 1996, Whittemore et al., 1997; Richard Woodward, CoCensys, Inc., personal communication). Finally, riluzole, a compound that inhibits glutamate release, was tested (Martin et al., 1993). In all of our studies, convulsions and lethality were used as the behavioral endpoints because many cocaine users convulse after a mild, but not necessarily fatal overdose, while after severe overdoses of cocaine, death can ultimately ensue. Initially, the antagonists were administered as a pre-treatment to ensure that the binding sites of interest were occupied at the time of the overdose. However, to be of practical use in the clinic, the antagonists must be effective when administered after an overdose. Therefore, in later experiments, the most effective antagonists were also administered as a post-treatment. This work has been described in abstract form (Matsumoto et al., 1998).