SENSITIVE AND RAPID BEHAVIORAL DIFFERENTIATION OF N-METHYL-D-ASPARTATE RECEPTOR ANTAGONISTS

Behavioral effects of PCP-type noncompetitive antagonists of N-methyl-D-aspartate (NMDA) receptors overlap with those of a host of other centrally acting compounds. In the present experiment, locomotor activity and performance on an inverted screen test in untrained mice were used to differentiate PCP-type noncompetitive NMDA antagonists from other drug classes. These uncompetitive NMDA antagonists [PCP, dizocilpine, (-)-MK-801, TCP, (+)-SKF 10,047, dextrorphan, ketamine] produced dose-related increases in locomotor activity and the percentage of mice falling off an inverted, elevated wire mesh screen. Both effects demonstrated stereoselectivity, occurred at comparable dose levels, and were within the range of doses producing other biological effects (e.g., anticonvulsant). The potencies of these drugs for producing behavioral effects were positively correlated with affinities for PCP ([H-3]MK-801) but not sigma([H-3]SKF 10,047) receptors. Although muscarinic antagonists (benactyzine, atropine) produced effects in the same direction, locomotor stimulation was small and occurred at lower doses than those inducing screen failures. Competitive NMDA antagonists (LY 274614, LY 233536, CPP, NPC 12626), sigma receptor ligands (DTG, dextromethorphan), postsynaptic dopamine agonists (quinpirole, SKF 38393) and antagonists (haloperidol, SCH 39166), and some depressant compounds (morphine, diazepam) increased failures on the screen test but decreased locomotor activity. Ligands of the polyamine regulatory site of the NMDA receptor (ifenprodil, SL 82.0715-10) and the AMPA receptor antagonist NBQX decreased locomotor activity without increasing screen failures. An antagonist of the strychnine-insensitive glycine receptor (7-chlorokynurenic acid) did not affect performance on either test. Psychomotor stimulants (cocaine and methamphetamine) stimulated locomotor activity without affecting screen performance. The only false positives occurred with barbiturates (pentobarbital, phenobarbital). Nonetheless, the present procedure demonstrates excellent sensitivity and power for rapid discrimination of uncompetitive NMDA antagonists.