Evaluation of the phencyclidine-like discriminative stimulus effects of novel NMDA channel blockers in rats.
BACKGROUND
Because of their potential therapeutic effects, N-methyl-D-aspartate (NMDA) receptor antagonists have been investigated for clinical use. Unfortunately, many channel-blocking antagonists have been associated with the production of side effects, including motor impairment and phencyclidine (PCP)-like subjective effects.


OBJECTIVE
This study investigated the relationship between NMDA receptor channel blockade and production of PCP-like side effects by evaluating a variety of NMDA channel blockers with different binding characteristics for the production of PCP-like discriminative stimulus effects.


METHODS
The NMDA channel blockers were tested in rats trained to discriminate 2 mg/kg PCP, i.p., from saline using a standard two-lever drug discrimination procedure with responding under a fixed ratio (FR) 32 schedule of food reinforcement.


RESULTS
The high-affinity channel blockers PD 138289, PD 137889 and FR 115427, produced full, dose-dependent substitution for PCP. Of the moderate-affinity channel blockers, MRZ 2/579 fully substituted for PCP while 1-(4-methoxyphenyl)-1,2,3,4-tetrahydroisoquinoline, 8-(2-methoxyphenyl)-1,2,3,4-tetrahydroisoquinoline and alaproclate produced partial substitution. Drugs with the lowest affinity for the channel site and/or higher affinity for non-NMDA CNS sites, antazoline, idazoxan, 1-phenyl-1,2,3,4-tetrahydroisoquinoline, alpha-benzyl- N-methylphenethylamine and orphenadrine, failed to substitute for PCP.


CONCLUSIONS
The results demonstrate that the cellular actions of the individual channel-blocking NMDA antagonists, in particular affinity for the channel site and NMDA receptor specificity, are important determinants of their discriminative stimulus effects. While higher affinity channel blockers show a correlation between affinity and PCP-like discriminative stimulus effects, behavioral disruption through action at non-NMDA receptors probably prevents achieving sufficient concentrations of the lower affinity compounds at NMDA receptors to produce PCP-like discriminative stimulus effects.
Nicholson
Katherine L
KL
Department of Pharmacology and Toxicology, Medical College of Virginia, Virginia Commonwealth University, VA 23298-0613, Richmond, USA. klnichol@hsc.vcu.edu
Balster
Robert L
RL
D000818:Animals; D004192:Discrimination, Psychological / Q000187:drug effects*; D004305:Dose-Response Relationship, Drug; D004341:Drug Evaluation; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology*; D008297:Male; D010622:Phencyclidine / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D016194:Receptors, N-Methyl-D-Aspartate / Q000037:antagonists & inhibitors* / Q000378:metabolism; D012055:Reinforcement Schedule; D012054:Reinforcement, Psychology; D044005:Tetrahydroisoquinolines / Q000494:pharmacology
D018691:Excitatory Amino Acid Antagonists; D016194:Receptors, N-Methyl-D-Aspartate; D044005:Tetrahydroisoquinolines; C084456:1-methyl-1-phenyl-1,2,3,4-tetrahydroisoquinoline; D010622:Phencyclidine