dextromethorphan and dioxadrol

The ability of non-competitive NMDA antagonists and other selected compounds to inhibit [3H]MK-801 binding to the NMDA receptor in brain membranes was evaluated in female, dark Agouti rats. In homologous competition binding studies the average apparent affinity (KD) of [3H]MK-801 for its binding site was 5.5 nM and the binding site density (Bmax) was 1.83 pmol/mg protein. Inhibition of [3H]MK-801 binding by non-competitive NMDA antagonists was best described with a one-site competition model and the average Hill coefficients were -1. A series of eight non-competitive NMDA antagonists inhibited [3H]MK-801 binding with the following rank order of affinity (K(i), nM): MK-801 (5.5) > dexoxadrol (21.5) > or = TCP (24.2) > phencyclidine (100.8) > (+)-SKF 10,047 (357.7) > dextrorphan (405.2) > ketamine (922.2) > dextromethorphan (2913). These inhibition binding constants determined in dark Agouti rat brain membranes were significantly correlated (P = 0.0002; r2 = 0.95) with previously reported values determined in Sprague-Dawley rats [Wong et al., 1988, J. Neurochem. 50, 274-281]. Despite significant differences in metabolic capability between these strains, the central nervous system NMDA receptor ion channel shares similar characteristics.

Topics: Animals; Binding, Competitive; Brain; Dextromethorphan; Dioxolanes; Dizocilpine Maleate; Dose-Response Relationship, Drug; Female; Ketamine; N-Methylaspartate; Phenazocine; Phencyclidine; Piperidines; Rats; Rats, Mutant Strains; Receptors, N-Methyl-D-Aspartate; Scintillation Counting; Tritium

The dizocilpine-like discriminative stimulus effects of a variety of channel blocking (uncompetitive) N-methyl-D-aspartate (NMDA) receptor antagonists were examined in rats trained to discriminate dizocilpine (0.17 mg/kg, i.p) from saline in a two-lever operant procedure. The dissociative anesthetic-type NMDA antagonists dizocilpine (ED50 0.05 mg/kg), phencyclidine (ED50 3.4 mg/kg) and ketamine (ED50 14 mg/kg) showed complete substitution without producing significant decreases in response rates, whereas dexoxadrol (ED50 4.3 mg/kg) also produced complete substitution with a concomitant decrease (35%) in response rate. Similarly, the low-affinity antagonist memantine resulted in complete substitution (ED50 9.7 mg/kg) at doses that significantly reduced (68%) the response rate. All other low-affinity antagonists resulted in either partial or no substitution for the discriminative stimulus effects of dizocilpine at doses that significantly decreased average response rates. These include (ED50 values in parentheses) remacemide (29 mg/kg), the remacemide metabolite 1,2-diphenyl-2-propylamine (ARL 12495) (14 mg/kg), phencylcyclopentylamine (25 mg/kg), dextromethorphan (46 mg/kg), (+/-)-5-aminocarbonyl-10,11-dihydro -5H-dibenzo-[a,d]cyclohepten-5,10-imine (ADCI; no substitution) and levoxadrol (no substitution). We conclude that low-affinity uncompetitive NMDA antagonists have discriminative stimulus properties distinct from dissociative anesthetic-type uncompetitive NMDA antagonists. The lowest-affinity antagonists show virtually no substitution for dizocilpine, whereas the relatively more potent low-affinity antagonists (such as memantine) exhibit greater substitution, but complete substitution is obtained only at rate-reducing doses.

Topics: Acetamides; Analysis of Variance; Anesthetics, Dissociative; Animals; Conditioning, Operant; Dextromethorphan; Dioxolanes; Discrimination Learning; Dizocilpine Maleate; Dose-Response Relationship, Drug; Excitatory Amino Acid Antagonists; Ketamine; Male; Memantine; Phencyclidine; Piperidines; Rats; Receptors, N-Methyl-D-Aspartate