EFFECTS OF THE NMDA ANTAGONIST, DIZOCILPINE, IN VARIOUS DRUG DISCRIMINATIONS - CHARACTERIZATION OF INTERMEDIATE LEVELS OF DRUG LEVER SELECTION

In each of different groups of rats trained to discriminate either 8-O H-DPAT, DOI, d-amphetamine, cocaine, chlordiazepoxide, or ethanol from saline, dizocilpine produced maximum percentages of drug lever (DL) s election that were intermediate between those produced by the training conditions. Dizocilpine also decreased DL selection produced by the t raining dose in each of the discriminations, except in ethanol-trained rats. In all discriminations, with the exception of ethanol-trained r ats, the intermediate levels of DL selection produced by dizocilpine w ere associated with increased FRF values (sum of the responses made on either lever before the first reinforcement occurred), increased leve r selection latencies, and increased responding on the nonselected lev er At doses that, in general, had effects on response rate similar to those of dizocilpine, intermediate levels of Db, selection were produc ed by BMY 7378 in 8-OH-DPAT-trained rats, by WY 50,324 in DOI-trained rats, by (-)3-PPP in d-amphetamine- and in cocaine-trained rats, by al pidem in chlordiazepoxide-trained rats, and by PCP in ethanol-trained rats, The intermediate levels of DL selection produced by these latter drugs were not associated with simultaneous increases of FRF values, selection latencies, and responding on the nonselected lever. The resu lts suggest that dizocilpine produces intermediate levels of drug-appr opriate responding through the behavioral mechanism of partial general ization only in ethanol-trained rats; in all other discriminations exa mined here, the effects of dizocilpine appear to involve (1) pharmacol ogical effects that differ from those of the training drug, and (2) be havioral mechanisms that are unrelated to stimulus generalization. The differentiation of partial generalization and other mechanisms whereb y intermediate responding can occur in the drug discrimination paradig m requires analyses that are more detailed than those commonly used in drug discrimination research.