BEHAVIORAL-EFFECTS OF ENANTIOMERS OF DIZOCILPINE UNDER 2 COUNTING PROCEDURES IN RATS

Stereoisomers of the N-methyl-D-aspartate antagonist dizocilpine (MK-8 01) were studied to determine whether behavioral effects on complex op erants depend on reinforcement loss accompanying behavioral disruption . Rats earned food pellets if the run of consecutive left-lever presse s preceding a trial-terminating right-lever press approximated a targe t of 12. A percentile schedule reinforced any run closer to the target than two-thirds of the runs on the most recent 24 trials. Once the se quence was learned, half the subjects were shifted to a procedure that yoked reinforcement for each length run to the probability that lengt h generated pellets during asymptotic percentile performance. Although these two procedures generate similar control run and reinforcement d istributions, disrupting behavior reduced reinforcement probability fa r more under the yoked than the percentile procedure. Despite this dif ference in drug-induced reinforcement loss, both enantiomers produced similar dose-related decreases in run length and response rate under b oth procedures, with the (-) isomer approximately one log unit less po tent than the (+) isomer. The absence of differential effects under th ese procedures diminishes the likelihood that reinforcement loss contr ibutes to dizocilpine's effects, indirectly bolstering claims that diz ocilpine directly affects learning.