Drug effects on response duration differentiation. I: Differential effects of drugs of abuse.

Rats were trained to respond under a response duration differentiation schedule in which responses on a lever were reinforced if lever press durations were greater than or equal to 1.00 s but were also less than 1.30 s. Dose-effect curves were generated for cocaine, methamphetamine, pentobarbital, phencyclidine, delta-9-tetrahydrocanninabol (delta 9-THC), and morphine. All drugs produced dose-dependent decreases in accuracy (the percentage of total response durations that were reinforced); however, the degree to which changes in accuracy were accompanied by changes in response rates varied among drugs. Pentobarbital and morphine affected primarily longer (> 1.3 s) response durations, phencyclidine and delta 9-THC affected primarily shorter response durations, whereas cocaine and methamphetamine affected both shorter and longer response durations. High doses of methamphetamine and cocaine increased the dispersion of response duration distributions with increasing dose, whereas higher doses of pentobarbital, delta 9-THC and morphine did not increase dispersion of response duration distributions as much. These data show that behavior under this novel schedule is differentially sensitive to a number of pharmacologic manipulations, and that the schedule can provide a useful addition to the analysis of drug effects upon behavior.