Pitts, R. C. & McKinney, A. P. (2005).
Effects of methylphenidate and morphine on delay-discount functions obtained within sessions.
Journal of the Experimental Analysis of Behavior, 83, 297-314.
Four rats responded under a “self-control” procedure designed to obtain delay-
discount functions within sessions. Each session consisted of seven blocks, with
seven trials within each block. Each block consisted of two initial forced-
choice trials followed by five free-choice trials. On choice trials, the rats
could press either of two retractable levers. A press on one lever was followed
by presentation of a smaller reinforcer (a single dipper presentation of a
sucrose solution); a press on the other lever was followed by presentation of a
larger reinforcer (four consecutive dipper presentations). The delay associated
with the smaller reinforcer always was 0 s, whereas the signaled delay
associated with the larger reinforcer increased across blocks (from 0 to 50 s).
Under these conditions, the percentage of choices of the larger reinforcer
decreased across blocks, and relatively reliable delay-discount functions were
obtained within sessions. Doses of methylphenidate (1.0 to 17.0 mg/kg) and
morphine (0.3 to 17.0 mg/kg) were then administered prior to selected sessions.
Typically, intermediate doses of methylphenidate shifted the discount functions
to the right (increased choices of the larger reinforcer). For 2 of the rats,
this effect was pronounced; for the other 2 rats, this effect occurred after the
range of delays for the larger reinforcer was decreased (0 to 20 s). On the
other hand, in most cases morphine produced a slight leftward shift in the
discount function (decreased choices of the larger reinforcer). The present
procedure appears to be a useful and efficient method to characterize drug
effects on an entire delay-discount function. As with many procedures used to
study self-control choices, however, sources of control other than reinforcement
delay and amount may have been operating in the present study, and these sources
must be considered when interpreting drug effects.
Key words: delay discounting, self-control, choice, methylphenidate, morphine,
lever press, rats