Rationale
Repeated amphetamine (AMPH) exposure is known to cause long-term changes in AMPH-induced locomotor behavior (i.e., sensitization)
that are associated with similarly long-lasting changes in brain function. It is not clear, however, if such exposure produces
long-lasting changes in a cognitive behavior that, in humans, is hypothesized to contribute to addiction.
Objectives
To examine whether repeated AMPH exposure induces both locomotor sensitization and alters impulsive choice in a delay-discounting
task.
Materials and methods
Adult, male Sprague–Dawley rats (n = 29) were pretreated with 3.0 mg/kg AMPH or saline every other day for 20 days and were then trained to lever press for
small, immediately delivered food reinforcement or larger reinforcements delivered after delays. We subsequently assessed
the effects of acute AMPH (0.1–2.0 mg/kg) on delay-discounting. Lastly, we tested for long-lasting effects of pretreatment
by giving an AMPH challenge (3.0 mg/kg) 1 week after the final delay-discounting session.
Results
Repeated AMPH produced sensitization to the drug’s stereotypy-inducing effects but did not alter acquisition or baseline behavior
in the delay-discounting task. Following acute AMPH, impulsive choice and other measures of delay-discounting were altered,
but to a similar extent in both saline- and AMPH-pretreated groups. The AMPH challenge, given ∼3 months after the last pretreatment
injection, revealed that sensitization was still evident.
Conclusions
Our results suggest that one behavioral consequence of repeated AMPH exposure—sensitization—does not overlap with another
potential outcome—increased impulsivity. Furthermore, the neuroadaptations known to be associated with sensitization may be
somewhat distinct from those that lead to changes in impulsive choice.
Keywords Delay-discounting - Amphetamine - Rat - Decision making - Stereotypy