Effects of amphetamine on delay discounting in rats depend upon the manner in which delay is varied

Abstract

Whether stimulant drugs like amphetamine increase or decrease choice of larger delayed reinforcers over smaller immediately available reinforcers under delay discounting procedures can depend on several factors, including the order in which delay is presented. This study examined whether the order of delay presentation impacts drug effects on discounting in rats (n=8) trained and tested under an ascending order, a descending order, as well as under a fixed delay condition. Responses on one lever delivered 1 food pellet immediately and responses on the other lever delivered 3 food pellets immediately or after a delay (4–32 s). In Experiment 1, the delay to the larger reinforcer varied within session and the order of delay presentation (ascending or descending) varied across conditions. In Experiment 2, the same delay value was presented in all blocks of the session (i.e., delay was fixed), and delay varied across phases. Under the ascending order of delay, amphetamine (0.32–1.78 mg/kg) increased choice of the larger reinforcer in some rats and decreased choice in others. In the same rats responding under the descending and fixed delay conditions, amphetamine markedly decreased choice of the larger reinforcer even in the component associated with no delay. In some subjects, the effects of amphetamine differed depending on the manner in which delay was presented, indicating that drug-induced changes in performance were due, in part, to mechanisms other than altered sensitivity to reinforcer delay. These results also suggest that a history of responding under both orders of delay presentation can modulate drug effects.

Effects of GluN2B-selective antagonists on delay and probability discounting in male rats: Modulation by delay/probability presentation order.

“…These results are consistent with other reports showing that delay presentation order alters the effects of amphetamine (Maguire et al, 2014; Tanno et al, 2014; but see Slezak & Anderson, 2009), methylphenidate (Tanno et al, 2014), and JNJ 16259685, a metabotropic glutamate receptor (mGluR) 1 antagonist (Yates, Rogers, et al, 2017), on delay discounting performance. Importantly, the effects reported with Ro 63–1908 in the current study are not due to increased perseverative responding, as Ro 63–1908 did not increase responses for the large magnitude reinforcer when the delays increased across the session.…”

“…Because rats trained on the descending schedule responded more for the large magnitude reinforcer relative to rats trained on the ascending schedule, the present results could reflect a baseline effect, as opposed to a delay presentation order effect. However, this seems unlikely, as Maguire et al (2014) and Tanno et al (2014) report differential effects of psychostimulants on delay discounting performance even though baseline performance was similar across each schedule. Related to this potential caveat, rats exhibited near exclusive preference for the small, immediate reinforcer for three of the five delays tested; thus, we cannot rule out the possibility that Ro 63–1908 causes a general increase in impulsive choice.…”

Effect of daily morphine administration and its discontinuation on delay discounting of food in rhesus monkeys

“…For example, other effects might have overshadowed effects on discounting, such as changes in sensitivity to reinforcer amount or increased perseveration, causing increased choice of the large delayed reinforcer. As is the case with stimulant drugs (e.g., Cardinal et al, 2000; Tanno et al, 2014; Maguire et al 2014), the effects of opioids on discounting might vary markedly across different experimental conditions; however, effects of opioids on discounting have been studied under a very narrow range of conditions compared with studies using stimulant drugs.…”

Adolescent methylmercury exposure affects choice and delay discounting in mice

“…Briefly, sessions were structured such that (a) subjects could lever-press for access to a small or large reinforcer, (b) the smaller reinforcer was always delivered after a 1.26-s delay, (c) six different delays to the larger reinforcer were presented in a random order across a session, with each delay signaled by a unique auditory stimulus, and (d) mice experienced the smaller and larger reinforcers an equal number of times—an important variable to control in delay-discounting procedures (Cardinal, Daw, Robbins, & Everitt, 2002; Maguire, Henson, & France, 2014; Tanno, Maguire, Henson, & France, 2014). …”