Effects of amphetamine and methylphenidate on delay discounting in rats: interactions with order of delay presentation

Abstract

Rationale Drug effects on delay discounting are thought to reflect changes in sensitivity to reinforcer delay, although other behavioral mechanisms might be involved. One strategy for revealing the influence of different behavioral mechanisms is to alter features of the procedures in which they are studied. Objective This experiment examined whether the order of delay presentation under within-session delay discounting procedures impacts drug effects on discounting. Methods Rats responded under a discrete-trial choice procedure in which responses on one lever delivered 1 food pellet immediately and responses on the other lever delivered 3 food pellets either immediately or after a delay. The delay to the larger reinforcer (0, 4, 8, 16, and 32 s) was varied within session and the order of delay presentation (ascending or descending) varied between groups. Results Amphetamine (0.1–1.78 mg/kg) and methylphenidate (1.0–17.8 mg/kg) shifted delay functions upward in the ascending group (increasing choice of the larger reinforcer) and downward in the descending group (decreasing choice of the larger reinforcer). Morphine (1.0–10.0 mg/kg) and delta-9-tetrahydrocannabinol (0.32–5.6 mg/kg) tended to shift the delay functions downward, regardless of order of delay presentation, thereby reducing choice of the larger reinforcer, even when both reinforcers were delivered immediately. Conclusion The effects of amphetamine and methylphenidate under delay discounting procedures differed depending on the order of delay presentation indicating that drug-induced changes in discounting were due, in part, to mechanisms other than altered sensitivity to reinforcer delay. Instead, amphetamine and methylphenidate altered responding in a manner consistent with increased behavioral perseveration.

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

“…The behavioral procedure used in the current study was based on the procedure developed by Evenden and Ryan (1996) and recently described by Tanno et al (2014). Daily sessions were divided into 5 blocks, each of which comprised 2 forced trials followed by 5 choice trials.…”

“…For example, the effects of stimulant drugs such as amphetamine on delay discounting can differ qualitatively, either increasing or decreasing discounting, depending upon whether the delay period is paired with a unique stimulus (e.g., Cardinal et al 2000). A recent study (Tanno et al 2014) showed that the effects of amphetamine and methylphenidate on performance under a delay discounting procedure vary depending on the order in which delays are presented within the session. Both drugs increased choice of the larger delayed reinforcer in rats responding under an ascending order of delay, consistent with effects reported by others (e.g., Barbelivien et al 2008; Cardinal et al 2000; Huskinson et al 2012; Pitts and McKinney 2005; Slezak and Anderson 2011; Slezak et al 2013; Van Gaalen et al 2006; Winstanley et al 2003; 2005), but markedly decreased choice of the larger reinforcer in a separate group of rats responding under a descending order.…”

“…For example, stimulant drugs might increase perseveration, alter the estimation of the passage of time, or change sensitivity to reinforcer amount [see discussions by Pitts and Febbo (2004), Pitts and McKinney (2005), Richards et al (1997), and Slezak and Anderson (2009)]. Because Tanno et al (2014) assessed the interaction between drug effects and delay order using a between-groups design, differences might be due to factors other than, or in addition to, delay order; it might be the case that a history of responding exclusively with one order of delay enhances the apparent perseverative effects of amphetamine. One goal of the current study (Experiment 1) was to examine whether the delay-order effect reported previously could be demonstrated for an individual subject; therefore, the effects of amphetamine were assessed in rats that were trained and tested under both ascending and descending orders of delay presentation.…”

“…Studies using either a between-groups (Tanno et al 2014) or within-subject (Slezak and Anderson 2009) design failed to show that amphetamine increases choice of larger delayed reinforcers (e.g., reduces delay discounting) when delay is presented in a descending order within session. If drug effects are mediated through changes in sensitivity to reinforcer delay, then such changes should be evident under various other conditions in which delay impacts behavior.…”

Lorcaserin and CP-809101 reduce motor impulsivity and reinstatement of food seeking behavior in male rats: Implications for understanding the anti-obesity property of 5-HT2C receptor agonists

“…It should be noted that procedural variables such as order of delay presentation (ascending vs. descending delay; St. Onge et al 2010;Tanno et al 2014) and signaled vs. unsignaled trials (e.g. Cardinal et al 2000) can influence experimental outcomes.…”