Effects of Methylphenidate and Morphine on Delay‐discount Functions Obtained Within Sessions

Abstract

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.

Impulsive choice and environmental enrichment: Effects of d-amphetamine and methylphenidate

“…Specifically, amphetamine increased impulsive choice in EC rats, whereas it decreased impulsive choice in IC rats; methylphenidate did not significantly alter impulsive choice in EC rats, but similar to amphetamine it decreased impulsive choice in IC rats. It is notable that previous reports have yielded mixed results about the effects of amphetamine and methylphenidate on impulsive choice in rats, with some showing decreased impulsive choice [68,87,90,93], and others reporting no effect [15,27]. The current study suggests that one potential reason for these discrepant findings is that environmental factors may modulate the baseline levels of impulsivity, and thus alter the sensitivity to amphetamine and methylphenidate.…”

“…However, since these studies were conducted in individuals with prior drug experience, preclinical research may be better suited to determine the acute effects of drugs on impulsive choice. In rats, some reports have found impulsive choice to be decreased by amphetamine [87,90,93], methylphenidate [68,87], atomoxetine [76], or methamphetamine [73]. However, other reports have found that impulsive choice was not altered by amphetamine [15,27] or cocaine [46].…”

RAPID ACQUISITION OF PREFERENCE IN CONCURRENT CHAINS: EFFECTS OF d‐AMPHETAMINE ON SENSITIVITY TO REINFORCEMENT DELAY

“…Several lines of evidence indicate that this effect occurs with considerable generality. As noted in the introduction, these drugs usually increase choices of a larger, more delayed reinforcer under self-control procedures (Pietras et al, 2003;Pitts & Febbo, 2004;Pitts & McKinney, 2005;Richards et al, 1999;Wade et al, 2000), and quantitative analyses of choice are consistent with this view (Pitts & Febbo, 2004). Interestingly, quantitative analyses also indicate that lesions of the orbital prefrontal cortex can increase estimates of sensitivity to reinforcement delay (e.g., Mobini et al, 2002;Kheramin et al, 2003).…”

“…Much of this work has focused on effects of drugs under ''self-control'' preparations in which subjects choose between a larger, more delayed reinforcer and a smaller, more immediate one. Drugs classified as psychomotor stimulants (e.g., amphetamines, methylphenidate) typically increase the likelihood of choosing the larger, more delayed reinforcer (Pietras, Cherek, Lane, Tcheremissine, & Steinberg, 2003;Pitts & Febbo, 2004;Pitts & McKinney, 2005;Richards, Sabol, de Wit, 1999;Wade, de Wit, & Richards, 2000; but see Charrier & Thiebot, 1996;Evenden & Ryan, 1996). There are a number of potential behavioral mechanisms of this effect (see Pitts & Febbo, 2004;Richards et al, 1999).…”

A long-term study of the impulsive choices of Lewis and Fischer 344 rats

“…Choice is estimated by analyzing the distribution of responses on two levers concurrently available in the initial link (e.g., Grace, 1994), preventing the development of an exclusive preference for one or the other alternative, which is observed when only one response is required to choose and produce the SSR and the LLR (Mazur, 1987(Mazur, , 2010. Another characteristic of the present procedure is that the presentation order of delays to deliver the LLR was manipulated across conditions (i.e., ascending, descending, or random presentation orders), avoiding the possibility of creating a single history of increasing delay to deliver the LLR; this characteristic is important because, in studies in which discrete trials have been used to create a history of increasing delay to LLR delivery, choice for the LLR decreased across blocks of trials during probe sessions in which the delay to LLR delivery remained at 0 s throughout the session (e.g., Evenden & Ryan, 1996;Pitts & McKinney, 2005;Slezak & Anderson, 2009). In addition, the present procedure required locomotion; rats traveled from the front to the back wall of the chamber to press a lever to restart each cycle of the concurrent-chains procedure, which implied effort, representing some cost to reach the choice point.…”