Delay Discounting in Lewis and Fischer 344 Rats: Steady‐state and Rapid‐determination Adjusting‐amount Procedures

Abstract

Lewis rats have been shown to make more impulsive choices than Fischer 344 rats in discrete-trial choice procedures that arrange fixed (i.e., nontitrating) reinforcement parameters. However, nontitrating procedures yield only gross estimates of preference, as choice measures in animal subjects are rarely graded at the level of the individual subject. The present study was designed to examine potential strain differences in delay discounting using an adjusting-amount procedure, in which distributed (rather than exclusive) choice is observed due to dynamic titration of reinforcer magnitude across trials. Using a steady-state version of the adjusting-amount procedure in which delay was manipulated between experimental conditions, steeper delay discounting was observed in Lewis rats compared to Fischer 344 rats; further, delay discounting in both strains was well described by the traditional hyperbolic discounting model. However, upon partial completion of the present study, a study published elsewhere (Wilhelm & Mitchell, 2009) demonstrated no difference in delay discounting between these strains with the use of a more rapid version of the adjusting-amount procedure (i.e., in which delay is manipulated daily). Thus, following completion of the steady-state assessment in the present study, all surviving Lewis and Fischer 344 rats completed an approximation of this rapid-determination procedure in which no strain difference in delay discounting was observed.

Mechanisms of Individual Differences in Impulsive and Risky Choice in Rats

“…In addition, the Lewis rats displayed delay aversion that developed over the course of the session in the SS delay manipulation, and this may be an important factor in their increased impulsive choice. These results substantiate the Lewis strain as a possible model for ADHD (see also García-Lecumberri et al, 2010;Stein et al, 2012;Wilhelm & Mitchell, 2009). Early rearing environment.…”

“…While much of our work has examined impulsive choice in outbred populations, we have also assessed impulsive choice in inbred strains of rats that are potential animal models of ADHD (Garcia & Kirkpatrick, 2013). The spontaneously hypertensive (SHR) and Lewis strains have been derived from their respective control strains, the Wistar Kyotos (WKY) and Wistars, and both have been reported to demonstrate possible markers of increased impulsive choice in previous studies (Anderson & Diller, 2010;Anderson & Woolverton, 2005;Bizot et al, 2007;Fox, Hand, & Reilly, 2008;García-Lecumberri et al, 2010;Hand, Fox, & Reilly, 2009;Huskinson, Krebs, & Anderson, 2012;Madden, Smith, Brewer, Pinkston, & Johnson, 2008;Stein, Pinkston, Brewer, Francisco, & Madden, 2012). Garcia and Kirkpatrick (2013) sought to potentially isolate the source of impulsive choice behaviors to either deficits in delay or magnitude sensitivity by delivery of two different impulsive choice tasks modeled after previous research (Galtress & Kirkpatrick, 2010;Roesch, Takahashi, Gugsa, Bissonette, & Schoenbaum, 2007).…”

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

“…The accuracy of the hyperbolic-decay model in describing the data from numerous studies with human (e.g., Myerson & Green, 1995;Rachlin, Raineri, & Cross, 1991) and nonhuman (e.g., Aparicio, Hughes, & Pitts, 2013;Farrar, Kieres, Hausknecht, de Wit, & Richards, 2003;Green, Myerson, Shah, Estle, & Holt, 2007;Mazur, 2012;Stein, Pinkston, Brewer, Francisco, & Madden, 2012) animals is remarkably general, and this model does so with a single free parameter (k). Studies of impulsivity using changes in k to assess the effects of drugs or other neurobiological variables on impulsive choice have often used the adjusting-delay (i.e.…”

“…, Mazur 1987) and the adjusting-amount titration procedures (i.e., Richards, Mitchell, de Wit, & Seiden, 1997;Green et al, 2007), or the method developed by Evenden and Ryan (1996). One important distinction between these procedures is that the former generate graded discounting functions between sessions (e.g., Stein et al, 2012), and the latter generates an entire delay-of-reinforcement function within each session (e.g., Evenden & Ryan, 1999); these methods serve different purposes when examining variables affecting impulsive choice. All of the procedures, however, share the following characteristics: (1) Discrete trials offer a choice between the SSR and the LLR; (2) forced-choice trials precede free-choice trials, exposing subjects to the contingencies associated with the SSR and the LLR; (3) forced-and free-choice trials require a single response to produce either the SSR or the LLR; and (4) an intertrial interval follows each SSR and LLR, keeping constant the time between choices.…”

“…Moreover, studies of impulsive choice in LEWs and F344s have shown steeper discounting functions for the former than for the latter strain of rats (e.g., Anderson & Diller, 2010;Anderson & Woolverton, 2005;Huskinson, Krebs, & Anderson, 2012;Madden, Smith, Brewer, Pinkston, & Johnson, 2008). Nonetheless, the generality of this finding has been compromised by research that failed to show differences in discounting functions between LEWs and F344s (e.g., Stein et al, 2012;Wilhelm & Mitchell, 2009).…”