Isolating Behavioral Mechanisms of Intertemporal Choice: Nicotine Effect on Delay Discounting and Amount Sensitivity

Abstract

Many drugs of abuse produce changes in impulsive choice, that is, choice for a smaller-sooner reinforcer over a larger-later reinforcer. Because the alternatives differ in both delay and amount, it is not clear whether these drug effects are due to the differences in reinforcer delay or amount. To isolate the effects of delay, we used a titrating delay procedure. In phase 1, 9 rats made discrete choices between variable delays (1 or 19 s, equal probability of each) and a delay to a single food pellet. The computer titrated the delay to a single food pellet until the rats were indifferent between the two options. This indifference delay was used as the starting value for the titrating delay for all future sessions. We next evaluated the acute effects of nicotine (subcutaneous 1.0, 0.3, 0.1, and 0.03 mg/kg) on choice. If nicotine increases delay discounting, it should have increased preference for the variable delay. Instead, nicotine had very little effect on choice. In a second phase, the titrated delay alternative produced three food pellets instead of one, which was again produced by the variable delay (1 s or 19 s) alternative. Under this procedure, nicotine increased preference for the one pellet alternative. Nicotine-induced changes in impulsive choice are therefore likely due to differences in reinforcer amount rather than differences in reinforcer delay. In addition, it may be necessary to include an amount sensitivity parameter in any mathematical model of choice when the alternatives differ in reinforcer amount.Key words: risk, impulsive choice, reinforer delay, reinforcer amount, nicotine, lever press, ratsGiven a choice between two reinforcers, the preferred reinforcer is said to have a greater ''value.'' Prediction of choice depends upon identifying the function that accurately describes how different variables contribute to reinforcer value. Several mathematical models of choice have been proposed for that purpose (e.g., Bateson & Kacelnik, 1995;Davison, 1988;Gibbon, Church, & Fairhurst, 1988;Grace, 1993;Herrnstein, 1970;Hursh & Fantino, 1973;Killeen, 1968;1982;Mazur, 1984). An assumption of most of these models is that the value of a reinforcer decreases as the delay to that reinforcer increases. The mathematical function that describes this relation is typically referred to as a delay discounting function. One such model that has been very successful in describing and predicting choices in discrete trial situations is Mazur's (1987) hyperbolic discounting function:where V represents the current value of a delayed reinforcer, A represents the reinforcer amount, D represents the delay to the reinforcer, and k is a free parameter which reflects the rate at which the reinforcer loses value with increases in delay. One prediction of Equation 1 is preference for a smaller-sooner reinforcer over a largerlater reinforcer, when the delay of the latter is such that its value is relatively lower than that of the former. The point at which this happens is determined in part by how quickly reinforcer value dec...

Determinants of choice, and vulnerability and recovery in addiction

“…If both life-course-persistent and adolescence-limited deviancy are a result of the decreased effectiveness of delayed rewards at controlling behavior in each group, a differing etiology of risk between the two groups may indicate that the mechanism underlying rapid discounting in each group also differs. The potential for multiple mechanisms controlling the rate of discounting is consistent with the multiple processes shown to contribute to discounting: working memory (Bobova et al, 2009); timing (Galtress et al, 2012); reward sensitivity (Locey & Dallery, 2009); as well as distinct brain regions/neurotransmitter systems (MacKillop, 2013; Monterosso et al, 2007; Stanger et al, 2013). Animal models of the rapid discounting seen in life-course-persistent and adolescence-limited deviancy might help us to discover the mechanisms underlying the rapid discounting seen in each group; and thus, aid in developing ways of ameliorating the consequences of this rapid discounting in each group, and in particular mouse models might be especially useful given the powerful tools for studying genetic factors in mice.…”

Comparing hyperbolic, delay-amount sensitivity and present-bias models of delay discounting

“…1), and q is a fitted parameter that moderates the effects of Q , reward quality, which contributes to the incentive value of the reward. A slightly different approach is taken by Locey & Dallery (2009), in which a sensitivity exponent ( z ) is added to A to modulate an individual’s response to both reward amount and delay in combination: $$V=A^z\cdot \frac{1}{1+k\cdot D}$$…”

Monetary-Based Consequences for Drug Abstinence: Methods of Implementation and Some Considerations about the Allocation of Finances in Substance Abusers

“…Several studies using non-human subjects have shown that exposure to some drugs result in an increase in delay discounting (e.g., methamphetamine and nicotine with rats (89,90), cf. (91), although such effects can differ depending on the drug, drug dose, procedure, and testing conditions. Other studies using rats have shown that pre-drug discounting rates can be used to predict various aspects of drug taking and relapse to drug use (86,92,93).…”