Delay discounting of food by rhesus monkeys: Cocaine and food choice in isomorphic and allomorphic situations.

Abstract

Research on delay discounting has focused largely on non-drug reinforcers in an isomorphic context in which choice is between alternatives that involve the same type of reinforcer. Less often, delay discounting has been studied with drug reinforcers in a more ecologically valid allomorphic context where choice is between alternatives involving different types of reinforcers. The present experiment is the first to examine discounting of drug and non-drug reinforcers in both isomorphic and allomorphic situations using a theoretical model (i.e., the hyperbolic discounting function) that allows for comparisons of discounting rates between reinforcer types and amounts. The goal of the current experiment was to examine discounting of a delayed, non-drug reinforcer (food) by male rhesus monkeys when the immediate alternative was either food (isomorphic situation) or cocaine (allomorphic situation). In addition, we sought to determine whether there was a magnitude effect with delayed food in the allomorphic situation. Choice of immediate food and immediate cocaine increased with amount and dose, respectively. Choice functions for immediate food and cocaine generally shifted leftward as delay increased. Compared to isomorphic situations in which food was the immediate alternative, delayed food was discounted more steeply in allomorphic situations where cocaine was the immediate alternative. Notably, discounting was not affected by the magnitude of the delayed reinforcer. These data indicate that how steeply a delayed non-drug reinforcer is discounted may depend more on the qualitative characteristics of the immediate reinforcer and less on the magnitude of the delayed one.

Choice between delayed food and immediate opioids in rats: treatment effects and individual differences

“…Bickel et al (2011) found a trend for cocaine addicts to discount delayed money more when the alternative was immediate cocaine than when the alternative was immediate money. Huskinson et al (2015) found that rhesus monkeys discounted delayed food more steeply when the alternative was immediate cocaine than when the alternative was immediate food. These findings are consistent with the steep discounting of delayed food that we observed when the alternative to delayed food was immediate delivery of a 16 µg/kg/inj dose of remifentanil, and they are consistent with similar effects in rhesus monkeys (Maguire et al 2013).…”

“…Since the percent choice curve and the area under this curve could be affected by the nondelayed values of the drug and nondrug options, or by the difference between these two values, the shape of the curves obtained in these experiments might not generalize to other amounts of food. More complete information could be obtained by determining indifference points between delayed and nondelayed rewards over a wider range of values (Bickel et al 2011; Huskinson et al 2015; Mazur 1987). However, despite this limitation, the comparisons obtained with the streamlined procedure clearly demonstrate that the nature of the immediate drug option strongly affects the value of the delayed nondrug option.…”

“…This maladaptive choice behavior involves delay discounting, the tendency to place less value on rewards that are not received immediately (Bickel et al 2014; Lamb et al 2016). From this perspective, procedures that allow animals to choose between immediate drug injection and delayed delivery of a salient nondrug reward could provide a useful model of addiction (Huskinson et al 2015; Maguire et al 2013; Woolverton and Anderson 2006), with certain advantages over procedures that only involve drug reward or that only provide a choice between immediate drug and immediate nondrug reward.…”

Delay discounting of the μ-opioid receptor agonist remifentanil in rhesus monkeys

“…Moreover, the current findings with remifentanil, taken together with studies using cocaine, support the hypothesis that in nonhuman primates drug reinforcers in general might be discounted less steeply than non-drug reinforcers such as sweetened solutions (see Freeman et al, 2009; 2012). A recent study reported that the rate at which food is discounted depends upon whether food or drug (cocaine) is available as the immediately delivered alternative (Huskinson et al, 2015), demonstrating that the types of commodities that are available, as either the immediate or the delayed option, also impact discounting. It is not known whether monkeys in the current study might discount other opioids (e.g., heroin), drugs with different pharmacological mechanisms (e.g., cocaine), or non-drug reinforcers (e.g., food) at different rates.…”

“…Studies to date indicate that delaying delivery of an otherwise preferred reinforcer, whether food (Maguire et al, 2013b; Woolverton and Anderson, 2006; Huskinson et al 2015) or a dose of drug (Maguire et al, 2013a; Woolverton and Anderson, 2006; Woolverton et al, 2007), increases responding for a small and less preferred reinforcer (e.g., small dose of drug). Delaying presentation of preferred commodities increases the effectiveness of small immediately-available commodities.…”