Group I metabotropic glutamate receptor antagonists impair discriminability of reinforcer magnitude, but not risky choice, in a probability-discounting task

Abstract

The glutamatergic system has been identified as an important mediator of risky choice. However, previous studies have focused primarily on ionotropic glutamate receptors (e.g., NMDA receptors). Little research has examined the contribution of metabotropic glutamate receptors (mGluRs) on risky choice. The goal of the current experiment was to determine the effects of mGluR₁ and mGluR₅ antagonism on risky choice as assessed in probability discounting (PD). Male Sprague Dawley rats (n = 24) were trained in PD, in which consistently choosing a large, probabilistic reward (LR) reflects risky choice. For half of the rats, the odds against (OA) receiving the LR increased across blocks of trials, whereas the OA decreased across the session for half of the rats. Following training, rats received injections of the mGluR₁ antagonist JNJ 16,259,685 (JNJ; 0, 0.1, 0.3, or 1.0 mg/kg; i.p) and the mGluR₅ antagonist MTEP (0, 1.0, 3.0, or 10.0 mg/kg; i.p.). Regardless of which schedule was used, JNJ and MTEP decreased preference for the LR when its delivery was guaranteed. In contrast to delay discounting, in which blocking the mGluR₁ has been shown to alter impulsive choice, these results show that the Group I mGluR family does not selectively alter risky choice. Instead, blocking these receptors appears to impair discriminability of reinforcers of varying magnitudes in PD.

Keywords: Discriminability of reinforcer magnitude; Metabotropic glutamate receptor; Probability discounting; Rat; Risky choice; Sensitivity to probabilistic reinforcement.

Figure 1.

Schematic of the PD task. (a) Forced-choice trial. (b) Free-choice trial. Abbreviations: FR = fixed ratio; HL = house light; LL = left lever; LR = large magnitude reinforcer; LSL = left stimulus light; OA = odds against; RL = right lever; RSL = right stimulus light.

Figure 2.

(a) Mean (±SEM) raw proportion of responses for the LR as a function of the OA receiving reinforcement. The closed symbols represent rats trained on the ascending schedule, and open symbols represent rats trained on the descending schedule. (b) Mean (±SEM) A parameter estimates (preference for LR when its delivery is guaranteed) for rats trained on the ascending schedule (white bars) and on the descending schedule (black bars). (c) Mean (±SEM) h parameter estimates (sensitivity to probabilistic reinforcement) for rats trained on the ascending schedule (white bars) and on the descending schedule (black bars). *p < .05, relative to rats trained on the ascending schedule.

Figure 3.

(a) Mean (±SEM) raw proportion of responses for the LR as a function of the OA receiving reinforcement following injections of JNJ 16259685. (b) Mean (±SEM) A parameter estimates (preference for LR when its delivery is guaranteed; left y-axis; closed symbols) and h parameter estimates (sensitivity to probabilistic reinforcement; right y-axis; open symbols). (c) Mean (±SEM) number of completed trials. *p < .05, relative to vehicle. Note, data from the ascending and descending schedules were averaged together.

Figure 4.

(a) Mean (±SEM) raw proportion of responses for the LR as a function of the OA receiving reinforcement following injections of MTEP. (b) Mean (±SEM) A parameter estimates (preference for LR when its delivery is guaranteed; left y-axis; closed symbols) and h parameter estimates (sensitivity to probabilistic reinforcement; right y-axis; open symbols). (c) Mean (±SEM) number of completed trials. *p < .05, relative to vehicle. Note, data from the ascending and descending schedules were averaged together.