Effects of N-methyl-d-aspartate receptor (NMDAr) uncompetitive antagonists in a delay discounting paradigm using a concurrent-chains procedure

A method for studying reinforcement factors controlling impulsive choice for use in behavioral neuroscience

Although procedures originating within the experimental analysis of behavior commonly are used in behavioral neuroscience to produce behavioral endpoints, they are used less often to analyze the behavioral processes involved, particularly at the level of individual organisms (see Soto, 2020). Concurrent-chains procedures have been used extensively to study choice and to quantify relations between various dimensions of reinforcement and preference. Unfortunately, parametric analysis of those relations using traditional steady-state, single-subject experimental designs can be time-consuming, often rendering these procedures impractical for use in behavioral neuroscience. The purpose of this paper is to describe how concurrent-chains procedures can be adapted to allow for parametric examination of effects of the reinforcement dimensions involved in impulsive choice (magnitude and delay) within experimental sessions in rats. Data are presented indicating that this procedure can produce relatively consistent within-session estimates of sensitivity to reinforcement in individual subjects, and that these estimates can be modified by neurobiological manipulation (drug administration). These data suggest that this type of procedure offers a promising approach to the study of neurobiological mechanisms of complex behavior in individual organisms, which could facilitate a more fruitful relationship between behavior analysis and behavioral neuroscience.

GABAB receptors in prelimbic cortex and basolateral amygdala differentially influence intertemporal decision making and decline with age

The ability to decide adaptively between immediate vs. delayed gratification (intertemporal choice) is critical for well-being and is associated with a range of factors that influence quality of life. In contrast to young adults, many older adults show enhanced preference for delayed gratification; however, the neural mechanisms underlying this age difference in intertemporal choice are largely un-studied. Changes in signaling through GABA_B receptors (GABA_BRs) mediate several age-associated differences in cognitive processes linked to intertemporal choice. The current study used a rat model to determine how GABA_BRs in two brain regions known to regulate intertemporal choice (prelimbic cortex; PrL and basolateral amygdala; BLA) contribute to age differences in this form of decision making in male rats. As in humans, aged rats showed enhanced preference for large, delayed over small, immediate rewards during performance in an intertemporal choice task in operant test chambers. Activation of PrL GABA_BRs via microinfusion of the agonist baclofen increased choice of large, delayed rewards in young adult rats but did not influence choice in aged rats. Conversely, infusion of baclofen into the BLA strongly reduced choice of large, delayed rewards in both young adult and aged rats. Aged rats further showed a significant reduction in expression of GABA_BR1 subunit isoforms in the prefrontal cortex, a discovery that is consonant with the null effect of intra-PrL baclofen on intertemporal choice in aged rats. In contrast, expression of GABA_BR subunits was generally conserved with age in the BLA. Jointly, these findings elucidate a role for GABA_BRs in intertemporal choice and identify fundamental features of brain maturation and aging that mediate an improved ability to delay gratification.

Characterization of a differential reinforcement of low rates of responding task in non-deprived male and female rats: Role of Sigma-1 receptors

Impulsive action can be defined as the inability to withhold a response and represents one of the dimensions of the broad construct impulsivity. Here, we characterized a modified differential reinforcement of low rates of responding (DRL) task developed in our laboratory, in which impulsive action is measured in ad libitum fed/watered subjects. Specifically, we first determined the effects of both sex and estrous cycle on impulsive action by systematically comparing male and estrous-synchronized female subjects. In addition, we evaluated the convergent validity of this modified DRL task by testing the effects of the D2R/5HT2AR antagonist, aripiprazole, and the noncompetitive NMDAR antagonist, MK-801. Finally, we tested the effects of the selective antagonist BD-1063 and agonist PRE-084 of Sigma-1 receptor (Sig-1R) on impulsive action using this modified DRL task. We found that female rats showed and increased inability to withhold a response when compared to males, and this effect was driven by the metestrus/diestrus phase of the estrous cycle. In addition, aripiprazole and MK-801 fully retained their capability to reduce and increase impulsive action, respectively. Finally, the selective Sig-1R antagonist, BD-1063 dose-dependently reduced the inability to withhold a response in both sexes, though more potently in female rats. In summary, we show that impulsive action, as measured in a modified DRL task which minimizes energy-homeostatic influences, is a function of both sex and estrous cycle. Furthermore, we validate the convergent validity of the task and provide evidence that Sig-1R antagonism may represent a novel pharmacological strategy to reduce impulsive action.

Pair housing, but not using a controlled reinforcer frequency procedure, attenuates the modulatory effect of probability presentation order on amphetamine-induced changes in risky choice

Probability discounting is often measured with independent schedules. Independent schedules have several limitations, such as confounding preference for one alternative with frequency of reward presentation and generating ceiling/floor effects at certain probabilities. To address this potential caveat, a controlled reinforcer frequency schedule can be used, in which the manipulandum that leads to reinforcement is pseudo-randomly determined before each trial. This schedule ensures subjects receive equal presentations of the small and large magnitude reinforcers across each block of trials. A total of 24 pair-housed and 11 individually housed female Sprague Dawley rats were tested in a controlled reinforcer frequency procedure. For half of the rats, the odds against (OA) receiving the large magnitude reinforcer increased across the session (ascending schedule); the OA decreased across the session for half of the rats (descending schedule). Following training, rats received treatments of amphetamine (AMPH; 0, 0.25, 0.5, 1.0 mg/kg; s.c.). For pair-housed rats, AMPH (0.5 mg/kg) increased risky choice, regardless of probability presentation order, whereas a higher dose of AMPH (1.0 mg/kg) decreased discriminability of reinforcer magnitude for rats trained on the descending schedule only. For individually housed rats, probability presentation order modulated the effects of AMPH on probability discounting, as AMPH (0.25 and 0.5 mg/kg) increased risky choice in rats trained on the ascending schedule but not on the descending schedule. These results show that pair-housing animals, but not using a controlled reinforcer frequency procedure, attenuates the modulatory effects of probability presentation order on drug effects on risky choice.

Using a dependent schedule to measure risky choice in male rats: Effects of d-amphetamine, methylphenidate, and methamphetamine

Risky choice is the tendency to choose a large, uncertain reward over a small, certain reward, and is typically measured with probability discounting, in which the probability of obtaining the large reinforcer decreases across blocks of trials. One caveat to traditional procedures is that independent schedules are used, in which subjects can show exclusive preference for one alternative relative to the other. For example, some rats show exclusive preference for the small, certain reinforcer as soon as delivery of the large reinforcer becomes probabilistic. Therefore, determining if a drug increases risk aversion (i.e., decreases responding for the probabilistic alternative) is difficult (due to floor effects). The overall goal of this experiment was to use a concurrent-chains procedure that incorporated a dependent schedule during the initial link, thus preventing animals from showing exclusive preference for one alternative relative to the other. To determine how pharmacological manipulations alter performance in this task, male Sprague-Dawley rats (n = 8) received injections of amphetamine (0, 0.25, 0.5, 1.0 mg/kg), methylphenidate (0, 0.3, 1.0, 3.0 mg/kg), and methamphetamine (0, 0.5, 1.0, 2.0 mg/kg). Amphetamine (0.25 mg/kg) and methylphenidate (3.0 mg/kg) selectively increased risky choice, whereas higher doses of amphetamine (0.5 and 1.0 kg/mg) and each dose of methamphetamine impaired stimulus control (i.e., flattened the discounting function). These results show that dependent schedules can be used to measure risk-taking behavior and that psychostimulants promote suboptimal choice when this schedule is used. (PsycInfo Database Record (c) 2020 APA, all rights reserved).