Neural basis of operant behaviors maintained on the differential-reinforcement-of-low-rate (DRL) schedule in rodents

Phasic dopamine release in two different rat models of attention-deficit/hyperactivity disorder: Spontaneously hypertensive rats (SHR) versus Lphn3 knockout rats

We examined DA activity in the medial prefrontal cortex (mPFC) and nucleus accumbens core (NAcc) in two Different Rat Models of Attention-Deficit/Hyperactivity Disorder: Spontaneously Hypertensive Rats (SHR) Versus Lphn3 Knockout Rats. We examined baseline stimulation-evoked phasic DA release, half-life, and DA autoreceptor (DAR) functioning in the mPFC and NAcc, as well as the response to nomifensine (10 mg/kg, IP), a DA transporter (DAT) blocker, on these measures in the NAcc. Both rat models were hypodopaminergic, with notable regional and mechanistic differences. The SHRs displayed decreased DA release in the NAcc compared to their control strain (i.e., WKY rats), with no differences in the mPFC, leading a much lower NAcc-to-PFC DA release ratio in SHRs compared to controls suggesting an imbalance in DA transmission between these regions. The Lphn3 KO rats were considered hypodopaminergic based on the reduced summed DA release in the mPFC and NAcc compared to WT controls, although differences were not observed when examining each site independently. Lphn3 KOs displayed increased DA half-life in the mPFC compared with Lphn3 WT rats, an indication of decreased DAT reuptake, with no differences in the NAcc. DAT blockade by nomifensine had a similar effect on DA release in the NAcc of SHRs and WKYs, but increased DA release in the NAcc of Lphn3 KOs to a greater extent than in WTs. These results suggest that the efficacy of pharmacotherapies used to treat externalizing disorders such as ADHD and/or SUD, likely differ between SHRs and Lphn3 KO rats.

Modular Maximization Theory: A functional account of economic behavior in laboratory animal models with applications to drug-seeking behavior

Substance abuse research depends on precise and sensitive assessments of reinforcer efficacy in animal models. However, conventional methods often lack theoretical rigor and specificity to support these assessments. To address these gaps, the Modular Maximization Theory (MMT) is introduced as a comprehensive framework for understanding instrumental behavior. Like earlier maximization theories, MMT posits that behavior is distributed across alternatives to maximize utility over time. This concept is structured through five foundational postulates that define alternative actions and rules for choosing between them as budget constraints and utility functions. A key innovation of MMT is its incorporation of reinforcer utilization-encompassing both consummatory and post-consummatory activities-into the budget-constraint function. A model of ratio-schedule performance is developed under the assumption that utilization is proportional to demand, with utility represented as an additive power function of reinforcer magnitude. This model, termed PURSPU (Proportional Utilization, Ratio Schedule, Power Utility), effectively explains how reinforcer magnitude, response effort, non-contingent reinforcement, and income influence demand curves, behavior-output functions, dose-response relationships, and progressive-ratio breakpoints, while accounting for rate-dependent effects. The model also offers novel insights into choice behavior, including concurrent-schedule performance, income dependency, and delay discounting, as well as post-reinforcement pauses and run rates. Variations in budget constraints and utility functions are proposed as alternative models. Potential theoretical advancements, more targeted assessments of drug abuse liability, and the broader role of MMT in understanding human drug abuse are explored.

Prenatal Alcohol Exposure Disrupts Performance in a Differential Reinforcement of Low Rates Task Specifically in Adolescent Male Rats

Prenatal alcohol exposure (PAE) can lead to a wide range of adverse effects in humans, including impaired self-control and increased impulsive behavior. Deficits in self-control can interfere with academic performance and have lasting impacts. In the present study, a rodent model of PAE was used to assess impulsivity through operant conditioning. Pregnant rats were assigned to one of three groups: ad-lib control (CON), pair-fed (PF), and alcohol-exposed (ALC). ALC rats were given a liquid diet containing 6% alcohol, PF rats were yoked to an ALC rat and given a CON liquid diet, and CON rats received ad libitum food. Operant conditioning was used to evaluate extinction in adolescents (Experiment 1) and differential reinforcement of low rates (DRL) in adolescents and adults (Experiment 2). PAE resulted in an increase in responses and resets during DRL testing, indicative of impaired self-control, an effect that was only observed in adolescent males. Females, regardless of age, did not show increased impulsivity following PAE. This indicates that children with PAE may exhibit attentional deficits similar to those with attention deficit hyperactivity disorder, with males at a higher risk.

Investigating Temporal Memory Strength and Time-Based Impulse Control Using the DRL Task

Differential reinforcement of low rate (DRL) responding is a schedule-controlled behavior sometimes used in timing research, but also received critics of not providing a pure measure of timing due to the influence of the subject’s motivation or inhibitory control. Nevertheless, we argue that the DRL task provides a unique approach to study how timing and emotion interact with each other. Here, we review evidence showing that male rats prenatally treated with choline supplementation had difficulty in acquiring longer criterion times in the DRL task. This was possibly due to the stronger memory strength of their previously learned shorter criterion times. Female rats, in contrast, performed better than male rats in the same task, but those receiving prenatal choline supplementation were the best performers in this task with longer criterion times because they required less training. Like all female rats, male rats treated with prenatal choline supplementation made very few burst responses, suggesting that the treatment improved male rats’ emotional regulation when facing ‘frustrating’ outcomes. The differential impulse control plus different memory strength of the rats trained in the DRL task revealed the potential interaction of sex hormones and prenatal choline supplementation, a rare combination in a single animal study on timing and time perception. In summary, although the DRL task is certainly not the best timing task, it may be useful in assisting us in better understanding how time perception participates in emotional regulation, especially relevant when the emotion is triggered by a failure in timing.