Decoupling Dopamine Synthesis from Impulsive Action, Risk-Related Decision-Making, and Propensity to Cocaine Intake: A Longitudinal [18F]-FDOPA PET Study in Roman High- and Low-Avoidance Rats

Impulsive action and risk-related decision-making (RDM) are two facets of impulsivity linked to a hyperdopaminergic release in the striatum and an increased propensity to cocaine intake. We previously showed that with repeated cocaine exposure, this initial hyperdopaminergic release is blunted in impulsive animals, potentially signaling drug-induced tolerance. Whether such dopaminergic dynamics involve changes in dopamine (DA) synthesis as a function of impulsivity is currently unknown. Here, we investigated the predictive value of DA synthesis for impulsive action, RDM, and the propensity to take cocaine in a rat model of vulnerability to cocaine abuse. Additionally, we assessed the effects of cocaine intake on these variables. Rats were tested sequentially in the rat Gambling Task (rGT) and were scanned with positron emission tomography and [18F]-FDOPA to respectively assess both impulsivity facets and striatal DA synthesis before and after cocaine self-administration (SA). Our results revealed that baseline striatal levels of DA synthesis did not significantly predict impulsive action, RDM, or a greater propensity to cocaine SA in impulsive animals. Besides, we showed that impulsive action, but not RDM, predicted higher rates of cocaine taking. However, chronic cocaine exposure had no impact on DA synthesis, nor affected impulsive action and RDM. These findings indicate that the hyper-responsive DA system associated with impulsivity and a propensity for cocaine consumption, along with the reduction in this hyper-responsive DA state in impulsive animals with a history of cocaine use, might not be mediated by dynamic changes in DA synthesis.

Motor impulsivity but not risk-related impulsive choice is associated to drug intake and drug-primed relapse

Introduction

Motor impulsivity and risk-related impulsive choice have been proposed as vulnerability factors for drug abuse, due to their high prevalence in drug abusers. However, how these two facets of impulsivity are associated to drug abuse remains unclear. Here, we investigated the predictive value of both motor impulsivity and risk-related impulsive choice on characteristics of drug abuse including initiation and maintenance of drug use, motivation for the drug, extinction of drug-seeking behavior following drug discontinuation and, finally, propensity to relapse.

Methods

We used the Roman High- (RHA) and Low- Avoidance (RLA) rat lines, which display innate phenotypical differences in motor impulsivity, risk-related impulsive choice, and propensity to self-administer drugs. Individual levels of motor impulsivity and risk-related impulsive choice were measured using the rat Gambling task. Then, rats were allowed to self-administer cocaine (0.3 mg/kg/infusion; 14 days) to evaluate acquisition and maintenance of cocaine self-administration, after which motivation for cocaine was assessed using a progressive ratio schedule of reinforcement. Subsequently, rats were tested for their resistance to extinction, followed by cue-induced and drug-primed reinstatement sessions to evaluate relapse. Finally, we evaluated the effect of the dopamine stabilizer aripiprazole on reinstatement of drug-seeking behaviors.

Results

We found that motor impulsivity and risk-related impulsive choice were positively correlated at baseline. Furthermore, innate high levels of motor impulsivity were associated with higher drug use and increased vulnerability to cocaine-primed reinstatement of drug-seeking. However, no relationships were observed between motor impulsivity and the motivation for the drug, extinction or cue-induced reinstatement of drug-seeking. High levels of risk-related impulsive choice were not associated to any aspects of drug abuse measured in our study. Additionally, aripiprazole similarly blocked cocaine-primed reinstatement of drug-seeking in both high- and low-impulsive animals, suggesting that aripiprazole acts as a D_2/3R antagonist to prevent relapse independently of the levels of impulsivity and propensity to self-administer drugs.

Discussion

Altogether, our study highlights motor impulsivity as an important predictive factor for drug abuse and drug-primed relapse. On the other hand, the involvement of risk-related impulsive choice as a risk factor for drug abuse appears to be limited.

Concurrent measures of impulsive action and choice are partially related and differentially modulated by dopamine D1- and D2-like receptors in a rat model of impulsivity

Impulsivity is a multidimensional construct, but the relationships between its constructs and their respective underlying dopaminergic underpinnings in the general population remain unclear. A cohort of Roman high- (RHA) and low- (RLA) avoidance rats were tested for impulsive action and risky decision-making in the rat gambling task, and then for delay discounting in the delay-discounting task to concurrently measure the relationships among the three constructs of impulsivity using a within-subject design. Then, we evaluated the effects of dopaminergic drugs on the three constructs of impulsivity, considering innate differences in impulsive behaviors at baseline. Risky decision-making and delay-discounting were positively correlated, indicating that both constructs of impulsive choice are related. Impulsive action positively correlated with risky decision-making but not with delay discounting, suggesting partial overlap between impulsive action and impulsive choice. RHAs showed a more impulsive phenotype in the three constructs of impulsivity compared to RLAs, demonstrating the comorbid nature of impulsivity in a population of rats. Amphetamine increased impulsive action and had no effect on risky decision-making regardless of baseline levels of impulsivity, but it decreased delay discounting only in high impulsive RHAs. In contrast, while D₁R and D₃R agonism as well as D_2/3R partial agonism decreased impulsive action regardless of baseline levels of impulsivity, D_2/3R agonism decreased impulsive action exclusively in high impulsive RHAs. Irrespective of baseline levels of impulsivity, risky decision-making was increased by D₁R and D_2/3R agonism but not by D₃R agonism or D_2/3R partial agonism. Finally, while D₁R and D₃R agonism, D_2/3R partial agonism and D₂R blockade increased delay discounting irrespective of baseline levels of impulsivity, D_2/3R agonism decreased it in low impulsive RLAs only. These findings indicate that the acute effects of dopamine drugs were partially overlapping across dimensions of impulsivity, and that only D_2/3R agonism showed baseline-dependent effects on impulsive action and impulsive choice.