Baseline-dependent effects of amphetamine on attention are associated with striatal dopamine metabolism

Methamphetamine-induced adaptation of learning rate dynamics depend on baseline performance

The ability to calibrate learning according to new information is a fundamental component of an organism's ability to adapt to changing conditions. Yet, the exact neural mechanisms guiding dynamic learning rate adjustments remain unclear. Catecholamines appear to play a critical role in adjusting the degree to which we use new information over time, but individuals vary widely in the manner in which they adjust to changes. Here, we studied the effects of a low dose of methamphetamine (MA), and individual differences in these effects, on probabilistic reversal learning dynamics in a within-subject, double-blind, randomized design. Participants first completed a reversal learning task during a drug-free baseline session to provide a measure of baseline performance. Then they completed the task during two sessions, one with MA (20 mg oral) and one with placebo (PL). First, we showed that, relative to PL, MA modulates the ability to dynamically adjust learning from prediction errors. Second, this effect was more pronounced in participants who performed poorly at baseline. These results present novel evidence for the involvement of catecholaminergic transmission on learning flexibility and highlights that baseline performance modulates the effect of the drug.

Reward-enhancing effects of d-amphetamine and its interactions with nicotine were greater in female rats and persisted across schedules of reinforcement

Nicotine enhances the value of environmental stimuli and rewards, and reward enhancement can maintain nicotine consumption. Stimulants such as d-amphetamine are misused more by women and are commonly co-used with nicotine. d-Amphetamine potentiates nicotine's effects in human and animal research. To date, there are no published studies examining this interaction in a reward-enhancement task. The current study sought to investigate the reward-enhancing effects of nicotine alongside and coadministered with d-amphetamine. Further, we evaluated the persistence of reward enhancement across ratio and temporal schedules of reinforcement. We used 10 male and 10 female Sprague-Dawley rats. Enhancement was assessed within subjects by examining active lever pressing for a visual stimulus reinforcer on variable ratio 3, variable interval 30 s and variable time 30 s - variable ratio 3 schedules. Before 1-h sessions, rats received one injection of saline, 0.1 or 0.3 mg/kg d-amphetamine and one of saline or 0.4 mg/kg nicotine, making six possible drug combinations (saline + saline, saline + nicotine, 0.1 d-amphetamine + aline, 0.1 d-amphetamine + nicotine, 0.3 d-amphetamine + saline and 0.3 d-amphetamine + nicotine) experienced in a randomized order by each rat. When d-amphetamine was coadministered with nicotine, we found an interaction effect on reward enhancement that persisted across schedules of reinforcement. Males and females exhibited reward enhancement by 0.3 d-amphetamine, while only females showed reward enhancement by 0.1 d-amphetamine. Further, females responded more for the visual stimulus than males in all d-amphetamine conditions. Future studies should assess how reward enhancement is involved in high nicotine-amphetamine comorbidity rates and enhanced amphetamine misuse in women.

Cannabinoid Modulation of Dopamine Release During Motivation, Periodic Reinforcement, Exploratory Behavior, Habit Formation, and Attention

Motivational and attentional processes energize action sequences to facilitate evolutionary competition and promote behavioral fitness. Decades of neuropharmacology, electrophysiology and electrochemistry research indicate that the mesocorticolimbic DA pathway modulates both motivation and attention. More recently, it was realized that mesocorticolimbic DA function is tightly regulated by the brain's endocannabinoid system and greatly influenced by exogenous cannabinoids-which have been harnessed by humanity for medicinal, ritualistic, and recreational uses for 12,000 years. Exogenous cannabinoids, like the primary psychoactive component of cannabis, delta-9-tetrahydrocannabinol, produce their effects by acting at binding sites for naturally occurring endocannabinoids. The brain's endocannabinoid system consists of two G-protein coupled receptors, endogenous lipid ligands for these receptor targets, and several synthetic and metabolic enzymes involved in their production and degradation. Emerging evidence indicates that the endocannabinoid 2-arachidonoylglycerol is necessary to observe concurrent increases in DA release and motivated behavior. And the historical pharmacology literature indicates a role for cannabinoid signaling in both motivational and attentional processes. While both types of behaviors have been scrutinized under manipulation by either DA or cannabinoid agents, there is considerably less insight into prospective interactions between these two important signaling systems. This review attempts to summate the relevance of cannabinoid modulation of DA release during operant tasks designed to investigate either motivational or attentional control of behavior. We first describe how cannabinoids influence DA release and goal-directed action under a variety of reinforcement contingencies. Then we consider the role that endocannabinoids might play in switching an animal's motivation from a goal-directed action to the search for an alternative outcome, in addition to the formation of long-term habits. Finally, dissociable features of attentional behavior using both the 5-choice serial reaction time task and the attentional set-shifting task are discussed along with their distinct influences by DA and cannabinoids. We end with discussing potential targets for further research regarding DA-cannabinoid interactions within key substrates involved in motivation and attention.

Healthy Subjects With Extreme Patterns of Performance Differ in Functional Network Topology and Benefits From Nicotine

Do subjects with atypical patterns in attentional and executive behaviour show different brain network topology and react differently towards nicotine administration? The efficacy of pro-cognitive drugs like nicotine considerably varies between subjects and previous theoretical and empirical evidence suggest stronger behavioural nicotine effects in subjects with low performance. One problem is, however, how to best define low performance, especially if several cognitive functions are assessed for subject characterisation. We here present a method that used a multivariate, robust outlier detection algorithm to identify subjects with suspicious patterns of performance in attentional and executive functioning. In contrast to univariate approaches, this method is sensitive towards extreme positions within the multidimensional space that do not have to be extreme values in the individual behavioural distributions. The method was applied to a dataset of healthy, non-smoking subjects (n = 34) who were behaviorally characterised by an attention and executive function test on which N = 12 volunteers were classified as outliers. All subjects then underwent a resting-state functional magnetic resonance imaging (fMRI) scan to characterise brain network topology and an experimental behavioural paradigm under placebo and nicotine (7 mg patch) that gauged aspects of attention and executive function. Our results indicate that subjects with an atypical multivariate pattern in attention and executive functioning showed significant differences in nodal brain network integration in visual association and pre-motor brain regions during resting state. These differences in brain network topology significantly predicted larger individual nicotine effects on attentional processing. In summary, the current approach successfully identified a subgroup of healthy volunteers with low behavioural performance who differ in brain network topology and attentional benefit from nicotine.