Dissociable roles of dopamine and serotonin transporter function in a rat model of negative urgency

Assessing the nature of premature responses in the rodent continuous performance test variable intertrial interval schedule using atomoxetine and amphetamine

BACKGROUND

Rodent operant tests that include premature responses (PR) as a measure of impulsivity commonly use variable intertrial interval (vITI) schedules. The rodent continuous performance test (rCPT) is suitable for a vITI schedule.

NEW METHOD

We optimised the analysis for a rCPT vITI schedule with intertrial intervals (ITIs) of 3, 6, and 12 s. Examining the nature of first (FiT) and following touches (FoT) to the blank screen led to a separate quantification of these two behaviours into the first touches level (%FiT) and the following-to-first touches ratio (FoT/FiT).

RESULTS

FiTs occurred more frequently in the 12 s ITIs than at shorter ITIs. Within 12 s ITIs, %FiT was only moderately higher during the last half than the first half, suggesting that long ITIs have a minimal effect on impulsivity, but allow a longer time for its detection. %FiT and the FoT/FiT ratio were uncorrelated. %FiT was negatively correlated with response criterion (C) and uncorrelated with discriminability. Conversely, FoT/FiT ratio was negatively correlated with discriminability, without correlation to C. Atomoxetine decreased %FiT but did not affect FoT/FiT ratio. Amphetamine increased %FiT and decreased the FoT/FiT ratio.

COMPARISON WITH EXISTING METHOD(S)

The results suggest that %FiT is analogous to %PR in related tasks and is a more suitable measure of waiting impulsivity in the rCPT. FoT/FiT ratio is unrelated to %FiT.

CONCLUSIONS

Long ITIs increase the detectability of, but has minimal effect on, waiting impulsivity. %FiT is analogous to %PR in related tasks, while the FoT/FiT ratio is a separate behaviour requiring further characterization.

Unraveling the Contribution of Serotonergic Polymorphisms, Prefrontal Alpha Asymmetry, and Individual Alpha Peak Frequency to the Emotion-Related Impulsivity Endophenotype

The unique contribution of the serotonin transporter-linked polymorphic region (5-HTTLPR), intronic region 2 (STin2), and monoamine oxidase A (MAO-A) genes to individual differences in personality traits has been widely explored, and research has shown that certain forms of these polymorphisms relate to impulsivity and impulsivity-related disorders. Humans showing these traits are also described as having an asymmetrical prefrontal cortical activity when compared to others. In this explorative study, we examine the relationship between serotonergic neurotransmission polymorphisms, cortical activity features (prefrontal alpha asymmetry, individual alpha peak frequency [iAPF]), emotion-related and non-emotion-related impulsivity in humans. 5-HTTLPR, MAO-A, and STin2 polymorphisms were assessed in blood taken from 91 participants with high emotion-related impulsivity levels. Sixty-seven participants completed resting electroencephalography and a more comprehensive impulsivity index. In univariate analyses, iAPF correlated with both forms of emotion-related impulsivity. In multiple linear regression models, 5-HTTLPR polymorphism (model 1, adj. R² = 15.2%) and iAPF were significant interacting predictors of emotion-related impulsivity, explaining a large share of the results’ variance (model 2, adj. R² = 21.2%). Carriers of the low transcriptional activity 5-HTTPLR and MAO-A phenotypes obtained higher emotion-related impulsivity scores than others did. No significant results were detected for non-emotion-related impulsivity or for a form of emotion-related impulsivity involving cognitive/motivational reactivity to emotion. Our findings support an endophenotypic approach to impulsivity, showing that tri-allelic 5-HTTLPR polymorphism, iAPF, and their interaction are relevant predictors of one form of emotion-related impulsivity.

Impaired cognitive flexibility and heightened urgency are associated with increased alcohol consumption in rodent models of excessive drinking

Alcohol use disorder (AUD) is characterized by impairments in decision-making that can exist as stable traits or transient states. Cognitive inflexibility reflects an inability to update information that guides decision-making and is thought to contribute to the inability to abstain from drinking. While several studies have reported evidence of impaired cognitive flexibility following chronic alcohol exposure, evidence that a pre-existing impairment in cognitive flexibility is a heritable risk factor for AUD is scarce. Here, we found that cognitive flexibility was impaired in rodents selectively bred for excessive alcohol consumption (alcohol preferring (P) rats), on the attentional set-shifting task (ASST). Further, the degree of impairment is predictive of future ethanol consumption, thus suggesting that cognitive inflexibility is a stable trait capable of predisposing one for drinking. In a second set of experiments, we observed an impairment in the ability of P rats to use a previously learned rule to guide foraging in a simple discrimination task. Convergence across several behavioral measures suggested that this impairment reflected a state of heightened urgency that interfered with decision-making. A similar impairment on a simple discrimination task was observed in Wistar rats with a history of alcohol consumption. These findings indicate how trait and state variables-in this case, impaired cognitive flexibility and heightened urgency, respectively-may influence the risk for excessive drinking. Furthermore, our results suggest that cognitive inflexibility and urgency can exist as both risk factors for and the result of alcohol exposure.

Which came first: Cannabis use or deficits in impulse control?

Impulse control deficits are often found to co-occur with substance use disorders (SUDs). On the one hand, it is well known that chronic intake of drugs of abuse remodels the brain with significant consequences for a range of cognitive behaviors. On the other hand, individual variation in impulse control may contribute to differences in susceptibility to SUDs. Both of these relationships have been described, thus leading to a "chicken or the egg" debate which remains to be fully resolved. Does impulsivity precede drug use or does it manifest as a function of problematic drug usage? The link between impulsivity and SUDs has been most strongly established for cocaine and alcohol use disorders using both preclinical models and clinical data. Much less is known about the potential link between impulsivity and cannabis use disorder (CUD) or the directionality of this relationship. The initiation of cannabis use occurs most often during adolescence prior to the brain's maturation, which is recognized as a critical period of development. The long-term effects of chronic cannabis use on the brain and behavior have started to be explored. In this review we will summarize these observations, especially as they pertain to the relationship between impulsivity and CUD, from both a psychological and biological perspective. We will discuss impulsivity as a multi-dimensional construct and attempt to reconcile the results obtained across modalities. Finally, we will discuss possible avenues for future research with emerging longitudinal data.

Effects of Social Isolation on Perineuronal Nets in the Amygdala Following a Reward Omission Task in Female Rats

Negative urgency is a facet of impulsivity associated with negative affect and risky behavior that may involve the amygdala. The current study determined if social isolation during development alters negative urgency and c-Fos activity in the basolateral amygdala (BLA). Female Sprague-Dawley rats were raised in an isolated condition (IC), a standard social condition (SC), or an enriched condition (EC) and then were tested for locomotor activity, novelty place preference, and negative urgency using a reward omission task. Following performance on the reward omission task, the brains were analyzed for c-Fos expression in Ca2+/calmodulin kinase II (CaMKII) and calbindin (CB) neurons, as well as in parvalbumin (PV) neurons associated with perineuronal nets (PNNs) in BLA. IC rats exhibited enhanced locomotion compared with both SC and EC rats, as well as enhanced novelty place preference compared with EC rats; only IC rats showed increased responding following omission of an expected reward (negative urgency). Following completion of the reward omission task, IC rats also displayed increased percent of c-Fos neurons in BLA associated with CaMKII, CB, and PV neurons compared with SC and EC rats. In IC rats, c-Fos activation in BLA occurred following the omission of an expected reward. Finally, IC rats displayed reduced PNN intensity associated with PV neurons compared with EC rats, but the percent of these neurons co-expressing c-Fos was greater in IC rats; SC rats were intermediate between IC and EC rats. Negative urgency was observed in IC rats, but not SC or EC rats. While multiple mechanisms are likely involved, this behavioral effect was associated with an isolation-induced increase in activity of excitatory neurons in BLA, as well as decreased PNN intensity surrounding GABAergic neurons in the same region.

Dietary intake of tryptophan tied emotion-related impulsivity in humans

Emotion-related impulsivity, defined as the tendency to say or do things that one later regret during periods of heightened emotion, has been tied to a broad range of psychopathologies. Previous work has suggested that emotion-related impulsivity is tied to an impaired function of the serotonergic system. Central serotonin synthesis relies on the intake of the essential amino acid, tryptophan and its ability to pass through the blood brain barrier. Objective: The aim of this study was to determine the association between emotion-related impulsivity and tryptophan intake. Methods: Undergraduate participants (N = 25, 16 women, 9 men) completed a self-rated measure of impulsivity (Three Factor Impulsivity Index, TFI) and daily logs of their food intake and exercise. These data were coded using the software NutriNote to evaluate intakes of tryptophan, large neutral amino acids, vitamins B6/B12, and exercise. Results: Correlational analyses indicated that higher tryptophan intake was associated with significantly lower scores on two out of three subscales of the TFI, Pervasive Influence of Feelings scores r = -.502, p < .010, and (lack-of) Follow-Through scores, r = -.407, p < .050. Conclusion: Findings provide further evidence that emotion-related impulsivity is correlated to serotonergic indices, even when considering only food habits. It also suggests the need for more research on whether tryptophan supplements might be beneficial for impulsive persons suffering from a psychological disorder.

Impulsivity Derived From the Dark Side: Neurocircuits That Contribute to Negative Urgency

Negative urgency is a unique dimension of impulsivity that involves acting rashly when in extreme distress and impairments in inhibitory control. It has been hypothesized to derive from stress that is related to negative emotional states that are experienced during the withdrawal/negative affect stage of the addiction cycle. Classically, a transition to compulsive drug use prevents or relieves negative emotional states that result from abstinence or stressful environmental circumstances. Recent work suggests that this shift to the "dark side" is also implicated in impulsive use that derives from negative urgency. Stress and anxious, depressed, and irritable mood have high comorbidity with addiction. They may trigger bouts of drug seeking in humans via both negative reinforcement and negative urgency. The neurocircuitry that has been identified in the "dark side" of addiction involves key neuropeptides in the central extended amygdala, including corticotropin-releasing factor. The present review article summarizes empirical and conceptual advances in the field to understand the role of the "dark side" in driving the risky and detrimental substance use that is associated with negative urgency in addiction.

Shared Neural Correlates Underlying Addictive Disorders and Negative Urgency

Negative urgency is a personality trait reflecting the tendency to act rashly in response to extreme negative emotions and is considered a transdiagnostic endophenotype for problematic levels of addictive behaviors. Recent research has begun to identify the neural correlates of negative urgency, many of which appear to overlap with neural circuitry underlying addictive disorders associated with negative urgency. The goal of this qualitative review is to summarize the extant literature concerning the neural correlates of negative urgency, to compare these correlates with those implicated with addictive disorders, and to propose new ways to begin to leverage such findings in treatment and intervention approaches. We also address current limitations in the field and make recommendations for areas for future growth in this research domain. Patterns of structure and function in the ventral striatum, frontal regions, such as the prefrontal cortex (PFC) and orbitofrontal cortex (OFC), and amygdala are common across addictive disorders and are related to both real-world risky behaviors and self-report measures of negative urgency. We propose that the time has come to move past considering this trait and these disorders as completely separate entities, and instead for the field to consider how general patterns of convergence across these disorders can lead to a more transdiagnostic approach to treatment and intervention. We suggest future work utilize these convergent patterns in the development of animal models of negative urgency, in the identification and testing of prime pharmacological and physiological interventions, and as objective biomarkers to be used when testing behavioral, pharmacological, and physiological intervention effectiveness. Little empirical work has been done to date in these areas and advances in these nascent fields would advance understanding and applications of the neuroscience of negative urgency.

Integrating Preclinical and Clinical Models of Negative Urgency

Overwhelming evidence suggests that negative urgency is robustly associated with rash, ill-advised behavior, and this trait may hamper attempts to treat patients with substance use disorder. Research applying negative urgency to clinical treatment settings has been limited, in part, due to the absence of an objective, behavioral, and translational model of negative urgency. We suggest that development of such a model will allow for determination of prime neurological and physiological treatment targets, the testing of treatment effectiveness in the preclinical and the clinical laboratory, and, ultimately, improvement in negative-urgency-related treatment response and effectiveness. In the current paper, we review the literature on measurement of negative urgency and discuss limitations of current attempts to assess this trait in human models. Then, we review the limited research on animal models of negative urgency and make suggestions for some promising models that could lead to a translational measurement model. Finally, we discuss the importance of applying objective, behavioral, and translational models of negative urgency, especially those that are easily administered in both animals and humans, to treatment development and testing and make suggestions on necessary future work in this field. Given that negative urgency is a transdiagnostic risk factor that impedes treatment success, the impact of this work could be large in reducing client suffering and societal costs.

Negative Urgency Mediates the Relation between Genetically-Influenced Serotonin Functioning and Alcohol Problems

Serotonin (5-HT) functioning is associated with alcohol problems. However, the mechanisms underlying this association remain unclear. The current study tested whether five separate dimensions of impulsivity (UPPS-P) mediated the relation between a polygenic score indexing 5-HT functioning and alcohol problems and whether any of these paths were moderated by age. Results showed that a 5-HT polygenic score predicted alcohol problems indirectly through negative urgency, but not any other facet of impulsivity. The 5-HT polygenic score also directly predicted alcohol problems. No age moderation was found. Findings suggest that negative urgency might be one important mechanism underlying the relation between genetically-influenced 5-HT functioning and alcohol problems. However, genetically-influenced 5-HT functioning likely influences alcohol problems through additional mechanisms. More broadly, results suggest that the previously observed transdiagnostic nature of 5-HT functioning on diverse types of psychopathology might be, in part, explained by its effect on negative urgency.

Individual differences in impulsive action and dopamine transporter function in rat orbitofrontal cortex

Impulsivity, which can be subdivided into impulsive action and impulsive choice, is implicated as a factor underlying drug abuse vulnerability. Although previous research has shown that dopamine (DA) systems in prefrontal cortex are involved in impulsivity and substance abuse, it is not known if inherent variation in DA transporter (DAT) function contributes to impulsivity. The current study determined if individual differences in either impulsive action or impulsive choice are related to DAT function in orbitofrontal (OFC) and/or medial prefrontal cortex (mPFC). Rats were first tested both for impulsive action in a cued go/no-go task and for impulsive choice in a delay-discounting task. Following behavioral evaluation, in vitro [(3)H]DA uptake assays were performed in OFC and mPFC isolated from individual rats. Vmax in OFC, but not mPFC, was correlated with performance in the cued go/no-go task, with decreased OFC DAT function being associated with high impulsive action. In contrast, Vmax in OFC and mPFC was not correlated with performance in the delay-discounting task. The current results demonstrate that impulsive behavior in cued go/no-go performance is associated with decreased DAT function in OFC, suggesting that hyperdopaminergic tone in this prefrontal subregion mediates, at least in part, increased impulsive action.

Role of serotonin transporter function in rat orbitofrontal cortex in impulsive choice

Impulsivity is a multi-faceted personality construct that plays a prominent role in drug abuse vulnerability. Dysregulation of 5-hydroxytryptamine (serotonin, 5-HT) systems in subregions of the prefrontal cortex has been implicated in impulsivity. Extracellular 5-HT concentrations are regulated by 5-HT transporters (SERTs), indicating that these transporters may be important molecular targets underlying individual differences in impulsivity and drug abuse vulnerability. The present study evaluated the role of SERT in mediating individual differences in impulsivity. Rats were tested for both impulsive action using the cued go/no-go task and for impulsive choice using a delay discounting task in a counterbalanced design. Following behavioral evaluation, Km and Vmax were obtained from kinetic analysis of [(3)H]5-HT uptake by SERT using synaptosomes prepared from both orbitofrontal cortex (OFC) and medial prefrontal cortex (mPFC) obtained from each individual rat. Vmax for SERT in OFC, but not mPFC, was negatively correlated with mean adjusted delay scores in the delay discounting task. In contrast, Vmax for SERT in OFC and mPFC was not correlated with performance in the cued go/no-go task. To further evaluate the relationship between SERT function and impulsive choice, a selective SERT inhibitor, fluoxetine (0, 15, 50 and 150pmol/side) was microinjected bilaterally into OFC and effects on the delay discounting task determined. Following stabilization of behavior, fluoxetine increased mean adjusted delay scores (decreased impulsivity) in high impulsive rats compared to saline microinjection, but had no effect in low impulsive rats. These ex vivo and in vivo results suggest that enhanced SERT function in OFC underlies high impulsive choice behavior.