Methylphenidate and fluphenazine, but not amphetamine, differentially affect impulsive choice in spontaneously hypertensive, Wistar-Kyoto and Sprague-Dawley rats

Effects of methylphenidate on sensitivity to reinforcement delay, magnitude, and probability: Implications for impulsive and risky choice

Under rapid-acquisition, concurrent-chains choice procedures, psychomotor stimulants typically decrease the sensitivity of responding to changes in separate dimensions of reinforcement. Across two experiments, pigeons chose between outcomes that differed in terms of reinforcement delay and magnitude (the dimensions involved in delay discounting or "impulsive" choice; Experiment 1) or reinforcement probability and magnitude (the dimensions involved in probability discounting or "risky" choice; Experiment 2). Outcomes associated with each terminal link were varied independently and pseudorandomly across sessions such that in dominated sessions one terminal link was favorable in terms of both dimensions (sooner, larger in Experiment 1 and more likely, larger in Experiment 2) and in trade-off sessions each terminal link was favorable in terms of a different dimension. Response allocation in initial links tracked changes in terminal-link outcomes in a manner that suggests each dimension contributed additively and independently to choice. Methylphenidate decreased sensitivity to all dimensions of reinforcement at a dose (or doses) that did not substantially affect bias or initial-link response rates. The degree to which methylphenidate decreased sensitivity was related to baseline sensitivity for delay and magnitude but not for probability. Baseline dependency may be a more useful approach for predicting drug effects on delay/impulsive, rather than risky, choice.

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.

Cross-species translational paradigms for assessing positive valence system as defined by the RDoC matrix

Functions associated with processing reward-related information are fundamental drivers of motivation, learning, and goal-directed behavior. Such functions have been classified as the positive valence system under the Research Domain and Criteria (RDoC) criteria and are negatively impacted across a range of psychiatric disorders and mental illnesses. The positive valence system is composed of three comprehensive categories containing related but dissociable functions that are organized into either Reward Responsiveness, Reward Learning, or Reward Valuation. The presence of overlapping behavioral dysfunction across diagnostic mental disorders is in-part what motivated the RDoC initiative, which emphasized that the study of mental illness focus on investigating relevant behavior and cognitive functions and their underlying mechanisms, rather than separating efforts on diagnostic categories (i.e., transdiagnostic). Moreover, the RDoC approach is well-suited for preclinical neuroscience research, as the rise in genetic toolboxes and associated neurotechnologies enables researchers to probe specific cellular targets with high specificity. Thus, there is an opportunity to dissect whether behaviors and cognitive functions are supported by shared or distinct neural mechanisms. For preclinical research to effectively inform our understandings of human behavior however, the cognitive and behavioral paradigms should have predictive, neurobiological, and pharmacological predictive validity to the human test. Touchscreen-based testing systems provide a further advantage for this endeavor enabling tasks to be presented to animals using the same media and task design as in humans. Here, we outline the primary categories of the positive valence system and review the work that has been done cross-species to investigate the neurobiology and neurochemistry underlying reward-related functioning. Additionally, we provide clinical tasks outlined by RDoC, along with validity and/or need for further validation for analogous rodent paradigms with a focus on implementing the touchscreen-based cognitive testing systems.

The Effects of Chronic Psychostimulant Administration on Bone Health: A Review

(1) Background: Methylphenidate (MP) and amphetamine (AMP) are psychostimulants that are widely prescribed to treat Attention Deficit Hyperactivity Disorder (ADHD) and narcolepsy. In recent years, 6.1 million children received an ADHD diagnosis, and nearly 2/3 of these children were prescribed psychostimulants for treatment. The purpose of this review is to summarize the current literature on psychostimulant use and the resulting effects on bone homeostasis, biomechanical properties, and functional integrity. (2) Methods: Literature searches were conducted from Medline/PubMed electronic databases utilizing the search terms "methylphenidate" OR "amphetamine" OR "methylphenidate" AND "bone health" AND "bone remodeling" AND "osteoclast" AND "osteoblast" AND "dopamine" from 01/1985 to 04/2023. (3) Results: Of the 550 publications found, 44 met the inclusion criteria. Data from identified studies demonstrate that the use of MP and AMP results in decreases in specific bone properties and biomechanical integrity via downstream effects on osteoblasts and osteoclast-related genes. (4) Conclusions: The chronic use of psychostimulants negatively affects bone integrity and strength as a result of increased osteoclast activity. These data support the need to take this into consideration when planning the treatment type and duration for bone fractures.

Locomotion changes in methamphetamine and amphetamine withdrawal: a systematic review

Despite extensive preclinical research over the years, a significant gap remains in our understanding of the specific effects of methamphetamine (METH) and amphetamine (AMPH) withdrawal. Understanding these differences could be pivotal to unveiling the unique pathophysiology underlying each stimulant. This may facilitate the development of targeted and effective treatment strategies tailored to the specific characteristics of each substance. Following PRISMA guidelines, this systematic review was conducted to examine alterations in spontaneous locomotor activity, specifically horizontal activity, in animals experiencing withdrawal from extended and repeated administration of AMPH or METH. Original articles were retrieved from four electronic databases, supplemented by a review of the references cited in the published papers. A total of thirty-one full-length articles (n = 31) were incorporated in the analysis. The results indicated that six studies documented a significant increase in horizontal activity among animals, seven studies reported decreased locomotion, and eighteen studies (8 AMPH; 10 METH) reported no significant alterations in the animals' locomotor activity. Studies reporting heightened locomotion mainly employed mice undergoing withdrawal from METH, studies reporting diminished locomotion predominantly involved rats undergoing withdrawal from AMPH, and studies reporting no significant changes in horizontal activity employed both rats and mice (12 rats; 6 mice). Drug characteristics, routes of administration, animal models, dosage regimens, duration, and assessment timing seem to influence the observed outcomes. Despite more than 50% of papers enlisted in this review indicate no significant changes in the locomotion during the stimulant withdrawal, the unique reactions of animals to withdrawal from METH and AMPH reported by some underscore the need for a more nuanced understanding of stimulant withdrawal.

The novel peptide LCGM-10 attenuates metabotropic glutamate receptor 5 activity and demonstrates behavioral effects in animal models

We employed a structural bioinformatics approach to develop novel peptides with predicted affinity to the binding site for negative allosteric modulators (NAMs) of metabotropic glutamate receptor 5 (mGluR5). Primary screening in zebrafish (Danio rerio) revealed a stimulatory effect of two peptides, LCGM-10 and LCGM-15. Target validation studies using calcium ion flux imaging and a luciferase reporter assay confirmed mGluR5 as the target. LCGM-10 showed greater potency than LCGM-15; it was comparable to that of the mGluR5 NAM 2-methyl-6-(phenylethynyl) pyridine (MPEP). Rodent behavioral screening in the open field and elevated plus maze revealed increased locomotor activity in both tests after acute LCGM-10 treatment, supported by further analysis of home cage spontaneous locomotor activity (SLA). The stimulating effect of a single LCGM-10 administration on SLA was evident up to 60 min after administration and was not accompanied by hypokinetic rebound observed for caffeine. According to our results, LCGM-10 has therapeutic potential to treat hypo- and dyskinesias of various etiologies. Further investigation of LCGM-10 effects in the delay discounting model of impulsive choice in rats revealed reduced trait impulsivity after single and chronic administrations, suggesting potential implication for attention deficit hyperactivity disorder, obsessive compulsive disorder, and addictions.

Timing and delay discounting in attention-deficit/hyperactivity disorder: A translational approach

Attention-deficit/hyperactivity disorder (ADHD) is a common neurodevelopmental disorder that often presents with abnormal time perception and increased impulsive choice behavior. The spontaneously hypertensive rat (SHR) is the most widely used preclinical model of the ADHD-Combined and ADHD-Hyperactive/Impulsive subtypes of the disorder. However, when testing the spontaneously hypertensive rat from Charles River (SHR/NCrl) on timing and impulsive choice tasks, the appropriate control strain is not clear, and it is possible that one of the possible control strains, the Wistar Kyoto from Charles River (WKY/NCrl), is an appropriate model for ADHD-Predominately Inattentive. Our goals were to test the SHR/NCrl, WKY/NCrl, and Wistar (WI; the progenitor strain for the SHR/NCrl and WKY/NCrl) strains on time perception and impulsive choice tasks to assess the validity of SHR/NCrl and WKY/NCrl as models of ADHD, and the validity of the WI strain as a control. We also sought to assess impulsive choice behavior in humans diagnosed with the three subtypes of ADHD and compare them with our findings from the preclinical models. We found SHR/NCrl rats timed faster and were more impulsive than WKY/NCrl and WI rats, and human participants diagnosed with ADHD were more impulsive compared to controls, but there were no differences between the three ADHD subtypes.

Effects of methylphenidate on impulsive choice and delay aversion in Lewis rats

Attention-deficit/hyperactivity disorder (ADHD), a common behavioral disorder in children and young adults, is characterized by symptoms of impulsivity, inattention, and hyperactivity. The purpose of this study was to evaluate the Lewis rat strain as a model of ADHD by testing their impulsive choices. Lewis rats were compared to their source strain, the Wistar rat, on an impulsive choice task. Rats completed the tasks on and off methylphenidate, a commonly prescribed medication for ADHD. Off methylphenidate, Lewis rats made more impulsive choices than Wistar rats. Analyses of acquisition of choice behavior suggested that both strains were able to discriminate reward sizes, but Lewis rats still chose the smaller-sooner option more than the larger-later (LL) option when the delays to reward were the same. This may be due to an aversion to the LL lever, which was associated with the longest delays to reward. Higher doses of methylphenidate increased LL choices in Lewis rats but decreased LL choices in Wistar rats. Altogether, these results suggest Lewis rats may be a viable model for ADHD in individuals whose symptoms are characterized by impulsive choices.

Previous experience with delays affects delay discounting in animal model of ADHD

BACKGROUND

ADHD is a disorder where a common symptom is impulsive behaviour, a broad term associated with making sub-optimal choices. One frequently used method to investigate impulsive behaviour is delay discounting, which involves choosing between a small, immediate reinforcer and a delayed, larger one. Choosing the small immediate reinforcer is by itself, however, not sufficient for terming the choice impulsive, as all organisms eventually switch to choosing the small, immediate reinforcer when the delay to the larger reinforcer becomes long. This switch can be termed impulsive only when it occurs more frequently, or at shorter LL delays, than typically observed in normal controls. A poorly understood aspect is how choice is influenced by previous experience with delays. Using an animal model of Attention-Deficit/Hyperactivity Disorder, the Spontaneously Hypertensive Rat, we manipulated the order of exposure to delays in a delay discounting task. Following a preference test, the Ascending group experienced gradually increasing delays between choice and reinforcer while the Descending group were exposed to these delays in reverse order.

RESULTS

The results showed that the Descending group chose the small, immediate reinforcer over the larger delayed to a much larger extent than the Ascending group, and continued to do so even when the delay component was ultimately removed. Strain effects were found in the Ascending group, with SHRs switching to the small, immediate reinforcer earlier than controls as the delay to the larger reinforcer increased.

CONCLUSION

The data suggests that delay discounting is affected by history of exposure to delayed consequences. When reinforcement contingencies are incrementally changed from having no response-reinforcer delay to a long delay, discounting of delayed consequences is gradual. However, a sudden change from no delay to a long delay, without intermediate training, results in a rapid switch to the small, immediate reinforcer option, and this behaviour is somewhat resilient to the shortening and eventual removal of the large reinforcer delay. The implication is that attempting to reduce already existing impulsive behaviour in children with ADHD will require gradual habituation and not sudden changes in reinforcement contingencies.

Spontaneously Hypertensive Rat substrains show differences in model traits for addiction risk and cocaine self-administration: Implications for a novel rat reduced complexity cross

Forward genetic mapping of F2 crosses between closely related substrains of inbred rodents - referred to as a reduced complexity cross (RCC) - is a relatively new strategy for accelerating the pace of gene discovery for complex traits, such as drug addiction. RCCs to date were generated in mice, but rats are thought to be optimal for addiction genetic studies. Based on past literature, one inbred Spontaneously Hypertensive Rat substrain, SHR/NCrl, is predicted to exhibit a distinct behavioral profile as it relates to cocaine self-administration traits relative to another substrain, SHR/NHsd. Direct substrain comparisons are a necessary first step before implementing an RCC. We evaluated model traits for cocaine addiction risk and cocaine self-administration behaviors using a longitudinal within-subjects design. Impulsive-like and compulsive-like traits were greater in SHR/NCrl than SHR/NHsd, as were reactivity to sucrose reward, sensitivity to acute psychostimulant effects of cocaine, and cocaine use studied under fixed-ratio and tandem schedules of cocaine self-administration. Compulsive-like behavior correlated with the acute psychostimulant effects of cocaine, which in turn correlated with cocaine taking under the tandem schedule. Compulsive-like behavior also was the best predictor of cocaine seeking responses. Heritability estimates indicated that 22 %-40 % of the variances for the above phenotypes can be explained by additive genetic factors, providing sufficient genetic variance to conduct genetic mapping in F2 crosses of SHR/NCrl and SHR/NHsd. These results provide compelling support for using an RCC approach in SHR substrains to uncover candidate genes and variants that are of relevance to cocaine use disorders.

The irrelevancy of the inter-trial interval in delay-discounting experiments on an animal model of ADHD

Delay discounting involves choosing between a small, immediate reward, and a larger but delayed one. As the delay between choice and large reward gets longer, people with ADHD tend to become impulsive faster than controls, indicated by a switch in preference from the large to the smaller reward. Choosing the smaller reward when the larger is considered reward maximizing is labeled impulsive behaviour. It is well documented that increased delays between choice and reward affects choice preference in both humans and other animals. Other variables such as the inter-trial interval or trial length are observed to have an effect on human discounting, but their effect on discounting in other animals is largely assumed rather than tested. In the current experiment, we tested this assumption. One group of rats was exposed to increasing delays between choosing the large reward and receiving it, while another group experienced longer inter-trial intervals that were equal in length to the delays in the other group. This ensured that trial length was controlled for in delay discounting, but that the delay function and inter-trial intervals could be manipulated and measured separately. Results showed that while the delay between choice and reward caused impulsive behaviour in rats, the length of the inter-trial interval (and by extension trial length) had no impact on choice behaviour. A follow-up experiment found this to be the case even if the length of the inter-trial interval was signaled with audio cues. These results suggest that rats, and possibly animals in general, are insensitive to time between trials, and therefore cannot easily represent human counterparts on the task.

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).

Effect of Schedule-Induced Behavior on Responses of Spontaneously Hypertensive and Wistar-Kyoto Rats in a Delay-Discounting Task: A Preliminary Report

Delay discounting is the loss of the subjective value of an outcome as the time to its delivery increases. It has been suggested that organisms can become more tolerant of this delay when engaging in schedule-induced behaviors. Schedule-induced behaviors are those that develop at a high rate during intermittent reinforcement schedules without the need of arranged contingency to the reinforcer, and they have been considered as a model of compulsivity. There is evidence that relates compulsivity to greater delay discounting. The rate of delay discounting represents how impulsive the subject is, as the rate of discounting increases the higher the impulsivity. Thus, the main purpose of this study was to undertake a preliminary evaluation of whether developing schedule-induced behaviors affects performance in a delay-discounting task, by comparing spontaneously hypertensive rats (SHRs) and Wistar-Kyoto (WKY) rats. The rats were exposed to a task that consisted of presenting the subjects with two levers: one produced a small, immediate food reinforcer while the other one produced a larger, delayed reinforcer. During Condition A, the levers were presented, and a water bottle and a running wheel were available in the conditioning chambers; during Condition B, only the levers were presented. SHR and WKY rats developed schedule-induced behaviors during Condition A and showed no difference in discounting rates, contradicting previous reports. Lick allocation during response-reinforcer delays and the inter-trial interval (ITI) showed, respectively, pre- and post-food distributions. Discounting rates during Condition B (when rats could not engage in schedule-induced behaviors) did not reach statistical significance difference among strains of animals, although it was observed a tendency for WKY to behave more self-controlled. Likewise it was not found any effect of schedule-induced behavior on discounting rates, however, a tendency for WKY rats to behave more impulsive during access to drink and run seems to tentatively support the idea of schedule-induced behavior as a model of compulsivity in those rats, being impulsivity simply defined as an excess in behavior.

Effects of d-amphetamine and MK-801 on impulsive choice: Modulation by schedule of reinforcement and delay length

Procedural modifications can modulate drug effects in delay discounting, such as signaling the delay to reinforcement and altering the order in which delays are presented. Although the schedule of reinforcement can alter the rate at which animals discount a reinforcer, research has not determined if animals trained on different schedules of reinforcement are differentially affected by pharmacological manipulations. Similarly, research has not determined if using different delays to reinforcement can modulate drug effects in delay discounting. Male Sprague Dawley rats (n = 36) were split into four groups and were trained in a delay-discounting procedure. The schedule of reinforcement (fixed ratio [FR] 1 vs. FR 10) and delays to reinforcement (0, 5, 10, 20, and 50 s vs. 0, 10, 30, 60, 100 s) were manipulated for each group. Following behavioral training, rats were treated with d-amphetamine (0, 0.25, 0.5, and 1.0 mg/kg) and MK-801 (0, 0.03, and 0.06 mg/kg). Results showed that amphetamine decreased impulsive choice when a FR 1 schedule was used, but only when the short delay sequence was used. Conversely, amphetamine decreased impulsive choice when a FR 10 schedule was used, but only when rats were trained on the long delay sequence. MK-801 decreased impulsive choice in rats trained on a FR 1 schedule, regardless of delay sequence, but did not alter choice in rats trained on a FR 10 schedule. These results show that schedule of reinforcement and delay length can modulate drug effects in delay discounting.

Semi-Automated Biomarker Discovery from Pharmacodynamic Effects on EEG in ADHD Rodent Models

We propose a novel semi-automatic approach to design biomarkers for capturing pharmacodynamic effects induced by pharmacological agents on the spectral power of electroencephalography (EEG) recordings. We apply this methodology to investigate the pharmacodynamic effects of methylphenidate (MPH) and atomoxetine (ATX) on attention deficit/hyperactivity disorder (ADHD), using rodent models. We inject the two agents into the spontaneously hypertensive rat (SHR) model of ADHD, the Wistar-Kyoto rat (WKY), and the Wistar rat (WIS), and record their EEG patterns. To assess individual EEG patterns quantitatively, we use an integrated methodological approach, which consists of calculating the mean, slope and intercept parameters of temporal records of EEG spectral power using a smoothing filter, outlier truncation, and linear regression. We apply Fisher discriminant analysis (FDA) to identify dominant discriminants to be heuristically consolidated into several new composite biomarkers. Results of the analysis of variance (ANOVA) and t-test show benefits in pharmacodynamic parameters, especially the slope parameter. Composite biomarker evaluation confirms their validity for genetic model stratification and the effects of the pharmacological agents used. The methodology proposed is of generic use as an approach to investigating thoroughly the dynamics of the EEG spectral power.

Dissecting drug effects in preclinical models of impulsive choice: emphasis on glutamatergic compounds

RATIONALE

Impulsive choice is often measured with delay discounting paradigms. Because there are multiple discounting procedures, as well as different statistical analyses that can be applied to data generated from these paradigms, there are some inconsistencies in the literature regarding drug effects on impulsive choice.

OBJECTIVES

The goal of the current paper is to review the methodological and analytic approaches used to measure discounting and to discuss how these differences can account for differential drug effects observed across studies.

RESULTS

Because some procedures/analyses use a single data point as the dependent variable, changes in this value following pharmacological treatment may be interpreted as alterations in sensitivity to delayed reinforcement, but when other procedures/analyses are used, no changes in behavior are observed. Even when multiple data points are included, some studies show that the statistical analysis (e.g., ANOVA on raw proportion of responses vs. using hyperbolic/exponential functions) can lead to different interpretations. Finally, procedural differences (e.g., delay presentation order, signaling the delay to reinforcement, etc.) in the same discounting paradigm can alter how drugs affect sensitivity to delayed reinforcement.

CONCLUSIONS

Future studies should utilize paradigms that allow one to observe alterations in responding at each delay (e.g., concurrent-chains schedules). Concerning statistical analyses, using parameter estimates derived from nonlinear functions or incorporating the generalized matching law can allow one to determine if drugs affect sensitivity to delayed reinforcement or impair discrimination of the large and small magnitude reinforcers. Using these approaches can help further our understanding of the neurochemical underpinnings of delay discounting.

Pharmacotherapies for decreasing maladaptive choice in drug addiction: Targeting the behavior and the drug

Drug addiction can be conceptualized as a disorder of maladaptive decision making in which drugs are chosen at the expense of pro-social, nondrug alternatives. The study of decision making in drug addiction has focused largely on the role of impulsivity as a facilitator of addiction, in particular the tendency for drug abusers to choose small, immediate gains over larger but delayed outcomes (i.e., delay discounting). A parallel line of work, also focused on decision making in drug addiction, has focused on identifying the determinants underlying the choice to take drugs over nondrug alternatives (i.e., drug vs. nondrug choice). Both tracks of research have been valuable tools in the development of pharmacotherapies for treating maladaptive decision making in drug addiction, and a number of common drugs have been studied in both designs. However, we have observed that there is little uniformity in the administration regimens of potential treatments between the designs, which hinders congruence in the development of single treatment strategies to reduce both impulsive behavior and drug choice. The current review provides an overview of the drugs that have been tested in both delay-discounting and drug-choice designs, and focuses on drugs that reduced the maladaptive choice in both designs. Suggestions to enhance congruence between the findings in future studies are provided. Finally, we propose the use of a hybridized, experimental approach that may enable researchers to test the effectiveness of therapeutics at decreasing impulsive and drug choice in a single design.

Methylphenidate and Atomoxetine-Responsive Prefrontal Cortical Genetic Overlaps in "Impulsive" SHR/NCrl and Wistar Rats

Impulsivity, the predisposition to act prematurely without foresight, is associated with a number of neuropsychiatric disorders, including attention-deficit/hyperactivity disorder (ADHD). Identifying genetic underpinnings of impulsive behavior may help decipher the complex etiology and neurobiological factors of disorders marked by impulsivity. To identify potential genetic factors of impulsivity, we examined common differentially expressed genes (DEGs) in the prefrontal cortex (PFC) of adolescent SHR/NCrl and Wistar rats, which showed marked decrease in preference for the large but delayed reward, compared with WKY/NCrl rats, in the delay discounting task. Of these DEGs, we examined drug-responsive transcripts whose mRNA levels were altered following treatment (in SHR/NCrl and Wistar rats) with drugs that alleviate impulsivity, namely, the ADHD medications methylphenidate and atomoxetine. Prefrontal cortical genetic overlaps between SHR/NCrl and Wistar rats in comparison with WKY/NCrl included genes associated with transcription (e.g., Btg2, Fos, Nr4a2), synaptic plasticity (e.g., Arc, Homer2), and neuron apoptosis (Grik2, Nmnat1). Treatment with methylphenidate and/or atomoxetine increased choice of the large, delayed reward in SHR/NCrl and Wistar rats and changed, in varying degrees, mRNA levels of Nr4a2, Btg2, and Homer2, genes with previously described roles in neuropsychiatric disorders characterized by impulsivity. While further studies are required, we dissected potential genetic factors that may influence impulsivity by identifying genetic overlaps in the PFC of "impulsive" SHR/NCrl and Wistar rats. Notably, these are also drug-responsive transcripts which may be studied further as biomarkers to predict response to ADHD drugs, and as potential targets for the development of treatments to improve impulsivity.

Effects of N-methyl-D-aspartate receptor ligands on sensitivity to reinforcer magnitude and delayed reinforcement in a delay-discounting procedure

RATIONALE

The N-methyl-D-aspartate (NMDA) receptor has been recently identified as an important mediator of impulsive choice, as assessed in delay discounting. Although discounting is independently influenced by sensitivity to reinforcer magnitude and delayed reinforcement, few studies have examined how NMDA receptor ligands differentially affect these parameters.

OBJECTIVES

The current study examined the effects of various NMDA receptor ligands on sensitivity to reinforcer magnitude and delayed reinforcement in a delay-discounting procedure.

METHODS

Following behavioral training, rats received treatments of the following NMDA receptor ligands: the uncompetitive antagonists ketamine (0, 1.0, 5.0, or 10.0 mg/kg; i.p.), MK-801 (0, 0.003, 0.01, or 0.03 mg/kg; s.c.), and memantine (0, 2.5, 5.0, or 10.0 mg/kg; i.p.), the competitive antagonist CGS 19755 (0, 5.0, 10.0, or 20.0 mg/kg; s.c.), the non-competitive NR2B subunit-selective antagonist ifenprodil (0, 1.0, 3.0, or 10.0 mg/kg; i.p), and the partial agonist D-cycloserine (0, 3.25, 15.0, or 30.0 mg/kg; s.c.).

RESULTS

When an exponential model was used to describe discounting, CGS 19755 (5.0 mg/kg) increased impulsive choice without altering sensitivity to reinforcer magnitude. Conversely, ketamine (10.0 mg/kg), memantine (5.0 mg/kg), and ifenprodil (10.0 mg/kg) decreased sensitivity to reinforcer magnitude without altering impulsive choice. MK-801 and D-cycloserine did not alter delay-discounting performance, although two-way ANOVA analyses indicated D-cycloserine (15.0 mg/kg) decreased impulsive choice.

CONCLUSIONS

The behavioral changes observed in delay discounting following administration of NMDA receptor antagonists do not always reflect an alteration in impulsive choice. These results emphasize the utility in employing quantitative methods to assess drug effects in delay discounting.

Effects of Group I metabotropic glutamate receptor antagonists on sensitivity to reinforcer magnitude and delayed reinforcement in a delay-discounting task in rats: Contribution of delay presentation order

Metabotropic glutamate receptor 1 (mGluR₁) blockade has been shown to decrease impulsive choice, as measured in delay discounting. However, several variables are known to influence an animal's discounting, including sensitivity to delayed reinforcement and sensitivity to reinforcer magnitude. The goal of this experiment was to determine the effects of mGluR₁, as well as mGluR₅, antagonism on these parameters. Forty Sprague Dawley rats were trained in delay discounting, in which consistently choosing a small, immediate reward reflects impulsive choice. For half of the rats, the delay to the large reinforcer increased across blocks of trials, whereas the delay decreased across the session for half of the rats. Following training, half of the rats received injections of the mGluR₁ antagonist JNJ 16259685 (JNJ; 0, 0.1, 0.3, or 1.0mg/kg; i.p), and half received injections of the mGluR₅ antagonist MPEP (0, 1.0, 3.0, or 10.0mg/kg; i.p.). Administration of JNJ increased sensitivity to delayed reinforcement (i.e., promoted impulsive choice), regardless of which schedule was used. However, the order in which delays were presented modulated the effects of JNJ on sensitivity to reinforcer magnitude. Specifically, JNJ decreased sensitivity to reinforcer magnitude in rats trained on the descending schedule only. MPEP did not alter sensitivity to reinforcer magnitude or sensitivity to delayed reinforcement. These results show that mGluR₁ is an important mediator of impulsive choice, and they provide further evidence that delay order presentation is an important variable that influences drug effects in delay discounting.

Does impulsivity change rate dependently following stimulant administration? A translational selective review and re-analysis

RATIONALE

Rate dependence refers to an orderly relationship between a baseline measure of behavior and the change in that behavior following an intervention. The most frequently observed rate-dependent effect is an inverse relationship between the baseline rate of behavior and response rates following an intervention. A previous report of rate dependence in delay discounting suggests that the discounting of delayed reinforcers, and perhaps, other impulsivity measures, may change rate dependently following acute and chronic administration of potentially therapeutic medications in both preclinical and clinical studies.

OBJECTIVE

The aim of the current paper was to review the effects of stimulants on delay discounting and other impulsivity tasks.

METHODS

All studies identified from the literature were required to include (1) an objective measure of impulsivity; (2) administration of amphetamine, methylphenidate, or modafinil; (3) presentation of a pre- and postdrug administration impulsivity measure; and (4) the report of individual drug effects or results in groups split by baseline or vehicle impulsivity. Twenty-five research reports were then reanalyzed for evidence consistent with rate dependence.

RESULTS

Of the total possible instances, 67 % produced results consistent with rate dependence. Specifically, 72, 45, and 80 % of the data sets were consistent with rate dependence following amphetamine, methylphenidate, and modafinil administration, respectively.

CONCLUSIONS

These results suggest that rate dependence is a more robust phenomenon than reported in the literature. Impulsivity studies should consider this quantitative signature as a process to determine the effects of variables and as a potential prognostic tool to evaluate the effectiveness of future interventions.

D-amphetamine improves attention performance in adolescent Wistar, but not in SHR rats, in a two-choice visual discrimination task

The validity of spontaneous hypertensive rat (SHR) as a model of attention deficit hyperactivity disorder (ADHD) has been explored by comparing SHR with Wistar rats in a test of attention, the two-choice visual discrimination task (2-CVDT). Animals were 4-5 weeks old during the training phase of the experiment and 6-7 weeks old during the testing phase in which they were tested with D-amphetamine, a stimulant drug used for the treatment of ADHD. As compared to Wistar, SHR showed a slightly better attention performance, a slightly lower impulsivity level, and a lower general activity during the training phase, but these differences disappeared or lessened thereafter, during the testing phase. D-amphetamine (0.5, 1 mg/kg) improved attention performance in Wistar, but not in SHR, and did not modify impulsivity and activity in the two strains. In conclusion, the present study did not demonstrate that SHR represents a valid model of ADHD, since it did not show face validity regarding the behavioral symptoms of ADHD and predictive validity regarding the effect of a compound used for the treatment of ADHD. On the other hand, this study showed that the 2-CVDT may represent a suitable tool for evaluating in adolescent Wistar rats the effect on attention of compounds intended for the treatment of ADHD.

Impulsivity and sensitivity to amount and delay of reinforcement in an animal model of ADHD

Previous research has been inconclusive about the degree of impulsivity displayed by spontaneously hypertensive rats (SHR), an animal model of Attention Deficit Hyperactivity Disorder (ADHD). In the present set of experiments, concurrent-chains schedules were employed in order to explore SHR's impulsivity, sensitivity to delay, and sensitivity to amount of reinforcement; Wistar rats (WIS) were used as comparison group. In the three experiments - performed with different subjects - non-independent variable interval 30s schedules were presented in the initial links; the difference between experiments was in the terminal links. For exploring impulsivity, one of the terminal links (SS) was associated to a short delay (2s) and a small reinforcer (1 pellet), whereas the other terminal link (LL) was associated to a longer delay (28s) and a larger reinforcer (4 pellets). The results indicated a remarkably higher impulsivity in SHR. Because this impulsivity may have as potential mechanisms an increased sensitivity to delay and/or a decreased sensitivity to the amount of reinforcement, in experiments 2 and 3 these possibilities were examined. For assessing sensitivity to delay, the following pairs of fixed interval (FI) schedules were used in the terminal links in five conditions: 2-28, 6-24, 15-15, 24-6, 28-2s; the magnitude of reinforcement was 1 pellet in all conditions for both alternatives. For assessing sensitivity to amount, in five conditions the alternatives were associated with different magnitudes of reinforcement: 1-5 pellets, 2-4, 3-3, 4-2 and 5-1 in left-right alternatives, respectively; the delay to reinforcement was controlled by a FI 15s in all conditions and for both alternatives. The sensitivity to delay and the sensitivity to amount were calculated according to the Generalized Matching Law. The results indicated a higher sensitivity to delay in SHR, and the same sensitivity to amount in SHR and WIS rats. These results suggest that the increased sensitivity to delay influences the high level of impulsivity observed in SHR.

Recent Translational Findings on Impulsivity in Relation to Drug Abuse

Impulsive behavior is strongly implicated in drug abuse, as both a cause and a consequence of drug use. To understand how impulsive behaviors lead to and result from drug use, translational evidence from both human and non-human animal studies is needed. Here, we review recent (2009 or later) studies that have investigated two major components of impulsive behavior, inhibitory control and impulsive choice, across preclinical and clinical studies. We concentrate on the stop-signal task as the measure of inhibitory control and delay discounting as the measure of impulsive choice. Consistent with previous reports, recent studies show greater impulsive behavior in drug users compared with non-users. Additionally, new evidence supports the prospective role of impulsive behavior in drug abuse, and has begun to identify the neurobiological mechanisms underlying impulsive behavior. We focus on the commonalities and differences in findings between preclinical and clinical studies, and suggest future directions for translational research.

Convergent pharmacological mechanisms in impulsivity and addiction: insights from rodent models

Research over the last two decades has widely demonstrated that impulsivity, in its various forms, is antecedent to the development of drug addiction and an important behavioural trait underlying the inability of addicts to refrain from continued drug use. Impulsivity describes a variety of rapidly and prematurely expressed behaviours that span several domains from impaired response inhibition to an intolerance of delayed rewards, and is a core symptom of attention deficit hyperactivity disorder (ADHD) and other brain disorders. Various theories have been advanced to explain how impulsivity interacts with addiction both causally and as a consequence of chronic drug abuse; these acknowledge the strong overlaps in neural circuitry and mechanisms between impulsivity and addiction and the seemingly paradoxical treatment of ADHD with stimulant drugs with high abuse potential. Recent years have witnessed unprecedented progress in the elucidation of pharmacological mechanisms underpinning impulsivity. Collectively, this work has significantly improved the prospect for new therapies in ADHD as well as our understanding of the neural mechanisms underlying the shift from recreational drug use to addiction. In this review, we consider the extent to which pharmacological interventions that target impulsive behaviour are also effective in animal models of addiction. We highlight several promising examples of convergence based on empirical findings in rodent-based studies.

Cocaine-seeking behavior in a genetic model of attention-deficit/hyperactivity disorder following adolescent methylphenidate or atomoxetine treatments

BACKGROUND

Attention-deficit/hyperactivity disorder (ADHD) is often comorbid with cocaine abuse. Controversy exists regarding long-term consequences of ADHD medications on cocaine abuse liability. Whereas childhood methylphenidate treatment may be preventative, methylphenidate in teens appears to further increase later cocaine abuse risk. In rodents, adolescent methylphenidate treatment further increases adult cocaine self-administration in the Spontaneously Hypertensive Rat (SHR) model of ADHD, whereas adolescent atomoxetine treatment does not. Effects of ADHD medications on cocaine cue reactivity, a critical component of addiction, are unknown.

METHODS

To investigate this, SHR, Wistar-Kyoto (inbred control) and Wistar (outbred control) rats received therapeutically relevant doses of methylphenidate (1.5 mg/kg, oral) and atomoxetine (0.3 mg/kg, intraperitoneal), or respective vehicles from post-natal day 28-55. Cocaine seeking, reflecting cue reactivity, was measured in adulthood during self-administration maintenance and cue-induced reinstatement tests conducted under a second-order schedule.

RESULTS

Compared to control strains, SHR earned more cocaine infusions, emitted more cocaine-seeking responses during maintenance and reinstatement testing, and required more sessions to reach the extinction criterion. Compared to vehicle, adolescent methylphenidate, but not atomoxetine, further increased cocaine intake during maintenance testing in SHR. Adolescent atomoxetine, but not methylphenidate, decreased cocaine seeking during reinstatement testing in SHR. Neither medication had effects on cocaine intake or cue reactivity in control strains.

CONCLUSIONS

The SHR successfully model ADHD and cocaine abuse comorbidity and show differential effects of adolescent ADHD medications on cocaine intake and cue reactivity during adulthood. Thus, SHR have heuristic value for assessing neurobiology underlying the ADHD phenotype and for evaluating pharmacotherapeutics for ADHD.

Using the five-choice serial reaction time task to examine the effects of atomoxetine and methylphenidate in the male spontaneously hypertensive rat

Attention deficit hyperactivity disorder (ADHD) is the most common neurodevelopmental disorder and is normally treated with either stimulant or non-stimulant medication such as methylphenidate and atomoxetine respectively. The impact of these drugs on attention and impulsivity has been explored extensively in healthy animals but there is little research into their effects in an animal model of ADHD. In the present study we investigated the effects of both drugs on the spontaneously hypertensive rat (SHR) model of ADHD using the five-choice serial reaction time task (5CSRTT). We found a number of difficulties associated with training this vulnerable strain on such a complex task. However, where rats were able to learn the task we found very small effects of methylphenidate; increased incorrect responding and therefore decreased accuracy, a marker of attention at a single dose. There were no significant effects of atomoxetine on accuracy once multiple comparisons were taken into consideration. We found no effects of either drug on premature responding, a marker of impulsivity. These results indicate that the 5CSRTT may not be most sensitive to the impulsivity displayed in the SHR. Furthermore, they suggest that the SHR may lack predictive validity and further investigation is needed to optimise use of this model.

Different relations between schedule-induced polydipsia and impulsive behaviour in the Spontaneously Hypertensive Rat and in high impulsive Wistar rats: questioning the role of impulsivity in adjunctive behaviour

Rats belonging to three different strains (15 Wistar, 8 Spontaneously Hypertensive - SHR- and 8 Wistar Kyoto - WKY-) were used to evaluate the possible relationship between different levels of impulsivity and development of schedule-induced polydipsia (SIP). We first measured the rats' levels of impulsivity by means of delay-discounting and indifference-point procedures. Secondly, development of SIP was studied under a series of fixed time 15, 30, 60 and 120s food schedules, which were counterbalanced by means of a Latin-square design. Finally, we re-assessed the rats' levels of impulsivity by replicating the delay-discounting test. The findings showed that, starting from equivalent levels of impulsivity, development of SIP differed among the groups of rats. In comparison with the rest of the animals, the SHRs were observed to attain elevated drinking rates under SIP. On the other hand, the Wistar rats which had initial high impulsivity levels similar to those of the SHRs, displayed the lowest rates of induced drinking. Moreover, low levels of impulsivity in Wistar rats prior to SIP acquisition were reflected into high drinking rates. Relation of SIP and impulsivity is questioned by present results, which gives ground to the understanding of the behavioural mechanisms involved in adjunctive behaviour and its usefulness as an animal model of excessive behaviour.

Dissociations between interval timing and intertemporal choice following administration of fluoxetine, cocaine, or methamphetamine

The goal of our study was to characterize the relationship between intertemporal choice and interval timing, including determining how drugs that modulate brain serotonin and dopamine levels influence these two processes. In Experiment 1, rats were tested on a standard 40-s peak-interval procedure following administration of fluoxetine (3, 5, or 8 mg/kg) or vehicle to assess basic effects on interval timing. In Experiment 2, rats were tested in a novel behavioral paradigm intended to simultaneously examine interval timing and impulsivity. Rats performed a variant of the bi-peak procedure using 10-s and 40-s target durations with an additional "defection" lever that provided the possibility of a small, immediate reward. Timing functions remained relatively intact, and 'patience' across subjects correlated with peak times, indicating a negative relationship between 'patience' and clock speed. We next examined the effects of fluoxetine (5 mg/kg), cocaine (15 mg/kg), or methamphetamine (1 mg/kg) on task performance. Fluoxetine reduced impulsivity as measured by defection time without corresponding changes in clock speed. In contrast, cocaine and methamphetamine both increased impulsivity and clock speed. Thus, variations in timing may mediate intertemporal choice via dopaminergic inputs. However, a separate, serotonergic system can affect intertemporal choice without affecting interval timing directly. This article is part of a Special Issue entitled: Associative and Temporal Learning.

Methylphenidate and impulsivity: a comparison of effects of methylphenidate enantiomers on delay discounting in rats

RATIONALE

Current formulations of methylphenidate (MPH) used in treatment of attention-deficit/hyperactivity disorder (ADHD) result in significantly different bioavailability of MPH enantiomers. Daytrana®, a dl-MPH transdermal patch system, produces higher levels of l-MPH than when dl-MPH is administered orally (e.g., Ritalin®). One potential limitation of increased l-MPH was indicated in a preclinical study showing l-MPH may attenuate effects of d-MPH.

OBJECTIVES

The objective of the study was to investigate the interactive effects of MPH enantiomers by (1) assessing drug effects via a preclinical model of "impulsivity" and (2) performing a quantitative dose equivalence analysis of MPH enantiomer interactions.

METHODS

Sprague-Dawley rats were trained to emit either of two responses, one producing an immediate food pellet, the other producing four pellets delivered at increasing delays (0, 8, and 32 s). The percent selection of the larger food amount was graphed as a function of delay with the area under the curve (AUC) assessed. Increases in AUC are consistent with decreases in "impulsivity" (i.e., selection of the smaller, immediate over the larger, delayed reinforcer).

RESULTS

Systemic administration of dl-MPH and d-MPH dose-dependently increased AUC, while l-MPH, morphine, and pentobarbital did not alter AUC. An analysis based upon dose equivalence indicated that dl-MPH produced additive effects that were not different from that predicted from effects of the enantiomers administered alone.

CONCLUSIONS

The present results indicate pharmacologically selective effects in that only drugs prescribed for the treatment of ADHD symptoms decreased a measure of "impulsivity" and that l-MPH likely does not attenuate or enhance the effects of d-MPH in the current delay-discounting task.

Impulsive choice behavior in four strains of rats: evaluation of possible models of Attention-Deficit/Hyperactivity Disorder

Several studies have examined impulsive choice behavior in spontaneously hypertensive rats (SHRs) as a possible pre-clinical model for Attention-Deficit/Hyperactivity Disorder (ADHD). However, this strain was not specifically selected for the traits of ADHD and as a result their appropriateness as a model has been questioned. The present study investigated whether SHRs would exhibit impulsive behavior in comparison to their control strain, Wistar Kyoto (WKY) rats. In addition, we evaluated a strain that has previously shown high levels of impulsive choice, the Lewis (LEW) rats and compared them with their source strain, Wistar (WIS) rats. In the first phase, rats could choose between a smaller-sooner (SS) reward of 1 pellet after 10 s and a larger-later (LL) reward of 2 pellets after 30 s. Subsequently, the rats were exposed to increases in LL reward magnitude and SS delay. These manipulations were designed to assess sensitivity to magnitude and delay within the choice task to parse out possible differences in using the strains as models of specific deficits associated with ADHD. The SHR and WKY strains did not differ in their choice behavior under either delay or magnitude manipulations. In comparison to WIS, LEW showed deficits in choice behavior in the delay manipulation, and to a lesser extent in the magnitude manipulation. An examination of individual differences indicated that the SHR strain may not be sufficiently homogeneous in their impulsive choice behavior to be considered as a viable model for impulse control disorders such as ADHD. The LEW strain may be worthy of further consideration for their suitability as an animal model.

Performance on a strategy set shifting task during adolescence in a genetic model of attention deficit/hyperactivity disorder: methylphenidate vs. atomoxetine treatments

Research examining medication effects on set shifting in teens with attention deficit/hyperactivity disorder (ADHD) is lacking. An animal model of ADHD may be useful for exploring this gap. The spontaneously hypertensive rat (SHR) is a commonly used animal model of ADHD. SHR and two comparator strains, Wistar-Kyoto (WKY) and Wistar (WIS), were evaluated during adolescence in a strategy set shifting task under conditions of a 0s or 15s delay to reinforcer delivery. The task had three phases: initial discrimination, set shift and reversal learning. Under 0s delays, SHR performed as well as or better than WKY and WIS. Treatment with 0.3mg/kg/day atomoxetine had little effect, other than to modestly increase trials to criterion during set shifting in all strains. Under 15s delays, SHR had longer lever press reaction times, longer latencies to criterion and more trial omissions than WKY during set shifting and reversal learning. These deficits were not reduced systematically by 1.5mg/kg/day methylphenidate or 0.3mg/kg/day atomoxetine. Regarding learning in SHR, methylphenidate improved initial discrimination, whereas atomoxetine improved set shifting but disrupted initial discrimination. During reversal learning, both drugs were ineffective in SHR, and atomoxetine made reaction time and trial omissions greater in WKY. Overall, WIS performance differed from SHR or WKY, depending on phase. Collectively, a genetic model of ADHD in adolescent rats revealed that neither methylphenidate nor atomoxetine mitigated all deficits in SHR during the set shifting task. Thus, methylphenidate or atomoxetine monotherapy may not mitigate all set shift task-related deficits in teens with ADHD.

Highly impulsive rats: modelling an endophenotype to determine the neurobiological, genetic and environmental mechanisms of addiction

Impulsivity describes the tendency of an individual to act prematurely without foresight and is associated with a number of neuropsychiatric co-morbidities, including drug addiction. As such, there is increasing interest in the neurobiological mechanisms of impulsivity, as well as the genetic and environmental influences that govern the expression of this behaviour. Tests used on rodent models of impulsivity share strong parallels with tasks used to assess this trait in humans, and studies in both suggest a crucial role of monoaminergic corticostriatal systems in the expression of this behavioural trait. Furthermore, rodent models have enabled investigation of the causal relationship between drug abuse and impulsivity. Here, we review the use of rodent models of impulsivity for investigating the mechanisms involved in this trait, and how these mechanisms could contribute to the pathogenesis of addiction.

Consideration of species differences in developing novel molecules as cognition enhancers

The NIH-funded CNTRICS initiative has coordinated efforts to promote the vertical translation of novel procognitive molecules from testing in mice, rats and non-human primates, to clinical efficacy in patients with schizophrenia. CNTRICS highlighted improving construct validation of tasks across species to increase the likelihood that the translation of a candidate molecule to humans will be successful. Other aspects of cross-species behaviors remain important however. This review describes cognitive tasks utilized across species, providing examples of differences and similarities of innate behavior between species, as well as convergent construct and predictive validity. Tests of attention, olfactory discrimination, reversal learning, and paired associate learning are discussed. Moreover, information on the practical implication of species differences in drug development research is also provided. The issues covered here will aid in task development and utilization across species as well as reinforcing the positive role preclinical research can have in developing procognitive treatments for psychiatric disorders.

Dopamine, serotonin and impulsivity

Impulsive people have a strong urge to act without thinking. It is sometimes regarded as a positive trait but rash impulsiveness is also widely present in clinical disorders such as attention deficit hyperactivity disorder (ADHD), drug dependence, mania, and antisocial behaviour. Contemporary research has begun to make major inroads into unravelling the brain mechanisms underlying impulsive behaviour with a prominent focus on the limbic cortico-striatal systems. With this progress has come the understanding that impulsivity is a multi-faceted behavioural trait involving neurally and psychologically diverse elements. We discuss the significance of this heterogeneity for clinical disorders expressing impulsive behaviour and the pivotal contribution made by the brain dopamine and serotonin systems in the aetiology and treatment of behavioural syndromes expressing impulsive symptoms.