Alpha-adrenoceptor-mediated modulation of 5-HT2 receptor agonist induced impulsive responding in a 5-choice serial reaction time task

Effects of psilocybin, the 5-HT2A receptor agonist TCB-2, and the 5-HT2A receptor antagonist M100907 on visual attention in male mice in the continuous performance test

RATIONALE

Neuropsychiatric disorders such as depression are characterized in part by attention deficits. Attention is modulated by the serotonin (5-HT) neurotransmitter system. The 5-HT2A agonist and hallucinogen psilocybin (PSI) is a promising treatment for disorders characterized by attention changes. However, few studies have investigated PSI's direct effect on attention.

OBJECTIVE

Using the rodent continuous performance task (CPT), we assessed PSI's effect on attention. We also evaluated the impact of 5-HT2A receptor agonist TCB-2 and antagonist M100907 for comparative purposes.

METHODS

In the CPT, mice learned to distinguish visual targets from non-targets for milkshake reward. Performance was then tested following injections of PSI (0.3, 1, and 3 mg/kg), TCB-2 (0.3, 1, and 3 mg/kg), or M100907 (0.1, 0.3, and 1 mg/kg). Subsequently, drug effects were then evaluated using a more difficult CPT with variable stimulus durations. Mice were then tested on the CPT following repeated PSI injections. Drug effects on locomotor activity were also measured.

RESULTS

In the CPT, all three drugs reduced hit and false alarm rate and induced conservative responding. PSI also reduced target discrimination. These effects were seen primarily at doses that also significantly reduced locomotor activity. No drug effects were seen on the more difficult CPT or following repeated PSI injections.

CONCLUSIONS

Psilocybin, TCB-2, and M100907 impaired performance of the CPT. However, this may be in part due to drug-induced locomotor changes. The results provide little support for the idea that psilocybin alters visual attention, or that 5-HT2A receptors modulate this process.

Theory of visual attention (TVA) applied to rats performing the 5-choice serial reaction time task: differential effects of dopaminergic and noradrenergic manipulations

RATIONALE

Attention is compromised in many psychiatric disorders, including attention-deficit/hyperactivity disorder (ADHD). While dopamine and noradrenaline systems have been implicated in ADHD, their exact role in attentional processing is yet unknown.

OBJECTIVES

We applied the theory of visual attention (TVA) model, adapted from human research, to the rat 5-choice serial reaction time task (5CSRTT) to investigate catecholaminergic modulation of visual attentional processing in healthy subjects of high- and low-attention phenotypes.

METHODS

Rats trained on the standard 5CSRTT and tested with variable stimulus durations were treated systemically with noradrenergic and/or dopaminergic agents (atomoxetine, methylphenidate, amphetamine, phenylephrine and atipamezole). TVA modelling was applied to estimate visual processing speed for correct and incorrect visual perceptual categorisations, independent of motor reaction times, as measures of attentional capacity.

RESULTS

Atomoxetine and phenylephrine decreased response frequencies, including premature responses, increased omissions and slowed responding. In contrast, methylphenidate, amphetamine and atipamezole sped up responding and increased premature responses. Visual processing speed was also affected differentially. Atomoxetine and phenylephrine slowed, whereas methylphenidate and atipamezole sped up, visual processing, both for correct and incorrect categorisations. Amphetamine selectively improved visual processing for correct, though not incorrect, responses in high-attention rats only, possibly reflecting improved attention.

CONCLUSIONS

These data indicate that the application of TVA to the 5CSRTT provides an enhanced sensitivity to capturing attentional effects. Unexpectedly, we found overall slowing effects, including impaired visual processing, following drugs either increasing extracellular noradrenaline (atomoxetine) or activating the α1-adrenoceptor (phenylephrine), while also ameliorating premature responses (impulsivity). In contrast, amphetamine had potential pro-attentional effects by enhancing visual processing, probably due to central dopamine upregulation.

Contrasting effects of DOI and lisuride on impulsive decision-making in delay discounting task

RATIONALE

The 5-HT2A receptor is the major target of classic hallucinogens. Both DOI (2,5-dimethoxy-4-iodoamphetamine) and lisuride act at 5-HT2A receptors, and lisuride shares comparable affinity with DOI and acts as a partial agonist at 5-HT2A receptors. However, not like DOI, lisuride lacks hallucinogenic properties. Impulsive decision-making refers to the preference for an immediate small reinforcer (SR) over a delayed large reinforcer (LR).

OBJECTIVES

The current study aims to compare the effects of DOI and lisuride on impulsive decision-making and further to investigate the possible receptor mechanisms responsible for the actions of the two drugs.

METHODS

Impulsive decision-making was evaluated in male Sprague-Dawley rats by the percentage of choice for the LR in delay discounting task (DDT). Delay to the LR changed in an ascending order (0, 4, 8, 16, and 32 s) across one session.

RESULTS

DOI (0.5 and 1.0 mg/kg) increased impulsive decision-making, and the effects of DOI (1.0 mg/kg) were blocked by the 5-HT2A receptor antagonist ketanserin (1.0 mg/kg) rather than the 5-HT2C receptor antagonist SB-242084 (1.0 mg/kg). Contrarily, lisuride (0.1, 0.3, and 0.5 mg/kg) decreased impulsive decision-making. The effects of lisuride (0.3 mg/kg) were not antagonized by ketanserin (1.0 mg/kg), selective 5-HT1A antagonist WAY-100635 (1.0 mg/kg), or selective dopamine D4 receptor antagonist L-745870 (1.0 mg/kg) but were attenuated by the selective dopamine D2/D3 receptor antagonist tiapride (40 mg/kg).

CONCLUSIONS

DOI and lisuride have contrasting effects on impulsive decision-making via distinct receptors. DOI-induced increase of impulsivity is mediated by the 5-HT2A receptor, while lisuride-induced inhibition of impulsivity is regulated by the dopamine D2/D3 receptor.

Adaptive aspects of impulsivity and interactions with effects of catecholaminergic agents in the 5-choice serial reaction time task: implications for ADHD

BACKGROUND

Work in humans has shown that impulsivity can be advantageous in certain settings. However, evidence for so-called functional impulsivity is lacking in experimental animals.

AIMS

This study investigated the contexts in which high impulsive (HI) rats show an advantage in performance compared with mid- (MI) and low impulsive (LI) rats. We also assessed the effects of dopaminergic and noradrenergic agents to investigate underlying neurotransmitter mechanisms.

METHODS

We tested rats on a variable inter-trial interval (ITI) version of the 5-choice serial reaction time task (5CSRTT). Rats received systemic injections of methylphenidate (MPH, 1 mg/kg and 3 mg/kg), atomoxetine (ATO, 0.3 mg/kg and 1 mg/kg), amphetamine (AMPH, 0.2 mg/kg), the alpha-2a adrenoceptor antagonist atipamezole (ATI, 0.3 mg/kg) and the alpha-1 adrenoceptor agonist phenylephrine (PHEN, 1 mg/kg) prior to behavioural testing.

RESULTS

Unlike LI rats, HI rats exhibited superior performance, earning more reinforcers, on short ITI trials, when the task required rapid responding. MPH, AMPH and ATI improved performance on short ITI trials and increased impulsivity in long ITI trials, recapitulating the behavioural profile of HI. In contrast, ATO and PHEN impaired performance on short ITI trials and decreased impulsivity, thus mimicking the behavioural profile of LI rats. The effects of ATO were greater on MI rats and LI rats.

CONCLUSIONS

These findings indicate that impulsivity can be advantageous when rapid focusing and actions are required, an effect that may depend on increased dopamine neurotransmission. Conversely, activation of the noradrenergic system, with ATO and PHEN, led to a general inhibition of responding.

Involvement of noradrenergic and serotonergic systems in risk-based decisions between options of equivalent expected value in rats

Risk perception is an important factor that may mediate risk-based decision-making processes regulated by noradrenergic (NA) and serotonergic (5-HT) systems. Most risk-based decision-making models involve complex factors, such as risk perception or reward value, such that the final decision is the result of the interactions among these factors. However, the contribution of risk perception per se in risk decisions has remained unclear. Therefore, in the present study, we made some modifications to the classical probabilistic discounting task (PDT) to focus on the impact of risk perception and noradrenergic/serotonergic systems on decision-making behavior. Meanwhile, we conducted an elevated plus-maze (EPM) test to detect the correlation between anxiety and choice behavior. In the current study, rats had to choose between a "certain" lever that delivered a certain number of pellets and a "risky" lever that delivered eight pellets in a probabilistic manner (descending: 50%, 25%, 12.5% or ascending 12.5%, 25%, 50% of the time). The long-term rewarding values of the two levers were always identical in each block within each session. According to their baseline performances in choosing the risky lever, rats were divided into the risk-prefer group and risk-averse group. The results showed that there was a significant correlation between open arm time in EPM and risky choice for both descending order and ascending order, indicating that highly anxious rats more often preferred the safe option under risk. Pharmacological stimulation of α2-adrenergic receptors via dexmedetomidine (0.01 mg/kg) decreased the preference of probabilistic rewards in the risk-prefer group, while blocking α2 receptors by atipamezole (0.3 mg/kg) also reduced risky choices. The NA reuptake inhibitor, atomoxetine, increased the preference for risky choices in the risk-prefer group, the effect of which was attained via multiple superimposed doses. Administration of the 5-HT_2A receptor agonist, DOI (0.1 mg/kg), increased risk-taking behavior in the risk-prefer group. Taken together, these results suggest that NA may be more inclined to process negative information such as loss or uncertainty in the regulation of risk-related decision making, whereas 5-HT may function primarily to increase risk-taking behavior. Our findings may help to further elucidate how noradrenergic and serotonergic systems differentially affect individuals with different risk preferences in terms of regulating risk perception in risk-related decision making.

The serotonin 2A receptor agonist 25CN-NBOH increases murine heart rate and neck-arterial blood flow in a temperature-dependent manner

BACKGROUND

Serotonin 2A receptors, the molecular target of psychedelics, are expressed by neuronal and vascular cells, both of which might contribute to brain haemodynamic characteristics for the psychedelic state.

AIM

Aiming for a systemic understanding of psychedelic vasoactivity, here we investigated the effect of N-(2-hydroxybenzyl)-2,5-dimethoxy-4-cyanophenylethylamine - a new-generation agonist with superior serotonin 2A receptor selectivity - on brain-supplying neck-arterial blood flow.

METHODS

We recorded core body temperature and employed non-invasive, collar-sensor based pulse oximetry in anesthetised mice to extract parameters of local blood perfusion, oxygen saturation, heart and respiration rate. Hypothesising an overlap between serotonergic pulse- and thermoregulation, recordings were done under physiological and elevated pad temperatures.

RESULTS

N-(2-hydroxybenzyl)-2,5-dimethoxy-4-cyanophenylethylamine (1.5 mg/kg, subcutaneous) significantly increased the frequency of heart beats accompanied by a slight elevation of neck-arterial blood flow. Increasing the animal-supporting heat-pad temperature from 37°C to 41°C enhanced the drug's effect on blood flow while counteracting tachycardia. Additionally, N-(2-hydroxybenzyl)-2,5-dimethoxy-4-cyanophenylethylamine promoted bradypnea, which, like tachycardia, quickly reversed at the elevated pad temperature. The interrelatedness of N-(2-hydroxybenzyl)-2,5-dimethoxy-4-cyanophenylethylamine's respiro-cardiovascular effects and thermoregulation was further corroborated by the drug selectively increasing the core body temperature at the elevated pad temperature. Arterial oxygen saturation was not affected by N-(2-hydroxybenzyl)-2,5-dimethoxy-4-cyanophenylethylamine at either temperature.

CONCLUSIONS

Our findings imply that selective serotonin 2A receptor activation modulates systemic cardiovascular functioning in orchestration with thermoregulation and with immediate relevance to brain-imminent neck (most likely carotid) arteries. As carotid branching is a critical last hub to channel cardiovascular output to or away from the brain, our results might have implications for the brain haemodynamics associated with psychedelia.

Pramipexole-induced impulsivity in mildparkinsonian rats: a model of impulse control disorders in Parkinson's disease

Treatment with dopaminergic agonists such as pramipexole (PPX) contributes to the development of impulse control disorders (ICDs) in patients with Parkinson's disease (PD). As such, animal models of abnormal impulse control in PD are needed to better study the pathophysiology of these behaviors. Thus, we investigated impulsivity and related behaviors using the 5-choice serial reaction time task, as well as FosB/ΔFosB expression, in rats with mild parkinsonism induced by viral-mediated substantia nigra overexpression of human A53T mutated α-synuclein, and following chronic PPX treatment (0.25 mg/kg/d) for 4 weeks. The bilateral loss of striatal dopamine transporters (64%) increased the premature response rate of these rats, indicating enhanced waiting impulsivity. This behavior persisted in the OFF state after the second week of PPX treatment and it was further exacerbated in the ON state throughout the treatment period. The enhanced rate of premature responses following dopaminergic denervation was positively correlated with the premature response rate following PPX treatment (both in the ON and OFF states). Moreover, the striatal dopaminergic deficit was negatively correlated with the premature response rate at all times (pretreatment, ON and OFF states) and it was positively correlated with the striatal FosB/ΔFosB expression. By contrast, PPX treatment was not associated with changes in compulsivity (perseverative responses rate). This model recapitulates some features of PD with ICD, namely the dopaminergic deficit of early PD and the impulsivity traits provoked by dopaminergic loss in association with PPX treatment, making this model a useful tool to study the pathophysiology of ICDs.

Interactions of Hallucinogens with the Glutamatergic System: Permissive Network Effects Mediated Through Cortical Layer V Pyramidal Neurons

Recordings made from layer V (L5) pyramidal cells of the prefrontal cortex (PFC) and neocortex in rodent slice preparations have shown that serotonin (5-hydroxytryptamine, 5-HT) and serotonergic hallucinogens induce an increase in the frequency of spontaneous excitatory postsynaptic currents (EPSCs) in the apical dendritic field by activating 5-HT_2A receptors. Serotonergic hallucinogens induce late EPSCs and increase recurrent network activity when subcortical or mid-cortical regions are stimulated at low frequencies (e.g., 0.1 Hz). A range of agonists or positive allosteric modulators (PAMs) for mostly G_i/o-coupled receptors, including metabotropic glutamate₂ (mGlu₂), adenosine A₁, or μ-opioid receptors, suppress these effects of 5-HT_2A receptor stimulation. Furthermore, a range of mostly G_q/11-coupled receptors (including orexin₂ [OX₂]; α₁-adrenergic, and mGlu₅ receptors) similarly induce glutamate (Glu) release onto L5 pyramidal cells. Evidence implicates a number of brain regions in mediating these effects of serotonergic hallucinogens and G_q/11-coupled receptors including the midline and intralaminar thalamic nuclei, claustrum, and neurons in deep PFC. These effects on 5-HT_2A receptors and related GPCRs appear to play a major role in the behavioral effects of serotonergic hallucinogens, such as head twitches in rodents and higher order behaviors such as rodent lever pressing on the differential-reinforcement-of-low rate 72-s (DRL 72-s) schedule. This implies that the effects of 5-HT_2A receptor activation on the activity of L5 pyramidal cells may be responsible for mediating a range of behaviors linked to limbic circuitry with connectivity between the PFC, striatum, thalamus, claustrum, striatum, amygdala, and the hippocampal formation.

Pharmacological investigations of a yohimbine-impulsivity interaction in rats

Both impulsivity and stress are risk factors for substance abuse, but it is not clear how these two processes interact to alter susceptibility for the disorder. The aim of this project was to examine the pharmacology of a stress-impulsivity interaction in rats. To do so, we tested the effects of yohimbine on impulsive action and then assessed whether behavioural changes could be reduced by antagonists at different receptor subtypes. Male Long-Evans rats were injected with various doses of yohimbine (0-5.0 mg/kg) before testing in the response-inhibition task. In subsequent experiments, yohimbine (2.5 mg/kg) was injected following pretreatment with the following receptor antagonists: corticotropin-releasing factor receptor 1, antalarmin (0-20 mg/kg); glucocorticoid, mifepristone (0-30 mg/kg); noradrenergic (NA) α1, prazosin (0-2 mg/kg); NA α2, guanfacine (0-0.5 mg/kg); NA β2, propranolol (0.5-2.0 mg/kg); dopamine D1/5, SCH 39166 (0-0.0625 mg/kg); μ opioid, naloxone (0-2 mg/kg); or 5-HT2A, M100907 (0.005-0.05 mg/kg). In all experiments, impulsive action was measured as increased premature responding. Yohimbine dose dependently increased impulsive action, but the effect was not reversed by antagonist pretreatment. None of the drugs altered any other behavioural measure. We conclude that stress-impulsivity interactions are likely mediated by a synergy of multiple neurotransmitter systems.

Hierarchical investigation of genetic influences on response inhibition in healthy young adults

Poor inhibitory control is a known risk factor for substance use disorders, making it a priority to identify the determinants of these deficits. The aim of the current study was to identify genetic associations with inhibitory control using the stop signal task in a large sample (n = 934) of healthy young adults of European ancestry. We genotyped the subjects genome-wide and then used a hierarchical approach in which we tested seven a priori single nucleotide polymorphisms (SNPs) previously associated with stop signal task performance, approximately 9,000 SNPs designated as high-value addiction (HVA) markers by the SmokeScreen array, and approximately five million genotyped and imputed SNPs, followed by a gene-based association analysis using the resultant p values. A priori SNP analyses revealed nominally significant associations between response inhibition and one locus in HTR2A (rs6313; p = .04, dominance model, uncorrected) in the same direction as prior findings. A nominally significant association was also found in one locus in ANKK1 (rs1800497; p = .03, uncorrected), although in the opposite direction of previous reports. After accounting for multiple comparisons, the HVA, genome-wide, and gene-based analyses yielded no significant findings. This study implicates variation in serotonergic and dopaminergic genes while underscoring the difficulty of detecting the influence of individual SNPs, even when biological information is used to prioritize testing. Although such small effect sizes suggest limited utility of individual SNPs in predicting risk for addiction or other impulse control disorders, they may nonetheless shed light on complex biological processes underlying poor inhibitory control. (PsycINFO Database Record

Contribution of Impulsivity and Serotonin Receptor Neuroadaptations to the Development of an MDMA ('Ecstasy') Substance Use Disorder

As is the case with other drugs of abuse, a proportion of ecstasy users develop symptoms consistent with a substance use disorder (SUD). In this paper, we propose that the pharmacology of MDMA, the primary psychoactive component of ecstasy tablets, changes markedly with repeated exposure and that neuroadaptations in dopamine and serotonin brain systems underlie the shift from MDMA use to MDMA misuse in susceptible subjects. Data from both the human and laboratory animal literature are synthesized to support the idea that (1) MDMA becomes a less efficacious serotonin releaser and a more efficacious dopamine releaser with the development of behaviour consistent with an SUD and (2) that upregulated serotonin receptor mechanisms contribute to the development of the MDMA SUD via dysregulated inhibitory control associated with the trait of impulsivity.

5-HT2A and mGlu2/3 receptor interactions: on their relevance to cognitive function and psychosis

Serotonin [5-hydroxytryptamine (5-HT)] and glutamate have both been implicated in the pathophysiology of neuropsychiatric disorders but also in the mechanism of antipsychotic and hallucinogenic drug actions. Furthermore, close antagonistic interactions between 5-HT2A and metabotropic glutamate (mGlu)2/3 receptors have been established over the last decades on the basis of numerous electrophysiological, biochemical, and behavioral studies. Besides synaptic mechanisms, more recent findings suggested that heterodimeric 5-HT2A-mGlu2 receptor complexes in the prefrontal cortex may account for the functional crosstalk between these two receptor subtypes. In this review, we focus on in-vitro and in-vivo studies documenting the important relationship between 5-HT2A and mGlu2/3 receptors, with relevance to both normal behavioral function and psychosis.

Serotonin (5-HT) 5-HT2A Receptor (5-HT2AR):5-HT2CR Imbalance in Medial Prefrontal Cortex Associates with Motor Impulsivity

A feature of multiple neuropsychiatric disorders is motor impulsivity. Recent studies have implicated serotonin (5-HT) systems in medial prefrontal cortex (mPFC) in mediating individual differences in motor impulsivity, notably the 5-HT2AR receptor (5-HT2AR) and 5-HT2CR. We investigated the hypothesis that differences in the ratio of 5-HT2AR:5-HT2CR protein expression in mPFC would predict the individual level of motor impulsivity and that the engineered loss of the 5-HT2CR would result in high motor impulsivity concomitant with elevated 5-HT2AR expression and pharmacological sensitivity to the selective 5-HT2AR antagonist M100907. High and low impulsive rats were identified in a 1-choice serial reaction time task. Native protein levels of the 5-HT2AR and the 5-HT2CR predicted the intensity of motor impulsivity and the 5-HT2AR:5-HT2CR ratio in mPFC positively correlated with levels of premature responses in individual outbred rats. The possibility that the 5-HT2AR and 5-HT2CR act in concert to control motor impulsivity is supported by the observation that high phenotypic motor impulsivity associated with a diminished mPFC synaptosomal 5-HT2AR:5-HT2CR protein:protein interaction. Knockdown of mPFC 5-HT2CR resulted in increased motor impulsivity and triggered a functional disruption of the local 5-HT2AR:5-HT2CR balance as evidenced by a compensatory upregulation of 5-HT2AR protein expression and a leftward shift in the potency of M100907 to suppress impulsive behavior. We infer that there is an interactive relationship between the mPFC 5-HT2AR and 5-HT2CR, and that a 5-HT2AR:5-HT2CR imbalance may be a functionally relevant mechanism underlying motor impulsivity.

Individual Differences in Impulsive Action Reflect Variation in the Cortical Serotonin 5-HT2A Receptor System

Impulsivity is an important feature of multiple neuropsychiatric disorders, and individual variation in the degree of inherent impulsivity could play a role in the generation or exacerbation of problematic behaviors. Serotonin (5-HT) actions at the 5-HT2AR receptor (5-HT2AR) promote and 5-HT2AR antagonists suppress impulsive action (the inability to withhold premature responses; motor impulsivity) upon systemic administration or microinfusion directly into the medial prefrontal cortex (mPFC), a node in the corticostriatal circuit that is thought to play a role in the regulation of impulsive action. We hypothesized that the functional capacity of the 5-HT2AR, which is governed by its expression, localization, and protein/protein interactions (eg, postsynaptic density 95 (PSD95)), may drive the predisposition to inherent impulsive action. Stable high-impulsive (HI) and low-impulsive (LI) phenotypes were identified from an outbred rodent population with the 1-choice serial reaction time (1-CSRT) task. HI rats exhibited a greater head-twitch response following administration of the preferential 5-HT2AR agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) and were more sensitive to the effects of the selective 5-HT2AR antagonist M100907 to suppress impulsive action relative to LI rats. A positive correlation was observed between levels of premature responses and 5-HT2AR binding density in frontal cortex ([(3)H]-ketanserin radioligand binding). Elevated mPFC 5-HT2AR protein expression concomitant with augmented association of the 5-HT2AR with PSD95 differentiated HI from LI rats. The observed differential sensitivity of HI and LI rats to 5-HT2AR ligands and associated distinct 5-HT2AR protein profiles provide evidence that spontaneously occurring individual differences in impulsive action reflect variation in the cortical 5-HT2AR system.

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.

Impaired flexibility in decision making in rats after administration of the pharmacological stressor yohimbine

RATIONALE

Stress-induced disruption of decision making has been hypothesized to contribute to drug-seeking behaviors and addiction. Noradrenergic signaling plays a central role in mediating stress responses. However, the effects of acute stress on decision making, and the role of noradrenergic signaling in regulating these effects, have not been well characterized.

OBJECTIVE

To characterize changes in decision making caused by acute pharmacological stress, the effects of yohimbine (an α2-adrenergic antagonist) were examined in a delay discounting task. Noradrenergic contributions to decision making were further characterized by examining the effects of propranolol (a β antagonist), prazosin (an α1 antagonist), and guanfacine (an α2 agonist).

METHODS

Sprague-Dawley rats were administered drugs prior to performance on a delay discounting task, in which the delay preceding the large reward increased within each session (ascending delays). To dissociate drug-induced changes in delay sensitivity from behavioral inflexibility, drug effects were subsequently tested in a modified version of the discounting task, in which the delay preceding the large reward decreased within each session (descending delays).

RESULTS

Yohimbine increased choice of the large reward when tested with ascending delays but decreased choice of the same large reward when tested with descending delays, suggesting that drug effects could be attributed to perseverative choice of the lever preferred at the beginning of the session. Propranolol increased choice of the large reward when tested with ascending delays. Prazosin and guanfacine had no effect on reward choice.

CONCLUSIONS

The stress-like effects of yohimbine administration may impair decision making by causing inflexible, perseverative behavior.

Serotonin at the nexus of impulsivity and cue reactivity in cocaine addiction

Cocaine abuse and addiction remain great challenges on the public health agendas in the U.S. and the world. Increasingly sophisticated perspectives on addiction to cocaine and other drugs of abuse have evolved with concerted research efforts over the last 30 years. Relapse remains a particularly powerful clinical problem as, even upon termination of drug use and initiation of abstinence, the recidivism rates can be very high. The cycling course of cocaine intake, abstinence and relapse is tied to a multitude of behavioral and cognitive processes including impulsivity (a predisposition toward rapid unplanned reactions to stimuli without regard to the negative consequences), and cocaine cue reactivity (responsivity to cocaine-associated stimuli) cited as two key phenotypes that contribute to relapse vulnerability even years into recovery. Preclinical studies suggest that serotonin (5-hydroxytryptamine; 5-HT) neurotransmission in key neural circuits may contribute to these interlocked phenotypes well as the altered neurobiological states evoked by cocaine that precipitate relapse events. As such, 5-HT is an important target in the quest to understand the neurobiology of relapse-predictive phenotypes, to successfully treat this complex disorder and improve diagnostic and prognostic capabilities. This review emphasizes the role of 5-HT and its receptor proteins in key addiction phenotypes and the implications of current findings to the future of therapeutics in addiction. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

α₂- and β-adrenoceptors involvement in nortriptyline modulation of auditory sustained attention and impulsivity

RATIONALE

The catecholamine innervation of the prefrontal cortex controls attentional focus and inhibits inappropriate behavioral responses. The mechanism of action with which norepinephrine (NE) reuptake inhibitors modulate these cognitive functions has not been fully investigated.

OBJECTIVE

We investigated the effect of systemic administration of the NE reuptake blocker nortriptyline (NT) on attention and impulsivity using an auditory sustained attention task. The task was designed to assess impulsive behavior and the maintenance of attentional focus to an auditory stimulus presented at interresponse time durations (IRT) between 5 and 80 s.

RESULTS

NT (2.0 but not 3.0 mg/kg) improved sustained attention and decreased the percentage of premature responses without changing their latency. To better understand the adrenergic component of NT action, we tested the effect of noradrenergic receptor antagonists alone or together with NT. The α(2)-receptor antagonist yohimbine, the α(1)-receptor antagonist prazosin, or the β-receptor antagonist propranolol alone did not significantly affect attentive performance or premature responses. However, the beneficial effects of NT on sustained attention and premature responses were attenuated by pretreatment with either yohimbine or propranolol. On the contrary, prazosin did not affect the NT-mediated improvement in sustained attention.

CONCLUSIONS

We conclude that sustained attention displays an inverse U-shaped dependence on NT, mediated-at least in part-by α(2)- and β-adrenoceptors. We speculate that low doses of NT improve performance by maximizing the phasic release of NE, while higher doses of NT would elevate tonic levels of NE, thus producing suboptimal levels of phasically released NE.

The utility of rat models of impulsivity in developing pharmacotherapies for impulse control disorders

High levels of impulsive behaviours are a clinically significant symptom in a range of psychiatric disorders, such as attention deficit hyperactivity disorder, bipolar disorder, personality disorders, pathological gambling and substance abuse. Although often measured using questionnaire assessments, levels of different types of impulsivity can also be determined using behavioural tests. Rodent analogues of these paradigms have been developed, and similar neural circuitry has been implicated in their performance in both humans and rats. In the current review, the methodology underlying the measurement of different aspects of impulsive action and choice are considered from the viewpoint of drug development, with a focus on the continuous performance task (CPT), stop-signal task (SST), go/no-go and delay-discounting paradigms. Current issues impeding translation between animal and human studies are identified, and comparisons drawn between the acute effects of dopaminergic, noradrenergic and serotonergic compounds across species. Although the field could benefit from a more systematic determination of different pharmacological agents across paradigms, there are signs of strong concordance between the animal and human data. However, the type of impulsivity measured appears to play a significant role, with the SST and delay discounting providing more consistent effects for dopaminergic drugs, while the CPT and SST show better predictive validity so far for serotonergic and noradrenergic compounds. Based on the available data, it would appear that these impulsivity models could be used more widely to identify potential pharmacotherapies for impulse control disorders. Novel targets within the glutamatergic and serotonergic system are also suggested.

A modified adjusting delay task to assess impulsive choice between isocaloric reinforcers in non-deprived male rats: effects of 5-HT₂A/C and 5-HT₁A receptor agonists

RATIONALE

Existing animal models of impulsivity frequently use food restriction to increase subjects' motivation. In addition, behavioral tasks that assess impulsive choice typically involve the use of reinforcers with dissimilar caloric content. These factors represent energy-homeostasis limitations, which may confound the interpretation of results and limit the applicability of these models.

OBJECTIVES

This study was aimed at validating face and convergent validities of a modified adjusting delay task, which assesses impulsive choice between isocaloric reinforcers in ad libitum fed rats.

METHODS

Male Wistar rats (n = 18) were used to assess the preferredness and reinforcing efficacy of a "supersaccharin" solution (1.5% glucose/0.4% saccharin) over a 1.5% glucose solution. A separate group of rats (n = 24) was trained in a modified adjusting delay task, which involved repeated choice between the glucose solution delivered immediately and the supersaccharin solution delivered after a variable delay. To pharmacologically validate the task, the effects of the 5-HT(2A/C) receptor agonist (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane [(±)-DOI] and the 5-HT(1A) receptor agonist (±)-8-hydroxy-2-(dipropylamino)tetralin hydrobromide [(±)-8-OH-DPAT] on impulsive choice were then evaluated.

RESULTS

Supersaccharin was highly reinforcing and uniformly preferred over the glucose solution by all subjects. Rats quickly learned the task, and impulsivity was a very stable and consistent trait. DOI and 8-OH-DPAT significantly and dose dependently increased impulsive choice in this modified adjusting delay task.

CONCLUSIONS

We validated a rodent task of impulsive choice, which eliminates typical energy-homeostasis limitations and, therefore, opens new avenues in the study of impulsivity in preclinical feeding and obesity research.

Pre-treatment with the mGlu2/3 receptor agonist LY379268 attenuates DOI-induced impulsive responding and regional c-Fos protein expression

RATIONALE

Overactivation of serotonin (5-hydroxytryptamine, 5-HT)(2A) receptors causes impulsivity and attentional deficits. Since 5-HT(2A) receptors are known to entertain antagonistic interactions with metabotropic glutamate (mGlu)2/3 receptors, this interaction may provide an alternative target for a novel class of antipsychotics.

OBJECTIVES/METHODS

The study characterizes interactions between 5-HT(2A) and mGlu2/3 receptors implicated in impulse control. Hooded Lister rats were trained in a 5-choice serial reaction time task (5-CSRTT) and treated with the 5-HT(2A/2C) receptor agonist (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropan hydrochloride (DOI, 0.1 mg/kg) and the mGlu2/3 receptor agonist LY379268 (1 mg/kg). In addition, associated drug-induced changes in neuronal activity were assessed via c-Fos immunoreactivity (Fos IR), and co-localization of c-Fos and GABAergic markers was detected using double immunofluorescence labeling.

RESULTS

Systemic DOI caused impulsive overresponding that was attenuated in animals pre-treated with LY379268. LY379268 itself had no significant effect on the rats' performance in the 5-CSRTT. DOI enhanced Fos IR within fronto-cortical and limbic brain structures, and this effect was blocked by LY379268 pre-treatment. Double immunofluorescence labeling showed a specific co-localization of DOI-elicited Fos IR with GABAergic (GAD(67)-positive) cells lacking the calcium-binding protein parvalbumin while LY379268 increased Fos IR in GABAergic and non-GABAergic cells.

CONCLUSION

Our results suggest that impulsivity is possibly due to a primary increase in Glu transmission mediated via 5-HT(2A) receptor activation. Thus, mGlu2/3 receptor agonists might have some potential for treating motor impulsivity-related impairments while their cognitive enhancing effects were not confirmed in this study.

Serotonin (5-hydroxytryptamine) 5-HT(2A) receptor: association with inherent and cocaine-evoked behavioral disinhibition in rats

Alterations in the balance of functional activity within the serotonin [5-hydroxytryptamine (5-HT)] system are hypothesized to underlie impulse control. Cocaine-dependent subjects consistently show greater impulsivity relative to nondrug using control subjects. Preclinical studies suggest that the 5-HT(2A) receptor (5-HT(2A)R) contributes to the regulation of impulsive behavior and also mediates some of the behavioral effects of cocaine. We hypothesized that the selective 5-HT(2A)R antagonist M100907 would reduce inherent levels of impulsivity and attenuate impulsive responding induced by cocaine in two animal models of impulsivity, the differential reinforcement of low rate (DRL) task and the one-choice serial reaction time (1-CSRT) task. M100907 reduced rates of responding in the DRL task and premature responding in the 1-CSRT task. Conversely, cocaine disrupted rates of responding in the DRL task and increased premature responding in the 1-CSRT task. M100907 attenuated cocaine-induced increases in specific markers of behavioral disinhibition in the DRL and 1-CSRT tasks. These results suggest that the 5-HT(2A)R regulates inherent impulsivity, and that blockade of the 5-HT(2A)R alleviates specific aspects of elevated levels of impulsivity induced by cocaine exposure. These data point to the 5-HT(2A)R as an important regulatory substrate in impulse control.

A neurochemical yin and yang: does serotonin activate and norepinephrine deactivate the prefrontal cortex?

INTRODUCTION

The prefrontal cortex (PFC) receives serotonergic input from the dorsal raphe nucleus of the brainstem, as well as noradrenergic input from another brainstem nucleus, the locus coeruleus. A large number of studies have shown that these two neurotransmitter systems, and drugs that affect them, modulate the functional properties of the PFC in both humans and animal models.

RESULTS

Here I examine the hypothesis that serotonin (5-HT) plays a general role in activating the PFC, whereas norepinephrine (NE) plays a general role in deactivating this brain region. In this manner, the two neurotransmitter systems may have opposing effects on PFC-influenced behavior. To assess this hypothesis, three primary lines of evidence are examined comprising the effects of 5-HT and NE on impulsivity, cognitive flexibility, and working memory.

DISCUSSION

While all of the existing data do not unequivocally support the activation/deactivation hypothesis, there is a large body of support for it.

Prefrontal cortex and drug abuse vulnerability: translation to prevention and treatment interventions

Vulnerability to drug abuse is related to both reward seeking and impulsivity, two constructs thought to have a biological basis in the prefrontal cortex (PFC). This review addresses similarities and differences in neuroanatomy, neurochemistry and behavior associated with PFC function in rodents and humans. Emphasis is placed on monoamine and amino acid neurotransmitter systems located in anatomically distinct subregions: medial prefrontal cortex (mPFC); lateral prefrontal cortex (lPFC); anterior cingulate cortex (ACC); and orbitofrontal cortex (OFC). While there are complex interconnections and overlapping functions among these regions, each is thought to be involved in various functions related to health-related risk behaviors and drug abuse vulnerability. Among the various functions implicated, evidence suggests that mPFC is involved in reward processing, attention and drug reinstatement; lPFC is involved in decision-making, behavioral inhibition and attentional gating; ACC is involved in attention, emotional processing and self-monitoring; and OFC is involved in behavioral inhibition, signaling of expected outcomes and reward/punishment sensitivity. Individual differences (e.g., age and sex) influence functioning of these regions, which, in turn, impacts drug abuse vulnerability. Implications for the development of drug abuse prevention and treatment strategies aimed at engaging PFC inhibitory processes that may reduce risk-related behaviors are discussed, including the design of effective public service announcements, cognitive exercises, physical activity, direct current stimulation, feedback control training and pharmacotherapies. A major challenge in drug abuse prevention and treatment rests with improving intervention strategies aimed at strengthening PFC inhibitory systems among at-risk individuals.

Opposing effects of 5-HT(2A) and 5-HT(2C) receptor antagonists in the rat and mouse on premature responding in the five-choice serial reaction time test

RATIONALE

Serotonin (5-HT) has been linked to impulsivity with recent data suggesting that different receptor sub-types exert opposing influences on this behaviour.

OBJECTIVES

This work characterised the effects of 5-HT(2A) (ketanserin, (+/-)2,3-dimethoxyphenyl-1-[2-4-(piperidine)-methanol] [M100907]), 5-HT(2B) (6-chloro-5-methyl-1-(5-quinolylcarbamoyl) indoline [SB215505]) and 5-HT(2C) (6-chloro-5-methyl-1-[2-(2-methylpyridyl-3-oxy)-pyrid-5-yl carbomyl] indoline [SB242084]) receptor antagonists on impulsive behaviour, measured in the five-choice serial reaction time test (5CSRTT), in rats and mice. The effects of (+/-)-2,5-dimethoxy-4-iodoamphetamine (DOI) and (S)-2-(chloro-5-fluoro-indol-1-yl)-1-methylethylamine 1:1 C4H4O4 (Ro60-0175), two compounds that have been used extensively as agonists for the 5-HT(2A) and 5-HT(2C) receptor, were also measured.

MATERIALS AND METHODS

Rats and mice were trained on the 5CSRTT in which reinforcement is earned for detecting and correctly responding to brief presentations of a stimulus light. Impulsivity in this task is measured as premature responding, before stimulus presentation. Several variants of the task were used in which the inter-trial interval (ITI) length was manipulated to alter basal levels of premature responding.

RESULTS

In the rat, ketanserin and M100907 reduced and SB242084 enhanced premature responding. SB215505 had no effect. DOI generally disrupted responding, while Ro60-0175 reduced premature responding when a long ITI was used. In mice, M100907 reduced and SB242084 increased premature responding when the ITI was lengthened. The effects of these drugs on other aspects of performance were less robust. M100907 and ketanserin did not affect response accuracy but tended to slow speed of responding; SB242084 occasionally increased speed of responding and slightly reduced accuracy.

CONCLUSIONS

Serotonin exerts both excitatory and inhibitory influences on motor impulsivity via 5-HT(2A) and 5-HT(2C) receptors in both rats and mice.

Acute serotonin and dopamine depletion improves attentional control: findings from the stroop task

Schizophrenia is associated with impairments of attentional control on classic experimental paradigms such as the Stroop task. However, at a basic level the neurochemical mechanisms that may be responsible for such impairments are poorly understood. In this study, we sought to investigate the influence of brain monoamine function on Stroop task performance in healthy participants using the established methods of acute dietary serotonin, dopamine, and combined monoamine depletion. The study was a double-blind placebo controlled design in which 12 healthy male participants completed the Stroop task under four acute treatment conditions: (a) balanced/placebo control, (b) acute tryptophan depletion, (c) acute tyrosine/phenylalanine depletion, and (d) acute tyrosine/phenylalanine/tryptophan depletion (combined monoamine depletion). Decreased Stroop interference indicating improved attentional control was observed after both tryptophan depletion and tyrosine/phenylalanine depletion, while there was no significant change in interference after combined monoamine depletion. Findings suggest that reduced tonic dopamine or serotonin activity within specific neural circuits (such as the striatum, anterior cingulate, or prefrontal cortex) may play a critical role in attentional control, possibly by improving gating of information via reducing noise in monoaminergic systems. These findings enhance our understanding of the neurochemical basis of attentional control and the possible cause of attentional control deficits in schizophrenia.

Role of atypical opiates in OCD. Experimental approach through the study of 5-HT(2A/C) receptor-mediated behavior

RATIONALE

The selective serotonin (5-HT) reuptake inhibitors (SSRIs) represent the first-line pharmacotherapy for obsessive-compulsive disorder (OCD), and atypical antipsychotic drugs, which block 5-HT2A receptors, are used in augmentation strategies. Opiate drugs are also effective in treatment-refractory OCD and Tourette syndrome. The 5-HT2A-related behavior (i.e., head twitch) has been related with tics, stereotypes, and compulsive symptoms observed in Tourette syndrome and OCD.

OBJECTIVES

The aim of this study was to explore whether 5-HT2A-related behavior is affected by atypical opiate drugs.

MATERIALS AND METHODS

Head-twitch response was induced in mice by administration of either 5-hydroxytryptophan (5-HTP) or the 5-HT2A/C agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI). Dose-effect curves of atypical opiate drugs [(+/-)-tramadol, (-)-methadone and levorphanol], morphine, and other psychoactive drugs (fluvoxamine, desipramine, nefazodone, and clozapine) were performed. Opioid mechanisms were investigated by administration of naloxone.

RESULTS

All the opiates tested reduced both 5-HTP and DOI-induced behavior in a naloxone-reversible fashion, atypical opiates being more effective. The effects of the other drugs depended on the protocol, clozapine being the most effective.

CONCLUSIONS

Combined 5-HT and opioid properties result in a greater efficacy in antagonizing 5-HT2A-related behavior. These results provide behavioral evidence to support convergent effects of the 5-HT and opioid systems in discrete brain areas, offering the potential for therapeutic advances in the management of refractory stereotypes and compulsive behaviors.

Behavioral disinhibition requires dopamine receptor activation

RATIONALE

Behavioral disinhibition is a manifestation of impulsive behavior that is prominent in the psychopathology of various psychiatric disorders, but the underlying neural mechanisms are unclear. Behavioral disinhibition can be investigated by measuring premature responding in the 5-choice serial reaction time task (5-CSRTT) in which attentional parameters can be measured as well.

OBJECTIVE

The objective of the study was to investigate the involvement of dopamine neurotransmission in behavioral disinhibition using the 5-CSRTT in rats.

METHODS

The effects of amphetamine, cocaine, nicotine, the dopamine reuptake inhibitor GBR 12909, the noradrenaline reuptake inhibitor desipramine, the dopamine D1 receptor antagonist SCH 23390, and the dopamine D2 receptor antagonist eticlopride were studied in rats that were well-trained in the 5-CSRTT. Subsequently, the effects of amphetamine, cocaine, and nicotine were tested after pretreatment with SCH 23390 or eticlopride.

RESULTS

What amphetamine, cocaine, and nicotine had in common is that they increased premature responding. However, these drugs had distinct effects on attentional parameters. GBR 12909 also enhanced premature responding, whereas desipramine reduced it. Eticlopride by itself had no effect on premature responding but it attenuated the increases in this parameter evoked by amphetamine, cocaine, or nicotine. SCH 23390 reduced premature responding on its own and also reduced its drug-induced enhancement.

CONCLUSIONS

The present data show that behavioral disinhibition, i.e., the inability to withhold a premature response, is a common effect of drugs of abuse and that this effect is the result of enhanced dopaminergic neurotransmission. In addition, dopamine D1 and D2 receptors play important, but perhaps distinct roles, in inhibitory control of behavior.