Abnormal brain activity in prefrontal brain regions in abstinent marijuana users

Evidence that the COMT(Val158Met) polymorphism moderates sensitivity to stress in psychosis: an experience-sampling study

Gene-environment interactions involving the catechol-O-methyltransferase Val(158)Met polymorphism (COMT(Val158Met)) have been implicated in the causation of psychosis. Evidence from general population studies suggests that Met/Met subjects are sensitive to stress, a trait associated with psychosis. We hypothesized that the Met allele would moderate the effects of stress on negative affect (NA) in controls, and on NA and psychosis in patients with a psychotic disorder. Thirty-one patients with a psychotic disorder and comorbid cannabis misuse and 25 healthy cannabis users were studied with the experience sampling method (ESM), a structured diary technique assessing current context and emotional and psychotic experiences in daily life. A significant interaction between COMT(Val158Met) genotype and ESM stress in the model of NA was found for patients (interaction chi(2) = 7.4, P = 0.02), but not for controls (interaction chi(2) = 3.8, P = 0.15). In the model of ESM psychosis, a significant interaction between COMT(Val158Met) genotype and ESM stress was also apparent (interaction chi(2) = 11.6, P < 0.01), with Met/Met patients showing the largest increase in psychotic experiences as well as NA in reaction to ESM stress. The findings suggest that the COMT(Val158Met) polymorphism moderates affective and psychotic responses to stress in patients with psychosis, providing evidence for gene-environment interaction mechanisms in the formation of psychotic symptoms.

Understanding drug addiction: a neuropsychological perspective

The purpose of the present review is to describe the neuropsychological correlates of long-term substance abuse and to discuss the findings within the context of premorbid vulnerabilities, comorbidity and adolescent neurodevelopment. The authors critically review key findings from the neuropsychological literature related to the long-term sequelae of alcohol, cannabis, inhalant, opiates, psychostimulants and ecstasy use. Leading electronic databases such as PubMed were searched to identify relevant studies published in the past 20 years. References identified from bibliographies of pertinent articles and books in the field were also collected and selectively reviewed. Across substances, individuals with long-term abuse consistently demonstrate neuropsychological impairments of executive (inhibitory) control, working memory and decision making, together with neurobiological abnormalities involving frontotemporal and basal ganglia circuits. In some instances these deficits are dose dependent, implying that they are a direct consequence of prolonged drug exposure. However, comorbid behavioural, personality and mental health problems are common among drug-using populations and are associated with similar neuropsychological deficits. Presented herein is a neuropsychological model of addictive behaviour that highlights the complex interplay between cognition, brain maturation, psychopathology and drug exposure.

Cerebral metabolism and mood in remitted opiate dependence

BACKGROUND

Opiate-dependent individuals are prone to dysphoria that may contribute to treatment failure. Methadone-maintenance therapy (MMT) may mitigate this vulnerability, but controversy surrounds its long-term use. Little is known about the neurobiology of mood dysregulation in individuals receiving or removed from MMT.

METHODS

Fifteen opiate-abstinent and 12 methadone-maintained, opiate-dependent subjects, who lacked other Axis I pathology, and 13 control subjects were compared on the Cornell Dysthymia Rating Scale (CDRS) and regional cerebral glucose metabolism (rCMRglc) using [(18)F]fluorodeoxyglucose positron emission tomography.

RESULTS

CDRS scores showed no group differences. Opiate-abstinent subjects had lower rCMRglc than control subjects in the bilateral perigenual anterior cingulate cortex (ACC), left mid-cingulate cortex, left insula and right superior frontal cortex. Methadone-maintained subjects exhibited lower rCMRglc than control subjects in the left insula and thalamus. In opiate-abstinent subjects, rCMRglc in the left perigenual ACC and mid-cingulate cortex correlated positively with CDRS scores.

CONCLUSIONS

In remitted heroin dependence, opiate-abstinence is associated with more widespread patterns of abnormal cortical activity than MMT. Aberrant mood processing in the left perigenual ACC and mid-cingulate cortex, seen in opiate-abstinent individuals, is absent in those receiving MMT, suggesting that methadone may improve mood regulation in this population.

The differential relationship between cocaine use and marijuana use on decision-making performance over repeat testing with the Iowa Gambling Task

Decision-making deficits are a robust cognitive correlate of substance abuse, but few studies have addressed the long-term differential associations of cocaine use and marijuana (MJ) use on decision-making. This study utilized the Iowa Gambling Task (IGT), a widely used measure of decision-making, to investigate the relationship between cocaine and MJ use and IGT learning. We analyzed between and within group differences across two consecutive testing sessions in abstinent users of either MJ or cocaine. We assessed long-term correlates of the use of these drugs by evaluating users after 25 days of enforced abstinence. Results showed that both cocaine users and MJ users performed worse than controls on the total IGT net score. All groups showed learning between Session 1 and Session 2, but the cocaine users showed the smallest increase in performance. The pattern of learning from the beginning to the end (block x block) of the IGT (Session 2) was different for the drug groups, with the cocaine group showing more learning than the MJ group. Dose-related measures of cocaine use (g/week) and MJ use (joints/week) predicted IGT performance (the heavier the drug use the lower the performance). Differential correlates of cocaine use and MJ use on decision-making learning may have important implications for the development of novel treatment interventions.

Neuropsychological functioning in adolescent marijuana users: subtle deficits detectable after a month of abstinence

In adults, studies examining the long-lasting cognitive effects of marijuana use demonstrate subtle deficits in attention, executive function, and memory. Because neuromaturation continues through adolescence, these results cannot necessarily generalize to adolescent marijuana users. The goal of this study was to examine neuropsychological functioning in abstinent marijuana using and demographically similar control adolescents. Data were collected from 65 adolescent marijuana users (n=31, 26% females) and controls (n=34, 26% females) 16-18 years of age. Extensive exclusionary criteria included independent psychiatric, medical, and neurologic disorders. Neuropsychological assessments were conducted after>23 days of monitored abstinence. After controlling for lifetime alcohol use and depressive symptoms, adolescent marijuana users demonstrated slower psychomotor speed (p<.05), and poorer complex attention (p<.04), story memory (p<.04), and planning and sequencing ability (p<.001) compared with controls. Post hoc analysis revealed that the number of lifetime marijuana use episodes was associated with poorer cognitive function, even after controlling for lifetime alcohol use. The general pattern of results suggested that, even after a month of monitored abstinence, adolescent marijuana users demonstrate subtle neuropsychological deficits compared with nonusers. It is possible that frequent marijuana use during adolescence may negatively influence neuromaturation and cognitive development.

The role of cognitive control in cocaine dependence

While hedonic and reward-related processes are central to drug use and dependence, this article focuses on the contribution that cognitive processes may make to addiction. In particular, attention is drawn to those processes involved in exercising control over behavior as drug dependence is characterized by risky, impulsive behavior. Functional neuroimaging implicates prefrontal deficits in cocaine dependence with an emerging picture of cocaine users having attentional biases towards drug-related stimuli, poor performance in laboratory tests of inhibitory control, and compromised monitoring and evaluation of their behavior. Combined, these deficits may contribute to the continuation of use in dependent individuals and may qualify as important targets for therapeutic interventions.

Effect of cigarette smoking on prefrontal cortical function in nondeprived smokers performing the Stroop Task

Some reports indicate that cigarette smoking can help smokers focus attention, even when they have not abstained from smoking for a substantial period of time (eg, >1 h). Understanding the mechanisms by which smoking affects attention may help in designing smoking cessation treatments. Thirteen nonsmokers and nine smokers participated in two tests of blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI). During fMRI, the participants performed the Stroop Task. There was a 15-min break between the two tests. During the break, each smoker smoked one cigarette. For smokers, the first test began 45-60 min after the last cigarette of ad libitum smoking. The differences in BOLD signal changes between Stroop conditions (ie, incongruent minus congruent) showed a group x test interaction in the right precentral sulcus, including the putative human frontal eye field (FEF). Smokers, but not nonsmokers, showed greater changes (relative to rest) in BOLD signal in the incongruent than in the congruent condition in the first fMRI test but not in the second. Even after brief abstinence from smoking, therefore, smokers exhibit compromised functional efficiency in the right FEF and adjacent precentral sulcus in a test of selective attention; and smoking ameliorates this condition.

Location, location: using functional magnetic resonance imaging to pinpoint brain differences relevant to stimulant use

AIMS

The purpose of this review is to summarize the neural substrate dysfunctions and disrupted cognitive, affective and experiential processes observed in methamphetamine and cocaine-dependent individuals.

METHODS

We reviewed all publications in PubMed that conducted comparison studies between healthy volunteers and cocaine-, amphetamine- or methamphetamine-dependent individuals using functional magnetic resonance imaging.

RESULTS

Stimulant dependence is characterized by a distributed alteration of functional activation to a number of experimental paradigms. Attenuated anterior and posterior cingulate activation, reduced inferior frontal and dorsolateral prefrontal cortex activation and altered posterior parietal activation point towards an inadequate demand-specific processing of information. Processes reported most consistently to be deficient in these functional neuroimaging studies include inhibitory control, executive functioning and decision-making.

CONCLUSION

One emerging theme is that stimulant-dependent individuals show specific, rather than generic, brain activation differences, i.e. instead of showing more or less brain activation regardless of task, they exhibit process-related brain activation differences that are consistent with a shift from context-specific, effortful processing to more stereotyped, habitual response generation.

Effects of frequent cannabis use on hippocampal activity during an associative memory task

Interest is growing in the neurotoxic potential of cannabis on human brain function. We studied non-acute effects of frequent cannabis use on hippocampus-dependent associative memory, investigated with functional Magnetic Resonance Imaging (fMRI) in 20 frequent cannabis users and 20 non-users matched for age, gender and IQ. Structural changes in the (para)hippocampal region were measured using voxel-based morphometry (VBM). Cannabis users displayed lower activation than non-users in brain regions involved in associative learning, particularly in the (para)hippocampal regions and the right dorsolateral prefrontal cortex, despite normal performance. VBM-analysis of the (para)hippocampal regions revealed no differences in brain tissue composition between cannabis users and non-users. No relation was found between (para)hippocampal tissue composition and the magnitude of brain activity in the (para)hippocampal area. Therefore, lower brain activation may not signify neurocognitive impairment, but could be the expression of a non-cognitive variable related to frequent cannabis use, for example changes in cerebral perfusion or differences in vigilance.

Anterior cingulate grey-matter deficits and cannabis use in first-episode schizophrenia

BACKGROUND

Despite the high prevalence of cannabis use in schizophrenia, few studies have examined the potential relationship between cannabis exposure and brain structural abnormalities in schizophrenia.

AIMS

To investigate prefrontal grey and white matter regions in patients experiencing a first episode of schizophrenia with an additional diagnosis of cannabis use or dependence (n=20) compared with similar patients with no cannabis use (n=31) and healthy volunteers (n=56).

METHOD

Volumes of the superior frontal gyrus, anterior cingulate gyrus and orbital frontal lobe were outlined manually from contiguous magnetic resonance images and automatically segmented into grey and white matter.

RESULTS

Patients who used cannabis had less anterior cingulate grey matter compared with both patients who did not use cannabis and healthy volunteers.

CONCLUSIONS

A defect in the anterior cingulate is associated with a history of cannabis use among patients experiencing a first episode of schizophrenia and could have a role in poor decision-making and in choosing more risky outcomes.

Functional correlates of verbal memory deficits emerging during nicotine withdrawal in abstinent adolescent cannabis users

BACKGROUND

Cannabis remains the most widely used illicit substance by adolescents and is typically consumed by this population in the context of ongoing tobacco use. Human studies have shown that both cannabis and tobacco exert effects on cognitive function; however, little is known about possible interacting effects of these drugs on brain function and cognition during adolescent development.

METHODS

Verbal learning and memory were assessed in 20 adolescent users of tobacco and cannabis and 25 adolescent tobacco users with minimal history of cannabis use. Functional magnetic resonance imaging was used to examine brain function and functional connectivity while a subset of these subjects performed a verbal working memory task.

RESULTS

Delayed recall of verbal stimuli deteriorated during nicotine withdrawal among cannabis users but not among comparison subjects. During high verbal working memory load, nicotine withdrawal selectively increased task-related activation of posterior cortical regions and was associated with disruption of frontoparietal connectivity in adolescent cannabis users relative to comparison subjects.

CONCLUSIONS

These observations suggest that cannabis use during adolescent development may disrupt neurocircuitry supporting verbal memory formation and that deficits associated with disruption of these neurocircuits are unmasked during nicotine withdrawal.

Insights from recent positron emission tomographic studies of drug abuse and dependence

PURPOSE OF REVIEW

Recent positron emission tomographic studies demonstrate a variety of abnormalities in the brains of addicted individuals. This review aims to discuss and highlight these findings.

RECENT FINDINGS

The recent findings are as follows: (a) the reward response to an addictive substance is associated with increased dopamine release in the striatum. (b) Activation of the orbitofrontal region is involved in the reinforcing properties of a drug. (c) Behavioral, cognitive and affective abnormalities are associated with alterations in specific brain networks and regions (e.g., prefrontal cortices) in drug abusers. (d) Personality traits may play a role in the susceptibility to addiction and the brain's responses to drugs. (e) Sex-differences exist for cue-induced craving. (f) Several studies have confirmed decreased D2 receptors in drug users, which is associated with increased salience to drug cues. (g) Serotonergic transporters are decreased in the current users of 3,4-methylene-deoxy-methamphetamine but found normal in the past users of 3,4-methylene-deoxy-methamphetamine.

SUMMARY

Abnormalities in the dopaminergic, opioid, and serotonergic systems in drug abusers are seen in positron emission tomography scans. Decreased D2 receptor densities in drug users, whether premorbid or the consequence of substance misuse, imply a source of the susceptibility to relapse of this population. Insights from these studies could lead to better treatment approaches targeting specific neurotransmitter systems.

Long-term effects of frequent cannabis use on working memory and attention: an fMRI study

RATIONALE

Excessive use of cannabis may have long-term effects on cognitive abilities. Mild impairments have been found in several cognitive domains, particularly in memory and attention. It is not clear, however, whether these effects also occur with moderate, recreational use of cannabis. Furthermore, little is known about underlying brain correlates.

OBJECTIVES

The aim of this study is to assess brain function in frequent but relatively moderate cannabis users in the domains of working memory and selective attention.

METHODS

Functional magnetic resonance imaging was used to examine verbal working memory and visuo-auditory selective attention in ten frequent cannabis users (after 1 week of abstinence) and ten non-using healthy controls. Groups were similar in age, gender and estimated IQ.

RESULTS

Cannabis users and controls performed equally well during the working memory task and the selective attention task. Furthermore, cannabis users did not differ from controls in terms of overall patterns of brain activity in the regions involved in these cognitive functions. However, for working memory, a more specific region-of-interest analysis showed that, in comparison to the controls, cannabis users displayed a significant alteration in brain activity in the left superior parietal cortex.

CONCLUSION

No evidence was found for long-term deficits in working memory and selective attention in frequent cannabis users after 1 week of abstinence. Nonetheless, frequent cannabis use may affect brain function, as indicated by altered neurophysiological dynamics in the left superior parietal cortex during working memory processing.

Combined and independent effects of chronic marijuana use and HIV on brain metabolites

The effects of chronic marijuana (MJ) use on brain function remain controversial. Because MJ is often used by human immunodeficiency virus (HIV) patients, the aim of this study was to evaluate whether chronic MJ use and HIV infection are associated with interactive or additive effects on brain chemistry and cognitive function. We evaluated 96 subjects (30 seronegative nondrug users, 24 MJ users, 21 HIV without MJ use, 21 HIV + MJ) using proton magnetic resonance spectroscopy and a battery of neuropsychological tests. The two primarily abstinent MJ user groups showed no significant differences on calculated estimates of lifetime grams of delta9-tetrahydrocannabinol exposure, despite some differences in usage pattern. The two HIV groups also had similar HIV disease severity (CD4 cell count, plasma viral load, HIV dementia staging, Karnofsky score). On two-way analyses of covariance, HIV infection (independent of MJ) was associated with trends for reduced N-acetyl aspartate (NA) in the parietal white matter and increased choline compounds (CHO) in the basal ganglia. In contrast, MJ (independent of HIV) was associated with decreased basal ganglia NA (-5.5%, p = 0.05), CHO (-10.6%, p = 0.04), and glutamate (-9.5%, p = 0.05), with increased thalamic creatine (+6.1%, p = 0.05). HIV + MJ was associated with normalization of the reduced glutamate in frontal white matter (interaction p = 0.01). After correction for age, education, or mood differences, MJ users had no significant abnormalities on neuropsychological test performance, and HIV subjects only had slower reaction times. These findings suggest chronic MJ use may lead to decreased neuronal and glial metabolites, but may normalize the decreased glutamate in HIV patients.

Is there evidence for neural compensation in attention deficit hyperactivity disorder? A review of the functional neuroimaging literature

This article reviews evidence for the presence of a compensatory, alternative, neural system and its possible link to associated processing strategies in children and adults with attention deficit hyperactivity disorder (ADHD). The article presents findings on a region by region basis that suggests ADHD should be characterized not only by neural hypo-activity, as it is commonly thought but neural hyperactivity as well, in regions of the brain that may relate to compensatory brain and behavioral functioning. In this context studies from the functional neuroimaging literature are reviewed. We hypothesize that impaired prefrontal (PFC) and anterior cingulate (ACC) cortex function in ADHD reduces the ability to optimally recruit subsidiary brain regions and strategies to perform cognitive tasks. The authors conclude that healthy individuals can recruit brain regions using visual, spatial or verbal rehearsal for tasks as needed. In contrast, individuals with ADHD may be less able to engage higher order executive systems to flexibly recruit brain regions to match given task demands. This may result in greater reliance on neuroanatomy that is associated with visual, spatial, and motoric processing rather than verbal strategies. The authors speculate that this impaired flexibility in recruiting brain regions and associated strategies limits adaptation to new cognitive demands as they present and may require more effortful processing.

Cannabinoids and prefrontal cortical function: Insights from preclinical studies

Marijuana use has been associated with disordered cognition across several domains influenced by the prefrontal cortex (PFC). Here, we review the contribution of preclinical research to understanding the effects of cannabinoids on cognitive ability, and the mechanisms by which cannabinoids may affect the neurochemical processes in the PFC that are associated with these impairments. In rodents, acute administration of cannabinoid agonists produces deficits in working memory, attentional function and reversal learning. These effects appear to be largely dependent on CB1 cannabinoid receptor activation. Preclinical studies also indicate that the endogenous cannabinoid system may tonically regulate some mnemonic processes. Effects of cannabinoids on cognition may be mediated via interaction with neurochemical processes in the PFC and hippocampus. In the PFC, cannabinoids may alter dopaminergic, cholinergic and serotonergic transmission. These mechanisms may underlie cognitive impairments observed following marijuana intake in humans, and may also be relevant to other disorders of cognition. Preclinical research will further enhance our understanding of the interactions between the cannabinoid system and cognitive functioning.

Recent cannabis abuse decreased stress-induced BOLD signals in the frontal and cingulate cortices of cocaine dependent individuals

Previous neuroimaging studies showed that use of marijuana can alter patterns of cortical activation during rest or a task challenge. We used functional magnetic resonance imaging to examine whether recent cannabis abuse contributed to stress-induced blood-oxygen-level-dependent (BOLD) contrast in a group of cocaine-dependent individuals. Emotional stress was induced using the script-guided imagery paradigm, in which subjects imagined being in a real-life stressful situation and, as a control, in a neutral situation, while BOLD signals of their brain were acquired with a 1.5 T scanner. Abstinent cocaine-dependent subjects with recent marijuana abuse (n=8) were compared with abstinent cocaine-dependent subjects who had not abused marijuana recently (n=18). The two groups were otherwise matched in their demographic characteristics and drug use history. All subjects were abstinent for at least 15 days and drug free as confirmed by urine drug screening before the imaging session. Recent cannabis abusers demonstrated hypo-activation in frontal cortical areas including the perigenual anterior cingulate during increased emotional stress. In contrast, at the same statistical threshold, no brain regions showed increased activation in recent cannabis abusers compared with non-abusers. The group difference in the perigenual anterior cingulate remained even when lifetime cocaine and alcohol consumption was accounted for in covariance analysis. These results provide evidence that recent cannabis abuse is associated with decreased activation in the frontal cortex during an emotional stress task. The results suggest an abnormal cognitive control mechanism during affective processing in association with heavy cannabis use.

Stroop interference task and single-photon emission tomography in anorexia: a preliminary report

OBJECTIVE

The aim of this preliminary study was to investigate the physiologic substrate of executive function in anorexia nervosa (AN) by assessing the relation between brain perfusion and Stroop interference task (SIT).

METHOD

The classical SIT test and brain single-photon emission tomography (SPET) were evaluated in 16 AN females (mean age = 23.69 +/- 8.68 years; mean body mass index [BMI] = 16.19 +/- 1.53 kg/m2). The relation between the two examinations was searched by statistical parametric mapping (SPM 99) with a height threshold of p = .001.

RESULTS

An abnormally low or a borderline SIT value was found in 25% of patients. A significant correlation between the SIT score and brain perfusion was found in the superior frontal gyrus of both hemispheres (Brodmann's area [BA] 6 in both hemispheres and BA 8 in the right hemisphere). No correlation was found in the anterior cingulate gyrus.

CONCLUSION

BA 6 and BA 8 and the anterior cingulate are believed to be the basis of both error detection and immediate correction. Activity of BA 6 and BA 8 reflects this executive task in AN patients as well, whereas the lack of correlation in the anterior cingulate may suggest its blunted activity in AN patients, similarly to what is shown in other conditions characterized by impaired executive function, such as patients with depression, patients with schizophrenia, and abstinent drug abusers. However, these findings should still be quoted as preliminary, given some limitations of the study design, such as the lack of a control group, and the unfeasibility of controlling some relevant confounding variables, such as psychiatric comorbidity, medication, and the time interval between examinations, mainly deriving from the relatively few patients studied.

Neurocognitive insights into substance abuse

Cognitive studies are revealing key aspects of how drug abusers monitor and respond to negative feedback differently from non-abusers, and in doing so are adding an important piece to the conceptual puzzle that must be solved to understand, treat, and prevent drug abuse. In this review, we bring together two quite different lines of research, one addressing the selection of gambles in a risky decision task, and the other focused on imaging neural systems related to the detection and processing of errors. We suggest that diminished behavioural control, which is a cardinal feature of drug abuse, may be linked to alterations in the psychological and neural mechanisms that detect error signals and which, in turn, lead to optimization of behavioural responses.

A role for cannabinoid CB1 receptors in mood and anxiety disorders

Mood and anxiety disorders, the most prevalent of the psychiatric disorders, cause immeasurable suffering worldwide. Despite impressive advances in pharmacological therapies, improvements in efficacy and side-effect profiles are needed. The present literature review examines the role that the endocannabinoid system may play in these disorders and the potential value of targeting this system in the search for novel and improved medications. Cannabis and its major psychoactive component (-)-trans-delta9-tetrahydrocannabinol, have profound effects on mood and can modulate anxiety and mood states. Cannabinoid receptors and other protein targets in the central nervous system (CNS) that modulate endocannabinoid function have been described. The discovery of selective modulators of some of these sites that increase or decrease endocannabinoid neurotransmission, primarily through the most prominent of the cannabinoid receptors in the CNS, the CB1 receptors, combined with transgenic mouse technology, has enabled detailed investigations into the role of these CNS sites in the regulation of mood and anxiety states. Although data point to the involvement of the endocannabinoid system in anxiety states, the pharmacological evidence seems contradictory: both anxiolytic- and anxiogenic-like effects have been reported with both endocannabinoid neurotransmission enhancers and blockers. Due to advances in the development of selective compounds directed at the CB1 receptors, significant progress has been made on this target. Recent biochemical and behavioural findings have demonstrated that blockade of CB1 receptors engenders antidepressant-like neurochemical changes (increases in extracellular levels of monoamines in cortical but not subcortical brain regions) and behavioural effects consistent with antidepressant/antistress activity in rodents.

Brain activity in cigarette smokers performing a working memory task: effect of smoking abstinence

BACKGROUND

When nicotine-dependent human subjects abstain from cigarette smoking, they exhibit deficits in working memory. An understanding of the neural substrates of such impairments may help to understand how nicotine affects cognition. Our aim, therefore, was to identify abnormalities in the circuitry that mediates working memory in nicotine-dependent subjects after they initiate abstinence from smoking.

METHODS

We used blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI) to study eight smokers while they performed a letter version of the N-Back working memory task under satiety (< or = 1.5 hours abstinence) and abstinence (> or = 14 hours abstinence) conditions.

RESULTS

Task-related activity in the left dorsal lateral prefrontal cortex (DLPFC) showed a significant interaction between test session (satiety, abstinence) and task load (1-back, 2-back, and 3-back). This interaction reflected the fact that task-related activity in the satiety condition was relatively low during performance of the 1-back task but greater at the more difficult task levels, whereas task-related activity in the abstinence condition was relatively high at the 1-back level and did not increase at the more difficult task levels.

CONCLUSIONS

We conclude that neural processing related to working memory in the left DLPFC is less efficient during acute abstinence from smoking than at smoking satiety.

Neuroimaging of marijuana smokers during inhibitory processing: a pilot investigation

Neuropsychological investigations of substance abusers have reported impairments on tasks mediated by the frontal executive system, including functions associated with behavioral inhibition and decision making. The higher order or executive components which are involved in decision making include selective attention and short term storage of information, inhibition of response to irrelevant information, initiation of response to relevant information, self-monitoring of performance, and changing internal and external contingencies in order to "stay the course" towards the ultimate goal. Given the hypothesized role of frontal systems in decision making and the previous evidence that executive dysfunctions and structural brain changes exist in subjects who use illicit drugs, we applied fMRI and diffusion tensor imaging (DTI) techniques in a pilot investigation of heavy cannabis smokers and matched control subjects while performing a modification of the classic Stroop task. Marijuana smokers demonstrated significantly lower anterior cingulate activity in focal areas of the anterior cingulate cortex and higher midcingulate activity relative to controls, although both groups were able to perform the task within normal limits. Normal controls also demonstrated increased activity within the right dorsolateral prefrontal cortex (DLPFC) during the interference condition, while marijuana smokers demonstrated a more diffuse, bilateral pattern of DLPFC activation. Similarly, although both groups performed the task well, marijuana smokers made more errors of commission than controls during the interference condition, which were associated with different brain regions than control subjects. These findings suggest that marijuana smokers exhibit different patterns of BOLD response and error response during the Stroop interference condition compared to normal controls despite similar task performance. Furthermore, DTI measures in frontal regions, which include the genu and splenium of the corpus callosum and bilateral anterior cingulate white matter regions, showed no between group differences in fractional anisotropy (FA), a measure of directional coherence within white matter fiber tracts, but a notable increase in trace, a measure of overall isotropic diffusivity in marijuana smokers compared to controls. Overall, results from the present study indicate significant differences in the magnitude and pattern of signal intensity change within the anterior cingulate and the DLPFC during the Stroop interference subtest in chronic marijuana smokers compared to normal controls. Furthermore, although chronic marijuana smokers were able to perform the task reasonably well, the functional activation findings suggest they utilize different cortical processes from the control subjects in order to do so. Findings from this study are consistent with the notion that substance abusers demonstrate evidence of altered frontal neural function during the performance of tasks that involve inhibition and performance monitoring, which may affect the ability to make decisions.

[Brain effects of cannabis--neuroimaging findings]

Cannabis is the most widely used illicit drug. Despite this, only a small number of studies have investigated the long-term neurotoxic consequences of cannabis use. Structural and functional neuroimaging techniques are powerful research tools to investigate possible cannabis-induced pathophysiological changes. A computer literature review was conducted in the MEDLINE and PsycLIT databases between 1966 and November of 2004 with the search terms 'cannabis', 'marijuana', 'neuroimaging', 'magnetic resonance', 'computed tomography', 'positron emission tomography', 'single photon emission computed tomography", 'SPET', 'MRI' and 'CT'. Structural neuroimaging studies have yielded conflicting results. Most studies report no evidence of cerebral atrophy or regional changes in tissue volumes, and one study suggested that long-term users who started regular use on early adolescence have cerebral atrophy as well as reduction in gray matter. However, several methodological shortcomings limit the interpretation of these results. Functional neuroimaging studies have reported increases in neural activity in regions that may be related with cannabis intoxication or mood-change effects (orbital and mesial frontal lobes, insula, and anterior cingulate) and decreases in activity of regions related with cognitive functions impaired during acute intoxication. The important question whether residual neurotoxic effects occur after prolonged and regular use of cannabis remains unclear, with no study addressing this question directly. Better designed neuroimaging studies, combined with cognitive evaluation, may be elucidative on this issue.

Risky decision making and the anterior cingulate cortex in abstinent drug abusers and nonusers

Risky decision making is a hallmark behavioral phenotype of drug abuse; thus, an understanding of its biological bases may inform efforts to develop therapies for addictive disorders. A neurocognitive task that measures this function (Rogers Decision-Making Task; RDMT) was paired with measures of regional cerebral perfusion to identify brain regions that may underlie deficits in risky decision making in drug abusers. Subjects were abstinent drug abusers (> or =3 months) and healthy controls who underwent positron emission tomography scans with H(2)(15)O. Drug abusers showed greater risk taking and heightened sensitivity to rewards than control subjects. Both drug abusers and controls exhibited significant activations in a widespread network of brain regions, primarily in the frontal cortex, previously implicated in decision-making tasks. The only significant group difference in brain activation, however, was found in the left pregenual anterior cingulate cortex, with drug abusers exhibiting less task-related activation than control subjects. There were no significant correlations between neural activity and task performance within the control group. In the drug abuse group, on the other hand, increased risky choices on the RDMT negatively correlated with activation in the right hippocampus, left anterior cingulate gyrus, left medial orbitofrontal cortex, and left parietal lobule, and positively correlated with activation in the right insula. Drug abuse severity was related positively to right medial orbitofrontal activity. Attenuated activation of the pregenual ACC in the drug abusers relative to the controls during performance on the RDMT may underlie the abusers' tendency to choose risky outcomes.