Effects of chronic marijuana use on human cognition

Acute and non-acute effects of cannabis on brain functioning and neuropsychological performance

Cannabis has an ancient history of human use and is currently one of the most commonly used drugs worldwide. Understanding its impact on neurobehavioral functioning is of significant public health concern. In recent decades, substantial progress has been made in understanding the impact of cannabis use on neurobehavioral functioning. This has been fueled, in part, by characterization of an endocannabinoid signaling system in the brain through which cannabis exerts its psychoactive effects. Acute intoxication with cannabis causes marked changes in subjective mental status, brain functioning, and neuropsychological performance. Some of these changes are consistently detected and well characterized, yet others are not. Changes in brain functioning and neuropsychological performance are also reported after abstinence, but appear to be mild, circumscribed, and transient. On the other hand, functional neuroimaging often reveals subtle differences in the brain functioning of abstinent cannabis users compared with controls. The persistence and clinical significance of these differences, however, remains to be determined. Neuropsychological deficits and differences in brain functioning are most consistently observed only among frequent, heavy users, who are those most likely addicted to cannabis. The dire impact of drug addiction on a person's life and everyday functioning suggests that the large number of individuals addicted to cannabis experience substantial negative effects from its use. This manuscript reviews the scientific literature on the aforementioned topics in detail, providing evidence for converging findings, and highlighting areas in need of further investigation.

Brain imaging study of the acute effects of Delta9-tetrahydrocannabinol (THC) on attention and motor coordination in regular users of marijuana

PROCEDURE

Twelve regular users of marijuana underwent two positron emission tomography (PET) scans using [18F] Fluorodeoxyglucose (FDG), one while subject to the effects of 17 mg THC, the other without THC. In both sessions, a virtual reality maze task was performed during the FDG uptake period.

RESULTS

When subject to the effects of 17 mg THC, regular marijuana smokers hit the walls more often on the virtual maze task than without THC. Compared to results without THC, 17 mg THC increased brain metabolism during task performance in areas that are associated with motor coordination and attention in the middle and medial frontal cortices and anterior cingulate, and reduced metabolism in areas that are related to visual integration of motion in the occipital lobes.

CONCLUSION

These findings suggest that in regular marijuana users, the immediate effects of marijuana may impact on cognitive-motor skills and brain mechanisms that modulate coordinated movement and driving.

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.

The relationship between non-acute adolescent cannabis use and cognition

Research indicates that cannabis continues to be a popular illegal drug internationally. Furthermore, adolescent rates of use appear to be significant. Whilst the non-acute effect of cannabis use on adult cognition has been extensively researched, there has been less examination of adolescents. This study aimed to investigate the non-acute relationship between cannabis and cognitive function in a sample of adolescents with a continuum of cannabis use, taking into account additional predictor variables (psychiatric functioning, general functioning, demographics and other drug use). Seventy adolescents were recruited from clinical and community sources as well as through newspaper advertisements. After 12 hours abstinence from cannabis, adolescents completed a two-hour interview covering: demographics; alcohol and drug use history; drug use in the past 28 days; depression; further psychiatric functioning (including ADHD and Conduct Disorder); and cognitive functioning as measured by computerised tasks (CANTAB) and traditional pen and paper tests. Adolescents who were regular cannabis users (more than once a week) had a significantly poorer performance on four measures of cognitive function reflecting attention, spatial working memory and learning. Cannabis use remained an independent predictor of performance on the working memory and strategy measures after additional predictor variables were included in a multivariate regression analysis. The results suggest that aspects of adolescent cognitive function are independently related to the frequency of cannabis use beyond acute intoxication.

Effects of smoking marijuana on focal attention and brain blood flow

Using an attention task to control cognitive state, we previously found that smoking marijuana changes regional cerebral blood flow (rCBF). The present study measured rCBF during tasks requiring attention to left and right ears in different conditions. Twelve occasional marijuana users (mean age 23.5 years) were imaged with PET using [15O]water after smoking marijuana or placebo cigarettes as they performed a reaction time (RT) baseline task, and a dichotic listening task with attend-right- and attend-left-ear instructions. Smoking marijuana, but not placebo, resulted in increased normalized rCBF in orbital frontal cortex, anterior cingulate, temporal pole, insula, and cerebellum. RCBF was reduced in visual and auditory cortices. These changes occurred in all three tasks and replicated our earlier studies. They appear to reflect the direct effects of marijuana on the brain. Smoking marijuana lowered rCBF in auditory cortices compared to placebo but did not alter the normal pattern of attention-related rCBF asymmetry (i.e., greater rCBF in the temporal lobe contralateral to the direction of attention) that was also observed after placebo. These data indicate that marijuana has dramatic direct effects on rCBF, but causes relatively little change in the normal pattern of task-related rCBF on this auditory focused attention task.

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.

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.

Anxiety sensitivity interacts with marijuana use in the prediction of anxiety symptoms and panic-related catastrophic thinking among daily tobacco users

The present investigation evaluated whether anxiety sensitivity interacted with marijuana use in relation to the prediction of panic-relevant variables among young adult tobacco smokers (n=265). Approximately 73% of the sample was composed of current marijuana smokers, with 78.5% of this sub-sample using marijuana more than once per week. As expected, after covarying cigarettes per day, alcohol use, and negative affectivity, the interaction between marijuana use and anxiety sensitivity predicted anxiety symptoms and agoraphobic cognitions. Partially consistent with prediction, the interaction between frequency of marijuana use and anxiety sensitivity predicted only anxiety symptoms. These results are discussed in relation to better understanding the potential role of regular marijuana use and anxiety sensitivity for panic-relevant emotional vulnerability among regular tobacco smokers.

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.

Self-change: A pathway to cannabis abuse resolution

Long-term daily cannabis abusers (N = 25) who without treatment stopped using cannabis for at least one year were interviewed about their past substance use, antecedents to change, and factors supportive of change. Respondents' cannabis problems decreased in the year prior to their recovery compared to their lifetime use. Respondents described their successful quit attempts through structured interviews and autobiographical narratives. The narratives were content analyzed for factors related to recovery. The reports indicated that marijuana cessation was motivated more by internal than external factors, and the most common precipitants of quit attempts were cognitive anti-cannabis factors. The major reason reported by respondents for stopping cannabis was a change in how they viewed their cannabis use, followed by negative personal effects. The most common reported maintenance factors were avoidance of situations in which cannabis was used, changes in lifestyle, and the development of non-cannabis-related interests. Cognitive and respiratory functioning were also assessed. Lastly, more than 75% of respondents reported not seeking treatment because they believed it was not needed or because they wanted to quit on their own. Directions for future research are offered.

Nicotine and cognitive efficiency in alcoholics and illicit stimulant abusers: implications of smoking cessation for substance users in treatment

Cigarette smoking is prevalent among alcoholics and illicit substance abusers. However, the potentially confounding effect of nicotine on studies of cognition in detoxified substance users has rarely been addressed. In the current study of 87 participants, behavioral and electrophysiological indices of cognitive efficiency were measured in tobacco smokers from four groups: alcoholics, illicit stimulant abusers, concurrent abusers, and control subjects. Data were collected from 2001 to 2003. We hypothesized that acute nicotine administration would modify cognitive deficits in alcoholics and illicit stimulant abusing groups. An adaptation of the Rapid Visual Information Processing task was administered after stabilization of nicotine levels via a high- or low-dose transdermal nicotine patch. Across groups, increased nicotine dose was associated with decreased reaction time (p = .03). A group x nicotine dose interaction trend was noted in which increased nicotine was associated with increased correct responding within the alcoholic group (p = .02). No significant differences in electrophysiology were observed. These results suggest that nicotine may modify cognitive efficiency in alcoholics and illicit stimulant abusers, a concept with relevance to both the design of experimental work and the treatment of alcohol and illicit stimulant dependence. Further work is needed to determine whether this effect predominantly reflects facilitation of cognition function or alleviation of nicotine withdrawal.

Cannabis use, cognitive performance and mood in a sample of workers

There are well documented acute and chronic effects of cannabis use on mental functioning. However, less is known about any effects on cognition within the context of work and everyday life. The aim of the study was to examine any association between cannabis use and cognitive performance, mood and human error at work. Cannabis users and controls completed a battery of laboratory based computer tasks measuring mood and cognitive function pre- and post-work at the start and end of a working week. They also completed daily diaries reporting their work performance. Cannabis use was associated with impairment in both cognitive function and mood, though cannabis users reported no more workplace errors than controls. Cannabis use was associated with lower alertness and slower response organization. In addition, users experienced working memory problems at the start, and psychomotor slowing and poorer episodic recall at the end of the working week. This pattern of results suggests two possible effects. First a 'hangover'-type effect which may increase with frequency of use. Second a subtle effect on cognitive function, perhaps more apparent under cognitive load and/or fatigue, which may increase with more prolonged use. The results also highlight the importance of the timing of testing within the context and routine of everyday life.

A community based investigation of the association between cannabis use, injuries and accidents

There are well documented acute and chronic effects of cannabis use. However, less is known about any effects on safety within the context of work and everyday life. The aim of the study was to examine any association between cannabis use and injuries and accidents. A postal questionnaire survey was conducted among people selected at random from the electoral registers of Cardiff and Merthyr Tydfil. Cannabis use was associated with both minor injuries and accidents, particularly among those with high levels of other associated risk factors. Cannabis use was associated with a significant detrimental impact on safety. It is possible that this is linked to an amplification of other risk factors associated with accidents and injuries. This has potentially wide reaching implications particularly in the context of other work and lifestyle characteristics.

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.

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.

Differential effects of THC- or CBD-rich cannabis extracts on working memory in rats

Cannabinoid receptors in the brain (CB(1)) take part in modulation of learning, and are particularly important for working and short-term memory. Here, we employed a delayed-matching-to-place (DMTP) task in the open-field water maze and examined the effects of cannabis plant extracts rich in either Delta(9)-tetrahydrocannabinol (Delta(9)-THC), or rich in cannabidiol (CBD), on spatial working and short-term memory formation in rats. Delta(9)-THC-rich extracts impaired performance in the memory trial (trial 2) of the DMTP task in a dose-dependent but delay-independent manner. Deficits appeared at doses of 2 or 5 mg/kg (i.p.) at both 30 s and 4 h delays and were similar in severity compared with synthetic Delta(9)-THC. Despite considerable amounts of Delta(9)-THC present, CBD-rich extracts had no effect on spatial working/short-term memory, even at doses of up to 50 mg/kg. When given concomitantly, CBD-rich extracts did not reverse memory deficits of the additional Delta(9)-THC-rich extract. CBD-rich extracts also did not alter Delta(9)-THC-rich extract-induced catalepsy as revealed by the bar test. It appears that spatial working/short-term memory is not sensitive to CBD-rich extracts and that potentiation and antagonism of Delta(9)-THC-induced spatial memory deficits is dependent on the ratio between CBD and Delta(9)-THC.

Spatial working memory in heavy cannabis users: a functional magnetic resonance imaging study

RATIONALE

Many neuropsychological studies have documented deficits in working memory among recent heavy cannabis users. However, little is known about the effects of cannabis on brain activity.

OBJECTIVE

We assessed brain function among recent heavy cannabis users while they performed a working memory task.

METHODS

Functional magnetic resonance imaging was used to examine brain activity in 12 long-term heavy cannabis users, 6-36 h after last use, and in 10 control subjects while they performed a spatial working memory task. Regional brain activation was analyzed and compared using statistical parametric mapping techniques.

RESULTS

Compared with controls, cannabis users exhibited increased activation of brain regions typically used for spatial working memory tasks (such as prefrontal cortex and anterior cingulate). Users also recruited additional regions not typically used for spatial working memory (such as regions in the basal ganglia). These findings remained essentially unchanged when re-analyzed using subjects' ages as a covariate. Brain activation showed little or no significant correlation with subjects' years of education, verbal IQ, lifetime episodes of cannabis use, or urinary cannabinoid levels at the time of scanning.

CONCLUSIONS

Recent cannabis users displayed greater and more widespread brain activation than normal subjects when attempting to perform a spatial working memory task. This observation suggests that recent cannabis users may experience subtle neurophysiological deficits, and that they compensate for these deficits by "working harder"-calling upon additional brain regions to meet the demands of the task.

Repeated, intermittent delta(9)-tetrahydrocannabinol administration to rats impairs acquisition and performance of a test of visuospatial divided attention

The residual neuropsychological effects of marijuana abuse in man indicate a dysfunction of the attentional/executive systems. Moreover, experimental investigations suggest that repeated, intermittent (subchronic) Delta(9)-tetrahydrocannabinol (THC), the main psychoactive ingredient of marijuana, alters neurotransmission in the frontal cortex of rats and humans, a key neural site mediating attention and executive functions. In the present studies, the acquisition and performance of a test of visuospatial attention (the lateralized reaction time task) after subchronic THC administration (10.0 mg/kg twice daily for 14 days) was examined. Rats previously administered THC showed impairments in this self-paced version of the classic multiple-choice serial reaction time task, which persisted 14 days after the final drug administration. Longer time points were not examined. These attentional impairments were transiently reversible with an acute amphetamine (0.5 mg/kg) challenge. These behavioral data demonstrate that chronic THC administration to rats induces an attentional deficit, similar to that observed in humans who abuse marijuana. Finally, amphetamine's ability to reverse the attentional impairments provides indirect evidence that monoaminergic deficits may be linked to the cognitive dysfunction.

Relation of premorbid cognitive abilities to substance users' problems at treatment intake and improvements with substance abuse treatment and case management

Substance users' premorbid cognitive skills and abilities, assessed by their scores on standardized achievement tests (Iowa Tests of Basic Skills) during the fourth grade of elementary school, were examined as predictors of their problems at treatment intake and improvements with substance abuse treatment and case management over a 12-month follow-up period. Clients were assigned to four groups differing in the type of case management provided. Among 213 clients meeting criteria for the study, problems in several areas at treatment intake (assessed by the Addiction Severity Index) were milder for those with higher, relative to lower, premorbid cognitive abilities. Relationships of premorbid cognitive abilities to improvements at follow-ups were more prominent for clients who worked with case managers primarily via telecommunications than for clients who generally met with case managers at the substance abuse treatment or an independent agency, or who received only limited case management services from primary drug counselors. Thorough utilization of an interactive voice response system available to the telecommunications group might have placed demands on cognitive abilities that were less emphasized in the other groups. Alcohol differed markedly from other drugs in how improvements in problems were related to premorbid cognitive abilities among clients in the telecommunications group, and, to a lesser extent, among clients who generally met with case managers in person. Premorbid cognitive abilities should be considered when examining how outcomes are related to clients' characteristics and how maximally effective treatments can be selected based on clients' characteristics.

Early-onset cannabis use and cognitive deficits: what is the nature of the association?

BACKGROUND

Individuals who initiate cannabis use at an early age, when the brain is still developing, might be more vulnerable to lasting neuropsychological deficits than individuals who begin use later in life.

METHODS

We analyzed neuropsychological test results from 122 long-term heavy cannabis users and 87 comparison subjects with minimal cannabis exposure, all of whom had undergone a 28-day period of abstinence from cannabis, monitored by daily or every-other-day observed urine samples. We compared early-onset cannabis users with late-onset users and with controls, using linear regression controlling for age, sex, ethnicity, and attributes of family of origin.

RESULTS

The 69 early-onset users (who began smoking before age 17) differed significantly from both the 53 late-onset users (who began smoking at age 17 or later) and from the 87 controls on several measures, most notably verbal IQ (VIQ). Few differences were found between late-onset users and controls on the test battery. However, when we adjusted for VIQ, virtually all differences between early-onset users and controls on test measures ceased to be significant.

CONCLUSIONS

Early-onset cannabis users exhibit poorer cognitive performance than late-onset users or control subjects, especially in VIQ, but the cause of this difference cannot be determined from our data. The difference may reflect (1). innate differences between groups in cognitive ability, antedating first cannabis use; (2). an actual neurotoxic effect of cannabis on the developing brain; or (3). poorer learning of conventional cognitive skills by young cannabis users who have eschewed academics and diverged from the mainstream culture.

Visual search in long-term cannabis users with early age of onset

The present research tested the hypothesis that there is a specific deficit in visual scanning in chronic users of cannabis with early onset of their drug consumption (age 14 to 16). 17 users and 20 control participants were asked to search for targets on a 5 x 5 stimulus array while their eye movements were monitored. Cannabis users showed less effective search behavior, including longer response times and more fixations at about the same error level. Search patterns were more conservative and included more frequent reinspections of previously fixated areas. In sum, the results point to two loci of adverse effects: an impairment in visual short-term memory, and less effective visual processing at a more strategic, top down controlled level.

Nonacute (residual) neuropsychological effects of cannabis use: a qualitative analysis and systematic review

Because there is a possibility that cannabis or cannabis-like molecules might be used as treatments for certain conditions in the future, it becomes important to consider the possible adverse effects of these compounds. In this paper, the authors review the evidence for persisting effects of nonacute cannabis use on the central nervous system, as reflected by alteration in neuropsychological performance. From the 40 articles that met criteria for inclusion in this review, the authors could not detect consistent evidence for persisting neuropsychological deficits in cannabis users; however, 22 of the 40 studies reported at least some subtle impairments. The inability to reach a firm conclusion results largely from methodological limitations inherent in most studies. These are considered in detail to inform future studies on (nonacute) consequences of cannabis consumption on cognitive abilities.

Functions of cannabinoid receptors in the hippocampus

Marijuana smoking is recognised to impair human cognition and learning, but the mechanisms by which this occurs are not well characterised. This article focuses exclusively on the hippocampus to review the effects of cannabinoids on hippocampal function and evaluate the evidence that hippocampal cannabinoid receptors play a role in learning and formation of memory. Activation of cannabinoid receptors inhibits release of a variety of neurotransmitters, and modulates a number of intrinsic membrane conductances. Suppression of inhibitory GABAergic synaptic transmission has been repeatedly described, but whether there is also control of excitatory glutamatergic transmission is more controversial. The recognition that the commonly used WIN55,212-2 also acts via non-cannabinoid receptors may help resolve this issue. The involvement of endocannabinoids in depolarisation induced suppression of inhibition (DSI) and the demonstration that activation of metabotropic glutamate receptors can stimulate endocannabinoid release have provided the first insights into the physiological roles of the cannabinoids. Cannabinoids have consistently been reported to inhibit high frequency stimulation induced synaptic long-term potentiation but the experimental design of most behavioural experiments have meant it is not possible to categorically demonstrate a role for hippocampal cannabinoid receptors in learning and memory.

Effects of frequent marijuana use on memory-related regional cerebral blood flow

It is uncertain whether frequent marijuana use adversely affects human brain function. Using positron emission tomography (PET), memory-related regional cerebral blood flow was compared in frequent marijuana users and nonusing control subjects after 26+ h of monitored abstention. Memory-related blood flow in marijuana users, relative to control subjects, showed decreases in prefrontal cortex, increases in memory-relevant regions of cerebellum, and altered lateralization in hippocampus. Marijuana users differed most in brain activity related to episodic memory encoding. In learning a word list to criterion over multiple trials, marijuana users, relative to control subjects, required means of 2.7 more presentations during initial learning and 3.1 more presentations during subsequent relearning. In single-trial recall, marijuana users appeared to rely more on short-term memory, recalling 23% more than control subjects from the end of a list, but 19% less from the middle. These findings indicate altered memory-related brain function in marijuana users.

Marijuana: a decade and a half later, still a crude drug with underappreciated toxicity

In 1984, I published in this journal a review entitled "Marijuana: A Crude Drug With a Spectrum of Underappreciated Toxicity." In the introduction to that article, I disclosed that our son Keith, who was 15 years old at the time, was in a long-term, modified outpatient adolescent drug and alcohol rehabilitation program because he had become dependent on marijuana with its associated behavioral, interpersonal, scholastic, and antisocial problems. Keith and most of his friends had experimented several times with LSD, beer, and several other drugs but never used injection drugs. Marijuana was clearly Keith's drug of choice and the only drug he used with regularity. Approximately 1 year later, Keith graduated from the treatment program. He completed the early aftercare component, relapsed several times, and completed a 4-month refresher drug rehabilitation program in another state. Nine years after admission to the first rehabilitation program, Keith finally attained some adult goals. Now 34 years old, he has been drug-free for 10 years. He is the president and owner of a successful discount cellular phone business that he started. More important, a decade ago, he reestablished an excellent and close relationship with his parents. As far as I can tell, Keith remains drug-free except for an occasional beer.

Delta 9-tetrahydrocannabinol induces the apoptotic pathway in cultured cortical neurones via activation of the CB1 receptor

Delta 9-tetrahydrocannabinol, the principal psychoactive component of marijuana, exerts a variety of effects on the CNS, including impaired cognitive function and neurobehavioural deficits. The mechanisms underlying these neuronal responses to tetrahydrocannabinol are unclear but may involve alterations in neuronal viability. Tetrahydrocannabinol has been shown to influence neuronal survival but the role of the cannabinoid receptors in the regulation of neuronal viability has not been fully clarified. In this study we demonstrate that tetrahydrocannabinol promotes the release of cytochrome c, activates caspase-3, promotes cleavage of the DNA repair enzyme poly-ADP ribose polymerase and induces DNA fragmentation in cultured cortical neurones. These effects of tetrahydrocannabinol were completely abrogated by the CB(1) receptor antagonist AM-251. The findings of this study demonstrate that tetrahydrocannabinol induces apoptosis in cortical neurones in a manner involving the CB1 subtype of cannabinoid receptor.

Residual neuropsychologic effects of cannabis

Acute intoxication with cannabis clearly produces cognitive impairment, but it is less clear how long cognitive deficits persist after an individual stops regular cannabis use. Numerous methodologic difficulties confront investigators in the field attempting to assess the residual neuropsychologic effects of cannabis among heavy users, and these must be understood to properly evaluate available studies. At present, it appears safe to conclude that deficits in attention and memory persist for at least several days after discontinuing regular heavy cannabis use. Some of these deficits may be caused or exacerbated by withdrawal effects from the abrupt discontinuation of cannabis; these effects typically peak after 3 to 7 days of abstinence. It is less clear, however, whether heavy cannabis use can cause neurotoxicity that persists long after discontinuation of use. It seems likely that such long-term effects, if they exist, are subtle and not clinically disabling--at least in the majority of cases.

Neurobehavioural consequences of substance abuse and HIV infection

Although our understanding of how human immunodeficiency virus (HIV)-related neurobehavioural deficits develop is nascent and preliminary, some clues have emerged which may clarify lingering uncertainties. In particular, HIV seems to yield brain dysfunction by mediating pathological changes upon neuronal function. HIV also compromises immunological integrity, thereby resulting in secondary infections that may further increase brain dysfunction. Notably, many individuals with HIV tend to be current or past abusers of drugs, and, in some cases, their drug use may have actually presented a pathway for initial HIV infection. Similar to HIV, many drugs tend to yield pathological changes upon neuronal function. Further paralleling HIV, some drugs seem to compromise immune function, which in turn may yield secondary detrimental effects upon the brain. Yet, despite the relatively high comorbidity rates of HIV infection and substance abuse, few investigations have addressed the potential interaction between these two factors upon neurobehavioural status. Towards this end, the present paper reviews the existing literature concerning neuropsychological effects of HIV and substance use, and suggests potential mechanisms whereby substance use may potentiate and exacerbate the onset and severity of neurobehavioural abnormalities in HIV infection.

Tetrahydrocannabinol-induced apoptosis of cultured cortical neurones is associated with cytochrome c release and caspase-3 activation

Delta(9)-tetrahydrocannabinol (THC), the principal psychoactive component of marijuana, is associated with impaired cognition and altered cortical function. THC transduces its central effects via activation of the G-protein linked cannabinoid receptor CB1. In this study we report that THC induces morphological degenerative changes in cultured cortical neurones, such as membrane blebbing and formation of apoptotic bodies, that are consistent with the apoptotic pathway of cell death. The THC-induced apoptosis was blocked by the CB1 receptor antagonist AM251 and pertussis toxin (PTX), suggesting that this effect of THC involves receptor-mediated activation of the G-protein subtypes G(i) or G(o). THC also promoted translocation of mitochondrial cytochrome c to the cytosol and increased the activity of the cysteine protease caspase-3, in a PTX-sensitive manner. The results from this study suggest that coupling of THC to a PTX-sensitive G-protein promotes cytochrome c release, caspase-3 activation and subsequent degeneration of cultured cortical neurones. This apoptotic pathway may underlie the compromised neuronal function that is associated with marijuana usage.

Investigating the neurocognitive deficits associated with chronic drug misuse

Cognitive deficits associated with the chronic abuse of drugs have important theoretical and clinical significance: such deficits reflect changes to the underlying cortical, sub-cortical and neuromodulatory mechanisms that underpin cognition, and also interfere directly with rehabilitative programs. Recent investigations have been made into the neuropsychology of chronic abuse of cannabis, stimulants and opiates. It is suggested that future progress in this area, involving developing advances in brain-imaging and neuropharmacology, will capitalize on experimental demonstrations of specific patterns of impairments in decision-making, attention and memory function.

Effects of frequent marijuana use on brain tissue volume and composition

To investigate CNS effects of frequent marijuana use, brain tissue volume and composition were measured using magnetic resonance imaging (MRI) in 18 current, frequent, young adult marijuana users and 13 comparable, non-using controls. Automated image analysis techniques were used to measure global and regional brain volumes, including, for most regions, separate measures of gray and white matter. The marijuana users showed no evidence of cerebral atrophy or global or regional changes in tissue volumes. Volumes of ventricular CSF were not higher in marijuana users than controls, but were, in fact, lower. There were no clinically significant abnormalities in any subject's MRI. Sex differences were detected in several global volume measures.

The relationship between marijuana initiation and dropping out of high school

The prevalence of marijuana use among young people has risen rapidly in recent years, causing concern over the potential impact on academic performance of such use. While recent studies have examined the effect of alcohol use on educational attainment, they have largely ignored the potential negative effects of other substances, such as marijuana. This paper examines whether the relationship between the initiation of marijuana use and the decision to drop out of high school varies with the age of dropout or with multiple substance use. Data are from a longitudinal survey of 1392 adolescents aged 16-18 years. The results suggest that marijuana initiation is positively related to dropping out of high school. Although the magnitude and significance of this relationship varies with age of dropout and with other substances used, it is concluded that the effect of marijuana initiation on the probability of subsequent high school dropout is relatively stable, with marijuana users' odds of dropping out being about 2.3 times that of non-users. Implications of these conclusions are considered for both policy makers and researchers.

Cognitive efficiency in alcoholics and polysubstance abusers

Previous studies have found alcoholics to be impaired on tests of cognitive efficiency. However, it is unclear to what extent individuals who abuse drugs in addition to alcohol exhibit similar deficits. To answer this question, 63 healthy control subjects were compared with 40 individuals who abused alcohol only, 24 individuals who abused alcohol and stimulants, 16 individuals who abused alcohol and marijuana, and 41 individuals who abused alcohol and depressants/narcotics, or alcohol and two or more other drugs. All subjects were administered tests of short-term memory, spatial orientation, visual-spatial perception, and problem-solving. Results from the study indicated that control subjects and individuals who abused both alcohol and marijuana performed significantly better than the other groups on most tests. These results were not attributable to differences on measures of affect or chronicity of alcohol consumption.

Marijuana and Job Performance: Comparing the Major Streams of Research

Though some estimate that marijuana use has cost U.S. companies billions of dollars in lost productivity, the relationship between use of marijuana and job performance is far from clear. In this paper, I review studies on this topic from four different methodological perspectives and discuss models to account for their apparently contradictory findings. This discussion suggests hypotheses for future research addressing possible reasons for inconsistencies in past findings.

Increase in meso-prefrontal dopaminergic activity after stimulation of CB1 receptors by cannabinoids

The intravenous administration of the psychoactive constituent of marijuana, delta9-tetrahydrocannabinol (delta9-THC) (62.5-1000 microg/kg), and the synthetic cannabinoid agonist WIN 55212,2 (WIN) (62.5-500 microg/kg), produced a dose-related increase in the firing rate and burst firing in the majority of antidromically identified meso-prefrontal dopaminergic neurons. In a restricted number of neurons (n=4), WIN administration did not increase firing rate but produced an increment of bursting activity. These effects of the cannabinoids were reversed by the intravenous administration of SR 141716 A, a selective cannabinoid antagonist (1 mg/kg), per se ineffective to modify the electrical activity of dopaminergic neurons. The results indicate that stimulation of cannabinoid CB1 receptors produces an activation of meso-prefrontal dopaminergic transmission. Considering that supranormal stimulation of D1 dopamine receptors in the prefrontal cortex has been shown to impair working memory, the present results suggest that the negative effects of cannabinoids on cognitive processes might be related to the activation of dopaminergic transmission in the prefrontal cortex.

Hippocampal neurotoxicity of Delta9-tetrahydrocannabinol

Marijuana consumption elicits diverse physiological and psychological effects in humans, including memory loss. Here we report that Delta9-tetrahydrocannabinol (THC), the major psychoactive component of marijuana, is toxic for hippocampal neurons. Treatment of cultured neurons or hippocampal slices with THC caused shrinkage of neuronal cell bodies and nuclei as well as genomic DNA strand breaks, hallmarks of neuronal apoptosis. Neuron death induced by THC was inhibited by nonsteroidal anti-inflammatory drugs, including indomethacin and aspirin, as well as vitamin E and other antioxidants. Furthermore, treatment of neurons with THC stimulated a significant increase in the release of arachidonic acid. We hypothesize that THC neurotoxicity is attributable to activation of the prostanoid synthesis pathway and generation of free radicals by cyclooxygenase. These data suggest that some of the memory deficits caused by cannabinoids may be caused by THC neurotoxicity.

Brain aging in a sample of normal Egyptians cognition, education, addiction and smoking

The impact of duration of education, cannabis addiction and smoking on cognition and brain aging is studied in 211 normal Egyptian volunteers with mean age 46.4+/-3.6 years (range: 20-76 years). Subjects were classified into two groups: Gr I (non-addicts) with 174 subjects, mean age 49.9+/-3.8 years (range 20-76 years), smokers and non-smokers, educated and non-educated and Gr II (cannabis addicts) with 37 subjects, mean age 43.6+/-2.6 years (range 20-72 years) all smokers, educated and non-educated. Outcome measures included the Paced Auditory Serial Addition test (PASAT) for testing attention and the Trailmaking test A, and B (TMa and TMb) for testing psychomotor performance. Age correlated positively with score of Trailmaking test (TMb) in the non-addict group and in the addict group (TMa and TMb). Years of education correlated negatively with scores of Trialmaking test (TMb) in the non-addict group (Gr I) but not the addict group (Gr II). However, in both groups mean scores of the Trailmaking test (TMa) were significantly lower in subjects with a primary level of education than those with higher levels of education. No significant difference was detected between male smokers and nonsmokers of Gr I (non-addicts) regarding any of the neuropsychological tests. Yet, smokers and the non-educated group had poorer attention compared to non-smokers of the same group. Cannabis addicts (Gr II) had significantly poorer attention than non-addict normal volunteers (Gr I). It is concluded that impairment of psychomotor performance is age related whether in normal non-addicts or in cannabis addicts. A decline in attention was detected in cannabis addicts and has been considered a feature of pathological aging. Education in early life as well as the duration of education are neuroprotectors for brain aging more so in the non-addict than addict group. Though cigarette smoking per se has no effect on cognitive abilities in normal aging, it becomes evident that its association with lack of education impairs attention.

Marijuana Use: Current Issues and New Research Directions

Due to recent evidence of the increasing frequency of marijuana use among adolescents and young adults, and due to the many remaining research issues regarding this drug, it is appropriate to summarize some of the important data about marijuana and to suggest some directions for research and service. This article first provides a review of the history, botany, active ingredients, effects, and negative consequences of marijuana use. Next, prevention and cessation of marijuana use are discussed. Finally, this paper provides a selective examination of current issues in marijuana research. Several salient issues are highlighted including its preference among certain subgroups (high risk youth and ethnic differences), its relations with illegal behavior (marijuana use and driving, current marijuana-related legislation, and marijuana use and violence), and its recent portrayal in the media.

Delta-9-tetrahydrocannabinol: an inhibitor of STAT1 alpha protein tyrosine phosphorylation

Tyrosine-phosphorylated signal transducer and activator of transcription 1 alpha (STAT1 alpha) is a 91-kDa protein responsible for interferon-gamma (IFN-gamma)-dependent transcription. The present study demonstrates that activation by IFN-gamma of murine macrophages resulted in tyrosine phosphorylation of STAT1 alpha identified by immunoprecipitation. The tyrosine phosphorylation of STAT1 alpha was found highly sensitive to treatment by delta-9 tetrahydrocannabinol (THC), a major marijuana component. Subsequently, the isoform formation of p91 due to tyrosine phosphorylation was reduced in THC-treated macrophages. Although inhibition by THC of the tyrosine phosphorylation of STAT1 alpha induced by IFN-gamma was in a THC concentration-related manner, the tyrosine phosphorylation of other proteins induced by lipopolysaccharide/IFN-gamma treatment of macrophages appeared insensitive to THC treatment. Our data suggest that blockade by THC of tyrosine phosphorylation of STAT1 alpha may be an important mechanism involved in the broad immunosuppressive effects of THC.

Acute effects of marijuana on cognition: relationships to chronic effects and smoking techniques

A double-blind, placebo-controlled study assessed acute effects on human cognition of marijuana smoking involving long or short durations of inhalation and breath holding. During eight test sessions, 48 adult, male volunteers completed standardized, pencil-and-paper tests of educational development and ability, as well as computerized tests of learning, associative processes, abstraction, and psychomotor performance. Marijuana impaired all capabilities except abstraction and vocabulary. These impairments were more pervasive than those associated with heavy, chronic marijuana use in a previous study involving the same tests, but showed some similarities. Marijuana altered associative processes, encouraging more uncommon associations. Marijuana-induced impairment in learning pairs of words was influenced by associative relationships between the words. There were a few hints that prolonged breath holding increased marijuana's effects under some test conditions, but in general it did not. Prolonged breath holding itself affected performance in four tests, regardless of whether subjects smoked marijuana or placebo. Whether physiological or psychological factors (e.g., exposure to carbon monoxide in smoke or subjects' expectations) produced these effects could not be determined.