Taken together,findings underscore that alterations in cortical metrics are likely dynamic and influenced by age,pre-existing vulnerabilities,and exogenous factors such as marijuana use.Continuing to study associations between cortical metrics and substance use is important given estimates have been linked to cognitive functioning in several studies in our laboratory and others.Alcohol likely has similar deleterious consequences on the brain.The present dose-dependent associations are consistent with our previous findings,as Squeglia et al.,found decreases in cortical thickness estimates associated with heavy episodic alcohol use in males,and accelerated declining brain volume trajectories in a large prospective investigation examining individuals who transitioned to heavy drinking.Alcohol likely interferes with neural development of the cerebral cortex,and thinner cortices observed with more cumulative use reported may represent non-beneficial pruning and/or inhibition of cell generation or cell death.Limitations of the present study include self-report of substance use,which can introduce measurement error.Further,while this study was prospective,participants were not assessed prior to initiation of substance use.However,previous work in our laboratory finds marijuana-related associations with white matter integrity in a sample of individuals assessed pre- and post-initiation of substance use.Nevertheless,future work should determine the influence of pre-existing differences on cortical metrics.The current investigation included users of both marijuana and alcohol,and despite controlling for alcohol use,it remains unclear what is precisely the result of marijuana as compared to the combination of co-occurring marijuana andalcohol use.Our sample was predominately male,however gender should be evaluated and future studies will focus on differential gender effects on brain morphometry in adolescent marijuana users.
Group did not statistically differ on days since last use of marijuana and alcohol use,likely influenced by the monitored abstinen ceperiod,ebb and flow therefore acute effects may not have been captured in our reported findings.Astatistically significant within-subjects effect was not widely observed,which may be attributed to the smaller sample size combined with a more restricted age range.We tried to reduce the number of correlational analysis that were conducted,however given that effects were modest,future work should replicate findings.Studies should continue to follow existing adolescent cohorts to understand neural and behavioral changes that occur into young adulthood.Understanding how co-occurring marijuana and alcohol use influences both macro-structural and micro-structural brain development,along with structural and functional connectivity,will help clinical interventions target neural vulnerabilities to develop novel and effective interventions to reduce marijuana misuse as prevalence rates of marijuana continue to increase.Marijuana is the most commonly used illicit drug in the United States,with 36.4% of high school seniors reporting past-year use.Recent trends show an increase in marijuana use coupled with a substantial decrease in perceptions of harm.This is concerning given the adverse outcomes associated with marijuana use,including cognitive impairment,lower lifetime achievement,and increased risk for addiction.Adolescent marijuana users may be at particular risk for adverse outcomes.Compared to adult-onset users,adolescent-onset marijuana users are more likely to experience symptoms of dependence within two years of use onset,are at increased risk of developing other drug dependence,and show increased deficits in executive functioning.Furthermore,studies of brain functioning during cognitive tasks have demonstrated differences in brain activation and connectivity in marijuana users compared with controls,some of which are more apparent in those with adolescent-onset use.Of particular relevance to the present work is evidence that marijuana use during adolescence may have long-lasting effects on emotion.Marijuana use is often co-morbid with mood disorders,and facets of negative affectivity have been correlated with marijuana use among both adults and adolescents.Although it has been suggested that these associations may be explained by the use of marijuana as self-medication for depression and anxiety symptoms,consistent support for this interpretation is lacking.Indeed,emerging evidence suggests that early marijuana use may contribute to the development of depression and anxiety later in life.
This is supported in animal models,which have demonstrated that early exposure to cannabinoids disrupts emotional processes and leads to later depressive phenotypes and increased social anxiety.Delta-9-tetrahydrocannabinol,the main psychoactive component of marijuana,binds to CB1 cannabinoid receptors in the brain.Endogenous cannabinoids are involved in the regulation of emotional responses,including mood,anxiety,and aggression,and laboratory studies support an acute impact of THC on mood and emotion.CB1 receptor expression is highest during adolescence,dropping thereafter into adulthood with the most pronounced decreases observed in limbic regions critically involved in emotion regulation.Thus,adolescent exposure to THC may have lasting consequences on the developing brain that specifically impact the regulation of emotion.Some support for this comes from structural imaging studies showing volumetric differences in adolescent marijuana users compared with controls inlimbic regions,including the amygdala,hippocampus,and insula.For example,larger amygdala volumes were observed in female marijuana users compared with controls,which was further associated with depression and anxiety symptoms.Other work has observed that marijuana users have differences in cerebral blood flow and resting connectivity compared with controls in brain regions involved in emotion,including the insula and temporal cortex.Together,the evidence supports an association between marijuana use during adolescence and an alteration of the neural systems supporting emotion regulation.However,to date only one study has investigated the effects of marijuana use on brain functioning during an emotion task.This study of adult heavy marijuana smokers found decreases in anterior cingulate and amygdala activation during the viewing of masked affective faces,suggesting a difference in the way marijuana users process emotional information.To date,no studies have investigated how the use of marijuana specifically during adolescence impacts these processes; thus,one goal of the current study was to address this gap by investigating brain functioning during emotion arousal in 17–22 year-old heavy marijuana smokers who began their use earlier in adolescence.Furthermore,although there is evidence for a prospective relationship between early marijuana smoking and later negative emotionality,the literature regarding the intermediary brain processes in this relationship has been less clear.
The work reviewed above has been cross sectional,and consequently,inferences cannot be made regarding causal relationships among history of marijuana use,brain functioning,and negative affect.Therefore,this study uses a prospective design to better address the nature of the relationship and to investigate whether emotion-related brain function in late adolescence/emerging adulthood mediates a relationship between prior marijuana use and later emotional functioning.We investigate two facets of emotional functioning,which are grounded in the temperament and personality literature—negative emotionality and resiliency.Negative emotionality is the propensity to experience depressed mood,anxiety,and irritable anger.Resiliency is the ability to flexibly adapt one’s level of control—in either direction—in response to the demands of the environment.It involves thoughtful,deliberate control of behavior in challenging or stressful circumstances and freer expression in circumstances where it is appropriate.This type of self-regulation is a critical aspect of emotional regulation.Note that the construct of resiliency is not directly related to the idea of resilience to adversity.Rather,resiliency has its conceptual roots in the temperament-based work of the Blocks,who identified the related construct of ego resiliency.Using a prospective,longitudinal design,we investigate negative emotionality and resiliency measured at three time points: at the approximate age when the heavy marijuana smokers initiated use ; within one year prior to participation in the functional magnetic resonance imaging study of emotion arousal ; and approximately three years after participation in the fMRI study.Information on occasions of marijuana use was collected prospectively on an annual basis from age 11 up to the time of participation in the fMRI study.This design allows us to investigate the impact of marijuana use during adolescence on the development of negative emotionality and resiliency and on emotion-related brain function,and to investigate whether emotion-related brain function mediates a relationship between prior marijuana use and later emotional functioning.Forty participants were selected from an ongoing fMRI study of adolescents and young adults recruited from the Michigan Longitudinal Study.The MLS is an ongoing,dry racks prospective community-recruited study of families with parental alcohol use disorder along with a contrast sample of families without AUD drawn from the same neighborhoods.All parent diagnoses were ascertained by a clinical psychologist based on Diagnostic Interview Schedule – Version 4 and established at time of recruitment and via multiple face-to-face diagnostic assessments of the parents over the course of the youth’s life.Families in which the target offspring exhibited signs of fetal alcohol syndrome were excluded from the original ascertainment.
Exclusionary FAS characteristics included prenatal or postnatal growth retardation or both,central nervous system involvement,and characteristic facial dysmorphology.From the time of enrollment,all family members are assessed at 3-year intervals with an extensive psychosocial battery of measures assessing temperament,behavioral symptomatology,IQ,school performance,social interaction,etc.During the 11–26 year-old period,all offspring are also assessed annually on substance use and problems.Full details on the prospective assessment and data collection protocol in the MLS can be found elsewhere.One hundred and thirty 17–22 year old offspring from the MLS have completed an emotion arousal task during fMRI.Exclusionary criteria for the fMRI study included neurological,acute,uncorrected,or chronic medical illness,current or recent treatment with centrally active medications,or history of psychosis in first-degree relatives.The presence of most active primary Axis I disorders was also exclusionary; this did not include unmedicated mood and anxiety disorders,antisocial personality disorder,or substance use disorder.These were allowed because their exclusion would preferentially eliminate part of the phenomena of interest.Diagnosis was determined using the Diagnostic Interview Schedule-Child for participants under the age of 18 and the Diagnostic Interview Schedule-Version IV for participants 18 and older.All participants were right-handed as determined with the Edinburgh Handedness Inventory.Participants were told to abstain from alcohol and illicit substances/recreational drugs for 48 h prior to scanning.Participants were given a multi-drug 5-panel urine screen before scanning.Because THC metabolites are detectible in urine for a week or longer,if a participant tested positive for marijuana,we relied on self-report regarding abstinence in the 2 days prior to the study; in this case,report of marijuana use in the prior 48 h was exclusionary.Three participants included in this study tested positive for marijuana.All analyses were performed without the three participants who tested positive for marijuana and results were substantively the same.All participants over the age of 18 provided written informed consent after explanation of the experimental protocol.Participants under the age of 18 signed their assent to participate in the study and at least one parent gave written informed consent.The focus of the current work is to investigate the impact of a history of marijuana use during adolescence on emotional functioning and brain functional mediators of this effect.Information regarding marijuana use and emotional functioning were collected prospectively beginning in childhood as part of the MLS.Functional neuro imaging data were collected at one time point.Of the 130 MLS offspring in the fMRI study,15% reported heavy marijuana use up until the age of participation in the fMRI study.These participants comprised the heavy marijuana user group.We matched 20 controls with minimal or no marijuana use to the heavy marijuana user group.Because marijuana use is highly comorbid with alcohol and other drug use,we took steps to reduce the potential confound of other substance use by identifying controls with similar alcohol and nicotine use profiles as the heavy marijuana users.No heavy marijuana users reported consuming fewer than 100 lifetime drinks; therefore,<100 lifetime drinks was exclusionary for the control group.From the remaining participants,20 controls were individually matched to the heavy marijuana group based on age at the time of the fMRI scan and parental AUD during the participant’s lifetime.Binge drinking history,smoking status,and gender were matched across groups as closely as possible.See Table 1 for demographic,substance use,and diagnostic characteristics for the final control group compared with the heavy marijuana user group.Between ages 6 and 10,alcohol and drug use was assessed at 3-year intervals with a health and daily living questionnaire as part of the MLS.specifically,children were asked if they ever used marijuana,had more than a sip of alcohol,smoked cigarettes,or used other drugs.If yes,the age at which this occurred and quantity/frequency of use were recorded.Beginning at age 11,substance use was assessed annually using the self-report Drinking and Drug History Form for Children.This form provides measures of quantity and frequency of alcohol and nicotine use and frequency of marijuana and other illicit drug use.The California Q-Sort is an examinerrated measure that permits the observer to systematically describe the subject’s personality and functioning with a standardized language.It is collected at 3-year intervals as part of the MLS beginning at ages 6–8.It is completed by clinically-trained assessors following a 3–4 h session with the subject.specifically,100 statements that portray a variety of behavioral adaptations are placed in a forced-choice,nine-category normal distribution by assigning rankings to the statements,ranging from 1 to 9.Scores for negative emotionality and resiliency were derived from the Q-Sort based on Eisenberg et al..