Death of the 'stepping-stone' hypothesis and the 'gateway' model? Comments on Morral et al

Associations of polygenic risk scores for smoking heaviness and lifetime cannabis use with tobacco and cannabis co-use trajectories among African Americans

BACKGROUND

We aimed to identify distinct trajectories of tobacco, cannabis, and their co-use among African Americans, and to investigate whether these patterns were associated with polygenic risk scores (PRS) for tobacco and cannabis use.

METHOD

Participants (N=428 participants; 50.9% male) were initially recruited for an elementary school-based prevention in a Mid-Atlantic city when they were in first grade. From ages 14-26, participants reported on their frequency of tobacco and cannabis use in the past year during annual assessments. DNA was collected from participants at age 21. PRS for smoking heaviness (i.e., cigarettes per day) and lifetime cannabis use were created based on genome-wide association study results derived from Liu et al. (2019) and Pasman et al. (2018), respectively.

RESULTS

We identified five distinct trajectories of tobacco and cannabis co-use, including (1) Low Tobacco and Cannabis Use, (2) Adolescent Limited Tobacco and Cannabis Use, (3) Experimental Cannabis, Young Adult Increasing Tobacco, (4) Experimental Tobacco, Young Adult Increasing Cannabis, and (5) High, Chronic Tobacco and Cannabis Use. Compared to the Low Tobacco and Cannabis Use subgroup, individuals in the High, Chronic Tobacco and Cannabis Use subgroup had greater PRS for smoking heaviness, and individuals in the Experimental Cannabis, Young Adult Increasing Tobacco subgroup had higher PRS for lifetime cannabis use.

CONCLUSIONS

Polygenic risk for lifetime cannabis use and smoking heaviness is associated with the developmental progression of tobacco and cannabis co-use among African Americans, furthering knowledge on the etiology of co-use in this population.

Effect of chronic binge-like ethanol consumption on subsequent cocaine reinforcement in rhesus monkeys

BACKGROUND

Although most individuals with cocaine use disorder also abuse alcohol, little is known about the behavioral and pharmacological mechanisms that promote co-abuse. For example, it is unclear whether prior experience with alcohol renders individuals more sensitive to cocaine when it is subsequently experienced.

METHODS

This study examined the effects of chronic ethanol consumption on subsequent cocaine reinforcement in rhesus monkeys. Six monkeys consumed 2.0 g/kg ethanol in a binge-drinking paradigm and 6 monkeys drank a non-alcoholic solution 5 days per week. After 9 months, each monkey's sensitivity to acquiring cocaine self-administration was determined. Monkeys performed an operant response resulting in food pellet delivery under a fixed-ratio 30 schedule of reinforcement. Saline, then ascending doses of cocaine, were substituted for food pellets until a cocaine dose was reached at which the number of cocaine injections delivered differed significantly from saline injections delivered. Following acquisition, a complete cocaine dose-effect curve was generated to determine whether ethanol consumption altered the reinforcing potency of cocaine determined by calculating the ED₅₀ of the ascending limb of each subject's curve.

RESULTS

Although individual variability was observed, the cocaine dose which initially served as a reinforcer did not differ between ethanol-drinking and control groups and, within the ethanol-drinking group, was not related to the amount of ethanol consumed. Moreover, the reinforcing potency of cocaine did not differ between groups.

CONCLUSION

Taken together, the data suggest that a history of binge-like alcohol consumption does not affect sensitivity to cocaine when it is subsequently first experienced.

Progression to regular heroin use: examination of patterns, predictors, and consequences

BACKGROUND

The present study retrospectively evaluated the chronology and predictors of substance use progression in current heroin-using individuals.

METHODS

Out-of-treatment heroin users (urinalysis-verified; N=562) were screened for laboratory-based research studies using questionnaires and urinalysis. Comprehensive substance use histories were collected. Between- and within-substance use progression was analyzed using stepwise linear regression models.

RESULTS

The strongest predictor of onset of regular heroin use was age at initial heroin use, accounting for 71.8% of variance. The strongest between-substance predictors of regular heroin use were ages at regular alcohol and tobacco use, accounting for 8.1% of variance. Earlier onset of regular heroin use (≤20 years) vs. older onset (≥30 years) was associated with a more rapid progression from initial to regular use, longer duration of heroin use, more lifetime use-related negative consequences, and greater likelihood of injecting heroin. The majority of participants (79.7%) reported substance use progression consistent with the gateway hypothesis. Gateway-inconsistent individuals were more likely to be African-American and to report younger age at initial use, longer duration of heroin use, and more frequent past-month heroin use.

CONCLUSIONS

Our findings demonstrate the predictive validity and clinical relevance of evaluating substance use chronology and the gateway hypothesis pattern of progression.

Does cannabis onset trigger cocaine onset? A case-crossover approach

Psychiatric researchers tend to select the discordant co-twin design when they seek to hold constant genetic influence while estimating exposure-associated disease risk. The epidemiologic case-crossover research design developed for the past two decades represents a viable alternative, not often seen in psychiatric studies. Here, we turn to the epidemiologic case-crossover approach to examine the idea that cannabis onset is a proximal trigger for cocaine use, with the power of "subject-as-own-control" research used to hold constant antecedent characteristics of the individual drug user, including genetic influence and other traits experienced up to the time of the observed hazard and control intervals. Data are from newly incident cocaine users identified in the 2002-2006 US National Surveys on Drug Use and Health. Among these cocaine users, 48 had both cannabis onset and cocaine onset in the same month-long hazard interval; the expected value is 30 users, based on the control interval we had pre-specified for case-crossover estimation (estimated relative risk, RR = 1.6; exact mid-p = 0.042). Within the framework of a subject-as-own-control design, the evidence is consistent with the hypothesis that cannabis onset is a proximal trigger for cocaine use, with genetic influences (and many environmental conditions and processes) held constant. Limitations are noted and implications are discussed.

Periadolescent exposure to cannabinoids alters the striatal and hippocampal dopaminergic system in the adult rat brain

In a previous work, we have shown that chronic administration of the cannabinoid agonist CP 55,940 (CP) during periadolescence increases cocaine self-administration in adult female rats, while it produces no such effect in males (Higuera-Matas et al., 2008). To extend these findings, we have analysed here the brains of the rats used as subjects in this previous work to evaluate the impact of the interaction between CP exposure and cocaine self-administration on dopaminergic parameters. We evaluated the levels of the dopamine transporter (DAT), and the D1- (D1R) and D2-type (D2R) dopaminergic receptors, as well as tyrosine hydroxylase (TH) mRNA in dopaminergic areas of the adult, cocaine self-administered, rat brain that had been chronically exposed to CP or vehicle (VH) during periadolescence. Control groups with CP/VH exposure and no self-administration experience were also included. In adult females, CP administration induced an up-regulation of DAT in the caudate-putamen that was maintained after cocaine self-administration. In males, CP induced an increase in the D1Rs content in the nucleus accumbens shell, which was not evident after cocaine self-administration. CP also reduced the expression of D2Rs in CA1 irrespective of sex. Finally, an increase in D1Rs was observed in the substantia nigra following cocaine self-administration. These findings suggest that a dopaminergic component modulated by cannabinoids may underlie the enhanced cocaine self-administration previously observed in adult female rats.

Tobacco, cannabis, and other illicit drug use among Finnish adolescent twins: causal relationship or correlated liabilities?

OBJECTIVE

Among Finnish adolescent twins, we compared (a) a model that describes a direct impact of liability to tobacco use on cannabis and other illicit drug use with (b) a model that included a shared underlying liability for these substances. Furthermore, the extent to which genetic and environmental influences contribute to the covariation between liabilities to use these substances was examined.

METHOD

Tobacco and illicit drug use were assessed at age 17.5 years. Twin data on 3,744 individuals were analyzed using standard biometrical methods. Two alternative multivariate models were fi t and compared with Mx, a statistical program for genetic model fitting.

RESULTS

The multivariate model, including a direct impact of the initiation of tobacco use on illicit drug use, provided the best fit to the data. In this model, the total variation in the initiation of illicit drugs was decomposed to genetic factors (32%), common environmental factors (20%), unique environmental factors (8%), and a component due to initiation of smoking (40%). Most variation in the progression of illicit drug use was the result of initiation of smoking and illicit drug use (83%).

CONCLUSIONS

Liability to initiate smoking directly affects illicit drug use in our best-fitting model. Our findings suggest that several common genetic influences may be related to tobacco use and illicit drugs but that a search for specific genes underlying illicit drug use is justifi ed as well. Such specific genes may hold a key to understanding biological vulnerabilities that lead to illicit drug use, which could aid in the development of targeted interventions.

Cannabis consumption initiation among adolescents: a longitudinal study

This study aimed to investigate factors related to initiation of cannabis consumption among adolescents. A questionnaire was administered to 2043 14-15-year-olds from Barcelona who were followed-up and re-interviewed after 15 months. A bivariate analysis was performed to identify the factors associated with consumption, and multivariate logistic regression was carried out to model cannabis initiation. Among matched students, 23.7% of non-users at baseline had started to consume 15 months later (23.0% boys and 24.2% girls). Among those who had reported occasional cannabis use, 30.3% reported consumption during the previous month at the follow-up survey. Factors associated with cannabis initiation among boys and girls were smoking, risky alcohol use and intention to consume cannabis. Among boys, other associated factors were frequenting bars or discotheques and not having organized activities in leisure time. Among girls, another risk factor for initiation was having cannabis-using friends. Cannabis initiation was facilitated by legal drug use, favorable attitudes and context-related variables. These results highlight the role of behavioral and contextual variables and support the importance of reinforcing social skills in preventive programs.

[Comparison of two methods to identify factors associated with the onset of cannabis use in a cohort study]

OBJECTIVE

To compare the utility of two multivariate analytic methods, case-control (CC) analysis and nested case-control (NCC) analysis in a cohort, to identify the factors associated with the onset of cannabis use.

METHODS

A longitudinal cohort study of a sample of secondary school students (n = 1,056) in the first year of secondary school was carried out. Participating students were followed-up annually until the fourth year of secondary school. In the CC analysis, students in the fourth year who reported having consumed cannabis at some time were considered cases and those who had never consumed cannabis were considered controls. Logistic regression (LR) models were estimated. In the NCC analysis, cases in each year of follow-up were compared with a random sample of controls at risk in the same year and conditional LR models were estimated.

RESULTS

In the CC analysis, 6 variables in boys and 9 variables in girls in bivariate models and 3 variables in boys and 4variables in girls in multivariate models were identified. In the NCC analysis, 17 variables in univariate models and 4 in multivariate models were obtained in both boys and girls. The estimators of the CC analysis showed an average of 1.2-fold more variability.

CONCLUSIONS

A higher number of factors associated with cannabis use were identified in the NCC analysis and the estimators were more precise. NCC could be a more efficient option than CC analysis.

Age and sex differences in factors associated with the onset of cannabis use: a cohort study

OBJECTIVE

To investigate age and sex differences in factors associated with the onset of cannabis use among adolescents.

METHODS

A representative sample of schoolchildren from secondary schools in Barcelona (n=1056) was selected and followed-up from the first to the fourth year of secondary education (7-10th grades). The participants completed a self-administered lifestyle questionnaire each year. Multilevel logistic regression models were used for each year and sex to analyse predictors of cannabis use; the second level was adjusted by school.

RESULTS

Onset of cannabis use during follow-up was associated with a prior history of tobacco smoking [odds ratio (OR)=7.7 in boys; OR=3.8 in girls], alcohol consumption (OR=6.4 in boys; OR=3.2 in girls), antisocial behavior (OR=2.8 in boys; OR=2.2 in girls), intention to use drugs (OR=3.5 in boys; OR=4.2 in girls), drug use among friends (OR=2.5 in boys; OR=3.7 in girls) and spending leisure time in bars or discos (OR=2.1 in boys; OR=3.8 in girls). Moreover, among girls, attending state schools (OR=2.9), low academic performance (OR=5.7) and living in a single-parent family (OR=2.0) also independently predicted cannabis use.

CONCLUSIONS

This study reveals a wide array of predictive factors in cannabis use onset that largely differed by age and sex. The results support the role of tobacco and alcohol, as well as the influence of drug use among friends, and the importance of leisure time patterns as facilitators of cannabis use.

Factors associated with the onset of cannabis use: a systematic review of cohort studies

OBJECTIVE

To determine the factors associated with the onset of cannabis use through a systematic review of cohort studies.

METHODS

An internet-based search was performed using several keywords and their combinations. Original studies with longitudinal design and the onset of cannabis use as dependent variable, as well as review studies were included, published between January 1980 and May 2004. Methodology quality of the studies was assessed independently by two reviewers, according to pre-established criteria, in order to classify studies in high, mid or low quality. Agreement between reviewers was assessed through kappa coefficient.

RESULTS

A total of 32 relevant studies were identified, of which 13 were of higher quality. Selection bias for the inclusion of consumers at the baseline measurement and lack or insufficient adjustment for confounders were the causes of exclusion. The factors of great evidence related to the onset of cannabis use were masculine sex, consumption of tobacco or alcohol, having a problematic relationship with parents, and cannabis consumption by friends.

CONCLUSION

Results highlight the importance of different individual, family and environmental factors on the onset of cannabis use. These must be considered to properly arrange intervention programs focusing on primary prevention among teenagers.

Early alcohol or tobacco onset and transition to other drug use among students in the state of Morelos, Mexico

In this study, we explored if patterns in the transition from alcohol/tobacco in the Mexican State of Morelos, Mexico are similar to those observed in other countries.

METHODS

The data were from a representative sample of youth age 11-21 years (n = 13,105), who participated in a paper-and-pencil survey in middle schools, high schools, and colleges in the State of Morelos, Mexico. Drug use was assessed via the standardized instrument mostly used in Mexican student surveys. Cox's models for discrete time-survival analyses, stratified by school and age group were used to estimate the risk of drug use in relation to age of alcohol and tobacco use initiation by gender, while accommodating the complex survey design.

RESULTS

About 5% of the students were estimated to have used drugs in their life. Male early users of alcohol or tobacco were more likely to use other drugs, compared to students who did not have an early alcohol or tobacco onset.

COMMENT

Further studies on social mechanisms might help to account for observed similarities in patterns of drug involvement in different countries, even in the context of important differences in rates of drug use.

Developmentally inspired drug prevention: middle school outcomes in a school-based randomized prevention trial

Prior investigations have linked behavioral competencies in primary school to a reduced risk of later drug involvement. In this randomized prevention trial, we sought to quantify the potential early impact of two developmentally inspired universal preventive interventions on the risk of early-onset alcohol, inhalant, tobacco, and illegal drug use through early adolescence. Participants were recruited as they entered first grade within nine schools of an urban public school system. Approximately, 80% of the sample was followed from first to eighth grades. Two theory-based preventive interventions, (1) a family-school partnership (FSP) intervention and (2) a classroom-centered (CC) intervention, were developed to improve early risk behaviors in primary school. Generalized estimating equations (GEE) multivariate response profile regressions were used to estimate the relative profiles of drug involvement for intervention youths versus controls, i.e. youth in the standard educational setting. Relative to control youths, intervention youths were less likely to use tobacco, with modestly stronger evidence of protection associated with the CC intervention (RR=0.5; P=0.008) as compared to protection associated with the FSP intervention (RR=0.6; P=0.042). Intervention status was not associated with risk of starting alcohol, inhalants, or marijuana use, but assignment to the CC intervention was associated with reduced risk of starting to use other illegal drugs by early adolescence, i.e. heroin, crack, and cocaine powder (RR=0.32, P=0.042). This study adds new evidence on intervention-associated reduced risk of starting illegal drug use. In the context of 'gateway' models, the null evidence on marijuana is intriguing and merits attention in future investigations.