Cannabis consumption and risk of developing schizophrenia: myth or reality?

Interaction Between Functional Genetic Variation of DRD2 and Cannabis Use on Risk of Psychosis

Both cannabis use and the dopamine receptor (DRD2) gene have been associated with schizophrenia, psychosis-like experiences, and cognition. However, there are no published data investigating whether genetically determined variation in DRD2 dopaminergic signaling might play a role in individual susceptibility to cannabis-associated psychosis. We genotyped (1) a case-control study of 272 patients with their first episode of psychosis and 234 controls, and also from (2) a sample of 252 healthy subjects, for functional variation in DRD2, rs1076560. Data on history of cannabis use were collected on all the studied subjects by administering the Cannabis Experience Questionnaire. In the healthy subjects' sample, we also collected data on schizotypy and cognitive performance using the Schizotypal Personality Questionnaire and the N-back working memory task. In the case-control study, we found a significant interaction between the rs1076560 DRD2 genotype and cannabis use in influencing the likelihood of a psychotic disorder. Among cannabis users, carriers of the DRD2, rs1076560, T allele showed a 3-fold increased probability to suffer a psychotic disorder compared with GG carriers (OR = 3.07; 95% confidence interval [CI]: 1.22-7.63). Among daily users, T carrying subjects showed a 5-fold increase in the odds of psychosis compared to GG carriers (OR = 4.82; 95% CI: 1.39-16.71). Among the healthy subjects, T carrying cannabis users had increased schizotypy compared with T carrying cannabis-naïve subjects, GG cannabis users, and GG cannabis-naïve subjects (all P ≤ .025). T carrying cannabis users had reduced working memory accuracy compared with the other groups (all P ≤ .008). Thus, variation of the DRD2, rs1076560, genotype may modulate the psychosis-inducing effect of cannabis use.

What can we learn about schizophrenia from studying the human model, drug-induced psychosis?

When drug-induced psychoses were first identified in the mid-20th century, schizophrenia was considered a discrete disease with a likely genetic cause. Consequently, drug-induced psychoses were not considered central to understanding schizophrenia as they were thought to be phenocopies rather than examples of the illness secondary to a particular known cause. However, now that we know that schizophrenia is a clinical syndrome with multiple component causes, then it is clear that the drug-induced psychoses have much to teach us. This article shows how the major neuropharmacological theories of schizophrenia have their origins in studies of the effects of drugs of abuse. Research into the effects of LSD initiated the serotonergic model; amphetamines the dopamine hypothesis, PCP and ketamine the glutamatergic hypothesis, while most recently the effects of cannabis have provoked interest in the role of endocannabinoids in schizophrenia. None of these models account for the complete picture of schizophrenia; rather the various drug models mimic different aspects of the illness. Determining the different molecular effects of those drugs whose pharmacological effects do and do not mimic the various aspects of schizophrenia has much to teach us concerning the pathogenesis of the illness.

Psychiatry's next top model: cause for a re-think on drug models of psychosis and other psychiatric disorders

Despite the widespread application of drug modelling in psychiatric research, the relative value of different models has never been formally compared in the same analysis. Here we compared the effects of five drugs (cannabis, psilocybin, amphetamine, ketamine and alcohol) in relation to psychiatric symptoms in a two-part subjective analysis. In the first part, mental health professionals associated statements referring to specific experiences, for example 'I don't bother to get out of bed', to one or more psychiatric symptom clusters, for example depression and negative psychotic symptoms. This measured the specificity of an experience for a particular disorder. In the second part, individuals with personal experience with each of the above-listed drugs were asked how reliably each drug produced the experiences listed in part 1, both acutely and sub-acutely. Part 1 failed to find any experiences that were specific for negative or cognitive psychotic symptoms over depression. The best model of positive symptoms was psilocybin and the best models overall were the acute alcohol and amphetamine models of mania. These results challenge current assumptions about drug models and motivate further research on this understudied area.

Avoidable mortality of psychiatric patients in an area with a community-based system of mental health care

OBJECTIVE

To ascertain the existence of an excess of avoidable mortality among psychiatric patients in an area with a community-based system of care, to identify predictors of higher risk of avoidable mortality and to provide some possible indication to reduce avoidable mortality in modern psychiatric services.

METHOD

All patients with an ICD-10 psychiatric diagnosis, living in a catchment area of about 75,000 inhabitants, seeking care in 1982-2001 were included (n = 6956). Mortality and causes of death were ascertained using linkage procedures with other local health databases. Standardized mortality ratios (SMRs) were calculated for each avoidable cause of death.

RESULTS

The observed number of deaths for those causes considered avoidable by the European Community was four times greater than the expected (P < 0.01). SMR was higher for deaths preventable with adequate health promotion policies than for those preventable with appropriate health care. Males, alcohol/drug addicted and young patients have the highest avoidable SMRs.

CONCLUSION

These findings urgently call for the implementation of health promotion and preventive programs targeted to psychiatric patients. Moreover, mental health services should improve the capacity to manage medical health problems of their patients.

A public health genetic approach for schizophrenia

BACKGROUND

Schizophrenia is a complex disease resulting from the interplay of genetic and environmental factors. However, psychiatric genetics and epidemiology have often worked as independent research fields.

AIMS

To review the evidence about the gene-environment interplay involved in the development of schizophrenia.

METHODS

Systematic review of medical and psychological databases.

RESULTS

On one hand, quantitative and molecular genetics showed high heritability for schizophrenia and identified genes likely to be involved in its pathophysiology. The strength of the association between candidate genes and schizophrenia is however modest, and the need for a more appropriate conceptualization of the genetic risk has been claimed. On the other hand, psychiatric epidemiology described several environmental factors liked with the onset or the course of schizophrenia. The observational nature of epidemiology, however, may hamper inference on causation. Gene-environment correlations and interactions influence the exposure and the vulnerability to the environment, respectively. Current findings suggest that gene-environment correlations and interactions may be common phenomena in the pathophysiology of schizophrenia. The consideration of gene-environment interplay may help to overcome many limitations of genetic and epidemiological studies in psychiatry and suggest innovative preventive and therapeutic strategies.

CONCLUSIONS

Taking into account the complexity of schizophrenia pathophysiology, mental health genetics may provide a comprehensive and heuristic model of disease.