There is growing evidence of high rates of substance use disorders among individuals with psychotic disorders especially in young people with predisposition for psychosis. There is some genetic evidence that carriers of the valine158 allele of the catechol-O-methyltransferase (COMT) gene had increased risk to exhibit psychotic symptoms and to develop schizophrenia if they used cannabis by the age of 18. It was also shown that carriers of the COMT val/val genotype were most sensitive to THC-induced psychotic experiences but this was conditional on pre-existing susceptibility to psychosis. The investigators propose to use brain-imaging and molecular genetics to investigate whether genetic factors may contribute to the THC-induced dopamine release and possibly to cannabis- induced psychosis.

Genetic association study will be performed in 100 young cannabis users (age 18-26 years) for genes that are related to the neurotransmitters dopamine (D2, DAT, COMT), GABA, glutamate and the cannabinoid receptor CB1. Out of this cohort, 24 male subjects without history of cannabis or drug-induced psychosis will undergo brain imaging procedure in order to measure THC-induced dopamine release using [11C] Raclopride in PET imaging. Carriers of high activity COMT158Val allele are expected to show higher meso-limbic DA release than carriers of low activity COMT 158Met homozygotes and after smoking a cigarette with THC which in turn are thought to underlie the risk for THC-induced psychotic symptoms. The aim of the present study is to evaluate the relationship between the COMT genotype and cannabis-induced meso-limbic dopamine release as measured by D2 occupancy. In addition, the association between predisposition to psychosis, age of onset of cannabis dependence, and genotype of COMT and other neurotransmitters-related genes will be evaluated. Such finding could provide novel pharmaco-genetic explanation for the psychogenic effects of cannabis in vulnerable individuals.

Inclusion Criteria:

• age 18-26 meeting DSM-IV-TR (American Psychiatric Association, 1994) diagnosis of THC dependence will be recruited from the general population by advertisement in the newspapers.

Exclusion Criteria:

• dependence on other substances or alcoholism
• a history of CNS disease
• a history of infection that might affect CNS (HIV, syphilis, cytomegalovirus, herpes)
• a history of head injury with loss of consciousness and pregnancy

100 current users of cannabis