Cannabis use disorder (CUD) is up to ten times more common in schizophrenia (SCZ) than in the general population, and substantially worsens the course of this severe psychiatric disorder. Since SCZ occurs in 1% of the population, the comorbidity of CUD in 13% to 42% of people with this disorder presents society with an important public health problem. At present, treatments available for these "dual diagnosis" patients are inadequate. New treatments to limit cannabis use in patients with schizophrenia are sorely needed.

While the basis of substance use in patients with SCZ is not clear, some have suggested that use of substances may "self-medicate" negative symptoms or the side effects they experience from antipsychotic treatment. We have proposed an alternative formulation of this "self-medication hypothesis" -- a neurobiological formulation suggesting that a dysregulated mesocorticolimbic "brain reward circuit" (BRC) in patients with SCZ underpins their substance use, and that cannabis or other substance use ameliorates this dysregulated circuitry.

Our formulation is based on literature suggesting that the reinforcing effects of substances of abuse, including cannabis, may be related to their stimulation of dopamine (DA) neurons in the prefrontal cortex (PFC) and the mesolimbic system, key components of the BRC. Thus, according to this formulation, cannabis use "medicates" the dysregulated brain reward circuitry in patients with SCZ and allows them to have more normal responses to naturally rewarding events. Using a monetary probe linked to fMRI, we have demonstrated that patients with SCZ and co-occurring CUD (in agreement with preliminary studies from other investigators of non substance abusing patients) do indeed have a deficit within their BRC (reduced activation of the nucleus accumbens) as compared to normal subjects. This proposal will allow us to directly test the effects of cannabis on the BRC in patients with SCZ and CUD and thus to confirm our hypothesis regarding its effects in these patients. In addition, the proposal seeks to assess whether the cannabinoid agonist dronabinol, when given to patients with SCZ and CUD, will also ameliorate this BRC deficit, and, thus, whether dronabinol could be considered as a potential adjunctive treatment (given with an antipsychotic medication) to decrease their cannabis use.

The study will consist of two phases - a Pilot Study and the Main Study. The Pilot Study, completed in 10 "dual diagnosis" patients prior to the initiation of the Main Study, will establish the dose of oral dronabinol and the THC concentration of the cannabis cigarette to be used in the subsequent Main Study. The Main Study will involve 3 groups of subjects: two groups of dual diagnosis patients (with SCZ and co-occurring CUD), randomly assigned to one of the groups, and a group of healthy control patients. All subjects will be studied at baseline (T1) and 4 days later (T2) with a monetary probe linked to fMRI to evaluate their brain reward circuitry. At T1 all subjects will be tested without any intervention. At T2, patients in Groups 1 and 2 will receive both a dronabinol (or placebo) pill and a cannabis (or placebo cannabis) cigarette in a blinded fashion before testing. Group 1 patients will receive an active cannabis cigarette and a placebo pill; Group 2 patients will receive an active dronabinol pill and a placebo cannabis cigarette. Multiple measures will be taken to insure the safety of these patients during the use of cannabis and dronabinol. Group 3 (healthy controls) will not receive pill or cannabis cigarette and will serve as a control for repeated testing. Analyses will assess whether baseline BRC activation is different between patients and the control group, and whether use of cannabis and of dronabinol at T2 normalizes activation of BRC relative to T1 and relative to controls at T2.