PROJECT SUMMARY The efficacious treatment of psychotic disorders remains a significant challenge in psychiatry. Current treatment approaches rely on compounds discovered serendipitously many decades ago and there is an urgent need for new therapeutic approaches that target the neurobiology of psychosis. Postmortem studies of schizophrenia and psychotic bipolar disorder suggest a significant role for dendritic spine abnormalities in the prefrontal cortex (PFC) in psychosis. Brains of patients with psychotic disorders show well-replicated neuronal abnormalities in dendritic spine density, specifically in upper-layer cortical pyramidal neurons. Additionally, in animal models of psychosis, brain tissue show decreased dendritic spine synapses in the PFC, with a more pronounced effect in the upper-layer cortical neurons. Furthermore, the decrease in spine synapses in these animal models was reversed with antipsychotic treatment. In vitro studies of cultured rodent neurons show that the antipsychotic clozapine modulates dendritic spines, with two independent studies showing increase in dendritic spine density in rodent neurons with clozapine treatment. These studies provide a strong impetus to test the hypothesis that dendritic spine biology plays an important role in the biology and treatment of psychosis. The studies to date have focused on postmortem brains, animal models and rodent neuronal cultures. To extend these findings, we seek to study cortical neurons generated from subjects with psychotic disorders. We have reprogrammed induced pluripotent stem cells (iPSCs) from human subjects with schizophrenia and bipolar disorder as well as from matched healthy controls. We have differentiated human iPSCs along the forebrain lineage to generate mature cortical neurons. Based on our preliminary studies of patient-derived neurons and of the effects of clozapine on dendritic spine density, we hypothesize that upper- layer cortical neurons generated from subjects with psychotic disorders will show decreased dendritic spine density compared to such neurons from healthy subjects. We further hypothesize that clozapine exposure in vitro will have differential effects on dendritic spine density in neurons derived from patients who are clozapine responders when compared to clozapine non-responders.