The present invention provides a method for identifying an agent to be tested for an ability to treat a psychotic disorder in a patient in need of such treatment. The invention provides a method for screening candidate drugs for anti-psychotic drug activity, preferably atypical anti-psychotic activity, comprising contacting cells or tissues with a candidate drug, determining levels of phosphorylation of the intracellular signaling proteins, DARPP-32, ERK1, ERK2 and CREB, in said cells or tissues and determining the pattern of the levels of phosphorylation of the proteins. The pattern of the levels of phosphorylation of DARPP-32, ERK1, ERK2 and CREB is, in certain embodiments, compared with the pattern of the levels of phosphorylation of DARPP-32, ERK1, ERK2 and CREB in the presence of an atypical anti-psychotic drug.