Abstract: The activation of neurons in specific areas of a mammalian brain are determined by measurement of levels of mRNA following administration of an agent that potentially affects neuronal activity. Differential levels of mRNA caused by an agent can be measured, as can the specific distribution of the alteration of neuronal activity. Many mental disorders are associated with the abnormal function of a particular area of the brain. The present invention allows the determination of which area of the brain, if any, is affected by an administered agent. Accordingly, the invention provides a valuable tool for assaying compounds as potential therapeutic agents.