The repeated, intermittent administration of cocaine and other psychostimulants produces an augmentation in locomotor behavior upon subsequent exposure to these drugs, a phenomenon known as behavioral sensitization. Associated with behavioral sensitization by cocaine is an alteration in dopamine neurotransmission primarily in the mesolimbic dopamine system. This system consists of dopamine cell bodies located in the A10 region of the ventromedial mesencephalon and their projections to the nucleus accumbens. The molecular mechanisms mediating the development of sensitization to cocaine in the mesolimbic dopamine pathways are not characterized, but it is known that changes occurring in the cells within the A10 region are critical for the development of sensitization. As a first step towards examining the phenomenon of sensitization at the molecular level, changes in the levels of mRNAs coding for tyrosine hydroxylase (TH), cholecystokinin (CCK), and neurotensin (NT) will be assessed. All of these gene products are localized in the A10 region and are important modulators of dopamine cell function and therefore may play a crucial role in mediating sensitization. Messenger RNA coding for TH, the rate-limiting enzyme in dopamine synthesis, is decreased in animals which demonstrate behavioral sensitization to cocaine. This effect has been demonstrated early after withdrawal, but whether it is a permanent effect is not known. this proposal will examine the time-course of changes in TH mRNA levels in the A10 region by Northern blot analysis, and whether these changes are accompanied by altered TH enzyme activity in the A10 region and nucleus accumbens, as measured by DOPA accumulation using in vivo microdialysis. Subsequently, it will be determined whether altered TH mRNA levels are a result of reduced TH gene transcription and/or a reduction in TH mRNA half- life. Finally, the same Northern blots used to measure TH mRNA levels will be used to examine the levels of CCK and NT mRNAs in the A10 region at various times following acute and chronic cocaine. These studies will define the changes in gene expression of three molecules that are important for modulating mesolimbic dopamine transmission and thus may provide insight into the altered function of dopamine neurons in response to repeated cocaine exposure. Defining the molecular mechanisms relevant to sensitization by cocaine in an animal model could help explain the mechanisms involved in the development of paranoid psychosis in humans, and may have implications for pharmacological treatment of this disorder.
