Inhibition of neuronal uptake of dopamine (DA) appears to be a primary and fundamental component of the reinforcing and stimulant actions of cocaine. Cocaine also inhibits the neuronal uptake of serotonin (5-HT) and this effect may contribute to the total pattern of cocaine actions in mammalian brain. A major focus of this proposal is a comparison of the factors regulating DA and 5-HT transport systems, especially in nucleus accumbens (NACC). The neuronal DA uptake systems in striatum (STR) and NACC play a major role in the removal of released DA from the extracellular space. Recent evidence suggests that DA transport in STR and NACC may be modified after exposure to drugs. In this application, we propose several studies designed to determine the extent to which DA and 5-HT transport in NACC and STR can be modified by receptor-mediated second messenger activation or other regulatory processes. The neuronal DA transporter has not yet been purified or sequenced; its subunit structure is unknown. Further characterization of the functions and regulation of the DA transporter would be facilitated by the availability of specific antisera, and by knowledge of factors regulating expression of its mRNA in response to treatments. We propose to continue our studies on the characterization of the transporter protein and to develop systems for the cloning of the mRNA for the DA transporter. It remains to be determined if cocaine also influences the release of DA from mesolimbic or other DA pathways, perhaps by potentiating or antagonizing the effects of neuroactive substances (such as 5-HT) in NACC that exert a local regulatory effect on DA release. We will continue experiments evaluating effects of cocaine on DA and 5-HT release mechanisms in NACC and STR. We have recently shown that a cocaine treatment regimen inducing behavioral sensitization results in a reduction in DA uptake and a hypersensitivity to cocaine-induced inhibition of transport in NACC. The mechanisms underlying the reduction in uptake are not clear, but the results again appear to point to a regulation of DA transporter function. These studies will be extended to include different cocaine treatment regimens and studies of chronic cocaine treatments on the regulation of DA and 5-HT release in NACC. We tentatively suggest that the inhibition of DA uptake in NACC is one component of a series of changes in DA and 5-HT neuron function that result from chronic cocaine administration. Collectively, these studies are designed to consider at a neurochemical level the effects of single doses and repeated cocaine dose schedules on specific DA and 5-HT pathways in brain. A greater knowledge of the effects of cocaine in brain may lead to a better understanding of reinforcement mechanisms in general, and should facilitate the identification of treatments than might be of value in the treatment of cocaine addiction.
