Monitoring Extracellular DOPAC Following Stimulated Release of Dopamine

Abstract

To understand the relationship of dopamine (DA) release to behavior, studies in which DA release, or an index of DA release, is monitored in intact animals are needed to complement studies using lesions of dopaminergic systems. Monitoring DA release directly is relatively difficult, particularly in behaving animals. One approach to the problem of monitoring DA release has been to use metabolite levels, either total tissue levels or extracellular levels, as indices of DA release. It has been known for some time that dopamine turnover in nigrostriatal neurons is dependent on nervous impulse flow (Anden et al., 1971) and electrical stimulation of these neurons produces an elevated turnover (Korf et al., 1976). Tissue measurements are useful for spatial mapping of activity, but require large numbers of animals for temporal information. In order to use extracellular metabolite information effectively, the temporal relationship of the extracellular levels of the metabolite to neurotransmitter release must be known. In this chapter the temporal relationship of extracellular dihydroxyphenylacetic acid (DOPAC) to stimulated dopamine release is described. In vivo voltammetry is used to observe the change in extracellular DOPAC following brief electrical stimulation of the medial forebrain bundle (MFB) (Michael et al., 1985). A similar method has been used to observe homovanillic acid in cerebrospinal fluid (Wightman et al., 1978). Our results indicate that there is considerable delay in the appearance of DOPAC in the extracellular fluid (ECF) following stimulated dopamine release. Because released dopamine is rapidly cleared from ECF following short periods of electrical stimulation (Ewing and Wightman, 1984), the changes in extracellular dopamine and DOPAC following stimulation occur at different times. This has enabled us to monitor directly the appearance and clearance of DOPAC in striatal ECF. The time course of the change in extracellular DOPAC is then used to characterize the kinetics of dopamine metabolism to extra-cellular DOPAC. Using this technique, rate constants for formation of DOPAC and its clearance from the extracellular space have been determined. This approach represents a new, nonpharmacological method for studying neurotransmitter metabolism. When combined with additional information, the compartmentation of dopamine may also be examined. The voltammetric data are validated by pharmacological manipulation of dopamine synthesis and metabolism. This approach should also work for examining serotonin and norepinephrine metabolism and compartmentation. The equivalence of rate constants determined by voltammetry and tissue assays has been demonstrated for clearance of the serotonin metabolite 5-hydroxindole acetic acid (5-HIAA) (Echizen and Freed, 1984).