N~~o.r~irnw Vol. 51, No. I, pp. 55-64, 1992 Printed in Great Britam 0306-4522!92 $5.00 + 0.00 Pergamon Press Ltd i 1992 1BRO zyxwvu REGULATION OF TRANSIENT DOPAMINE CONCENTRATION GRADIENTS TN THE MICROENVIRONMENT SURROUNDING NERVE TERMINALS IN THE RAT STRIATUM K. T. KAWAGOE, P. A. GARRIS, D. J. WIEDE~A~~ and R. M. WIGHTMA~* Department of Chemistry and Curriculum In Neurobiology, CB # 3290, Venable Hall, University of North Carolina at Chapel Hill, Chapel Hill. NC 27599.3290, IJ.S.A. Abstract-Synaptic overflow of dopamine in the striatum has been investigated during electrical stimulation of the medial forebrain bundle in anesthetized rats. Dopamine has been detected with Nafion-coated, carbon-fiber electrodes used with fast-scan voltammetry. In accordance with previous results, dopamine synaptic overflow is a function of the stimulation frequency and the anatomical position of the carbon-fiber electrode. In some positions the concentration of dopamine is found to respond instantaneously to the stimulus when the time-delay for diffusion through the Nafion film is accounted for. In these locations the measured rates of change of dopamine are sufficiently rapid such that extracellular diffusion is not apparent. The rate of dopamine overflow can be described by a model in which each stimulus pulse causes instantaneous release, and cellular uptake decreases the concentration between stimulus pulses. Uptake is found to be described by a constant set of Michaelis-Menten kinetics at each location for concentrations of dopamine from 100 nM to I5 p M. The concentration of dopamine released per stimulus pulse is found to be greatest at low frequency ( < IO Hz) with stimulus trains, and with single-pulse stimulations in nomifensine-treated animals. The frequency dependence of release is not an effect of dopamine receptor activation; haloperidol (2.5 mg:kg) causes a uniform increase in release at ail frequencies. The absence of diffusional effects in the measurement locations means that the constants determined with the electrode are those operant inside intact striatal tissue during stimulated overflow. These values are then extrapolated to the case where a single neuron fires alone. The extrapolation shows that while the transient concentration of dopamine may be high (200 nM) at the interface of the synapse and the extrasynaptic region, it is normally very low ( -C 6 nM) in the bulk of extracellular fluid. An understanding of the factors which influence dopamine concentration in the extracellular fluid is important to discern the role of dopamine in neurotransmission. If appreciable concentrations of dopamine exist only in synaptic regions, then dopa- mine acts as a classical transmitter. Alternatively, if dopamine released from one terminal reaches concentrations in the extrasynaptic region that are sufficiently high to activate receptors over a wide spatial range, it may act as a neuromodulator.34 The concentration of dopamine in the extracellu- lar fluid of the rat brain has been the subject of several investigations,“,‘4,42.54 and current estimates range from 5 to 50 nM. Extracellular concen- trations in these ranges have been demonstrated to affect postsynaptic neurons in the striatum with the combined use of electrophysiology and in ciao voltammetry.5’~53 The synaptic concentration of dopamine is a func- tion of the balance of release from terminals, cellular uptake, and metabolism, I7 although the latter process operates on a significantly longer time-scale.” Since release and uptake are rapid events, dopamine which .__~__ *To whom correspondence should be addressed. leaves the synapse will generate a transient concen- tration gradient controlled by extracellular diffusion. Thus, to understand the transient regulation of dopa- mine the central issues are release, uptake, and diffu- sion. Many techniques such as in uico dialysis sample the spatially and temporally averaged extracellular concentration.~.~’ However, a much different picture may emerge if viewed in real time at the outer edge of a synapse. Fast-scan cyclic voltammetry25,32 pro- vides the temporal resolution to aid in such a view. In previous work we have examined the rates of dopamine release, uptake and diffusion from synaptic overflow measurements made with carbon-fiber elec- trodes in the caudate nucleus during stimulation of the ascending dopaminergic fibers.49 The stimulation is designed so that the neurons are forced to fire in synchrony for a short interval f < 5 s). The concen- tration of dopamine measured during stimulated synaptic overflow varies with electrode location29.“.‘2 and is a function of the frequency and duration of the stimulus train.‘4.24 After stimulation, the dopamine rapidly disappears from the extracellular space as a result of cellular uptake, and can be described by Michaelis-Menten kinetics. Based on the assump- tions that each stimulus pulse releases an instan- taneous concentration from a group of neurons