RELEASE AND UPTAKE RATES OF 5-HYDROXYTRYPTAMINE IN THE DORSAL RAPHE AND SUBSTANTIA-NIGRA RETICULATA OF THE RAT-BRAIN

Fast scan cyclic voltammetry with carbon fiber electrodes has been use d to investigate the dynamics of the neurotransmitter 5-hydroxytryptam ine (5-HT) in the extracellular fluid of two brain regions: the dorsal raphe and the substantia nigra reticulata. The method used previously was shown to be optimized to allow the time course of 5-HT concentrat ion changes to be measured rapidly. Measurements were made in slices p repared from the brains of rats with the carbon fiber electrode insert ed into the tissue and a bipolar stimulating electrode placed on the s lice surface. Identification of 5-HT as the detected substance in both regions was based on voltammetric, anatomical, physiological, and pha rmacological evidence. Autoradiography using [H-3]paroxetine revealed highest 5-HT transporter binding densities in the regions in which vol tammetric measurements were made. Evaluation of the pharmacological ac tions of tetrodotoxin and tetrabenazine, as well as the effects of cal cium removal, suggested that 5-HT storage was vesicular and that the r elease process was exocytotic. The effects of fluoxetine (0.5 mu M) we re typical of a competitive uptake inhibitor, changing K-m with little effect on V-max. Release of 5-HT was found to be maximal with wide (2 -ms) stimulus pulses in both regions, as expected for release from sma ll unmyelinated processes, and to increase linearly with the number of pulses when high frequencies (100 Hz) were used. At lower frequencies , the concentration observed was a function of both release and uptake . Kinetic simulations of the data revealed that the major difference i n 5-HT neurotransmission between the two regions was that release and uptake rates are twice as large in the dorsal raphe ([5-HT] per pulse = 100 +/- 20 nM, V-max = 1,300 +/- 20 nM/s for dorsal raphe; [5-HT] pe r pulse = 55 +/- 7 nM, V-max = 570 +/- 70 nM/s for substantia nigra re ticulata). When normalized to tissue content, uptake rates in both reg ions were identical and similar to rates previously reported for dopam ine in dopamine terminal regions. Nonetheless, compared with dopaminer gic transmission in terminal regions such as the striatum, the absolut e clearance rates in the substantia nigra reticulata and dorsal raphe were lower, resulting in a longer lifetime of 5-HT in the extracellula r fluid and allowing long-range interactions.