Effects of low extracellular chloride on dopamine release and the dopamine transporter

PA Diliberto, RA Jeffs and LX Cubeddu

Division of Clinical Pharmacology, University of North Carolina, Chapel Hill.

Removal of chloride (Cl-) from the superfusion medium results in increased spontaneous efflux of dopamine (DA) but not acetylcholine from rabbit striatal slices prelabeled with [3H]DA and [14C]choline. Cl- was substituted to varying degrees with the impermeant anion, isethionate (IS-), or the permeant anion, nitrate (NO3-). The magnitude of low Cl(-)-induced DA efflux was inversely related to the external [Cl-] and was greater with IS- than with NO3-. Analysis of the composition of the 3H efflux in terms of DA and its metabolites revealed an increase in [3H]DA with decreasing Cl- concentration. Reduction of external Ca++ from 1.3 to 0.13 mM increased low Cl(-)- induced DA efflux. In slices depleted of vesicular DA by reserpine pretreatment and subsequently labeled and superfused in the presence of monoamine oxidase and catechol-O-methyltransferase inhibitors, the same inverse relationship between [Cl-] and spontaneous DA efflux was observed. Neuronal DA uptake inhibitors, nomifensine, mazindol, GBR- 12909 and cocaine, all increased the rate of low Cl(-)-induced DA efflux in the reserpinized preparation. Cl(-)-induced DA efflux in untreated and reserpinized preparations was not blocked by tetrodotoxin, amiloride, furosemide, picrotoxin or 4-acetamido-4'- isothiocyanatostilbene-2,2'-disulfonic acid). Low Cl- inhibited initial rates of [3H]DA uptake. At Cl- concentrations producing significantly different efflux rates (0 and 7.4 mM Cl-, IS- and NO3- substitution), DA uptake was inhibited in all cases by greater than 90%. Additionally, the binding of [3H]mazindol, one of the uptake inhibitors, to striatal membranes was unaffected by removal of Cl-. In summary, low Cl- produces a nonexocytotic rapid outward transport of DA. Extracellular Cl- is not required for the binding to transporter sites or for the inhibition of neuronal uptake produced by neuronal uptake inhibitors.

Volume 248, Issue 2, pp. 644-653, 02/01/1989
Copyright © 1989 by American Society for Pharmacology and Experimental Therapeutics

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