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

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 >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.