Effects of Several Cations on the Neuronal Uptake of Dopamine and the Specific Binding of [³H]GBR 12783: Attempts to Characterize the Na⁺ Dependence of the Neuronal Transport of Dopamine

Abstract: We have studied the effects of several cations on (1) the neuronal uptake of [³H]dopamine ([³H]DA) and (2) the specific binding of 1‐[2‐(diphenylmethoxy)ethyl]‐4‐(3‐phenyl‐2‐[1‐³H]propenyl)piperazine ([³H]GBR 12783) to a site associated with the neuronal carrier of DA, in preparations obtained from rat striatum. When studied under the same experimental conditions, both the uptake of [³H]DA and the binding of [³H]GBR 12783 were similarly impaired by the gradual replacement of NaCl by sucrose. In both processes, no convenient substitute for N^a+ was found. Furthermore, potential substitutes of Na⁺ acted as inhibitors of the uptake with a rank order of potency as follows: K⁺= Li⁺ gtm Cs⁺ gtm Rb⁺ > choline+ > Tris⁺ > sucrose, which was somewhat different from that observed in binding studies, i.e., Cs⁺ > Rb⁺ > choline⁺ gtm K⁺ > Li⁺ > Tris⁺ > sucrose. In the presence of either 36 mM or 136 mM Na⁺, [³H]DA uptake was optimal with 2 mM Mg²⁺, 1 mM K⁺, or 1 mM Ca²⁺. In contrast, higher concentrations of divalent cations competitively blocked the uptake process. K⁺ concentrations > 50 mM impaired the specific binding, whereas in the millimolar range of concentrations, K⁺ noncompetitively inhibited the uptake. Decreasing the Na⁺ concentration increased the inhibitory effect of K⁺, Ca²⁺, and Mg²⁺ on the specific uptake. An increase in NaCl concentration from 0 to 120 mM elicited a significant decline in the affinity of some substrates for the [³H]GBR 12783 binding site. An uptake study performed using optimal experimental conditions defined in the present study revealed that decreasing Na⁺ concentration reduces the affinity of DA for the neuronal transport. We propose a hypothetical model for the neuronal transport of DA in which both Na⁺ and K⁺ membrane gradients are involved.