A model of the sodium dependence of dopamine uptake in rat striatal synaptosomes

Abstract

Initial velocity of uptake of dopamine (DA) has been measured in rat striatal synaptosomes as a function of both [DA] and [Na]. Carrier mediated uptake is totally dependent on external sodium. The data were fitted to a rapid equilibrium model which has been found in previous studies to fit, with appropriate simplification, uptake data for glutamate, GABA, and choline in several brain regions under varying conditions. This model also gives a good fit to the dopamine data. The minimal best fit simplification of this model allows for DA uptake along with two sodium ions and predicts that apparent maximal velocity of uptake should increase with [Na], while the Michaelis-Menten constant should decrease. The minimal best fit model for DA, and a number of kinetic parameters which quantitate the model, are compared to those for the GABA, glutamate, and choline transporters. The results are consistent with a symmetrical, rapid equilibrium model, which has been presented previously for other neurotransmitters and precursors (18). This model offers a unifying basis for understanding the sodium and membrane potential dependence of neurotransmitter transport and the possible participation of transporters in depolarization induced release throughout the CNS.

Multisubtrate mechanism for the inward transport of dopamine by the human dopamine transporter expressed in HEK cells and its inhibition by cocaine

“…Inward transport could be blocked with 100 µM cocaine, and is dependent on Na ϩ and Cl -with reaction orders of 2 and unity, respectively. These results are in agreement with previously published studies (Krueger, 1990;Amejdki-Chab et al, 1992a;Wheeler et al, 1993) on the uptake of [ 3 H]dopa- mine into striatal preparations as well as those studies where nonradiolabeled dopamine was monitored (McElvain and Schenk, 1992;Povlock and Schenk, 1997). In HEK hDAT cell suspensions, [Na ϩ ] or [Cl -] reduction decreases the V max and increases the K m for dopamine inward transport (Fig.…”

“…3), which is inconsistent with observations reported in the literature. Our observed effects on the kinetic parameters are in agreement with those reported by Wheeler et al (1993) in studies of striatal synaptosomes where inward transport was measured using [ 3 H]dopamine. In contrast Holz and Coyle (1974), also measuring inward transport of [ 3 H]dopamine in striatal synaptosomes, found a decrease in V max when [Na ϩ ] was decreased, but observed no change in K m .…”

“…It is well established that the DAT in the striatum and nucleus accumbens of the rat requires Na ϩ and Cl -as cosubstrates during dopamine inward transport (Holz and Coyle, 1974;Harris and Baldessarini, 1973;Kuhar and Zarbin, 1978). The reaction orders for Na ϩ and Cl -are 2 and unity, respectively, and the kinetic mechanism is a partially random, sequential process where dopamine or Na ϩ bind first and Cl -binds last (McElvain and Schenk, 1992;Wheeler et al, 1993;Povlock and Schenk, 1997). Cocaine is thought to be an uncompetitive inhibitor with respect to dopamine, but competitive with respect to both Na ϩ and Cl -in the striatum and cocaine is thought to bind at the Na ϩ site on the dopamine occupied transporter (McElvain and Schenk, 1992).…”

Regulation of the Dopamine and Vesicular Monoamine Transporters: Pharmacological Targets and Implications for Disease

“…Substrate release shifts the DAT from its inward-facing conformation to the outward-facing conformation. In this model, DA is stoichiometrically transported across the membrane and is highly dependent upon the concentration of extracellular Na + (Wheeler et al, 1993). Inward and outward conformations also expose different DAT residues to the extracellular space, which affects the binding properties of specific pharmacologic agents (e.g., cocaine), as discussed in section VI.A.2.…”

Normal and Physio-Pathological Striatal Dopamine Homeostasis

“…In this process, transmitter molecules are transported outside the cell, most likely via an exchangediffusion process. This mode of transfer has been extensively studied and various mathematical models have been proposed over the course of the last four decades (Chubb et al, 1972;Stein, 1990, for a review; Thoenen et al, 1969;Wheeler et al, 1993;Ziance and Rutledge, 1972).…”