Ion Dependence of Neurotransmitter Uptake: Inhibitory Effects of Ion Substitutes

Abstract

Several ions commonly used as substitutes for Na+ or Cl- were found to inhibit directly the high-affinity uptake of norepinephrine, dopamine, serotonin, and gamma-aminobutyric acid, but not glutamate or glutamine. When Na+ was partially replaced by any of several different cations or sucrose the uptake of all neurotransmitters studied except that of serotonin was reduced more than could be accounted for by just the inhibitory effect of the cation substitute. In contrast, when Cl- was partially replaced by any of several anions only the uptake of dopamine was reduced more than could be accounted for by the inhibitory effect of the anion substitute. These results suggest that for most neurotransmitters the electrochemical potential for Na+, but not for Cl-, contributes to the uptake driving force. When either Na+ or Cl- was totally replaced by an ion substitute or by sucrose the high-affinity uptake was virtually abolished, an exception being that glutamate uptake was not affected when isethionate was substituted for Cl-. The lack of uptake in the absence of either Na+ or Cl- may reflect a specific role for these ions in either increasing the affinity between the substrate and the carrier, or facilitating the translocation process. Alternatively, the transport carriers may undergo a nonspecific conformational change to an inactive form in the absence of Na+ or Cl-. A partial substitution of Na+ with Li+ or sucrose differentially affected the kinetics of uptake in that replacement with Li+, but not sucrose, usually resulted in a marked increase in the Km values.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding of the [125I]3?-(Iodophenyl)tropan-2?-carboxylic acid isopropyl ester to the dopamine transporter at a physiologically relevant temperature: Mutually exclusive binding and different ionic requirements for various uptake blockers and substrates

“…Wall et al (1993) reported a different Cl 2 requirement for [ 125 I]RTI-55 and DA binding. However, they did not compare the change in binding affinities, and the effect of the substitute anion (SO 4 22 ) remains to be clarified (see Shank et al, 1987). The present observation of a differential NaCl sensitivity of several uptake blockers and substrates in binding to the DAT is not affected by the substitution issue, because substitutes were not used.…”

Characterization of a genetically reconstituted high-affinity system for serotonin transport.

“…This system is characterized by ligand specificity, Na+ and temperature dependence, antagonizability by known blockers (nortriptyline, imipramine, and desipramine), and saturable and high-affinity uptake kinetics. In particular, the Km and Vma values are well within the range of such parameters measured for 5-HT-uptake systems in the central nervous system and blood platelets (6)(7)(8)(9)(10)(11)(12)(13).…”

“…Although the physiology and pharmacology of a number of neurotransmitter transport systems including that for serotonin have been extensively studied (6)(7)(8)(9)(10)(11)(12)(13), little is known about the biochemistry and molecular biology of any of these systems. With the hope of attaining such information, we report here the use of gene transfer (14)(15)(16)(17) in the establishment and identification of a clonal mouse fibroblast cell line, L-S1, that stably expresses a presumptively human highaffinity uptake system for serotonin (5-HT).…”

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

“…In studies of the ion dependencies of dopamine transport, Na ϩ was substituted with choline and Clwith isethionate (Holz and Coyle, 1974;Shank et al, 1987;Kreuger, 1990;McElvain and Schenk, 1992;Povlock and Schenk, 1997). The resulting initial velocities were analyzed as described above, and in some experiments the effects of cocaine on the ion dependencies were investigated.…”