Allosteric regulation by sodium of the binding of [3H]cocaine and [3H]GBR 12935 to rat and bovine striata.
Sodium regulation of ligand binding to the dopamine transporter of rat and/or bovine striata was investigated using a filtration binding assay. In low Na+ phosphate or bicarbonate-buffered sucrose (300 mOsm), the tissue exhibited high affinity for [3H]cocaine which was reduced by the addition of Na+ in a dose-dependent manner. However, [3H]GBR 12935 binding was insensitive to Na+ in these physiological buffers. Although binding of [3H]GBR 12935 was displaced by cocaine in a manner consistent with competitive displacement, a non-linear affinity shift of the displacement of [3H]GBR 12935 by cocaine suggests that the two ligands bind to distinct sites. Binding of both radioligands was suppressed when measured in sodium-free 50 nM Tris-sucrose and increased with the addition of Na+. Scatchard analysis indicated that Bmax for [3H]cocaine binding in Tris plus 120 mM NaCl reached the same level as in the physiological buffers. In Krebs-Ringer buffer with phosphate, bicarbonate or Tris, which contained 120 nM NaCl, both [3H]cocaine and [3H]WIN 35428 binding exhibited lower affinities than in Na(+)-deficient phosphate buffer. It is suggested that the cation form of Tris binds to the dopamine transporter and that the Tris-receptor complex does not bind [3H]cocaine or [3H]GBR 12935. Na+ displaces Tris, forming a Na(+)-receptor complex which binds these ligands. Thus, it is suggested that the Na(+)-dependent binding of cocaine to the dopamine transporter is observed only in Tris.
Eshleman
A J
AJ
Department of Pharmacology and Experimental Therapeutics, University of Maryland School of Medicine, Baltimore 21201.
Calligaro
D O
DO
Eldefrawi
M E
ME
D000494:Allosteric Regulation / Q000187:drug effects; D000818:Animals; D002021:Buffers; D002417:Cattle; D003042:Cocaine / Q000378:metabolism*; D003342:Corpus Striatum / Q000378:metabolism*; D004298:Dopamine / Q000378:metabolism; D008024:Ligands; D008297:Male; D010879:Piperazines / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012964:Sodium / Q000494:pharmacology*; D014316:Tritium
D002021:Buffers; D008024:Ligands; D010879:Piperazines; D014316:Tritium; C044630:1-(2 (diphenylmethoxy)ethyl)-4-(3-phenylpropyl)piperazine; D012964:Sodium; D003042:Cocaine; D004298:Dopamine