ALLOSTERIC REGULATION BY SODIUM OF THE BINDING OF [H-3] COCAINE AND [H-3] 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 ass ay. In low Na+ phosphate or bicarbonate-buffered sucrose (300 mOsm), t he tissue exhibited high affinity for [H-3]cocaine which was reduced b y the addition of Na+ in a dose-dependent manner. However, [H-3]GBR 12 935 binding was insensitive to Na+ in these physiological buffers. Alt hough binding of [H-3]GBR 12935 was displaced by cocaine in a manner c onsistent with competitive displacement, a non-linear affinity shift o f the displacement of [H-3]GBR 12935 by cocaine suggests that the two ligands bind to distinct sites. Binding of both radioligands was suppr essed when measured in sodium-free 50 mm Tris-sucrose and increased wi th the addition of Na+. Scatchard analysis indicated that B(max) for [ H-3]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 mm NaCl, both [H-3]cocaine an d [H-3]WIN 35428 binding exhibited lower affinities than in Na+-defici ent phosphate buffer. It is suggested that the cation form of Tris bin ds to the dopamine transporter and that the Tris-receptor complex does not bind [H-3]cocaine or [H-3]GBR 12935. Na+ displaces Tris, forming a Na+-receptor complex which binds these ligands. Thus, it is suggeste d that the Na+-dependent binding of cocaine to the dopamine transporte r is observed only in Tris.