High affinity stereospecific binding of [3H] cocaine in striatum and its relationship to the dopamine transporter

A high affinity (KD 35 nM) binding site for 3H]cocaine is detected in rat brain striatum present at 2-3 pmol/mg protein of synaptic membranes. This binding is displaced by cocaine analogues with the same rank order as their inhibition of 3H]dopamine (3H]DA) uptake into striatal synaptosomes (r = 0.99), paralleling the order of their central stimulant activity. The potent DA uptake inhibitors nomifensine, mazindol, and benztropine are more potent inhibitors of this high affinity 3H]cocaine binding than desipramine and imipramine. Cathinone and amphetamine, which are more potent central stimulants than cocaine, displace the high affinity 3H]cocaine binding stereospecifically, but with lower potency (IC50 approximately equal to 1 microM) than does cocaine. It is suggested that the DA transporter in striatum is the putative "cocaine receptor." Binding of 3H]cocaine, measured in 10 mM Na2HPO4-0.32 M sucrose, pH 7.4 buffer, is inhibited by physiologic concentrations of Na+ and K+ and by biogenic amines. DA and Na+ reduce the affinity of the putative "cocaine receptor" for 3H]cocaine without changing the Bmax, suggesting that inhibition may be competitive. However, TRIS reduces 3H]cocaine binding noncompetitively while Na+ potentiates it in TRIS buffer. Binding of 3H]mazindol is inhibited competitively by cocaine. In phosphate-sucrose buffer, cocaine and mazindol are equally potent in inhibiting 3H]mazindol binding, but in TRIS-NaCl buffer cocaine has 10 times lower potency. It is suggested that the cocaine receptor in the striatum may be an allosteric protein with mazindol and cocaine binding to overlapping sites, while Na+ and DA are allosteric modulators, which stabilize a lower affinity state for cocaine.