Calligaro D O, Eldefrawi M E
Department of Pharmacology and Experimental Therapeutics, University of Maryland School of Medicine, Baltimore 21201.
Membr Biochem. 1987;7(2):87-106. doi: 10.3109/09687688709039986.
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.
在大鼠脑纹状体中检测到一个对[³H]可卡因具有高亲和力（解离常数KD为35 nM）的结合位点，其在突触膜蛋白中的含量为2 - 3 pmol/mg。这种结合可被可卡因类似物取代，取代顺序与其抑制[³H]多巴胺（[³H]DA）摄取到纹状体突触小体中的顺序相同（r = 0.99），与它们的中枢兴奋活性顺序平行。强效的DA摄取抑制剂诺米芬辛、马吲哚和苯海索对这种高亲和力的[³H]可卡因结合的抑制作用比地昔帕明和丙咪嗪更强。卡西酮和苯丙胺作为比可卡因更强效的中枢兴奋剂，能立体特异性地取代高亲和力的[³H]可卡因结合，但效力比可卡因低（半数抑制浓度IC50约为1 μM）。有人提出纹状体中的DA转运体可能是假定的“可卡因受体”。在10 mM Na₂HPO₄ - 0.32 M蔗糖、pH 7.4缓冲液中测得的[³H]可卡因结合受到生理浓度的Na⁺和K⁺以及生物胺的抑制。DA和Na⁺降低了假定的“可卡因受体”对[³H]可卡因的亲和力，而不改变最大结合量（Bmax），这表明抑制作用可能是竞争性的。然而，三羟甲基氨基甲烷（TRIS）以非竞争性方式降低[³H]可卡因结合，而在TRIS缓冲液中Na⁺则增强其结合。[³H]马吲哚的结合受到可卡因的竞争性抑制。在磷酸盐 - 蔗糖缓冲液中，可卡因和马吲哚在抑制[³H]马吲哚结合方面效力相当，但在TRIS - NaCl缓冲液中，可卡因的效力低10倍。有人提出纹状体中的可卡因受体可能是一种变构蛋白，马吲哚和可卡因结合到重叠位点，而Na⁺和DA是变构调节剂，它们稳定了可卡因的低亲和力状态。
