EFFECTS OF COCAINE AND RELATED DRUGS IN NONHUMAN-PRIMATES .1. [H-3] COCAINE BINDING-SITES IN CAUDATE-PUTAMEN

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  • 1 October 1989
    • journal article
    • research article
    • Vol. 251  (1) , 131-141
Abstract
Specific binding sites for [3H]cocaine were identified in caudate-putamen membranes prepared from nonhuman prepared from nonhuman primate brains (Macaca fascicularis and Saimiri sciureus). Saturation of the sites was determined in competiton studies using a fixed concentration of [3H]cocaine (2.7 nM) and increasing concentrations of unlabeled cocaine (1 pM-100 .mu.M). Computer resolution of the shallow displacement curve (nH, 0.58) revealed that a two-component binding model [Kd1, 19.2 mM, maximum binding1 (Bmax1), 28.3 pmpl/g of tissue; Kd2, 1120 nM, Bmax2, 431 pmol/g of tissue] was statistically preferred over a one-component model (K.50, 283 mM, Bmax, 471 pmol/g of tissue). Binding of [3H]cocaine was NaCl-dependent, with specific binding reduced by 72% when NaCl (100 mM) was omitted from the incubation medium. [3H]Cocaine was displaced stereoselectively by the enantiomers of cocaine and by the diastereoisomers of cocaine and its phenyltropane analog. Cocaine congeners displaced specifically bound [3H]cocaine with IC50 values ranging from 17 mM to over 100 .mu.M in the following rank order of potency: WIN 35,428 > WIN 35,065-2 > (-)-cocaine > WIN 35,981 > (-)-norcocaine > WIN 35,140 > (+)-cocaine, (+)-pseudococaine > 3.alpha.-tropanyl-1H-indole-carboxylic acid ester > 1.alpha.H-3.alpha.-5.alpha.H-tropan-3-yl-3,5-dichlorobenzoate > benzoylecgonine, benzoylnorecgonine and (-)-pseudococaine. Several monoamine uptake inhibitors structurally unrelated to cocaine also displaced [3H]cocaine with IC50 values ranging from 1.6 nM to 50 .mu.M. The rank order of potency was: (.+-.)-trans-3-(3'',4''-dichlorophenyl)-N-methyl-1-indanamine > mazindol > nomifensine > methylphenidate, 1-[2-(bis(4-fluorophenyl)methoxy]ethyl]-4-(3-phenylpropyl)piperine, N-[1-(2-benzo(b)thiophenyl)cyclohexyl]piperidine > (-)-cocaine > 1-amino-4-phenylbicyclo-[2,2,2]-octane > bupropion, nisoxetine > desipramine, talsupram > citalopram. Other drugs, including the dopamine releasing agent (+)-amphetamine and the dopamine receptor agonists (-)-apomorphine, (+)-4-propyl-9-hydroxynaphthoxazine, quinpirole and SKF 38393 were weak displacers of [3H]cocaine. Monoamine neurotransmitters also were relatively weak, but dopamine was considerably more potent than either norepinephrine or serotonin. The affinities of the cocaine congeners and other monoamine uptake inhibitors at [3H]cocaine binding sites correspond closely with their reported potencies for inhibiting uptake of dopamine (r, 0.93, P < .0001), but not norepinephrine or serotonin (r, 0.54 and 0.38, respectively). The results are consistent with the view that [3H]cocaine bindings sites in monkey caudate-putamen are associated with elements of the dopamine uptake system and mediate the inhibitory effects of cocaine on dopamine transport.