Effects of cocaine and related drugs in nonhuman primates. I. [3H]cocaine binding sites in caudate-putamen

Abstract

Specific binding sites for [3H]cocaine were identified in caudate-putamen membranes prepared from nonhuman primate brains (Macaca fascicularis and Saimiri sciureus). Saturation of the sites was determined in competition studies using a fixed concentration of [3H]cocaine (2.7 nM) and increasing concentrations of unlabeled cocaine (1 pM-100 microM). Computer resolution of the shallow displacement curve (nH, 0.58) revealed that a two-component binding model [Kd1, 19.2 nM, maximum binding1 (Bmax1), 28.3 pmol/g of tissue; Kd2, 1120 nM, Bmax2, 431 pmol/g of tissue] was statistically preferred over a one-component model (K.50, 283 nM, 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 nM to over 100 microM in the following rank order of potency: WIN 35,428 greater than WIN 35,065-2 greater than (-)-cocaine greater than WIN 35,981 greater than (-)-norcocaine greater than WIN 35,140 greater than (+)-cocaine, (+)-pseudococaine greater than 3 alpha-tropanyl-1H-indole-carboxylic acid ester greater than 1 alpha H-3 alpha-5 alpha H-tropan-3-yl-3,5-dichlorobenzoate greater than 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 microM. The rank order of potency was: ( +/- )-trans-3-(3',4'-dichlorophenyl)-N-methyl-1-indanamine greater than mazindol greater than nomifensine greater than methylphenidate 1-[2-[bis(4-fluorophenyl)methoxy]ethyl]- 4-(3-phenylpropyl)piperazine, N-[1-(2- benzo(b)thiophenyl)cyclohexyl]piperidine greater than (-)-cocaine greater than 1-amino-4-phenylbicyclo-[2,2,2]-octane greater than bupropion, nisoxetine greater than desipramine, talsupram greater than citalopram. Other drugs, including the dopamine releasing agent (+)-amphetamine and the dopamine receptor agonists (-)-apomorphine, (+)-4-propyl-9-hydroxy-naphthoxazine, 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.(ABSTRACT TRUNCATED AT 400 WORDS)