[3H]cocaine labels a binding site associated with the serotonin transporter in guinea pig brain: Allosteric modulation by paroxetine

We studied the characteristics of ³H]cocaine binding to membranes prepared from whole guinea pig brain. Cocaine binding was specific and saturable. A one-site binding model fit the data adequately: the Kd value of ³H]cocaine was 44 nM with a Bmax value of 280 fmol/mg protein. The rank order of potency for the ³H]cocaine binding site was paroxetine > clomipramine > (–)-cocaine > fluoxetine > mazindol > desipramine > GBR12909 > phencyclidine > benztropine > GBR12935 > (+)-cocaine. The IC50 values of these drugs for inhibition of ³H]cocaine binding were highly correlated with their IC50 values for inhibition of ³H]5-HT uptake into synaptosomes prepared from whole guinea pig brain. High affinity 5-HT uptake inhibitors produced dose-dependent wash-resistant (pseudoirreversible) inhibition of ³H]cocaine binding. The wash-resistant inhibition produced by paroxetine was due to an increase in the Kd of ³H]cocaine binding sites, and was accompanied by an increase in the dissociation rate, consistent with an allosteric mechanism. These studies suggest that, using membranes prepared from whole guinea pig brain, ³H]cocaine labels a binding site associated with serotonin transporter and that paroxetine and cocaine bind to different sites on the serotonin transporter.Abbreviations GBR12909 1-(2-{bis(4-fluorophenyl)methoxy}ethyl)-4-{3-phenylpropyl}piperazine - TCP 1-{1-(2-thienyl)cyclohexyl}piperidine - BTCP N-{1-(2-benzo(b)thiophenyl)cyclohexyl}piperidine - PCP 1-(1-phenylcyclohexyl)piperidine - GBR12935 (1-2-(diphenylmethoxy)ethyl]-4-(3-phenylpropyl)piperazine) - CMI clomipramine