Inhibitory effects of clozapine and other antipsychotic drugs on noradrenaline transporter in cultured bovine adrenal medullary cells

The effects of clozapine and other antipsychotic drugs on noradrenaline (NA) transport were examined in cultured bovine adrenal medullary cells and in transfected Xenopus laevis oocytes expressing the bovine NA transporter. Incubation of adrenal medullary cells with clozapine (30–1000 ng/ml) inhibited desipramine (DMI)-sensitive uptake of [³H]NA in a concentration-dependent manner (IC₅₀=110 ng/ml or 336 nM). Other antipsychotic drugs such as haloperidol, chlorpromazine, and risperidone also decreased [³H]NA uptake (IC₅₀= 144, 220, and 210 ng/ml or 383, 690, and 512 nM, respectively). Eadie-Hofstee analysis showed that clozapine reduced V_max of uptake of [³H]NA and increased K_m. Furthermore, clozapine inhibited specific binding of [³H]DMI to plasma membranes isolated from bovine adrenal medulla (IC₅₀=48 ng/ml or 146 nM). Scatchard plot analysis of [³H]DMI binding revealed that clozapine decreased both B_max and K_d. Other antipsychotic drugs, including haloperidol, chlorpromazine, and risperidone, also reduced [³H]DMI binding to the membranes. In transfected Xenopus oocytes expressing the bovine NA transporter, clozapine inhibited [³H]NA uptake in a concentration-dependent manner similar to that observed in adrenal medullary cells. These results suggest that clozapine and haloperidol directly inhibit transport of NA by acting on the site of an NA transporter that influences both substrate transport and binding of tricyclic antidepressants. Received: 13 April 1999 / Final version: 2 November 1999