A comparison of the effects of sibutramine hydrochloride,bupropion and methamphetamine on dopaminergic function: evidence that dopamine is not a pharmacological target for sibutramine

Sibutramine hydrochloride, a novel monoamine reuptake inhibitor antidepressant, has been studied to determine whether it alters dopaminergic function in the brain. Its effects have been compared with bupropion, a dopamine reuptake inhibitor, and methamphetamine, a dopamine reuptake inhibitor and releasing agent. Sibutramine (0.1–3 mg/kg PO) and methamphetamine (0.3–30 mg/kg PO) both prevented reserpine (0.75 mg/kg IV) ptosis in rats with ED₅₀ values of 0.6 mg/kg and 4.2 mg/kg, respectively. Bupropion (10–100 mg/kg PO) was ineffective against reserpine ptosis. The efflux of [³H]-dopamine from preloaded rat striatal slices was not altered by 10^–7–10^–5 M concentrations of sibutramine, BTS 54 354, BTS 54 505 (secondary and primary amine metabolites, respectively) or bupropion. In contrast, methamphetamine (10^–8–10^–4 M) caused a significant concentration-dependent increase in [³H]-dopamine release. Sibutramine (3 mg/kg IP or 6 mg/kg PO) and bupropion (10 mg/kg IP or 430 mg/kg PO) did not alter 3-methoxy-tyramine (3-MT) levels in rat striatum. Striatal 3-MT concentrations were, however, dose-dependently increased by methamphetamine (0.3–10 mg/kg IP or 0.42–4.2 mg/kg PO). Sibutramine (6 mg/kg PO) did not induce circling in rats with unilateral 6-hydroxydopamine lesions of the nigrostriatal dopaminergic neuronal tract. Bupropion (10–100 mg/kg PO) did not induce circling at the lowest dose, but caused increasing ipsilateral rotation at higher doses. Methamphetamine (0.42 or 4.2 mg/kg PO) induced ipsilateral circling with marked effects at the higher dose. In a two-choice lever pressing model using rats trained to discriminated-amphetamine (0.5 mg/kg IP) from saline, sibutramine (0.3–3 mg/kg IP) generalised to the saline lever. Bupropion (3–30 mg/kg IP) generalised tod-amphetamine at the highest dose, while methamphetamine (0.1–5 mg/kg IP) generalised to this lever at doses as low as 0.3 mg/kg. Overall, the rank order of potency for enhancing central dopaminergic function is methamphetamine > bupropion >> sibutramine. The data therefore indicate that dopamine is unlikely to be an important pharmacological target for reuptake inhibition by sibutramine.