Monoamine reuptake site occupancy of sibutramine: Relationship to antidepressant-like and thermogenic effects in rats.

Sibutramine was formerly marketed as an anti-obesity agent. The current study investigated the relationships between monoamine reuptake site occupancy for sibutramine and both its antidepressant-like efficacy and thermogenic effects. Sibutramine's effects on locomotor activity (LMA) and food intake were also evaluated. Sibutramine occupied monoamine reuptake binding sites with the rank order of potency of NET>SERT>DAT; at 10mg/kg, po, occupancy was 95% NET, 81% SERT and 73% DAT. Sibutramine produced antidepressant-like behavior in the forced swim test; at the lowest effective dose (3mg/kg, po) occupancy was 61%, 90% and 23% at SERT, NET and DAT sites, respectively. Sibutramine also increased body core temperature in a dose- and time-dependent manner; at the lowest effective dose (30mg/kg) SERT, NET and DAT occupancies were respectively 78%, 86% and 59%. A significant decrease in food consumption was observed at 3 and 10mg/kg, po. LMA was increased at ≥10mg/kg, sc. The relationship between efficacy in the FST and occupancy was also determined for citalopram, fluoxetine and reboxetine. Similarly, the relationship between thermogenesis and target occupancy for several single or double/triple reuptake inhibitors was measured and showed that >40-50% DAT binding was required for thermogenesis. Thermogenesis was blocked by the D1 antagonist SCH39166 (3mg/kg, sc). Our findings indicate that the antidepressant-like effect of sibutramine may result from additive or synergistic actions on the three reuptake binding targets. At higher doses, sibutramine produces thermogenesis; DAT inhibition and activation of dopamine D1 receptors are required for this effect.

Ex vivo assessment of binding site occupancy of monoamine reuptake inhibitors: methodology and biological significance.

The goal of this study was to develop and validate ex vivo binding assays for serotonin (SERT), norepinephrine (NET) and dopamine (DAT) transporters, and to use these assays to evaluate the binding site occupancy of triple and double monoamine reuptake inhibitors in rat brains. This study demonstrated that while autoradiographic methods provided anatomic precision and regional resolution, the homogenate binding method for site occupancy assessment yielded comparable sensitivity with markedly improved throughput. For ex vivo binding assays, the reduction of temperature and time during the in vitro process (primarily incubation with a radioligand) markedly decreased the dissociation of test agents from binding sites in brain tissues. This reduction, in turn, minimized the potential for underestimation of site occupancy in vivo especially for test compounds with affinity >10nM. The ratios of measured occupancy ED(50) values (doses at which 50% occupancy occurs) among SERT, NET and DAT sites for duloxetine, venlafaxine, nomifensine, indatraline, DOV 21,947 and DOV 216,303 were consistent with the ratios of the in vitro affinities between these target binding sites. The biological relevance of the monoamine transporter occupancy for these compounds is discussed.