Sibutramine does not decrease the number of 5-HT re-uptake sites in rat brain and, like fluoxetine, protects against the deficits produced by dexfenfluramine.

The effect of sibutramine and dexfenfluramine on 5-HT re-uptake sites, labelled with [(3)H]paroxetine, have been determined in various rat brain regions. In addition, the ability of fluoxetine and sibutramine to protect against the changes in [(3)H]paroxetine binding produced by dexfenfluramine was examined. Sibutramine (9 mg/kg, p.o.) and dexfenfluramine (1, 3 and 10 mg/kg, p.o.) were administered twice daily (before 09.00 h and after 16.00 h) for four days, followed by a 14 day drug-free period. In the protection studies, fluoxetine (10 mg/kg, i.p.) and sibutramine (9 mg/kg, p.o.) were given 1 h prior to dexfenfluramine (10 mg/kg, p.o.) using the same dosing regimen as described above. Sibutramine (9 mg/kg, p.o.; three times its ED(50) to inhibit food intake at 2 h) had no significant effect on the number or affinity of 5-HT re-uptake sites the brain regions studied. In contrast, dexfenfluramine at an equivalent dose (3 mg/kg, p.o.) significantly decreased the number of 5-HT re-uptake sites in frontal cortex (by 35%), hippocampus (by 47%) and hypothalamus (by 27%). This effect was dose-dependent with marked decreases (by 58-84%) in the number of sites following 10 mg/kg, p.o. These effects were not associated with changes in binding affinity. Fluoxetine (10 mg/kg, i.p.) completely blocked the effect of dexfenfluramine (10 mg/kg, p.o.) without having any significant effect alone. Sibutramine (9 mg/kg, p.o.) also blocked the effects of dexfenfluramine, although the reversal was only partial in frontal cortex, hippocampus and hypothalamus. Thus sibutramine, unlike dexfenfluramine, does not alter brain 5-HT re-uptake sites. Furthermore, sibutramine and fluoxetine protect against the deficits in 5-HT re-uptake sites produced by dexfenfluramine. These data provide further evidence that sibutramine is a 5-HT re-uptake inhibitor and it does not have neurotoxic potential.

[3H]paroxetine binding in rat frontal cortex strongly correlates with [3H]5-HT uptake: effect of administration of various antidepressant treatments.

Paroxetine is a selective and potent inhibitor of 5-hydroxytryptamine (5-HT) uptake into serotonergic neurones. [3H]Paroxetine binding to rat frontal cortex was of high affinity with a high percentage of specific binding. The binding data of both competition and saturation studies fitted a single site binding model. [3H]Paroxetine binding was potently inhibited by the selective 5-HT uptake inhibitors. In addition, a very good correlation was demonstrated between the ability of twenty-three compounds to inhibit [3H]paroxetine binding to rat frontal cortical membranes and [3H]5-HT uptake into rat frontal cortical synaptosomes. These data support the view that [3H]paroxetine binds to a single site which corresponds to the 5-HT uptake site. Using this ligand, the effects of repeated administration of antidepressant drugs with a wide range of pharmacological actions and electroconvulsive shock on 5-HT reuptake sites were examined. [3H]Paroxetine binding parameters (Kd and Bmax) were unaltered by all treatments. It would, therefore, appear that antidepressant therapy does not produce adaptive changes in 5-HT uptake sites.

Effect on radiolabelled-monoamine uptake in vitro of plasma taken from healthy volunteers administered the antidepressant sibutramine HCl.

Sibutramine HCl, a monoamine reuptake inhibitor type of antidepressant, was administered to healthy male volunteers as either a single dose (12.5 or 50 mg) or repeated treatment (5-20 mg once daily or 15 mg twice daily). Plasma, obtained at regular intervals during and after sibutramine HCl or placebo treatment, was assayed in vitro for its ability to inhibit the uptake of [3H]-noradrenaline (NA) by rat cortical synaptosomes, [3H]-5-hydroxytryptamine (5HT) by human platelets and [14C]-dopamine (DA) by rat striatal synaptosomes. After both single and repeated sibutramine HCl administration, the rank order of uptake inhibition was [3H]-NA greater than [3H]-5HT greater than [14C]-DA. The level of monoamine uptake inhibition increased on daily administration to a plateau 4-6 days after initiation of treatment, for example, approximately 60% and 40% inhibition of [3H]-NA and [3H]-5HT, respectively, following 15 mg sibutramine HCl twice daily. The pattern of monoamine uptake inhibition following sibutramine HCl administration to man is similar to that observed in sibutramine HCl-treated rats, and probably at least partly reflects inhibition of uptake by drug metabolites in both species. The inhibition of monoamine uptake following sibutramine HCl administration to man is consistent with an antidepressant effect.