Binding of pinoline on the 5-hydroxytryptamine transporter: competitive interaction with [3H] citalopram

Pinoline (6-methoxy-1,2,3,4-tetrahydro-beta-carboline) is a naturally occurring compound in the mammalian body which inhibits 5-hydroxytryptamine (5-HT) uptake and exerts antidepressant-like behavioural effects in rats. The present study investigates the effects of pinoline on [3H]citalopram binding to the 5-HT transporter on rat brain. Our experiments revealed that pinoline inhibits [3H]citalopram binding with IC50 1255 +/- 167 nM and Ki 572 +/- 76 nM; Hill coefficient for inhibition was close to 1. In saturation experiments, pinoline co-incubated with [3H]citalopram, increased dose-dependently the Kd value but had no effect on the Bmax value of [3H]citalopram binding. Micromolar concentrations of pinoline did not have influence on the dissociation rate of specifically bound [3H]citalopram. Binding parameters of [3H]citalopram did not differ significantly in cerebral cortex and hippocampus of rats treated for 10 days with pinoline or vehicle. These results indicate that pinoline did not have any modulative influence on the activity of 5-HT transporter and it interacts competitively with citalopram on the substrate recognition site of the 5-HT transporter.