Comparison of [3H]paroxetine and [3H]cyanoimipramine for quantitative measurement of serotonin transporter sites in human brain.

Previous studies have demonstrated that [3H]paroxetine and [3H]cyanoimipramine ([3H]CN-IMI) are highly selective ligands for the serotonin (5-HT) transporter. Using membrane preparation from the putamen, we confirmed that in human brain [3H]paroxetine labeled with high affinity one class of site associated with the 5-HT transporter. [3H]CN-IMI labeled two classes of sites in human brain. The one displaceable by 5-HT and with high affinity to 5-HT uptake inhibitors accounts for about 60% of the [3H]CN-IMI binding and, presumably, is associated with the 5-HT transporter. From the competition experiments, citalopram was selected to define [3H]CN-IMI binding to the 5-HT transporter in tissue sections because of its high selectivity to 5-HT transporter sites. A good correlation of the regional distribution patterns for [3H]CN-IMI and [3H]paroxetine was found using quantitative autoradiography. However, [3H]paroxetine underestimated high concentrations of the 5-HT transporter comparing to [3H]CN-IMI. This is likely to be due to the higher specific activity of [3H]CN-IMI. There is a good correlation between the regional distribution of 5-HT transporter sites labeled with either [3H]paroxetine or [3H]CN-IMI and the density of serotonergic innervation. This suggests that the brain areas that receive numerous serotonergic afferents, such as the hypothalamus and basal forebrain, might be common targets of these antidepressant drugs. Pharmacologic similarity of the sites labeled by both ligands as well as their similar distribution in the brain suggests that both antidepressant drugs interact with the same protein, thereby eliciting a similar neurochemical response.