[3H]Imipramine Labels Sites on Brain Astroglial Cells Not Related to Serotonin Uptake

Brain astroglial cells, whether from a bulk isolated preparation or in culture, have been shown to take up serotonin actively. 3H]imipramine has been proposed as a specific label for serotonin uptake sites in brain. We therefore studied the binding of 3H]imipramine to C6 astroglial cells in culture to determine if some of the binding of this radioligand in brain homogenates is actually to serotonin transporting sites on glia. 3H]Imipramine binds saturably (Bmax = 202 fmol/mg protein) and with high affinity (KD = 1.72 nM) to C6 cells. This binding is competitively inhibited by other tricyclic antidepressants. The C6 cells actively transport 3H]serotonin with a Km of 2 microM and a Vmax of 1080 fmol/10(6) cells/min. However, the pharmacological profile for inhibition of serotonin uptake does not correlate with the pharmacological profile for inhibition of 3H]imipramine binding. These results suggest that the binding of 3H]imipramine to astroglial cells is not related to their capacity for active uptake of serotonin. Further, in brain homogenates, some of the binding of 3H]imipramine may not be to neuronal uptake sites but rather may be to sites on astroglial cells.