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

Abstract

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 erotonin transporting sites on glia. [³H]Imipramine binds saturably (B_max= 202 fmol/mg protein) and with high affinity (K_D= 1.72 nM) to C₆ cells. This binding is competitively inhibited by other tricyclic antidepressants. The C₆ cells actively transport [³H]serotonin with a K_m of 2 μ.M and a V_max of 1080 fmol/10⁶ 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.