Seasonal variation in platelet 3H-imipramine binding: comparable values in control and depressed populations.

Recent findings of reduced 3H-imipramine binding in platelets of depressed patients compared to healthy controls have been proposed as a biological marker of depression. However, these studies failed to consider the possible occurrence of seasonal variation in the binding characteristics. Our results show a highly significant seasonal variation in platelet 3H-imipramine binding which occurs in both normal and depressed populations. Furthermore, when annual rhythms are taken into account, there is no difference in the binding parameters in the two populations.

[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.