Enhanced displacement of [3H]imipramine, but not [3H]paroxetine binding by plasma from depressed patients

Plasma from healthy subjects inhibited the binding of [3H]imipramine competitively and [3H]paroxetine non-competitively to platelet membranes in a volume-dependent manner. Plasma from 40 depressed patients was more effective at inhibiting [3H]imipramine, but not [3H]paroxetine, binding compared to plasma from matched controls. This difference was not related to recent antidepressant treatment. The results suggest that the concentration of an endogenous modulator of [3H]imipramine binding is increased in depressive illness. The concentration of alpha 1-acid glycoprotein, a proposed endogenous modulator of [3H]imipramine binding, did not differ between depressed patients and controls. Our results suggest that factors other than alpha 1-acid glycoprotein may modulate [3H]imipramine binding.