Platelet 3H-imipramine binding: a state-dependent marker in depression.

Reduced density of 3H-imipramine binding sites (Bmax) to platelets has been reported in depressed patients during an episode of illness. In the present study we assessed the usefulness of decreased Bmax of platelet 3H-imipramine binding as an indicator of the depressed state. We also investigated the effect of long-term treatment with imipramine on platelet 3H-imipramine binding. A comparison of platelet 3H-imipramine binding in 10 drug-free depressed patients and 8 normal volunteers revealed significantly lower mean Bmax values in depressed patients, whereas the affinity (Kd) of 3H-imipramine binding was identical in both groups. A longitudinal study of platelet 3H-imipramine binding in 10 depressed patients during and after imipramine treatment (125-200 mg/day) revealed consistently low Bmax values despite clinically meaningful improvement. However, Bmax values increased significantly following complete remission and remained elevated even after imipramine had been discontinued for 4 weeks. These findings suggest that decrease in the sensity of platelet 3H-imipramine binding sites in depressed patients is not likely to be a direct drug effect and that normalization of this variable may follow clinical remission.