Effects of chronic imipramine treatment on subclasses of platelet 3H-imipramine binding sites and plasma cortisol.

A decreased density of platelet 3H-imipramine (3H-IMI) binding sites has been proposed as a putative trait marker of major depressive illness. However, subsequent studies have demonstrated that the number of such sites is increased so as to be more like normal controls upon chronic treatment with antidepressant drugs. In addition, there is some evidence to suggest that altered 3H-IMI binding may be secondary to elevated plasma cortisol levels which are common in depressed patients and which normalize with remission. The present study compares platelet 3H-imipramine binding, plasma cortisol levels, and clinical improvement of 10 endogenous depressed patients before and after 6 weeks of treatment with imipramine-HCl. Total high affinity 3H-IMI binding sites were further differentiated into two subclasses on the basis of their relative sensitivities to cyanoimipramine (CNIMI) inhibition. Treatment was associated with a significant increase (134%) in CNIMI resistant binding but a decrease (45.2%) in CNIMI sensitive binding. While the former was significantly correlated with posttreatment cortisol levels, no significant correlation was found between cortisol and CNIMI specific binding. Neither site appeared to be directly related to mood state. The significance of these findings to the evaluation of platelet binding as a trait dependent marker is discussed.