Platelet imidazoline receptors as state marker of depressive symptomatology

Abstract

Objective: Previous studies have shown that imidazoline receptors (IR-1) are increased in platelets and frontal cortex of depressed patients, and this up-regulation is normalized (down-regulated) after antidepressant drug treatments. It has been hypothesized that IR-1 up-regulation during the depressive episode may be a state marker for depressive symptomatology. The goal of the present study was to address the state versus trait question.

Method: Twelve healthy subjects (six males and six females) met stringent inclusion and exclusion criteria for physical and mental health. They received desipramine for 6 weeks in order to simulate the length of time and dosing used previously to obtain an IR-1 down-regulation and a therapeutic response in depressed patients. Outcome and safety measures included clinical, psychological, and cardiovascular assessments obtained throughout the study. Plasma concentrations of desipramine were measured throughout the 6 weeks of treatment and again after 2 weeks following tapered discontinuation of desipramine. Platelet receptors were assessed by Western blotting and radioligand binding assays.

Results: Healthy subjects taking desipramine experienced mild dysphoric effects but there were no adverse events. The binding of 8 nM p-[(125)I]clonidine to IR-1 and alpha(2)-adrenoceptors in healthy subjects did not change during desipramine treatment. The immunodensity of the 33 kDa band associated with IR-1 gradually increased to a maximum, by week-6, of 26% higher than baseline (p < 0.01 compared to baseline). Two weeks after desipramine discontinuation, there was a decline in alpha(2)-adrenoceptor binding and 33 kDa band's immunodensity (p = 0.04).

Conclusions: The findings support the hypothesis that platelet IR-1 binding sites are a marker of mood state rather than of antidepressant-induced pharmacological regulation. By comparison, platelet alpha(2)-adrenoceptors appear to be regulated by desipramine as a pharmacological effect independent of mood state.

Figure 1

Effects of desipramine treatment on immunodensities of the 33 kD IRAS band in platelet total membranes of healthy subjects. Western blotting was performed with IRBP antiserum and immunodensities were determined by densitometry of films. Values represent integrated optical density units (O.D.) normalized to a 5-point standard curve on each blot of 2.5 – 12.5 μg total protein standard (the standard was a total membrane preparation of platelets from the Mississippi Blood Services, Jackson, MS). Bars represent mean ±SEM of values from 12 subjects. * Compared with week-0, p < 0.01 (Repeated measures ANOVA with Dunnet’s multiple comparison test.

Figure 2

Binding of p-[¹²⁵I]clonidine binding to platelet imidazoline-1 receptors (IR-1) and α₂-adrenoceptors (α₂–AR) in desipramine-treated depressed patients (n= 6) and desipramine-treated healthy subjects (n= 12). The specific binding of 8 nM p-[¹²⁵I]clonidine to α₂–AR was defined by subtracting non-specific binding in the presence of 10 μM NE, and to IR-1 by subtracting additional non-specific binding in the presence of 10 μM moxonidine. Values are expressed as fmoles of radioligand specifically bound to each site per mg membrane protein. (A) IR-1 comparisons in depressed patients before desipramine (pre-DMI) and after 6 weeks of desipramine treatment. (B) IR-1 comparisons in healthy subjects before desipramine (pre-DMI), after 6 weeks of desipramine treatment, and after additional 2 weeks of desipramine discontinuation (ns: non-significance). (C) α₂–AR comparisons in depressed patients before desipramine (pre-DMI) and after 6 weeks of desipramine treatment. (D) α₂–AR comparisons in healthy subjects before desipramine (pre-DMI), after 6 weeks of desipramine treatment, and after additional 2 weeks of desipramine discontinuation.