Comparison of 3H-para-aminoclonidine binding to different platelet preparations

Alpha 2-adrenoceptors and I1-imidazoline binding sites: relationship with catecholamines in women of reproductive age

A comparison is presented between plasma catecholamine concentrations and platelet [125I]-p-iodoclonidine binding sites in 16 healthy women. Blood samples were obtained at six regularly spaced intervals over two consecutive menstrual cycles from healthy women with regular menstrual periods. Although no cycle-related changes were observed per se, there were significant correlations between the platelet binding sites and plasma norepinephrine and epinephrine concentrations. The densities of platelet alpha 2-adrenoceptors were negatively correlated in an exponential fashion (r2 = 0.694, P = 0.009) with plasma epinephrine concentrations, implying agonist-induced downregulation. On the other hand, platelet I1-imidazoline binding sites were positively correlated with plasma concentrations of norepinephrine in a linear fashion (r2 = 0.326, P = 0.021). This is the first indication that I1 binding sites might be upregulated by a physiological factor. Furthermore, the data suggest that elevations in plasma norepinephrine might explain reports of upregulated I1 binding sites in depressed patients.

Imidazoline receptor proteins are regulated in platelet-precursor MEG-01 cells by agonists and antagonists

The I1-imidazoline receptor is a novel brainstem modulator of sympathetic outflow that is elevated on platelets and in brains of depressed patients. A positive correlation has been reported (accompanying manuscript) between plasma norepinephrine (NE) concentrations and the densities (Bmax) of platelet I1 binding sites (I1 sites). I1-candidate proteins of 33 kDa and 85 kDa are now identified on Western blots probed with anti-imidazoline receptor antiserum (IRBP antiserum), that correlate with Bmax values for I1 sites. Furthermore, a human megakaryoblastoma cell line (MEG-01) has been used to study the regulation of these proteins on megakaryocytic cells, while bovine adrenal chromaffin cells provide a standard I1 cell type for comparison. Both the 33 kDa and 85 kDa IRBP-immunoreactive bands were enriched in plasma membrane fractions. IRBP antiserum did not cross-react with I2 imidazoline binding sites located on platelet mitochondrial membranes. The 85 kDa band was enhanced under conditions lacking fetal bovine serum (FBS) from the culture medium 6 h prior to harvesting. Conversely, 33 kDa protein was enhanced on MEG-01 cells grown in the presence of 10% FBS; suggesting that a precursor (85 kDa) and product (33 kDa) relationship might be induced by serum. The 85 kDa band was robustly up-regulated in response to imidazoline receptor-sensitive ligands; moxonidine, idazoxan and agmatine (10 microM each for 6 h). NE also up-regulated the 85 kDa IRBP-immunoreactive protein on MEG-01 membranes, but to a lesser extent. Idazoxan, an imidazoline alpha 2-antagonist, off-set its induction of 85 kDa protein by reducing the 33 kDa band. Yohimbine, a non-imidazoline alpha 2-antagonist, was ineffective alone, or in combination with moxonidine (up to 40 microM), but yohimbine blocked NE's induction of the 85 kDa band. Therefore, a rise in either plasma NE and/or endogenous I-site ligands (i.e. agmatine) could explain an elevation of imidazoline receptors observed in depression.

Platelet I1-imidazoline binding sites are decreased by two dissimilar antidepressant agents in depressed patients

Previous studies have indicated that there may be a dysregulation of alpha 2-adrenoceptors and imidazoline receptors in depression. This study compares the effects of chronic antidepressant treatment with a serotonin reuptake inhibitor (fluoxetine) versus a noradrenaline reuptake inhibitor (desipramine) on the binding parameters of the platelet imidazoline binding site (subtype I1) and of the platelet alpha 2-adrenoceptor in depressed patients. After 6 weeks of treatment with either antidepressant, platelet I1 binding sites became normalized (i.e. downregulated). A negative correlation was obtained between plasma epinephrine concentrations and platelet alpha 2-adrenoceptor Bmax values within the samples, but no correlation was obtained between any plasma catecholamine and a platelet I1 binding parameter. An additional finding was the increased affinity of alpha 2-adrenoceptors for p125I-clonidine in untreated depressed patients compared to healthy subjects. Because of the density of platelet I1 binding sites was downregulated by both of the antidepressants, we postulate that a decrease in platelet I1 binding site density may be related to an improved state from depression that these antidepressants produce.

Platelet I1-imidazoline binding sites are elevated in depression but not generalized anxiety disorder

Depressed patients have been reported to have a higher than normal density of platelet binding sites for 3H-clonidine, an alpha 2-adrenoceptor agonist. Paradoxically, other studies using 3H-alpha 2, antagonists have found no differences from controls. Because 3H-clonidine interacts with platelet alpha 2-adrenoceptors to form G-protein complexes, whereas 3H-alpha 2-antagonists bind with uncoupled receptors, an elevation in G-protein coupling might explain this paradox. Another possibility is that depression might be associated with increased non-adrenergic I1-imidazoline binding sites, which are also clonidine sensitive. To distinguish these possibilities, we utilized p125I-clonidine to measure density (Bmax) and affinity (KD) of platelet G-protein coupled alpha 2-adrenoceptors as well as platelet I1 binding sites, and compared diagnostic groups of major depressive disorder (MDD), generalized anxiety disorder (GAD) and healthy subjects. Specific inhibition of binding by norepinephrine (NE = 10 microM) was used to selectively quantify alpha 2-adrenoceptors, whereas inhibition by 10 microM moxonidine (a > 100-fold selective I1 ligand) quantified I1 binding sites under a NE mask. I1 sites were found to be markedly elevated by, on average, +136% in MDD patients (p = .0007), whereas there was only a marginal increase in alpha 2-adrenoceptor Bmax values in MDD patients (p = .08; GAD and healthy subjects did not differ). Treatment of MDD patients for 6-8 weeks with desipramine downregulated I1 sites as well as alpha 2-adrenoceptors. Positive correlations were also noted for both sites: (a) between Bmax values and the severity of depression (using the Hamilton Depression Rating Scale); and (b) between end-of-treatment plasma desipramine concentrations and the extent of downregulation in Bmax values when subject groups were pooled. None of the binding parameters was associated with plasma catecholamine concentrations. The results suggest that an increased density of platelet I1 binding sites may partially explain the utility of radiolabeled clonidine as a potential biological marker for depressive illness, although an additional increase in G-protein coupling cannot be excluded.

Increased imidazoline and alpha 2 adrenergic binding in platelets of women with dysphoric premenstrual syndromes

An association between dysphoric premenstrual syndromes (PMS) and a lifetime history of major depressive disorders has previously been documented. Other studies have demonstrated an increase in the binding of radiolabeled imidazoline compounds to platelets of depressed patients. Clonidine and related imidazoline compounds interact with alpha 2 adrenoceptors to inhibit neuronal noradrenergic activity and in higher concentrations, they stimulate noradrenergic activity through their interaction with imidazoline receptors. Here we report increased 3H para-aminoclonidine binding to high affinity alpha 2 adrenoceptor sites as well as to nonadrenergic imidazoline binding sites in platelets of women with dysphoric PMS. This higher binding was most pronounced during the late-luteal-symptomatic phase of the menstrual cycle and, to a lesser degree, during the non-symptomatic mid-follicular phase. Binding to the imidazoline site distinguished women with dysphoric PMS from women with no such symptoms, was highly positively correlated with the severity of symptoms, and was negatively correlated with plasma levels of progesterone. These findings suggest that platelet imidazoline binding sites might be a biological marker for dysphoric states in PMS or for the vulnerability to develop them. These findings also point to a possible biological link between dysphoric PMS and major depressive disorders.

Alpha 2-agonist binding sites in brain: [125I]para-iodoclonidine versus [3H]para-aminoclonidine

The localization of alpha 2-receptors was determined by quantitative autoradiography using [125I]para-iodoclonidine ([125I]PIC) and [3H]para-aminoclonidine ([3H]PAC). In cortical tissue, [125I]PIC and [3H]PAC were equipotent in their capacity to bind sites recognized by oxymetazoline (preferentially binds to the alpha 2A receptor subtype). The iodinated ligand was about 10 times more potent than [3H]PAC for binding to the heterogenous receptor population labeled by ARC-239 (alpha 2B and alpha 2C). The density of [125I]PIC binding was found to be two-fold higher than that of [3H]PAC in many brain areas and the disparity was even greater in regions such as the dentate gyrus, stria terminalis, and granular layer of the cerebellum. By contrast, other regions of the brain such as the laterodorsal thalamic nucleus, the locus coeruleus, and several amygdaloid areas had equivalent levels of binding. These observations indicate that [3H]PAC has selectivity for the alpha 2A receptor subtype and thus offer a restricted view of alpha 2-adrenergic receptor distribution. The iodinated ligand provides a more complete picture of the overall alpha 2 receptor population.

Platelet membrane alpha 2-adrenergic receptors in depression

The platelet membrane was used as a model system to examine alpha 2-adrenergic receptors in 30 depressed patients and 30 healthy control subjects. The number of binding sites and their affinity for 3H-UK 14304 (5-bromo-6-(2-imidazoline-2-ylamino)-quinoxaline), a potent, highly selective alpha 2-adrenergic receptor agonist, was measured. Plasma magnesium and free 3-methoxy-4-hydroxyphenylglycol (MHPG) concentrations were assayed in the same sample. A decreased agonist-receptor affinity was found in depressed patients, whereas receptor density was not significantly altered compared with that in control subjects. In bipolar depressed and dysthymic patients, there was a tendency toward a higher density of alpha 2-adrenergic receptors. This trend was not apparent in unipolar, recurrent depressed subjects. Moreover, a positive correlation between Bmax and Kd values was observed in patients but not in control subjects--a finding that suggests that a compensatory phenomenon occurs in depression. After the patients were treated with antidepressant drugs, an increased affinity (decrease in Kd) was observed, together with a decrease in binding sites. Plasma magnesium concentrations were higher in drug-free depressed patients than in control subjects. In addition, magnesium concentrations were negatively correlated with the density of alpha 2-adrenergic receptor binding sites in depressed patients, both before and during treatment. Lastly, a trend toward a negative correlation between plasma MHPG concentration and the number of binding sites was also observed. These results suggest a complex multifactorial regulation of alpha 2-adrenergic receptors, which are probably hyposensitive in depressive syndromes.

Intracellular membranes are more fluid in platelets of Alzheimer's disease patients

We studied 12 patients with probable Alzheimer's disease versus age- and sex-matched healthy control subjects. Platelets were subfractionated into intracellular membranes and plasma membranes, and steady-state anisotropy of diphenylhexatriene was measured on the preparations as an index of membrane fluidity. Fluidity was higher in intracellular membranes from platelets of Alzheimer's patients compared to controls (P = 0.016). However, no difference was observed in purified plasma membrane's fluidity from the same patients versus controls. Neither the platelet counts, platelet volumes, percent of larger platelets, nor the amount of internal membrane protein per platelet were different between groups. There was no correlation between intracellular membrane anisotropy and severity of dementia as measured on the Mini-Mental State Exam. The results extend previous studies suggesting that there is an intracellular membrane alteration in platelets in Alzheimer's disease.

Binding of [3H]-p-aminoclonidine to alpha 2-adrenoceptor states plus a non-adrenergic site on human platelet plasma membranes

Characterization of the binding of [3H]p-aminoclonidine ([3H]PAC) to purified plasma membranes from human platelets has revealed multiple binding sites. [3H]PAC identified site-1 in the picomolar affinity range (site-1 KD estimates ranged from 13 to 94 pM). Site-1 displayed a rank order of competition by various compounds for [3H]PAC, indicative of an alpha 2-adrenoceptor, and was sensitive to 0.1 mM GTP. [3H]PAC also identified a second site with nanomolar affinity (site-2 KD estimates ranged from 0.7 to 1.7 nM). In the presence of 0.1 mM GTP, site-2 was not diminished significantly. Also in contrast to site-1, site-2 displayed low affinity for yohimbine (YOH), (-)-epinephrine and (-)-norepinephrine (NE). Therefore, site-2 could not be an active alpha 2-adrenoceptor; instead it had properties similar to a previously reported imidazoline-preferring binding site. A third site (site-3) bound [3H]PAC with a KD for site-3 of 26.6 +/- 10.0 nM (SD). Site-3 had a rank order of competition by various compounds for 5 nM [3H]yohimbine ([3H]YOH) binding which was indicative of an alpha 2-adrenoceptor. (-)-NE competed for 5 nM [3H]YOH binding at two sites: site-1 Ki = 32 pM, site-3 Ki = 239 nM. Treatment with 0.1 mM GTP completely removed site-1 and transferred the competitive binding of (-)-NE to low affinity (Ki = 437 nM). Thus, site-3 appears to be a free alpha 2-adrenoceptor. Bmax estimates for untreated membranes, derived from simultaneous multi-experiment curve-fitting analyses, were site-1 = 36 +/- 29 fmol/mg plasma membrane protein, site-2 = 95 +/- 34 fmol/mg and site-3 = 154 +/- 35 fmol/mg. We are the first to report a site for [3H]PAC binding on platelets (site-2) with properties uncharacteristic of an adrenoceptor. This observation appears to be due to our use of purified plasma membrane and low ionic strength buffer. These studies relate to reports of increased binding of [3H]PAC to platelets from depressed patients.

Relationship between membrane fluidity and adrenoceptor binding in depression

Membrane fluidity and adrenergic receptor binding were studied in platelets of depressed patients before and during treatment with desmethylimipramine to investigate the relationship between the alpha 2-adrenergic receptor and its membrane environment in depression. Most samples came from a previous study in which we observed higher 3H-para-aminoclonidine (3H-PAC) binding in platelets from depressed patients compared to healthy subjects. Fluidity was measured by steady state diphenylhexatriene (DPH) anisotropy in both purified plasma membranes and in intracellular membrane preparations from platelets. No differences were observed in DPH membrane fluidity, per se, indicating that fluidity changes probably do not underlie either the increased alpha 2-adrenergic receptor binding in depression or the normalization of binding during treatment. However, lower intracellular membrane fluidity was correlated with higher binding to 3H-PAC site-1 in healthy subjects, but not in depressed patients. Thus, during depression there may be a disruption in the normal relationship between the adrenergic receptor and its membrane environment.