A comparison of alpha 2-adrenoceptor regulation in brain and platelets

Assessment of noradrenergic functioning in patients with non-combat-related posttraumatic stress disorder: a study with desmethylimipramine and orthostatic challenges

The functioning of the noradrenergic system was assessed in 16 patients with non-combat-related posttraumatic stress disorder (PTSD) and the same number of age- and sex-matched healthy subjects by measuring (1) plasma norepinephrine (NE) levels in supine and upright postures, and (2) growth hormone (GH) responses to challenge with desmethylimipramine (DMI), a NE reuptake inhibitor. Subjects were cannulated at 08:30 h after an overnight fast. Blood samples were drawn for NE levels with subjects in a supine position and after 5 min of standing. After subjects were allowed to rest for 30 min in a supine position, a blood sample was drawn for basal GH (T-15) levels. The second baseline sample was drawn 15 min later (T0), at which time DMI (1 mg/kg) was given orally, and further blood samples were drawn at 90, 120, and 180 min. PTSD patients had significantly higher baseline NE levels and blunted NE responsivity to postural challenge compared with normal subjects. Basal and DMI-induced GH levels, on the other hand, did not differ in PTSD versus normal subjects. Overall, these findings suggest that non-combat-related PTSD patients have peripheral noradrenergic dysregulation, but central postsynaptic alpha 2-adrenergic receptor sensitivity is not altered in this patient 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.

Alpha 2 adrenergic receptor subtypes in depression: a candidate gene study

Alpha 2 adrenergic receptors play an important role in regulating the neuronal release of norepinephrine through presynaptic feedback inhibition in the locus ceruleus. Therefore, alpha 2 adrenergic autoreceptors may underlie some aspects of the pathogenesis and symptomatic expression of depressive illness. We studied two brain-expressed alpha 2 adrenergic receptor genes as genetic markers in linkage analyses in 17 multiplex pedigrees of unipolar depression. Neither of the genes was supportive of linkage to depression. Lod scores of less than -2 were found in both familial pure depressive disease pedigrees and in depression spectrum disease pedigrees. Therefore, we conclude that depression in our pedigrees is not related to mutations in the two alpha 2 adrenergic receptor genes tested.

Adrenergic status in anxiety disorders: platelet alpha 2-adrenergic receptor binding, blood pressure, pulse, and plasma catecholamines in panic and generalized anxiety disorder patients and in normal subjects

In order to evaluate adrenergic function in anxiety disorders, platelet alpha 2-adrenergic binding parameters and supine and standing blood pressure, pulse, and venous plasma epinephrine and norepinephrine were determined in patients with panic attacks or generalized anxiety disorder and in normal subjects. The maximum number of binding sites (Bmax) for the partial agonist tritiated clonidine was significantly lower for both patient groups than for normal subjects, and the Bmax for the antagonist tritiated yohimbine was significantly lower for panic patients. There were no other substantive differences across groups. Prior exposure to psychotropic drugs might account for the results for clonidine binding, but not for yohimbine. The Bmax for clonidine was correlated with norepinephrine increases upon standing and, for panic patients, with the severity of full unexpected panic attacks. These data provide further evidence of adrenergic receptor abnormalities in people with anxiety disorders.

Platelet alpha-2-receptor binding and adenylate cyclase activity in panic disorder

The function of the alpha-2-receptor and intracellular effector systems was examined in 39 panic disorder patients and 30 healthy subjects using the platelet as a model system. Alpha-2-receptor density, as reflected by 3H-yohimbine binding, was not different between the two groups. Platelet alpha-2-receptor affinity was decreased (higher Kd) in the panic disorder patients. Other significant abnormalities in the patients included a decreased basal adenylate cyclase activity, reduced EC50 for the epinephrine inhibition of adenylate cyclase activity, and decreased stimulation of adenylate cyclase activity by prostaglandin E1 and sodium fluoride. These results are consistent with a dysfunction in some panic disorder patients at the level of the stimulatory GTP binding regulatory protein, Gs, or the adenylate cyclase catalytic unit. The relationship of these findings to previous studies of noradrenergic function in panic disorder patients is discussed.

Regulation of adrenergic receptor number following chronic noradrenaline infusion in the rabbit

In order to study noradrenaline-induced regulation of alpha- and beta-adrenoceptors, groups of male New Zealand White rabbits (n = 8) were treated with intravenous noradrenaline (0.09 mumol/kg x h) or ascorbate (0.1%) for 10 days via osmotic minipumps implanted in the femoral vein, and the number of cardiac, lung and lymphocyte beta-adrenoceptors as well as renal and platelet alpha 2-adrenoceptors were determined. 1. The mean arterial blood pressure, heart rate and catecholamine levels were measured before commencing, and after 24 h and 10 days infusion. Circulating noradrenaline concentrations were elevated approximately 6-fold at 24 h and were sustained at these levels after 10 days administration of noradrenaline. There were no significant alterations in the blood pressure while a significant decrease in the heart rate was observed at 24 h. 2. Alpha 2-adrenoceptor density was assessed using [3H]-yohimbine. A significant decrease in the number of alpha 2-adrenoceptors in the kidney was observed following the 10 days infusion with noradrenaline. This down-regulation was in marked contrast to the lack of alteration in platelet alpha 2-adrenoceptor number and the platelet alpha 2-adrenoceptor mediated aggregatory response. 3. The density of beta-adrenoceptors in lymphocytes, heart and lung were quantified using (-)[125I]iodocyanopindolol (ICYP). The noradrenaline infusions caused significant reductions in beta-adrenoceptor number in the heart and lung (containing predominantly beta 1-adrenoceptors) but not in lymphocytes (possessing mainly beta 2-adrenoceptors). The KD-values (pM) for ICYP binding to heart and lung were also significantly decreased in the present studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Platelet research in psychiatry

The platelet is one of the most researched biological markers in psychiatry. Characteristics of MAO activity, 5-HT uptake, imipramine and alpha 2-adrenergic receptor binding, for example, are similar in platelet and CNS. Methodological factors are not negligible, and range from diagnostic specificity and drug effects to the normal physiological variability of age and hormone-related changes, circadian and seasonal rhythms. As yet, there are no clear state or trait platelet markers in affective disorders and schizophrenia that can be unequivocally used to detect vulnerability to the illness, predict therapeutic response, define clinical diagnostic entities or follow the course of the illness. However, platelet markers are increasingly being used in careful studies to monitor psychopharmacological effects (an in vivo assay of all active metabolites), different ligands can be specific markers for certain aspects of a psychiatric illness (e.g. alpha 2-adrenergic receptors and weight loss), and this homogeneous preparation of human cells is an increasingly important tool in studying mechanisms in pathophysiology. More longitudinal studies are required to establish functional relationships between platelet variables and psychopathology.

Platelet alpha 2-adrenergic receptor function in psychiatric disorders

The inhibitory effects of norepinephrine (NE) on the cyclic adenosine-3',5'-monophosphate (cAMP) response to prostaglandin E1 (PGE1), a measure of alpha 2-adrenergic receptor function, have been compared in platelets from drug-free schizophrenic patients, depressive patients, and normal controls. The absolute value of the inhibition by NE of the cAMP response to PGE1 was smaller in platelets from schizophrenic and depressive patients than in controls. However, this result was secondary to the smaller baseline platelet cAMP response to PGE1 in patients with these disorders. Effects of NE on cAMP production did not discriminate between actively ill and remitted patients with either schizophrenia or depression. Platelet alpha 2-receptor sensitivity, as measured by the effects of NE on cAMP production, does not appear to be altered in these psychiatric disorders.