Platelet hyperaggregability and increased alpha-adrenoceptor density in anorexia nervosa

The involvement of the adrenergic system in feeding and eating disorders. A systematic review

BACKGROUND

Adrenergic dysregulation has been proposed as a possible underlying mechanism in feeding and eating disorders (FED). This review aims to synthesise the current evidence on the role of adrenergic dysregulation in the pathogenesis and management of FED.

METHODS

A systematic review was conducted in MEDLINE, Cochrane Library, and Clinicaltrials.gov. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) was adopted. Preclinical, clinical, and pharmacological studies assessing the adrenergic system in FED were included.

RESULTS

Thirty-one out of 1415 recognised studies were included. Preclinically, studies on adrenaline's anorectic impact, receptor subtypes, and effects on hepatic function in rats show that catecholamine anorexia is primarily alpha-adrenergic, whereas beta-adrenergic anorexia can be obtained only after puberty, implying an impact of sexual hormones. Clinically, catecholamine levels may be higher in FED patients than in healthy controls (HC). Individuals with anorexia nervosa (AN) may show higher epinephrine-induced platelet aggregability response than HC. Pharmacological trials suggest that the alpha-2-adrenergic medication clonidine may not lower AN symptoms, but agents regulating the adrenaline-noradrenaline neurotransmission (bupropion, reboxetine, duloxetine, sibutramine) have been found to improve binge eating symptoms.

CONCLUSION

Adrenergic dysregulation may be involved in the pathophysiology of FED. More research is needed to comprehend underlying mechanisms and treatment implications.

Autonomic Nervous System Function in Anorexia Nervosa: A Systematic Review

Background: Autonomic nervous system (ANS) dysfunction has been suggested to contribute to the high prevalence of cardiovascular complications in individuals with anorexia nervosa (AN), yet has not been thoroughly investigated. The current review aimed to synthesize the evidence of basal ANS function in individuals with a current diagnosis of AN and those with a previous diagnosis who had achieved weight restoration, as compared to controls.

Methods: A systematic review of nine databases was conducted and studies that were published in a peer-review journal, in English, that included at least one assessment of ANS function in individuals with a current or previous diagnosis of AN were selected. Forty-six studies were included with a total of 811 participants with a current diagnosis of AN and 123 participants with a previous diagnosis of AN.

Results: ANS function was assessed through heart rate variability (n = 27), orthostatic challenge, blood pressure variability or baroreflex sensitivity (n = 11), adrenergic activity (n = 14), skin conductance level (n = 4), and pupillometry (n = 1). Individuals with AN demonstrated increased parasympathetic activity and decreased sympathetic activity, suggestive of autonomic dysregulation. Following weight restoration, autonomic function trended toward, or was equivalent to, control levels.

Discussion: Autonomic dysregulation is indicated through a range of assessments in individuals with AN. Future investigations should utilize a variety of assessments together in order to conclusively establish the nature of autonomic dysfunction in AN, and following extended weight restoration. Moreover, investigation into the co-occurrence of ANS function and cardiovascular risk is required.

Anorexia nervosa depends on adrenal sympathetic hyperactivity: opposite neuroautonomic profile of hyperinsulinism syndrome

OBJECTIVE

The aim of our study was to determine the central and peripheral autonomic nervous system profiles underlying anorexia nervosa (AN) syndrome, given that affected patients present with the opposite clinical profile to that seen in the hyperinsulinism syndrome.

DESIGN

We measured blood pressure and heart rate, as well as circulating neurotransmitters (noradrenaline, adrenaline, dopamine, plasma serotonin, and platelet serotonin), using high-performance liquid chromatography with electrochemical detection, during supine resting, one minute of orthostasis, and after five minutes of exercise. In total, 22 AN patients (12 binge-eating/purging type and 10 restricting type) and age-, gender-, and race-matched controls (70 ± 10.1% versus 98 ± 3.0% of ideal body weight) were recruited.

RESULTS

We found that patients with AN had adrenal sympathetic overactivity and neural sympathetic underactivity, demonstrated by a predominance of circulating adrenaline over noradrenaline levels, not only during the supine resting state (52 ± 2 versus 29 ± 1 pg/mL) but also during orthostasis (67 ± 3 versus 32 ± 2 pg/mL, P < 0.05) and after exercise challenge (84 ± 4 versus 30 ± 3 pg/mL, P < 0.01).

CONCLUSION

Considering that this peripheral autonomic nervous system disorder depends on the absolute predominance of adrenomedullary C1 adrenergic nuclei over A5 noradrenergic pontine nucleus, let us ratify the abovementioned findings. The AN syndrome depends on the predominance of overwhelming adrenal sympathetic activity over neural sympathetic activity. This combined central and autonomic nervous system profile contrasts with that registered in patients affected by hyperinsulinism, hypoglycemia, and bulimia syndrome which depends on the absolute predominance of neural sympathetic activity.

Central and peripheral noradrenalin regulation in eating disorders

Norepinephrine and its metabolites were studied in various body fluids (plasma, urine and cerebrospinal fluid) of patients with anorexia nervosa, bulimia nervosa and healthy young women. The reaction of plasma norepinephrine to different stimuli like orthostatic challenge, test meals, standardized exercise, mental challenge tests etc. were studied. All results indicate a reduced noradrenergic activity in the central and peripheral nervous system of patients with eating disorders. The clinical consequences of these changes are hypotension, bradicardia, hypothermia and depression. Evidence is presented that the reduced activity of the sympathetic nervous system is caused by starvation (anorexia nervosa) or intermittent dieting (bulimia nervosa).

The use of platelets as models for neurons: possible applications to the investigation of eating disorders

The pathogenesis of eating disorders is still unclear. However, there is evidence that neurotransmitters may be involved in the control of satiety/appetite. Investigating these disorders could be facilitated if a readily available human cell model were to reflect central nervous system (CNS) function. Platelets may satisfy, at least partially, this requirement. We discuss here the use of platelets as neuronal models with an emphasis on eating disorders.

The effect of ethanol on platelet function and vascular prostanoids

The present review will discuss the effects of ethanol on platelet function and vascular prostanoids. Whenever possible we have considered human studies because there are marked species differences in platelet function and vascular prostanoid release even in the absence of ethanol. Because of the specialised nature of some parts of the text, we have included brief introductions to help readers who are not familiar with this field.

[3H]yohimbine binding to platelet alpha 2-adrenoceptors in depression

[3H]Yohimbine binding to platelet alpha 2-adrenoceptors was studied in depressed patients and healthy volunteers. Where possible platelet binding measurement was repeated in depressed patients following treatment. Bmax of [3H]yohimbine binding did not differ significantly between depressed patients and control subjects and did not change with treatment in depressed patients. KD was significantly lower in female depressed patients, particularly in those who were post-menopausal. Multivariate analysis showed significant effects on KD of depression, season of testing and assay protein concentration.

86Rb(K) influx and [3H]ouabain binding by human platelets: evidence for beta-adrenergic stimulation of Na-K ATPase activity

Although active transport of potassium into human platelets has been demonstrated previously, there is hitherto no evidence that human platelets have an ouabain-inhibitable Na-K ATPase in their membrane. The present study demonstrates active rubidium (used as an index of potassium influx), 86Rb(K), influx into platelets, inhibitable by ouabain, and also demonstrates the presence of specific [3H]ouabain binding by the human platelet. This 86Rb(K) influx was stimulated by adrenaline, isoprenaline, and salbutamol, but noradrenaline caused a mild inhibition. Active 86Rb(K) influx by platelets was inhibited markedly by timolol, mildly by atenolol, but not by phentolamine. Therefore, active 86Rb(K) influx in human platelets is enhanced by stimulation of beta adrenoceptors of the beta 2 subtype. The platelet may therefore replace the leukocyte in future studies of Na-K ATPase activity. This would be a considerable advantage in view of the ease and rapidity of preparation of platelets.

Alpha 2-adrenoceptor sensitivity in anorexia nervosa: GH response to clonidine or GHRH stimulation

Growth hormone (GH) response to clonidine and growth hormone-releasing hormone (GHRH) stimulation, together with baseline somatomedin C (SmC) levels, were examined in parallel in a group of 21 patients with anorexia nervosa (AN) and in 10 controls. In addition, the Hamilton Rating Scale for Depression (HRS) was administered to the patients. Clonidine (2.5 micrograms/kg body weight, iv) induced GH elevations that were not significantly different between patients and controls. In contrast, GHRH (1 microgram/kg body weight, iv) produced a significantly higher GH response in anorectics than in controls. The ratio between GH responses (area under the curve, or AUC) to GHRH and to clonidine was significantly higher in patients than in controls. Baseline SmC levels (6 patients) were significantly lower in anorectics than in controls. Minor depressive symptomatology was present in all patients. When viewed in relation to the GH hyperresponsiveness to GHRH, the apparent normality of the response to clonidine in anorectics reflects the existence of an actual alpha 2-adrenoceptor subsensitivity. As clonidine reportedly acts via release of endogenous GHRH, an excessive, rather than a normal, GH response to clonidine was to be anticipated.

Diminished [3H] ouabain binding and 86Rb influx by leukocytes in anorexia nervosa

Leukocyte Na-K ATPase was assessed in ten patients with anorexia nervosa. The indices of Na-K ATPase measured were 86Rb influx and [3H]-ouabain binding. Both 86Rb influx and [3H]-ouabain binding were significantly lower in patients with anorexia nervosa than in controls. Following weight gain (4.1 to 11.9 kg) in seven patients both 86Rb influx and [3H]-ouabain binding increased in each patient to levels similar to those in controls, although the patients still remained underweight. Acute oral glucose challenge (75 g) also resulted in an increase in 86Rb influx and [3H]-ouabain binding in each of the eight patients tested. There was a significant correlation between 86Rb influx and [3H]-ouabain binding. We conclude that the leukocytes of patients with anorexia nervosa have a significantly diminished number of Na-K ATPase units with a parallel decrease in 86Rb influx. Weight gain and acute glucose challenge result in an increase in both indices of Na-K ATPase. Nutrition appears to play an important role in the modulation of this enzyme.

Hypothalamus, frontal cortex and lymphocyte beta 2-adrenergic receptors in acute and chronic starvation in rat

Hypothalamus and frontal cortex beta-adrenergic receptors, as measured by [125I]cyanopindolol binding, were investigated in male rats both after 4 days of acute starvation and after 2 weeks of semistarvation with dietary manipulation. Moreover, to test whether lymphocyte beta 2-adrenergic receptors can be used as a marker for brain beta-receptors, the parallel measurement of beta-adrenoceptors of lymphocyte, hypothalamus and frontal cortex was carried out in acutely starved rats. T3 and corticosterone in rat serum were also determined in this study. Our experiments showed that beta-adrenergic receptors in hypothalamus and frontal cortex, as well as those in lymphocytes remain unaltered in rats starved acutely for 4 days when compared with controls, despite reduced T3 and increased corticosterone levels. Chronic semistarvation on either a protein-rich or a carbohydrate-rich diet also resulted in decreased T3, increased corticosterone and no alterations of beta-adrenergic receptors in hypothalamus and frontal cortex.

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.

Dieting and weight loss in volunteers increases the number of alpha 2-adrenoceptors and 5-HT receptors on blood platelets without effect on [3H]imipramine binding

Platelet monoamine receptor binding was determined in normal male and female subjects before and at the end of a 2- to 3-week weight reducing diet (1200 kcal daily). Dieting was associated with an increase in platelet binding sites for both [3H]yohimbine and [125I]iodolysergic acid diethylamide (iodoLSD). The affinity at the platelet [125I]iodoLSD binding site was reduced. In contrast, [3H]imipramine binding was unchanged. These results have important implications for studies that employ platelet binding as a peripheral marker of neurotransmitter function in psychiatric illness.

A placebo-controlled crossover study of oral clonidine in acute anorexia nervosa

The alpha 2-adrenergic agonist clonidine has been reported to increase feeding in several species. This study evaluated the effects of clonidine (500-700 micrograms/day), administered per os, to four treatment-resistant anorexia nervosa patients in a long-term placebo-controlled crossover trial. All patients increased their body weight significantly. Clonidine administration, however, did not influence the rate of weight gain, nor did clonidine affect hunger or satiety sensations. Similarly, 24-hour urinary 3-methoxy-4-hydroxyphenylglycol levels and levels of anxiety and depression were unchanged by clonidine. By contrast, clonidine showed significant hemodynamic effects; clonidine lowered systolic and diastolic blood pressure, reduced pulse rate, and produced sedation. Discontinuation of clonidine was associated with a small but significant weight loss compared to a small weight increase during the initiation of clonidine treatment. The results suggest that clonidine may not be indicated in the treatment of anorexia nervosa.

Platelet hyperaggregability in cystic fibrosis

Platelet function was investigated in 15 patients with cystic fibrosis (CF) and in ten age-matched controls. Marked hyperaggregability of platelets to adrenaline, collagen and arachidonic acid was observed in platelet rich plasma (PRP) prepared from patients with cystic fibrosis. Thromboxane A2 (TXA2) release from these platelets was also markedly enhanced. Hyperaggregability and increased TXA2 release observed in patients with CF was not due to the higher platelet counts in these patients since hyperaggregability was observed even in those patients whose platelet counts were similar to those in controls. Platelet hyperaggregability and increased thromboxane release in these patients were also independent of their body weight and occurred despite supplementation with vitamin E. Hyperaggregability of platelets in CF may be clinically relevant since it may contribute to the pathogenesis of bronchoconstriction through the release of TXA2 and other bronchoconstrictor platelet products such as serotonin.

Beta 2-adrenergic receptor regulation in circulating mononuclear leukocytes in anorexia nervosa and bulimia

Mononuclear leukocyte (MNL) beta 2-adrenergic receptors and their coupled adenylate cyclase system were studied in underweight anorectics (n = 12), weight-recovered anorectics (n = 8), bulimics (n = 8), and age- and sex-matched controls (n = 39). Compared with controls, underweight anorectics had significantly fewer MNL beta 2-adrenergic receptor sites (Bmax) but did not differ in binding affinity (Kd). Weight-recovered anorectics and bulimics did not differ from controls on either Bmax or Kd. Compared with controls, all three patient groups had significantly reduced plasma levels of triiodothyronine (T3), while only underweight anorectics had significantly elevated plasma levels of cortisol. Plasma norepinephrine (NE) response to orthostasis was significantly lower in the three patient groups than in controls. The reduction in beta 2-adrenergic receptor number in underweight anorectics could reflect their elevated cortisol and reduced T3 levels. The decrease in beta 2-adrenergic receptor sites, together with the lower NE response to orthostasis, could be responsible for the reduced sympathetic activity of underweight anorectics.

Adrenaline-induced hyperaggregability of platelets and enhanced thromboxane release in anorexia nervosa

Platelet aggregation and thromboxane A2 release in response to adrenaline and the relationship of this response to body weight was investigated in female patients with anorexia nervosa. Platelets obtained from patients with body weights below 75% of the expected average weight (Group I) showed significantly greater aggregation and TXA2 release in response to adrenaline when compared with controls matched for sex and age. Patients with body weights 75-95% of the expected average weight (Group II) did not show enhancement of platelet aggregation or TXA2 release. In Group I patients, platelet hyperaggregability and enhanced TXA2 release induced by adrenaline tended to normalise following inpatient treatment and weight gain: with 0.5 mumol/l adrenaline, the pre-weight gain median aggregation was 65%, whereas the post-weight gain value was 37% - P less than 0.01. Markedly underweight patients also had hyperaggregability following stimulation with ADP and collagen. This hyperaggregability also tended to normalise after weight gain but these changes were not statistically significant. Platelet hyperaggregability (especially in response to adrenaline) in anorexia nervosa is therefore secondary to weight loss and reverts to normal with normalisation of weight. These changes may reflect the previously documented increase in platelet alpha-adrenoceptors in thin patients with anorexia nervosa and their normalisation following weight gain. However, the hyperaggregability in response to agonists other than adrenaline suggests that an additional post-receptor mechanism may be involved.

Heparin-induced platelet aggregation in anorexia nervosa and in severe peripheral vascular disease

We have previously demonstrated that platelets obtained from patients with anorexia nervosa or severe peripheral vascular disease are hyperaggregable. Since conventional heparins are known to activate platelets in vitro and occasionally induce thrombosis and consumptive thrombocytopenia in vivo, we have investigated the direct effect of a conventional heparin on platelets obtained from patients with anorexia nervosa or severe peripheral vascular disease. Heparin at therapeutic concentrations was found to induce platelet aggregation of such platelets in vitro. In contrast, a recently developed low molecular weight heparinoid (Org 10172), at therapeutic concentrations, had no effect on these hyperaggregable platelets. We conclude that: heparin may be potentially harmful to patients with hyperaggregable platelets; thrombocytopenia and thrombosis associated with heparin therapy may be mediated through a direct effect of heparin on platelets; it is unlikely that heparin induced thrombocytopenia is always mediated by classical immunological mechanisms, especially in patients with hyperaggregable platelets; and low molecular weight heparinoids may be safer anticoagulants in patients with platelet hyperaggregability.

Fibrinogen mediated activation of platelet aggregation and thromboxane A2 release: pathological implications in vascular disease

The effect of a human fibrinogen preparation on in vitro platelet aggregation was assessed. Platelets were obtained from healthy volunteers. Human fibrinogen induced platelet aggregation in 65% of platelet rich plasma samples and enhanced submaximal platelet aggregation induced by heparin or by several conventional agonists in all samples. Aggregation induced by fibrinogen alone was reversed by the in vitro addition of human albumin. Fibrinogen induced aggregation was associated with the release of the vasoconstrictor, thromboxane A2. Preincubation with indomethacin inhibited both the aggregation and the release of thromboxane A2. Fibrinogen had no effect on in vitro vascular prostaglandin I2 synthesis (rat aortic rings) during a 60 minute incubation. The observed effects of fibrinogen on platelet function may be relevant to clinical conditions in which hyperaggregability of platelets is associated with hyperfibrinogenemia and thrombosis.

Response to platelet-activating factor of platelets from patients with multiple sclerosis

The response of Multiple Sclerosis (MS) and control platelets to different concentrations of platelet-activating factor was studied. At concentrations in the range 10(-7) to 10(-5)M, it was found that the MS platelets tended to aggregate fully at lower concentrations than was the case with control platelets. At a concentration of 10(-6)M, it was found that in 19 cases the MS platelets gave a full aggregation response whilst 4 cases showed biphasic but full aggregation, whereas at this concentration the control platelets showed full aggregation in only 2 cases, biphasic but less complete aggregation in 5 cases and reversible aggregation in 16 cases. In crossover studies, it could be shown that MS platelets resuspended in control platelet-poor plasma still showed enhanced aggregability and that the response of control platelets was unaffected by resuspension in MS platelet-poor plasma. Differences were also seen in susceptibility of platelets of MS and control subjects to inhibition by indomethacin, bromophenacyl bromide (a phospholipase inhibitor) and verapamil (a Ca2+ antagonist). It is suggested that the hyperaggregability of the MS platelets could reside in the platelets themselves, and may be associated with enhanced phospholipase activity.

Platelet alpha 2-adrenoceptors in major depression: relationship with urinary 4-hydroxy-3-methoxyphenylglycol and age at onset

Platelet alpha 2-adrenoceptor number and affinity were measured in 31 drug-free patients with major depressive illness utilizing 3H-clonidine as ligand. A significant negative correlation was found between number of alpha 2-adrenoceptors, baseline urinary 4-hydroxy-3-methoxyphenylglycol (MHPG) excretion, present age and age at onset of the disease. Kd did not correlate with any of these variables not with the Bmax of platelet alpha 2-adrenergic binding. Multiple regression analysis, with MHPG and age at onset as independent variables, explained variance for alpha 2-adrenoceptor density better than single regression (from 19% for MHPG and 30% for age at onset to 40%), with the addition of both these variables being significant.

Neurotransmitter markers of depression

The role of antidepressants in bringing about adaptive changes in peripheral and central neurotransmitter receptors is briefly reviewed and extended to show how changes in platelet and lymphocyte adrenoceptor and serotonin receptor activity may correlate with the depressed state and normalise following effective treatment. By contrast, the 3H-imipramine binding sites on the platelet membrane do not change following successful treatment and may therefore reflect a trait-dependent (genetic) change. The neuroendocrine markers, such as the dexamethasone suppression test (DST), have the advantage over the receptor markers in that they require less sophisticated laboratory facilities. It is concluded that a better insight into the aetiology and progress of depression may be gained by combining receptor markers with such neuroendocrine markers as the DST.