Fluoxetine induces vasopressin and oxytocin abnormalities in food-restricted rats given voluntary exercise: relationship to anorexia nervosa

Is there a hypothalamic basis for anorexia nervosa?

The hypothalamus has long been known to control food intake and energy metabolism through a complex network of primary and secondary neurons and glial cells. Anorexia nervosa being a complex disorder characterized by abnormal feeding behavior and food aversion, it is thus quite surprising that not much is known concerning potential hypothalamic modifications in this disorder. In this chapter, we review the recent advances in the fields of genetics, epigenetics, structural and functional imaging, and brain connectivity, as well as neuroendocrine findings and emerging animal models, which have begun to unravel the importance of hypothalamic adaptive processes to our understanding of the pathology of eating disorders.

Activity Based Anorexia as an Animal Model for Anorexia Nervosa-A Systematic Review

Anorexia nervosa (AN) is a severe eating disorder affecting around 1 per 100 persons. However, the knowledge about its underlying pathophysiology is limited. To address the need for a better understanding of AN, an animal model was established early on in the late 1960's: the activity-based anorexia (ABA) model in which rats have access to a running wheel combined with restricted food access leading to self-starving/body weight loss and hyperactivity. Both symptoms, separately or combined, can also be found in patients with AN. The aim of this systematic review was to compile the current knowledge about this animal model as well as to address gaps in knowledge. Using the data bases of PubMed, Embase and Web of science 102 publications were identified meeting the search criteria. Here, we show that the ABA model mimics core features of human AN and has been characterized with regards to brain alterations, hormonal changes as well as adaptations of the immune system. Moreover, pharmacological interventions in ABA animals and new developments, such as a chronic adaptation of the ABA model, will be highlighted. The chronic model might be well suited to display AN characteristics but should be further characterized. Lastly, limitations of the model will be discussed.

A rat in the labyrinth of anorexia nervosa: contributions of the activity-based anorexia rodent model to the understanding of anorexia nervosa

Activity-based anorexia (ABA) is an analogous animal model of anorexia nervosa where food-restricted rats develop excessive running activity when given free access to a running wheel; their body weight sharply decreases, and finally self-starvation and death ensue unless animals are removed from the experimental conditions. The parallel of this animal model with major signs in the human disorder has been the focus of much attention from researchers and clinicians as a platform for translational research. The paper reviews the historical antecedents of ABA, research characterizing its occurrence, and its main limitations and strengths as a model of AN. As a symptomatic model of AN, the ABA model can provide clinicians with innovative and alternative routes for improving the treatment of AN.

Effect of serotonergic system on AVP secretion induced by physical exercise

The present study was undertaken in order to establish the possible involvement of serotonergic receptors in the control of physical exercise-stimulated vasopressin secretion. Twenty-one healthy men (divided in three groups of seven) underwent bicycle-ergometer tests until exhaustion: exercise control test (n=21), exercise plus ondansetron, selective 5-HT3 antagonist (n=7), exercise plus buspirone, selective 5-HT1A receptor agonist (n=7), exercise plus sumatriptan, selective 5-HT1D receptor agonist (n=7). AVP levels, physiological and biochemical variables were measured and compared during tests. Results showed that exercise-induced AVP rise did not change after the administration of buspirone and sumatriptan. In contrast, the administration of ondansetron significantly reduced physical exercise-induced AVP rise. Mean peak levels during physical exercise were 4.9 times higher than basal values in the control test and 2.6 times higher than basal values in the ondansetron plus exercise test. These data demonstrate that 5-HT3 serotonergic receptors at least partially mediate the AVP response to physical exercise. On the other hand, 5-HT1A and 5-HT1D serotonergic receptors do not appear to be involved in the control of AVP secretion during exercise.

The continuum model of obligatory exercise: a preliminary investigation

OBJECTIVE

The aim of this study was to carry out a preliminary study of the proposed Continuum Model of Obligatory Exercise, which states that obligatory exercise lies on a continuum--a continuum that is characterized not only by differences in severity, but by qualitative differences in the way the syndrome manifests, as the behaviour changes from mildly obsessive attitudes to exercise, to extremely disordered attitudes to exercise, which are accompanied by an eating disorder.

METHOD

Structural equation modelling (SEM) was used to investigate the Continuum Model of Obligatory Exercise utilizing data from a longitudinal study of female triathletes.

RESULTS

The structural model specifying the hypothesized causal pathways confirmed many of the postulates of the Continuum Model of Obligatory Exercise, including the role of weight and shape preoccupation in predicting food restriction, eating disorder behaviour, and increased obligatory exercise behaviour, as well as the role of obsessive-compulsiveness and increased physical activity in predicting eating disorder behaviour in exercisers.

CONCLUSION

The structural model investigated is reasonable and can serve as a starting point for a theory-based empirical exploration of the notion that obligatory exercise behaviour lies on a continuum.

Oxytocin levels in the posterior pituitary and in the heart are modified by voluntary wheel running

We hypothesized that voluntary wheel running results in increased secretion of oxytocin, a peptide involved in the stress response. An additional hypothesis was that prolonged exercise affects oxytocin levels in the heart, which is in line with the potential role of oxytocin in cardiovascular functions. Voluntary wheel running lasted 3 weeks and daily running distances increased progressively reaching maximum levels about 8 km (Sprague-Dawley rats) and 4 km (Lewis strain). The exercise resulted in significant reduction of epididymal fat, slight increase in glucose transporter GLUT4 mRNA levels and significant enhancement of plasma density. Voluntary exercise failed to influence plasma oxytocin levels either in Lewis or Sprague-Dawley rats, but it resulted in a significant decrease of oxytocin concentrations in the posterior pituitary. Plasma oxytocin concentrations were not modified even if the measurements were made in the dark phase of the day. In voluntary wheel running Sprague-Dawley rats, the content of oxytocin in the right heart atrium was lower than in controls. Thus, the present findings demonstrate that prolonged voluntary wheel running results in a decrease in pituitary oxytocin content without evident changes in hormone concentrations in peripheral blood. However, prolonged exercise used has a significant impact on oxytocin levels in the heart.

Activity-based anorexia: ambient temperature has been a neglected factor

Activity-based anorexia refers to the self-starvation of rats exposed to experimental conditions that combine restricted access to food with access to an activity wheel. This paper compares previous studies of this phenomenon in relation to the ambient temperatures (AT) that were employed. On this basis, and from some more direct evidence, we argue that AT is an important, but neglected, factor in activity-based anorexia research. More attention to AT is needed in future research, since its neglect threatens the validity of conclusions drawn from those studies. Furthermore, direct examination of the effect of AT on activity-based anorexia will allow a better understanding of the mechanisms underlying this phenomenon and the possible clinical implications for the treatment of human anorexia nervosa.

Food-deprived activity stress decreased the activity of the histaminergic neuron system in rats

The hypothalamus, which is rich in histaminergic neurons, is highly sensitive to aversive stimuli such as stress. Histamine H3 receptors, which regulate histamine release from the presynaptic site, are associated with stress-induced brain activity. In this study, we investigated the changes of histamine content and histamine H1 and H3 receptors in the brains of rats subjected to stress induced through food deprivation and physical activity on a running wheel (food-deprived activity stress). For purposes of comparison, we also examined the stressful effects of forced swimming on the histaminergic neuron system of rats. The H3 receptor density rapidly declined in the acute phase of stress but gradually returned to the control level in the chronic phase. On the other hand, the H1 receptor slowly decreased and remained at a low level during the chronic phase. These results reveal that there is a discrepancy between the levels of H1 and H3 receptors in the acute and chronic phases of stress. Brain histamine content gradually increased during the late phase of both food-deprived activity stress and forced swimming stress. These changes presumably resulted in the inhibition of histaminergic neuronal activity in the chronic stress condition. In accordance with this hypothesis, the intraventricular administration of histamine significantly reduced the hyperactivity caused by food-deprived activity stress. Since extensive exercise and restricted feeding are thought to be associated with anorexia nervosa, the abnormalities in the histaminergic neuron system might contribute to trait status in anorexia nervosa.

Selective serotonin reuptake inhibitors and neuroendocrine function

Selective serotonin (5-HT) reuptake inhibitors (SSRIs) are effective drugs for the treatment of several neuropsychiatric disorders associated with reduced serotonergic function. Serotonergic neurons play an important role in the regulation of neuroendocrine function. This review will discuss the acute and chronic effects of SSRIs on neuroendocrine function. Acute administration of SSRIs increases the secretion of several hormones, but chronic treatment with SSRIs does not alter basal blood levels of hormones. However, adaptive changes are induced by long-term treatment with SSRIs in serotonergic, noradrenergic and peptidergic neural function. These adaptive changes, particularly in the function of specific post-synaptic receptor systems, can be examined from altered adrenocorticotrophic hormone (ACTH), cortisol, oxytocin, vasopressin, prolactin, growth hormone (GH) and renin responses to challenges with specific agonists. Neuroendocrine challenge tests both in experimental animals and in humans indicate that chronic SSRIs produce an increase in serotonergic terminal function, accompanied by desensitization of post-synaptic 5-HT1A receptor-mediated ACTH, cortisol, GH and oxytocin responses, and by supersensitivity of post-synaptic 5-HT2A (and/or 5-HT2C) receptor-mediated secretion of hormones. Chronic exposure to SSRIs does not alter the neuroendocrine stress-response and produces inconsistent changes in alpha2 adrenoceptor-mediated GH secretion. Overall, the effects of SSRIs on neuroendocrine function are dependent on adaptive changes in specific neurotransmitter systems that regulate the secretion of specific hormones.

Patterns of c-fos expression induced by fluvoxamine are different after acute vs. chronic oral administration

Fluvoxamine is a selective serotonin (5-HT) reuptake inhibitor (SSRI) with a broad spectrum of behavioral and therapeutic effects, e.g. in depressive illness. We used the expression of c-fos, after both acute and chronic oral administration of fluvoxamine in the rat, to study its immediate and long-term effects, in relation to the distribution of Galanin (GAL) and Vasoactive Intestinal Polypeptide (VIP). After acute oral administration, most consistent increases were apparent in (parts of); the nucleus of the solitary tract, medial part; the lateral parabrachial nucleus, external part; the bed nucleus of the stria terminalis, dorsolateral part; and the central nucleus of the amygdala, lateral part. After chronic administration, distribution of Fos-IR was similar to acute administration, although numbers of Fos-IR neurons were no longer significantly different from control values. It is concluded that activation of 5-HT3-receptors in the caudal brainstem or gastro-intestinal afferents of the vagal nerve may play a role in the observed pattern of Fos-IR after fluvoxamine administration. The relationship with the antidepressant effects of fluvoxamine needs further investigations.

5-HT3 serotonergic receptor mediation of hypoglycemia-induced arginine-vasopressin but not oxytocin secretion in normal men

The present study was undertaken in order to establish the possible involvement of 5-HT3 serotonergic receptors in the control of basal and/or hypoglycemia-stimulated arginine vasopressin (AVP) and/or oxytocin (OT) secretion. For this purpose, 12 normal men were injected intravenously with a bolus of 4 mg ondansetron, a specific 5-HT3 receptor antagonist, under basal conditions (n = 6) or 30 min before insulin (0.15 IU/kg body weight) administration (n = 6) (insulin tolerance test (ITT)). Control experiments with normal saline instead of ondansetron treatment were performed. Furthermore, on a different occasion, the same subjects were tested in identical experimental conditions with 8 mg ondansetron. Our results showed that the hypoglycemic response to insulin was similar during the ITT and ondansetron plus ITT. Inhibition of 5-HT3 serotonergic receptors with ondansetron (4 or 8 mg) did not modify the basal secretion of AVP and OT and the OT response to insulin-induced hypoglycemia. In contrast, the administration of 4 or 8 mg ondansetron significantly reduced in a similar manner hypoglycemia-induced AVP rise. Mean peak level at 45 min after insulin injection was 2.25 times higher than baseline in the control ITT and 1.5 times higher than basal value in the ondansetron (4 or 8 mg) plus ITT. These data demonstrate that 5-HT3 serotonergic receptors at least partially mediate the AVP response to hypoglycemia, without modifying the simultaneous OT response. On the other hand, 5-HT3 receptors do not appear to be involved in the control of basal posterior pituitary hormone secretions.

Alterations in behavior and opioid gene expression induced by the novel tropane analog WF-31

The effects of the acute administration of the serotonin-selective tropane analog, [2beta-propanoyl-3beta-(4-isopropylphenyl)-tropane, WF-31, on spontaneous locomotor activity were measured and compared to those of the highly selective serotonin uptake inhibitor, fluoxetine and cocaine, a non-selective re-uptake inhibitor of dopamine and serotonin. WF-31 (1, 10 and 30 mg/kg)-elicited increases in locomotor behaviors when compared to vehicle-treated rats. This increased activity was blocked by pre-treatment with the dopaminergic antagonist, flupenthixol, suggesting that these effects may be mediated by dopaminergic mechanisms. Cocaine, but not fluoxetine, also elicited increases in behaviors. In addition, the effects of these three compounds on opioid peptide gene expression were also assessed using in situ hybridization histochemistry in the same animals. The acute administration of both WF-31 and cocaine increased the expression of preprodynorphin mRNA in the dorsal striatum whereas fluoxetine had no effect. Expression of striatal preproenkephalin mRNA was augmented by all three compounds. Within the nucleus accumbens, PPD mRNA levels were affected only by treatment with WF-31, an effect that was blocked by pre-treatment with flupenthixol. In contrast, the acute administration of both WF-31 and fluoxetine, but not cocaine, increased the expression of preproenkephalin mRNA. These increases, however, were not reversed by pre-treatment with flupenthixol. Despite its profile in vitro as a relatively selective serotonin re-uptake inhibitor, some of the in vivo actions of WF-31 appear to be mediated by dopaminergic mechanisms. These data further suggest that the mechanisms underlying expression of the opioid peptides in the nucleus accumbens may vary from those in the dorsal striatum.

Eating disorders and hyperactivity: a psychobiological perspective

OBJECTIVE

To review the eating disorder research investigating the psychobiological connections between self-starvation and high-level exercising, including both animal experimentation and clinical field studies. In recent years it has been proposed that physical activity plays a central role in the pathogenesis and progression of the eating disorders-in particular, anorexia nervosa.

METHOD

A review of research from animal experimentation and from clinical field studies investigating the biological and psychological implications of physical activity and starvation in the pathogenesis of eating disorders.

RESULTS

Animal research indicates that physical activity and starvation seem to potentiate one another and that alterations in the serotonergic system may underlie this process. Similar behavioural results have been found in recent clinical studies with eating-disordered patients, which suggests that physical activity plays a more central role in the development and maintenance of the eating disorders than had previously been thought.

CONCLUSIONS

The emerging picture is that psychosocial factors seem to provide the most compelling factors in the etiology and onset of the disorder, while biological factors-in most cases induced by severe malnutrition and strenuous overexercising-predominate in the maintenance of the disorder.

Effects of tryptophan and/or acute running on extracellular 5-HT and 5-HIAA levels in the hippocampus of food-deprived rats

The present microdialysis study has examined whether exercise-elicited increases in brain tryptophan availability (and in turn 5-HT synthesis) alter 5-HT release in the hippocampus of food-deprived rats. To this end, we compared the respective effects of acute exercise, administration of tryptophan, and the combination of both treatments, upon extracellular 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) levels. All rats were trained to run on a treadmill before implantation of the microdialysis probe and 24 h of food deprivation. Acute exercise (12 m/min for 1 h) increased in a time-dependent manner extracellular 5-HT levels (maximal increase: 47%), these levels returning to their baseline levels within the first hour of the recovery period. Besides, exercise-induced increases in extracellular 5-HIAA levels did not reach significance. Acute administration of a tryptophan dose (50 mg/kg i.p.) that increased extracellular 5-HIAA (but not 5-HT) levels in fed rats, increased within 60 min extracellular 5-HT levels (maximal increase: 55%) in food-deprived rats. Whereas 5-HT levels returned toward their baseline levels within the 160 min that followed tryptophan administration, extracellular 5-HIAA levels rose throughout the experiment (maximal increase: 75%). Lastly, treatment with tryptophan (60 min beforehand) before acute exercise led to marked increases in extracellular 5-HT and 5-HIAA levels (maximal increases: 100% and 83%, respectively) throughout the 240 min that followed tryptophan administration. This study indicates that exercise stimulates 5-HT release in the hippocampus of fasted rats, and that a pretreatment with tryptophan (at a dose increasing extracellular 5-HT levels) amplifies exercise-induced 5-HT release.

Obsessive compulsiveness and physical activity in anorexia nervosa and high-level exercising

Although excessive physical activity and obsessive compulsiveness are both prevalent in anorexia nervosa (AN), to date, the association between these two factors has not been systematically investigated. The aim of the present study was to investigate the relationship between obsessive compulsiveness and both behavioral and psychological aspects of exercise in women with AN, and to compare them to a nonclinical sample of females classified as either moderate or high-level exercisers. Results indicated that obsessive compulsiveness, weight preoccupation, and pathological aspects of exercise were significantly related to the level of physical activity among the eating disorder patients. For the high-level exercisers, only obsessive compulsiveness was significantly related to the amount of physical activity. The findings are discussed in terms of a model in which physical activity, starvation, and obsessive compulsiveness are reciprocally and dynamically related, with each factor creating a destructive bidirectional loop that is resistant to change and difficult to break. We propose that this self-perpetuating loop may be a significant influence in the development and maintenance of eating disorders in a certain subgroup of women.

Effects of rearing condition on activity-induced weight loss

This study evaluated the effects of differential rearing conditions on a rat protocol for various human syndromes. Subjects were 26 male Sprague-Dawley rats, 24 days old at the start of the experiment, matched according to weight, and randomly assigned to an isolation- or group-reared (4 rats/cage) condition. At 60 days of age (273 +/- 13 g), subjects were individually housed in cages allowing access to running wheels. Weight loss was produced through voluntary exercise and restricted food access. Animals in the isolation-reared condition lost weight at a faster rate and had heavier relative thymus weights than those in the group-reared condition. Animals in both conditions ran equivalent distances and ate equivalent amounts of food. The data show that postweaning rearing conditions impact the interpretation of behavioral and physiological outcomes of animal models. The results implicate a shift from maternal regulation of pup physiological and behavioral systems to the broader social niche.

Exercise in food-restricted rats produces 2DG feeding and metabolic abnormalities similar to anorexia nervosa

Anorexia nervosa (AN) is associated with a paradoxical reduction in hunger ratings following 2-deoxy-D-glucose (2DG) induced glucose insufficiency. Because of the relationship between exercise and AN, there is interest in the weight-loss phenomenon produced by exercise in food restricted rats. This investigation determined if the weight-loss phenomenon is associated with a paradoxical suppression of food intake following 2DG and if the effect is related to reductions in prevailing glucose and insulin levels. Weight-matched, normal-weight exercised and normal-weight unexercised rats served as controls. As predicted, 2DG reduced food intake in animals subjected to the phenomenon (1.5 h/day food access and 22.5 h/day running wheel access). This effect was related to reductions in plasma glucose and insulin under the conditions that prevailed at the time of injection. Since these changes also occurred in weight-matched controls, they were attributed to the general effects of weight loss. A situational specificity for the "anorexia" of the weight-loss syndrome was also demonstrated. Finally, the strengths and weaknesses of the phenomenon as a model of AN were considered.

Systemic clonidine increases feeding and wheel running but does not affect rate of weight loss in rats subjected to activity-based anorexia

Activity-based anorexia (ABA) is an animal model of anorexia nervosa with two characteristics of the disorder, decreased food intake and increased activity. We have shown that chronic noradrenergic stimulation of the paraventricular hypothalamus exacerbates ABA rather than ameliorates it. This study determined if peripheral chronic administration of clonidine affects ABA. Rats were implanted SC with osmotic minipumps infusion 0, 30, or 300 micrograms/kg/day of clonidine and exposed to ABA (1.5 h/day ad lib food, 22.5 h/day ad lib wheel access). Results showed that clonidine did not affect the rate of weight loss during ABA, but increased food intake at the lower dose and wheel activity at the higher dose. It is proposed that increased energy expenditure due to wheel running is counteracted by an inhibition of sympathetically mediated diet-induced thermogenesis, and that the elevation in running by the higher dose potentially increases the risk of developing ABA.

Endogenous opiates: 1993

This paper is the sixteenth installment of our annual review of research concerning the opiate system. It is restricted to papers published during 1993 that concern the behavioral effects of the endogenous opiate peptides, and does not include papers dealing only with their analgesic properties. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; development; immunological responses; and other behaviors.

Fluoxetine induced insulin-like growth factor II (IGF-II) changes in hypothalami of normal, exercised and food restricted rats

Hypothalamic and pituitary insulin-like growth factor II (IGF-II) peptide concentrations are differentially regulated by factors associated with metabolism such as insulin and glucoprivation. However, the effects of other metabolic stressors such as food restriction or exercise on hypothalamic IGF-II concentrations remain largely to be explored. In order to assess whether metabolic stress alters central nervous system IGF-II secretion, peptide analysis was conducted in rats exhibiting activity-based anorexia (ABA) compared to exercised-matched, body weight-matched or ad libitum fed controls. Further, the possibility of serotonergic control of IGF-II secretion was examined by determining IGF-II response to fluoxetine (FLX) injections (15 mg/kg body wt., i.p.). While ABA and body weight loss altered peripheral IGF-II concentrations compared to ad libitum fed or exercised controls, these treatments had no effect on hypothalamic or posterior pituitary IGF-II content. However, FLX administration increased IGF-II concentrations in the ventromedial hypothalamus and decreased IGF-II content in the lateral hypothalamus compared to vehicle injected. Anterior pituitary levels of IGF-II were also decreased by FLX. These data suggest that a serotonergic influence on CNS IGF-II exists and that IGF-II secretion may be altered by factors affecting serotonin metabolism or efficacy.

Beta-endorphin and dynorphin abnormalities in rats subjected to exercise and restricted feeding: relationship to anorexia nervosa?

Exercise and the endogenous opioids have been linked to anorexia nervosa. This investigation determined the effects of the weight-loss syndrome induced by voluntary exercise (22.5 h/day) in food-restricted rats (1.5 h/day food access) on the endogenous opioids. The animals were tested under resting-fed and 2-deoxy-D-glucose (2DG) stimulated conditions. Weight-matched, freely fed exercised and ad libitum fed unexercised groups served as controls. Specific opioid abnormalities were found in the syndrome. These included a basal elevation in plasma beta-endorphin, which was abnormally suppressed by 2DG, and 2DG-induced elevations in arcuate hypothalamic beta-endorphin content and supraoptic hypothalamic dynorphin-A content. None of these changes occurred in controls. Finally, it was found that short-term moderate exercise itself chronically reduced adenohypophysial beta-endorphin content and elevated supraoptic dynorphin-A content. The relationship of the syndrome's hyperendorphinism to the hypothalamo-pituitary-adrenal axis and the auto-addiction hypothesis of anorexia nervosa was considered, as was the significance of the supraoptic dynorphin-A abnormality to the hypothalamo-neurohypophysial system. The differential sensitivity of the supraoptic dynorphin-A system compared to the arcuate hypothalamic beta-endorphin system to moderate exercise was also discussed.