Effect of fenfluramine on sympathetic firing rate

Serotonergic Control of Metabolic Homeostasis

New treatments are urgently needed to address the current epidemic of obesity and diabetes. Recent studies have highlighted multiple pathways whereby serotonin (5-HT) modulates energy homeostasis, leading to a renewed interest in the identification of 5-HT-based therapies for metabolic disease. This review aims to synthesize pharmacological and genetic studies that have found diverse functions of both central and peripheral 5-HT in the control of food intake, thermogenesis, and glucose and lipid metabolism. We also discuss the potential benefits of targeting the 5-HT system to combat metabolic disease.

Peripheral Serotonin: a New Player in Systemic Energy Homeostasis

Whole body energy balance is achieved through the coordinated regulation of energy intake and energy expenditure in various tissues including liver, muscle and adipose tissues. A positive energy imbalance by excessive energy intake or insufficient energy expenditure results in obesity and related metabolic diseases. Although there have been many obesity treatment trials aimed at the reduction of energy intake, these strategies have achieved only limited success because of their associated adverse effects. An ancient neurotransmitter, serotonin is among those traditional pharmacological targets for anti-obesity treatment because it exhibits strong anorectic effect in the brain. However, recent studies suggest the new functions of peripheral serotonin in energy homeostasis ranging from the endocrine regulation by gut-derived serotonin to the autocrine/paracrine regulation by adipocyte-derived serotonin. Here, we discuss the role of serotonin in the regulation of energy homeostasis and introduce peripheral serotonin as a possible target for anti-obesity treatment.

An assessment of the acute effects of the serotonin releasers methylenedioxymethamphetamine, methylenedioxyamphetamine and fenfluramine on immunity in rats

The purpose of the present study was to examine the effect of the serotonin releasing amphetamine derivatives methylenedioxymethamphetamine (MDMA), methylenedioxyamphetamine (MDA) and fenfluramine (FEN) on immunity in rats. Similar to MDA and MDMA, FEN reduced the number of circulating lymphocytes, provoked a suppression of Con A-stimulated lymphocyte proliferation and total IFN-gamma and IL-10 production in diluted whole blood cultures. Thus the non-psychostimulant amphetamine derivative FEN, shares the ability of the psychostimulant methylenedioxy-substituted amphetamine derivatives to alter these indices of immune function in the rat. However, when Con A-stimulated cytokine production was normalised for the number of lymphocytes in culture in order to examine cytokine production at a cellular level, the effect of the amphetamine derivatives begins to diverge. FEN shares with MDMA and MDA the ability to suppress production of the Th2 type cytokine IL-10. However the effect of these drugs on Th1 type cytokine secretion was much more complex. While the methylendioxy-substituted amphetamines increases the secretion of the Th1 type cytokine IL-2 without altering the related Th1 type cytokine IFN-gamma, FEN did not alter IL-2 secretion, but suppressed IFN-gamma secretion. In addition to these effects on T-cell responses, all three drugs inhibited LPS-induced TNF-alpha secretion from diluted whole blood cultures suggesting that macrophage activity is impaired following treatment. In all, these data extend our previous findings concerning the effects of MDMA on the immune system and demonstrate that the related serotonin releasers MDA and FEN also provoke immunological changes in rats.

Effect of galanin and enterostatin on sympathetic nerve activity to interscapular brown adipose tissue

The effects of galanin and enterostatin on sympathetic activity have been examined in rats using electrophysiological techniques. Galanin, in doses of 25-300 pmol, and enterostatin, in doses of 0.5-10 nmol, were injected into the third ventricle of anesthetized male Sprague-Dawley rats in 1-microliter volumes. Galanin produced a dose-dependent suppression (ranging between 20 and 80%) of sympathetic firing rate of nerves innervating interscapular brown adipose tissue. In rats fed a chow diet, injection of enterostatin produced only a transient 10% rise in firing rate which returned to baseline within 10-15 min. In contrast, animals fed a high-fat diet showed a dose-dependent increase in firing rate lasting for 60 min. The results of this experiment are consistent with the hypothesis that food intake and sympathetic nervous system activity have a reciprocal relationship. The implications of this relationship are discussed.

Serotonin uptake inhibitors: uses in clinical therapy and in laboratory research

Fluoxetine, zimelidine, sertraline, paroxetine, fluvoxamine, indalpine and citalopram are the selective inhibitors of serotonin uptake that have been most widely studied. Some of these compounds are or have been used clinically in the treatment of mental depression, obsessive-compulsive disorder and bulimia, and therapeutic benefit has been claimed in additional diseases as well. By blocking the membrane uptake carrier which transports serotonin from the extracellular space to inside the serotonin nerve terminals, these compounds increase extracellular concentrations of serotonin and amplify signals sent by serotonin neurons. Because serotonin neurons are widespread in the central nervous system, the functional consequences of blocking serotonin uptake are diverse, but are generally subtle. Animals treated with serotonin uptake inhibitors look normal in gross appearance, but effects such as reduced aggressive behavior, decreased food intake and altered food selection, analgesia, anticonvulsant activity, endocrine changes and neurochemical changes have been demonstrated and characterized. Serotonin uptake inhibitors have helped in revealing some dynamics of serotonin neurons; for example, when uptake is inhibited and extracellular serotonin concentration increases, presynaptic as well as postsynaptic receptors for serotonin are activated to a greater degree. A consequence of increased activation of autoreceptors on serotonin cell bodies and nerve terminals is a reduction in firing of serotonin neurons and a decrease in serotonin synthesis and release. The result is a limit on the degree to which extracellular serotonin and serotonergic neurotransmission are increased.(ABSTRACT TRUNCATED AT 250 WORDS)

d-fenfluramine in a rat model of dietary fat-induced obesity

d-Fenfluramine is an appetite suppressant drug that acts by releasing serotonin from axon terminals and inhibiting its reuptake. S 5B/P1 rats, which are resistant to dietary-fat induced obesity, and Osborne-Mendel rats, which are sensitive, were adapted to ad lib feeding of either a low- or high-fat diet. d-Fenfluramine (10 mg/kg, IP) was injected daily for 12 days. Other than a slightly greater suppression of food intake in Osborne-Mendel rats, there was little difference in response to d-fenfluramine between S 5B/P1 and Osborne-Mendel rats eating the low-fat diet. However, in Osborne-Mendel rats d-fenfluramine completely abolished the excess food intake and weight gain associated with the high-fat diet. Purine nucleotide (GDP) binding on day 13 was higher in S 5B/P1 rats than in Osborne-Mendel rats and was increased by d-fenfluramine in animals of both strains eating the low-fat diet. The high-fat diet increased GDP binding only in S 5B/P1 rats and blocked the fenfluramine-induced increase in GDP binding in both strains. We speculate that d-fenfluramine blocks a feeding reward system stimulated by the high-fat diet.

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

Anorexia nervosa is associated with vasopressin, oxytocin and serotonin abnormalities. Because of the relationship between exercise and anorexia nervosa, we explored the weight-loss syndrome produced by wheel running in food-deprived rats. Its effects on regional vasopressin and oxytocin concentrations were determined under basal conditions and following systemic fluoxetine. Weight-matched, exercised and unexercised rats served as controls. Fluoxetine caused abnormalities in suprachiasmatic vasopressin and dynorphin A content and in thymus oxytocin content that did not occur in weight-matched or exercised controls. No syndrome-specific anomalies occurred in the hypothalamo-neurohypophysial system or dorsal vagal complex (DVC). However, weight reduction and fluoxetine increased circulating vasopressin; moderate exercise caused fluoxetine-induced elevations in posterior pituitary vasopressin and oxytocin; and, unlike the other groups, fluoxetine increased DVC oxytocin in freely fed unexercised rats. It was concluded that syndrome-specific vasopressin and oxytocin abnormalities occur that are not secondary to weight loss or moderate exercise; that weight loss or fluoxetine increases circulating vasopressin; that moderate exercise alters neurohypophysial vasopressin and oxytocin content; and that weight loss or exercise inhibits a fluoxetine-stimulated increase in DVC oxytocin. Finally, it was argued that the fluoxetine abnormalities indicate possible serotonin dysfunction in the syndrome.

The nutrient balance hypothesis: peptides, sympathetic activity, and food intake

Food intake can be increased or decreased after either central or peripheral administration of peptides. Galanin, neuropeptide Y, opioid peptides, growth hormone releasing hormone and desacetyl-MSH increase food intake whereas insulin, glucagon, cholecystokinin, anorectin, corticotropin releasing hormone, neurotensin, bombesin, enterostatin, cyclo-his-pro and thyrotropin-releasing hormone reduce food intake. A number of these peptides also affect the activity of the sympathetic nervous system. The peptides which have been tested have a reciprocal effect on food intake and sympathetic activity. Opioids, NPY and GHRH, which increase food intake, decrease sympathetic activity. Conversely, peptides which reduce food intake, increase sympathetic activity, with glucagon, cholecystokinin, corticotropin releasing hormone, calcitonin, neurotensin and bombesin being examples, Several of these peptides also affect the intake of specific nutrients. Insulin reduces food intake in animals fed a high carbohydrate diet, but not when fed a high fat diet. Neuropeptide Y increases carbohydrate intake. Galanin and opioid peptides increase fat intake. Enterostatin and cyclo-His-Pro, on the other hand reduce fat intake. Glucagon decreases protein intake. The effect of peptides on the intake of specific nutrients suggests that peptides may work in part by modulating basic feeding mechanisms to lead to the selection of specific nutrients from the diet. This hypothesis might be called a nutrient specific model of peptide-induced food intake.

Food intake, sympathetic activity, and adrenal steroids

Food Intake is reciprocally related to the activity of sympathetic nerves to brown adipose tissue. This reciprocal or feedback relation is shown for hypothalamic lesions, drugs, and many peptides. These peptides also modulate intake of specific nutrients. Galanin and opioids increase fat intake, whereas enterostatin decreases fat intake. NPY increases carbohydrate intake and growth hormone releasing hormone decreases protein intake. The activity of the sympathetic nervous system is low in obesity and adrenalectomy reverses this decrease in sympathetic activity and reverses or stops the progression of obesity. One mechanism for this effect of adrenal steroids is through a transacting substance which is involved in steroid actions and the production of obesity.

Effects of cholecystokinin on sympathetic activity to interscapular brown adipose tissue

The effects of injecting cholecystokinin (CCK) into the third ventricle or into selected hypothalamic sites on electrical firing rate of sympathetic nerves to interscapular brown fat (IBAT) has been investigated in anesthetized rats. The hypothesis for these experiments was that there was a reciprocal relationship between sympathetic activity and food intake. Since CCK reduces food intake we predicted that CCK would stimulate sympathetic activity to IBAT. Following the injection of CCK into the third ventricle there was an increase in firing rate of sympathetic nerves to IBAT. When the peptide was injected into either the ventromedial hypothalamic nucleus (VMH) or lateral hypothalamic area (LHA), there was likewise an increase in sympathetic firing rate. The injection of CCK into the paraventricular nucleus produced a small decrease in sympathetic firing rate. In contrast, no effect was seen following injection of CCK into the preoptic area or dorsomedial hypothalamic nucleus. Thus, the VMH or LHA appear to be the principal hypothalamic areas mediating the stimulation of sympathetic activity to IBAT which is observed following the third ventricular injection of CCK. These studies support the hypothesis of a reciprocal relationship between the effects of CCK on the thermogenic component of the sympathetic nervous system and food intake and identify the VMH and LHA as the primary sites for this effect.

Reciprocal relation between the sympathetic nervous system and food intake

The present studies have examined the hypothesis that food intake and sympathetic activity are inversely related. Following lateral hypothalamic lesions, food intake decreases and sympathetic activity rises. When pair-gained animals are used, the sympathetic activity of the LH-lesioned animals is above that of ad lib fed controls, and the sympathetic activity of the pair-gained animals is below that of the controls. With VMH-lesions, food intake rises and sympathetic activity falls. Several drugs, including 2-deoxy-D-glucose, fenfluramine, corticotropin releasing hormone, and neuropeptide Y, all show this reciprocal relationship. Three exceptions to this rule, the rat with a PVN lesion, cold exposure and animals eating a highly palatable diet, are discussed. Under the usual circumstances, the sympathetic activity appears to play a tonic role in regulating the onset of a meal, and the acute changes in sympathetic activity after eating may play a role in the onset of satiety.

Fluoxetine, a selective inhibitor of serotonin uptake

In summary, fluoxetine is a highly selective serotonin uptake inhibitor in vitro and in vivo. The conformation of fluoxetine, which resembles that of sertraline and other serotonin uptake inhibitors, appears to be a key feature that enables its high affinity and selective interaction with the serotonin transporter. The para-trifluoromethyl substituent, however, is also a pivotal structural element. The molecular pharmacology of fluoxetine has been well-defined, and its in vivo pharmacological effects appear to be mediated almost exclusively by serotonin uptake inhibition. Its selectivity for the serotonin transporter, lack of affinity for neurotransmitter receptors, and retention of selectivity following metabolism to norfluoxetine make fluoxetine a useful tool to explore pharmacologically induced increases in serotonin neurotransmission. Fluoxetine has found a variety of therapeutic application. Its use in treating depression has been most extensively studied, but controlled clinical studies also suggest the drug may have a role in treating obesity and bulimia. Moreover, a variety of other psychiatric disorders may be treatable with this drug. Regardless of the outcome of these clinical trials, it is apparent that fluoxetine has found a useful niche in therapy, and can be used as a probe to determine the role of serotonin in modulating human pathophysiologies.

Relationship between uptake of norepinephrine by hypothalamic homogenates and the activity of brown adipose tissue

It has previously been established that norepinephrine (NE) in the central nervous system is involved in feeding and the development of obesity. The present experiments were carried out to investigate the relationship between the uptake of NE by a crude hypothalamic homogenate and NE-mediated sympathetic activity in interscapular brown adipose tissue (IBAT). Sympathetic nervous system activity was assessed by measuring the binding of the purine nucleotide guanosine-5'-diphosphate (GDP) to mitochondria isolated from IBAT. Four situations known to alter food intake and sympathetic activity, namely, corticotropin releasing factor infusion, adrenalectomy, fenfluramine treatment and obesity due to genetic transmission were studied. In each case, [3H]NE uptake by the hypothalamic preparation and GDP binding to IBAT mitochondria were measured. A highly significant negative correlation between the uptake of NE by hypothalamic homogenates and the binding of GDP to IBAT mitochondria was obtained in both lean and obese animals. These findings are discussed with regard to the regulation of food intake and sympathetic nervous system mediated thermogenesis.

Effect of norepinephrine, serotonin and tryptophan on the firing rate of sympathetic nerves

The firing rate of sympathetic nerves innervating interscapular brown adipose tissue (IBAT) has been recorded following microinjection of monoamines into the ventromedial (VMN) and paraventricular nuclei (PVN). Microinjection of norepinephrine (10 nmol) into the paraventricular nucleus produced a biphasic pattern in the firing rate of the sympathetic efferent nerves to IBAT. There was an initial dose-related 20% inhibition of firing rate followed 2 min later by 10% increase above control. When 10 times as much norepinephrine was injected into the ventromedial hypothalamus, there was a small 5% decrease in firing rate and a later significant dose-related increase in firing rate. These effects of norepinephrine were blocked by phentolamine, an alpha-adrenergic blocking drug, but not by propranolol, a beta-adrenergic blocking drug. Injection of serotonin into the paraventricular nucleus produced a short-lived but significant increase in firing rate of sympathetic nerves to brown adipose tissue. Comparable amounts of serotonin injected into the ventromedial nucleus produced a similar magnitude of increase in firing rate which lasted longer. There was a clear dose-response effect of serotonin injected into the PVN, but a much less impressive response when serotonin was injected into the VMN. The response to injections of tryptophan in both the VMN and PVN was similar to those seen with serotonin. When serotonin or tryptophan were injected into the PVN and VMN simultaneously, there was a synergistic increase in sympathetic firing rate. These data are consistent with the hypothesis that both norepinephrine and serotonin can modulate sympathetic firing rate through interaction with neurons in either the VMN or PVN.

Effects of corticotropin releasing factor on genetically obese (fatty) rats

Corticotropin releasing factor (CRF) has been administered into the third ventricle of lean and genetically obese Zucker fatty rats in both acute and chronic experiments. Following a single injection of CRF (5 micrograms or approximately 1 nmole) there was an acute reduction of food intake in both the lean and obese animals, but the effect was greater in the obese. This effect persisted for the first three hours but was no longer detectable in either lean or genetically obese animals at 6 hours. Binding of GDP to mitochondria from interscapular brown adipose tissue in 21-hour deprived animals was lower in the fatty rats than in the lean controls. The injection of CRF significantly increased GDP binding in both the lean and fatty rats. During chronic infusion of CRF into the third ventricle of fatty rats, there was a significant decrease in food intake in the obese rats and fall of body weight in both groups. The basal levels of GDP binding were significantly lower in the saline-infused fatty rats than in the saline-infused lean controls. The chronic infusion of CRF increased GDP binding in the fatty rats but not in the lean animals. The CRF-treated values for GDP binding in fatty rats however, remained significantly below the baseline values in the control animals. Chronic CRF infusion also significantly lowered glucose levels in the fatty rat. These studies are consistent with the hypothesis that CRF may be involved in the decreased food intake and increased sympathetic activity observed in genetically obese fatty rats following adrenalectomy.

Interaction of adrenalectomy and fenfluramine treatment on body weight, food intake and brown adipose tissue

Three experiments have examined the interaction of adrenalectomy and fenfluramine on food intake, body weight and the binding of guanosine-5'-diphosphate (GDP) to interscapular brown adipose tissue (IBAT). In the first experiment, GDP-binding by IBAT mitochondria from adrenalectomized or sham-operated animals was measured for 3 hr after one of 3 doses of fenfluramine. Fenfluramine stimulated GDP-binding at lower doses in the adrenalectomized animals than in the controls. In the first chronic experiment, adrenalectomy prevented the restoration of normal food intake observed 8-10 days after the beginning of fenfluramine treatment. Adrenalectomy also increased weight loss and enhanced GDP binding to mitochondria from IBAT in rats treated with fenfluramine. In the second chronic experiment, the combination of fenfluramine and adrenalectomy led to a progressive weight loss, continuing hypophagia and stimulation of GDP-binding by IBAT, whereas rats treated with fenfluramine alone showed a recovery of food intake at a stabilized but lower body weight. These data are consistent with the hypothesis that adrenalectomy and fenfluramine disable two separate components of the food intake system and that when combined, produce a profound and persisting disturbance in energy or nutrient balance.

Nutrient balance and obesity: an approach to control of food intake in humans

This article has examined the regulated systems that control nutrient balance. From this analysis, the following conclusions may be suggested: 1. Each nutrient is regulated separately in a feedback system. 2. The control of glucose is regulated by the size of the glycogen stores; the size of the fat depots, by the rate of hepatic fatty acid oxidation; and protein, by the size of the protein depots. 3. Obesity can occur as a result of hyperphagia or from repartitioning the deposition of nutrients. In either case, there is a relative or absolute reduction in the activity of the sympathetic nervous system, requiring adequate levels of circulating corticosteroids.