Medial hypothalamic serotonin: effects on deprivation and norepinephrine-induced eating

Hypothalamic TRH mediates anorectic effects of serotonin in rats

Among the modulatory functions of thyrotropin-releasing hormone (TRH), an anorectic behavior in rodents is observed when centrally injected. Hypothalamic PVN neurons receive serotonergic inputs from dorsal raphe nucleus and express serotonin (5HT) receptors such as 5HT1A, 5HT2A/2C, 5HT6, which are involved in 5HT-induced feeding regulation. Rats subjected to dehydration-induced anorexia (DIA) model show increased PVN TRH mRNA expression, associated with their decreased food intake. We analyzed whether 5HT input is implicated in the enhanced PVN TRH transcription that anorectic rats exhibit, given that 5HT increases TRH expression and release when studied in vitro By using mHypoA-2/30 hypothalamic cell cultures, we found that 5HT stimulated TRH mRNA, pCREB and pERK1/2 levels. By inhibiting basal PKA or PKC activities or those induced by 5HT, pCREB or pERK1/2 content did not increase suggesting involvement of both kinases in their phosphorylation. 5HT effect on TRH mRNA was not affected by PKA inhibition, but it diminished in the presence of PKCi suggesting involvement of PKC in 5HT-induced TRH increased transcription. This likely involves 5HT2A/2C and the activation of alternative transduction pathways than those studied here. In agreement with the in vitro data, we found that injecting 5HT2A/2C antagonists into the PVN of DIA rats reversed the increased TRH expression of anorectic animals, as well as their decreased food intake; also, the agonist reduced food intake of hungry restricted animals along with elevated PVN TRH mRNA levels. Our results support that the anorectic effects of serotonin are mediated by PVN TRH in this model.Significance statementInteraction between brain peptides and neurotransmitters' pathways regulates feeding behavior, but when altered it could lead to the development of eating disorders, such as anorexia. An abnormal increased TRH expression in hypothalamic PVN results in dehydration-induced anorectic rats, associated to their low food intake. The role of neurotransmitters in that alteration is unknown, and since serotonin inhibits feeding and has receptors in PVN, we analyzed its participation in increasing TRH expression and reducing feeding in anorectic rats. By antagonizing PVN serotonin receptors in anorectic rats, we identify decreased TRH expression and increased feeding, suggesting that the anorectic effects of serotonin are mediated by PVN TRH. Elucidating brain networks involved in feeding regulation would help to design therapies for eating disorders.

Rheumatological features of Whipple disease

Whipple disease (WD) is a rare infectious systemic disease. Rheumatologists are at the frontline of WD diagnosis due to the early rheumatological manifestations. An early diagnosis is crucial, as usual anti-rheumatic drugs, especially TNF inhibitors, may worsen the disease course. We conducted a retrospective multicentre national study from January 2010 to April 2020 to better characterize the rheumatological features of WD. Classic WD (CWD) was defined by positive periodic acid-Schiff (PAS) staining of a small-bowel biopsy sample, and non-CWD (NCWD) was defined by negative PAS staining of a small-bowel biopsy sample but at least one positive Tropheryma whipplei (TW) polymerase chain reaction (PCR) for a digestive or extradigestive specimen. Sixty-eight patients were enrolled, including 11 CWD patients. Twenty patients (30%) received TNF inhibitors during the WD course, with inefficacy or symptom worsening. More digestive symptoms and systemic biological features were observed in CWD patients than in NCWD patients, but both patient groups had similar outcomes, especially concerning the response to antibiotics and relapse rate. Stool and saliva TW PCR sensitivity were both 100% for CWD and 75% for NCWD and 89% and 60% for small-bowel biopsy sample PCR, respectively. WD encountered in rheumatology units has many presentations, which might result from different pathophysiologies that are dependent on host immunity. Given the heterogeneous presentations and the presence of chronic carriage, multiple TW PCR tests on samples from specific rheumatological sites when possible should be performed, but samples from nonspecific digestive and extradigestive sites also have great value.

Serotonin (5-HT) activation of immortalized hypothalamic neuronal cells through the 5-HT1B serotonin receptor

Serotonin [or 5-hydroxytryptamine or (5-HT)] has been implicated as a key modulator in energy homeostasis and a primary focus in the treatment of obesity. There is growing evidence that 5-HT, acting through the 5-HT 1B receptor (5-HT(1B)R) in the paraventricular nucleus of the hypothalamus (PVN), is important to this regulation. However, there is some contention as to whether 5-HT(1B)R action occurs directly on PVN neurons or indirectly via inhibitory inputs into the PVN. To address these questions, we used a novel clonal, hypothalamic neuronal cell model, adult mouse hypothalamic-2/30 (mHypoA-2/30), expressing a PVN-specific marker, single-minded homolog 1, as well as a complement of PVN neuropeptides, including TRH, vasopressin, ghrelin, nucleobindin-2, and galanin. Adult mouse hypothalamic-2/30 neurons were also found to express the 5-HT(1B)R and 5-HT 6 receptor, but not 2C, all previously linked to feeding regulation. Direct serotonergic stimulation (100 nm to 10 μm) of these neurons resulted in dose-dependent cFos activation. 5-HT (10 μm) suppressed forskolin-induced cAMP levels and induced a rise in intracellular Ca(2+) through ER Ca(2+) release, effects that were mimicked by the 5-HT(1B)R agonists, CGS12066B and CP93129, and that were attenuated in the presence of the 5-HT(1B)R-specific inhibitors, GR55562 and isamoltane hemifumarate. Modest transcriptional changes in ghrelin and nucleobindin-2 were also observed in response to 100 nm and 10 μm 5-HT, respectively. These findings support the model wherein 5-HT action through the 1B receptor subtype occurs directly on PVN neurons, leading to potential modification of neuronal transcriptional and secretory machinery.

Serotonin mediated changes in corticotropin releasing factor mRNA expression and feeding behavior isolated to the hypothalamic paraventricular nuclei

Fenfluramine reduces hunger and promotes body weight loss by increasing central serotonin (5-HT) signaling. More recently, neuropeptides have been linked to the regulation of feeding behavior, metabolism and body weight. To examine possible interactions between 5-HT and neuropeptides in appetite control, fenfluramine (200 nmol/0.5 μl/side) was administered directly into the hypothalamic paraventricular nuclei (PVN) of male rats. Bilateral fenfluramine produced significant hypophagia and increased expression of PVN corticotropin releasing factor (CRF) mRNA and neuropeptide Y (NPY) mRNA in the arcuate nucleus within the first hour after drug administration. Fenfluramine's effects on feeding behavior and mRNA expression were blocked by PVN injections of a 5-HT(1-2) receptor antagonist, metergoline (15 nmol/0.5 μl/side). These data suggest that 5-HT neurons targeting hypothalamic paraventricular CRF neurons may participate in an appetite control circuit for reducing food intake.

Depression-like behavior and mechanical allodynia are reduced by bis selenide treatment in mice with chronic constriction injury: a comparison with fluoxetine, amitriptyline, and bupropion

RATIONALE

Neuropathic pain is associated with significant co-morbidities, including depression, which impact considerably on the overall patient experience. Pain co-morbidity symptoms are rarely assessed in animal models of neuropathic pain. Neuropathic pain is characterized by hyperexcitability within nociceptive pathways and remains difficult to treat with standard analgesics.

OBJECTIVES

The present study determined the effect of bis selenide and conventional antidepressants (fluoxetine, amitriptyline, and bupropion) on neuropathic pain using mechanical allodynic and on depressive-like behavior.

METHODS

Male mice were subjected to chronic constriction injury (CCI) or sham surgery and were assessed on day 14 after operation. Mice received oral treatment with bis selenide (1-5 mg/kg), fluoxetine, amitriptyline, or bupropion (10-30 mg/kg). The response frequency to mechanical allodynia in mice was measured with von Frey hairs. Mice were evaluated in the forced swimming test (FST) test for depression-like behavior.

RESULTS

The CCI procedure produced mechanical allodynia and increased depressive-like behavior in the FST. All of the drugs produced antiallodynic effects in CCI mice and produced antidepressant effects in control mice without altering locomotor activity. In CCI animals, however, only the amitriptyline and bis selenide treatments significantly reduced immobility in the FST.

CONCLUSION

These data demonstrate an important dissociation between the antiallodynic and antidepressant effects in mice when tested in a model of neuropathic pain. Depressive behavior in CCI mice was reversed by bis selenide and amitriptyline but not by the conventional antidepressants fluoxetine and buproprion. Bis selenide was more potent than the other drugs tested for antidepressant-like and antiallodynic effects in mice.

Contrasting effects of systemic and central sibutramine administration on the intake of a palatable diet in the rat

Sibutramine hydrochloride monohydrate is the only centrally active weight-modifying agent currently approved by the FDA for long-term use in the treatment of obesity. Systemic sibutramine treatment has been shown to reduce food intake in humans and rodent models in a manner that is consistent with the enhancement of satiety mechanisms. Although it is generally assumed that the hypophagic effects of the drug are mediated by actions within the brain, the locus or loci of these effects remains unclear. These experiments compared the effects of systemic and intracranial injections of sibutramine on the intake of a palatable diet in non-deprived animals. Consistent with prior reports, systemic injections of sibutramine hydrochloride (at 0, 0.5, 1.0, or 3.0mg/kg sibutramine i.p.) dose-dependently reduced feeding on a high fat/high sucrose diet across a 2-h feeding session, but did not alter water intake or locomotor activity. In contrast, bilateral injections of sibutramine (at 0.0, 2.0, 4.0 and 10.0μg/0.5μl/side) into either the paraventricular nucleus of the hypothalamus (PVN) or the medial nucleus accumbens shell (ACb) significantly and dose-dependently increased food intake of the sweetened fat diet. ACb treatment also modestly inhibited locomotor behavior; intracranial injections had no effect on water consumption. These experiments are the first to suggest that sibutramine treatment may have distinct actions upon separate neural circuits that modulate food intake behavior in the rat.

Serotonergic regulation of galanin-induced selective macronutrient intake in self-selecting rats

We investigated macronutrient intake after intraventricular injection of galanin (GAL, 3 nmol/5 microl/rat) and/or serotonin (5-HT, 50 nmol/5 microl/rat) in self-selecting fasted rats with preferences for either carbohydrates or fats. GAL injection significantly increased carbohydrate and total intake in all rats irrespective of macronutrient preference, whereas 5-HT alone did not affect macronutrient intake. The GAL-induced increase in total intake decreased to the level of saline controls when GAL was coinjected with 5-HT. The ratio of kilocalories of carbohydrates, fats and proteins (macronutrient energy ratio) after injections of GAL and/or 5-HT was similar to the saline control. In carbohydrate-preferring rats, GAL increased carbohydrate, protein and fat intake as well as total intake. Coinjection of GAL and 5-HT tended to decrease carbohydrate intake, but increase protein and fat intake. The macronutrient energy ratio after injection of GAL did not change, but the carbohydrate energy ratio decreased after 5-HT was injected, with or without GAL. In contrast, in fat-preferring rats, GAL significantly increased carbohydrate intake. Injection of 5-HT with or without GAL did not change total macronutrient intake. The macronutrient energy ratio did not change after GAL injection with or without 5-HT. These differences suggest that macronutrient preferences should be considered in any macronutrient intake study, and that serotonergic neurons play a part in the regulation of GAL-induced macronutrient intake.

Loss of cholecystokinin and glucagon-like peptide-1-induced satiation in mice lacking serotonin 2C receptors

To investigate the role of serotonin 2C receptors (2CR), which are expressed only in the central nervous system, in the satiating actions of the gut peptides CCK and glucagon-like peptide 1 (GLP-1), we examined 1) the effect of null mutations of serotonin 2CR (2CR KO) on the eating-inhibitory potencies of dark-onset intraperitoneal injections of 0.9, 1.7, or 3.5 nmol/kg (1, 2, or 4 μg/kg) CCK and 100, 200, and 400 nmol/kg (33, 66, or 132 μg/kg) GLP-1, and 2) the effects of intraperitoneal injections of 1.7 nmol//kg CCK and 100 nmol/kg GLP-1 on neuronal activation in the brain, as measured by c-Fos expression. All CCK and GLP-1 doses decreased 30-min food intake in wild-type (WT) mice, but none of them did in 2CR KO mice. CCK increased the number of cells expressing c-Fos in the nucleus tractus solitarii (NTS) of WT, but not 2CR KO mice. CCK induced similar degrees of c-Fos expression in the paraventricular (PVN) and arcuate (Arc) nuclei of the hypothalamus of both genotypes. GLP-1, on the other hand, increased c-Fos expression similarly in the NTS of both genotypes and increased c-Fos expression more in the PVN and Arc of 2CR KO mice, but not WT mice. These results indicate that serotonin signaling via serotonin 2CR is necessary for the full satiating effects of CCK and GLP-1. In addition, they suggest that the satiating effects of the two peptides are mediated by different neural mechanisms.

NPY ablation in C57BL/6 mice leads to mild obesity and to an impaired refeeding response to fasting

Neuropeptide Y (NPY) is an orexigenic (appetite-stimulating) peptide that plays an important role in regulating energy balance. When administered directly into the central nervous system, animals exhibit an immediate increase in feeding behavior, and repetitive injections or chronic infusions lead to obesity. Surprisingly, initial studies of Npy(-/-) mice on a mixed genetic background did not reveal deficits in energy balance, with the exception of an attenuation in obesity seen in ob/ob mice in which the NPY gene was also deleted. Here, we show that, on a C57BL/6 background, NPY ablation is associated with an increase in body weight and adiposity and a significant defect in refeeding after a fast. This impaired refeeding response in Npy(-/-) mice resulted in a deficit in weight gain in these animals after 24 h of refeeding. These data indicate that genetic background must be taken into account when the biological role of NPY is evaluated. When examined on a C57BL/6 background, NPY is important for the normal refeeding response after starvation, and its absence promotes mild obesity.

Cholecystokinin combined with serotonin in the hypothalamus limits accumbens dopamine release while increasing acetylcholine: a possible satiation mechanism

Serotonin (5-HT) or cholecystokinin (CCK) injected in the hypothalamic paraventricular nucleus (PVN) inhibits feeding, but the mechanism is unknown. Prior research suggests that dopamine (DA) input to the nucleus accumbens (NAc) motivates behavior, and a component of that motivation circuit includes hypothalamic feeding systems. Acetylcholine (ACh) in the NAc, on the other hand, may act in part to inhibit feeding and generate satiety. If so, 5-HT and/or CCK in the PVN should lower extracellular DA or release ACh in the NAc. Rats were prepared with microdialysis probes in the NAc and injectors in the PVN. Serotonin (7.75 microg) or CCK-8 (0.12 microg) injected in the PVN significantly decreased ipsilateral accumbens DA (63 and 73% of baseline, respectively, without effect on ACh). However, 5-HT plus CCK injected in combination decreased DA to 72% (P<0.001) and simultaneously increased extracellular ACh to 128% of baseline (P<0.001). In later tests with the same doses and the same animals, unilateral PVN injections of 5-HT, CCK, or both combined, significantly inhibited food intake in the early dark period. The results suggest that 5-HT in the PVN acts as a neural modulator that primes a hypothalamic satiation system to respond to CCK when the gastrointestinal tract contains food to be digested. The synergistic action of 5-HT plus phasic CCK may then activate a circuit that simultaneously limits DA and releases ACh in the accumbens as part of the satiation process.

Parabrachial infusion of D-fenfluramine reduces food intake. Blockade by the 5-HT(1B) antagonist SB-216641

Systemic administration of the serotonin (5-HT) releaser/reuptake inhibitor, D-fenfluramine decreases consumption of food in mammals. This hypophagic action involves loci at several levels of the neuraxis. Indirect evidence implicates the parabrachial nucleus (PBN) of the pons as one of these regions. Consistent with this hypothesis, unilateral infusion of D-fenfluramine (200, 280, and 400 nmol/0.5 microl) directly into the lateral PBN (LPBN) of male rats reduced food intake by 33%, 56%, and 66% from baseline (7.3 +/- 0.7 g) during a 30-min test with chow. Infusions lateral, medial, and dorsal to the PBN were ineffective. Stimulating 5-HT(1B) receptors in the PBN also reduces feeding. Administration of the selective 5-HT(1B) agonist CP-93,129 (3-(1,2,5,6-tetrahydropyrid-4-yl)pyrrolo[3,2-b]pyrid-5-one) (0, 0.625, 2.5, and 10 nmol/0.5 microl) into the PBN reduced food intake by 25--79%. The selective 5-HT(1B) antagonist SB-216641 (N-[3-[3-(dimethylamino(ethoxy]-4-methoxyphenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-[1,1'-biphenyl]-4-carboxamide) (2.5 nmol) completely blocked the hypophagic action of the approximate ED(50) doses of CP-93,129 (2.5 nmol) and D-fenfluramine (280 nmol). These data strongly suggest that directly or indirectly activating 5-HT(1B) receptors in the LPBN inhibits feeding and implicates this pontine region in the serotonergic regulation of eating and satiation.

Feeding increases 5-hydroxytryptamine and norepinephrine within the hypothalamus of chicks

It is thought that hypothalamic 5-hydroxytryptamine (5HT) and norepinephrine (NE) are involved in the regulation of feeding in chicks. The present study was conducted to elucidate changes in the levels of extracellular 5HT and NE in the hypothalamus during feeding of chicks. In order to measure 5HT, NE and 4-hydroxy-3-methoxyphenylglycol (MHPG), which is a major metabolite of NE, we used brain microdialysis and high-pressure liquid chromatography with an electrochemical detector. After collecting samples to determine the basal levels of 5HT, NE and MHPG, food-deprived birds were given access to food. 5HT levels in the medial hypothalamus (MH) and lateral hypothalamus (LH) increased during the first 30 min of feeding, and then returned to basal levels. NE and MHPG in the LH increased during feeding, and remained elevated throughout the experiment. This study supports an idea that hypothalamic monoamines in the chick brain are involved in the regulation of feeding.

Meal-induced changes in extracellular 5-HT in medial hypothalamus of lean (Fa/Fa) and obese (fa/fa) Zucker rats

Hypothalamic serotonin (5-HT) is involved in appetite regulation and sympathetic stimulation of thermogenesis. This study tested the hypothesis that the enhanced energetic efficiency of obese Zucker rats involves blunted serotonergic release within the medial hypothalamus (MH). We used microdialysis and HPLC-EC to measure dynamic changes in extracellular 5-HT levels in the MH of 10-13-week-old male lean (Fa/Fa) and obese (fa/fa) Zucker rats before and after a meal. No differences were noted in basal levels of 5-HT between lean and obese rats. Consistent with the suggestion that hypothalamic 5-HT plays a physiological role in feeding, extracellular 5-HT levels increased significantly in both lean and obese rats given a meal. This increase was observed in the 20 min interval in which they ate the 8.1 kcal meal and remained for an additional 60 min. The net release of 5-HT during the meal interval was comparable in the lean (1.46+/-0.38 fmol/microl) and obese (1.21+/-0.82 fmol/microl) rats. However, the 5-HT levels of the leans (1.80+/-0.29 fmol/microl) plateaued in the next 20 min interval, whereas they continued rising (2.74+/-0.53 fmol/microl) in obese rats and were significantly higher than those in the leans during the 40 and 60 min intervals after the meal was presented. This resulted in a total net release during the meal plus the next three 20 min intervals that was significantly higher in obese (9.83+/-1.16 fmol/microl) than in lean (5.59+/-0.85 fmol/microl) rats. Thus, the enhanced energetic efficiency of the obese Zucker rats may not be associated with attenuated serotonin release in response to a meal. Rather their enhanced release of 5-HT in the MH may reflect compensatory mechanisms for the elevated orexigen NPY, the reduction in meal-induced CCK release, and/or a functional resistance to 5-HT.

Posterodorsal amygdala lesions reduce feeding stimulated by 8-OH-DPAT

Injections of the serotonin (5-HT)(1A) agonist, 8-hydroxy-2(di-n-propylamino)tetralin, (8-OH-DPAT), either systemically or into the midbrain raphe nuclei, elicit food intake in otherwise satiated rats. Lesions of the paraventricular nucleus of the hypothalamus are well known for producing long-term overeating, but past research has excluded this site as a potential locus for short-term 8-OH-DPAT feeding effects. More recent work shows that small lesions of the posterodorsal amygdala (PDA) elicit overeating in their own right. Since this and related regions of the amygdala receive 5-HT innervations from the dorsal raphe nucleus (DRN), we determined if PDA lesions might alter feeding after injecting 8-OH-DPAT into this midbrain region. Adult female rats received either bilateral electrolytic lesions of the PDA or sham lesions. After recording weight gains for over 1 month, all rats were implanted with DRN cannulae, then randomly tested every 3-4 days for 1 h intake of standard lab chow after 0, 0.4, 0.8 or 1.6 nmol injections of 8-OH-DPAT. Additional 90 min measures of intake were also made after 0 vs. 250 microg (760 nmol) 8-OH-DPAT s.c. At the two highest DRN doses tested, lesioned rats showed 50% less intake compared to shams. A similar profile emerged after the single s.c. dose. These results suggest that the PDA may be an important locus at which reduced release of endogenous 5-HT stimulates feeding. Alternatively, the PDA may represent part of a larger brain circuit whose integrity is necessary for eliciting intake in response to a variety of feeding stimuli.

Effects of chronic administration of sibutramine on body weight, food intake and motor activity in neonatally monosodium glutamate-treated obese female rats: relationship of antiobesity effect with monoamines

When the hypothalamic ventromedial nucleus and arcuate nucleus were destroyed in rats by treatment with monosodium glutamate in the neonatal stage, increase in the Lee index (body weight 1/3/body length) and in retroperitoneal fat as well as decreases in spontaneous motor activity, food consumption and growth hormone secretion function associated with hypothalamic low body length obesity (monosodium glutamate-treated obesity; MSG-OB) were observed as these rats grew. Treatment with sibutramine at 3 and 10 mg/kg p.o. once a day continuously for 14 days improved these parameters, and the degree of improvement was dose related. The plasma lipid values in MSG-OB rats, which were the same as those in normal rats, were decreased by consecutive administration of sibutramine. Levels of hypothalamic monoamines (MAs) such as norepinephrine, 5-HT (serotonin) and dopamine and their metabolites DOPAC, HVA and 5-HIAA were decreased in MSG-OB rats, and further decrease in them, though slight, was observed with consecutive daily administration of sibutramine, probably as a result of the feedback attributable to an increase in MA in synapses caused by inhibition of MA uptake by sibutramine. These results suggest that sibutramine can activate the MA nervous system by MA uptake inhibition in regions of the brain such as the lateral hypothalamic area and the paraventricular nucleus, which control food intake and sympathetic nerve activity, and the nigrostriatal area related to the extrapyramidal motor system, and thereby exhibit anti-obesity effects in the MSG-OB rat.

Decreased 5-HT transporter mRNA in neurons of the dorsal raphe nucleus and behavioral depression in the obese leptin-deficient ob/ob mouse

The neurotransmitter serotonin (5-hydroxytryptamine; 5-HT) is an important regulator of feeding behavior. A hypothalamic site of action for 5-HT in body weight control is supported by the presence of 5-HT receptors in hypothalamic regions which are intimately associated with regulation of food intake. In the present study we have investigated whether there may be an interaction between the hormone leptin, an adipose tissue-derived cytokine signaling factor that inhibits food intake and lowers body weight, and the brain serotonergic system. Immunohistochemical analysis of colchicine-treated rats showed colocalization of 5-HT transporter- and leptin receptor-immunoreactivity in cell bodies of the dorsal raphe nucleus, suggesting that dorsal raphe neurons are targets for circulating leptin. Levels of 5-HT transporter mRNA expression were compared in neurons of the dorsal raphe nucleus of obese leptin-deficient ob/ob mice and their lean littermates using in situ hybridization. 5-HT transporter mRNA levels were significantly down-regulated in neurons of the dorsal raphe nucleus of obese ob/ob mice as compared to lean control mice. Behavioral analysis showed that obese ob/ob mice had significantly lower locomotor activity and exhibited increased immobility in Porsolt's test, a model for depression. Taken together, these results suggest that serotonergic cell bodies in the rodent dorsal raphe nucleus possess leptin receptors and that the serotonergic system, as reflected by expression levels of 5-HT transporter mRNA, is down-regulated in the obese behaviorally depressed ob/ob mouse.

Hypothalamic serotonin in control of eating behavior, meal size, and body weight

Serotonin (5-HT) has been implicated in the control of eating behavior and body weight. Stimulants of this monoamine reduce food intake and weight gain and increase energy expenditure, both in animals and in humans. This article reviews evidence that supports a role for hypothalamic serotonergic receptor mechanisms in the mediation of these effects. A variety of studies in rodents indicate that, at low doses, 5-HT or drugs that enhance the release of this neurotransmitter preferentially inhibit the ingestion of carbohydrate, more than fat or protein. This phenomenon is mediated, in part, by 5-HT receptors located in various medial hypothalamic nuclei. A negative feedback loop exists between the consumption of this macronutrient and the turnover of 5-HT in the hypothalamus. That is, carbohydrate ingestion enhances the synthesis and release of hypothalamic 5-HT, which in turn serves to control the size of carbohydrate-rich meals. A model is described that proposes the involvement of circulating hormones and glucose in this feedback process. These hormones, including insulin, corticosterone, and the adipose tissue-derived hormone, leptin, have impact on serotonergic function as well as satiety. This model further suggests that 5-HT exerts its strongest effect on appetite at the start of the natural feeding cycle, when carbohydrate is normally preferred. Clinical studies provide evidence that is consistent with the proposed model and that implicates 5-HT in disturbances of eating and body weight disorders.

The 5-HT1A and 5-HT2A/2C receptor antagonists WAY-100635 and ritanserin do not attenuate D-fenfluramine-induced fos expression in the brain

D-Fenfluramine is a serotonin (5-hydroxytryptamine, 5-HT) releaser and reuptake inhibitor. It is used to study the neurochemical control of feeding and has been used to treat obesity. It has also been employed as a pharmacological tool to study changes in serotonergic function in psychiatric patients. Brain sites activated by D-fenfluramine via the release of 5-HT have been mapped via the expression of the immediate early gene c-fos. Studies in our laboratory have indicated that D-fenfluramine induces Fos in the hypothalamus and cortex through 5-HT release. The present study investigated whether 5-HT released by D-fenfluramine induces Fos expression in the brain by activating 5-HT1A or 5-HT2A/2C receptors. Rats were pretreated either with WAY-100635, a 5-HT1A antagonist, or ritanserin, a 5-HT2A/2C antagonist, prior to d-fenfluramine injection. Blockade of either 5-HT1A or 5-HT2A/2C receptors was not sufficient to suppress the Fos response to D-fenfluramine in any region of the brain examined, including the cingulate cortex, frontal cortex, caudate-putamen, paraventricular nucleus of the hypothalamus, amygdala, and brainstem. These results indicate that Fos response elicited by D-fenfluramine may be mediated by other receptors, in addition to the 5-HT1A or 5-HT2A/2C receptors.

Conditioned taste aversion induced by fluoxetine

The present study assessed the ability of the serotonin reuptake inhibitor fluoxetine (FLX) and lithium chloride (LiCl) to induce conditioned taste aversion (CTA) to a novel 20% sucrose solution. FLX (2, 5, or 8 mg/kg) or LiCl (10 mg/kg) was administered 30 min after an initial exposure to the solution. A single-bottle test of CTA 24 h after the initial exposure indicated that rats that received FLX, at any dose, or LiCl consumed significantly less solution than did those that received a vehicle treatment following the initial exposure. To examine the possibility that decreased consumption during the CTA test exposure was associated with lasting hypophagia and/or hypodipsia induced by FLX, separate groups of rats, without any prior exposure to the solution, were administered FLX (2, 5, or 8 mg/kg) and given access 24 h later to a 20% sucrose solution. FLX failed to suppress consumption of the solution at any dose. These data suggest that FLX induces an aversive drug state in rats, similar to that induced by LiCl, which serves as a potent conditioned stimulus in CTA. In addition, this CTA is independent of FLX-induced hypophagia and/or hypodipsia. The relevance of these results to the study of hypophagia induced by FLX administration is discussed.

Stimulation of 5-HT2A receptors in the paraventricular hypothalamus attenuates neuropeptide Y-induced hyperphagia through activation of corticotropin releasing factor

The effect of 5-HT1 and 5-HT2 receptor agonists administered into the paraventricular hypothalamus was studied on the hyperphagia caused by neuropeptide Y (NPY) injected into the same area. The 5-HT2A/2C receptor agonist DOI (10-20 nmol/0.5 microliter) significantly reduced NPY overeating while the 5-HT1A/1B receptor agonist RU 24969 (3.5-14 nmol/0.5 microliter) and the 5-HT1B/2C receptor agonist mCPP (5-20 nmol/0.5 microliter) had no such effect. The 5-HT2A receptor antagonist spiperone (5 microgram/0.5 microliter) and the corticotropin releasing factor antagonist alpha-helical-CRF9-41 (0.5-1 micrograms/0.5 microliter) completely antagonized the effect of 10 nmol DOI.

Effects of systemic phenylpropanolamine and fenfluramine on serotonin activity within rat paraventricular hypothalamus

Phenylpropanolamine (PPA) anorexia has been linked to activation of alpha 1-adrenergic receptors within rat paraventricular hypothalamus (PVN) by studies documenting that intra-PVN injection of PPA and other alpha 1-adrenergic agonists suppress food intake. The present experiments examine the hypothesis that PPA may suppress appetite indirectly via release of serotonin (5-HT) within the PVN. In Experiment 1, we compare the effects of PPA (20 mg/kg, IP) and of d,l-fenfluramine (FEN: 7.5 mg/kg, IP), relative to a vehicle treatment, on extracellular levels of 5-HT and the 5-HT metabolite 5-HIAA in adult male rats prepared with concentric microdialysis probes aimed at either the PVN or the perifornical hypothalamus (PFH). Injection of either vehicle or 20 mg/kg PPA had no significant effect on extracellular 5-HT within the PFH or the PVN. In contrast, a subsequent injection of 7.5 mg/kg FEN produced significant increases in 5-HT and significant decreases in 5-HIAA within the PVN and the PFH. In Experiment 2, the 5-HT1a autoreceptor agonist 8-OH-DPAT (0.25 mg/kg, SC) was used to suppress presynaptic release of 5-HT prior to systemic injection of either fenfluramine (5 mg/kg, IP) or PPA (5, 10, 20, and 30 mg/kg, IP). The anorexic action of FEN, but not PPA, was reduced by pretreatment with 8-OH-DPAT. These results suggest that the anorexic action of PPA is not mediated by an indirect effect of PPA on presynaptic release of 5-HT.

Multiple serotonin receptors: opportunities for new treatments for obesity?

Recent progress in the molecular pharmacology of 5-HT receptors and the development of selective ligands for various 5-HT receptor subtypes has advanced our understanding of the role of 5-HT mechanisms in the control of food intake and bodyweight. The most intensively investigated 5-HT receptor subtypes have been the 5-HT1A receptor, the 5-HT1B receptor and the 5-HT2C receptor. The overall pattern of results to date suggests that selective 5-HT2C agonists may be novel anorectic drugs and prove useful in the treatment of obesity. However, a number of issues remain unresolved, particularly regarding potential side-effects, as the 5-HT2C receptor agonist mCPP has been reported to induce anxiety and nausea in humans, actions that would clearly limit its therapeutic utility. In addition, the possible role of recently cloned 5-HT receptor subtypes such as 5-ht5, 5-ht6 and 5-ht7, remains unexplored and the development of selective ligands for these sites has the potential to lead to new treatments for obesity.

Alterations in deprivation, glucoprivic and sucrose intake following general, mu and kappa opioid antagonists in the hypothalamic paraventricular nucleus of rats

While opioid agonists administered into the hypothalamic paraventricular nucleus increase food intake in rats, naloxone reduces deprivation-induced intake. Ventricular administration of either mu (beta-funaltrexamine) or kappa (nor-binaltorphamine) opioid antagonists reduces spontaneous, deprivation, glucoprivic and palatable intake. The present study assessed whether microinjections of either general, mu or kappa opioid antagonists into the paraventricular nucleus altered either deprivation (24 h) intake, 2-deoxy-D-glucose hyperphagia or sucrose intake in rats. Deprivation intake was significantly reduced by nor-binaltorphamine (5 micrograms, 68 nmol, 30-33%), beta-funaltrexamine (5 micrograms, 100 nmol, 26-29%) or naltrexone (10 micrograms, 260 nmol, 26%) in the paraventricular nucleus. 2-Deoxy-D-glucose hyperphagia was significantly reduced only after 2 h by naltrexone (10 micrograms, 260 nmol, 69%), norbinaltorphamine (20 micrograms, 272 nmol, 69%) or beta-funaltrexamine (20 micrograms, 400 nmol, 83%) in the paraventricular nucleus. Sucrose intake was significantly reduced by nor-binaltorphamine (5 micrograms, 68 nmol, 27-36%), naltrexone (5-10 micrograms, 130-260 nmol, 18-31%) and beta-funaltrexamine (5 micrograms, 100 nmol, 20%) in the paraventricular nucleus. These data indicate that general, mu and kappa opioid antagonists administered into the hypothalamic paraventricular nucleus produce similar patterns of effects upon different forms of food intake as did ventricular administration, implicating this nucleus as part of the circuitry underlying opioid mediation of ingestion.

Morphine induced changes in ethanol-and water-intake are attenuated by the 5-HT3/4 antagonist tropisetron (ICS 205-930)

The opiate agonist morphine has been shown to increase ethanol intake and mesolimbic dopamine (DA) levels. Conversely, the 5-HT3/4 antagonist tropisetron has been shown to decrease ethanol intake and morphine-induced increases in mesolimbic DA levels. This study was designed to test the effects of acutely administered tropisetron on morphine-induced changes in ethanol (6% v/v) and water intake in a two-bottle test procedure. Ten water restricted male rats were injected with combinations of morphine (0.0, 0.56, 1.0, 1.5, 10.0, and 17.0 mg/kg, SC) and tropisetron (0.0, 1.0, 10.0, and 17.0 mg/kg, SC) prior to test sessions. Morphine (1.0 and 1.5 mg/kg) significantly increased absolute (g/kg) and relative ethanol intake (ethanol/total fluid). Tropisetron alone did not affect ethanol or water intake. When tropisetron (10.0 and 17.0 mg/kg) was administered in combination with morphine (1.5 mg/kg), the increase in ethanol intake induced by morphine was attenuated. Tropisetron (1.0 mg/kg) reversed a decrease in ethanol intake induced by morphine (17.0 mg/kg). The two highest doses of tropisetron partially attenuated a significant decrease in water intake produced by morphine (17.0 mg/kg). These data suggest that opiate and 5-HT3 mechanisms could interact in the regulation of ethanol intake. However, the doses of tropisetron tested were high and, therefore, the potential involvement of 5-HT4 receptors or other neurotransmitter systems in regulating ethanol intake is discussed.

d-Fenfluramine and d-norfenfluramine hypophagias do not require increased hypothalamic 5-hydroxytryptamine release

d-Fenfluramine (2.5 mg/kg i.p.) caused marked hypophagia in food-deprived rats and significantly increased medial hypothalamic extracellular 5-hydroxytryptamine (5-HT) as indicated by in vivo microdialysis. When the drug was given after the 5-HT synthesis inhibitor p-chlorophenylalanine (150 mg/kg per day x 3) the hypophagic response was unimpaired but dialysate 5-HT concentration no longer rose. The d-fenfluramine metabolite d-norfenfluramine (1.5 mg/kg i.p.) caused slightly greater hypophagia than the parent drug and completely blocked feeding in animals pretreated with p-chlorophenylalanine, but dialysate 5-HT was increased in neither circumstance. The results provide evidence against mediation of the hypophagic effects of d-fenfluramine and d-norfenfluramine by increased availability of 5-HT to receptors.

Radiofrequency lesions of the PVN fail to modify the effects of serotonergic drugs on food intake

Past research has suggested that the paraventricular nucleus (PVN) of the hypothalamus is an important brain site mediating changes in feeding induced by drugs that modify 5-hydroxytryptamine (5-HT; serotonin) neurotransmission. To test this possibility, several experiments examined the impact of lesions of the PVN on both decreases and increases in feeding following treatment with 5-HT-acting drugs. Rats with free access to standard lab chow were given access also to a wet mash diet for 1 h each day. When intakes of this diet had stabilised, rats were divided into two groups: one group received bilateral radiofrequency lesions of the PVN, the other served as a sham-operated control group. The PVN-lesioned group consumed more lab chow and gained significantly more weight over a 10-week period than the control group. Clonidine stimulated feeding in the sham-operated group, but did not do so in the lesioned group. These findings confirmed that the PVN lesions disrupted the control of food intake, as well as body weight regulation. The indirect 5-HT agonists D-fenfluramine (0.63, 1.25 and 2.5 mg/kg) and fluoxetine (2.5, 5 and 10 mg/kg), and the 5-HT1 agonist 1-(m-trifluoromethylphenyl)piperazine (TFMPP, 0.63, 1.25 and 2.5 mg/kg) dose dependently reduced the intake of the wet mash diet in sham-operated animals. This action was not modified by the PVN lesions. The highest doses of D-fenfluramine and fluoxetine also suppressed intake of chow over the 23-h period subsequent to the wet mash presentations, but the magnitude of this effect was similar in sham-operated and PVN-lesioned animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Paraventricular nucleus injections of idazoxan block feeding induced by paraventricular nucleus norepinephrine but not intra-raphe 8-hydroxy-2-(di-n-propylamino)tetralin

Previous work has shown that injection of the 5-hydroxytryptamine (5-HT)1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) into the midbrain raphe nuclei activates somatodendritic 5-HT autoreceptors leading to decreased 5-HT synthesis and release in terminal forebrain regions and an increase in feeding behaviour. Since 5-HT is believed to function antagonistically with norepinephrine (NE) in the hypothalamic paraventricular nucleus (PVN) to control feeding, it has been proposed that 8-OH-DPAT elicits food intake by removing the inhibitory influence of 5-HT over PVN alpha 2-adrenergic feeding mechanisms. This hypothesis was tested by examining the ability of PVN injection of the alpha 2-adrenoceptor antagonist idazoxan (IDAZ) to attenuate the feeding stimulant action induced by raphe injection of 8-OH-DPAT. In the first series of experiments the dose-response effects of dorsal and median raphe injection of 8-OH-DPAT in addition to PVN NE on feeding were examined. Injection of NE (5-40 nmol) and 8-OH-DPAT (0.4-1.6 nmol) both elicited reliable dose-dependent increases in 1 h food intake compared to saline control. Similar doses of 8-OH-DPAT injected into the PVN failed to alter baseline feeding. A second series of experiments examined the effects of IDAZ on 8-OH-DPAT and NE-stimulated food intake in rats implanted with dorsal or median raphe cannulae as well as cannulae aimed at the PVN.(ABSTRACT TRUNCATED AT 250 WORDS)

Injecting 5-HT into the PVN does not prevent feeding induced by injecting 8-OH-DPAT into the raphe

The selective 5-hydroxytryptamine1A (5-HT1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) activates raphe somatodendritic autoreceptors, leading to an inhibition of 5-HT neuronal activity and reduced synthesis and release of 5-HT in forebrain terminal areas. One behavioural consequence of this is increased feeding in satiated rats. Because injections of 5-HT agonists into the medial hypothalamus suppress feeding, it has been proposed that 8-OH-DPAT-induced feeding may involve a reduction of 5-HT release within this area. This hypothesis was tested by examining the ability of 5-HT injected into the medial hypothalamus to reverse the feeding-stimulant action of 8-OH-DPAT following injection into the dorsal raphe or median raphe. Two groups of rats, maintained with free access to food at all times, were used. Each was prepared with two cannulae, one aimed at the paraventricular nucleus (PVN) of the medial hypothalamus and the other at either the dorsal raphe nucleus or median raphe nucleus. Food intake over the next hour was increased following dorsal raphe or median raphe injections of 8-OH-DPAT (1 and 0.5 microgram, respectively). These effects were not blocked by injections of 7.5 or 15 micrograms 5-HT into the PVN. However, 15 micrograms 5-HT did attenuate the feeding-stimulant action of 10 micrograms norepinephrine injected into the PVN. These results do not support the hypothesis that a reduction in 5-HT release within the medial hypothalamus is responsible for the feeding-stimulant action of 8-OH-DPAT.

d-Fenfluramine- and d-norfenfluramine-induced hypophagia: differential mechanisms and involvement of postsynaptic 5-HT receptors

Severe depletion of 5-hydroxytryptamine (5-HT) by para-chlorophenylalanine (pCPA, 150 mg/kg per day x3) did not alter the hypophagic effect of d-fenfluramine (1-3 mg/kg i.p.) 1 h after food presentation in 24-h food-deprived rats, and moderately and comparably increased the hypophagic effects of its metabolite, d-norfenfluramine (0.35-1.0 mg/kg i.p.), and of the 5-HT1C receptor agonist, 1-(3-chlorophenyl)piperazine (mCPP; 1.5, 2.0 mg/kg i.p.). Chronic treatment with mCPP (2.5 mg/kg i.p. x 14) attenuated the hypophagia induced by d-norfenfluramine (1, 1.5 mg/kg) but not d-fenfluramine (1, 3 mg/kg). 1-(1-Naphthyl)piperazine (3, 8 mumol/kg s.c.), which has greater affinity for 5-HT1C than for 5-HT2 receptors, had no effect on the hypophagia induced by d-fenfluramine (1.25, 2.0 mg/kg), but 1.3 and 3 mumol/kg 1-(1-naphthyl)piperazine largely and comparably attenuated the substantial hypophagic effect of d-norfenfluramine (0.75 mg/kg). The essentially complete hypophagic action of d-norfenfluramine (1.25 mg/kg) was inhibited by 1-(1-naphthyl)piperazine with ID50 = 2.13 mumol/kg. Ketanserin, which binds more weakly than 1-(1-naphthyl)piperazine to 5-HT1C receptors and more strongly to 5-HT2 receptors, attenuated weaker but not stronger hypophagic effects of d-fenfluramine (1.25, 2.0 mg/kg) when given at high dosage (8, 16 mumol/kg s.c.). Ketanserin (16 mumol/kg) also weakly attenuated the hypophagia due to d-norfenfluramine (0.75 mg/kg), but not the essentially complete hypophagia due to d-norfenfluramine (1.25 mg/kg).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of serotonin and the serotonin blocker metergoline on meal patterns and macronutrient selection

Serotonin [5-hydroxytryptamine(5-HT)] in the paraventricular nucleus (PVN) of rats has a suppressive effect on feeding behavior and causes a selective decrease in carbohydrate ingestion, specifically at the onset of the natural (dark) feeding period. Studies conducted here provide further evidence for this phenomena, showing a similar dose-related decrease in carbohydrate ingestion at dark onset after PVN injection of 5-HT or of the agonists, d-norfenfluramine or fluoxetine, which act through endogenous 5-HT. To further characterize the effects of this indoleamine on the macrostructure of feeding, a computer-automated data acquisition system was used to analyze macronutrient feeding patterns in freely feeding animals maintained on the pure diets of protein, carbohydrate, and fat. Results indicate that PVN administration of 5-HT at dark onset decreases intake of the carbohydrate nutrient by decreasing meal size, feeding time, and feeding rate for this nutrient and increasing the satiating effect of carbohydrate. These effects, which occur specifically during the first meal after injection, are opposite those seen after peripheral administration of the 5-HT receptor antagonist, metergoline. This drug stimulates feeding through a selective increase in carbohydrate intake, characterized by an increase in meal size, percent composition, and feeding time for this nutrient and a decrease in the satiety ratio for carbohydrate. These results implicate the serotonergic system in the termination of carbohydrate-rich meals that are prevalent during the early hours of the natural feeding cycle.

Lack of synergistic feeding enhancement by systemic clonidine and 8-OH-DPAT

To expand on recent suggestions that brain alpha 2-adrenergic and serotonergic systems may interact in their controls over feeding, three experiments were conducted to determine if combining clonidine and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) injections subcutaneously might produce exponential feeding enhancements greater than responses elicited by either agent alone. Ad lib fed adult male rats were tested in 2 x 2 drug designs in all studies. In Experiment 1, 30 micrograms/kg clonidine reliably enhanced feeding over a 6-hr test period. However, 250 micrograms/kg 8-OH-DPAT showed no signs of enhancing this response, but instead seemed to impede it as determined by analyses of cumulative intake curves. In Experiment 2, either 10 micrograms/kg clonidine or 250 micrograms/kg 8-OH-DPAT reliably enhanced 6-h interval intakes, but their combination again failed to synergize other than to interact with time in suppressing intake. To determine if feeding synergy might occur if subthreshold doses of each agent were combined, Experiment 3 tested 1 microgram/kg clonidine and 15 micrograms/kg 8-OH-DPAT in 4-h feeding tests. Neither agent alone elicited reliable feeding as planned, but their combination also failed to stimulate feeding. These findings, combined with other recent work, do not support the possibility that previously demonstrated interactions between brain alpha 2-adrenergic and serotonergic systems extend to circumstances wherein enhancements of the former combined with suppressions of the latter might underly robust overeating responses.

Modulation of feeding by hypothalamic paraventricular nucleus alpha 1- and alpha 2-adrenergic receptors

Noradrenergic receptor populations within the paraventricular hypothalamus (PVN) modulate feeding. Satiated rats exhibit enhanced feeding subsequent to activation of alpha 2-adrenergic receptors within the PVN induced by exogenous infusion of either norepinephrine (NE) or clonidine (CLON). The feeding-stimulatory effect of alpha 2-adrenergic agents presumably reflects an inhibitory action on receptors located on medial hypothalamic "satiety" cells. Adrenergic receptors of the alpha 1-subclass have been identified within the PVN which are excitatory and which may function to suppress food intake. Microinjection into rat PVN of various alpha 1-adrenergic agonists including cirazoline, methoxamine, phenylpropanolamine and phenylephrine suppress feeding; an effect that is reversed by pretreatment with alpha 1-adrenergic receptor antagonists. The present review argues that alpha 1- and alpha 2-adrenoceptors within brain and specifically within the PVN are organized in an antagonistic fashion and that the effects of various adrenergic agonists on feeding may reflect the degree to which these agonists act at alpha 1- and alpha 2-adrenoceptors as well the relative balance of these receptors and their activity within the PVN.

Effects of intracisternal vs. intrahypothalamic 5,7-DHT on feeding elicited by hypothalamic infusion of NE

A variety of evidence has led to suggestions that brain serotonin (5-HT) and norepinephrine (NE) interact within the medial hypothalamus to control food intake. To test the possibility that chronic decrements in 5-HT might enhance NE-induced feeding, adult male rats were prepared with permanently indwelling cannulae aimed at the paraventricular nucleus (PVN), then received either intracisternal (IC) or PVN injections of the 5-HT neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT) vs. its vehicle, 1% ascorbic acid. Over a 4-week period, IC-5,7-DHT rats showed no signs of enhanced daily feeding or drinking. However, in 40-min intake tests, feeding but not drinking was enhanced by injecting 20 nmol NE into the PVN commencing 2 weeks after neurotoxin treatment. Terminal monoamine assays confirmed that IC-5,7-DHT produced large (80-90%) depletions of brain regional 5-HT. A functional index of 5-HT terminal damage was also implied by the impaired short-term feeding responses IC-5,7-DHT rats showed to the systemic administration of the 5-HT1A agonist, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) when tested between 3 and 4 weeks after IC treatment. Over a comparable 4-week period, PVN-5,7-DHT rats also showed no tendencies to overeat or overdrink on a daily basis. However, in contrast to IC-5,7-DHT rats, they also showed no differences in their feeding or drinking responses to NE injections into the PVN. This was so despite reliable depletions of 5-HT in the hypothalamus (-28%) and hippocampus (-71%). These results support earlier work showing that neither widespread nor localized hypothalamic damage to brain 5-HT neurons produce chronic overeating. However, the data suggest that phasic enhancements of PVN NE activity may trigger enhanced feeding when there is widespread damage to brain 5-HT neurons, although the PVN does not appear to be the brain site mediating this effect.

Concurrent measurement of serotonin metabolism and single neuron activity changes in the lateral hypothalamus of freely behaving rat

To further investigate the activity of serotonin neurons in relation to feeding behavior, the metabolic activity of the serotonergic system and single neuron activity changes in the lateral hypothalamic area (LHA) were investigated concurrently in freely behaving rats. The extracellular concentration of 5-hydroxyindoleacetic acid (5-HIAA), a metabolic product of serotonin in the LHA, began to increase concomitantly with the early stage of nocturnal eating. The increased 5-HIAA returned to the basal level within 3 or 4 h. In conjunction with the increase in serotonin metabolism, activity of 12 out of 30 LHA neurons (40%) increased, whereas it decreased in 7 (23%), and in 11 (37%) it showed no change. An intracerebroventricular injection of lisuride suppressed the increased activity in 7 of the 12 neurons, but had no effect on the others. These results suggest that the concurrent increase in serotonin metabolism and neuron activity changes in the LHA may occur in the early portion of the nocturnal eating period, and may be important in controlling feeding behavior.

Naltrexone, serotonin receptor subtype antagonists, and glucoprivic intake: 1. 2-Deoxy-D-glucose

Inhibition of deprivation-induced intake by naloxone was significantly enhanced by the 5-hydroxytryptamine3 (5-HT3) antagonist ICS-205,930. Interactions between naloxone and either the general 5-HT antagonist methysergide or the 5-HT2 antagonist ritanserin or ketanserin produced smaller effects. The present study evaluated whether 2-deoxy-D-glucose (2DG, 400 mg/kg) hyperphagia was affected by methysergide (0.5-5 mg/kg), ritanserin (0.25-2.5 mg/kg), or ICS-205,930 (0.5-5 mg/kg) alone or in combination with naltrexone (0.25 and 2.5 mg/kg). Only ICS-205,930 stimulated spontaneous intake for up to 4 h in the light cycle. Only ritanserin (1.25 mg/kg) transiently reduced 2DG hyperphagia. The dose-dependent decreases in 2DG hyperphagia by naltrexone were significantly enhanced by the dose range of ICS-205,930. The inhibition of 2DG hyperphagia by the low naltrexone dose was enhanced by methysergide (5 mg/kg) and ritanserin (1.25 mg/kg). These data suggest that the 5-HT3 receptor primarily interacts with opioid systems to modulate 2DG hyperphagia and that one possible locus of interaction is in the caudal brainstem.

A comparison of the effects of the 5-HT1 agonists TFMPP and RU 24969 on feeding following peripheral or medial hypothalamic injection

Experiments were conducted to compare the food intake suppressant effects of the 5-hydroxytryptamine (5-HT)1 agonists 1-3-trifluoromethylphenylpiperazine hydrochloride (TFMPP) and 5-methoxy-3-(1,2,3,6-tetrahydropyridinyl)1H indole (RU 24969) following either peripheral or medial hypothalamic injections. The effects of these manipulations were examined in 3 different paradigms involving the stimulation of feeding by: (1) infusion of 25 nmol noradrenaline (NA) into the medial hypothalamus, (2) adaptation to a 20 h food deprivation schedule, and (3) the presentation of a palatable wet mash diet for 1 h each day to ad libitum-fed rats. In all 3 paradigms TFMPP and RU 24969 (0.31-5 mg/kg, i.p.) induced dose-dependent reductions of food intake. Both drugs were somewhat less potent at inhibiting feeding that resulted from food deprivation. In contrast to these results medial hypothalamic infusion of TFMPP or RU 24969 (12.5-50 nmol) failed to affect food intake in any of the 3 tests. This occurred in spite of the fact that both 5-HT (12.5-50 nmol) and fluoxetine (12.5-50 nmol) mildly attenuated the feeding that resulted from NA infusion into the same site. The results provide clear evidence that the food intake suppressant effects of peripherally injected TFMPP and RU 24969 are not mediated in the medial hypothalamus. They also suggest that even though manipulations of serotonergic function within the medial hypothalamus can alter food intake, this probably does not involve selective activation of 5-HT1C and/or 5-HT1B receptors.

Serotonin may play a role in the anorexia induced by amphetamine injections into the lateral hypothalamus

Amphetamine (AMPH) injections into the lateral hypothalamus (LH) are known to inhibit feeding and this effect has been shown to be mediated by the release and the reuptake blockade of catecholamines. LH serotonin (5-HT) has been suggested to be involved in feeding inhibition and a recent study showed that LH amphetamine infusion increases extracellular dopamine (DA), norepinephrine (NE) and 5-HT, which suggests that 5-HT might also be involved in amphetamine anorexia. The present study investigated this possibility. A correlational study was performed between the anorectic effect of LH unilateral microinjections of each monoamine and the anorectic effect of AMPH. Six groups of male rats were used. The rats in each group were submitted to 2 series of 6 experimental sessions. Each session consisted of one microinjection in 24 h food-deprived rats, followed by the measurement of food intake 30 min later. The first series was similar for all groups and explored the AMPH effect (difference between the mean food intake after 3 AMPH injections (40 micrograms/0.5 microliter) and the mean food intake after 3 saline injections (0.5 microliter)). The second series explored the effects of DA (40 micrograms), NE (25 micrograms), EPI (25 micrograms), 5-HT (25 micrograms) or AMPH again, in a similar way as described for AMPH in the first series. Linear regression analysis on the first series AMPH effect and the amine effect of the 2nd series showed a positive correlation between both series of AMPH, AMPH and DA, and AMPH and 5-HT. This last correlation was replicated in a different group. No correlation was found between AMPH and NE or AMPH and epinephrine (EPI).(ABSTRACT TRUNCATED AT 250 WORDS)

Naloxone and serotonin receptor subtype antagonists: interactive effects upon deprivation-induced intake

Whereas opiate receptor antagonists generally act to inhibit food intake under a variety of physiological conditions in rats, agonists of some serotonin (5-HT) receptor subtypes appear to stimulate intake, and others appear to inhibit intake. The present study evaluated the effects of the general 5-HT receptor antagonist, methysergide (1-5 mg/kg), the 5-HT2 receptor antagonists, ketanserin (1-2.5 mg/kg) and ritanserin (1-2.5 mg/kg), and the 5-HT3 receptor antagonist, ICS 205930 (1-5 mg/kg) upon deprivation (24 h)-induced intake themselves, and upon the hypophagic properties of the general opiate receptor antagonist, naloxone (1-5 mg/kg). Whereas the high doses of methysergide (0.5-4 h, 34%) and ketanserin (0.5 h, 28%) significantly decreased deprivation-induced intake themselves, ritanserin and ICS 205930 were without effect. Naloxone produced dose-dependent reductions in deprivation-induced intake (24-45%). Methysergide (1 mg/kg) significantly potentiated naloxone (5 mg/kg) hypophagia after 0.5 h. Significant potentiations of hypophagia occurred following pairing the 1 mg/kg ketanserin dose with the 1 and 5 mg/kg naloxone doses at 2 and 4 h respectively, and pairing the 2.5 mg/kg ketanserin and 1 mg/kg naloxone doses at 0.5 and 2 h. Whereas the 1 mg/kg dose of ritanserin eliminated naloxone (1 mg/kg) hypophagia over a 2-h time course, ritanserin failed to exert further effects in other dose conditions. The differences between ketanserin and ritanserin in their effects upon deprivation-induced feeding and naloxone hypophagia suggest that the former's antagonistic actions upon alpha-adrenergic receptors may be responsible for its effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of hypothalamic and peripheral fluoxetine injection on natural patterns of macronutrient intake in the rat

The effect of the serotonin (5-HT) reuptake inhibitor, fluoxetine (FLU), on nutrient intake was examined in rats given free access to three pure macronutrient diets (protein, carbohydrate and fat). Fluoxetine was administered either peripherally or directly into the hypothalamic paraventricular nucleus (PVN) at three different times of the rats' nocturnal cycle. Using a range of doses for IP (0.6-10 mg/kg) and PVN injection (3.2-100 nmol), FLU exerted a selective, dose-dependent suppression (-20% to -60%) of carbohydrate intake only during the first hour of the dark. No change in the consumption of protein or fat was observed. This suppressive effect in the early dark period was not observed during the late dark phase, after either IP or PVN administration. In animals with brain cannulae aimed at different hypothalamic nuclei, the nutrient-suppressive effect of FLU was found to be localized to the medial hypothalamic nuclei, namely, the ventromedial, dorsomedial and suprachiasmatic nuclei, in addition to the PVN. These results, along with other published work, support a role for hypothalamic 5-HT systems in the control of nutrient intake in a circadian-related manner and in mediating the central action of the anorectic compound FLU.

Medial hypothalamic nuclei mediate serotonin's inhibitory effect on feeding behavior

Previous studies have demonstrated that injection of serotonin (5-HT) into the paraventricular nucleus (PVN), specifically at the onset of the active feeding cycle, causes a strong and selective suppression of carbohydrate intake, while producing no change in fat intake and, in some cases, enhancing protein consumption. The purpose of the present investigation was to determine whether this selective inhibitory effect of 5-HT on macronutrient ingestion is localized to a specific brain region, perhaps the PVN, or whether it can also occur in other sites throughout the hypothalamus or in regions outside this structure. A total of 7 hypothalamic and 5 extrahypothalamic areas were examined in brain-cannulated, freely feeding rats maintained on pure macronutrient diets of protein, carbohydrate and fat. The effect of 5-HT, a selective suppression (-55%) of carbohydrate feeding, was replicated in the PVN with a relatively low dose of 2.5 nmoles. Tests in 11 other brain sites demonstrated that this action of 5-HT is not unique to the PVN but is anatomically localized to the medial nuclei of the hypothalamus. Sites outside the hypothalamus, namely, the amygdala, nucleus accumbens, septum, diagonal band of Broca and nucleus reuniens dorsal to the PVN, failed to exhibit any response to 5-HT injection. Within the hypothalamus, the ventromedial (VMN) and suprachiasmatic (SCN) nuclei each responded to 5-HT in a manner similar to the PVN, producing a suppression of carbohydrate intake (-50% to -70%) with little or no change in either protein, fat or total kcal intake. The dorsomedial nucleus showed a somewhat smaller response relative to these other medial hypothalamic areas.(ABSTRACT TRUNCATED AT 250 WORDS)

Tryptophan increases extracellular serotonin in the lateral hypothalamus of food-deprived rats

Although it is well established that increases in tryptophan availability can increase brain serotonin synthesis, the effect of tryptophan loads on serotonin release is not as clear. We have used in vivo microdialysis in order to monitor extracellular serotonin in the lateral hypothalamus to examine this issue. Tryptophan methyl ester (100 mg/kg IP) was administered to ad lib-fed and 48-h food-deprived rats. The results suggest that a peripheral tryptophan load can elevate extracellular serotonin in food-deprived subjects more effectively than in food-replete subjects.

Serotonin release in lateral and medial hypothalamus during feeding and its anticipation

In the present experiments we extend previous findings that established a relationship between feeding behavior and hypothalamic serotonin as measured by in vivo microdialysis. The new result is hypothalamic release of serotonin in anticipation of eating when the animal sees and smells food. We have now verified brain serotonin peaks in four different ways: 1) a serotonergic reuptake blocker (fluoxetine 1 or 10 microM) in the perfusion medium raised basal levels of serotonin, 2) every sample was oxidized at two potentials using a dual potentiostat to confirm the voltage characteristics of each peak, 3) serotonin peaks were reduced by the selective serotonin cell body agonist, 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), thus helping confirm that most of the serotonin observed in these experiments was neuronal in origin, and 4) lateral and medial hypothalamic microdialysis probes were used simultaneously to monitor the degree of diffusion from one to the other. The results show that extracellular serotonin increases at both sites during preingestive events as well as during eating, but not afterwards.

Effects of systemic 8-OH-DPAT on the feeding induced by hypothalamic NE infusion

Past research suggests that activating brain serotonin (5-hydroxytryptamine or 5-HT) systems can inhibit feeding induced by activating brain norepinephrine (NE) systems. To explore this interaction more fully, we tested the capacity of the endogenous 5-HT release inhibitor, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), to enhance feeding stimulated by infusing NE into the medial hypothalamus. All experiments were conducted using ad lib-fed adult male rats with indwelling cannulae aimed at the paraventricular nucleus (PVN). In the first study, proven PVN-NE responders were tested for 40-min food intake after receiving 20 nanomoles (nmol) 1-NE or saline in the PVN following subcutaneous (SC) pretreatment with 250 micrograms/kg 8-OH-DPAT or saline. Both drugs produced equivalent, reliable increments in feeding compared to PVN-saline. However, no additivity or synergy was seen when they were combined. Short-term water intake was unaffected by these treatments as was subsequent food or water intake over the next 22 hr. In a second study, additional proven PVN-NE responders were tested under two comparable conditions when 1) the 8-OH-DPAT dose was left at 250 micrograms/kg but the NE dose was lowered to 10 nmol, and 2) the 8-OH-DPAT dose was lowered to 120 micrograms/kg and the NE dose was increased to 40 nmol. In the first case, no reliable feeding was seen in response to either agent alone or combined. In the second case, NE alone enhanced feeding but 8-OH-DPAT did not. The combination of both produced the same enhanced feeding as seen with NE alone.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of serotonin in eating disorders

Recent pharmacological studies have more precisely characterised the nature of the inhibitory effect of brain serotonin (5-hydroxytryptamine) on feeding behaviour. Thus, the brain sites and receptors involved have been identified, and a possible physiological role of endogenous serotonin in controlling natural patterns of eating and nutrient selection has been defined. The medial hypothalamus is believed to be a critical location in the mediation of serotonin's action. Specifically, the paraventricular and ventromedial nuclei are known to be involved in controlling energy balance, while the suprachiasmatic nucleus determines circadian patterns of eating. Serotonergic stimulation of these 3 nuclei with exogenous serotonin or drugs that release endogenous serotonin, preferentially reduces carbohydrate intake in naturally feeding animals through satiety mechanisms involved in the termination of feeding. This phenomenon is mediated by serotonin and possibly serotonin receptors, in contrast to serotonin autoreceptors which potentiate feeding possibly by inhibiting serotonin release. The activity of serotonergic function in the medial hypothalamus exhibits a circadian rhythm which is characterised by a peak at the beginning of the active cycle when the motivation to eat is strongest and is triggered by deficits in energy stores. At this time, carbohydrate is found to be the naturally preferred macronutrient, and it appears that serotonin becomes most activated under these conditions to terminate the carbohydrate-rich meal, possibly by activating satiety neurons localised in the medial hypothalamus. In this process, serotonin may interact antagonistically with noradrenaline (norepinephrine) and its alpha 2-noradrenergic receptors that normally function to enhance carbohydrate intake at the onset of the natural feeding cycle. Moreover, while inducing satiety for carbohydrate, serotonin may also play a role in switching the animal's preference towards protein. The regulation of this macronutrient is closely linked to that of carbohydrate, and it is normally preferred in the second meal of the natural feeding cycle. Most of the pharmacological evidence to date generally supports the hypothesis that disturbances in serotonin function occur in eating disorders. Decreases in plasma tryptophan, urinary 5-hydroxyindoleacetic acid (5-HIAA), platelet serotonin binding and basal cerebrospinal fluid 5-HIAA in anorexia nervosa normalise upon weight restoration and appear to be starvation effects. These alterations in serotonergic function may however perpetuate the symptomatology of anorexia nervosa once the illness is set in motion. Some drugs which in part affect serotonergic function facilitate weight gain in conjunction with an integrated psychotherapeutic and behavioural programme. Patients with bulimia nervosa, regardless of the presence of anorexia nervosa or major depression, who have been relatively weight stable and free of binge/vomit episodes for at least 3 weeks, have significantly blunted prolactin responses to the serotonin agonists. These findings indicate that post-synaptic responsiveness in hypothalamic-pituitary serotonergic pathways is reduced in bulimia. Similar alterations in other serotonin pathways at or above the level of the hypothalamus may contribute to binge eating and other behavioural symptoms in bulimic patients. The clinical response to several psychotropic agents known to potentiate serotonergic transmission further substantiates a serotonin dysregulation hypothesis of bulimia nervosa.(ABSTRACT TRUNCATED AT 400 WORDS)

Failure of serotonin antagonist pizotifen to stimulate feeding or weight gain in free-feeding rats

The serotonin antagonist pizotifen (BC-105) is prescribed as an appetite and weight enhancer (Mosegor--Wander, also commercialized under brand names Sanmigran or Sandomigran--Sandoz, Switzerland) for anorectic and convalescent humans. There has been, however, difficulty in demonstrating any orexigenic effect of pizotifen in laboratory animals. In the present report, the influence of chronic administration of pizotifen (0.1-30.0 mg/kg b.wt. per day, SC) on food intake and body weight gains was studied in rats given a standard diet (SD-energy content 14.5 kJ/g, 9% fibre), and in rats either habituated to a low energy content, carbohydrate-free diet (DD-7.3 kJ/g, 45% fibre), or given the DD after habituation to the SD. Pizotifen failed to increase food intake or weight gain. Nor did it shorten a period of initial depression of intake of the unfamiliar DD. On the contrary, pizotifen seemed to diminish food intake and weight gain in rats fed the low energy content diet. Since it has been reported that other 5-HT antagonists, e.g., cyproheptadine, methysergide, and ritanserin can enhance feeding, it is of some interest that pizotifen failed to affect food intake or weight gain in rats. The results suggest that the effects of pizotifen (and, possibly, of serotonin) in rats may differ from those in man. The possibility that feeding in the rat is mediated by 5-HT1 rather than 5-HT2 receptors is discussed.

Microdialysis studies of brain norepinephrine, serotonin, and dopamine release during ingestive behavior. Theoretical and clinical implications

This minireview deals with the possible roles of monoamines in feeding and feeding disorders. The introduction sketches the results of earlier studies with local drug injections and selective neurotoxins which provided pharmacological evidence that monoamines can influence food intake and body weight. A table summarizing this evidence is used to list monoamine changes that could underlie anorexia or hyperphagia. It is apparent that abnormalities in the monoamines, along with their cotransmitters, could cause many forms of feeding disorder. It is proposed as a working hypothesis that several varieties of hyperphagia leading to obesity have a common element. This common factor is a change in excitability of a lateral hypothalamic reinforcement system as manifested in self-stimulation at a stimulation-bound feeding site. Understanding this feeding reward-aversion system helps us understand hyperphagia and anorexia. The neurochemistry of reward and aversion involves the monoamines. This paper focuses on dopamine and serotonin. The data support the hypothesis that dopamine systems projecting to the nucleus accumbens and other forebrain areas from the mid-brain ventral tegmental area (VTA) are important for approach and positive reinforcement in ingestive behavior and self-stimulation. Serotonin is hypothesized to facilitate satiety and inhibition of feeding reward in the hypothalamus. The next section abstracts our recent experiments that measured pharmacological and physiological release of the monoamines in the hypothalamus and nucleus accumbens during ingestive behavior and self-stimulation. In vivo microdialysis in freely moving rats suggested the following: (1) Norepinephrine was released in the paraventricular nucleus during the active, feeding period of the circadian cycle. (2) The serotonin metabolite 5-HIAA also increased in the PVN at the same time if there was food to eat. (3) Amphetamine infused into the lateral hypothalamus (LH) by reverse dialysis increased synaptic dopamine, norepinephrine, and serotonin. (4) The anorectic drug d-fenfluramine increased synaptic serotonin in the LH and also increased the dopamine metabolite DOPAC, suggesting that serotonin and dopamine in the LH might contribute to fenfluramine-induced satiety. Local d-fenfluramine injection into the LH or local infusion by reverse dialysis again increased serotonin and decreased 5-HIAA and interfered with local dopamine metabolism as reflected in decreased DOPAC and HVA. (5) Tryptophan, a serotonin precursor, given systemically at an anorectic dose, increased extracellular serotonin in the LH, but this effect was only detectable in food-deprived rats. This was seemingly pH independent (between 5.8 and 8). The passage other cations through CFo is strictly suppressed (even at pH 8 and with 300 mM NaCl in the medium).(ABSTRACT TRUNCATED AT 400 WORDS)