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

Delineating a serotonin 1B receptor circuit for appetite suppression in mice

Triptans are a class of commonly prescribed antimigraine drugs. Here, we report a previously unrecognized role for them to suppress appetite in mice. In particular, frovatriptan treatment reduces food intake and body weight in diet-induced obese mice. Moreover, the anorectic effect depends on the serotonin (5-HT) 1B receptor (Htr1b). By ablating Htr1b in four different brain regions, we demonstrate that Htr1b engages in spatiotemporally segregated neural pathways to regulate postnatal growth and food intake. Moreover, Htr1b in AgRP neurons in the arcuate nucleus of the hypothalamus (ARH) contributes to the hypophagic effects of HTR1B agonists. To further study the anorexigenic Htr1b circuit, we generated Htr1b-Cre mice. We find that ARH Htr1b neurons bidirectionally regulate food intake in vivo. Furthermore, single-nucleus RNA sequencing analyses revealed that Htr1b marks a subset of AgRP neurons. Finally, we used an intersectional approach to specifically target these neurons (Htr1bAgRP neurons). We show that they regulate food intake, in part, through a Htr1bAgRP→PVH circuit.

Role of impulsivity and reward in the anti-obesity actions of 5-HT2C receptor agonists

The selective 5-HT_2C receptor agonist lorcaserin entered clinical obesity trials with the prevalent view that satiety was a primary mechanism of action. Subsequent Phase II and III trials demonstrated efficacy in terms of weight loss, although the overall effect size (~3% placebo-corrected change) is considered modest. Lorcaserin has been approved by the FDA for the treatment of obesity with lifestyle modification, but since its introduction in 2013 its sales are in decline, probably due to its overall modest effect. However, in some individuals, lorcaserin has a much more clinically significant effect (i.e. >10% placebo-corrected change), although what common features, if any, define these high responders is presently unknown. In the present article we highlight the evidence that alternative mechanisms to satiety may contribute to the anti-obesity effect of lorcaserin, namely effects on constructs of primary and conditioned reward and impulsivity. This may better inform the clinical evaluation of lorcaserin (and any future 5-HT_2C receptor agonists) to subgroups of obese subjects characterized by overeating due to maladaptive impulsivity and reward mechanisms. One such population might be individuals diagnosed with binge eating disorder.

Pro-TRH and pro-CRF expression in paraventricular nucleus of small litter-reared fasted adult rats

Neuroendocrine axes adapt to nutrient availability. During fasting, the function of the hypothalamus-pituitary-thyroid axis (HPT) is reduced, whereas that of the hypothalamus-pituitary-adrenal axis (HPA) is increased. Overfeeding-induced hyperleptinemia during lactation may alter the regulatory set point of neuroendocrine axes and their adaptability to fasting in adulthood. Hyperleptinemia is developed in rodents by litter size reduction during lactation; adult rats from small litters become overweight, but their paraventricular nucleus (PVN) TRH synthesis is unchanged. It is unclear whether peptide expression still responds to nutrient availability. PVN corticotropin-releasing factor (CRF) expression has not been evaluated in this model. We analyzed adaptability of HPT and HPA axes to fasting-induced low leptin levels of reduced-litter adult rats. Offspring litters were reduced to 2-3/dam (early-overfed) or maintained at 8/dam (controls, C). At 10 weeks old, a subset of animals from each group was fasted for 48 h and leptin, corticosterone, and thyroid hormones serum levels were analyzed. In brain, expressions of leptin receptor, NPY and SOCS3, were evaluated in arcuate nucleus, and those of proTRH and proCRF in PVN by real-time PCR. ProTRH expression in anterior and medial PVN subcompartments was assayed by in situ hybridization. Early-overfed adults developed hyperphagia and excessive weight, together with decreased proTRH expression in anterior PVN, supporting the anorexigenic effects of TRH. Early-overfed rats presented low PVN proTRH synthesis, whereas fasting did not induce a further reduction. Fasting-induced stress was unable to increase corticosterone levels, contributing to reduced body weight loss in early-overfed rats. We concluded that early overfeeding impaired the adaptability of HPT and HPA axes to excess weight and fasting in adults.

The 5-HT2C receptor agonist lorcaserin reduces nicotine self-administration, discrimination, and reinstatement: relationship to feeding behavior and impulse control

Lorcaserin ((1R)-8-chloro-1-methyl-2,3,4,5-tetrahydro-1H-3-benzazepine HCl) is a selective 5-HT(2C) receptor agonist with clinical efficacy in phase-III obesity trials. Based on evidence that this drug class also affects behaviors motivated by drug reinforcement, we compared the effect of lorcaserin on behavior maintained by food and nicotine reinforcement, as well as the stimulant and discriminative stimulus properties of nicotine in the rat. Acutely administered lorcaserin (0.3-3 mg/kg, subcutaneous (SC)) dose dependently reduced feeding induced by 22-h food deprivation or palatability. Effects up to 1 mg/kg were consistent with a specific effect on feeding motivation. Lorcaserin (0.6-1 mg/kg, SC) reduced operant responding for food on progressive and fixed ratio schedules of reinforcement. In this dose range lorcaserin also reversed the motor stimulant effect of nicotine, reduced intravenous self-administration of nicotine, and attenuated the nicotine cue in rats trained to discriminate nicotine from saline. Lorcaserin also reduced the reinstatement of nicotine-seeking behavior elicited by a compound cue comprising a nicotine prime and conditioned stimulus previously paired with nicotine reinforcement. Lorcaserin did not reinstate nicotine-seeking behavior or substitute for a nicotine cue. Finally, lorcaserin (0.3-1 mg/kg) reduced nicotine-induced increases in anticipatory responding, a measure of impulsive action, in rats performing the five-choice serial reaction time task. Importantly, these results indicate that lorcaserin, and likely other selective 5-HT(2C) receptor agonists, similarly affect both food- and nicotine-motivated behaviors, and nicotine-induced impulsivity. Collectively, these findings highlight a therapeutic potential for 5-HT(2C) agonists such as lorcaserin beyond obesity into addictive behaviors, such as nicotine dependence.

Tryptophan hydroxylase gene (TPH1) and peripheral tryptophan levels in depression

BACKGROUND

Genetic variants have been discovered in two genes encoding for tryptophan hydroxylase (TPH)-TPH1 and TPH2. Low tryptophan (TRP) levels are associated with depression and may arise because of stress. Evidence suggests that hypothalamic and peripheral 5HT systems have a significant role in appetite regulation, possibly a homeostatic mechanism in regulating peripheral TRP levels.

METHODS

We examined the association between a polymorphism in intron 7 of TPH1, 218A>C and plasma total TRP levels in 118 patients with major depression.

RESULTS

There was an interaction between 218A>C and gender in determining plasma TRP whereby presence of the 218C allele, in women, was associated with markedly reduced plasma TRP.

LIMITATIONS

The study investigated only the TRP1 gene and did not use a haplotype analysis. The results only apply to a population of subjects suffering from major depression.

CONCLUSIONS

TPH1 may be associated with the regulation of peripheral tryptophan levels and therefore availability of tryptophan to the brain. This may have relevance to a range of neuropsychiatric conditions.

Feeding behavior after metergoline or GR-46611 injections into the paraventricular nucleus of the hypothalamus in the pigeon

The present study examined changes in spontaneous behavior of free-feeding pigeons in response to local injections of metergoline (MET, an antagonist of 5-HT(1/2) receptors; 5, 10 and 20 nmol), GR-46611 (GR, a 5-HT(1B/1D) agonist; 0.6 and 6 nmol) or vehicle into the paraventricular hypothalamic nucleus (PVN). When infused into the PVN, MET and GR promptly and reliably elicited feeding at their higher doses, without affecting drinking or non-ingestive behaviors (locomotion, exploration, preening, sleep) during the first hour after injection. Both GR- and MET-evoked ingestive responses were associated only with an increase in feeding duration, with no changes in latency to start feeding. In a second series of experiments, the effective doses of MET (20 nmol) and GR (6 nmol) were injected into other diencephalic areas. This exploratory study revealed that intense feeding responses to both MET and GR local injections are also observed in the n. medialis hypothalami posterioris and in the adjacent n. lateralis hypothalami posterioris (PMH/PLH complex, in the caudoventral hypothalamus) and in the n. magnocellularis preopticus (PPM, in the caudal preoptic region). The behavioral profiles associated with these hyperphagic responses were nucleus-specific: in the PMH/PLH, MET-induced feeding was accompanied by an increase in total feeding duration and by a reduction in the latency to start feeding, while ingestive responses evoked by MET in the PPM were associated only with an increase in feeding duration (similar to that observed in the PVN experiments). No ingestive effects were observed after intracerebroventricular (ICV, lateral ventricle) injections of MET (10, 30, 100 or 300 nmol), while ICV injections of GR (3, 15 or 30 nmol) increased feeding only at the higher dose [Da Silva RA, De Oliveira ST, Hackl LPN, Spilere CI, Faria MS, Marino-Neto J, Paschoalini MA. Ingestive behaviors and metabolic fuels after central injections of 5-HT1A and 5-HT1D/1B receptors agonists in the pigeon. Brain Res, 2004;1026:275-283]. These data indicate the presence of a tonic inhibitory influence on feeding behavior exerted by 5-HT afferents on these hypothalamic areas, and suggest that these inputs, possibly mediated by non-rodent-type 5-HT1D/1B receptors, can affect both satiety and satiation mechanisms.

GABAA receptor antagonists prevent abnormalities in leptin, insulin and amylin actions on paraventricular hypothalamic neurons of overweight rats

The hypothalamic regulatory system of body weight which develops in rats during critical periods of early postnatal life seems to express plastic changes depending on nutrition at that time. Adult rats previously exposed to early postnatal overnutrition by raising them in small litters become persistently predisposed to overweight, hyperphagia and hyperleptinaemia. The hypothesis was raised that feeding-related peptides could be involved through altered effects on neuronal activity of the regulatory systems of such rats. This was studied on brain slices of small-litter rats and normal-weight controls between days 60 and 120 of life. Neurons of the medial parvocellular part of the paraventricular nucleus were significantly activated by the adiposity signals leptin, insulin and amylin in controls. This is a kind of negative feedback, because activation of these neurons is known to be followed in vivo by increased energy expenditure. GABAergic mechanisms seem to affect these neuronal responses because the activating effects of insulin and amylin were reduced in the presence of a GABA(A) receptor antagonist. In overweight small-litter rats, however, the neuronal responses to the adiposity signals were significantly changed; activating effects were reduced and inhibitory effects increased. By means of blockade of GABA(A) receptors, significant alterations in the neuronal responses to leptin, insulin and amylin in small-litter rats were prevented. Responses to the peptides were reversed and now resembled those of controls. In conclusion, changes in neuronal wiring with GABAergic interneurons seem to contribute to a persistently reduced negative feedback of adiposity signals in early postnatally overfed rats.

Altered responses to orexigenic (AGRP, MCH) and anorexigenic (alpha-MSH, CART) neuropeptides of paraventricular hypothalamic neurons in early postnatally overfed rats

Food intake and energy expenditure are regulated by neuropeptides in the hypothalamus. While cocaine- and amphetamine-regulated transcript (CART) peptide and melanocortins such as alpha-melanocyte-stimulating hormone (alpha-MSH) are anorexigenic and increase energy expenditure, the endogenous melanocortin receptor antagonist agouti gene-related protein (AGRP), melanin-concentrating hormone (MCH) and neuropeptide Y (NPY) are orexigenic, anabolic peptides. Alterations in the regulatory balance may promote excessive weight gain. The action of these peptides on paraventricular hypothalamic neurons was studied in brain slices of overweight, adult rats previously subjected to early postnatal overfeeding in small litters of only three pups per mother, compared to 12 pups per dam in control litters. CART, melanocortins and NPY significantly excited paraventricular neurons of controls, whereas neurons of small-litter rats were mainly inhibited. Inhibition was dominant following administration of AGRP, MCH and NPY. The altered responses of paraventricular neurons in adult small-litter rats might reflect a general mechanism of neurochemical plasticity and 'malprogramming' of hypothalamic neuropeptidergic systems acquired during the postnatal critical differentiation period, thus leading to permanently altered function of these regulatory systems of body weight.

Hypothalamic paraventricular 5-hydroxytryptamine: receptor-specific inhibition of NPY-stimulated eating and energy metabolism

The feeding effects of 5-hydroxytryptamine (5-HT)(1) and 5-HT(2) receptor agonists injected into the hypothalamic paraventricular nucleus (PVN) immediately prior to PVN administration of neuropeptide Y (NPY) were examined. The impact of these same compounds on NPY-induced alterations in energy metabolism was also assessed in an attempt to characterize further the potential interactive relationship of PVN NPY and 5-HT on feeding and whole body calorimetry. Specifically, several experiments examined the effect of various 5-HT receptor agonists on NPY-stimulated eating and alterations in energy substrate utilization [respiratory quotient (RQ)]. This included the 5-HT(1A) receptor agonist 8-OH-DPAT, the 5-HT(1B/1A) agonist RU 24969, the 5-HT(1D) agonist L-694,247, the 5-HT(2A/2C) agonist DOI, the 5-HT(2B) agonist BW 723C86 and the 5-HT(2C) agonist mCPP. In feeding tests conducted at the onset of the dark cycle, drugs were administered 5 min prior to PVN injection of NPY and food intake was measured 2 h postinjection. The metabolic effects of NPY following a similar pretreatment were monitored using an open-circuit calorimeter measuring the volume of oxygen consumed (VO(2)), carbon dioxide produced (VCO(2)) and RQ (VCO(2)/VO(2)). PVN injection of NPY (100 pmol) potentiated feeding and evoked reliable increases in RQ. Only DOI (2.5--5 nmol) pretreatment antagonized NPY-induced eating and blocked the peptide's effect on energy substrate utilization. Direct PVN pretreatment with spiperone (SPRN), a 5-HT(2A) receptor antagonist, and ketanserin (KTSN), a 5-HT(2A/2C) antagonist, but not SDZ SER 082, a 5-HT(2B/2C) antagonist, or the 5-HT(2C) antagonist RS 102221, blocked the effect of DOI in both feeding and metabolic tests providing additional evidence that activation of PVN 5-HT(2A) receptors inhibits NPY's action on feeding and substrate utilization.

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.

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 three neurochemical stimuli on delayed feeding and energy metabolism

Infusions of norepinephrine (NE), the gamma-aminobutyric acid agonist, muscimol (MUS), or neuropeptide Y (NPY) into the paraventricular nucleus (PVN) of the hypothalamus all increase food intake. Such feeding may be due to direct activation of behavioral processes driving ingestion and/or to alterations in nutrient metabolism that feeding serves to normalize. To examine these possibilities, male Sprague-Dawley rats received PVN infusions of vehicle, 20 nmol NE, 1 nmol MUS or 100 pmol NPY at dark onset, then food intake was measured under three feeding conditions: (1) 1 and 2 h immediately after injections, (2) 1 h after a 1 h delay between injections and access to food, and (3) 1 h after a 1 h feeding delay, but with injections occurring just before presenting food. Measures of energy expenditure (EE) and respiratory quotients (RQs) in the absence of food were made over 2 h in parallel experiments. Results confirmed that NE, MUS and NPY all increased dark-onset feeding, but only NPY increased intake above control levels after a 1 h feeding delay. No neurochemically-induced changes in EE were observed, nor were there changes in RQs after NE or MUS. However, NPY reliably enhanced RQs from 30 to 120 min of testing. Our findings imply that NE and MUS initiate relatively immediate, short-term feeding that is not associated with changes in nutrient metabolism and does not summate with cues stimulated by delayed access to food. NPY initiates more protracted feeding temporally linked to enhanced carbohydrate metabolism. This may indicate that part of NPY's feeding stimulatory effects are secondary to physiological processes driving ingestion.

Effects of selected serotonin 5-HT(1) and 5-HT(2) receptor agonists on feeding behavior: possible mechanisms of action

Serotonin (5-HT) receptor agonists with high affinity for the different subtypes (i.e. 5-HT(1A-1F), 5-HT(2A-2C)) of the 5-HT(1)- and 5-HT(2) receptor families have been shown to affect ingestive behavior. It has been assumed that: (1) stimulation of hypothalamic 5-HT(2C) or 5-HT(1B) receptors leads to a behaviorally specific hypophagic effect by accelerating satiety processes; (2) stimulation of 5-HT(2A) receptors leads to a disruption of the feeding cascade; and (3) stimulation of 5-HT(1A) and 5-HT(2B) receptors leads to a hyperphagic effect. The present paper reviews studies performed with the relatively selective receptor agonists ipsapirone (5-HT(1A)), CP-94,253 (5-HT(1B)), BW 723C86 (5-HT(2B)) and ORG 37684 (5-HT(2C)), as well as the nonselective receptor agonists TFMPP (5-HT(1B/2C)), m-CPP (5-HT(2C/1B)) and DOI (5-HT(2A/2C)) in a variety of feeding paradigms in rats, both after systemic and local injection. These studies support a role for other neuroanatomical regions (i.e. brain stem) and behavioral mechanisms (i.e. appetitive processes) in the hypophagic effects of these compounds, possibly as a function of the administered dose. Studies with 5-HT receptor antagonists indicate that the proposed role of particular 5-HT(1/2) receptor subtypes in the hypophagic effects of these 5-HT receptor agonists may be more complicated than originally thought. Further characterization of the role of 5-HT(1/2) receptor subtypes in the control of ingestive behavior will require extensive pharmacological and behavioral studies, using more selective receptor agonists and antagonists and different behavioral procedures, as well as verification in transgenic animals.

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 spectrum of behaviors influenced by serotonin

The diverse array of behavioral effects of serotonin form the basis for understanding its potential role as an etiological marker in psychiatric disorders and for the successful pharmacologic intervention of drugs regulating serotonin neurotransmission in behavior. General theories of the behavioral functions of serotonin have implicated serotonin as a general inhibitor of behavioral responding and in modulating motor behavior. The ability of serotonin to regulate behavioral satiety and macronutrient selection provides the basis for pharmacologic treatment of obesity and eating disorders. The role of serotonin in behavioral suppression may be important in social behavior involving aggression and anxiety. The role of serotonin in neuroendocrine regulation provides a basis for understanding serotonin dysregulation in depression. Animal behavior tests are being used to better understand the neural substrates underlying the behavioral effects of antidepressant drugs and to address important issues in clinical treatment. The integration of information between basic and clinical studies provides the basis for future development of more sophisticated pharmacologic treatments of psychiatric disorders.

Absence of fenfluramine-induced anorexia and reduced c-Fos induction in the hypothalamus and central amygdaloid complex of serotonin 1B receptor knock-out mice

Fenfluramine, a serotonin releaser and uptake inhibitor, has been widely prescribed as an appetite suppressant. Despite its popular clinical use, however, the precise neural pathways and specific 5-HT receptors that account for its anorectic effect have yet to be elucidated. To test the hypothesis that stimulation of 5-HT1B receptors is required for the anorectic effect of fenfluramine, we assessed food intake in wild-type and 5-HT1B knock-out mice. Next, to determine possible brain structures and pathways that may contribute to the 5-HT1B-mediated effects of fenfluramine, we studied by immunohistochemistry the induction of the immediate early gene c-fos. Although the effect of fenfluramine on locomotion was indistinguishable between both wild-type and 5-HT1B knock-out mice, the anorectic effect of the drug was absent in only the knock-out mice. Furthermore, the induction of c-Fos immunoreactivity found in the paraventricular nucleus of the hypothalamus (PVN) of wild-type mice was substantially reduced in the knock-outs. Induction in the central amygdaloid nucleus (CeA) and in the bed nucleus of the stria terminalis (BNST), although robust in wild-type animals, was completely absent in knock-out animals. The mixed 5-HT1A/1B agonist RU24969 was able to mimic both the hypophagia and c-fos induction elicited by fenfluramine in wild-type mice, but not in the 5-HT1B knock-out mice. Our results thus demonstrate that stimulation of 5-HT1B receptors is required for fenfluramine-induced anorexia and suggest a role for the PVN, CeA, and BNST in mediating this effect.

Reversal of fenfluramine and fluoxetine anorexia by 8-OH-DPAT is attenuated following raphe injection of 5,7-dihydroxytryptamine

Drugs that enhance serotonergic neurotransmission reduce food intake by directly or indirectly activating serotonergic receptors. In contrast drugs that inhibit serotonergic neurotransmission such as the 5-HT1A agonist 8-hydroxy-2-(di-n-propyl-amino)tetralin (8-OH-DPAT) stimulate food intake. The present study examined the effects of 8-OH-DPAT on the feeding suppressant action of the indirect 5-HT agonists fenfluramine (FEN; 0.63-2.5 mg/kg) and fluoxetine (FLU; 2.5-10 mg/kg), as well as the 5-HT1B/2C agonist 1-(3-trifluoromethylphenyl)piperazine (TFMPP; 0.5-2 mg/kg). 8-OH-DPAT (62.5-250 microg/kg) was administered 5 min prior to FEN, FLU or TFMPP, injected 30 min before food access. While FEN, FLU and TFMPP dose-dependently reduced 2 h food intake, 8-OH-DPAT stimulated eating behavior. 8-OH-DPAT (62.5-250 microg/kg) pretreatment reversed the anorectic action of FEN (1.25 mg/kg) and FLU (5 mg/kg) but not TFMPP (1 mg/kg). Separate groups of rats were injected with 5,7-dihydroxytryptamine (5,7-DHT; 3 microg free base) into both the dorsal and median raphe, which resulted in extensive 5-HT depletion in hypothalamus (80%), striatum and hippocampus (90%). In both 5, 7-DHT and vehicle-injected rats, FEN (1.25 mg/kg) and FLU (5 mg/kg) suppressed feeding. In 5,7-DHT treated rats, however, the ability of 8-OH-DPAT (125 microg/kg) to block FEN and FLU induced anorexia was attenuated. That is, 8-OH-DPAT pretreatment did not reverse the feeding inhibitory effects of either FEN or FLU. Further, the ability of FEN and FLU to suppress food intake was not altered by the 5,7-DHT lesion. These findings suggest that the reversal of FEN and FLU anorexia by 8-OH-DPAT is partially dependent on the integrity of brain 5-HT systems since their disruption compromises the ability of this 5-HT1A agonist to antagonize the feeding suppressant action of either FEN or FLU. However, the ability of treatments which impair 5-HT neurotransmission to reverse FEN and FLU induced suppression of food intake may depend upon whether this impairment is acute and reversible (8-OH-DPAT), or chronic and irreversible (5,7-DHT).

Dietary and hypothalamic changes in delta 4-androstenedione-treated Zucker rats

Dehydroepiandrosterone (DHEA) has been shown to alter hypothalamic monoamines and reduce energy intake (EI) in Zucker rats (ZRs). We hypothesized that a metabolite of DHEA, delta 4-Androstenedione (delta 4), may mediate these effects. Male lean and obese ZRs (LZR, OZR) were fed control chow (CC) for 7 days, during which basal EI was recorded, various concentrations of delta 4 for 7 days, during which 0.6 and 0.3% delta 4 reduced EI significantly, and CC for 7 days, which resulted in a return of EI to basal levels. After delta 4 administration, neurotransmitter contents of various hypothalamic areas were determined. Serotonin (5-HT) has been shown to be correlated with feeding inhibition, and we have shown DHEA to increase lateral hypothalamic 5-HT synthesis; however, after 1 day and 7 days of delta 4, the OZR exhibited an increased metabolism, not synthesis, of 5-HT in the lateral and paraventricular hypothalamus, respectively, delta 4 was compared to DHEA in a macronutrient self-selection study with female OZRs. One group was injected intraperitoneally (IP) with sesame oil (control), another with DHEA (100 mg/kg), and another with delta 4 (100 mg/kg). Previous studies have shown that DHEA decreases both EI and % calories from fat. In this study, delta 4 decreased % calories from fat, but did not decrease total EI. Contrary to DHEA's effect of reducing serum insulin through 28 days of treatment, delta 4 in chow reduced insulin only acutely (1 day). We conclude, based on these differences, that DHEA has unique effects not mediated by its metabolite, delta 4-Androstenedione.

Appetite suppression by commonly used drugs depends on 5-HT receptors but not on 5-HT availability

The widely prescribed appetite suppressants D-fenfluramine and fluoxetine not only decrease feeding and body weight but also increase extracellular brain 5-HT. As central injection of 5-HT also decreases feeding, the drugs are often thought to require an increase of 5-HT at receptors in order to exert their hypophagic effect. However, much evidence now suggests that D-fenfluramine and its metabolite D-norfenfluramine can cause hypophagia by acting directly at unspecified 5-HT receptors and at 5-HT2C receptors, respectively, while fluoxetine may act independently of 5-HT receptors. These hypophagias may involve interference with the hyperphagic action of neuropeptide Y.

Serotonergic control of the organization of feeding and satiety

Studies in which serotonergic drugs were administered either systemically or directly into central sites have implicated 5-HT in the inhibitory control of feeding in mammals. In animal models and in humans, 5-HT agonists such as fenfluramine, fluoxetine and sertraline reduced the rate of eating and the size of meals in a manner suggesting that increasing serotonergic neurotransmission specifically enhanced satiation. In rodents, directly acting agonists at 5-HT1B, 5-HT2C or 5-HT2A receptors decreased food intake but by different behavioral mechanisms. Stimulation of the 1B and 2C subtypes may probe physiological roles in feeding and satiety. The former receptors may be involved primarily in regulating meal size and the latter more in controlling eating rate. Activation of both may be required for complete expression of behavioral satiety. By contrast, stimulating 2A sites may simply disrupt the continuity of feeding. Drugs that stimulate 5-HT1A autoreceptors increase food intake, presumably by acutely reducing the firing of serotonergic neurons in the brain. The hypothalamic paraventricular nucleus (PVN) has been proposed as an important terminal field in the forebrain that is involved in 5-HT's satiety role although recent studies implicate extra-PVN regions in this function. Peripherally administered 5-HT also decreases food intake in rats in a behaviorally specific manner. Studies with antagonists and with structural analogs of 5-HT revealed that 5-HT's peripheral satiety action involves 5-HT1-like and 5-HT2-like mechanisms. Thus, within and outside the brain, multiple pharmacological and behavioral mechanisms contribute to serotonergic functions in ingestion. The rich body of data from preclinical investigation in animals provides the foundation for therapeutic development in humans.

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.

The 5-HT1B receptor mediates the effect of d-fenfluramine on eating caused by intra-hypothalamic injection of neuropeptide Y

d-Fenfluramine (0.63 mg/kg i.p.), a serotonin (5-hydroxytryptamine, 5-HT) releaser and re-uptake inhibitor, reduced the eating caused by neuropeptide Y (235 pmol) injected into the paraventricular nucleus of the hypothalamus. The 5-HT1 and 5-HT2 receptor antagonist metergoline (1.0 and 2.0 mg/kg i.p.) and the 5-HT1A and 5-HT1B receptor antagonist (+/-)-cyanopindolol (3.0 and 8.0 mg/kg s.c.) significantly antagonized the effect of d-fenfluramine. The 5-HT2A and 5-HT2C receptor antagonist mesulergine (0.1 and 0.3 mg/kg s.c.) and the 5-HT2A receptor antagonist ketanserin (2.5 and 5.0 mg/kg i.p.) did not significantly modify the effect, nor did the 5-HT1A and 5-HT1B receptor antagonist (-)-propranolol (20-40 nmol), injected bilaterally into the paraventricular nucleus of the hypothalamus. The results suggest that d-fenfluramine reduces neuropeptide Y's hyperphagia by indirectly stimulating 5-HT1B receptors outside the paraventricular nucleus of the hypothalamus.

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.