Fluoxetine: A serotonergic appetite suppressant drug

Serotonergic regulation of appetite and sodium appetite

Serotonin is a neurotransmitter that is synthesized and released from the brainstem raphe nuclei to affect many brain functions. It is well known that the activity of raphe serotonergic neurons is changed in response to the changes in feeding status to regulate appetite via the serotonin receptors. Likewise, changes in volume status are known to alter the activity of raphe serotonergic neurons and drugs targeting serotonin receptors were shown to affect sodium appetite. Therefore, the central serotonin system appears to regulate ingestion of both food and salt, although neural mechanisms that induce appetite in response to hunger and sodium appetite in response to volume depletion are largely distinct from each other. In this review, we discuss our current knowledge regarding the regulation of ingestion - appetite and sodium appetite - by the central serotonin system.

Metabolic and behavioral effects of olanzapine and fluoxetine on the model organism Caenorhabditis elegans

The use of many psychotropic drugs (PDs) is associated with increased caloric intake, significant weight gain, and metabolic disorders. The nematode Caenorhabditis elegans (C. elegans) has been used to study the effects of PDs on food intake. However, little is known about PDs effects on the body fat of C. elegans. In C. elegans, feeding behavior and fat metabolism are regulated through independent mechanisms. This study aims to evaluate the body fat and food intake of C. elegans in response to treatment olanzapine and fluoxetine. Here we report that, with careful consideration to the dosage used, administration of fluoxetine and olanzapine increases body fat and food intake in C. elegans.

Effect of permeation enhancers on transdermal delivery of fluoxetine: in vitro and in vivo evaluation

The aim of this study was to investigate the feasibility of transdermal fluoxetine (FX) delivery. The effects of chemical forms (base or salt) and permeation enhancers on in vitro skin permeation of FX were assessed using hairless mouse, rat and human cadaver skin. The optimized formulations from the in vitro studies were then evaluated in an in vivo pharmacokinetic study in rats. The in vitro skin permeation studies suggested that the FX base (FXB) and isopropyl myristate (IPM)-limonene mixture could be suitable for transdermal delivery of FX. The permeation parameters of FX through human cadaver skin were well correlated with that through hairless mouse and rat skin, suggesting that these animal models can be used for predicting the permeability of FX through human skin. After transdermal administration of FX with IPM or the IPM-limonene mixture to rats, the mean steady-state plasma concentration (Css) was 66.20 or 77.55 ng/mL, respectively, which was maintained over 36 h and had a good correlation with the predicted Css from the in vitro data. These in vitro and in vivo data demonstrated that permeation enhancers could be a potential strategy for transdermal delivery of FX.

Serotonergic drugs : effects on appetite expression and use for the treatment of obesity

Over 35 years of research suggests that endogenous hypothalamic serotonin (5-hydroxytryptamine) plays an important part in within-meal satiation and post-meal satiety processes. Thus, the serotonin system has provided a viable target for weight control, critical to the action of at least two effective anti-obesity treatments, both producing clinically significant weight loss over a year or more. Numerous serotonin receptor subtypes have been identified; of these, serotonin 5-HT1B and 5-HT2C receptors have been specifically recognised as mediators of serotonin-induced satiety.A number of serotonergic drugs, including selective serotonin reuptake inhibitors (SSRIs), dexfenfluramine and 5-HT2C receptor agonists, have been shown to significantly attenuate rodent bodyweight gain. This effect is strongly associated with marked hypophagia and is probably mediated by the hypothalamic melanocortin system. Additionally, sibutramine, dexfenfluramine, fluoxetine and the 5-HT2C receptor agonist chlorophenylpiperazine (mCPP) have all been shown to modify appetite in both lean and obese humans, resulting in reduced caloric intake. Clinical studies demonstrate serotonergic drugs specifically reduce appetite prior to and following the consumption of fixed caloric loads, and cause a reduction in pre-meal appetite and caloric intake at ad libitum meals. Weight loss in the obese has also been produced by treatment with both the serotonin precursor 5-hydroxytryptophan and the preferential 5-HT2C receptor agonist mCPP.A new generation of 5-HT2C receptor selective agonists have been developed and at least one, lorcaserin (APD356), is currently undergoing clinical trials. In addition, 5-HT6 receptor antagonists such as PRX-07034 and BVT74316 have been shown to potently reduce food intake and bodyweight gain in rodent models and have recently entered clinical trials. However, the role of the 5-HT6 receptor in the expression of appetite remains to be determined. The hope is that these drugs will not only be free of their predecessors' adverse effect profiles, but will also be equally or more effective at regulating appetite and controlling bodyweight.

Antiobesity pharmacotherapy in the management of type 2 diabetes

Obesity is a well-known risk factor for the development of Type 2 diabetes mellitus. The management of the obese diabetic patient remains a challenge for the clinician but, in any case, weight reduction should be considered as a key objective. In this respect, several antiobesity drugs have demonstrated potential. However, while fenfluramine and dexfenfluramine have been shown to promote weight loss and to directly improve insulin sensitivity, being two mechanisms contributing to better blood glucose control in obese Type 2 diabetic patients, they were recently withdrawn due to safety problems. Sibutramine, a new selective norepinephrine and serotonin reuptake inhibitor, promotes weight loss by decreasing food intake, an effect which leads to a mild improvement (significant in patients losing > or =5% of initial body weight) of blood glucose control in obese diabetic patients. Similarly, orlistat, a selective gastrointestinal lipase inhibitor which increases faecal fat losses, enhances diet-induced weight reduction and improves both blood glucose control and vascular risk profile, especially dyslipidaemia, in obese Type 2 diabetic patients. Further studies are required to better identify good responders to pharmacotherapy and specify the role of antiobesity agents in the overall long-term management of obese subjects with Type 2 diabetes. Other novel pharmacological approaches deserve further consideration, for instance beta-3 agonists aiming to increase energy expenditure, drugs interfering with tumor necrosis factor-alpha (TNF-alpha) or free fatty acid release by the adipose tissue or agents that slow gastric emptying. However, until now, results regarding efficacy and/or safety have been disappointing or preliminary in humans.

A comparison of the effects on central 5-HT function of sibutramine hydrochloride and other weight-modifying agents

1. Effects on 5-HT function of sibutramine and its active metabolites, BTS 54 354 and BTS 54 505, were compared with fluoxetine, (+)-fenfluramine and (+)-amphetamine. 2. In vitro sibutramine weakly inhibited [3H]-5-HT uptake into brain synaptosomes. BTS 54 354, BTS 54 505 and fluoxetine were powerful [3H]-5-HT uptake inhibitors, whereas (+)-fenfluramine and (+)-amphetamine were very much weaker. Conversely, whilst sibutramine, its metabolites and fluoxetine did not release [3H]-5-HT from brain slices at < or = 10(-5)M, (+)-fenfluramine and (+)-amphetamine concentration-dependently increased [3H]-5-HT release. 3. Sibutramine and fluoxetine had no effect on 5-hydroxytryptophan (5-HTP) accumulation in either frontal cortex or hypothalamus at doses < 10 mg kg(-1). In contrast, (+)-amphetamine ( > or = 3 mg kg(-1)) reduced 5-HTP in hypothalamus, whilst (+)-fenfluramine (> or =1 mg kg(-1)) decreased 5-HTP in both regions. 4. Sibutramine (10 mg kg(-1) i.p.) and fluoxetine (10 mg kg(-1) i.p.) produced slow, prolonged increases of extracellular 5-HT in the anterior hypothalamus. In contrast, (+)-fenfluramine (3 mg kg(-1) i.p.) and (+)-amphetamine (4 mg kg(-1) i.p.) induced rapid, short-lasting increases in extracellular 5-HT. 5. Only (+)-fenfluramine (10 mg kg(-1)) altered 5-HT2A receptors in rat frontal cortex when given for 14 days, producing a 61% reduction in receptor number and a 18% decrease in radioligand affinity. 6. These results show that sibutramine powerfully enhances central 5-HT function via its secondary and primary amine metabolites; this effect, like that of fluoxetine, is almost certainly mediated through 5-HT uptake inhibition. By contrast, (+)-fenfluramine enhances 5-HT function predominantly by increasing 5-HT release. (+)-Amphetamine, though weaker than (+)-fenfluramine, also enhances 5-HT function by release.

Evaluation of the neuropharmacodynamics of paroxetine in vivo utilizing microdialysis

The effect of paroxetine, a selective serotonin reuptake inhibitor used in the treatment of depression, on extracellular serotonin levels was evaluated in freely moving conscious rats. Microdialysis, a powerful in vivo technique to monitor the extracellular levels of neurotransmitters, was used to monitor the baseline changes in the levels of serotonin in rat brain anterior lateral striatum post paroxetine administration, which is a measure of the neuropharmacodynamic effect of the drug. Microdialysis sampling was performed for 210 min prior to and for 240 min after intraperitoneal administration of paroxetine (10 mg/kg). Paroxetine caused a statistically significant increase in the extracellular levels of serotonin in the anterior lateral striatum sampled by microdialysis. The present study demonstrates the utility of microdialysis for studying the in vivo neuropharmacodynamics of paroxetine in conscious rats.

Neuropharmacological effects of low and high doses of repeated oral dexfenfluramine in rats: a comparison with fluoxetine

The neuropharmacological effects of repeated oral doses of dexfenfluramine (DF; 1.25-10 mg/kg, twice daily for 21 days) were examined in rats and related to the drug brain levels. Results were compared with fluoxetine (FL) given at similar doses relative to its anorectic ED50. Both drugs dose-dependently slowed body weight gain and reduced brain serotonin (5-HT). However, at 1.25 mg/kg DF caused only a slight and transient decrease in cortical 5-HT. Comparable doses of FL (6.25-12.5 mg/kg) lowered 5-HT more than DF, besides slightly reducing striatal dopamine. At higher doses DF markedly reduced 5-HT in all regions, and to a lesser extent noradrenaline in hippocampus. There was a negative relationship between 5-HT and log total active drug levels and the indole was approximately halved at drug levels about 50 times lower with DF than FL. However, the ratio between drug levels causing marked 5-HT reductions and those considered anorectic was similar for DF and FL because brain levels at the anorectic ED50 were higher with FL than DF. Long-lasting reductions of 5-HT were also observed but recovery was only consistently slow beginning from 5 mg/ kg DF. Comparable doses of FL could not be used because its general toxicity leads to the death of rats after only 2-4 multiples of its anorectic ED50.

Plasma amino acid ratios related to brain serotonin synthesis in response to food intake in bulimia nervosa

Fifteen bulimic women (BN) and 19 healthy female controls (CO) were studied. The subjects were cross-over treated with either fluoxetine (FXT) or placebo during 4 days. They received, in randomized order, a breakfast containing pure carbohydrate (CHO) or a protein-rich (PROT) breakfast following day 3 and 4 of each treatment period. Twenty-nine different food items were offered for lunch. The fasting serum glucose and insulin concentrations and the fasting plasma tryptophan (Trp)/large neutral amino acid (LNAA) ratio were slightly higher in BN. The changes of these metabolic parameters in response to a CHO or PROT breakfast were similar in both groups. Across breakfast type, the plasma (Trp)/(LNAA) ratio at 120 min after breakfast was higher in BN. Total caloric intake at lunchtime was less in BN. In CO, less carbohydrate was selected at lunchtime following the CHO breakfast, an effect that was abolished by FXT. Breakfast type or FXT did not have any apparent effect on food intake at lunchtime in BN. This might indicate that bulimic subjects are less sensitive to serotoninergic stimuli than control subjects.

Efficacy and safety of long-term fluoxetine treatment of obesity--maximizing success

Obesity is a major health care concern because of its associated medical complications and increased mortality. Despite a myriad of short-term weight loss strategies and the motivation of improving health, patients have difficulty maintaining reduced weight. Pharmacologic agents, such as fluoxetine, a selective serotonin uptake inhibitor, have been investigated as adjunctive therapy to standard weight management programs. Extended therapy with fluoxetine has demonstrated clinically meaningful benefits on weight loss and obesity-associated medical conditions in double-blind placebo-controlled studies. However, the magnitude of these benefits for individuals vary. Such findings are consistent with the belief that the obesity syndrome has differing etiologies. Accordingly not all patients are likely to benefit from a particular therapy. Studies should identify patient subgroups that are more likely to respond to a specific therapy. In this study of 719 fluoxetine-treated and 722 placebo treated patients in four multicenter, randomized, double-blind, long-term clinical trials, we investigated possible predictors of a beneficial long-term outcome from fluoxetine therapy. Patients' age, current smoking activity, and baseline uric acid concentration were predictors of a meaningful long-term treatment effect. Further review of the weight loss patterns of patients achieving long-term success provided the basis for a treatment monitor. Use of the predictors and the treatment monitor are strategies to maximize the benefits of therapy through improved patient selection and monitoring during a therapeutic program.

Prozac (fluoxetine, Lilly 110140), the first selective serotonin uptake inhibitor and an antidepressant drug: twenty years since its first publication

In this review, we describe the evolutionary process involved in the discovery of the selective 5-HT uptake inhibitor, fluoxetine, and summarize some of the large body of scientific research performed on fluoxetine in the 20 years since the first publication. The historical background of the proposed involvement of 5-HT in psychiatric disorders and the activity of tricyclic antidepressants in depression is reviewed. The effects of fluoxetine in various in vitro assays and in animal studies including receptor down-regulation, neurochemical and behavioral models are summarized. In addition, the clinical effectiveness of fluoxetine in depression and obsessive compulsive disorders and its potential use in other disorders are examined.

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

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

Hyperphagia and weight loss during fluoxetine treatment

OBJECTIVE

To report the unusual coincidence of weight loss with increased appetite and food intake in a patient treated for depression on two separate occasions with fluoxetine.

CASE SUMMARY

A 27-year-old woman experienced a modest weight loss during treatment for depression with fluoxetine. The weight loss was associated with a reported increase in daily caloric intake and consumption of a greater proportion of dietary fat than usual for the patient. The same patient was treated again with fluoxetine more than a year later and again experienced weight loss associated with an increase in appetite, caloric intake, and dietary fat consumption.

DISCUSSION

Fluoxetine is a selective serotonin reuptake inhibitor that often is associated with a modest weight loss when used for the treatment of depression, although it also has been reported to have the opposite effects of weight gain and hyperphagia in some patients. The effects on weight usually are assumed to be the result of primary effects on appetite, but the discrepancy between the appetite and weight changes in this case challenges the applicability of that assumption in all cases.

CONCLUSIONS

The effects of fluoxetine on appetite and weight may be mediated by partially distinct mechanisms and might conceivably involve a direct metabolic effect in some patients.

Fluoxetine at anorectic doses does not have properties of a dopamine uptake inhibitor

Although fluoxetine is a highly selective inhibitor of serotonin uptake in vitro and in vivo, some investigators have suggested that dopamine uptake inhibition may contribute to anorectic actions of fluoxetine. The present experiments were done to determine fluoxetine's effects in some animal protocols in which dopamine uptake inhibitors have characteristic actions. Mazindol prevented the depletion of striatal dopamine and its metabolites by amphetamine in iprindole-pretreated rats, but fluoxetine had no effect. Mazindol prevented the depletion of striatal dopamine and its metabolites by 6-hydroxydopamine injected intracerebroventricularly into rats, but fluoxetine had no effect. Mazindol enhanced the elevation of 3,4-dihydroxyphenylacetic acid concentration in rat brain after spiperone injection, but fluoxetine did not cause that effect. Fluoxetine did not mimic amfonelic acid in antagonizing the retention of alpha-methyl-m-tyramine invant striatum after the injection of alpha-methyl-m-tyrosine. These results show that fluoxetine, at doses that are effective in blocking the serotonin uptake carrier and causing anorexia, does not block the dopamine uptake carrier.

Short-term fluoxetine treatment alters monoamine levels and turnover in discrete brain nuclei

The effects of short-term fluoxetine administration on monoamine levels and turnover were assessed in discrete brain nuclei. Adult male rats received fluoxetine HCl (10 mg/kg) or saline injections intraperitoneally for 4 days and monoamine levels determined by high performance liquid chromatography. The major metabolite of 5-HT, 5-hydroxyindoleacetic acid (5-HIAA), was decreased by fluoxetine treatment in the ventromedial hypothalamic nucleus (VMN), the lateral hypothalamic area and the CA1 region of the hippocampus. Fluoxetine treatment significantly increased serotonin (5-HT) levels in the VMN but did not change 5-HT levels in any other area examined. Norepinephrine (NE) levels were higher in fluoxetine-treated rats in the dorsomedial hypothalamic nucleus, dorsal raphe nucleus and parietal motor cortex (MCTX). 5-HT and NE turnover were also determined by the pargyline method. Fluoxetine treatment decreased 5-HT turnover in the VMN and increased 5-HT turnover in the median raphe. NE turnover was decreased in the preoptic area, the MCTX and parietal sensory cortex by fluoxetine administration. These results demonstrate that brain areas with similar 5-HT innervation respond differently to fluoxetine administration and fluoxetine, which selectively alters 5-HT uptake, also affects NE levels and turnover in several brain nuclei.

Therapy for obesity--today and tomorrow

Obesity poses a serious health hazard and its treatment is often disappointing. Besides conservative methods, the place of pharmacotherapy, very-low-calorie diets, and even, in selected cases, mechanical means or surgery can be considered. Effective drug treatment for obesity must reduce energy intake, or increase energy expenditure, or increase energy losses in faeces. All these possibilities have potential activities but also serious limitations. Current pharmacotherapy essentially uses anorectic drugs and the other approaches, although promising, are still under investigation. Of the anorectic compounds currently available, serotoninergic agents, like dexfenfluramine and fluoxetine, appear to have the most suitable pharmacological profile. Very-low-calorie diets could help in the short-term but should be associated with other approaches to increase the rate of long-term success. They must be well-balanced as macronutrients and micronutrients are concerned, be prescribed in well-selected patients under careful medical supervision, and not be followed longer than a few weeks. Surgery can provide palliation for severe obesity when all medical approaches have failed. It may consist in decreasing food intake (gastric procedures), affecting calorie absorption (intestinal shunting, biliopancreatic bypass), or removing localized excess fat (lipectomy, liposuction). Gastric reduction operations are safe and effective provided they are performed by experienced surgeons in well-selected patients. They can be considered now as the best option for a minority of patients with morbid and refractory obesity. Finally, in very selected patients, mechanical means (such as the waist cord) may also help losing weight and/or avoiding weight regain. Even if all these therapeutic approaches can be helpful, at least in some obese individuals, they also have important limitations so that prevention remains up to now the 'treatment' of choice for obesity.

Changes in dopamine metabolism in rat forebrain regions after cessation of long-term fluoxetine treatment: relationship with brain concentrations of fluoxetine and norfluoxetine

We examined the effects of repeated administration of the selective serotonin uptake inhibitor (SSRI) fluoxetine (Flx) (5, 10, or 15 mg/kg i.p., twice daily for 21 days) on brain and plasma concentrations of the parent drug and its active desmethyl metabolite, norfluoxetine (NFlx), in rats during the 21-day regimen as well as after cessation of drug treatment. We also measured dopamine (DA) levels in 2 midbrain regions (the striatum, St and nucleus accumbens, NAc) in rats killed 1-14 days after the last dose. NFlx concentrations in plasma and brain were ten times higher than those of Flx during the period of drug treatment. Although Flx accumulated more markedly in the rat brain than NFlx, it disappeared completely from plasma and brain after treatment stopped, while NFlx persisted up to Day P7. Chronic Flx treatment caused a persistent decrease in brain DA levels of -60% to -70% in St and NAc; this lasted for 7-14 days after cessation of treatment, depending on the dose used. The levels of DA metabolites decreased by 20-40%, and, except for 3-MT, tended to overshoot during the recovery period. Our data suggest that the long-term inhibition of DA neurons after cessation of Flx treatment parallels the inhibition previously observed for 5-HT neurons. Thus, besides blocking 5-HT uptake, Flx is likely to also inhibit in vivo DA uptake in forebrain regions, following prolonged administration.

Time course of brain serotonin metabolism after cessation of long-term fluoxetine treatment in the rat

The effects of repeated fluoxetine (Flx) administration (5, 10, or 15 mg/kg i.p., twice daily for 21 days) on serotonin and 5-HIAA metabolism were examined in the hypothalamus, hippocampus, pons medulla and cerebral cortex of rats killed 1-28 days after the last dose. Dose-dependent weight loss was observed during treatment, followed by gradual and complete recovery of body weight over the following two weeks. Chronic Flx treatment caused a dose-dependent decrease in brain 5-HT levels (by between 10 and 50% depending on the region examined), lasting for 3-7 days after cessation of treatment with the lowest and intermediate doses, and for 7-14 days after cessation of the highest dose. 5-HIAA levels decreased more markedly (-20; -60% depending on the region examined) than those of 5-HT, and tended to overshoot during the recovery period. The prolonged reduction in brain 5-HT levels after chronic Flx treatment was similar to that seen in rats given very high doses of dexfenfluramine (d-fen), a drug which both blocks 5-HT uptake and increases its release. These data suggest that brain 5-HT and 5-HIAA depletion may reflect similar dose-related expressions of the drug's mechanisms of action.

A molecular recognizing system of serotonin in rat fetal axonal growth cones: uptake and high affinity binding

Axonal growth cone particles (AGCP) isolated from prenatal and postnatal rat brain had different high-affinity 5-HT uptake characteristics. In postnatal AGCP the uptake behaves as in the adult rat brain, while in the prenatal AGCP the uptake characteristics seem to be in a transitional stage. Also in prenatal AGCP we observed specific, high-affinity 5-HT binding sites. These results support the idea of an important role for 5-HT during axogenesis.

Food intake and rumen motility in dwarf goats. Effects of some serotonin receptor agonists and antagonists

The serotonergic regulation of feeding behaviour has not so far been studied in ruminants. Therefore, the effects of some serotonin (5-HT) receptor agonists and antagonists on food intake and forestomach motility were studied in dwarf goats. Goats ate less food when treated intravenously (IV) with the 5-HT precursor 5-HTP (25 micrograms, 50 micrograms or 100 micrograms kg-1 min-1 over 15 min) than when they were treated with 5-HT (which does not pass the blood-brain barrier) or with saline. Accordingly, IV dexfenfluramine infusions (50 micrograms or 100 micrograms kg-1 min-1 over 15 min), which induces release of brain 5-HT, also led to dose-related reductions in food intake. In contrast, no anorectic effects were observed after IV infusions with the selective 5-HT reuptake inhibitor fluoxetine (100 micrograms kg-1 min-1 over 15 min), the selective 5-HT1A agonist 8-OH-DPAT (0.5 micrograms kg-1 min-1 over 15 min), or eltoprazine (4 or 8 micrograms kg-1 min-1 over 15 min), a mixed 5-HT1A/5HT1B receptor agonist. None of the 5-HT antagonists tested gave any increase in food consumption in this model. Interestingly, the non-selective 5-HT receptor antagonist methysergide (360 micrograms/kg IV) reduced food intake. This effect was most noticeable at 3 h after injection. The 5-HT3 receptor antagonist ondansetron (IV 10 micrograms kg-1 min-1 over 15 min) and the peripheral 5-HT2 receptor antagonist xylamidine (IV 100 micrograms kg-1 min-1 over 10 min) failed to modify food intake. These results provide evidence for central serotonergic involvement in the control of feeding. However, this control system differs markedly in goats and rodents. Dexfenfluramine, 5-HTP and eltoprazine administered at similar dose rates to those used in the food intake experiments induced some clinical signs including inhibition of forestomach contractions. These results, together with our earlier in vivo and in vitro observations, suggest that the inhibitory effects of serotonin receptor agonists on forestomach contractions are due to interactions with both peripheral and central serotonergic receptors. The change in smooth muscle tension, which leads to a change in the signals transmitted via vagal afferents to the central nervous system, appears not to modify feeding behaviour in dwarf goats.

Effects of methiothepin on changes in brain serotonin release induced by repeated administration of high doses of anorectic serotoninergic drugs

We previously observed, using in vivo microdialysis, that the potassium-evoked release of frontocortical serotonin (5-HT) is suppressed after rats receive high doses (30 mg/kg, i.p., daily for 3 days) of fluoxetine, a selective blocker of 5-HT reuptake. We now describe similar impairments in 5-HT release after repeated administration of two other 5-HT uptake blockers, zimelidine and sertraline (both at 20 mg/kg, i.p. for 3 days) as well as after dexfenfluramine (7.5 mg/kg, i.p. daily for 3 days), a drug which both releases 5-HT and blocks its reuptake. Doses of these indirect serotonin agonists were about 4-6 times the drug's ED50 in producing anorexia, a serotonin-related behavior. In addition, methiothepin (20 microM), a non-selective receptor antagonist, locally perfused through the dialysis probe 24 h after the last drug injection, enhanced K(+)-evoked release of 5-HT at serotoninergic nerve terminals markedly in control rats and slightly in rats treated with high doses of dexfenfluramine or fluoxetine. On the other hand, pretreatment with methiothepin (10 mg/kg, i.p.) one hour before each of the daily doses of fluoxetine or dexfenfluramine given for 3 days, totally prevented the decrease in basal and K(+)-evoked release of 5-HT. Finally, when methiothepin was injected systemically the day before the first of 3 daily injections of dexfenfluramine, it partially attenuated the long-term depletion of brain 5-HT and 5-HIAA levels induced by repeated administration of high doses of dexfenfluramine. These data suggest that drugs which bring about the prolonged blockade of 5-HT reuptake - such as dexfenfluramine and fluoxetine - can, by causing prolonged increases in intrasynaptic 5-HT levels as measured by in vivo microdialysis, produce receptor-mediated long-term changes in the processes controlling serotonin levels and dynamics.

Differential effects of serotonergic and catecholaminergic drugs on ingestive behavior

The serotonergic agonists fenfluramine and fluoxetine and the catecholaminergic agonists amphetamine and phenylpropanolamine are well known to cause a reduction in intake in rats. In the studies reported here we investigated the effects of these drugs on the microstructure of licking behavior of the rat ingesting 0.4 M sucrose. The purpose was to examine the similarities in the behavioral effects within and between these two classes of anorectic agents. The serotonergic agonists fenfluramine and fluoxetine caused a reduction in intake primarily by reducing the size of bursts and clusters of licking within the test meal without affecting the duration of the meal, suggesting a reduction in the palatability of the test solution. The catecholamine agonists amphetamine and phenylpropanolamine reduced intake primarily by reducing the number of bursts and clusters without affecting their size, suggesting a fractionation in the organization of the normal pattern of ingestion. The differences between the two serotonin and the two catecholamine agonists on the microstructure of the licking behavior suggest a different effect of the two neurotransmitters on the motor system that controls ingestive behavior. The similarities between the two different agonists within each class suggests a common neurotransmitter mechanism responsible for these two different effects on the behavior of the animals.

Effect of fluoxetine on serotonin and dopamine concentration in microdialysis fluid from rat striatum

Fluoxetine injected i.p. into rats at a dose of 10 mg/kg rapidly increased serotonin concentration in microdialysis fluid from the striatum by at least 4-fold, an increase that was maintained throughout the 3 hr observation period. Dopamine concentration in the microdialysis fluid did not change. The concentration of the two dopamine metabolites, 3,4-dihydroxyphenylacetic acid and homovanillic acid, was not changed in the microdialysis fluid, whereas the concentration of the serotonin metabolite, 5-hydroxyindoleacetic acid, was significantly decreased after fluoxetine injection. The increased extracellular concentration of serotonin no doubt resulted from inhibition of the serotonin uptake carrier by fluoxetine, and the lack of change in dopamine is evidence for the specificity of action of this uptake inhibitor.

Persistent blockade of potassium-evoked serotonin release from rat frontocortical terminals after fluoxetine administration

We examined 5-HT and 5-HIAA release from frontal cortex evoked by high potassium chloride concentrations in rats pretreated for 3 days with high doses of the 5-HT uptake blocker fluoxetine or of dexfenfluramine, which both releases 5-HT and blocks its reuptake. The standard fluoxetine dose (30 mg/kg i.p.) was about 4 times the drug's ED50 in producing a serotonin-related behavioral effect, anorexia, while the dexfenfluramine dose (7.5 mg/kg i.p.) was about 6 times its ED50. These high doses were chosen in order to elucidate the mechanism by which similar doses of fluoxetine and dexfenfluramine had been found to produce long-term changes in serotonin dynamics. Fluoxetine decreased the basal release of both compounds; dexfenfluramine decreased basal 5-HIAA efflux without affecting the release of 5-HT release. Potassium-evoked 5-HT release was unchanged after dexfenfluramine pretreatment but was suppressed by fluoxetine doses as low as 7.5 mg per kg per day. Basal release of 5-HT and 5-HIAA returned to normal after 7 days of fluoxetine pretreatment, but evoked release continued to be suppressed. These data suggest that long-term changes in brain serotonin dynamics after high doses of dexfenfluramine or fluoxetine are related to the drug's mechanisms of action, specifically their blockade of 5-HT reuptake.

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.

Differential effects of d-fenfluramine and p-chloroamphetamine on H75/12-induced depletion of 5-hydroxytryptamine and dopamine in the rat brain

The effects of the two 5-HT-releasing drugs, p-chloroamphetamine and d-fenfluramine, on central serotoninergic and dopaminergic systems were compared in adult rats. Both drugs (0.5-5.0 mg/kg i.p., 2 hr before death) produced a dose-dependent reduction in levels of 5-HT, but only p-chloroamphetamine decreased the levels of 5-hydroxyindoleacetic acid (5-HIAA) in the hippocampus, striatum and cerebral cortex. Within the dose range tested, d-fenfluramine did not affect the levels of DA and of its metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in brain. By contrast, p-chloroamphetamine significantly increased the levels of DA and HVA and decreased the levels of DOPAC, notably in the striatum. As expected of a 5-HT uptake inhibitor, d-fenfluramine at small doses (0.2-0.5 mg/kg) prevented the depletion of 5-HT due to 4-methyl-alpha-ethyl-meta-tyramine (H75/12, 40 mg/kg i.p.), whereas at large doses (1.0-5.0 mg/kg) d-fenfluramine, like p-chloroamphetamine (0.2-1.0 mg/kg), slightly enhanced the effect of H75/12. Neither d-fenfluramine (0.5 mg/kg) nor p-chloroamphetamine (0.5 mg/kg) affected the depletion of DA due to H75/12. These data indicate that p-chloroamphetamine is a 5-HT-releasing drug, at any dose between 0.2 and 5.0 mg/kg, whereas d-fenfluramine acts as a 5-HT uptake inhibitor at 0.2-0.5 mg/kg and as a 5-HT releasing drug at larger doses. On account of the potential neurotoxicity of 5-HT-releasing drugs but not 5-HT uptake inhibitors, it can be inferred that d-fenfluramine is very probably devoid of any neurotoxic action in the dose range (less than 1.0 mg/kg) required for its anorectic action.

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)

Effects of fluoxetine and desipramine on palatability-induced ethanol consumption in the alcohol-nonpreferring (NP) line of rats

Three groups of NP rats (n = 5/group) received food, water and one of 3 Polycose solutions ad lib. One group received a solution containing 3% (w/v) Polycose, 0.125% (w/v) saccharin, 0.5% (w/v) NaCl (3% POL solution) to which ethanol was gradually added over three weeks until the concentration of 10% (v/v) ethanol (E) was reached (3% POL + E group). Alcohol ingestion by the 3% POL + E group reached an average of 9 g of ethanol/kg b. wt./day; the rats attained average blood alcohol concentrations of 61 +/- 8 mg%. One control group (3% POL) was given the same solution as above but without ethanol. The second control group (17% POL) had access to a 17.6% Polycose solution supplemented with 0.125% saccharin and 0.5% NaCl and was isocaloric to the 3% POL + E solution. Although the three groups differed significantly in the amounts of food and Polycose solutions consumed, their total caloric intakes were equivalent. The IP administration of the serotonin (5-HT) uptake inhibitor fluoxetine (5 and 10 mg/kg) significantly reduced drinking of the group receiving the 3% POL + E solution by 23% and 67%, respectively, but did not alter intakes of the Polycose solutions by the 3% or 17% POL control groups. The IP administration of the norepinephrine (NE) uptake inhibitor desipramine (5 and 10 mg/kg) significantly reduced intake of the Polycose solution by the 17% POL group by 52 and 83%, respectively, but only the 10 mg/kg dose attenuated drinking of the solutions by the 3% POL and 3% POL + E groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of fluoxetine or D-fenfluramine on serotonin release from, and levels in, rat frontal cortex

Using in vivo microdialysis of frontal cortex in anesthetized rats, as well as analysis of frontal cortex homogenates, we examined the effects of chronic administration of fluoxetine (30 mg/kg, i.p.) or D-fenfluramine (7.5 mg/kg, i.p.), administered daily for 3 days, on serotonin and 5-HIAA levels a day later. Measurements were also taken after 3-, 7- , and 21-day recovery periods. Neither chronic fluoxetine nor D-fenfluramine changed basal serotonin release. Both treatments, however, transiently decreased the release of serotonin evoked by an acute dose of D-fenfluramine (10 mg/kg, i.p.). Release initially was completely suppressed in fluoxetine-pretreated animals but returned to normal by the 21st day of washout; following D-fenfluramine pretreatment, normal release was attained by the 7th day of washout. Both fluoxetine and D-fenfluramine transiently decreased 5-HIAA levels in the dialysates and tissues. Both drugs also caused prolonged changes in frontal cortex serotonin levels, D-fenfluramine lowering them but fluoxetine elevating them. These results suggest that, at comparable dosage levels relative to their ED50s, fluoxetine and D-fenfluramine cause comparable reversible effects on brain serotonin release. The drugs also cause prolonged but opposite changes in brain serotonin levels, probably reflecting differences in the extents to which they or their principal metabolites release serotonin and block its reuptake.