Fluoxetine reduces food intake by a cholecystokinin-independent mechanism

Vers un élargissement des indications des antidépresseurs

Les données neurochimiques directes ou indirectes obtenues dans différentes affections du système nerveux central (SNC) constituent la base rationnelle de I'utilisation des médicaments dans des indications précises. Les antidépresseurs (AD) tricycliques agissant sur les principales amines du SNC trouvent ainsi des applications thérapeutiques variées en dehors de la dépression. L'arrivée de nouveaux AD au mécanisme d'action, soit plus cibé (la sérotonine), soit différent (IMAO) pose de nouveau cette question des indications psychiatriques et neurologiques, qui ne peut être résolue que par des essais méthodologiquement irréprochables. Une telle approche évitera d'ignorer pendant des années des effets thérapeutiques incontestables d'une molécule ancienne, tel que ce fut le cas de la clomipramine dans les troubles obsessifs-compulsifs.

Combined naloxone and fluoxetine on deprivation-induced binge eating of palatable foods in rats

Opioid antagonism and serotonergic stimulation is associated with macronutrient-specific hypophagia in animals. In the present study we evaluated their systemic effect alone, and in combination, at various doses, on the intake of sweet carbohydrate-rich and sweet fat-rich foods, tastes, and nutrients that are typical of binge-food items. Low-dose (1 mg/kg) naloxone, alone, preferentially suppressed fat-rich intake while low-dose (2.5 mg/kg) fluoxetine, alone, preferentially suppressed carbohydrate-rich intake. Each drug at these doses, combined with various doses of the other (2.5-10 mg/kg fluoxetine; 0.01-1 mg/kg naloxone) additively suppressed both kinds of the sweet foods. Naloxone and fluoxetine have therapeutic potential in treating binge-eating disorders. This animal study suggests what shortcomings and benefits might be expected when combining these two agents.

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.

New drugs for the treatment of experimental alcoholism

This article presents a current overview of the efforts to suppress pharmacologically the craving, dependence, or other factors associated with the self-selection of alcohol in an experimental animal. The contemporary status of the pharmacotherapy of experimental alcoholism similarly is described for different animal models of alcohol drinking. An evaluation is presented of several classes of drug for their efficacy in ameliorating the volitional ingestion of alcohol in the presence of an alternative fluid. Currently, two main experimental animal models of alcoholism are being used in this endeavor: (a) genetic lines or substrains of high alcohol preferring or high drinking rats; and (b) strains of nondrinking or low alcohol preferring rats which are induced chemically to prefer alcohol. Because of technical, methodological, and other issues surrounding the procedures used to assess the efficacy of a drug in reducing alcohol intake, several of the newer findings remain controversial. For example, serious side effects on the intake of food, caloric regulation, motor activity, or other functions would preclude the clinical utility of the drug. However, several drugs which affect monoaminergic neurons as well as opioid systems in the brain now seem to offer promise as agents which do possess clinical benefits. Two of these drugs, FG5606 (amperozide) and FG 5893 are essentially "antialcoholic" or anticraving and are without any significant side effects on cerebral mechanisms responsible for hunger, caloric intake, motor activity, or other physiological process. Amperozide, a 5-HT2 receptor antagonist with dopamine releasing properties, is particularly notable because of its irreversible nature in attenuating alcohol preference for months after its administration. It is concluded that future pharmacological research on presently available and newly developed compounds will provide exciting opportunities to the clinician who can utilize a particular drug as an adjunctive tool in the therapeutic treatment of the alcoholic individual.

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.

Fenfluramine-induced modification of palatability: analysis by the taste reactivity test

The ability of various doses (0, 1.25, 2.5, and 5.0 mg/kg) of fenfluramine to modify the palatability of sucrose and quinine solutions was assessed by means of the taste reactivity test. Although fenfluramine did not modify the positive hedonic ingestive reactions elicited by sucrose solution, it consistently enhanced the negatively hedonic aversive reactions elicited by unpalatable 0.05% quinine solution and moderately palatable 2% sucrose solution. The results suggest that fenfluramine enhances the aversive properties of tastants without suppressing the positive hedonic properties of tastants. The results support a two-dimensional model of palatability.

Individual differences in schedule-induced and conditioned behaviors

Previous experiments have shown that subjects which exhibit a high locomotor response to novelty (HR) also show a greater locomotor response to psychomotor stimulants than subjects which have a low locomotor response to a novel environment (LR). The current experiments were designed to examine in more detail the behavioral differences between HR and LR rats in non-drug paradigms. In the first experiment HR rats acquired schedule-induced polydipsia (SIP) more readily than LR rats. Panel pressing to gain access to the food pellets, however, was greater in LR rats compared to HR rats, especially after stable levels of SIP had been attained. In the second experiment one group of rats were fed daily after a 30-min period in photocell-cages (food conditioning; FC) while a control group was fed in the home-cage (non-conditioned; NC). FC subjects developed heightened locomotor activity in anticipation of feeding in the initial 30 min in the test-cage compared to NC rats. This anticipatory locomotor activity developed more rapidly and to a greater level in HR rats than in LR rats. The concentrations of dopamine, dihydroxyphenylacetic acid, homovanillic acid, serotonin, 5-hydroxyindoleacetic acid, and norepinephrine were determined at the completion of behavioral testing in both the food conditioned and non-conditioned rats. The food conditioned experiment showed that variations in both the dopaminergic and serotoninergic systems may underlie individual differences in behavioral responsiveness. However, no clear pattern of neurochemical differences emerged. The current set of experiments have demonstrated differences between HR and LR rats in non-drug related paradigms and that HR rats appear to show a greater motivational excitement induced by periodic food delivery than LR rats.

Biological actions of cholecystokinin

Cholecystokinin (CCK) has emerged as an important mammalian neuropeptide, localized in peripheral organs and in the central nervous system. This review presents an overview of the molecular aspects of CCK peptides and CCK receptors, the anatomical distribution of CCK, the neurophysiological actions of CCK, release of CCK and effects of CCK on release of other neurotransmitters, and the actions of CCK on digestion, feeding, cardiovascular function, respiratory function, neurotoxicity and seizures, cancer cell proliferation, analgesia, sleep, sexual and reproductive behaviors, memory, anxiety, and dopamine-mediated exploratory and rewarded behaviors. Human clinical studies of CCK in feeding disorders and panic disorders are described. New findings are presented on potent, nonpeptide CCK antagonists, selective for the two CCK receptor subtypes, which demonstrate that endogenous CCK has biologically important effects on physiology and behavior.

The CCK-A receptor antagonist, devazepide, blocks the anorectic action of CCK but not peripheral serotonin in rats

A role has been proposed for cholecystokinin (CCK)-A-type receptors in mediating the anorectic action produced by serotonergic stimulation in rats. We examined the effect of pretreatment with the CCK-A antagonist devazepide (DVZ) on anorexia produced by peripheral administration of serotonin [5-hydroxytryptamine (5-HT)] or CCK-8 in 3-h food-deprived rats consuming a 30-min test meal of sweetened mash. The anorectic effect of CCK-8 (4.0 nmol/kg, IP) was antagonized in a dose-dependent manner by DVZ (0.03, 0.10, and 0.30 mumol/kg, IP), with even the lowest dose producing a significant reversal. Under identical testing conditions, a supramaximal dose of DVZ (.75 mumol/kg) did not attenuate the reductions in food intake produced by either a moderate (4.0 mumol/kg) or a high dose (10.0 mumol/kg) of 5-HT. These data confirm established findings that the anorectic action of peripheral CCK depends upon CCK-A receptors. However, peripherally administered 5-HT reduces food intake independently of CCKergic function.

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.

Alcohol drinking attenuated by sertraline in rats with 6-OHDA or 5,7-DHT lesions of N. accumbens: a caloric response?

The purpose of this study was to elucidate further the role of serotonin (5-HT) in the preference for ethyl alcohol induced in the Sprague-Dawley rat by lesions of the N. accumbens. Following a standard preference test for 3-30% alcohol, dopaminergic or serotonergic neurons in the N. accumbens of the rat were lesioned bilaterally by 6-hydroxydopamine (6-OHDA) or 5,7-dihydroxytryptamine (5,7-DHT), respectively. After recovery postoperatively, each rat was offered water and its maximally preferred concentration of alcohol, which ranged from 7% to 11%. Following a 4-day pretest, either the saline control vehicle or the 5-HT reuptake inhibitor, sertraline, was injected subcutaneously in a dose of either 3.0 or 10 mg/kg b.i.d. at 0800 and 2000 h over the next 3 days. Alcohol preference during the injection sequence and for 4 days thereafter was significantly reduced by sertraline in terms of both absolute g/kg and proportion of alcohol to water intakes. Saline was without effect on alcohol drinking. Comparisons of the drinking profiles of serotonergic versus dopaminergic lesioned rats revealed a dose dependent response to sertraline only in the 5,7-DHT lesioned animals. Although sertraline did not alter water drinking, the consumption of food declined significantly during and after its administration with a decline in body weight also observed at the higher dose. These results suggest that in addition to dopaminergic neurons in the N. accumbens, the synaptic activity of 5-HT in this structure contributes to the aberrant drinking of alcohol. However, this interpretation is tempered by the fact that caloric intake was suppressed concomitantly by the drug.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of repeated administration of serotonergic agonists on diet selection and body weight in rats

Food intake, diet selection and body weight gain were examined in three separate experiments in which rats received saline or one of three serotonergic agonists, dexfenfluramine, RU 24969 and fluoxetine. In all experiments, food was available only in the dark period during which time rats were given simultaneous access to two isoenergetic diets which differed in their protein and carbohydrate content. After habituation to this feeding paradigm and intraperitoneal injections, rats were assigned to control or drug group. Saline or a serotonergic agonist was given to the same rat once daily, 15 min prior to feeding, for six consecutive days. All three agonists (1.5 mg/kg for dexfenfluramine and RU 24969; 3 mg/kg for fluoxetine) caused immediate (first two h of feeding) hypophagia which was accounted for by the selective suppression in intake of the high-carbohydrate-low-protein diet. This selective shift in diet choice was sustained upon repeated exposure. Although the effects of these agonists on daily (12-h) feeding was less pronounced, appetite suppression was due entirely to reduced intake of the high-carbohydrate-low-protein diet. Of the three agonists tested, partial tolerance was observed only after dexfenfluramine. Nevertheless, all three agonists caused comparable declines in weight gain. These results suggest that repeated administration of serotonergic agonists has sustained impacts on food intake, diet choice and weight gain.

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.

Behavioral mechanisms for the anorectic action of the serotonin (5-HT) uptake inhibitor sertraline in rats: comparison with directly acting 5-HT agonists

The 5-HT uptake inhibitor, sertraline (5-40 mumol/kg, IP) reduced the volume of milk consumed by food-deprived rats during a 30-min test (ID50 = 12 mumol/kg). Observations using a time-sampling method revealed that sertraline shortened meal duration (ID50 = 14 mumol/kg) by decreasing feeding and increasing resting without altering nonfeeding activity or the overall sequence of behavior that characterizes normal satiety. In separate experiments, analysis of videotapes demonstrated that sertraline (10 mumol/kg) decreased not only the time that rats fed but also their actual rate of intake. In comparison, doses of the direct 5-HT agonists, mCPP (1-[3-chlorophenyl]piperazine), RU 24969 (5-methoxy-3-[1,2,3,6-tetrahydropyridin-4-yl]-1H-indole), and DOI (1-[2,5-dimethoxy-4-iodophenyl]-2-amino-propane) that produced similar anorectic effects altered either feeding time or rate but not both. DOI also disrupted the continuity of feeding and the 5-HT agonist, 8-OH-DPAT (8-hydroxy-di-N-propylamino tetralin) produced marked stereotypy at anorectic doses. Together, these results imply that stimulating a number of different serotonergic mechanisms can reduce food intake in rats. Sertraline appears to accelerate the onset of normal satiety, presumably by enhancing physiological actions of endogenous 5-HT.