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

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

The appetite suppressant d-fenfluramine induces apoptosis in human serotonergic cells

Fenfluramine is an amphetamine analogue which has been widely used in the treatment of obesity. In rodents, non-human primates, and humans, fenfluramine is associated with some indices of neurotoxicity, as well as pulmonary hypertension and cardiac valve pathology. In the present study, d-fenfluramine was found to be cytotoxic to the serotonin (5-HT) transporter (5-HTT) expressing human placental choriocarcinoma cells. d-Fenfluramine caused DNA fragmentation and apoptosis. Apoptosis was not observed after the 5-HTT had been blocked by fluoxetine, indicating that intact 5-HTT function is required for d-fenfluramine to induce programmed cell death. These observations in a human cell line may reflect a possible mechanism associated with the risks of fenfluramine administration in several species, including humans.

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).

Cocaine and selective monoamine uptake blockers (sertraline, nisoxetine, and GBR 12935) prevent the d-fenfluramine-induced head-twitch response in mice

Serotonin release subsequent to 5-HT precursor loading mainly occurs via exocytosis. Acute cocaine or sertraline administration promote the ability of 5-HT precursors (e.g. L-tryptophan) to induce the 5-HT2A receptor-mediated head-twitch response (HTR) in rodents. The 5-HT releaser, d-fenfluramine, at behaviorally active doses, can induce the head-twitch response in rodents by releasing cytoplasmic 5-HT via the serotonin uptake carrier working in reverse. The purpose of the present study was to utilize the d-fenfluramine-induced HTR to determine the serotonergic and nonserotonergic components of cocaine's actions on the d-fenfluramine-sensitive pool of cytoplasmic 5-HT. Because a dramatic differential potentiation in HTR frequency is obtained when cocaine is administered prior relative to after L-tryptophan injection, the effects of varying doses of cocaine and the selective serotonin (sertraline), dopamine (DA) (GBR 12935), and norepinephrine (NE) (nisoxetine) uptake blockers were investigated on the d-fenfluramine-induced behavior in two experimental protocols. Thus, each uptake inhibitor was administered either 10 min following (protocol 1) or 10 min prior to (protocol 2) d-fenfluramine injection. All the tested uptake inhibitors attenuated the d-fenfluramine-induced HTR in a dose-dependent manner in both experimental protocols. However, their order of potency in either protocol 1 (nisoxetine > GBR 12935 > cocaine > sertraline) or protocol 2 (cocaine > GBR 12935 > nisoxetine = sertraline) does not agree with in vitro affinity of these drugs for the 5-HT transporter. In addition, the potency order for cocaine and nisoxetine in protocol 1 was significantly reversed in protocol 2. The inhibitory effects of the cited drugs on the d-fenfluramine-induced HTR are discussed in terms of: 1) high doses of selective monoamine uptake blockers may not exhibit as much selectivity for their target uptake sites as indicated by in vitro tests; and 2) possible pharmacokinetic interactions between d-fenfluramine and the monoamine uptake blockers.

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

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

Prolactin response to fenfluramine is independent of serotonin release

To assess the role of serotonin release in the prolactin response to fenfluramine, rats were treated with fenfluramine alone or in combination with a dose of fluoxetine known to block fenfluramine-induced serotonin release. Fluoxetine pretreatment did not prevent fenfluramine-induced increases in prolactin. These findings indicate that fenfluramine-induced increases in prolactin are independent of serotonin release, and possibly involve direct post-synaptic actions of fenfluramine or one of its metabolites (norfenfluramine).

5-HT2a/2c receptor blockade by amesergide fully attenuates prolactin response to d-fenfluramine challenge in physically healthy human subjects

Prolactin responses to d-fenfluramine (d-FEN) challenge (0.5 mg/kg PO) were examined after pre-treatment with and without the 5-HT2a/2c receptor antagonist amesergide in eight physically healthy male volunteers. Compared to pretreatment with placebo, pre-treatment with amesergide completely blocked the prolactin (PRL) response to d-FEN challenge in all subjects. These data are consistent with data demonstrating a complete blockade of the PRL response to d-FEN with the 5-HT2a/2c receptor antagonist ritanserin, and suggest that the PRL response to d-FEN challenge in humans may largely be due to activation of the 5-HT2a/2c receptor.

In vivo and in vitro interaction of flunarizine with D-fenfluramine serotonergic effects

Flunarizine (35 mg/kg), but not haloperidol and trifluperazine, counteracted the initial indole depletion induced by D-fenfluramine (dF) in vivo (5 mg/kg), without affecting ex vivo [3H]-serotonin (5-HT) uptake by synaptosomes or changing the brain concentrations of the parent drug and its main active metabolite, D-norfenfluramine (dNF). The long-term indole depletion induced by repeated doses of dF (5 mg/kg, b.i.d. for 4 days) was also reversed by flunarizine pretreatment. Flunarizine, methiothepin, and trifluperazine, but not haloperidol, reduced in vitro the Ca(2+)-dependent [3H]5-HT release stimulated by 0.5 microM dF and dNF from superfused synaptosomes. At the concentrations used in release experiments the drugs were not active on [3H]5-HT uptake nor on the calcium-calmodulin protein kinase activity, thus excluding an effect on the uptake carrier or on phosphorylation of synaptic proteins involved in exocytosis, respectively. The drugs did not consistently affect [3H]5-HT release induced by depolarization, or dNF-induced [3H]dopamine release in vitro. The fact that flunarizine, as methiothepin and 5-HT uptake inhibitors, counteract dF-induced indole depletion in vivo suggests a relation between the reduction of the Ca(2+)-dependent release of [3H]5-HT induced by dF in vitro and the protective effect on the short- and long-lasting depletion of indoles induced in vivo by high doses of dF.

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.

Dopamine agonists prevent or counteract the suppression of brain stimulation reward by fenfluramine

The interaction between the serotonin (5-HT) and the dopamine (DA) systems in the modulation of intracranial self-stimulation (ICSS), a DA-dependent behavior, was investigated. Chronically implanted rats for ICSS in the medial forebrain bundle were tested for the effects of fenfluramine at a dose of 20 mg/kg, and then for the effects of 10 mg/kg piribedil plus 2 mg/kg amphetamine, injected 30 min before fenfluramine or 60 min after fenfluramine. Our aim was to determine whether the action of fenfluramine at the DA binding site could be blocked by prior occupation, or whether if it were occupied by fenfluramine it could be reversed. Fenfluramine, 20 mg/kg, injected alone, suppressed ICSS for 5-7 h. The suppression was followed by a prolonged recovery during which ICSS was profounded depressed. Repeating the treatment 7 days later produced the same response, except that the suppression was of shorter duration. In another group of animals, pretreatment with piribedil plus amphetamine 30 min before fenfluramine prevented the suppression of ICSS. Instead, ICSS was briefly attenuated, then restored to baseline levels, and then facilitated. Repeating the treatment 7 days after the first treatment potentiated this response. The attenuation was now even briefer, the recovery more rapid, and the facilitation more robust. In still another group of animals, fenfluramine was given just before the ICSS session began. Predictably, the effect was a total cessation of ICSS. At 60 min into the session, piribedil plus amphetamine was injected. The response showed a rapid recovery of ICSS followed by facilitation.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of indorenate on food intake: a comparison with fenfluramine and amphetamine

Indorenate (TR3369, 5-methoxytryptamine b-methylcarboxylate HCl) is a 5-HT1-like receptor agonist with hypotensive activity. Here, we describe that indorenate also decreases food intake (ED50 26.1 mg/kg) without an appreciable effect in water intake (the estimated ED50 for water was 589.8 mg/kg). The anorectic activity of indorenate was compared to the effects of amphetamine and other serotonin agonists; the effect of indorenate was smaller than those of the other compounds; however, the effect of indorenate was specific to food, whereas all the other drugs also produced significant decrements in water intake. The serotonin antagonists cinanserin, cyproheptadine, methergoline and methysergide effectively prevented the decrease in food intake produced by indorenate and fenfluramine. Haloperidol, a dopaminergic antagonist, was ineffective in preventing the effect of indorenate although it prevented the anorectic effect of amphetamine. The present results suggest the participation of serotoninergic, but not dopaminergic mechanisms, in the decrease in food intake produced by indorenate.

p-Chloroamphetamine (PCA), 3,4-methylenedioxy-methamphetamine (MDMA) and d-fenfluramine pretreatment attenuates d-fenfluramine-evoked release of 5-HT in vivo

Previous work has suggested that repeated treatment with substituted amphetamines including PCA, MDMA and d-fenfluramine produces a persistent neurodegeneration which is relatively selective for the fine serotoninergic terminals arising from the dorsal raphe nucleus. The aim of the present study was to investigate whether the acute releasing effect of d-fenfluramine might also be sensitive to lesions produced by PCA, MDMA and d-fenfluramine itself. Basal and 5-HT release evoked by d-fenfluramine or 100 mM KCl was measured by microdialysis in frontal or parietal cortex of rats 2 weeks after they had been treated with a neurodegenerative regime of PCA, MDMA, d-fenfluramine, or vehicle. In frontal cortex of vehicle controls, d-fenfluramine (10 mg/kg IP) and KCl (100 mM via microdialysis probe) evoked an increase in 5-HT of 1740% and 779% of basal, respectively. PCA pretreatment reduced d-fenfluramine-evoked 5-HT release by 90.9% while potassium-evoked release was reduced by only 66.8%. Similar results were obtained in parietal cortex. MDMA (20 mg/kg x 8) and d-fenfluramine (1.25 mg/kg x 8) pretreatment reduced d-fenfluramine-evoked release of 5-HT in frontal cortex by 45.2% and 72.0%, respectively. Overall, the present data are consistent with the hypothesis that the acute release of 5-HT evoked by d-fenfluramine occurs via those terminals destroyed by pretreatment with PCA, MDMA and d-fenfluramine, while KCl evokes release from both PCA-sensitive and PCA-insensitive terminals. The significance of these results for the interpretation of neuroendocrine data from d-fenfluramine challenge tests is discussed.

Fluoxetine prevents the disruptive effects of fenfluramine on differential-reinforcement-of-low-rate 72-second schedule performance

This study compared the effects of fenfluramine and fluoxetine on the differential-reinforcement-of-low-rate 72-s schedule of reinforcement. Fluoxetine, a clinically effective antidepressant, increases extracellular serotonin (5-HT) by blocking the uptake of 5-HT after release. Fenfluramine increases extracellular 5-HT through transporter-mediated release (although it also blocks 5-HT uptake). The following characteristics were identified. First, fenfluramine and fluoxetine had two different effects on the differential-reinforcement-of-low-rate 72-s schedule. Fluoxetine had an antidepressant-like effect by increasing reinforcement rate without disrupting the interresponse time distribution. Fenfluramine's effect on the differential-reinforcement-of-low-rate 72-s schedule was not antidepressant-like: it did not increase the reinforcement rate, whereas it did disrupt the interresponse time distribution. Second, when fluoxetine and fenfluramine were given in combination, fluoxetine prevented the disruptive effects of fenfluramine. This result is consistent with fluoxetine's ability to block fenfluramine-induced 5-HT release, and supports the argument that the uptake transporter mediates fenfluramine's effects on both 5-HT release and behavior. Putative behavioral mechanisms (waiting capacity and temporal discrimination) which may mediate the acute effects of fluoxetine are discussed.

5-HT and carbohydrate suppression: effects of 5-HT antagonists on the action of d-fenfluramine and DOI

The effects of several 5-hydroxytryptamine (5-HT) receptor antagonists on the anorectic effect of d-fenfluramine and the 5-HT2/5-HT1C agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) were examined in a dietary paradigm that appears to be sensitive to 5-HT-induced carbohydrate suppression. In this paradigm, deprived rats are provided with a nutritionally complete hydrated chow mash diet together with an optional carbohydrate supplement of powdered Polycose. Both d-fenfluramine and DOI produced a clear suppression of total energy intake and carbohydrate (Polycose) intake. However, the mechanisms underlying these effects are different. The effect of d-fenfluramine in this paradigm was attenuated by the 5-HT1/5-HT2 receptor antagonist metergoline and partially attenuated by the 5-HT1A/5-HT1B receptor antagonist (+/-)cyanopindolol. In contrast, d-fenfluramine's effect was not antagonised by the 5-HT2 receptor antagonist ketanserin, the 5-HT3 receptor antagonist (3 alpha-tropanyl)-1H-indole-3-carboxylic acid ester (ICS-205,930), the 5-HT2/5-HT1C receptor antagonist ritanserin, or the peripheral 5-HT receptor antagonist xylamidine. However, the effect of DOI in this paradigm was significantly attenuated by ketanserin but was not antagonised by either ritanserin or (+/-)cyanopindolol. Therefore, the suppressive effect of these two 5-HT drugs on total and Polycose intake appears to be mediated, respectively, by 5-HT1B/5-HT1C receptors (d-fenfluramine) and 5-HT2 receptors (DOI).

5-HT loss in rat brain following 3,4-methylenedioxymethamphetamine (MDMA), p-chloroamphetamine and fenfluramine administration and effects of chlormethiazole and dizocilpine

1. The present study has investigated whether the neurotoxic effects of the relatively selective 5-hydroxytryptamine (5-HT) neurotoxins, 3,4-methylenedioxymethamphetamine (MDMA or 'Ecstasy'), p-chloroamphetamine (PCA) and fenfluramine on hippocampal and cortical 5-HT terminals in rat brain could be prevented by administration of either chlormethiazole or dizocilpine. 2. Administration of MDMA (20 mg kg-1, i.p.) resulted in an approximate 30% loss of cortical and hippocampal 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) content 4 days later. Injection of chlormethiazole (50 mg kg-1) 5 min before and 55 min after the MDMA provided complete protection in both regions, while dizocilpine (1 mg kg-1, i.p.) protected only the hippocampus. 3. Administration of a single dose of chlormethiazole (100 mg kg-1) 20 min after the MDMA also provided complete protection to the hippocampus but not the cortex. This regime also attenuated the sustained hyperthermia (approx +2.5 degrees C) induced by the MDMA injection. 4. Injection of PCA (5 mg kg-1, i.p.) resulted in a 70% loss of 5-HT and 5-HIAA content in hippocampus and cortex 4 days later. Injection of chlormethiazole (100 mg kg-1, i.p.) or dizocilpine (1 mg kg-1, i.p.) 5 min before and 55 min after the PCA failed to protect against the neurotoxicity, nor was protection afforded by chlormethiazole when a lower dose of PCA (2.5 mg kg-1, i.p.) was given which produced only a 30% loss of 5-HT content. Chlormethiazole did prevent the hyperthermia induced by PCA (5 mg kg-1), while the lower dose of PCA (2.5 mg kg-1) did not produce a change in body temperature.5. Neither chlormethiazole nor dizocilpine prevented the neurotoxic loss of hippocampal or cortical 5-HT neurones measured 4 days following administration of fenfluramine (25 mg kg-1, i.p.).6. In general, chlormethiazole and dizocilpine were effective antagonists of the 5-HT-mediated behaviours of head weaving and forepaw treading which appeared following injection of all three neurotoxins.7. Both chlormethiazole and dizocilpine have previously been shown to prevent the neurotoxic effects ofa high dose of methamphetamine on cerebral 5-HT and dopamine pathways. These drugs also prevent MDMA-induced neurotoxicity of 5-HT pathways, but not that induced by injection of PCA or fenfluramine. This suggests that the mechanisms of neurotoxic damage to 5-HT pathways produced by substituted amphetamines cannot be identical. The monoamine loss does not appear to result from the hyperthermia produced by the neurotoxic compounds.

Ritanserin attenuates anorectic, endocrine and thermic responses to d-fenfluramine in human volunteers

This study investigated the role of the 5-HT2/1C receptor antagonist ritanserin on d-fenfluramine (d-FF) induced changes in food intake, prolactin (PRL) secretion and oral temperature in 12 healthy male volunteers. The study was double blind and placebo controlled. Food intake was measured using an automated food dispenser. d-FF (30 mg) significantly reduced fat intake. While ritanserin (5 mg) had no effect when given alone it abolished the d-FF induced reduction in fat intake. In addition, ritanserin abolished the d-FF induced rise in PRL and oral temperature. The results suggest that 5-HT2 or 5-HT1C receptors mediate the effects of d-fenfluramine on appetite, prolactin secretion and temperature in humans.

Impaired growth hormone response to insulin-induced hypoglycaemia in obese patients: restoration blocked by ritanserin after fenfluramine administration

The aim of the present study was to test whether the serotoninergic system may be involved in the well known reduced growth hormone (GH) response to insulin-induced hypoglycaemia (IIH) in obese patients. Ten obese women and 10 normal-weight control women underwent three IIH tests, at 14-day intervals: the first in basal conditions, the other two after randomized administration of a serotoninergic drug, fenfluramine (FF, 120 mg/day for 7 days) and FF plus ritanserin (RIT, 30 mg/day for the first 2 days and 20 mg/day on the following days). Ritanserin is a new selective 5-HT2 blocker receptor agent. Both controls and obese patients showed similar normal basal GH levels before each test and insulin administration always effectively reduced glucose levels to values lower than 2.2 mmol/l. In the controls, the expected GH increase to IIH (peak value 56 +/- 13.4 mU/l, AUC 234.4 +/- 55 mU/min/ml) was unaffected by FF administration (peak value 43 +/- 11.4; AUC 216.8 +/- 34.8). In response to the first IIH, the obese patients showed a significantly lower GH increase than in the case of the controls (peak value 21.4 +/- 4.6 mU/l, P less than 0.02; AUC 93.2 +/- 18.6, P less than 0.02). However, in comparison with the basal test, FF administration significantly (P less than 0.001) enhanced GH response to insulin hypoglycaemia (peak value 33.4 +/- 4; AUC 150 +/- 14.6), reaching values not significantly different from those of the controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Ambient temperature modulation of fenfluramine-induced thermogenesis in the rat

The anti-obesity drug fenfluramine, promotes loss of weight by reducing food intake; however, there is controversy as to whether the drug can also elevate expenditure of energy. Resting consumption of oxygen (VO2) was measured in conscious rats to determine whether the injection of fenfluramine increased metabolic rate and whether prior fasting, or ambient temperature altered the response. Regardless of whether the rats were fed or had been fasted for 22 hr, in a thermoneutral environment (28 degrees C), the intraperitoneal injection of dl-fenfluramine (20 mg/kg) caused a raised oxygen consumption. This elevation was sustained to the end of the 60-min period of measurement after the injection, at which point the colonic temperature was found to be increased. This metabolic response to fenfluramine was largely attenuated when the drug was administered at 23 degrees C, and the colonic temperature of the rats was decreased by 60 min after the injection. At 4 degrees C, the injection of fenfluramine inhibited thermogenesis against cold, the oxygen consumption fell and the rats exhibited hypothermia. It was concluded that fenfluramine can increase the metabolic rate, but that this effect is not conditional on associated food intake, as has been reported. Rather, the ambient temperature governs whether stimulation or inhibition of thermogenesis will be evoked. These metabolic effects of fenfluramine explain, in part, its divergent effects on body temperature, reported previously.

Reversal of the anorectic effect of (+)-fenfluramine in the rat by the selective cholecystokinin receptor antagonist MK-329

1. Experiments were conducted to determine whether or not the effect of (+)-fenfluramine (3.0 mg kg-1, i.p.) on food intake can be antagonized by the selective cholecystokinin receptor antagonist MK-239 (formerly L364,718; (3S(-)-N-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1-H-1,4-benzodiazepin++ +-3-yl)-1H- indole-2-carboxamide). Two feeding paradigms were employed. In the first, non-deprived rats were familiarized with eating a highly palatable, sweetened mash in a 30 min test. In the second, freely-feeding rats were trained to consume powdered chow in their home-cages, and their intake was monitored over the first 6 h of the night-period. 2. In doses of 30.0 and 100.0 micrograms kg-1, s.c., MK-329 almost completely blocked the anorectic effect of (+)-fenfluramine in the palatable food intake test. These doses of MK-329 have previously been reported to antagonize the anorectic effect produced by exogenous cholecystokinin-octapeptide (CCK8) in rats. Both doses of MK-329 were also effective in significantly attenuating the anorectic effect of (+)-fenfluramine in nocturnal free-feeding animals over a 6 h-period. 3. MK-329 (10.0-100.0 micrograms kg-1, s.c.) failed to antagonize the anorectic effect of either the specific dopamine D2-receptor agonist quinpirole (0.3 mg kg-1, s.c.) or the beta-carboline FG 7142 (10.0 mg kg-1, i.p.) in the palatable food intake test. 4. MK-329 (10.0-300.Opgkg-1, s.c.) had no effect, when administered alone, on the level of palatable food intake in non-deprived rats, even when substantial satiation was produced by a pre-feeding procedure. Furthermore, MK-329 had no effect, when administered alone, on nocturnal food intake in freelyfeeding rats. 5. In conclusion, not only was MK-329 a potent antagonist of the effect of CCK8 on food intake, it also blocked the effect of (+)-fenfluramine to a significant degree. The effect of MK-329 was selective in that the anorectic effects of either quinpirole or FG 7142 remained unaffected. Administered alone, MK-329 did not affect food intake, indicating that its reversal of (+ -fenfluramine-induced anorexia was not secondary to an intrinsic hyperphagic effect. The results provide some evidence that the depressant effect of (+ )-fenfluramine on food intake depends on the activity of endogenous CCK.

Cholecystokinin-induced anorexia depends on serotoninergic function

The existence of a relationship between cholecystokinin (CCK)-induced satiety and the serotoninergic system was evaluated. The food intake of 3-h-fasted male rats was studied after treatment with the COOH-terminal octapeptide of CCK (CCK-8) alone or in combination with one of two blockers of serotonin (5-HT) receptors, metergoline (MET; 1.0 or 0.06 mg/kg), active in both the periphery and brain, or xylamidine tosylate (XYL; 1.5 mg/kg), active only in the periphery. CCK-8 reduced food intake in the 30 min after food presentation by 37% at 2 micrograms/kg, 68% at 4 micrograms/kg, and 80% at 8 micrograms/kg compared with controls. Both doses of MET attenuated CCK-8-induced satiety, increasing food intake of rats treated with all doses of CCK-8 to control values. Food intake was significantly increased over base line by the 1.0-mg/kg dose of MET alone but unaffected by the 0.06-mg/kg dose of MET alone. XYL had no effect either given alone or in combination with CCK-8. These results indicate that the inhibitory action of CCK-8 on food intake is dependent on intact functioning of the serotoninergic system, probably at central sites.

Ketanserin antagonises the anorectic effect of DL-fenfluramine in the rat

To determine the role played by 5-HT2 receptors in the anorectic action of DL-fenfluramine, the ability of the selective 5-HT2 receptor antagonist ketanserin to block the reduction in food intake produced by this drug was investigated in non-deprived rats. Ketanserin (1 and 2.5 mg/kg i.p.) produced a dose-dependent antagonism of the anorectic effect of DL-fenfluramine (3 mg/kg i.p.). Prazosin (1 mg/kg i.p.) did not antagonise this effect. It is concluded that the anorectic actions of DL-fenfluramine are mediated via 5-HT2 receptors.

Fenfluramine discrimination in obese and lean Zucker rats: serotonergic mediation of effect

Genetically obese Zucker rats and their lean littermates were trained to discriminate between the stimulus properties of 2.0 mg/kg fenfluramine and its vehicle in a two-lever, food-motivated operant task. Both groups learned the discrimination at the same rate and all rats showed a dose-related decrease in discriminative performance with lower fenfluramine doses. Analysis of the dose-response curves indicated an ED50 for the obese rats of 0.56 mg/kg and for the lean group of 0.42 mg/kg. Time-course experiments indicted that the obese rats maintain errorless discrimination through 90 min post-injection but discriminate significantly less than the lean rats at 960 min post-administration. These results suggest a similar sensitivity to fenfluramine in obese and lean rats with a difference in the time-course of drug action. Pre-treatment with the specific serotonin receptor antagonist pirenperone significantly attenuated fenfluramine discrimination in lean rats without a similar effect in the obese rats. Possible reasons for this observation are offered.

The effect of previous d-amphetamine treatment on the disposition and lethality of fenfluramine in the rat

In an earlier study, it was found that a 15-day ip postfeeding session d-amphetamine treatment rendered an apparent "tolerance" to the feed intake suppressant effects of fenfluramine. The purpose of the present study was to determine if such d-amphetamine treatment altered the disposition of fenfluramine in a way which might account for this decrement in the feed intake suppressant effect of fenfluramine. In addition, the effect of d-amphetamine on fenfluramine toxicity was also examined in an attempt to further characterize and correlate fenfluramine dispositional changes. It was found that plasma and, to some extent, brain concentrations of fenfluramine were elevated in the d-amphetamine-treated rats as compared to a saline control group. The acute, ip LD50 for fenfluramine in d-amphetamine-treated rats was 68 mg/kg (95% CI = 64.8 to 71.4), a value significantly lower than that observed in the saline-treated animals (97 mg/kg; 95% CI = 93.3 to 100.9). The overall results of this study suggest that the anorectic tolerance conferred toward fenfluramine by previous subchronic treatment with d-amphetamine does not result from decreased plasma or brain concentrations of fenfluramine. In fact, such concentrations are increased by prior d-amphetamine treatment, and this increase may in part account for the increased lethality of fenfluramine observed in these animals.

Fenfluramine inhibits insulin secretion and potentiates glucagon release by the perfused rat pancreas

Fenfluramine, an anorectic used in the treatment of obesity, in a final concentration of 1 mM strongly inhibited both phases of insulin release by the perfused rat pancreas. Insulin secretion resumed promptly after cessation of the drug infusion. This concentration of the drug markedly increased glucagon output. The blockade of alpha-adrenergic receptors and the use of antiserotonin agents did not alter the inhibitory effect of fenfluramine on insulin secretion. It is concluded that in the perfused rat pancreas 1 mM fenfluramine acutely inhibits glucose-induced insulin secretion and potentiates glucagon output. The direct effect of fenfluramine on insulin secretion is not related to alpha-adrenergic activity, nor is it mediated by serotonin.

Central action of amitriptyline N-oxide

The central action of amitriptyline N-oxide (AMINO) has been compared with amitriptyline (AMI) in biochemical and pharmacological studies in rats and mice. It has been found in rats that both drugs prevent 6-OH-dopamine-induced depletion of brain noradrenaline (NA). At the same time AMINO increases and AMI lowers the NA level, both being without effect on 3-methoxy-4-hydroxyphenylglycol concentrations in the brain. AMINO and AMI potentiate the depletion of 5-hydroxytryptamine (5-HT) induced by p-chloroamphetamine in the rat brain and it may be considered as evidence that both drugs do not inhibit 5-HT uptake in vivo. Neither AMINO nor AMI affects the rat brain level of 5-HT but at higher doses they elevate the 5-hydroxy-indoleacetic acid concentrations. AMINO antagonizes the head twitch reaction induced by 5-hydroxytryptophan in mice and tryptamine convulsions in rats. The hyperthermia induced by fenfluramine (in rats at a high ambient temperature) as well as the stimulation of the hind limb flexor reflex in spinal rats, induced by fenfluramine or LSD, are also inhibited. AMINO antagonizes the 5-HT-induced increase in blood pressure in pithed rats. All the above effects are similar to those induced by AMI, only the active doses of AMINO are higher. The results presented indicate that AMINO, like AMI, inhibits NA uptake and is a 5-HT antagonist.

The effect of viloxazine on drug-induced inhibition of food intake in the rat

In male Wistar rats trained to eat their normal daily dietary requirement in a restricted 2 h period, dose-dependent decreases in food consumption were produced by fenfluramine, tiflorex, mazindol and amphetamine. The antidepressant drug viloxazine (Vivalan) alone did not alter food intake significantly, nor did the drug prevent the inhibitory effects of either mazindol or amphetamine. However, complete prevention of the inhibitory effect of fenfluramine was achieved with 7.5 mg kg-1 viloxazine, while 40 mg kg-1 viloxazine similarly prevented the anorectic action of tiflorex. An interaction involving 5-hydroxytryptaminergic mechanisms is suggested, and since viloxazine given after fenfluramine or tiflorex produced no reversal of the inhibition of food intake, it is suggested that viloxazine prevents access of the anorectic agents to their site of action. The clinical significance of these interactions is discussed.

Characterisation of adjustments to the structure of feeding behaviour following pharmacological treatment: effects of amphetamine and fenfluramine and the antagonism produced by pimozide and methergoline

An observational procedure for examining the micro-structure of eating has been employed to establish the characteristic behaviour patterns displayed after various pharmacological manipulations. Using a double dissociation design it was shown that amphetamine and fenfluramine gave rise to quite distinctive readjustments to the structure of feeding behaviour. Amphetamine anorexia was characterised by a long initial delay, following which feeding was typified by infrequent short bursts of rapid eating. These effects were antagonised by the dopamine receptor blocking agent, pimozide. Fenfluramine exerted a more restricted pattern of action characterised by a marked slowing of the rate of eating. This effect was countered by the serotonin receptor blocking agent methergoline. These data throw light on the way in which pharmacological agents may impede food consumption and upon the neurochemical systems believed to be involved in the expression of feeding behaviour.

Tissue ascorbic acid, fenfluramine, and changes in fat metabolism

Following initial weight gain, reduction in appetite and pronounced weight loss occurred in scorbutic unsupplemented guinea-pigs. Hepatic ascorbic acid levels were significantly reduced and cholesterol concentration increased in the liver. Fenfluramine administration caused immediate loss of weight and appetite in the scorbutic guinea-pigs, these changes being more pronounced in the males. Hepatic ascorbic acid, cholesterol and triglycerides were reduced to lower levels in the fenfluramine-treated scorbutic animals than in the scorbutic guinea-pigs receiving diet alone. In contrast, weight and appetite increased in vitamin-C-supplemented animals while they were receiving fenfluramine. Their hepatic cholesterol and triglyceride levels became significantly reduced. It has been shown that supplementary vitamin C can inhibit the anti-obesity and anorectic actions of fenfluramine and counteract its effect in raising tissue cholesterol.

Anorexigenic and ancillary actions of MK-212 (6-chloro-2-(1-piperazinyl)-pyrazine; CPP)

In rats allowed to eat for 2 h/day and injected i.p. 30 min before feeding, MK-212, ED50 = 1.5 mg/kg, was two times more potent as an anorexigen than fenfluramine. However, the compounds were equiactive in the rat following p.o. administration 1.5 or 3 h before the test, while fenfluramine was more potent if the interval was extended to 6 h. In cats permitted to eat for 3 h/day, the ED50 dose (mg/kg p.o.) for MK-212 determined at 0.5, 1 and 3 h after feeding was, respectively, 15, 10, and 3 times less than that of fenfluramine. Emesis and diarrhea were frequently observed ancillary effects in cats treated with fenfluramine, whereas apparent sedation and salivation were commonly detected in animals after MK-212. In rats or cats pretreated with methergoline, the decrease in food consumption elicited by MK-212 was markedly inhibited, suggesting that the mechanism of action involves a serotoninlike effect. Compared with the marked stimulant action of amphetamine, MK-212 had only a minor and inconsistent effect on motor activity in rats and mice. Similar results were obtained with fenfluramine. MK-212 was not self-administered by rats, while the self-administration of amphetamine and morphine were demonstrated using the same experimental protocol.

The effect of methysergide and other receptor antagonists on fenfluramine-induced glucose uptake into the isolated rat hemidiaphragm

Fenfluramine in therapeutic concentrations causes a significant and dose related increase in glucose uptake into isolated rat and human skeletal muscle in the presence of insulin. Using fenfluramine, 100 ng/ml, on the rat hemidiaphragm preparation the effects of the following receptor blocking drugs in concentrations up to 250 ng/ml were investigated on this phenomenon: atropine, haloperidol, mepyramine, methysergide, propranolol and thymoxamine. Only methysergide, the 5-HT antagonist, reduced the action of fenfluramine in relatively low concentrations of the drug which were dose related (10 ng/ml causing approximately 40% inhibition). This suggests that the peripheral as well as the central actions of fenfluramine are mediated through 5-HT receptors.

Fenfluramine-induced hyperthermia and stimulation in the rabbit

Fenfluramine produces dose-related hyperthermia and behavioral stimulation in rabbits following intravenous administration. Both responses were antagonized by pretreatment with cinanserin, cyproheptadine, or fluxetine. Methergoline likewise abolished the stimulatory effect of fenfluramine and produced a partial blockade of the hyperthermia. Haloperidol was ineffective as a fenfluramine antagonist. This study demonstrates that fenfluramine-induced hyperthermia and stimulation in the rabbit result from an indirect action upon serotonergic receptors.

Inhibition of drug-induced anorexia in rats by methysergide

Iproniazid was found to reduce food consumption in fasting rats. Combined treatment of iproniazid with tryptophan resulted in a significantly greater anorexic action whilst tryptophan alone had no effect on food consumption. Iproniazid treatment was associated with a significant increase in brain 5-hydroxytryptamine (5-HT) concentration but in association with tryptophan higher brain 5-HT concentrations were recorded. The anorexic action of the iproniazid-tryptophan combination was antagonized in a dose-dependent fashion by methysergide. Equivalent levels of anorexia induced by fenfluramine and mazindol were similarly antagonized by methysergide in a dose-related manner. The results suggest a common role of 5-HT in the inhibition of eating behaviour in fasting rats when anorexia is induced by iproniazid, fenfluramine or mazindol, sensitive to a specific 5-HT antagonist.

Hyperthermia induced by amphetamine, p-chloroamphetamine and fenfluramine in the rat-1

Hyperthemia was elicited in rats by the subcutaneous injection of 4 mg/kg (+)-amphetamine, 4 mg/kg (+)-p-chloroamphetamine, or 8 mg/kg fenfluramine. This hyperthermia could be abolished by oral pretreatment with 300 mg/kg p-chlorophenylalanine. Cyproheptadine, at a dose of 0.05 mg/kg, completely inhibited the response induced by p-chloroamphetamine, but a dose of 0.4 mg/kg was necessary in the case of amphetamine and fenfluramine. Protection against hyperthermia by p-chloroamphetamine was also provided by chloroimipramine. Hypothalamic turnover of 5-HT was enhanced by all three amphetamines. alpha-Methyltyrosine and disulfiram had no effect on the hyperthemia induced by the amphetamines, FLA-63 seemed even to enhance it. Haloperidol, in the dose range of 0.1 -0.4 mg/kg, attenuated the hyperthermia induced by amphetamine and p-chloroamphetamine, but not that induced by fenfluramine. The results point to a central origin of the hyperthemia induced by amphetamine, p-chloroamphetamine and fenfluramine. In the mediation of this effect, 5-HT and, to a lesser degree, dopamine seem to play an important role.

Evidence for a difference in mechanism of action between fenfluramine- and amphetamine-induced anorexia

The influence of drugs, active on 5-hydroxytryptamine (5-HT) mechanisms, has been examined on the anorexigenic activity of fenfluramine and (+)-amphetamine in rats trained to consume their daily food ration during 6 h. Chlorimipramine, which inhibits the re-uptake mechanisms in central 5-HT neurons, and the 5-HT blocking drugs methergoline and methysergide were used. Fenfluramine, 7.5 mg kg−1, and amphetamine, 2.5 mg kg−1, given 1/2 h before feeding reduced the food intake during the following 2 h to approximately 40% compared with control days. Pretreatment with methergoline in the optimal dose (1 mg kg−1) produced only a weak but significant antagonism to amphetamine anorexia, whereas the fenfluramine anorexia was strongly antagonized by methergoline in all doses tested (0.3, 1 and 3 mg kg−1). Methysergide (0.1, 0.3, 1 and 3 mg kg−1) showed no significant antagonism against amphetamine or fenfluramine anorexia. Chlorimipramine produced a strong antagonistic effect to the fenfluramine anorexia, but showed no antagonism against amphetamine. In contrast the highest dose of chlorimipramine (20 mg kg−1) potentiated amphetamine anorexia. The present results together with other evidence discussed support the conclusion that 5-HT mechanisms are involved in fenfluramine anorexia, whereas amphetamine anorexia seems mainly correlated with catecholamine dependent mechanisms.