Effects of d-fenfluramine on feeding and hypothalamic 5-hydroxytryptamine and dopamine in male and female rats

Age-dependent effects of the cannabinoid CB1 antagonist SR141716A on food intake, body weight change, and pruritus in rats

RATIONALE

The cannabinoid CB1 selective antagonist SR141716A (Rimonabant) has been shown to decrease body weight in laboratory animals and humans. Furthermore, SR141716A can elicit scratching behavior in rodents, a behavior that has been hypothesized to contribute to SR141716A-induced decrease in food intake. Although childhood obesity is a rising health issue, it is unknown whether SR141716A is equipotent at modulating food intake and other CB1-mediated behaviors in younger subjects.

OBJECTIVE

To determine whether CB1 receptor blockade is equipotent at modulating food and water intake, body weight, and scratching behavior, the effect of a range of SR141716A doses on these behaviors in food-restricted postnatal day (P) 18, 28, and 60 male rats was investigated. Brain concentrations of SR141716A were determined in each age group.

RESULTS

SR141716A dose- and age-dependently suppressed food and water intake and body weight gain and elicited head scratching, with the most potent effects observed in P18 and P28 rats. Brain concentrations of SR141716A were significantly elevated in P18 rats relative to P28 and P60 rats. SR141716A-elicited head scratching was attenuated by the 5-HT(2A/2C) antagonist ketanserin.

CONCLUSIONS

SR141716A is more potent at modulating food intake and head scratching in very young animals; these differences can be attributed to an increase in brain penetration of SR141716A for P18 but not for P28 and P60 rats. In addition, SR141716-elicited head scratching is modulated by 5HT receptor antagonism and is not a contributing factor to SR141716A's anorectic effects.

The anorectic effect of fenfluramine is influenced by sex and stage of the estrous cycle in rats

The controls of food intake differ in male and female rats. Daily food intake is typically greater in male rats, relative to female rats, and a decrease in food intake, coincident with the estrous stage of the ovarian reproductive cycle, is well documented in female rats. This estrous-related decrease in food intake has been attributed to a transient increase in the female rat's sensitivity to satiety signals generated during feeding bouts. Here, we investigated whether sex or stage of the estrous cycle modulate the satiety signal generated by fenfluramine, a potent serotonin (5-HT) releasing agent. To examine this hypothesis, food intake was monitored in male, diestrous female, and estrous female rats after intraperitoneal injections of 0, 0.25, and 1.0 mg/kg d-fenfluramine. The lower dose of fenfluramine decreased food intake only in diestrous and estrous females, suggesting that the minimally effective anorectic dose of fenfluramine is lower in female rats, relative to male rats. Although the larger dose of fenfluramine decreased food intake in both sexes, the duration of anorexia was greater in diestrous and estrous female rats, relative to male rats. Moreover, the magnitude of the anorectic effect of the larger dose of fenfluramine was greatest in estrous rats, intermediate in diestrous rats, and least in male rats. Thus our findings indicate that the anorectic effect of fenfluramine is modulated by gonadal hormone status.

The serotonergic appetite suppressant fenfluramine. Reappraisal and rejection

Medical and social pressures have led to increased emphasis on dieting. However, there has been a concurrent world wide increase of obesity. Therefore, much attention has been paid to the development of drugs which decrease appetite. The most extensively used drug of this type over the past three decades has been the serotonergic compound fenfluramine. Recent findings have cast doubt on the previously accepted view that its action requires the release of central 5-HT. Instead, it seems likely that action on specific 5-HT receptors independently of 5-HT stores is involved. It is ironic that these new developments in understanding its mechanism of action have coincided with the recognition of its cardiovascular side-effect apparent especially in patients treated with d-fenfluramine combined with phentermine. This has forced the withdrawal of fenfluramine (both as racemate and d-isomer) from clinical use. The implications of these developments are commented upon.

Differences in hypothalamic serotonin between estrous phases and gender: an in vivo microdialysis study

The aim of the present study was to assess whether there are gender differences in (1) levels of extracellular serotonin (5-HT) in the forebrain, and (2) the effect on 5-HT of a reuptake inhibitor, paroxetine, or a releasing drug, fenfluramine. In vivo microdialysis was used to measure 5-HT in the hypothalamus of male and regularly cycling female rats. Hypothalamic 5-HT was significantly lower in estrous females (0.83 +/- 0.05 pg/sample, n=33) than in male rats (1.04 +/- 0.06 pg, n=38). Levels in diestrous females (0.98 +/- 0.09 pg, n=38) were not significantly different from males. Paroxetine (1 mg/kg) increased hypothalamic 5-HT in males, and diestrous and estrous females to approximately 2 pg/sample. However, the increase in hypothalamic 5-HT produced by a maximally effective dose of paroxetine (10 mg/kg) was significantly greater in male rats and during diestrous than during estrous. d,l-Fenfluramine (10 mg/kg) evoked an increase in extracellular 5-HT to approximately 15 pg/sample in all groups. A higher dose of d,l-fenfluramine (20 mg/kg) produced a significantly greater increase in hypothalamic 5-HT in males than in females during estrous or diestrous. These results are consistent with other evidence that during estrous, when rats are responding to peak levels of estrogen and progesterone, 5-HT release is decreased.

The effect of D-fenfluramine on brain 5-hydroxytryptamine and 5-hydroxyindoleacetic acid in male and female rats

Brain regional 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) concentrations were determined in freely feeding male and female rats 7 days after giving a single dose of D-fenfluramine (3.8 mg/kg, p.o.) or vehicle. Males showed negligible effects except for a significant decrease of 5-HT in the rest of the cortex, whereas females showed significant decreases of 5-HT and 5-HIAA in the frontal cortex, the rest of the cortex, hippocampus and hypothalamus; 5-HT was also decreased in female midbrain. Females had substantially higher plasma and brain concentrations of fenfluramine and moderately but significantly lower concentrations of norfenfluramine than the males. Plasma fenfluramine + norfenfluramine concentrations of the females were significantly higher than those of the males. Corresponding brain values showed smaller but significant differences. Female brain and plasma areas under the curve for fenfluramine + norfenfluramine (0-24 h after administration of D-fenfluramine) were 20 and 35% higher than male values. However, results suggest that the sex difference in the effect of D-fenfluramine on brain 5-HT metabolism is not due to differences in the metabolism of the drug.

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

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

Effect of p-chlorophenylalanine at moderate dosage on 5-HT and 5-HIAA concentrations in brain regions of control and p-chloroamphetamine treated rats

The effects of p-chlorophenylalanine (PCPA, 100-150 mg/kg x 1. i.p.), doses which decrease brain 5-hydroxytryptamine (5-HT) by 30-50%, were investigated in both intact rats and 14 days after giving p-chloroamphetamine (PCA, 10 mg/kg/day x 2, i.p.). The PCPA dose-dependently decreased brain regional 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) 24 hr later. As per cent decreases of 5-HIAA were greater than those of 5-HT in cortex, striatum and hippocampus 5-HIAA/5-HT ratios fell, suggesting that partial inhibition of 5-HT synthesis by PCPA increases 5-HT conservation in these terminal regions. In the hypothalamus and brain stem, decreases of the ratio were small or absent. The PCA given without subsequent PCPA treatment decreased 5-HT and 5-HIAA so that 5-HT fell by about 70% in the cortex, striatum and hippocampus, 55% in the brain stem but only by 27% in the hypothalamus. The PCPA given after PCA decreased 5-HT and 5-HIAA further but not the 5-HIAA/5-HT ratios and increased the ratio in the brain stem. The 5-HIAA/5-HT findings imply that the increase of 5-HT conservation after PCPA treatment does not occur after partial depletion of 5-HT by PCA. The increase of the 5-HIAA/5-HT ratio in the brain stem is explicable by the resistance to both PCA and PCPA of 5-HT in cell bodies where the ratio is high. Results are discussed in relation to the question of whether the PCA treatment used destroys axon terminals projecting from the dorsal but not from the median raphe.

Depletion and time-course of recovery of brain serotonin after repeated subcutaneous dexfenfluramine in the mouse. A comparison with the rat

The indole-depleting effects of repeated subcutaneous doses of dexfenfluramine (D-F) (2.5, 5, 10, 20 and 40 mg/kg/day, for four days) in mice were examined with regard to the initial response and time-course of recovery and related to the pharmacokinetics of D-F and its active metabolite dexnorfenfluramine (D-NF). Steady-state plasma and brain concentrations of D-F rose dose-dependently with a metabolite-to-drug ratio averaging 0.4 in brain. This confirmed that in mice D-NF contributes less than in other species to the effects of D-F. Regional serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) contents were decreased dose-dependently 4 hr after the last injection of D-F. However, two weeks after D-F (2.5-10 mg/kg/day) brain indoles had almost totally recovered, and the long-term effects of the 20 mg/kg/day dose were completely reversed by six weeks, when significant effects are still observable in rats. Although substantial recovery was evident even at 40 mg/kg/day, 5-HT but not 5-HIAA was still slightly reduced nine weeks later. Comparative studies in rats given 2.5-20 mg/kg/day D-F indicated much more severe initial indole depletions than in mice. Brain levels of D-F and D-NF were much higher in rats than in mice. The total active drug brain concentration (D-F + D-NF) was significantly correlated with 5-HT content in both species, with approx 20 nmol/g of total drug causing 50% reduction. These findings point to species differences in D-F kinetics as a main reason for differences in the neurochemical response, supporting the view that the recovery of indoles over time is related to the extent of initial depletion, which in turn depends on critical drug brain concentrations. In view of the qualitative and quantitative species differences in the pharmacodynamics and pharmacokinetics of D-F neither of these rodent species is a suitable model for predicting potential drug toxicity in humans.