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

Development of 5-HT1A receptor radioligands to determine receptor density and changes in endogenous 5-HT

[(18)F]FCWAY and [(18)F]FPWAY, analogues of the high affinity 5-HT(1A) receptor (5-HT(1A)R) antagonist WAY100635, were evaluated in rodents as potential radiopharmaceuticals for determining 5-HT(1A)R density and changes in receptor occupancy due to changes in endogenous serotonin (5-HT) levels. The in vivo hippocampus specific binding ratio [(hippocampus(uptake)/cerebellum(uptake))-1] of [(18)F]FPWAY was decreased to 32% of the ratio of [(18)F]FCWAY, indicating that [(18)F]FPWAY has lower affinity than [(18)F]FCWAY. The 5-HT(1A)R selectivity of [(18)F]FPWAY was confirmed using ex vivo autoradiography studies with 5-HT(1A)R knockout, heterozygous, and wildtype mice.Pre- or post-treatment of awake rodents in tissue dissection studies with paroxetine had no effect on hippocampal binding of [(18)F]FCWAY or [(18)F]FPWAY compared to controls, indicating neither tracer was sensitive to changes in endogenous 5-HT. In mouse ex vivo autoradiography studies in which awake mice were treated with fenfluramine following the [(18)F]FPWAY, a significant decrease was not observed in the hippocampus specific binding ratios. In rat dissection studies with fenfluramine administered following [(18)F]FPWAY or [(18)F]FBWAY ([(18)F]-MPPF) in awake or urethane-anesthetized rats, no significant differences in the specific binding ratios of the hippocampus were observed compared to their respective controls. [(18)F]FPWAY and [(18)F]FBWAY uptakes in all brain regions were increased variably in the anesthetized group (with the greatest increase in the hippocampus) vs. the awake group, but were decreased in the fenfluramine-treated anesthetized group vs. the anesthetized group. These data are best explained by changes in blood flow caused by urethane and fenfluramine, which varies from region to region in the brain.

Relationship between anorexia and loss of serotonin uptake sites in brain of mice and rats receiving d-norfenfluramine or d-fenfluramine

Previously, we have shown that repeated administration of d-fenfluramine (D-F) to rats is associated with development of tolerance to the initial anorexia, but that in mice no such tolerance seems to occur. In the first study, we show that chronic administration of neither d-norfenfluramine (D-NF; the principal metabolite of D-F) nor the serotonin (5-HT) 2C receptor agonist m-chlorophenyl-piperazine (mCPP) is associated with the development of anorectic tolerance tested using a dessert protocol. However, compared with mice receiving these drugs for the first time, both of these chronic treatments were associated with a significant attenuation of Fos immunoreactivity (Fos-ir) in several brain regions, including bed nucleus of the stria terminalis, paraventricular hypothalamus, and central nucleus of amygdala. This attenuation is similar to that described previously in rats. Because loss of efficacy of these agents could be related to loss of 5-HT transporter (5-HTT) sites, their presumptive primary mode of action, in the final study we determined the effect of various, low-dose regimens of D-F and D-NF on 5-HT uptake in frontal cortex of mice and rats. We show in mice that D-F causes a greater loss of 5-HT uptake than D-NF, and that at the lowest dose regimen used uptake was unaffected in rats but was reduced in mice. The data are discussed in terms of the species difference in behavioral tolerance and differences in neurochemical profile of D-F and D-NF.

Intermittent, chronic fenfluramine administration to rats repeatedly suppresses food intake despite substantial brain serotonin reductions

The mechanisms by which fenfluramine suppresses food intake and body weight have been linked to its ability to enhance transmission across serotonin synapses in brain. This drug initially lowers body weight and suppresses food intake, yet after repeated administration food intake soon returns to normal and body weight no longer decreases. Fenfluramine also causes rapid and prolonged reductions in brain serotonin concentrations, which might account for its loss of appetite suppression. This possibility has been evaluated in rats by assessing if intermittent, chronic fenfluramine administration could suppress food intake during each treatment period, and if so, whether such an effect occurs in the presence of reduced brain serotonin levels. Rats were injected once daily with 10 mg/kg D,L-fenfluramine for 5 days, and then received no injections for the next 5 days. Control rats received only vehicle injections. This 10-day sequence was repeated five more times. During each period of fenfluramine administration, daily food intake dropped markedly the first 1-2 days of treatment, but returned to pretreatment values by day 5. Daily food intake was normal or slightly above normal during non-injection periods. Body weight dropped modestly during each period of fenfluramine administration, and rose during each subsequent period when injections had ceased. Serotonin concentrations and synthesis rates in several brain regions were markedly reduced at early, middle, and late periods of the experiment. Despite the long-term reduction in brain serotonin pools produced by fenfluramine, the drug continues to reduce food intake and body weight. Several possible interpretations of these findings are considered, based on the multiple mechanisms through which this drug has been proposed to modify synaptic serotonin transmission.

Long-term impairment of anterograde axonal transport along fiber projections originating in the rostral raphe nuclei after treatment with fenfluramine or methylenedioxymethamphetamine

To further evaluate the serotonin (5-HT) neurotoxic potential of substituted amphetamines, we used tritiated proline to examine anterograde transport along ascending axonal projections originating in the rostral raphe nuclei of animals treated 3 weeks previously with (+/-)fenfluramine (FEN, 10 mg/kg, every 2 h x 4 injections; i.p.) or (+/-)3,4-methylenedioxymethamphetamine (MDMA, 20 mg/kg, twice daily for 4 days; s.c.). The documented 5-HT neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT, 75 microg; ICV; 30 min after pretreatment with pargyline, 50 mg/kg; i.p., and desipramine 25 mg/kg; i.p.), served as a positive control. Along with anterograde axonal transport, we measured two 5-HT axonal markers, 5-HT and 5-hydroxyindoleacetic acid (5-HIAA). Prior treatment with FEN or MDMA led to marked reductions in anterograde transport of labeled material to various forebrain regions known to receive 5-HT innervation. These reductions were associated with lasting decrements in 5-HT axonal markers. In general, decreases in axonal transport were less pronounced than those in 5-HT and 5-HIAA. However, identical changes were observed after 5,7-DHT. These results further indicate that FEN and MDMA, like 5,7-DHT, are 5-HT neurotoxins.

Functional consequences of central serotonin depletion produced by repeated fenfluramine administration in rats

Repeated administration of D,L-fenfluramine (FEN) is known to cause prolonged depletion of forebrain serotonin (5-HT) in animals. Ironically, few studies have evaluated functional consequences of such FEN-induced 5-HT loss. In the present work, we examined neuroendocrine and behavioral responses evoked by acute FEN injection in rats that had previously received a 4 d FEN-dosing regimen known to deplete forebrain 5-HT (D,L-FEN, 20 mg/kg, s.c., b. i.d.). Rats were fitted with indwelling jugular catheters before the study to allow for repeated intravenous challenge injections and stress-free blood sampling. At 1 and 2 weeks after the 4 d dosing regimen, acute FEN (1.5 or 3.0 mg/kg, i.v.) produced dose-related elevations in plasma corticosterone and prolactin; these hormonal responses were markedly attenuated in FEN-pretreated rats. Behavioral effects of acute FEN, namely flat body posture and forepaw treading, were also blunted in FEN-pretreated rats. Interestingly, rats exposed to repeated FEN did not display overt abnormalities in hormonal or behavioral parameters under basal (i.e., unprovoked) conditions, despite dramatic decreases in postmortem tissue levels of 5-HT in numerous brain areas. Our results suggest that FEN-induced 5-HT depletion is accompanied by multiple impairments in 5-HT function. Although the clinical relevance of our data are debatable, the findings clearly show the utility of the FEN challenge test for uncovering in vivo functional deficits that might otherwise go undetected. FEN should remain an important pharmacological tool for determining the role of 5-HT neurons in mediating diverse physiological and behavioral processes.

Neurotoxic effects of +/-fenfluramine and phenteramine, alone and in combination, on monoamine neurons in the mouse brain

Until recently, (+/-)fenfluramine (FEN) was widely prescribed as an appetite suppressant. In animals, FEN is a potent and selective brain serotonin neurotoxin. The present studies assessed the effects of phentermine (PHEN), an appetite suppressant frequently used clinically in combination with FEN, on FEN-induced serotonin neurotoxicity. Groups (n = 6/group) of mice were treated with FEN (10 mg/kg), PHEN (20 mg/kg or 40 mg/kg), FEN (10 mg/kg) plus PHEN (20 mg/kg or 40 mg/kg), or vehicle twice daily for four days. Food intake and body weight were measured during and after drug treatment. Brains were evaluated for regional brain serotonin and dopamine axonal markers two weeks after drug treatment. PHEN enhanced the anorectic and weight-reducing effects of FEN. PHEN also significantly enhanced FEN's long-term toxic effects on 5-HT axons. This effect was evident in some (hypothalamus, striatum) but not all (hippocampus, cortex) brain regions examined. PHEN alone produced no long-term effects on 5-HT axonal markers. However, whether given alone or in combination with FEN, PHEN produced significant, dose-related decreases in striatal DA axonal markers. These results, coupled with those from previous studies, suggest that PHEN has the potential to exacerbate FEN-induced serotonin neurotoxicity, if utilized in certain doses. Further, the present results indicate that PHEN possesses dopamine (DA) neurotoxic potential. The relevance of these data to humans previously treated with FEN/PHEN is discussed.

The effect of the spin trapping agent α-phenyl-n-tert-butyl nitrone on dexfenfluramine-induced serotonin depletion in rat brain

Oxygen-free radical formation from either the parent compound amphetamine, its metabolites or drug-released serotonin (5-HT) has been implicated in the reduction of serotoninergic markers caused by amphetamine derivatives. Therefore, the present study investigated the effects of the spin-trapping agent α-phenyl-tert-butyl nitrone (PBN) on the 5-HT-lowering action of dexfenfluramine (DF) in rats, compared with p-chloroamphetamine (PCA). PBN (150 mg/kg, i.p, divided in two doses) almost totally prevented the reduction of 5-HT in particularly sensitive regions of the rat brain (cortex and striatum) 1 and 7 days after DF (10 mg/kg, i.p.). It also provided complete protection against the acute 5-HT-depleting action of PCA (5 mg/kg, i.p.), reducing it at 7 days in striatum, although with the higher dose (300 mg/kg, divided in two doses) there was a tendency to antagonize the long-term effects in both regions. With DF, however, the antagonistic effect of PBN was associated with a marked reduction of the plasma and brain concentrations of the parent drug, but particularly its active metabolite dexnorfenfluramine (DNF). Thus, reduced brain availability of the total active drug (DF+DNF) may explain why PBN prevents the neurochemical effects of DF (but not PCA), including the long-term one which possibly depends on the extent of the initial 5-HT lowering.

Effects of repeated oral doses of dexnorfenfluramine on 5-HT levels and 5-HT uptake sites in rat brain

The effects of oral dexnorfenfluramine (DNF; 1-4 mg/kg, twice daily for 4 days), the active metabolite of dexfenfluramine, were examined on rat regional brain indole contents and [3H]citalopram binding. Two hours after the last dose, serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were dose-dependently lowered at doses above 1.5 mg/kg, with slight regional differences. Cortical 5-HT uptake sites were reduced only at the highest dose. Above 2 mg/kg DNF also caused a more lasting reduction (4 weeks) of regional indoles and cortical 5-HT uptake sites. At this longer time while the decrease in hippocampal 5-HT levels and cortical 5-HT uptake sites remained essentially constant, cortical and striatal 5-HT levels were lowered less than at 2 h, suggesting a return toward control values.

Effects of phentermine and cocaine on fenfluramine-induced depletion of serotonin in mouse brain

The combined administration of phentermine and fenfluramine (PHEN/FEN) has been used as a treatment for obesity. Recent evidence suggests that this drug mixture may also be an effective medication for substance abuse disorders, including cocaine dependence. It is well-established that repeated high-dose fenfluramine causes serotonin (5-HT) terminal degeneration in laboratory animals, and no studies have addressed possible interactions between phentermine and fenfluramine. The purpose of the present work was to examine the effect of phentermine coadministration on fenfluramine-induced depletion of 5-HT in mouse forebrain. In addition, because of the potential for cocaine abuse in drug addicts taking PHEN/FEN as a medication, we examined the effects of PHEN/FEN on forebrain 5-HT levels in the presence or absence of cocaine. Fenfluramine (0, 3, 10, 30 mg/kg, s.c. twice daily for 4 days) caused a dose-dependent reduction in forebrain 5-HT without affecting dopamine or norepinephrine. Phentermine coadministration (7 mg/kg, s.c. twice daily for 4 days) did not significantly alter the 5-HT-depleting effect of fenfluramine. Likewise, cocaine (10 mg/kg, i.p.), administered 60 min prior to or 60 min after PHEN/FEN, had no effect on the PHEN/FEN-induced decrease in central 5-HT. The present results indicate that doses of phentermine far above those typically administered to humans do not potentiate the 5-HT-depleting effect of repeated high-dose fenfluramine. Moreover, exposure to cocaine does not significantly alter the long-term neurochemical actions of the PHEN/FEN mixture.