Combined phentermine/fenfluramine administration enhances depletion of serotonin from central terminal fields

Neurochemical organization of the ventral striatum's olfactory tubercle

The ventral striatum is a collection of brain structures, including the nucleus accumbens, ventral pallidum and the olfactory tubercle (OT). While much attention has been devoted to the nucleus accumbens, a comprehensive understanding of the ventral striatum and its contributions to neurological diseases requires an appreciation for the complex neurochemical makeup of the ventral striatum's other components. This review summarizes the rich neurochemical composition of the OT, including the neurotransmitters, neuromodulators and hormones present. We also address the receptors and transporters involved in each system as well as their putative functional roles. Finally, we end with briefly reviewing select literature regarding neurochemical changes in the OT in the context of neurological disorders, specifically neurodegenerative disorders. By overviewing the vast literature on the neurochemical composition of the OT, this review will serve to aid future research into the neurobiology of the ventral striatum.

Metyrapone attenuates the sequential learning deficits but not monoamine depletions following d,l-fenfluramine administration to adult rats

Fenfluramine (FEN) is a substituted amphetamine known for its anorectic effects, without the stimulatory or abuse potential associated with other amphetamine derivatives. FEN is a potent serotonin (5-HT) releaser and reuptake inhibitor and has been shown to cause depletions of 5-HT that can last days and even weeks after administration. Administration of FEN four times on a single day also causes a prolonged increase of corticosterone (CORT) that lasts approximately 72 h following the first FEN dose. This dosing regimen also produces deficits in sequential learning as measured in the Cincinnati water maze (CWM). Adrenalectomy blocks this effect but removes more than CORT. Accordingly, the purpose of this study was to determine whether inhibiting glucocorticoid production, by administration of the 11 beta-hydroxylase inhibitor metyrapone (MET), will similarly attenuate or eliminate the sequential learning deficits seen with FEN exposure. MET (50 mg/kg) injections were administered 90 min prior to and for 3 days after FEN (four doses given at 2-h intervals). Animals pretreated with MET and treated with FEN showed no sequential learning deficits when tested 1 week following FEN administration compared to FEN alone. The depletions of monoamines were similar following FEN administration, regardless of MET treatment. Taken together, this suggests that a potential mechanism for the sequential learning deficits in FEN-treated animals is a result of prolonged increases in CORT output.

Liquid chromatography studies on the pharmacokinetics of phentermine and fenfluramine in brain and blood microdialysates after intraperitoneal administration to rats

A highly sensitive and simple HPLC method with fluorescence detection for the determination of phentermine (Phen), fenfluramine (Fen) and norfenfluramine (Norf, the active metabolite of Fen) in rat brain and blood microdialysates has been developed. The brain and blood microdialysates were directly subjected to derivatization with 4-(4,5-diphenyl-1H-imidazol-2-yl) benzoyl chloride (DIB-Cl) in the presence of carbonate buffer (0.1 M, pH 9.0) at room temperature. The chromatographic conditions consisted of an ODS column and mobile phase composition of acetonitrile and water (65:35, v/v) with flow rate set at 1.0 ml/min. The detection was performed at excitation and emission wavelengths of 325 and 430 nm, respectively. Under these conditions, the DIB-derivatives of Phen, Fen and Norf were well separated and showed good linearities in the studied ranges (5-2000 nM for Phen and 10-2000 nM for Norf and Fen) with correlation coefficients greater than 0.999. The obtained detection limits were less than 23 fmol on column (for the three compounds) in both brain and blood microdialysates at a signal-to-noise ratio of 3 (S/N=3). The intra- and the inter-assay precisions were lower than 10%. The method coupled with microdialysis was applied for a pharmacokinetic drug-drug interaction study of Phen and Fen following individual and combined intraperitoneal administration to rats. In addition, since the role of protein binding in drug interactions can be quite involved, the method was applied for the determination of total and free Phen and Fen in rat plasma and ultrafiltrate, respectively. The results showed that Fen and/or Norf significantly altered the pharmacokinetic parameters of Phen in both blood and brain but did not alter its protein binding. On the other hand, there was no significant difference in the pharmacokinetics of Fen when administered with Phen.

Effects of preexposure to dexfenfluramine, phentermine, dexfenfluramine-phentermine, or fluoxetine on sibutramine-induced hypophagia in the adult rat

The antiobesity drug sibutramine suppresses food intake via inhibition of reuptake of both norepinephrine (NE) and serotonin (5-HT) into brain terminals. The present study examined whether preexposure to other antiobesity drugs (fluoxetine [FLUOX], phentermine [PHEN], and dexfenfluramine [DEX]) that alter noradrenergic and/or serotonergic activity in brain induces tolerance or sensitization to the subsequent hypophagic action of sibutramine. Accordingly, adult male rats were treated (administered orally once per day for 21 days) with DEX (0, 1, or 3 mg/kg) and/or PHEN (0, 5, or 10 mg/kg), alone and in combination, or with the selective 5-HT reuptake inhibitor FLUOX (0, 15, or 30 mg/kg). Daily administration of PHEN persistently reduced food intake and body weight whereas tolerance developed to the hypophagic action of DEX or of FLUOX within the first week of daily administration. Moreover, low doses of DEX (1 mg/kg) and PHEN (5 mg/kg) interacted in a supra-additive manner to inhibit food intake and water intake and decrease body weight over the 21-day exposure period. After a recovery period of 9 days, a series of food intake trials were conducted to assess the hypophagic action of sibutramine (0, 1, 3, and 9 mg/kg po). Preexposure to PHEN (5 or 10 mg/kg), DEX (3 mg/kg), or FLUOX (30 mg/kg) resulted in a significant attenuation of the hypophagia induced by sibutramine over an 8-h, but not a 2-h, testing period. The pattern of cross-tolerance noted in this study is consistent with the observation that sibutramine inhibits eating via an interaction with noradrenergic and serotonergic mechanisms. Whether PHEN and DEX preexposure in humans alters subsequent sibutramine effectiveness is unknown.

Impaired spatial and sequential learning in rats treated neonatally with D-fenfluramine

D-Fenfluramine, a serotonin releaser, was administered to neonatal rats on postnatal days 11-20 (a stage of hippocampal development analogous to third trimester human ontogeny). As adults, the D-fenfluramine-treated offspring exhibited dose-related impairments of sequential and spatial learning and reference memory in the absence of sensorimotor impairments. Procedures to minimize stress and to control for other performance effects prior to testing for spatial learning demonstrated that nonspecific factors did not account for the selective effects of D-fenfluramine on learning and memory. Developmental D-fenfluramine-induced spatial and sequential learning deficits are similar to previous findings with developmental MDMA treatment. By contrast, recent findings with developmental D-methamphetamine treatment showed spatial learning deficits while sparing sequential learning. The spatial learning effects common to all three drugs suggest that they may share a common mechanism of action, however, the effects are not related to long-lasting changes in hippocampal 5-HT levels as no differences were found in adulthood. Whether the cognitive deficits are related to the effects of substituted amphetamines on corticosteroids, other aspects of the 5-HT system, or some unidentified neuronal substrates is not known, but the data demonstrate that these drugs are all capable of inducing long-term adverse effects on learning.

Effects on serotonin in rat hypothalamus of D-fenfluramine, aminorex, phentermine and fluoxetine

Hypothalamic 5-HT (serotonin) regulates food intake, energy expenditure and bodyweight. Using in vivo microdialysis, we determined the effects of various anorectic drugs on hypothalamic extracellular 5-HT levels during the dark phase when rats predominantly feed. Phentermine and aminorex, which were originally considered to be catecholaminergic drugs, markedly increased 5-HT efflux in rat hypothalamus. Their actions were less profound than D-fenfluramine, but considerably greater than that of the selective 5-HT reuptake inhibitor, fluoxetine. This suggests that enhanced hypothalamic 5-HT function could be involved in their anorectic actions. Pharmacological characterization revealed that D-fenfluramine, aminorex and probably also phentermine potentiate synaptic 5-HT function predominantly by release, whereas fluoxetine acts exclusively by reuptake inhibition. The results also revealed that the combined actions of phentermine and D-fenfluramine on hypothalamic extracellular 5-HT levels were additive, but not synergistic. In contrast, there was a significant negative cooperative effect on extraneuronal 5-HT of combining phentermine with fluoxetine.

Inhibition of cocaine self-administration by fluoxetine or D-fenfluramine combined with phentermine

Instrumental responding for intravenous cocaine in rats at 85% of free-feeding weight was significantly decreased 50% by D-fenfluramine plus phentermine (D-Fen/Phen, 5 mg/kg of each for 1 day). A similar effect was obtained in normal-weight rats self-administering a cocaine-heroin mixture. Treating normal-weight animals with fluoxetine (5 mg/kg) for 4 days also significantly decreased cocaine self-administration by half, and then adding phentermine caused an additional decrease in cocaine intake. Animals that were well trained to self-administer drug did not self-administer intravenous D-Fen/Phen or Flu/Phen. The present results confirm that serotonergic drugs can decrease cocaine, or cocaine/heroin, self-administration in rats, and that phentermine adds to the effect. Based on related research with the same dose of D-Fen/Phen, it is suggested that effectiveness in reducing cocaine reinforcement is due in part to a satiating effect in which dopamine and acetylcholine are released in the nucleus accumbens.

Rapid clot formation and abnormal fibrin structure in a symptomatic patient taking fenfluramine--a case report

A 35-year-old woman experienced symptomatic calf pain while taking a combination of fenfluramine and phentermine. All symptoms resolved when the medications were stopped, but pain returned when fenfluramine was restarted. Laboratory evaluation revealed mild elevations of aspartate aminotransferase and lactate dehydrogenase and a remarkably shortened prothrombin time (6.3 seconds). Additional studies revealed that the clots were composed of very thin fibrin fibers. All laboratory abnormalities, including the abnormal fibrin structure, completely resolved when fenfluramin was stopped. Direct addition of fenfluramine or phentermine to normal plasma did not alter either coagulation kinetics or fibrin structure, supporting the concept that the induced changes may have originated at the hepatic level. Clots composed of thin fibers are much more resistant to fibrinolysis, and could potentially put such patients at risk for thrombotic complications. This is the first report of clotting abnormalities associated with fenfluramine use. Subsequent to its use in this patient, fenfluramine was removed from clinical use due to reports of acquired valvular heart disease.

Fluoxetine increases the anorectic and long-term dopamine-depleting effects of phentermine

The anorectic drug phentermine produces dose-related toxic effects on brain dopamine (DA) neurons in animals. Until recently, phentermine was widely used in combination with fenfluramine for purposes of appetite suppression and weight loss. With the recent withdrawal of fenfluramine from the market, many people have begun combining phentermine with fluoxetine, a serotonin reuptake inhibitor which also produces mild anorectic effects. Fluoxetine, in addition to inhibiting serotonin reuptake, inhibits hepatic mixed function oxidase, which plays an important role in the metabolic degradation of amphetamines. The purpose of the present study was to assess the effects of fluoxetine on the anorectic and DA neurotoxic effects of phentermine in mice. Phentermine, in combination with fluoxetine, produced greater reductions in food intake and body weight than phentermine alone. The phentermine/fluoxetine combination also produced greater long-term reductions in brain DA levels than phentermine alone, likely reflecting greater DA neurotoxicity of the drug combination. Brain concentrations of phentermine were also found to be higher in animals pretreated with fluoxetine. These findings indicate that fluoxetine potentiates both the anorectic and DA neurotoxic effects of phentermine, probably by increasing phentermine brain levels. The clinical significance of these findings remains to be ascertained.

Fenfluramine-Phentermine (Fen-Phen) and Seizures: Evidence for an Association

Fenfluramine-phentermine combination therapy ("fen-phen") became a popular treatment for obesity in the 1990s. Although this treatment causes cardiac toxicity, use of these medications has not previously been associated with seizures. We report five cases with apparent association between use of fenfluramine-phentermine and occurrence of seizures. Three patients with a history of childhood-onset idiopathic generalized epilepsy in remission experienced a recrudescence of seizures following treatment with fenfluramine-phentermine. Two patients presented with new-onset seizures in midlife following use of fenfluramine-phentermine, and seizures persisted following discontinuation of this therapy. One of these patients restarted fenfluramine-phentermine months later, and experienced recurrent seizures. The nature of the association between fenfluramine-phentermine and seizures is uncertain from this preliminary report. There may be a specific association with idiopathic generalized epilepsies, which appeared to be overrepresented in this case series. An effect of fen-phen on seizure threshold appears most likely; however, an epileptogenic effect cannot be excluded.

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.

Selective neurotoxins, chemical tools to probe the mind: the first thirty years and beyond

For centuries, starting with the advent of the microscope, cytotoxins have been known to non-selectively destroy nerves and other tissue cells. However, neurotoxins restricted in effect to one kind of neuron are an invention of the 20th century. One might reasonably trace the origins of this field to 1960 when the Nobel Laureates, R. Levi- Montalcini and S Cohen, showed that an antibody to nerve growth factor effectively prevented development of sympathetic nerves in the absence of overt changes in dorsal root ganglia and other neural and non-neural tissues. The year 1967 marks discovery of 6-hydroxydopamine, the first of dozens of chemically-selective neurotoxins. As stated by the physiologist W.B. Cannon, neural function can be deduced by denoting absence-deficits. A wealth of knowledge in neuroscience has been realized through use of neurotoxins. In the 21st century we foresee neurotoxins for virtually all neurochemically-identifiable or receptor-specific neurons, acting at/via functional proteins or characteristic DNA sites. These tools will provide us with a better means to probe the mind and thereby lead to a fuller understanding of the intricate roles of identifiable neuronal systems in integrative neuroscience.

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.