Behavioural and neurochemical characteristics of phentermine and fenfluramine administered separately and as a mixture in rats

Topiramate-phentermine combinations reduce cocaine self-administration in humans

RATIONALE

Cocaine use disorder is an unrelenting public health concern. Despite nearly four decades of research, an FDA approved medication is not yet available.

OBJECTIVES

The objective of this human laboratory study was to demonstrate the initial efficacy, safety and tolerability of topiramate-phentermine combinations for cocaine use disorder.

METHODS

Thirty-one (31) participants with cocaine use disorder completed this mixed-model inpatient laboratory study. Participants were maintained on topiramate (0 [N = 11], 50 [N = 9] or 100 [N = 11] mg/day). Each topiramate group was concurrently maintained on phentermine (0, 15, 30 mg). Drug self-administration, subjective responses and cardiovascular effects following acute doses of intranasal cocaine (0, 40, 80 mg) were determined during separate experimental sessions after at least seven (7) days of maintenance on each condition.

RESULTS

The three groups of participants were well matched demographically and generally did not differ significantly in their responses to a range of doses of intranasal cocaine (0, 10, 20, 40, 80 mg) during a medical safety session. Maintenance on topiramate and phentermine alone significantly decreased cocaine self-administration although these effects were modest in magnitude. Combining topiramate and phentermine robustly decreased cocaine self-administration. Topiramate and phentermine were well tolerated alone and combined, as well as in conjunction with cocaine.

CONCLUSIONS

The results of the present study support advancing topiramate-phentermine combinations as a putative pharmacotherapeutic for cocaine use disorder.

Discriminative Stimulus Effects of Abused Inhalants

Inhalants are a loosely organized category of abused compounds defined entirely by their common route of administration. Inhalants include volatile solvents, fuels, volatile anesthetics, gasses, and liquefied refrigerants, among others. They are ubiquitous in modern society as ingredients in a wide variety of household, commercial, and medical products. Persons of all ages abuse inhalants but the highest prevalence of abuse is in younger adolescents. Although inhalants have been shown to act upon a host of neurotransmitter receptors, the stimulus effects of the few inhalants which have been trained or tested in drug discrimination procedures suggest that their discriminative stimulus properties are mediated by a few key neurotransmitter receptor systems. Abused volatile solvent inhalants have stimulus effects that are similar to a select group of GABAA positive modulators comprised of benzodiazepines and barbiturates. In contrast the stimulus effects of nitrous oxide gas appear to be at least partially mediated by uncompetitive antagonism of NMDA receptors. Finally, volatile anesthetic inhalants have stimulus effects in common with both GABAA positive modulators as well as competitive NMDA antagonists. In addition to a review of the pharmacology underlying the stimulus effects of inhalants, the chapter also discusses the scientific value of utilizing drug discrimination as a means of functionally grouping inhalants according to their abuse-related pharmacological properties.

Differential effects of 3,4-methylenedioxypyrovalerone (MDPV) and 4-methylmethcathinone (mephedrone) in rats trained to discriminate MDMA or a d-amphetamine + MDMA mixture

RATIONALE

Recent reports on the abuse of novel synthetic cathinone derivatives call attention to serious public health risks of these substances. In response to this concern, a growing body of preclinical research has characterized the psychopharmacology of these substances, particularly mephedrone (MEPH) or methylenedioxypyrovalerone (MDPV), noting their similarities to 3,4-methylenedioxymethamphetamine (MDMA) and cocaine. Few studies have utilized drug discrimination methodology to characterize the psychopharmacological properties of these substances.

OBJECTIVES

The present study employed a rodent drug discrimination assay to further characterize the stimulus effects of MEPH and MDPV in comparison to MDMA and to a drug mixture comprised of d-amphetamine and MDMA.

METHODS

Eight male Sprague-Dawley rats were trained to discriminate 1.5 mg/kg MDMA, and eight rats were trained to discriminate a mixture of 1.5 mg/kg MDMA and 0.5 mg/kg d-amphetamine (MDMA + AMPH) from vehicle. Substitution tests were conducted with MDMA, d-amphetamine, MDPV, MEPH, and cocaine.

RESULTS

Dose-response curves generated with MDMA and MEPH were comparable between training groups. In contrast, AMPH, MDPV, and cocaine produced only partial substitution in animals trained to discriminate MDMA but produced full substitution in animals trained to discriminate the MDMA + AMPH mixture.

CONCLUSIONS

These findings indicate that MDPV's effects may be more similar to those of traditional psychostimulants, whereas MEPH exerts stimulus effects more similar to those of MDMA. Additional experiments with selective DA and 5-hydroxytryptamine (5-HT) receptor antagonists are required to further elucidate specific receptor mechanisms mediating the discriminative stimulus effects of MDPV and mephedrone.

Investigating interactions between phentermine, dexfenfluramine, and 5-HT2C agonists, on food intake in the rat

RATIONALE

Synergistic or supra-additive interactions between the anorectics (dex)fenfluramine and phentermine have been reported previously in the rat and in the clinic. Studies with 5-HT2C antagonists and 5-HT2C knockouts have demonstrated dexfenfluramine hypophagia in the rodent to be mediated by actions at the 5-HT2C receptor. Given the recent FDA approval of the selective 5-HT2C agonist lorcaserin (BELVIQ®) for weight management, we investigated the interaction between phentermine and 5-HT2C agonists on food intake.

OBJECTIVES

This study aims to confirm dexfenfluramine-phentermine (dex-phen) synergy in a rat food intake assay, to extend these findings to other 5-HT2C agonists, and to determine whether pharmacokinetic interactions could explain synergistic findings with particular drug combinations.

METHODS

Isobolographic analyses were performed in which phentermine was paired with either dexfenfluramine, the 5-HT2C agonist AR630, or the 5-HT2C agonist lorcaserin, and inhibition of food intake measured in the rat. Subsequent studies assessed these same phentermine-drug pair combinations spanning both the full effect range and a range of fixed ratio drug combinations. Satellite groups received single doses of each drug either alone or in combination with phentermine, and free brain concentrations were measured.

RESULTS

Dex-phen synergy was confirmed in the rat and extended to the 5-HT2C agonist AR630. In contrast, although some synergistic interactions between lorcaserin and phentermine were observed, these combinations were largely additive. Synergistic interactions between phentermine and dexfenfluramine or AR630 were accompanied by combination-induced increases in brain levels of phentermine.

CONCLUSIONS

Dex-phen synergy in the rat is caused by a pharmacokinetic interaction, resulting in increased central concentrations of phentermine.

Serotonergic and dopaminergic mechanisms in graft-induced dyskinesia in a rat model of Parkinson's disease

Dyskinesia seen in the off-state, referred as graft-induced dyskinesia (GID), has emerged as a serious complication induced by dopamine (DA) cell transplantation in parkinsonian patients. Although the mechanism underlying the appearance of GID is unknown, in a recent clinical study the partial 5-HT(1A) agonist buspirone was found to markedly reduce GID in three grafted patients, who showed significant serotonin (5-HT) hyperinnervation in the grafted striatum in positron emission tomography scanning (Politis et al., 2010, 2011). Prompted by these findings, this study was performed to investigate the involvement of serotonin neurons in the appearance of GID in the rat 6-hydroxydopamine model. L-DOPA-primed rats received transplants of DA neurons only, DA plus 5-HT neurons or 5-HT neurons only into the lesioned striatum. In DA cell-grafted rats, with or without 5-HT neurons, but not in 5-HT grafts, GID was observed consistently after administration of amphetamine (1.5mg/kg, i.p.) indicating that grafted DA neurons are required to induce GID. Strikingly, a low dose of buspirone produced a complete suppression of GID. In addition, activation of 5-HT(1A) and 5-HT(1B) receptors by 8-OH-DPAT and CP 94253, known to inhibit the activity of 5-HT neurons, significantly reduced GID, whereas induction of neurotransmitter release by fenfluramine administration significantly increased GID, indicating an involvement of the 5-HT system in the modulation of GID. To investigate the involvement of the host 5-HT system in GID, the endogenous 5-HT terminals were removed by intracerebral injection of 5,7-dihydroxytryptamine, but this treatment did not affect GID expression. However, 5-HT terminal destruction suppressed the anti-GID effect of 5-HT(1A) and 5-HT(1B) agonists, demonstrating that the 5-HT(1) agonist combination exerted its anti-GID effect through the activation of pre-synaptic host-derived receptors. By contrast, removal of the host 5-HT innervation or pre-treatment with a 5-HT(1A) antagonist did not abolish the anti-GID effect of buspirone, showing that its effect is independent from activation of either pre- or post-synaptic 5-HT(1A) receptors. Since buspirone is known to also act as a DA D(2) receptor antagonist, the selective D(2) receptor antagonist eticlopride was administered to test whether blockade of D(2) receptors could account for the anti-dyskinetic effect of buspirone. In fact, eticlopride produced complete suppression of GID in grafted animals already at very low dose. Together, these results point to a critical role of both 5-HT(1) and D(2) receptors in the modulation of GID, and suggest that 5-HT neurons exert a modulatory role in the development of this side effect of neuronal transplantation.

Nicotine-induced conditioned place preference in rats: sex differences and the role of mGluR5 receptors

To elucidate sex differences in nicotine addiction and the underlying mechanisms of the conditioning aspects of nicotine, nicotine-induced conditioned place preference (CPP) was evaluated in male and female Sprague Dawley rats using a three-chambered CPP apparatus and a biased design. In a series of experiments, the dose-response curve was obtained, pairings between the drug and initially non-preferred versus preferred compartments were compared, and the involvement of mGluR5 receptors in nicotine-induced CPP was evaluated. Modulation of nicotine-induced CPP with mGluR5 inhibition was obtained by MPEP (2-methyl-6-(phenylethynyl)-pyridine hydrochloride). Our results show that nicotine induces CPP dose-dependently in male rats but not in female rats. The comparison of the biased protocol, pairing nicotine with the initially preferred and non-preferred chambers, indicated that nicotine-induced CPP in male rats under both conditions, but the effect was stronger when nicotine was paired with the initially non-preferred side. The selective mGluR5 antagonist MPEP inhibited nicotine-induced CPP in male rats. In conclusion, the results of the current study in rats demonstrate that the conditioning effect of nicotine is more important in males than in females. Furthermore, in line with reported findings, our results suggest that mGluR5 antagonism may be therapeutically useful in smoking cessation during the maintenance of smoking behavior when conditioning plays an important role, notwithstanding the fact that this effect is observed only in male rats, not in females.

The effects of the 5-HT(2C) receptor antagonist SB242084 on locomotor activity induced by selective, or mixed, indirect serotonergic and dopaminergic agonists

RATIONALE

The 5-HT(2C) receptor modulates mesolimbic dopamine (DA) function and the expression of DA-dependent behaviors, including stimulant-induced hyperactivity. The 5-HT(2C) receptor may also be involved in drug-induced locomotion that is 5-HT-dependent.

OBJECTIVES

This study investigated the effects of the 5-HT(2C) receptor antagonist 6-chloro-5-methyl-1-[[2-[(2-methyl-3-pyridyl)oxy]-5-pyridyl]carbamoyl]-indoline (SB242084) on hyperlocomotion induced by psychomotor stimulants with selective, or mixed, actions on serotonergic and/or dopaminergic systems.

MATERIALS AND METHODS

Male Sprague-Dawley rats were treated in the presence or absence of SB242084 with releasers/reuptake inhibitors of DA (amphetamine and methylphenidate), 5-HT (fenfluramine and citalopram), or both 5-HT and DA (MDMA and cocaine). In addition, the effects of SB242084 combined with nicotine, morphine, or the 5-HT(1A/1B) receptor agonist RU24969 were examined. Locomotor activity was recorded for 2 h.

RESULTS

SB242084 potentiated hyperactivity induced by MDMA (2.5-5 mg/kg), amphetamine (0.5 mg/kg), fenfluramine (5 mg/kg), cocaine (10 mg/kg), and methylphenidate (5 mg/kg). SB242084 modestly potentiated nicotine-induced (0.2-0.4 mg/kg) and morphine-induced (2.5 mg/kg) hyperactivity. SB242084 failed to influence hyperactivity induced by RU24969 (0.5-1 mg/kg) or citalopram (10-20mg/kg).

CONCLUSION

SB242084 potentiated the locomotor stimulant effects of both indirect DA and 5-HT agonists. This potentiation may reflect two distinct mechanisms. The first involves direct enhancement of DA activity as shown by potentiation of the effects of amphetamine and methylphenidate. The second mechanism reflects an unmasking of stimulatory 5-HT receptors activated by 5-HT releasers (possibly 5-HT(1B/2A)) through blockade of inhibitory 5-HT(2C) receptors. The failure of SB242084 to potentiate the effect of citalopram might reflect differences between changes in synaptic levels of 5-HT produced by release compared to reuptake inhibition.

Effects of amphetamine-CNS depressant combinations and of other CNS stimulants in four-choice drug discriminations

Three pigeons were trained to discriminate among 5 mg/kg pentobarbital, 2 mg/kg amphetamine, a combination of these two drugs at these doses, and saline using a four-choice procedure (amphetamine-pentobarbital group). Three other pigeons were trained to discriminate among 5 mg/kg morphine, 2 mg/kg methamphetamine, a combination of these two drugs at these doses, and saline (methamphetamine-morphine group). After 10 to 13 months of training, the pigeons averaged more than 90% of their responses on the appropriate key during training sessions. In subsequent testing, dose-response curves were determined for the individual drugs, for a wide range of dose combinations of the training drugs, and for two drugs to which the pigeons had not been exposed previously (pseudoephedrine and nicotine). After low test doses of the training drugs, pigeons responded on the saline key. As the dose increased, responding on the key associated with that drug during training sessions increased. When training drugs were combined at doses that were not discriminable when given alone, responding occurred on the saline key. When a discriminable dose of one training drug was combined with a nondiscriminable dose of the other training drug, responding occurred on the key associated with the discriminable dose. When both drugs were given at discriminable doses, responding almost always occurred on the drug-combination key. The response-rate decreasing effects of pentobarbital and amphetamine were mutually antagonized when the drugs were combined, but the rate-decreasing effects of morphine and methamphetamine were not. After low doses of pseudoephedrine and nicotine, pigeons in both groups responded on the saline key. After higher doses of pseudoephedrine and nicotine, responding in the amphetamine-pentobarbital group occurred primarily on the amphetamine key. In the methamphetamine-morphine group, higher doses of pseudoephedrine and especially nicotine engendered more responding on the combination key than had occurred in the other group. The four-choice procedure can reveal subtle effects in the discrimination of individual drugs and drug combinations that are not apparent with procedures offering fewer response alternatives.

Effects of anorectic drugs on food intake under progressive-ratio and free-access conditions in rats

The effects of two anorectic drugs, dexfenfluramine and phentermine, on food intake under different food-access conditions were examined. Experiment 1 compared the effects of these drugs on food intake under a progressive-ratio (PR) schedule and free-access conditions. Dexfenfluramine decreased food intake under both conditions, but the doses required to decrease intake under free-access conditions were higher than those required to reduce intake under the PR condition. Intermediate doses of phentermine sometimes increased breaking points, and higher doses decreased them. Phentermine decreased food intake at the same doses under both access conditions. Thus the potency of dexfenfluramine, but not phentermine, to decrease food-maintained behavior depended upon the food-access condition. Experiment 2 used a novel mixed progressive-ratio schedule of food delivery to study the duration of drug effects. Sessions consisted of five components separated by 3-hr timeouts. The ratio requirement reset at the beginning of each component and a new breaking point was obtained. Both dexfenfluramine and phentermine dose-dependently decreased breaking points early in the session. In some rats, compensatory increases in breaking point were observed. That is, breaking points later in the session increased over control levels, resulting in no change in the total number of food pellets earned for the session compared to control. The present findings suggest that the effects of some anorectic drugs depend upon the access conditions for food; increasing the effort to obtain food may enhance their ability to decrease food-maintained behavior.

Blunted fenfluramine-evoked prolactin secretion in hypertensive rats

Plasma prolactin (PRL) levels after acute administration of fenfluramine (FEN) have been used as a probe of brain serotonin activity. FEN-evoked increases in PRL levels inversely correlate with arterial blood pressure (ABP) in humans (Muldoon et al. Hypertension. 1998;32:972-975), thereby suggesting that brain serotonin activity may be reduced in hypertension. The present study sought to determine whether the relation between FEN-evoked PRL levels and ABP was present in two rat models of hypertension. Experiments were performed in awake male rats that were instrumented with femoral arterial and venous catheters 2 days before experiments. FEN (3.0 mg/kg IV) significantly increased plasma PRL levels in both spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY); however, FEN-evoked PRL levels were significantly lower in SHR compared with WKY, though baseline levels were similar between strains. Similar results were obtained in rats with chronic hypertension produced by figure-8 renal wrap plus contralateral nephrectomy. In contrast, the increase in PRL levels evoked by the serotonin receptor agonist m-CPP or the dopamine receptor antagonist eticlopride did not differ between SHR and WKY, indicating that PRL secretion is not generally blunted in chronic hypertensive rats. Furthermore, FEN-evoked PRL levels were not attenuated in rats made acutely hypertensive by an infusion of the alpha-adrenergic agonist phenylephrine. Thus, the present findings are consistent with the human data and suggest that chronic hypertension is associated with a presynaptic alteration in brain serotonin function.

Monoamine transporters and psychostimulant drugs

Most psychostimulants interact with monoamine transport proteins. This paper reviews work our laboratory has conducted to investigate the interaction of psychostimulants with monoamine transporters in order to advance our understanding of how these drugs affect the brain. We review two topics: (1) characterization of multiple binding sites for cocaine-like drugs and (2) an examination of the mechanisms of action of amphetamine-type anorectic agents. We conclude that the brain contains high abundance nonclassical binding sites for cocaine-like drugs that have micromolar affinity for cocaine and that none of the clinically available amphetamine-type appetite suppressants are equipotent substrates for dopamine transporter (DAT) and serotonin transporter (SERT) proteins. Future medications discovery efforts should focus on identifying new compounds which possess the equipotent substrate activity at DAT and SERT, but which lack the adverse effects of stimulants developed decades ago.

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.

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.

Appetite suppressants as agonist substitution therapies for stimulant dependence

Several lines of evidence support a dual-deficit model of stimulant withdrawal in which decreases in synaptic dopamine (DA) and serotonin (5-HT) contribute to withdrawal symptoms, drug craving, and relapse. According to the dual-deficit model, DA dysfunction during withdrawal underlies anhedonia and psychomotor disturbances, whereas 5-HT dysfunction gives rise to depressed mood, obsessive thoughts, and lack of impulse control. The model suggests that medications capable of normalizing stimulant-induced DA and 5-HT deficits should be effective treatment adjuncts. Furthermore, the model may explain why medications targeting only one neurotransmitter system (i.e., DA) have failed to treat cocaine dependence. Amphetamine-type appetite suppressants are logical choices for neurochemical normalization therapy of stimulant dependence, yet few clinical studies have tested anorectics in this regard. The chief purpose of the present work is to profile the activity of various anorectic agents at DA, 5-HT, and NE transporters, in order to identify possible medications for stimulant dependence. Compounds were tested in vitro for their ability to stimulate release and inhibit uptake of [(3)H]DA, [(3)H]NE, and [(3)H]5-HT. Selected compounds were tested in vivo for their ability to elevate extracellular levels of DA and 5-HT in rat nucleus accumbens. The results show that clinically available appetite suppressants display a wide range of activities at monoamine transporters. However, no single medication possesses equal potency at DA and 5-HT transporters, suggesting that none of the anorectics is ideally suited for treatment of stimulant addictions. Future efforts should focus on developing new medications that possess the desired therapeutic activity but lack the adverse effects associated with older amphetamine-type anorectics.

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.

Additive effects on rat brain 5HT release of combining phentermine with dexfenfluramine

OBJECTIVE AND DESIGN

This study examined the effects of the anti-obesity agents, phentermine and dexfenfluramine given alone or in combination, on in vitro and in vivo 5HT release from rat brain tissue.

RESULTS

In vitro, phentermine was without effect on basal [3H]5HT efflux from hypothalamic slices whereas dexfenfluramine (10 microM) evoked a 131% increase in [3H]5HT release. In combination, the two drugs did not alter [3H]5HT release beyond that caused by dexfenfluramine alone. At pharmacologically equivalent doses, phentermine (5.7 mg/kg, i.p.) caused a rapid, modest elevation, and dexfenfluramine (3 mg/kg, i.p.) a larger but equally rapid elevation of extracellular 5HT in the microdialysates from the rat anterior hypothalamus. In combination, the increase in extracellular 5HT evoked by these drugs was not significantly greater than the sum of their individual effects.

CONCLUSIONS

This study provides evidence that phentermine's actions are not restricted to catecholamine systems and indicates that combining phentermine with dexfenfluramine results in an additive increase in neuronal 5HT release.

Discrimination of gamma-hydroxybutyrate and ethanol administered separately and as a mixture in rats

The physiological effects of gamma-hydroxybutyrate (GHB) are complex and not yet clearly defined. GHB has been labeled as a recreational drug and is reported to be frequently coabused with ethanol (ETH). Other studies have yielded discrepant results as to the interaction between GHB and ETH. Thus, the present study investigated extensively the discriminative stimulus of GHB and ETH and a mixture of the two compounds. Thirty male Long-Evans rats were divided into three groups and trained to discriminate doses of either 300 mg/kg GHB, 1000 mg/kg ETH, or a mixture (MIX: 150 mg/kg GHB+500 mg/kg ETH) from vehicle on a two-lever fixed-ratio (FR) 10 schedule of food reinforcement. Dose-response curves were attained in each group with its respective training drugs. GHB and ETH did not cross-generalize in the ETH- and GHB-trained rats, respectively. However, when the effects of the MIX were tested in the GHB- and ETH-trained rats, a greater than additive response was observed. Testing also revealed that the MIX-trained rats did not perceive a novel stimulus but a near-equal contribution from GHB and ETH. This study provides evidence of a complex relationship between GHB and ETH and opposes previous work reporting cross-generalization between GHB and ETH.

Comparison of the effects of sibutramine and other weight-modifying drugs on extracellular dopamine in the nucleus accumbens of freely moving rats

The acute effects of systemic administration of the anti-obesity agent sibutramine on extracellular dopamine (DA) in the nucleus accumbens of freely moving rats were studied using in vivo microdialysis and compared with the actions of phentermine and d-amphetamine at doses 1x and 3x their respective 2 h ED(50) values to reduce food intake in rats. At the lower dose, sibutramine did not elevate extracellular DA concentrations; however, at the higher dose (6.0 mg kg(-1), i.p.) it caused a modest and prolonged increase in extraneuronal DA. A maximal rise was observed at 60 min post-sibutramine treatment (+231% compared to controls) with DA levels remaining elevated for up to 160 min post treatment. In contrast, phentermine and d-amphetamine significantly enhanced DA efflux at both the lower and higher doses. These elevations of DA levels were significantly greater than that seen with the corresponding dose of sibutramine over 0-80 min post treatment. Maximal rises in DA levels resulting from the higher dose of each drug were +733% (phentermine, 3.9 mg kg(-1), i.p.) and +603% (d-amphetamine, 1.5 mg kg(-1), i.p.) compared to controls 40 min post treatment. The highest doses of phentermine and d-amphetamine increased rat locomotor activity up to 100 min and 160 min post treatment, respectively, whereas the equivalent sibutramine dose had no effect. These findings therefore suggest that dopaminergic reward mechanisms are not involved in the reduction of food intake by sibutramine. Furthermore, they are consistent with the view that sibutramine lacks abuse potential.

Effects of phentermine and fenfluramine on extracellular dopamine and serotonin in rat nucleus accumbens: therapeutic implications

Combined administration of the amphetamine analogs phentermine and fenfluramine (PHEN/FEN) has been used in the treatment of obesity. While these medications are thought to modulate monoamine transmission, the precise neurochemical effects of the PHEN/FEN mixture have not been extensively studied. To assess the mechanism of PHEN/FEN action, in vivo microdialysis studies were performed in the nucleus accumbens of conscious freely moving rats. A series of amphetamine derivatives including phentermine, chlorphentermine, fenfluramine, and PHEN/FEN (1:1 ratio), were infused locally into the accumbens via reverse-dialysis (1, 10, 100 microM) or injected systemically (1 mg/kg, ip). Dialysate samples were assayed for dopamine (DA) and serotonin (5-HT) by high-performance liquid chromatography with electrochemical detection. When infused locally, phentermine preferentially increased extracellular DA, whereas fenfluramine selectively increased extracellular 5-HT. Local administration of chlorphentermine or the PHEN/FEN mixture caused parallel elevations of both transmitters. Analogous results were obtained when the drugs were injected systemically. Phentermine stimulated robust locomotor activity in mice, whereas chlorphentermine and fenfluramine did not. PHEN/FEN caused modest locomotor stimulation after a low dose, but had no effect at the highest dose. Accumulating evidence suggests that chronic drug and alcohol abuse is associated with deficits in both DA and 5-HT neuronal function. Thus, dual activation of DA and 5-HT neurotransmission with monoamine releasing agents may be an effective treatment strategy for substance use disorders, as well as for obesity. Synapse 36:102-113, 2000. Published 2000 Wiley-Liss, Inc.

Supraadditive effect of d-fenfluramine plus phentermine on extracellular acetylcholine in the nucleus accumbens: possible mechanism for inhibition of excessive feeding and drug abuse

The combination of d-fenfluramine plus phentermine (d-FEN/PHEN) provides a tool for exploring neural mechanisms that control food intake and drug abuse. Prior research suggests that dopamine (DA) in the nucleus accumbens can reinforce appetitive behavior and acetylcholine (ACh) inhibits it. When rats were given d-fenfluramine (5 mg/kg, IP) DA increased to 169% (p < 0.01), and ACh decreased slightly. Phentermine (5 mg/kg, IP) increased extracellular DA to 469% of baseline and ACh increased slightly to 124% (both p < 0.01). The d-FEN/PHEN combination, however, increased both DA and ACh with a supraadditive effect on ACh to 172%. One interpretation is that dFEN/PHEN increases DA like a meal or drug of abuse, while also increasing ACh to stop further approach behavior. This leaves the animal "satiated," as defined by reduced intake of food or drugs.

Effects of phentermine and fenfluramine on alcohol consumption and alcohol withdrawal seizures in rats

The drug combination of phentermine plus fenfluramine has been used clinically in both the treatment of obesity and alcoholism. The aim of the current study was to assess the interaction of the two drugs on consumption of both an alcohol-containing and a nonalcoholic diet. Furthermore, the efficacy of the drug combination on suppression of withdrawal seizures was determined. Animals were either maintained on a 6% alcohol-containing diet, free-fed an isocaloric control, or pair-fed the control diet. It was observed that, with regard to body weight growth curves, alcohol provides about 2.5 kcal/g. Both phentermine and fenfluramine caused a decrease in consumption 1 h after administration; however, during the next 23 h, 4 mg/kg phentermine significantly increased consumption of all diets. At doses of 1 and 2 mg/kg, fenfluramine selectively reduced consumption of the alcohol-containing diet as compared to the isocaloric diets. Lower doses of fenfluramine blocked the increases in consumption induced by phentermine. Furthermore, in animals fed the nonalcoholic diet, the drug combination of 2 mg/kg fenfluramine plus 8 mg/kg phentermine produced a 63-82% reduction in consumption, an effect not seen when either drug was administered alone. This greater than additive effect was also seen in the earlier time periods in animals pair-fed the control diet. Neurochemical analysis from these animals revealed that the alcohol-dependent animals displayed a significant reduction of DOPAC and 5-HIAA levels in the striatum, frontal cortex, and hypothalamus after a 9-h withdrawal period, further implicating the serotonergic and dopaminergic systems in mediation of withdrawal symptoms and alcohol craving. Finally, 8 mg/kg phentermine plus 8 mg/kg fenfluramine completely abolished alcohol withdrawal seizures, compared to a 78% rate in saline treated rats. In conclusion, the coadministration of phentermine plus fenfluramine produced a moderate reduction of alcohol consumption and was completely effective at reducing alcohol withdrawal seizures.

Influence of training paradigm on specificity of drug mixture discriminations

Generalization to different drugs and drug mixtures has been examined in rats trained to discriminate a mixture of amphetamine (0.4 mg/kg) plus pentobarbitone (10 mg/kg) from saline (AND discrimination, n = 8) or to discriminate the same mixture from its component drugs alone (AND-OR discrimination, n = 9). The studies used two-lever operant procedures with a tandem variable interval 1-min fixed-ratio 10 schedule of food reinforcement. There was partial generalization to nicotine and midazolam and no generalization to cocaine, caffeine, or ethanol under AND-discrimination conditions and no generalization to any of these drugs in the AND-OR discrimination. Nicotine or midazolam coadministered with the training doses of pentobarbitone and amphetamine, respectively, produced full generalization in the AND discrimination and partial generalization under AND-OR conditions. Cocaine coadministered with pentobarbitone generalized fully under both procedures, but at larger doses in the AND-OR than in the AND discrimination. Mixtures of either nicotine plus midazolam or caffeine plus ethanol produced very marked generalization under AND-discrimination conditions, but were without significant effect in the AND-OR procedure. The results consistently supported the hypothesis that the AND-OR discrimination procedure increases the specificity of discriminations based on drug mixtures.

Drug mixtures and ethanol as compound internal stimuli

Drug discrimination methods that entail training with mixtures of drugs may shed light on polydrug abuse and on the actions of single drugs that interact with more than one receptor. In AND-discrimination procedures (drug A + drug B vs. vehicle), mixtures are discriminated primarily on the basis of their component drugs: these discriminations may be useful for testing interactions between component drugs in mixtures. The role of training dose, overshadowing and associative blocking in AND-discriminations have been investigated. For example, after prior training with midazolam, it was possible to demonstrate associative blocking of the nicotine element of the mixture stimulus, and vice versa. Using the AND-OR discriminations (drug A + drug B vs. drug A or drug B) increased pharmacological specificity considerably, and these procedures may be valuable for determining whether the effects of a novel mixture are similar to the combined effects of the training drugs. Ethanol is an example of a single drug that may produce a compound cue; rats trained to discriminate ethanol from water generalize (asymmetrically) to GABA(A) enhancers such as chlordiazepoxide (CDP) or pentobarbitone, to NMDA antagonists such as dizocilpine (MK-801), and to some serotonin agonists, such as trifluoromethylphenylpiperazine (5-HT(1B/2C)). In addition, rats trained to discriminate mixtures of either CDP or pentobarbitone plus MK-801 generalize to ethanol. A previous history of training with MK-801 or CDP (prior to ethanol discrimination training) enhanced the MK-801-like and CDP-like effects of ethanol respectively, but associative blocking of proposed elements in the ethanol stimulus was not seen. These studies provide some support for the multielement concept of ethanol discrimination but also suggest that rules governing three-component stimuli (such as those putatively produced by ethanol) may differ from those for the two-component mixtures of drugs studied previously.

Aminorex, fenfluramine, and chlorphentermine are serotonin transporter substrates. Implications for primary pulmonary hypertension

BACKGROUND

Coadministration of phentermine and fenfluramine (phen/fen) effectively treats obesity and possibly addictive disorders. The association of fenfluramine and certain other anorexic agents with serious side effects, such as cardiac valvulopathy and primary pulmonary hypertension (PPH), limits the clinical utility of these drugs. Development of new medications that produce neurochemical effects like phen/fen without causing unwanted side effects would be a significant therapeutic breakthrough.

METHODS AND RESULTS

We tested the hypothesis that fenfluramine (and other anorexic agents) might increase the risk of PPH through interactions with serotonin (5-HT) transporters. Because 5-HT transporter proteins in the lung and brain are identical, we examined, in rat brain, the effects of selected drugs on 5-HT efflux in vivo and monoamine transporters in vitro as a generalized index of transporter function. Our data show that drugs known or suspected to increase the risk of PPH (eg, aminorex, fenfluramine, and chlorphentermine) are 5-HT transporter substrates, whereas drugs that have not been shown to increase the risk of PPH are less potent in this regard.

CONCLUSIONS

We speculate that medications that are 5-HT transporter substrates get translocated into pulmonary cells where, depending on the degree of drug retention, their intrinsic drug toxicity, and individual susceptibility, PPH could develop as a response to high levels of these drugs or metabolites. Emerging evidence suggests that it is possible to develop transporter substrates devoid of adverse side effects. Such medications could have therapeutic application in the management of obesity, drug dependence, depression, and other disorders.

Anorectic efficacy of the fenfluramine/phentermine combination in rats: additivity or synergy?

Fenfluramine + phentermine was a widely used combination for weight loss. Fenfluramine and phentermine are believed to act via serotonin and catecholamines, respectively. To what extent these drugs interact has not been well-established. We compared the anorectic efficacy of a range of doses of the combination (using dexfenfluramine instead of fenfluramine) relative to a range of doses of the individual drugs in 90 min sweetened milk intake tests in deprived and nondeprived rats. Results were plotted on isobolograms to determine whether the anorectic effects of the combination were either additive or synergistic. Collectively, the isobolographic analysis revealed that: (1) under acute conditions, dexfenfluramine and phentermine interact for the most part in a synergistic manner, and (2) with the exception of phentermine alone, deprivation state at time of testing did not alter the efficacy of the anorectics. These findings suggest that combined drug treatment for obesity is a theoretical approach that merits further investigation.

Is phentermine an inhibitor of monoamine oxidase? A critical appraisal

Phentermine produces a spectrum of concentration-dependent biochemical effects. It interacts with NE transporters at 0.1 microM, DA transporters at about 1 microM, 5-HT transporters at 15 microM and MAO-A at about 100 microM. When administered at typical anorectic doses, phentermine primarily interacts with DA and NE transporters and does not produce biochemical or neurochemical effects which would occur if it were inhibiting MAO-A. Some other explanation other than MAO inhibition must be sought to explain how oral phentermine increases platelet 5-HT, since platelet MAO-B does not metabolize platelet 5-HT, and since amphetamine-type drugs are even weaker inhibitors of MAO-B than MAO-A. Clinical studies in humans have shown that amphetamine, which is a more potent inhibitor of MAO-A than phentermine, does not inhibit MAO-A at therapeutic doses. Neither phentermine alone, fluoxetine alone or their combined use have been associated with cardiac valvulopathy, and clinical experience has shown their combined use to be free of significant adverse effects. Viewed collectively, there appears to be no data to support the hypothesis that phentermine inhibits MAO at typical therapeutic doses.

Effects of fenfluramine and phentermine (fen-phen) on dopamine and serotonin release in rat striatum: in vivo microdialysis study in conscious animals

We measured the effects of acute or chronic administration of fenfluramine and phentermine, alone or in combination, on brain dopamine and serotonin release into striatal dialysates of freely moving rats. Samples collected every 30 min were assayed in a single run by high-pressure liquid chromatography. Acute or chronic administration of fenfluramine (1 mg/kg, i.p.) did not significantly change dopamine concentrations in rat striatal dialysates, but increased those of serotonin by 182% (acute) and 124% (chronic). Phentermine (2 mg/kg, i.p.), on the other hand, significantly increased dopamine concentrations by 52% (acute) and 80% (chronic) without affecting those of serotonin. Administration of the drugs in combination (fenfluramine 1 mg/kg and phentermine 2 mg/kg) amplified the effects of each, increasing striatal dopamine concentrations by 209% (acute) and serotonin concentrations by 330% (acute) and 299% (chronic).