Biological actions of drugs affecting serotonin and eating

The present status of knowledge on drugs affecting food intake and presumably acting via a serotoninergic mechanism is reviewed. The mechanism of action of these drugs is analyzed at the neurochemical level. All the drugs, to various extents, inhibit the uptake of serotonin (5HT), increase the release of 5HT and decrease brain levels of 5HT and 5HIAA. However, the underlying mechanisms are not identical as exemplified by comparisons made with d-fenfluramine, d-norfenfluramine, fluoxetine, sertraline and paroxetine. An analysis of the role of 5HT in the inhibition of food intake reveals that only d-fenfluramine is inhibited by antiserotonin agents. The role of the different 5HT receptor-subtypes in this antagonism is discussed. More selective 5HT antagonists are needed to establish which 5HT receptor(s) controls food intake.

d-fenfluramine, but not d-norfenfluramine, uses calcium to increase extracellular serotonin

Microdialysis in the hippocampus of freely moving rats was used to assess extracellular serotonin (5-HT) in response to local infusion of d-fenfluramine and its metabolite d-norfenfluramine with and without local calcium depletion. Verapamil (1 mM) in calcium-free Ringer infused via the microdialysis probe increased extracellular 5-HT and prevented the full increase in extracellular 5-HT normally caused by 1 mM d-fenfluramine. The results suggest d-fenfluramine might act in part as a calcium channel agonist favoring a calcium influx that in turn would trigger the exocytotic process in 5-HT terminals. d-norfenfluramine, on the other hand, was capable of releasing 5-HT, in vivo, in spite of depleted Ca levels.

Effects of serotoninergic agents on downregulation of beta-adrenoceptors by the selective serotonin reuptake inhibitor sertraline

The results of the present study show that the down-regulation of beta-adrenoceptors of rat brain, induced by subacute administration of sertraline, is facilitated when this selective serotonin reuptake inhibitor was co-administered with the serotonin releaser, norfenfluramine, or the serotonin terminal autoreceptor antagonist, methiothepin. The respective drug combination produced a reduction in Bmax of [3H]dihydroalprenolol binding to cortical membranes of treated rats at a dose of the releaser, release enhancer, or sertraline, which was ineffective when administered alone. In a similar manner, the 5-HT1A agonists, gepirone and 8-OH-DPAT, were found to facilitate the downregulation of beta-adrenoceptors induced by sertraline. The 5-HT1B agonist, 3-trifluoromethylphenylpiperazine, and the 5-HT2 antagonist, ritanserin, showed neither facilitation nor antagonism of sertraline, but the 5-HT3 antagonist, ondansetron, attenuated the decrease of Bmax of [3H]dihydroalprenolol binding elicited by sertraline. Agents that putatively increase the serotoninergic activity facilitated the down-regulation of beta-adrenoceptors induced by sertraline, suggesting that the enhancement of serotonin transmission, expected of the selective serotonin reuptake inhibitor itself, may play a role in this effect of sertraline. Whether the downregulation of brain beta-adrenoceptors by sertraline plays any role in its antidepressant activity cannot be deduced from these experiments.

d-Fenfluramine and d-norfenfluramine hypophagias do not require increased hypothalamic 5-hydroxytryptamine release

d-Fenfluramine (2.5 mg/kg i.p.) caused marked hypophagia in food-deprived rats and significantly increased medial hypothalamic extracellular 5-hydroxytryptamine (5-HT) as indicated by in vivo microdialysis. When the drug was given after the 5-HT synthesis inhibitor p-chlorophenylalanine (150 mg/kg per day x 3) the hypophagic response was unimpaired but dialysate 5-HT concentration no longer rose. The d-fenfluramine metabolite d-norfenfluramine (1.5 mg/kg i.p.) caused slightly greater hypophagia than the parent drug and completely blocked feeding in animals pretreated with p-chlorophenylalanine, but dialysate 5-HT was increased in neither circumstance. The results provide evidence against mediation of the hypophagic effects of d-fenfluramine and d-norfenfluramine by increased availability of 5-HT to receptors.

Long-term administration of dexfenfluramine to genetically obese (ob/ob) and lean mice: body weight and brain serotonin changes

The weight-reducing and brain 5-HT-depleting properties of dexfenfluramine (DFEN) or dexnorfenfluramine (DNOR) were measured in genetically obese (ob/ob) and lean mice. These agents were infused for 14 days via osmotic minipumps to mice fed either a low fat or a moderate fat diet. Weight loss was observed in only the obese mice, with DNOR more potent than DFEN. At the end of 14 days, neither agent caused a consistent change in either plasma glucose or corticosterone concentrations, although some effects of diet and differences between batches of mice were apparent. The levels of brain 5-HT, or of paroxetine binding that correlates with 5-HT level, were reduced by 24 mg DFEN/kg/day, a decline that persisted for at least 14 days after the end of treatment. Plasma and brain concentrations of DFEN and DNOR were measured on the last day of pump function. DNOR accounted for about 30% of the total drug + metabolite content, a ratio comparable to that in human plasma. Brain concentrations exceeded plasma by 10-fold at 6 mg DFEN/kg/day and by 17-fold at 24 mg DFEN/kg/day. The levels were higher in mice fed the moderate-fat compared with the low-fat diet. Depletions in brain 5-HT parameters were found only in the high-dose groups, and at brain total levels above about 20 microM.

Serotonergic facilitation of acetylcholine release in vivo from rat dorsal hippocampus via serotonin 5-HT3 receptors

The serotonin (5-HT) releaser d-fenfluramine and its active metabolite d-norfenfluramine, or the 5-HT-uptake inhibitor citalopram, by increasing synaptic 5-HT availability, facilitated in vivo release of acetylcholine (ACh) from dorsal hippocampi of freely moving rats as determined by the microdialysis technique. The effects of d-norfenfluramine (7.5 mg/kg i.p.) and citalopram (10 microM, applied by reverse dialysis) were prevented by a 14-day chemical lesion of the raphe nuclei, suggesting mediation by the 5-HT system in the cholinergic action of the drugs. The increase in extracellular ACh content induced by d-norfenfluramine (5 mg/kg i.p.) was antagonized by the 5-HT3 receptor antagonists tropisetron (0.5 mg/kg i.p.) and DAU 6215 (60 micrograms/kg i.p.), but not by the mixed 5-HT1 and 5-HT2 receptor antagonist metergoline (2 mg/kg s.c.). In accordance with an involvement of the 5-HT3 receptor in the ACh facilitation induced by d-norfenfluramine is the finding that the selective 5-HT3 receptor agonist 2-methylserotonin (250 micrograms i.c.v., or 10 microM applied by reverse dialysis) raised ACh release. The effect of the intracerebroventricular drug was prevented by the 5-HT3 antagonists DAU 6215 (60 micrograms/kg i.p.) and ondansetron (60 micrograms/kg s.c.). These antagonists by themselves did not modify the basal ACh release, indicating that 5-HT does not tonically activate the 5-HT3 receptors involved. In conclusion, the overall regulatory control exerted by 5-HT in vivo is to facilitate hippocampal ACh release. This is mediated by 5-HT3 receptors probably located in the dorsal hippocampi.

Pharmacological evaluation of 2-amino-6(7)- and 9-amino-6-trifluoromethylbenzonorbornenes, the conformationally rigid analogues of norfenfluramine in mice

1. The possible role of conformational requirements which fenfluramine and norfenfluramine must satisfy to elicit its observed pharmacological activities was investigated in mice with the use of four conformationally-rigid norfenfluramine analogues. 2. In this study, both the syn-9-amino and endo-2-amino isomers, which structurally resemble the gauche conformation of norfenfluramine, were found to have little or no effect on spontaneous locomotor activity. 3. On the other hand, the isomers (i.e. the anti-9-amino and exo-2-amino isomers) that mimic the anti conformation of norfenfluramine were capable of causing a decrease in spontaneous motor activity similar to that of norfenfluramine. 4. The analgesic activities of these rigid analogues were also assessed and all of the isomers were found to be weakly analgesic. 5. Only the exo-2-amino isomer exhibited analgesic potency similar to that of fenfluramine. Furthermore all of these compounds were capable of enhancing the analgesic activity of morphine.

Effects of serotonin and the serotonin blocker metergoline on meal patterns and macronutrient selection

Serotonin [5-hydroxytryptamine(5-HT)] in the paraventricular nucleus (PVN) of rats has a suppressive effect on feeding behavior and causes a selective decrease in carbohydrate ingestion, specifically at the onset of the natural (dark) feeding period. Studies conducted here provide further evidence for this phenomena, showing a similar dose-related decrease in carbohydrate ingestion at dark onset after PVN injection of 5-HT or of the agonists, d-norfenfluramine or fluoxetine, which act through endogenous 5-HT. To further characterize the effects of this indoleamine on the macrostructure of feeding, a computer-automated data acquisition system was used to analyze macronutrient feeding patterns in freely feeding animals maintained on the pure diets of protein, carbohydrate, and fat. Results indicate that PVN administration of 5-HT at dark onset decreases intake of the carbohydrate nutrient by decreasing meal size, feeding time, and feeding rate for this nutrient and increasing the satiating effect of carbohydrate. These effects, which occur specifically during the first meal after injection, are opposite those seen after peripheral administration of the 5-HT receptor antagonist, metergoline. This drug stimulates feeding through a selective increase in carbohydrate intake, characterized by an increase in meal size, percent composition, and feeding time for this nutrient and a decrease in the satiety ratio for carbohydrate. These results implicate the serotonergic system in the termination of carbohydrate-rich meals that are prevalent during the early hours of the natural feeding cycle.

Metabolic and hormonal responses to hypothalamic administration of norfenfluramine in rats

The effects of intrahypothalamic administration of norfenfluramine (NFFL), an anorectic agent that increases serotonergic transmission, on plasma concentrations of glucose, free fatty acids (FFA), and their regulating hormones were investigated in resting and exercising rats. Infusion of 5 micrograms NFFL in 0.125 microliter aCSF/min into the nucleus paraventricularis of the hypothalamus (PVN) caused a significant increase of blood glucose, plasma epinephrine (E), and corticosterone concentrations. Plasma levels of FFA, insulin, or norepinephrine (NE) remained unchanged. Lower doses of NFFL (0.5 and 0.05 microgram/min) did not affect peripheral metabolism. The effects of NFFL in the PVN were completely prevented by prior administration of a 5-HT1 antagonist, (S)-(-)propranolol. The exercise-induced increase of plasma NE was reduced after prior administration of 5 micrograms NFFL/min into the PVN. Plasma E responses tended to be increased. The exercise-induced alterations in glucose, FFA, corticosterone, and insulin were not affected by NFFL infusion into the PVN. The data suggest that activation of serotonergic mechanisms in the PVN might change the neurohormonal response to a stressor favouring the release of adrenal hormones above activation of the neuronal branch of the sympathetic nervous system.

The role of d-norfenfluramine in the indole-depleting effect of d-fenfluramine in the rat

The importance of d-norfenfluramine in regard to the indole-depleting action of d-fenfluramine has not been well studied in sensitive animal species. The present study therefore examined the intensity and time course of the neurochemical effects of i.p. injected d-fenfluramine (2.5 and 5 mg/kg) and d-norfenfluramine (2.5 mg/kg) in vehicle- and SKF-525A-pretreated rats, relating the effects to the brain concentration-time profiles of the drug and its active metabolite. At the lower dose d-fenfluramine caused only a small, short-lasting decrease in brain serotonin (5-HT) without affecting the 5-hydroxyindoleacetic acid (5-HIAA). Higher doses affected both 5-HT and 5-HIAA (50-60 and 30-40% reductions, respectively), the effect being maximal for at least 8 h. d-Norfenfluramine reduced the brain content of 5-HT and 5-HIAA less (by about 30%) than 5 mg/kg d-fenfluramine did. Brain concentrations of d-norfenfluramine at the time of the maximal depletion of indoles were close to those of the metabolite after 5 mg/kg d-fenfluramine, indicating that the acute indole-depleting effects did not depend solely on the brain concentrations of its nor-metabolite. SKF-525A changed the metabolite-to-parent drug ratios in brain without appreciably influencing the action of d-fenfluramine. However, the maximum decrease in indole content caused by 2.5 mg/kg d-fenfluramine in SKF-525A-pretreated rats was only 12% of the control level, although the brain concentration of unchanged drug was comparable to that after 5 mg/kg d-fenfluramine in vehicle-pretreated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Anorectic effect and brain concentrations of D-fenfluramine in the marmoset: relationship to the in vivo and in vitro effects on serotonergic mechanisms

The present study investigated the anorectic activity of d-fenfluramine (d-F) and the relationship with brain levels of unchanged drug and its metabolite d-norfenfluramine (d-NF) in marmosets, relating them to neurochemical effects on the serotoninergic system. d-F and d-NF were equally active in reducing food intake (ED50 about 3 mg/kg, p.o.). However, the brain concentrations of the metabolite required to reduce food intake after synthetic d-NF were more than twice those after d-F, indicating that d-NF contributes to but does not completely explain the anorectic effect of d-F. At this dose d-F did not appreciably modify the serotonin (5-HT) and 5-hydroxyindoleacetic (5-HIAA) contents of the brain regions examined, except for a slight enhancement of 5-HIAA in hippocampus. In vitro in brain cortical synaptosomes d-F inhibited [3H]5-HT uptake more potently than d-NF, as in other species. d-F and d-NF showed similar potency in stimulating [3H]5-HT release, in a Ca++ dependent manner. The tritium released by d-F and d-NF appeared to be mainly unmetabolized [3H]5-HT. Like in other species the marmoset too has saturable and specific [3H]d-F binding sites, for which d-NF has lower affinity. d-F and d-NF have low affinities for 5-HT receptor subtypes, except that d-NF has appreciable affinity for 5-HT1C and 5-HT1D receptors. Unlike in rodents but similarly to primates in the striatum the pharmacology of 5-HT receptors seems to correspond to the 5-HT1D subtype.(ABSTRACT TRUNCATED AT 250 WORDS)

Suppression of behavioral activity by norfenfluramine and related drugs in rats is not mediated by serotonin release

Fenfluramine, a phenalkylamine with serotonin (5-HT) releasing properties, decreases motor activity in rats. The following studies assessed the contribution of 5-HT release to the behavioral effects of fenfluramine and norfenfluramine using a behavioral pattern monitor that simultaneously assesses locomotor and investigatory behavior. First, both fenfluramine and its active metabolite d-norfenfluramine dose-dependently reduced locomotor and investigatory activity. The norfenfluramine-induced reduction in activity was not antagonized by pretreatment with the 5-HT uptake inhibitor fluoxetine or the 5-HT synthesis inhibitor p-chlorophenylalanine, drugs that reduce drug-induced 5-HT release. Second, the d- and l-enantiomers of norfenfluramine were nearly equipotent at reducing behavioral activity, although d-norfenfluramine is more potent as a 5-HT releasing agent. Third, p-chloroamphetamine, a drug that shares the 5-HT releasing properties of fenfluramine produced locomotor hyperactivity in the same paradigm. Previous studies indicate that other 5-HT releasing phenalkylamines have behavioral effects resembling those of p-chloroamphetamine rather than those of fenfluramine. Finally, a structurally related drug, 4-methoxy-5-methyl-aminoindan (MMAI), produced dose-dependent reductions in behavioral activity that are similar to the effects of fenfluramine. The behavioral effects of MMAI were not antagonized by fluoxetine or by the 5-HT receptor antagonist methiothepin. These data suggest that the decrease in activity induced by fenfluramine, norfenfluramine and the related drug MMAI is not related to 5-HT release.

Serotonin uptake inhibition: in vivo effect of sertraline in rats

Sertraline, a potent and selective serotonin uptake inhibitor, was used to analyze the changes occurring in the serotonin system after uptake inhibition in vivo. Sertraline (11 mg/kg) lowered extracellular 5-hydroxyindolacetic acid (5-HIAA), measured in rat hippocampus by in vivo voltammetry, for about 3 h. The interaction between sertraline and drugs known to interfere with the release or uptake of serotonin (L-5-hydroxytryptophan (5-HTP), d-norfenfluramine and tianeptine) was then studied. The sertraline-induced decrease in extracellular 5-HIAA was related to the inhibition of uptake.

In vivo studies on the enhancement of serotonin reuptake by tianeptine

The present study investigates the in vivo effects of the serotonin uptake enhancer tianeptine. The serotonin metabolite, 5-hydroxy-indolacetic acid (5-HIAA) was measured by in vivo voltammetry and carbon fiber electrodes chronically implanted in different brain areas of freely moving rats. Tianeptine (10 mg/kg i.p.) increased extracellular 5-HIAA in the hippocampus and hypothalamus. The interaction between tianeptine and drugs known to interfere with the uptake or release of serotonin (sertraline, buspirone, D-norfenfluramine) was then studied and, to ascertain the in vivo pharmacological relevance of tianeptine's effects, its ability to reduce the serotoninergic syndrome was evaluated. Both the biochemical and behavioral data indicate that in vivo tianeptine's effects on the serotoninergic system are likely to be due to serotonin uptake enhancement.

Response of extrapyramidal and limbic neuropeptides to fenfluramine administration: comparison with methamphetamine

The responses of extrapyramidal and limbic neuropeptide and striatal dopamine and serotonin systems were evaluated after treatment with fenfluramine in rats. After multiple administrations of fenfluramine, its active metabolite, norfenfluramine, and methamphetamine (METH), striatal neurotensin (NT) content was similarly increased to approximately 200% of control. In contrast, nigral NT levels were unaltered by fenfluramine, intermediately increased by norfenfluramine (148% of control) and maximally increased by METH (267% of control). Striatal and nigral substance P (SP) and dynorphin A (Dyn) systems were unaltered by fenfluramine, whereas norfenfluramine caused an intermediate increase in striatal Dyn content but did not significantly alter striatal SP or nigral SP and Dyn levels. However, METH significantly elevated striatal and nigral Dyn and SP concentrations to 280 to 425% (Dyn) and 140% (SP) of control. For the most part, the response of the limbic peptides was similar to that seen in the striatum with a couple of notable differences. Further investigation of the striatal NT system showed that the increases induced by fenfluramine were completely blocked by the D1 antagonist, SCH 23390, and the noncompetitive N-methyl-D-aspartate antagonist, MK801. Depletion of 5-hydroxytryptamine with pretreatment by parachloroamphetamine did not alter the response of the striatal NT system to fenfluramine. The present results demonstrate common and unique features in the response of peptide systems to fenfluramine and methamphetamine, which might explain some of the similarities and differences between these two drugs.

Comparison of the effects of repeated oral versus subcutaneous fenfluramine administration on rat brain monoamine neurons: pharmacokinetic and dose-response data

The importance of the route of drug administration (oral vs. subcutaneous) on the neurochemical effects and pharmacokinetics of repeated d,1-fenfluramine administration in rats (1-24 mg/kg b.i.d., i.e., 2-48 mg/kg/day for 4 days) was examined. Overall, comparable dose-dependent alterations in brain monoamine markers were observed following repeated oral (PO) and subcutaneous (SC) administration of fenfluramine. Doses of 1 and 2 mg/kg fenfluramine were without significant effects on the density of 3H-paroxetine-labeled serotonin (5-HT) uptake sites. Higher doses of fenfluramine (4, 12 and 24 mg/kg) produced dose-dependent decreases in 5-HT, 5-hydroxyindoleacetic acid and 5-HT uptake sites with maximal decreases (80-90%) occurring at the 12 mg/kg dose. Fenfluramine administration produced dose-dependent and biphasic effects on brain dopamine markers with increases in homovanillic acid (HVA) observed at 2 hours, whereas decreases in the levels of dopamine, HVA and dihydroxyphenylacetic acid were evident at 18 hours posttreatment. Norepinephrine levels were only decreased at the highest dose of fenfluramine. Significantly higher levels of brain fenfluramine were observed following SC than following PO administration of the drug. On the other hand, comparable levels of its active metabolite norfenfluramine were present in the brain following the two routes of fenfluramine administration. These data suggest the importance of norfenfluramine levels in the brain in determining the high-dose neurotoxic effects of fenfluramine on brain 5-HT neurons in rats.

Comparative studies on the anorectic activity of d-fenfluramine in mice, rats, and guinea pigs

The present study compares the anorectic activity of d-fenfluramine and its metabolite d-norfenfluramine in three animal species. d-Fenfluramine and d-norfenfluramine show anorectic activity at increasing doses (ED50) in rats, guinea pigs, and mice, d-norfenfluramine being more active than d-fenfluramine in all three species. Equiactive anorectic activities are reached with different brain levels of d-fenfluramine and d-norfenfluramine, guinea pigs being the most sensitive species, followed by rats then mice. The metabolite most probably plays a major role in the anorectic effect of d-fenfluramine in guinea pigs, contributes to the anorectic activity in rats, but adds little to the action of the parent drug in mice. The different sensitivity to d-fenfluramine and d-norfenfluramine in these three species does not appear to be explained by a number of biochemical parameters, including serotonin uptake or release, receptor subtypes, or 3H-d-fenfluramine binding and uptake.

Effects of intracerebroventricular administration of d-fenfluramine and d-norfenfluramine, as a single injection or 2-hr infusion, on serotonin in brain: relationship to concentrations of drugs in brain

The effects of d-fenfluramine (DF) and d-norfenfluramine (DNF), administered intracerebroventricularly (i.c.v.) on levels of serotonin (5-HT) in the brain, was assessed in relation to levels of drugs in brain. d-Fenfluramine, as a single injection (500 micrograms/20 microliters), caused no significant changes in 5-HT in whole brain from 15 to 480 min after injection. When infused intraventricularly for 2 hr, DF and DNF at 500 but not at 125 250 micrograms/hr, markedly reduced concentrations of 5-HT in brain 4 hr after the end of the infusion. At this time levels of DNF in brain were similar (between 4 and 5 micrograms/g) with both compounds, whereas levels of DNF after single intraventricular injections of DF were below 2 micrograms/g at all times after injection. Infusion of 500 micrograms/hr of DNF for 2-hr reduced concentrations of 5-HT in various regions of the brain, with the exception of the brainstem, whereas 250 micrograms/hr of DNF significantly lowered levels of 5-HT only in the cortex. The effect of infusion of 500 micrograms/hr of DNF was specific for 5-HT (no effect on dopamine and norepinephrine) and lasted for at least 168 hr. The results suggest that the effect on 5-HT in brain of intraventricular infusion of DF, but not a single injection, was due to the fact that, only in the former condition were adequate levels of DNF, the active metabolite of DF, reached in the brain. These results are relevant to the interpretation of studies in which biochemical changes in the brain after intraventricular administration, are reported without any measurement of the drug or its active metabolites, in plasma and brain.

Impact of hypothalamic d-norfenfluramine and peripheral d-fenfluramine injection on macronutrient intake in the rat

Previous research with hypothalamic injection of serotonin (5-HT) has suggested that this monoamine may act within the medial hypothalamus to suppress carbohydrate intake in a selective, phasic and circadian-related fashion. To explore further the action of 5-HT in the brain, the present studies tested the serotonergic stimulants, d-norfenfluramine (DNF) and d-fenfluramine (DF), in freely feeding, brain-cannulated animals maintained on pure macronutrient diets (protein, carbohydrate and fat) and tested at different times of the diurnal cycle. The results show that administration of DNF into the paraventricular nucleus (PVN) potently influences appetite for a specific nutrient at a particular time of the light-dark cycle. Specifically, DNF injection at the onset of the nocturnal (active) period selectively and dose-dependently suppresses carbohydrate consumption, while leaving protein and fat intake unchanged. This drug, however, has no effect, even at high doses, on macronutrient intake in the middle and late h of the dark phase, strongly implicating a function for hypothalamic 5-HT in the control of carbohydrate ingestion at the beginning of the nocturnal cycle. The possibility that peripherally injected DF may act, in part, through this endogenous serotonergic system is supported by the additional finding that, at low doses of 0.06-0.5 mg/kg, DF preferentially modulates carbohydrate ingestion exclusively at the onset of the nocturnal period. However, at doses above 0.5 mg/kg, this compound produces a potent and general suppression of feeding of all macronutrients. In animals with brain cannulas aimed at different hypothalamic nuclei, the feeding-suppressive effect of DNF is found to be site specific; it is localized to the medial hypothalamic nuclei, including the ventromedial, suprachiasmatic and dorsomedial nuclei as well as the PVN. Serotonin in these nuclei may function to produce satiety specific for carbohydrate and, through the suprachiasmatic nucleus, control energy intake in a circadian-related manner.

Influence of fenfluramine and norfenfluramine stereoisomers on the firing rate of central monoaminergic neurons in the rat

The influence of acute administration of stereoisomers of fenfluramine and norfenfluramine on the firing rate of central monoaminergic neurons was investigated in rats anaesthetized with chloral hydrate. The firing rate of dorsal raphe (DR) and locus coeruleus (LC) neurons was inhibited. The parent drugs were more active on DR neurons than on LC neurons, and the converse was true for the demethylated metabolites. In both cases the d isomers were more active than the l isomers. No effect was observed on the electrical activity of A10 dopaminergic neurons. These differences in potency and selectivity could have therapeutic implications.

Temporal differences in behavioral effect of fenfluramine and norfenfluramine

Ten male rats were trained to discriminate the anorectic drug d,l-fenfluramine (2.0 mg/kg intraperitoneally administered) from its vehicle using a food-reinforced (fixed-ratio 10 schedule) two-lever operant task. Once learned, the fenfluramine stimulus was dose-dependent (ED50 = 0.8 mg/kg) and stereoselective with the d-isomer (ED50 = 0.6 mg/kg) approximate twice as potent as the l-isomer (ED50 = 1.2 mg/kg). Time-course data indicate that the fenfluramine metabolite norfenfluramine produces a significantly faster onset and longer duration of action than does the parent compound. The results suggest that both stereoisomers of fenfluramine have discriminative stimulus properties and that the fenfluramine metabolite, norfenfluramine, contributes to the discriminative stimulus properties of the parent drug.

Increased drug sensitivity in the drug discrimination procedure afforded by drug versus drug training

Rats were trained to discriminate norfenfluramine (NF) 1.4 mg/kg from its vehicle or amphetamine (AMPH) 0.8 mg/kg or pentobarbital (PB) 6.0 mg/kg in order to determine the role that drug combination training plays in the rate of learning and sensitivity to lower drug doses. The results suggest that drug versus drug training can increase the rate of drug discrimination learning for some drugs that are learned slowly when trained in a drug versus vehicle training procedure, whereas drug versus drug training does not increase the rate of learning for other drugs that are learned rapidly. Drug versus drug training does, however, appear to increase the level of stimulus control of the training drug for all drugs examined in this study.

Comparison of anorectic drugs in rats trained to discriminate between satiation and deprivation

Eight male rats were trained to discriminate between the internal states produced by food deprivation of 3 hours (satiation) and that produced by food deprivation of 27 hours duration (deprivation). One lever, in a two-lever operant chamber, had to be pressed to receive reinforcement in the satiation state, whereas pressing the other lever was required when the rat was in the deprivation state. Once the rats were trained, increasing the number of hours of food deprivation, from 1 to 48 hours, resulted in more deprivation-appropriate lever responses in the two-lever operant task. Administration of doses of fenfluramine (0.5-1.5 mg.kg), its active metabolite norfenfluramine (0.25-1.0 mg/kg) or d-amphetamine (0.5-1.5 mg/kg) produced a dose-responsive decrease in deprivation-appropriate responses when each drug/dose was injected (i.p.) 15 min prior to deprivation (27 hours) testing. Norfenfluramine was 1.5 times more potent than fenfluramine which was 1.5 times more potent than amphetamine.

Effects of the l isomer of fenfluramine on dopamine mechanisms in rat brain: further studies

Experiments were carried out to gain additional evidence that l-fenfluramine reduces the dopamine-mediated effects in intact animals. l-Fenfluramine 5 and 10 mg/kg i.p. dose dependently raised the levels of homovanillic acid in the striatum and nucleus accumbens of rats 1 h after injection. The effect of 5 mg/kg l-fenfluramine disappeared and was actually reversed 4 and 8 h after injection. The effect of 10 mg/kg l-fenfluramine, administered 48 h after the last haloperidol dose, was completely antagonized in both striatum and nucleus accumbens of animals made tolerant to the effect of haloperidol on homovanillic acid levels (through repeated treatment with 1 mg/kg haloperidol i.p. twice daily for 11 days). Unlike haloperidol (0.25 mg/kg), l-fenfluramine in various doses (2.5-20 mg/kg i.p.) did not modify the levels of striatal 3-methoxytyramine or change the decrease induced by a s.c. injection of 0.5 mg/kg apomorphine. The effect of apomorphine was not antagonized by 10 or 20 mg/kg l-norfenfluramine, an active metabolite of l-fenfluramine but 20 mg/kg l-norfenfluramine significantly raised striatal 3-methoxytyramine levels. l-Fenfluramine 20 mg/kg (but not 10 mg/kg) significantly enhanced the output of striatal acetylcholine assessed by trans-striatal microdialysis, for 60 min after injection. Apomorphine 1 mg/kg i.p. completely antagonized the increase of acetylcholine caused by 1 mg/kg haloperidol or 20 mg/kg l-fenfluramine. The results confirm that the l isomer of fenfluramine produces effects on the responses to dopamine and acetylcholine similar to those of neuroleptics by a mechanism not involving direct blockade of receptors.

Norfenfluramine, the fenfluramine metabolite, provides stimulus control: evidence for serotonergic mediation

Nine male rats were trained to discriminate 1.4 mg/kg norfenfluramine (NF) from its vehicle using a two-lever, food-motivated, operant discrimination task. Once trained, the rats showed a dose-dependent decrease in responding on the NF-correct lever following decreased doses of NF (ED50 = 0.71 mg/kg). Administration of 2.0 mg/kg fenfluramine (FEN) produced 100% responding on the NF-correct lever and decreasing doses of FEN, likewise, produced a dose-dependent decrease in responding on the NF-correct lever (ED50 = 1.30 mg/kg). Time-course data indicated that NF has a fast onset and a peak effect at 20-60 min after administration. Analysis of the time-course data provided a half-life of approximately 8 hr. In contrast, FEN did not show the rapid onset that was observed with NF. However, NF had a similar peak effect and half-life. These results indicate a pharmacological similarity between NF and FEN. However, the difference in onset of action suggests a possible difference between the parent drug and its metabolite. The serotonergic agonists mCPP, DOI, 5-MeODMT and LSD generalized to 1.4 mg/kg NF, whereas neither TFMPP nor 8-OHDPAT generalized to NF. The dopaminergic agonist AMPH also did not generalize to NF. The implications of these findings are discussed.