Biochemical and behavioral antagonism between fenfluramine and apomorphine in rats

Central 5-HT(4) receptors and dopamine-dependent motor behaviors: searching for a functional role

In this study, we evaluated the role of central 5-HT(4) receptors in the control of motor behaviors related to change of nigrostriatal dopamine (DA) transmission, namely, stereotyped behavior and catalepsy in rats. Indeed, given that 5-HT(4) receptors indirectly modulate nigrostriatal DA neuron activity, we hypothesized that these receptors would regulate nigrostriatal DA transmission in the basal ganglia, and consequently, associated motor responses. Stereotypy was induced either by an acute administration of apomorphine (0.3 and 1.5 mg/kg sc), or by a single morphine administration (15 mg/kg sc) in chronically morphine-treated (15 mg/kg sc, twice daily for 10 days) rats. Catalepsy was induced by the typical neuroleptic haloperidol (HAL; 1 mg/kg sc). The selective 5-HT(4) antagonist, GR 125487 (1 mg/kg ip), modified neither apomorphine- nor morphine-induced stereotypy. HAL-induced catalepsy, while reduced by the systemic administration of the 5-HT(1A) agonist 8-OH-DPAT (0.1 mg/kg sc), was insensitive to GR 125487, systemically (1, 3, 10 mg/kg ip) or locally (20 and 40 nmol/20 microl) administered into the third ventricle. Also, HAL-induced catalepsy was not affected by the selective 5-HT(4) antagonist GR 113808 (3 mg/kg ip). The obtained results indicate that 5-HT(4) receptor antagonism does not modulate motor behaviors related to change of striatal DA transmission.

Neuroleptic-like effects of the l-isomer of fenfluramine on striatal dopamine release in freely moving rats

l-Fenfluramine (l-F) was studied for its ability to release dopamine (DA) and its metabolites in freely moving rats through the trans-striatal dialysis technique. l-F's effect on striatal DA release was also studied in animals made tolerant to the effect of haloperidol by chronic treatment (1 mg/kg i.p. twice daily for 11 days and 48 hr wash-out) with the neuroleptic or pretreated with 300 mg/kg i.p. gamma-butyrolactone (GBL). Five and 10 mg/kg l-F dose-dependently increased the release of DA and its metabolites with a pattern of effects similar to that observed with neuroleptic drugs. The dose of 20 mg/kg l-F had the same effect as 10 mg/kg. Repeated haloperidol treatment reduced the basal release of DA and its metabolites and a much smaller amount of DA and metabolites was released by l-F (10 mg/kg i.p.) and haloperidol (0.1 mg/kg i.p.) in animals treated with haloperidol than in controls. GBL 300 mg/kg i.p. reduced basal DA release by about 50%. When 10 mg/kg l-F, 0.1 mg/kg haloperidol and 0.25 mg/kg d-amphetamine were injected i.p. 40 min after GBL, l-F and haloperidol did not significantly raise DA release in GBL-treated rats whereas a significant effect was observed at various times after d-amphetamine. The data show that l-F resembles haloperidol in its ability to release DA and its metabolites from the corpus striatum of freely moving rats. The cross-tolerance between haloperidol and l-F for their effect on DA release suggests that a common site is involved in the mechanism of these drugs.

Neuropharmacology of drugs affecting food intake

The importance of the central monoamines NE, DA and 5-HT in ingestive behavior has inevitably resulted in considerable effort being expended in attempting to implicate these monoamines in the mechanism of action of anorectic drugs. The statements that amphetamine-induced anorexia is unlikely to be due to central serotoninergic systems and that central noradrenergic and dopaminergic systems are not implicated in the appetite suppressant effect of fenfluramine are in all probability correct. However, to attribute the ability of drugs to decrease food intake unequivocally to a specific effect on central monoaminergic systems is almost certainly an oversimplification, due to the fact that other putative neurotransmitters, such as GABA and peptides, play a critical role in eating. This can be achieved either directly or by modulating the release of other transmitters. An added complication in attempting to correlate a specific neurochemical process to a behavioral effect, such as anorexia, is the complexity of the central actions of the drug. At best, a predominant but not an exclusive process can be identified. Perhaps the in-built constraint of attempting to correlate a specific neurochemical effect to the desired action of a drug is accountable for the absence of a second generation of centrally acting anorectic drugs. Dramatic progress has been made in elucidating the factors involved in ingestive behavior over the last 5-10 years. This information should, and must, provide the catalyst for more efficacious anorectic drugs because obesity represents one of the few major diseases for which adequate drug therapy does not exist.

Effect of substances influencing brain serotonergic transmission on plasma vasopressin levels in the rat

Studies were carried out in the rat to investigate whether serotonin (5-HT) is involved in the regulation of vasopressin (ADH) release. For this purpose plasma ADH levels were measured in rats treated with drugs enhancing 5-HT transmission, such as d-fenfluramine and quipazine and with 5-HT depleting drugs, p-chlorophenylalanine (PCPA) and 5,7-dihydroxytryptamine (5,7-DHT). Forebrain 5HT, noradrenaline (NA) and dopamine (DA) were also measured. d-Fenfluramine and quipazine induced dose-related increases in plasma ADH levels in normohydrated rats. The effects of quipazine and d-fenfluramine were prevented by PCPA, indicating that 5-HT transmission was required for the biologic response. 5,7-DHT and PCPA pretreatment decreased forebrain 5-HT content and prevented ADH increases involved by water deprivation, suggesting that 5HT is necessary for the hormonal response to osmotic stimuli. The results summarized imply that serotonin may have a role in the physiological release of ADH from neurohypophysis.

Study of fenfluramine in outpatients with the syndrome of autism

Fenfluramine was administered to 14 outpatient children with the syndrome of autism to determine whether previously produced decreases in blood serotonin concentrations and clinical improvements could be reinstituted. A double-blind medication-placebo crossover design was used. Each patient received fenfluramine 1.5 mg/kg daily (0.75 mg/kg twice daily) for 8 months, followed by placebo for 2 months. Blood serotonin levels promptly fell approximately 49% regardless of baseline levels. Clinical improvement returned, and on some scales gains after 8 months exceeded those noted after only 4 months of treatment. Significant correlations emerged among the amount of clinical response, initially high verbal IQs, and low blood serotonin concentrations. Compliance was excellent, and no clinically relevant side effects or weight changes occurred. It appears that fenfluramine is effective in ameliorating specific symptoms in certain autistic patients. The extent and mechanism of its action remain to be discovered.

Effect of repeated treatment with desipramine in the behavioral "despair" test in rats: antagonism by "atypical" but not "classical" neuroleptics or antiadrenergic drugs

A 7-day treatment with 20 mg/kg/day desipramine reduced the immobility time in the behavioral "despair" test in rats. The effect of DMI was antagonized by sulpiride (100 mg/kg i.p.), metoclopramide (20 mg/kg i.p.) and clopazine (20 mg/kg i.p.) but not by haloperidol (0.5 mg/kg i.p.) or chlorpromazine (5 mg/kg i.p.). Alpha-adrenoreceptor blockers (prazosin 3 mg/kg s.c.; aceperone 10 mg/kg i.p.; azapetine 24 mg/kg s.c.; phentolamine 20 mg/kg i.p.), dl-propranolol (5 mg/kg i.p.) and clonidine (0.1 mg/kg i.p.) failed to modify the anti-immobility effect of DMI. The data suggest that a particular subtype of dopamine receptors is involved in the anti-immobility effect of a 7-day treatment with DMI in the behavioral "despair" test in rats.

Effect of fenfluramine on prolactin secretion in obese patients: evidence for serotoninergic regulation of prolactin in man

The present investigation was carried out to study the effect of serotoninergic stimulation on prolactin (PRL) secretion in man. Fenfluramine (60 mg, orally), an anorexiant drug which under acute circumstances stimulates the serotoninergic system, was administered to eight obese patients. Compared with placebo, drug administration increased PRL significantly (P less than 0.05 at 180 and 300 min, P less than 0.01 at 240 min). No significant changes were observed after fenfluramine in blood pressure, plasma aldosterone (PA), plasma cortisol, plasma renin activity, serum electrolytes or growth hormone. Since it has been reported that dopaminergic blockade raises PA concentration, the lack of change in PA in obese patients treated with fenfluramine suggests that the observed increase in PRL induced by fenfluramine is likely to be mediated by serotoninergic stimulation.

Does acute L-DOPA increase active release of dopamine from dopaminergic neurons?

L-DOPA is believed to be decarboxylated by the residual striatal dopaminergic presynaptic terminals with formation of the putative neurotransmitter dopamine (DA) and with increased availability of DA at post-synaptic receptors. However there is no direct evidence that the DA formed is released into the synaptic cleft. We therefore investigated the biochemical modifications occurring in the dopaminergic system after acute administration of L-DOPA. After acute L-DOPA (100 mg/kg plus 25 mg/kg of benserazide p.o.) the levels of 3-methoxytyramine (3-MT), a metabolite reflecting release of the neurotransmitter DA, were significantly raised, following the same pattern as DA levels, indicating that DA release from DA nerve terminals is increased after L-DOPA administration. The increased DA release and 3-MT formation were not reduced by pretreatment with direct DA agonists such as apomorphine (5 mg/kg i.p.) or piribedil (120 mg/kg p.o.). Thus in this case DA release is not under the control of the compensatory mechanisms induced by post-synaptic receptor hyperstimulation.

A rapid and simple method for the determination of picogram levels of 3-methoxytyramine in brain tissue using liquid chromatography with electrochemical detection

A rapid and simple technique using solvent extraction, ion-pairing extraction, and high pressure liquid chromatography with electrochemical detection has been developed for the determination of 3-methoxytyramine in striata of rats killed by microwave irradiation. The method is specific and reproducible (coefficient of variation among replications, +/- 4%); recovery of authentic 3-methoxytyramine added to the samples is 45-50%. 3-Methoxytyramine levels found with this technique in rat striata were 15 +/- 1.7 ng/g. The method has a sensitivity of about 0.2 pmol per brain sample. Monoamine oxidase inhibition with pargyline increased 3-methoxytyramine levels in rat striata, while catechol-O-methyltransferase inhibition with 3',4'-dihydroxy-2 methylpropiophenone completely depleted 3-methoxytyramine. The effects of nomifensine, quipazine, caroxazone, piribedil, and D-amphetamine were also examined. The 3-methoxytyramine concentrations in the brains of animals killed by decapitation or by microwave irradiation were compared.

Comparison of the effects of the stereoisomers of fenfluramine on the acetylcholine content of rat striatum, hippocampus and nucleus accumbens

The (+)- and (-)- isomeric forms of fenfluramine were compared for their effects on rat brain area acetylcholine (ACh) content. The drugs showed similar patterns in increasing ACh content in the accumbens and hippocampus and in being ineffective in the brainstem. The actions differed in the striatum where the (+)-form markedly increased ACh content while the (-)-form produced no change. Both isomer-induced increases in ACh in the accumbens were prevented when 5-HT synthesis was blocked by p-chlorophenylalanine, thus denoting 5-hydroxytryptaminergic mediation of these effects. In striatum, the increase in ACh induced by (+)-fenfluramine was summated with the increase in ACh induced by dopamine receptor stimulation with apomorphine and was not prevented by dopamine receptor blockade with pimozide. On the other hand, apomorphine's effect was blocked by (-)-fenfluramine while pimozide pretreatment unmasked an increase in ACh induced by (-)-fenfluramine. The results favour the notion that there is a population of cholinergic neurons intrinsic to the striatum which is under inhibitory 5-HT regulation and independent of inhibitory dopamine regulation.

Increased plasma prolactin levels induced in rats by d-fenfluramine: relation to central serotonergic stimulation

d-Fenfluramine (7.5 and 10 mg/kg i.p.) and quipazine (10 and 20 mg/kg i.p.) increased plasma prolactin levels in male rats. Metergoline (3 mg/kg p.o.) or p-chlorophenylalanine (100 mg/kg X 3, orally) pretreatment markedly blocked the prolactin-releasing effect of both d-fenfluramine and quipazine. This result suggests that the effect of these drugs on prolactin secretion could be mediated through a serotonergic mechanism. Brain serotonin may thus exert a stimulatory role on prolactin secretion in rats.

Tolerance to the increase of striatal homovanillic acid elicited by several anorectic drugs

Several anorectic drugs affect dopamine metabolism in the rat striatum, increasing the concentration of homovanillic acid (HVA). However 1- and d-amphetamine and mazindol develop a tolerance to the HVA increase. On the contrary, the effect of fenfluramine and S 992 is not reduced by sub-chronic treatments. Moreover, a cross-tolerance to this biochemical effect develops between amphetamine and mazindol, but pretreatment with fenfluramine or with S 992 does not induce cross-tolerance to amphetamine.

Decrease in rat striatal acetylcholine levels by some direct- and indirect-acting dopaminergic antagonists

Several direct- or indirect-acting dopamine receptor antagonists were found to decrease rat striatal acetylcholine levels. The maximum decrease of about 50% was produced by pimozide (0.5 mg/kg), by haloperidol (0.5 mg/kg) and by reserpine (2.5 mg/kg). The decreases in acetylcholine produced by pimozide and by haloperidol were found to be specific for the striatum and did not alter diencephalonic, mesencephalonic, cerebellar or hemispheric acetylcholine levels. Furthermore, these two drugs completely blocked the increase in striatal acetylcholine produced by the dopamine receptor agonist, apomorphine, and had no effect on striatal choline acetyltransferase and cholinesterase. These data suggest that haloperidol and pimozide act on the striatal cholinergic neurons through strong blockade of dopamine receptors. Reserpine presumably decreased striatal acetylcholine levels indirectly by depleting biogenic amines. Clozapine and 1-fenfluramine were unable to block the action of apomorphine, as was shown previously for chlorpromazine. It is thus suggested that these drugs are reversible dopamine receptor antagonists. Their weaker action in decreasing striatal acetylcholine may depend upon this property.