Brain Res. 1996 Jun 10;724(1):84-8.
The anticonvulsant action of fluoxetine in substantia nigra is dependent upon endogenous serotonin.

Pasini A, Tortorella A, Gale K.

Georgetown University, Department of Pharmacology, Washington, DC 20007, USA.

Fluoxetine, a serotonin (5-HT) reuptake inhibitor, has been documented to exert a protective action against convulsive seizures in animal models, when administered either systemically, or focally into substantia nigra. It is likely that the mechanism of anticonvulsant action of fluoxetine is due to an enhancement of endogenous 5-HT transmission. To evaluate this possibility in the context of the anticonvulsant action of intranigral fluoxetine, we examined the influence of 5-HT-mediated transmission in substantia nigra on seizure susceptibility in a rat model of focally evoked complex partial seizures. In addition to fluoxetine (3.5 nmol), we found that the directly acting 5-HT receptor agonists, 1-[3-(trifluoromethyl)phenyl]piperazine (TFMPP) (10 nmol), 1-(3-chlorophenyl)piperazine (m-CPP) (7.4 nmol), gepirone (70 nmol) and 2-dipropylamino-8-hydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide (8-OH-DPAT) (10 nmol), when microinjected bilaterally into substantia nigra, protected rats from limbic motor seizures evoked focally from area tempestas, an epileptogenic site in the deep rostral piriform cortex. This indicates that multiple 5-HT receptor subtypes in substantia nigra may contribute to seizure regulation. Consistent with this, the 5-HT antagonist, metergoline, partially reversed the anticonvulsant action of intranigral fluoxetine. Depletion of endogenous 5-HT, by pretreatment with parachlorophenylalanine (PCPA), completely prevented the anticonvulsant action of intranigral fluoxetine, without modifying the anticonvulsant effect of intranigral TFMPP. These findings support the proposal that the anticonvulsant action of fluoxetine in substantia nigra is due to an enhancement of the synaptic action of endogenous 5-HT in substantia nigra which i




Naunyn Schmiedebergs Arch Pharmacol. 1996 Jun;354(1):17-24.
Inhibition of rat brain monoamine oxidase enzymes by fluoxetine and norfluoxetine.

Holt A, Baker GB.

Department of Psychiatry, Mackenzie Health Sciences Centre, University of Alberta, Edmonton, Canada.

Fluoxetine and its primary metabolite, norfluoxetine, are inhibitors of neuronal uptake of 5-hydroxytryptamine. While fluoxetine has also been reported to inhibit monoamine oxidase (MAO) in vitro at concentrations much lower than those measured in brain following chronic fluoxetine treatment, neurochemical profiles are not consistent with substantial MAO inhibition in vivo. In an attempt to explain this inconsistency, we have examined the interactions of fluoxetine and norfluoxetine with rat brain MAO-A and -B by a radiochemical assay method. Fluoxetine and norfluoxetine were competitive inhibitors of MAO-A in vitro, with Ki values of 76.3 microM and 90.5 microM, respectively. Both compounds were non-competitive or uncompetitive inhibitors of MAO-B in vitro. Inhibition of MAO-B was time-dependent and was very slowly reversible by dialysis. IC50 values versus metabolism of 50 microM beta-phenylethylamine were 17.8 microM (fluoxetine) and 18.5 microM (norfluoxetine). Analysis of the time-dependence of MAO-B inhibition by fluoxetine revealed that an initial competitive interaction between the enzyme and the inhibitor (Ki 245 microM) was followed by tight-binding enzyme inactivation (K(inact) 0.071 min-1). Following administration of fluoxetine (20 mg kg-1 day-1) for 7 days, the cortical concentration of fluoxetine + norfluoxetine was estimated by gas-liquid chromatography to be 700 microM. Such drug treatment reduced MAO-A activity by 23% in 1:8 (w/v) cortical homogenates, but not in 1:80 homogenates. Inhibition of MAO-B in 1:8 homogenates was modest (12%) and was not significantly reduced by homogenate dilution. The concentration of 5-hydroxyindole-3-acetic acid, measured by high pressure liquid chromatography, was reduced by 47% in cortices from drug-tre




Eur J Pharmacol. 1996 Jan 25;296(2):137-43.
An investigation of the mechanism responsible for fluoxetine-induced hypophagia in rats.

Lightowler S, Wood M, Brown T, Glen A, Blackburn T, Tulloch I, Kennett G.

SmithKline Beecham Pharmaceuticals, Harlow, Essex, UK.

The effect of fluoxetine on feeding in p-chlorophenylalanine (PCPA) pretreated rats and the nature of its interaction with 5-HT2C receptors have been investigated. Animals that received 3 days PCPA (150 mg/kg i.p.) pretreatment and vehicle on the test day consumed a similar amount as control, saline pretreated animals under the test paradigm used in this study. Fluoxetine (20 and 30 mg/kg p.o.) significantly reduced food intake in PCPA and control pretreated animals to a similar extent, despite an approximately 90% reduction in the levels of brain 5-hydroxytryptamine (5-HT) and 5-hydroxyindole acetic acid (5-HIAA) in the PCPA-pretreated animals. Thus, hypophagia is unlikely to be caused by inhibition of 5-HT reuptake. In the pig choroid plexus in vitro, fluoxetine and norfluoxetine inhibited specific [3H] mesulergine binding with pKI's (+/- S.E.M.) of 6.45 +/- 0.09 (n = 4) and 6.05 +/- 0.05 (n = 3), and slope factors (+/- S.E.M.) of 1.06 +/- 0.14 and 0.99 +/- 0.13, respectively. In slices of piglet choroid plexus fluoxetine (1, 10 and 33 microM) caused a rightward shift in the dose-response curve produced by 5-HT with no effect on the maximal response, and a mean pKB of 5.94 +/- 0.09. Norfluoxetine (10 microM) also produced a rightward shift in the 5-HT dose-response curve with no effect on the maximal response, and a pKB of 6.20. Thus, both compounds acted as surmountable antagonists with no agonist efficacy at 5-HT2C receptors present in choroid plexus. The hypophagic effect of fluoxetine (20 mg/kg p.o.) was also unaffected by the non-specific 5-HT2C receptor antagonist metergoline (2 and 5 mg/kg i.p.). These findings suggest that the hypophagic effect of fluoxetine is not likely to be dependent upon intact brain 5-hydroxytryptaminergi




Neuropsychopharmacology. 1996 Aug;15(2):143-51.
Chronic citalopram and fluoxetine treatments upregulate 5-HT2c receptors in the rat choroid plexus.

Laakso A, Palvimaki EP, Kuoppamaki M, Syvalahti E, Hietala J.

Department of Pharmacology, University of Turku, Finland.

The effects of chronic (for 14 days) citalopram and fluoxetine treatments with three doses (2.5, 10, and 20 mg/kg) and withdrawal times (24 hours, 68 hours, and 14 days) on 5-HT2C (formerly 5-HT1C) receptors in the rat brain choroid plexus were studied with quantitative receptor autoradiography in two separate experiments. Chronic citalopram treatment caused a consistent and dose-related increase in the density of 5-HT2C receptors (up to 90%). This effect was slightly more pronounced when measured with an antagonist ligand ([3H]mesulergine) than with an agonist ligand [(+/-)-1-(2,5-dimethoxy-4-[125I]iodophenyl)-2-aminopropane ([125I]DOI)]. The upregulation was most evident 24 hours after the last dose and disappeared thereafter rather rapidly. Chronic fluoxetine treatment also increased the density of 5-HT2C receptors 24 hours from the last dose, but the increase was accompanied by a reduced affinity and was less marked than that observed with citalopram. The changes in receptor characteristics were not observed consistently after the 68-hour withdrawal from fluoxetine. Furthermore, the upregulation of fluoxetine appeared not to be dose related or reflected by an increase in agonist binding. In conclusion, the results show that chronic citalopram and fluoxetine treatments induce an increase of choroid plexus 5-HT2C receptor density, but the effect is more marked with citalopram. These differences in the regulation of the 5-HT2C receptors may lead to pharmacodynamic differences between chronic citalopram and fluoxetine treatments.

Source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=8840350&dopt=Abstract fluoxetine




J Chromatogr B Biomed Appl. 1996 Jul 12;682(2):265-72.
Simultaneous determination of plasma levels of fluvoxamine and of the enantiomers of fluoxetine and norfluoxetine by gas chromatography-mass spectrometry.

Eap CB, Gaillard N, Powell K, Baumann P.

Departement Universitaire de Psychiatrie Adulte, Hopital de Cery, Prilly-Lausanne, Switzerland.

A gas chromatographic-mass spectrometric method is presented which allows the simultaneous determination of the plasma concentrations of fluvoxamine and of the enantiomers of fluoxetine and norfluoxetine after derivatization with the chiral reagent, (S)-(-)-N-trifluoroacetylprolyl chloride. No interference was observed from endogenous compounds following the extraction of plasma samples from six different human subjects. The standard curves were linear over a working range of 10 to 750 ng/ml for racemic fluoxetine and norfluoxetine and of 50 to 500 ng/ml for fluvoxamine. Recoveries ranged from 50 to 66% for the three compounds. Intra- and inter-day coefficients of variation ranged from 4 to 10% for fluvoxamine and from 4 to 13% for fluoxetine and norfluoxetine. The limits of quantitation of the method were found to be 2 ng/ml for fluvoxamine and 1 ng/ml for the (R)- and (S)-enantiomers of fluoxetine and norfluoxetine, hence allowing its use for single dose pharmacokinetics. Finally, by using a steeper gradient of temperature, much shorter analysis times are obtained if one is interested in the concentrations of fluvoxamine alone.

Source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=8844419&dopt=Abstract fluoxetine




Pharmacol Biochem Behav. 1996 Aug;54(4):745-51.
Metergoline antagonizes fluoxetine-induced suppression of food intake but not changes in the behavioural satiety sequence.

Halford JC, Blundell JE.

Department of Psychology, University of Leeds, UK.

In this study continuous monitoring was used to yield a true behavioural record. This allows a bidimensional account of drug effects on every unit of behaviour. Behavioural dimensions of duration (dur) and frequency (frq) measures were utilized to monitor the effects of an ED50 anorectic dose of fluoxetine (10 mg/kg i.p.) on the behavioural satiety sequence and the effect of a metergoline (1 mg/kg i.p.) challenge. Fluoxetine reduced food intake by 45% (p < 0.005). The local eating rate was also reduced (p < 0.001), demonstrating a marked slowing of eating behaviour. Eating behaviour was reduced (frq p < 0.05) as was grooming (frq p < 0.05) and activity. Resting was increased (dur p < 0.05) and temporally advanced. There was no gross disruption of behaviour and the profile was adjusted in a way consistent with the expression of satiety. Fluoxetine-induced changes were very similar to those produced by prefeeding. Metergoline antagonised fluoxetine's effect on intake and eating duration (dur p < 0.05). However, metergoline did not antagonise the effect of fluoxetine on the frequency of eating (frq p < 0.005), thus increasing the amount consumed per eating episode. Grooming (frq p < 0.005) and activity also remained reduced. At this dose fluoxetine-induced suppression of eating is serotonin dependent as it is reversed by metergoline. Fluoxetine-induced suppression of eating at this dose is consistent with the normal operation of satiety. Fluoxetine-induced slowing of behavior appears to be mediated by a separate mechanism.

Source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=8853199&dopt=Abstract fluoxetine




Psychopharmacology (Berl). 1996 Aug;126(3):234-40.
Interactions of selective serotonin reuptake inhibitors with the serotonin 5-HT2c receptor.

Palvimaki EP, Roth BL, Majasuo H, Laakso A, Kuoppamaki M, Syvalahti E, Hietala J.

Department of Pharmacology and Clinical Pharmacology, University of Turku, Finland.

Interactions of the selective serotonin reuptake inhibitors (SSRIs) citalopram, fluoxetine and its main metabolite norfluoxetine, and the tricyclic antidepressant (TCA) imipramine with the rat serotonin 5-HT2C receptor in a clonal cell line and in the rat choroid plexus were investigated by radioligand binding and phosphoinositide (PI) hydrolysis assays. For comparison, the affinities of a variety of other antidepressants of different chemical classes for the cloned rat 5-HT2C and 5-HT2A receptors were also determined by radioligand binding assays. Fluoxetine displayed relatively high affinity for the 5-HT2C receptor in the choroid plexus, with a Ki value for inhibition of [3H]mesulergine binding of 55.4 nM. The Ki values for imipramine, norfluoxetine and citalopram were 136 nM, 203 nM, and 298 nM, respectively. Similar rank order of potency was detected in PI hydrolysis assays, which showed that these drugs are antagonists at the 5-HT2C receptor without exhibiting inverse agonist activity. [3H]Ketanserin (5-HT2A) binding assays revealed that the SSRIs fluoxetine, norfluoxetine and citalopram show 10- to 23-fold selectivity for the 5-HT2C receptor in vitro, whereas the TCA imipramine does not. Many other TCAs also had high to intermediate affinity for both 5-HT2A and 5-HT2C receptors. The present data provide evidence that fluoxetine, norfluoxetine and citalopram, along with many other antidepressant compounds, interact directly with the 5-HT2C receptor.

Source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=8876023&dopt=Abstract fluoxetine




Psychopharmacology (Berl). 1996 Sep;127(2):83-7.
Chronic desipramine and fluoxetine differentially affect extracellular dopamine in the rat prefrontal cortex.

Tanda G, Frau R, Di Chiara G.

Department of Toxicology, University of Cagliari, Italy.

The effect of chronic administration of desipramine or fluoxetine (10 mg/kg IP once a day for 2 weeks) on extracellular noradrenaline; serotonin and dopamine in the rat prefrontal cortex was studied by transcerebral microdialysis. Chronic desipramine increased extracellular noradrenaline and dopamine by three-fold as compared to saline controls. Acute challenge with 10 mg/kg desipramine increased by more than three-fold extracellular noradrenaline and dopamine in saline controls, but failed further to increase extracellular noradrenaline and dopamine in rats chronically administered desipramine. Chronic fluoxetine more than doubled the extracellular concentrations of serotonin but failed to change the extracellular concentrations of dopamine as compared to saline controls. Challenge with 5 mg/kg fluoxetine while almost doubling extracellular serotonin and dopamine concentrations in saline controls, failed further to increase extracellular serotonin and did not change extracellular dopamine in rats chronically exposed to fluoxetine. In contrast, challenge with 10 mg/kg desipramine normally increased extracellular dopamine in rats chronically exposed to fluoxetine. Therefore, chronic fluoxetine is associated with normal presynaptic dopamine transmission in the prefrontal cortex as a result of tolerance to fluoxetine-induced increase of extracellular dopamine; in contrast, chronic desipramine is associated with an increase of pre-synaptic dopamine transmission in the prefrontal cortex up to a level that cannot be further elevated by acute desipramine challenge. The results suggest that prefrontal cortex dopamine plays a different role in the antidepressant properties of desipramine and fluoxetine.

Source: www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=8888371&dopt=Abstract fluoxetine




J Clin Pharmacol. 1996 Sep;36(9):783-91.
Midazolam hydroxylation by human liver microsomes in vitro: inhibition by fluoxetine, norfluoxetine, and by azole antifungal agents.

von Moltke LL, Greenblatt DJ, Schmider J, Duan SX, Wright CE, Harmatz JS, Shader RI.

Department of Pharmacology and Experimental Therapeutics, Tufts University School of Medicine, Boston, MA 02111, USA.

Biotransformation of the imidazobenzodiazepine midazolam to its alpha-hydroxy and 4-hydroxy metabolites was studied in vitro using human liver microsomal preparations. Formation of alpha-hydroxy-midazolam was a high-affinity (Km = 3.3 mumol/L) Michaelis-Menten process coupled with substrate inhibition at high concentrations of midazolam. Formation of 4-hydroxy-midazolam had much lower apparent affinity (57 mumol/L), with minimal evidence of substrate inhibition. Based on comparison of Vmax/Km ratios for the two pathways, alpha-hydroxy-midazolam formation was estimated to account for 95% of net intrinsic clearance. Three azole antifungal agents were inhibitors of midazolam metabolism in vitro, with inhibition being largely consistent with a competitive mechanism. Mean competitive inhibition constants (Ki) versus alpha-hydroxy-midazolam formation were 0.0037 mumol/L for ketoconazole, 0.27 mumol/L for itraconazole, and 1.27 mumol/L for fluconazole. An in vitro-in vivo scaling model predicted inhibition of oral midazolam clearance due to coadministration of ketoconazole or itraconazole; the predicted inhibition was consistent with observed interactions in clinical pharmacokinetic studies. The selective serotonin reuptake inhibitor (SSRI) antidepressant fluoxetine and its principal metabolite, norfluoxetine, also were inhibitors of both pathways of midazolam biotransformation, with norfluoxetine being a much more potent inhibitor than was fluoxetine itself. This finding is consistent with results of other in vitro studies and of clinical studies, indicating that fluoxetine, largely via its metabolite norfluoxetine, may impair