Isabelle Malagié

Effects of acute fluoxetine on extracellular serotonin levels in the raphe: an in vivo microdialysis study.

Acute administration of fluoxetine (1, 10 and 20 mg/kg i.p.) increased extracellular levels of serotonin (5-hydroxytryptamine, 5-HT) in the frontal cortex, ventral hippocampus and raphe nuclei as measured by in vivo microdialysis in anaesthetized rats. In the frontal cortex, fluoxetine showed a marked dose-response effect whereas in the ventral hippocampus and raphe nuclei the fluoxetine-induced effect was maximum at 10 mg/kg. However, the maximal increase in 5-HT was observed in the cell body-containing area, the raphe nuclei. The order of changes in extracellular 5-HT was raphe nuclei > ventral hippocampus > frontal cortex. Our results add further arguments in favour of the key role played by raphe nuclei in the mechanism of action of serotoninergic antidepressant drugs.

5-HT1B Autoreceptors limit the effects of selective serotonin re-uptake inhibitors in mouse hippocampus and frontal cortex

We used knockout mice and receptor antagonist strategies to investigate the contribution of the serotonin (5‐hydroxytryptamine, 5‐HT) 1B receptor subtype in mediating the effects of selective serotonin re‐uptake inhibitors (SSRIs). Using in vivo intracerebral microdialysis in awake mice, we show that a single systemic administration of paroxetine (1 or 5 mg/kg, i.p.) increased extracellular serotonin levels [5‐HT]ext in the ventral hippocampus and frontal cortex of wild‐type and mutant mice. However, in the ventral hippocampus, paroxetine at the two doses studied induced a larger increase in [5‐HT]ext in knockout than in wild‐type mice. In the frontal cortex, the effect of paroxetine was larger in mutants than in wild‐type mice at the 1 mg/kg, but not at 5 mg/kg. In addition, either the absence of the 5‐HT1B receptor or its blockade with the mixed 5‐HT1B/1D receptor antagonist, GR 127935, potentiated the effect of a single administration of paroxetine on extracellular 5‐HT levels more in the ventral hippocampus than in the frontal cortex. These data suggest that 5‐HT1B autoreceptors limit the effects of SSRIs on dialysate 5‐HT levels at serotonergic nerve terminals.

Effects of acute and chronic tianeptine administration on serotonin outflow in rats: comparison with paroxetine by using in vivo microdialysis

Using in vivo microdialysis, we compared the effects of tianeptine (an antidepressant drug which, in marked contrast with other antidepressants, is thought to increase the uptake of serotonin (5-hydroxytryptamine, 5-HT) on extracellular 5-HT concentrations ([5-HT](ext)) in the frontal cortex and raphe nuclei of freely moving rats with those of paroxetine, a potent selective serotonin reuptake inhibitor. A single paroxetine dose (1 mg/kg, i.p.) increased [5-HT](ext) over baseline in the frontal cortex and raphe nuclei, respectively. A single administration of tianeptine (10 mg/kg, i.p.) did not change [5-HT(ext)] in the two brain regions studied. Repeated exposure to paroxetine (0.5 mg/kg) b.i.d. for 14 days induced a sixfold significant increase in basal [5-HT](ext) in the raphe nuclei. Administration of tianeptine (5 mg/kg) b.i.d. for 14 days did not affect 5-HT baseline concentrations. In rats chronically treated with either paroxetine or tianeptine, drug challenge did not alter area under the curve values. Thus, our in vivo data indicate that tianeptine and paroxetine do not exert a similar in vivo effect on the serotonergic system in rat brain.

Regional differences in the effect of the combined treatment of WAY 100635 and fluoxetine: an in vivo microdialysis study

We studied the changes in extracellular serotonin (5-HT) levels in the frontal cortex (FC) and ventral hippocampus (vHi) in conscious rats, induced by the combined administration of a highly selective 5-HT1A receptor antagonist, WAY 100635 (0.1 mg/kg, i.v.), and fluoxetine (1 mg/kg, i.p.), a selective 5-HT reuptake inhibitor (SSRI). In the two brain areas studied, no change in extracellular 5-HT concentrations was observed following fluoxetine administration over the 210 min post-injection period. However, in animals co-administered with [WAY 100635 + fluoxetine], the maximal increase in 5-HT levels in the FC was to 215% of the respective basal value (100%), while no significant change in 5-HT was observed in dialysates from the vHi. Furthermore, the [norfluoxetine]-to-[fluoxetine] ratio in the FC was significantly higher than in the hippocampus as measured in homogenates of animals treated with either fluoxetine alone or a prior administration of WAY 100635. Thus, WAY 100635 made the fluoxetine short-lasting effect apparent in the FC, but not by interfering with pharmacokinetic parameters of fluoxetine. Taken together, our data suggest the possibility, that either 5HT-1A autoreceptor sensitivity or uptake carrier density or higher [metabolite]-to-[parent drug] ratios in the FC than in the hippocampus may be involved in regional specific responses to SSRIs.

Synergistic neurochemical and behavioral effects of fluoxetine and 5-HT1A receptor antagonists

We studied the ability of WAY 100635 [N-[4-(2-methoxyphenyl)-1-piperazinyl]-N-(2-pyridinyl) cyclo-hexanecarboxamide], 0.5 mg/kg, i.v. and (-)-5-Me-8-OH-DPAT [(-)-5-methyl-8-hydroxy-2-(di-n-propylamino)tetralin], 3 mg/kg, i.v. two selective 5-HT1A receptor antagonists, to potentiate: (1) the enhancement of extracellular 5-HT levels ([5-HT(ext)]) induced by a single administration of 5 mg/kg i.p. fluoxetine using in vivo microdialysis in the ventral hippocampus of conscious rats, (2) the decrease in food intake induced by this antidepressant drug in food-deprived rats. The effects of fluoxetine were significantly potentiated, by 30-40%, by WAY 100635 as well as by (-)-5-Me-8-OH-DPAT in the two sets of experiments. Thus, fluoxetine increased [5-HT(ext)] in serotonergic nerve terminal areas and consequently, induced hypophagia, both effects being limited by indirect activation of somatodendritic 5-HT1A autoreceptors.

Improved efficacy of fluoxetine in increasing hippocampal 5-hydroxytryptamine outflow in 5-HT1B receptor knock-out mice

To test for the contribution of the 5-HT(1B) receptor subtype in mediating the effects of fluoxetine, a selective serotonin reuptake inhibitor (SSRI), we used intracerebral in vivo microdialysis in awake, freely moving 5-HT(1B) receptor knock-out mice. We show that a single systemic administration of fluoxetine (1, 5 or 10 mg/kg, i.p.) increased extracellular serotonin levels [5-HT](ext) in the ventral hippocampus and frontal cortex of wild-type and mutant mice. However, in the ventral hippocampus, fluoxetine, at the three doses studied, induced a larger increase in [5-HT](ext) in knock-out than in wild-type mice. In the frontal cortex, the effect of fluoxetine did not differ between the two genotypes. The region-dependent response to fluoxetine described here in mutants confirms data we recently reported for another SSRI, paroxetine. These data suggest that 5-HT(1B) autoreceptors limit the effects of selective serotonin reuptake inhibitors on dialysate 5-HT levels at serotonergic nerve terminals located mainly in the ventral hippocampus. Alternative mechanisms, e.g., changes in 5-HT transporter and/or 5-HT(1A) receptor density in 5-HT(1B) receptor knock-out mice could also explain these findings.

Récepteurs 5-HT1B de la sérotonine et effets antidépresseurs des inhibiteurs de recapture sélectifs de la sérotonine

We used knockout mice and receptor antagonist strategies to investigate the contribution of the serotonin (5-hydroxytryptamine, 5-HT) 5-HT1B receptor subtype in mediating the effects of selective serotonin reuptake inhibitors (SSRIs). Using in vivo intracerebral microdialysis in awake mice, we show that a single systemic administration of paroxetine (1 or 5 mg/kg, i.p.) increased extracellular serotonin levels [5-HT]ext in the ventral hippocampus and frontal cortex of wild-type and mutant mice. However, in the ventral hippocampus, paroxetine at the two doses studied induced a larger increase in [5-HT]ext in knockout than in wild-type mice. In the frontal cortex, the effect of paroxetine was larger in mutants than in wild-type mice at the 1 mg/kg dose but not at 5 mg/kg. In addition, either the absence of the 5-HT1B receptor or its blockade with the mixed 5-HT1B/1D receptor antagonist, GR 127935, potentiates the effect of a single administration of paroxetine on [5-HT]ext more in the ventral hippocampus than in the frontal cortex. Furthermore, we demonstrate that SSRIs decrease immobility in the forced swimming test; this effect is absent in 5-HT1B knockout mice and blocked by GR 127935 in wild-type suggesting therefore that activation of 5-HT1B receptors mediate the antidepressant-like effects of SSRIs. Taken together these data demonstrate that 5-HT1B autoreceptors appear to limit the effects of SSRI on dialysate 5-HT levels particularly in the hippocampus while presynaptic 5-HT1B heteroreceptors are likely to be required for the antidepressant activity of SSRIs.

Effects of WAY 100635 and (−)-5-Me-8-OH-DPAT, a novel 5-HT1A receptor antagonist, on 8-OH-DPAT responses

The neurochemical profile at both post and presynaptic 5-HT1A receptors of a novel 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) analog, 5-methyl-8-hydroxy-2-(di-n-propylamino)tetralin ¿(+/-)-5-Me-8-OH-DPAT¿ and its stereoisomers was determined and compared to that of the highly selective 5-HT1A receptor antagonist, N-[4-(2-methoxyphenyl)-1-piperazinyl]-N-(2-pyridinyl) cyclo-hexanecarboxamide (WAY 100635). We evaluated their effects on 8-OH-DPAT-induced decrease in cAMP production, on 8-OH-DPAT-induced decrease in rat ventral hippocampal extracellular 5-hydroxytryptamine (5-HText) levels and in body temperature in mice. Both (+/-)- and (-)-5-Me-8-OH-DPAT blocked the 8-OH-DPAT-induced inhibition of forskolin-stimulated cAMP production. Moreover, while having no significant effect when injected alone, (+/-)-, (-)-5-Me-8-OH-DPAT and WAY 100635 antagonized the 8-OH-DPAT-induced decrease in 5-HText in rats and hypothermia in mice. By contrast, the (+) isomer inhibited the cAMP synthesis and did not modify the 8-OH-DPAT response on 5-HText in ventral hippocampus. These data suggest that (+/-)-5-Me-8-OH-DPAT acts selectively, its activity residing in the (-) enantiomer, this latter compound acting similarly to WAY 100635 as a full, selective and silent 5-HT1A antagonist.

8-OH-DPAT attenuates the dexfenfluramine-induced increase in extracellular serotonin: an in vivo dialysis study.

Rats with frontocortical microdialysis probes were treated with dexfenfluramine or dexfenfluramine with 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) pretreatment. Dexfenfluramine (10 mg/kg i.p.) increased extracellular serotonin (5-hydroxytryptamine, 5-HT) (calculated area under the curve (AUC) for the 0 to 105-min period after dexfenfluramine treatment = 8.22 +/- 2.66 pmol 5-HT). Systemic (0.025 mg/kg i.p.) or local (0.01 microM into the dorsal raphe nucleus) 8-OH-DPAT pretreatement decreased the dexfenfluramine response (AUC: 1.03 +/- 0.07 and 0.44 +/- 0.04 pmol 5-HT, respectively). This result might be explained by the decrease in 5-HT neuronal discharge caused by somatodendritic 5-HT1A autoreceptor activation, and suggests that the 5-HT releasing effect of dexfenfluramine in vivo depends on nerve terminal depolarization.

(-)-5-methyl-8-hydroxy-(di-n-propylamino)tetralin a new 5-HT1A receptor antagonist

Abstract − (±)-5-Me-8-OH-DPAT 4 was synthesized by a new synthetic pathway recently described by us. The (+)- and (−)-enantiomers 4 were prepared from the primary amine 8 by crystallisation of the (+)- and (−)-mandelic acid salts. The enantiomers reacted with propyl iodide and were demethylated by 48% HBr to the (+)- and (−)- 4 compounds. These compounds had good affinity for 5-HT 1A receptors ( K i = 32.9 ± 0.8 and 45.6 ± 2 nM, respectively) but lacked enantioselectivity. In contrast to 8-OH-DPAT, but similar to WAY 100635 and (+)-WAY 100135, the addition of GTP-γS did not decrease the affinity of these compounds for 5-HT 1A receptors, suggesting a partial agonist or antagonist profile. Adenylyl cyclase assays with rat hippocampal membranes showed that (−)- 4 was totally inactive as an agonist over a wide concentration range in contrast to (+)- 4 which was a partial agonist. (−)- 4 (1 and 10 μM) shifted the concentration—effect curve for the inhibition by 8-OH-DPAT of forskolin-stimulated cyclic AMP production to the right (pA 2 = 7.6), demonstrating a competitive interaction between the two drugs.