Christian Jacquot

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.

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.

Effects of chronic paroxetine treatment on dialysate serotonin in 5‐HT1B receptor knockout mice

The role of serotonin (5‐HT)1B receptors in the mechanism of action of selective serotonin re‐uptake inhibitors (SSRI) was studied by using intracerebral in vivo microdialysis in conscious, freely moving wild‐type and 5‐HT1B receptor knockout (KO 5‐HT1B) mice in order to compare the effects of chronic administration of paroxetine via osmotic minipumps (1 mg per kg per day for 14 days) on extracellular 5‐HT levels ([5‐HT]ext) in the medial prefrontal cortex and ventral hippocampus. Basal [5‐HT]ext values in the medial prefrontal cortex and ventral hippocampus, ≈ 20 h after removing the minipump, were not altered by chronic paroxetine treatment in both genotypes. On day 15, in the ventral hippocampus, an acute paroxetine challenge (1 mg/kg i.p.) induced a larger increase in [5‐HT]ext in saline‐pretreated mutant than in wild‐type mice. This difference between the two genotypes in the effect of the paroxetine challenge persisted following chronic paroxetine treatment. Conversely, in the medial prefrontal cortex, the paroxetine challenge increased [5‐HT]ext similarly in saline‐pretreated mice of both genotypes. Such a challenge produced a further increase in cortical [5‐HT]ext compared with that in saline‐pretreated groups of both genotypes, but no differences were found between genotypes following chronic treatment. To avoid the interaction with raphe 5‐HT1A autoreceptors, 1 µm paroxetine was perfused locally through the dialysis probe implanted in the ventral hippocampus; similar increases in hippocampal [5‐HT]ext were found in acutely or chronically treated wild‐type mice. Systemic administration of the mixed 5‐HT1B/1D receptor antagonist GR 127935 (4 mg/kg) in chronically treated wild‐type mice potentiated the effect of a paroxetine challenge dose on [5‐HT]ext in the ventral hippocampus, whereas systemic administration of the selective 5‐HT1A receptor antagonist WAY 100635 did not. By using the zero net flux method of quantitative microdialysis in the medial prefrontal cortex and ventral hippocampus of wild‐type and KO 5‐HT1B mice, we found that basal [5‐HT]ext and the extraction fraction of 5‐HT were similar in the medial prefrontal cortex and ventral hippocampus of both genotypes, suggesting that no compensatory response to the constitutive deletion of the 5‐HT1B receptor involving changes in 5‐HT uptake capacity occurred in vivo. As steady‐state brain concentrations of paroxetine at day 14 were similar in both genotypes, it is unlikely that differences in the effects of a paroxetine challenge on hippocampal [5‐HT]ext are due to alterations of the drugs pharmacokinetic properties in mutants. These data suggest that there are differences between the ventral hippocampus and medial prefrontal cortex in activation of terminal 5‐HT1B autoreceptors and their role in regulating dialysate 5‐HT levels. These presynaptic receptors retain their capacity to limit 5‐HT release mainly in the ventral hippocampus following chronic paroxetine treatment in mice.

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.

Microdialysis monitoring of variations in extracellular levels of serotonin, GABA and excitatory amino acids in the frontal cortex of awake rats in response to a single peripheral or central administration of dexfenfluramine

The effects of a single dexfenfluramine (D-fen) administration on the release of endogenous serotonin (5-hydroxytryptamine, 5-HT), excitatory (glutamate, Glu, aspartate, Asp) and inhibitory (gamma-aminobutyric acid, GABA) amino acids from the frontal cortex were studied by using in vivo microdialysis in freely-moving rats. Extracellular levels of these neurotransmitters were measured with HPLC coupled to electrochemical detection or with capillary electrophoresis coupled to laser-induced fluoresence detection (CE-LIFD). In a first study, single intraperitoneal administration of D-fen (0.5, 1.3, 5 and 10 mg/kg) increased extracellular 5-HT levels in a dose-dependent manner (maximal increase by 982% over baseline for the highest dose) while changes in Glu, Asp or GABA never reached statistical significance. In a second study, 73 nM of D-fen applied locally through the frontocortical dialysis probe, at a flow rate of 1.5 microliters/min in 30 microliters of perfusion fluid for 20 min, increased extracellular 5-HT and Asp levels [the maximal increases were to 1804% and 280% of the respective basal values (100%)] without altering extracellular levels of Glu and GABA. Thus, the order of magnitude of the changes induced by systemic administration or local infusion of D-fen on frontocortical extracellular levels of several neurotransmitters (5-HT > > Asp > GABA = Glu) demonstrate that D-fen, an indirect serotoninergic agonist, mainly increases 5-HT release while producing slight (Asp) or no (Glu, GABA) short-term in vivo variations in amino acid extracellular levels in the rat frontal cortex.

Benzodiazepine receptors are involved in tabernanthine-induced tremor: in vitro and in vivo evidence

Tabernanthine, an indol alkaloid, is structurally related to carbolines (harmane, harmaline) which, in vitro, displace specific flunitrazepam binding to brain benzodiazepine receptors. In vivo, both tabernanthine and carbolines cause a fine general tremor, suggesting that a possible interaction with benzodiazepine receptors could be involved in the activity of tabernanthine. This hypothesis was validated by the in vitro and in vivo antagonism of benzodiazepine by tabernanthine. In vitro, tabernanthine inhibited specific flunitrazepam binding in a competitive manner with an affinity (IC50 150 microM) in the same range as harmane. Tabernanthine appeared as a benzodiazepine receptor inverse agonist in a discriminant in vitro binding assay. In vivo, the time course of tremorigenic activity was related to the tabernanthine concentration in brain (half-life = 2 h). Moreover, tabernanthine-induced tremor was inhibited reversibly by flunitrazepam or by Ro-15 1788 (an antagonist of benzodiazepine-receptors). These results suggest that part of the action of tabernanthine may be mediated by an interaction at the benzodiazepine receptor level.

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.

Lesions of the Afferent Catecholaminergic Pathways Inhibit the Temporal Activation of the CRH and POMC Gene Expression and ACTH Release Induced by Human Interleukin-1β in the Male Rat

A number of recent studies have suggested that interleukin-1beta (IL-1beta) is a major mediator contributing to the recruitment of the hypothalamo-pituitary-adrenal (HPA) axis following infectious aggressions. Central catecholamines modulate the response of the HPA axis. To investigate the importance of the afferent catecholaminergic pathways in a pathophysiological situation, we used the intraperitoneal (i.p.) IL-1beta injection (mimicking peripheral infections) and we investigated the effects on the HPA responses to IL-1beta of bilateral neurotoxic (6-OHDA) deletion of the ventral noradrenergic ascending bundle (VNAB-X). The VNAB is an essential stimulating pathway linking the brainstem and the paraventricular nucleus (PVN). We determined the time courses of a number of HPA variables up to 240 min after i.p. injection of IL-1beta. We followed: plasma ACTH and corticosterone (CORT) concentrations, AP POMC nuclear primary RNA transcripts, AP POMC nuclear intermediate transcript RNA, AP POMC cytoplasmic mRNA, and hypothalamus (HT) CRH cytoplasmic mRNA. Compared to sham-lesioned male rats, VNAB-X animals displayed: (1) a reduced increase in plasma ACTH, and to a lesser extent in CORT throughout the experimental period with a 85% inhibition at the peak (90 min); (2) an increase in AP POMC primary nuclear transcript and in AP POMC nuclear intermediate transcript RNAs which last 60 min, instead of sustained significantly higher levels up to 240 min; (3) a similar, although reduced inhibition in the corresponding POMC cytoplasmic mRNA; (4) an almost complete abolishment of the marked biphasic rise in HT CRH mRNA. In conclusion, activation of the HPA axis by peritoneal IL-1beta challenge involves CRH-producing neurons, and afferent catecholaminergic innervation of the PVN plays a crucial role in the signaling machinery linking the peritoneal aggression to the HPA axis.