Denis J. David

Effects of nicotine in the dopaminergic system of mice lacking the alpha4 subunit of neuronal nicotinic acetylcholine receptors.

The mesostriatal dopaminergic system influences locomotor activity and the reinforcing properties of many drugs of abuse including nicotine. Here we investigate the role of the α4 nicotinic acetylcholine receptor (nAChR) subunit in mediating the effects of nicotine in the mesolimbic dopamine system in mice lacking the α4 subunit. We show that there are two distinct populations of receptors in the substantia nigra and striatum by using autoradiographic labelling with 125I α‐conotoxin MII. These receptors are comprised of the α4, β2 and α6 nAChR subunits and non‐α4, β2, and α6 nAChR subunits. Non‐α4 subunit‐containing nAChRs are located on dopaminergic neurons, are functional and respond to nicotine as demonstrated by patch clamp recordings. In vivo microdialysis performed in awake, freely moving mice reveal that mutant mice have basal striatal dopamine levels which are twice as high as those observed in wild‐type mice. Despite the fact that both wild‐type and α4 null mutant mice show a similar increase in dopamine release in response to intrastriatal KCl perfusion, a nicotine‐elicited increase in dopamine levels is not observed in mutant mice. Locomotor activity experiments show that there is no difference between wild‐type and mutant mice in basal activity in both habituated and non‐habituated environments. Interestingly, mutant mice sustain an increase in cocaine‐elicited locomotor activity longer than wild‐type mice. In addition, mutant mice recover from depressant locomotor activity in response to nicotine at a faster rate. Our results indicate that α4‐containing nAChRs exert a tonic control on striatal basal dopamine release, which is mediated by a heterogeneous population of nAChRs.

Effects of acute treatment with paroxetine, citalopram and venlafaxine in vivo on noradrenaline and serotonin outflow: a microdialysis study in Swiss mice.

This study investigated whether a single administration of a range of doses (1, 4 and 8 mg kg−1, i.p.) of paroxetine, citalopram or venlafaxine may simultaneously increase extracellular levels of 5‐HT ([5‐HT]ext) and noradrenaline ([NA]ext) by using in vivo microdialysis in the frontal cortex (FCx) of awake, freely moving Swiss mice. In vivo, paroxetine induced similar increases in cortical [5‐HT]ext at the three doses tested, and induced a statistically significant increase in cortical [NA]ext at 4 and 8 mg kg−1. Citalopram increased neither [5‐HT]ext nor [NA]ext at the lowest dose, but increased both neurotransmitter levels at 4 and 8 mg kg−1. At these doses, citalopram induced greater increases in cortical [5‐HT]ext than in [NA]ext. Venlafaxine increased [5‐HT]ext and [NA]ext to about 400 and 140% of the respective basal values at 8 mg kg−1. Citalopram and paroxetine have the highest potency to increase cortical [5‐HT]ext and [NA]ext, respectively. In addition, the rank of order of efficacy of these antidepressant drugs to increase [5‐HT]ext in vivo in the FCx of mice was as follows: venlafaxine>citalopram>paroxetine, while the efficacy to increase cortical [NA]ext in mice of paroxetine and citalopram is similar, and greater than that of venlafaxine. In conclusion, extracellular levels of cortical [NA]ext increase with the highest doses of the very selective SSRI citalopram, as well as with the very potent SSRI paroxetine. Surprisingly, the SNRI venlafaxine increased cortical [5‐HT]ext to a greater extent rather than [NA]ext in the range of doses studied in mice.

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 (KOu20035‐HT1B) mice in order to compare the effects of chronic administration of paroxetine via osmotic minipumps (1u2003mg per kg per day for 14u2003days) 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, ≈u200a20u2003h 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 (1u2003mg/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, 1u2003µm 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 GRu2003127935 (4u2003mg/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 WAYu2003100635 did not. By using the zero net flux method of quantitative microdialysis in the medial prefrontal cortex and ventral hippocampus of wild‐type and KOu20035‐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.

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.