Serotonin1B heteroreceptor activation induces an antidepressant-like effect in mice with an alteration of the serotonergic system.

OBJECTIVE: We sought to demonstrate whether the specific activation of serotonin1B (5-HT1B) heteroreceptors by systemic or local administration of the selective 5-HT1B receptor agonist anpirtoline could mediate antidepressant-like effects in mice. METHODS: We confirmed the selectivity of action of anpirtoline in the forced swim test (FST) in 5-HT1B knockout mice. We then evaluated the behavioural effects of anpirtoline on 5-HT-lesioned (5,7-dihydroxytryptamine creatinine [5,7-DHT]) and 5-HT-depleted (p-CPA) mice. We estimated the depletion level and selectivity of action of 5,7-DHT and p-CPA by measuring the neurotransmitter levels and [3H]-citalopram binding. We investigated the antidepressant-like effect of anpirtoline when locally perfused in an area of the brain where the response is mainly attributable to presynaptic (cortex and hippocampus) or postsynaptic receptors (substantia nigra and caudate putamen). Furthermore, we evaluated the effect of the 5-HT1B receptor antagonist GR127935 on the activity of various antidepressants in the FST. RESULTS: Anpirtoline was devoid of effects in 5-HT1B receptor knockout mice. It induced a greater effect in p-CPA and 5,7-DHT pretreated mice compared with control subjects, suggesting that the antidepressant-like activity of anpirtoline mainly depends on 5-HT1B heteroreceptor stimulation (autoreceptors being destroyed by 5,7-DHT). This observation was confirmed by the results showing the antidepressant-like effect of anpirtoline when locally perfused in areas of the brain that contain postsynaptic receptors. The blockade of 5-HT1B receptors antagonizes the effect of selective serotonin reuptake inhibitors (SSRIs). CONCLUSION: Our results demonstrate that the antidepressant-like effect of SSRIs in the FST requires the activation of 5-HT1B heteroreceptors.

Blockade of 5-HT1A receptors by (+/-)-pindolol potentiates cortical 5-HT outflow, but not antidepressant-like activity of paroxetine: microdialysis and behavioral approaches in 5-HT1A receptor knockout mice.

Selective serotonin reuptake inhibitors like paroxetine (Prx) often requires 4-6 weeks to achieve clinical benefits in depressed patients. Pindolol shortens this delay and it has been suggested that this effect is mediated by somatodendritic 5-hydroxytryptamine (5-HT) 1A autoreceptors. However clinical data on the beneficial effects of pindolol are conflicting. To study the effects of (+/-)-pindolol-paroxetine administration, we used genetical and pharmacological approaches in 5-HT1A knockout mice (5-HT1A-/-). Two assays, in vivo intracerebral microdialysis in awake mice and the forced swimming test (FST), were used to assess the antidepressant-like effects of this drug combination. Basal levels of extracellular serotonin, 5-HT ([5-HT]ext) in the frontal cortex (FCX) and the dorsal raphe nucleus (DRN) did not differ between the two strains of mice, suggesting a lack of tonic control of 5-HT1A autoreceptors on nerve terminal 5-HT release. Prx (1 and 4 mg/kg) dose-dependently increased cortical [5-HT]ext in both genotypes, but the effects were greater in mutants. The selective 5-HT1A receptor antagonist, WAY-100635 (0.5 mg/kg), or (+/-)-pindolol (5 and 10 mg/kg) potentiated the effects of Prx (4 mg/kg) on cortical [5-HT]ext in 5-HT1A+/+, but not in 5-HT1A-/- mice. Similar responses were obtained following local intra-raphe perfusion by reverse microdialysis of either WAY-100635 or (+/-)-pindolol (100 microM each). In the FST, Prx administration dose-dependently decreased the immobility time in both strains of mice, but the response was much greater in 5HT1A-/- mice. In contrast, (+/-)-pindolol blocked Prx-induced decreases in the immobility time while WAY-100635 had no effect in both genotypes. These findings using 5-HT1A-/- mice confirm that (+/-)-pindolol behaves as an antagonist of 5-HT1A autoreceptor in mice, but its blockade of paroxetine-induced antidepressant-like effects in the FST may be due to its binding to other neurotransmitter receptors.

Monoamine metabolism changes following the mouse forced swimming test but not the tail suspension test.

Microdialysis, binding and behavioural studies have shown that the dopaminergic system plays a role in antidepressant treatment. It has been suggested that stress may provoke a modification in dopamine (DA) release in different brain areas and that the forced swimming test (FST), in its own accord as a stressor, may be responsible for this modification. Naive male Swiss mice, receiving saline solution, were used in two animal models of depression, the FST and the tail suspension test (TST). In order to understand the locomotor aspect of each test, groups of mice were studied for effects on locomotor activity. Following each test, mice were killed by cervical dislocation, brains were removed and concentrations of amines in the whole brain were analysed by high-performance liquid chromatography. DA concentration increased from 5 min of the FST, dihydroxyphénylacetate (DOPAC), from 20 min of FST and serotonin, from 8 min of FST. No modification of noradrenaline was observed during the FST and no modification of the neurotransmitter concentrations was observed during the TST. Following an FST of 2-min duration, a hypolocomotor effect was observed in the subsequent actimeter test. The same effect was observed after a TST of 8 min and onwards. This study confirms the fact that although these two tests are used to study depression, they involve different neuronal mechanisms.

Characterization of 5-HT(1A/1B)-/- mice: an animal model sensitive to anxiolytic treatments.

Selective serotonin (5-HT) re-uptake inhibitors (SSRIs) are commonly used in the treatment of generalized anxiety disorder in Humans. However, because only few animal models display overt anxious-like behavior, detailed preclinical studies of the anxiolytic properties of antidepressants are still lacking. Here, we studied the neurochemical and behavioral effects of a double 5-HT(1A/1B) receptor knockout in mice (5-HT(1A/1B)-/-) as compared to their wild-type littermates (5-HT(1A/1B)+/+). It is known that single deletion of either 5-HT(1A) or 5-HT(1B) receptor induces behavioral changes that are not correlated with differences in brain serotonergic tone. Deletion of both receptors resulted in (i) higher emotionality of animals, as observed in three unconditioned paradigms of anxiety (open field, elevated plus maze and novelty suppressed feeding tests); (ii) a ≈200% increase in the mean spontaneous firing rate of 5-HT neurons in the dorsal raphe nucleus (DRN) compared to 5-HT(1A/1B)+/+ mice; (iii) elevated basal dialysate levels of 5-HT in the DRN and frontal cortex; (iv) an exaggerated response to acute paroxetine administration in microdialysis experiments, and (v) increased basal core body temperature. These findings suggest that the deletion of both autoreceptors induces a strong anxious-like behavioral state associated with increased 5-HT neurotransmission. Interestingly, 5-HT(1A/1B)-/- mice are still sensitive to the acute administration of diazepam. Moreover, while deletion of both receptors impacted on the response to acute SSRI treatment in the forced swim test, anxiolytic-like effects of a chronic SSRI treatment were still observed in 5-HT(1A/1B)-/- mice. Thus, the 5-HT(1A/1B)-/- mouse model could be of great interest to unveil the mechanisms of action of the anxiolytic effects of SSRIs.

Brain-derived neurotrophic factor-deficient mice exhibit a hippocampal hyperserotonergic phenotype.

Growing evidence supports the involvement of brain-derived neurotrophic factor (BDNF) in mood disorders and the mechanism of action of antidepressant drugs. However, the relationship between BDNF and serotonergic signalling is poorly understood. Heterozygous mutants BDNF +/- mice were utilized to investigate the influence of BDNF on the serotonin (5-HT) system and the activity of the serotonin transporter (SERT) in the hippocampus. The zero net flux method of quantitative microdialysis revealed that BDNF +/- heterozygous mice have increased basal extracellular 5-HT levels in the hippocampus and decreased 5-HT reuptake capacity. In keeping with these results, the selective serotonin reuptake inhibitor paroxetine failed to increase hippocampal extracellular 5-HT levels in BDNF +/- mice while it produced robust effects in wild-type littermates. Using in-vitro autoradiography and synaptosome techniques, we investigated the causes of attenuated 5-HT reuptake in BDNF +/- mice. A significant decrease in [3H]citalopram-binding-site density in the CA3 subregion of the ventral hippocampus and a significant reduction in [3H]5-HT uptake in hippocampal synaptosomes, revealed mainly a decrease in SERT function. However, 5-HT1A autoreceptors were not desensitized in BDNF +/- mice. These results provide evidence that constitutive reductions in BDNF modulate SERT function reuptake in the hippocampus.