Annie Daszuta

Depletion in serotonin decreases neurogenesis in the dentate gyrus and the subventricular zone of adult rats.

During adulthood, neuronal precursor cells persist in two discrete regions, the subventricular zone and the hippocampal subgranular zone, as recently demonstrated in primates. To date, a few factors such as adrenal steroids and trophic factors are known to regulate adult neurogenesis. Since neuronal activity may also influence cellular development and plasticity in brain, we investigated the effects of serotonin depletion on cell proliferation occurring in these regions. Indeed, in addition to its role as a neurotransmitter, 5-hydroxytryptamine (serotonin) is considered as a developmental regulatory signal. Prenatal depletion in 5-hydroxytryptamine delays the onset of neurogenesis in 5-hydroxytryptamine target regions and 5-hydroxytryptamine promotes the differentiation of cortical and hippocampal neurons. Although in the adult brain, a few studies have suggested that 5-hydroxytryptamine may play a role in neuronal plasticity by maintaining the synaptic connections in the cortex and hippocampus, no information is actually available concerning the influence of 5-hydroxytryptamine on adult neurogenesis. If further work confirms that new neurons can be produced in the adult human brain as is the case for a variety of species, it is particularly relevant to determine the influence of 5-hydroxytryptamine on neurogenesis in the hippocampal formation, a part of the brain largely implicated in learning and memory processes. Indeed, lack of 5-hydroxytryptamine in the hippocampus has been associated with cognitive disorders, such as depression, schizophrenia and Alzheimers disease. In the present study, we demonstrated that both inhibition of 5-hydroxytryptamine synthesis and selective lesions of 5-hydroxytryptamine neurons are associated with decreases in the number of newly generated cells in the dentate gyrus, as well as in the subventricular zone.

Serotonin-Induced Increases in Adult Cell Proliferation and Neurogenesis are Mediated Through Different and Common 5-HT Receptor Subtypes in the Dentate Gyrus and the Subventricular Zone

Increase in serotonin (5-HT) transmission has profound antidepressant effects and has been associated with an increase in adult neurogenesis. The present study was aimed at screening the 5-HT receptor subtypes involved in the regulation of cell proliferation in the subgranular layer (SGL) of the dentate gyrus (DG) and the subventricular zone (SVZ) and to determine the long-term changes in adult neurogenesis. The 5-HT1A, 5-HT1B, and 5-HT2 receptor subtypes were chosen for their implication in depression and their location in/or next to these regions. Using systemic administration of various agonists and antagonists, we show that the activation of 5-HT1A heteroreceptors produces similar increases in the number of bromodeoxyuridine-labeled cells in the SGL and the SVZ (about 50% over control), whereas 5-HT2A and 5-HT2C receptor subtypes are selectively involved in the regulation of cell proliferation in each of these regions. The activation of 5-HT2C receptors, largely expressed by the choroid plexus, produces a 56% increase in the SVZ, while blockade of 5-HT2A receptors produces a 63% decrease in the number of proliferating cells in the SGL. In addition to the influence of 5-HT1B autoreceptors on 5-HT terminals in the hippocampus and ventricles, 5-HT1B heteroreceptors also regulate cell proliferation in the SGL. These data indicate that multiple receptor subtypes mediate the potent, partly selective of each neurogenic zone, stimulatory action of 5-HT on adult brain cell proliferation. Furthermore, both acute and chronic administration of selective 5-HT1A and 5-HT2C receptor agonists produce consistent increases in the number of newly formed neurons in the DG and/or olfactory bulb, underscoring the beneficial effects of 5-HT on adult neurogenesis.

Agomelatine, a new antidepressant, induces regional changes in hippocampal neurogenesis.

BACKGROUND Antidepressant treatments increase neural plasticity and adult neurogenesis, especially in the hippocampus. Here, we determined the effects of agomelatine (S-20098), a new antidepressant, on various phases of neurogenesis in the dentate gyrus of adult rat. METHODS Animals were injected with agomelatine for different time periods. Immunostaining for bromodeoxyuridine, neuron specific nuclear protein, and glial fibrillary acid protein, as well as for the highly polysialylated form of neuronal cell adhesion molecule and doublecortin, was used to detect changes in cell proliferation, neurogenesis, and survival. Cell death was estimated by terminal deoxynucleotidyl transferase-mediated biotinylated deoxyuridine triphosphate nick end labeling and cresyl violet staining. RESULTS Chronic (3 weeks) but not acute (4 hours) or subchronic (1 week) administration of agomelatine increased cell proliferation and neurogenesis in the ventral dentate gyrus, a region notably implicated in response to emotion, which is consistent with the antidepressant-anxiolytic properties of the drug. Extending agomelatine treatment over several weeks, however, increases survival of newly formed neurons in the entire dentate gyrus. Finally, agomelatine treatment does not affect mature granule cells. CONCLUSIONS This study shows that an antidepressant can affect differentially various stages of neurogenesis in the dorsal and ventral hippocampus. Altogether, these changes lead to a pronounced augmentation in the total number of new granule cells.

Serotonin may stimulate granule cell proliferation in the adult hippocampus, as observed in rats grafted with foetal raphe neurons

The long‐term effects of hippocampal serotonergic denervation and reinnervation by foetal raphe tissue were examined in the dentate gyrus where neurons are continously born in the adult. Complete lesion of serotonin neurons following injections of 5,7‐dihydroxytryptamine in the dorsal and medial raphe nuclei produced long‐term decreases in the number of newly generated granule cells identified with 5‐Bromo‐2′‐deoxyuridine (BrdU) and the polysialylated form of neural cell adhesion molecule (PSA‐NCAM) immunostaining, as observed in 2‐month‐survival rats. The raphe grafts, but not the control grafts of embryonic spinal tissue, reversed the postlesion‐induced decreases in the density of BrdU‐ and PSA‐NCAM‐labelled cells detected in the granule layer. Inhibition of serotonin synthesis in animals with raphe grafts reversed back to lesion‐induced changes in granule cell proliferation. Furthermore, extensive serotonergic reinnervation of the dentate gyrus in the area proximal to the raphe graft could be associated with supranormal density of BrdU‐labelled cells. These results indicate that serotonin may be considered a positive regulatory factor of adult granule cell proliferation. Finally, the lack of effect of embryonic nonserotonergic tissue grafted to serotonin‐deprived rats suggests that neurotrophic factors may not be involved in the effects of serotonin on adult neurogenesis.

Serotonin mediates oestrogen stimulation of cell proliferation in the adult dentate gyrus

Characterizing the mechanisms by which endogenous factors stimulate neurogenesis is of special interest in view of the possible implication of newly generated cells in hippocampal functions or disorders. The aim of this study was to determine whether serotonin (5‐HT) and oestradiol (E2) act through a common pathway to increase cell proliferation in the adult dentate gyrus (DG). We also investigated the effects of long‐lasting changes in oestrogen levels on cell proliferation. Combining ovariectomy with inhibition of 5‐HT synthesis using p‐chlorophenylalanine (PCPA) treatment produced approximately the same decreases in the number of bromodeoxyuridine (BrdU) and PSA‐NCAM immunolabelled cells in the subgranular layer as ovariectomy alone. Administration of 5‐hydroxytryptophan (5‐HTP) restored cell proliferation primarily decreased by ovariectomy, whereas oestradiol was unable to reverse this change in ovariectomized rats treated with PCPA. These findings demonstrate that 5‐HT mediates oestrogen stimulation of cell proliferation in adult dentate gyrus. However, increase in ovarian hormones during pregnancy has no effect on dentate cell proliferation. This finding suggests that concomitant changes in other factors, such as glucocorticoids, may counterbalance the positive regulation of cell proliferation by 5‐HT and oestradiol. Finally, oestrogen may regulate structural plasticity by stimulating PSA‐NCAM expression independently of neurogenesis, as shown for instance by the increases in the number of PSA‐NCAM labelled cells in pregnants. As 5‐HT and oestrogen are involved in mood disorders, our data suggest that the positive regulation of cell proliferation and neuroplasticity by these two factors may contribute to restore hippocampal connectivity in depressive patients.

Mechanisms Contributing to the Phase-Dependent Regulation of Neurogenesis by the Novel Antidepressant, Agomelatine, in the Adult Rat Hippocampus

Agomelatine is a novel antidepressant acting as a melatonergic receptor agonist and serotonergic (5-HT2C) receptor antagonist. In adult rats, chronic agomelatine treatment enhanced cell proliferation and neurogenesis in the ventral hippocampus (VH), a region pertinent to mood disorders. This study compared the effects of agomelatine on cell proliferation, maturation, and survival and investigated the cellular mechanisms underlying these effects. Agomelatine increased the ratio of mature vs immature neurons and enhanced neurite outgrowth of granular cells, suggesting an acceleration of maturation. The influence of agomelatine on maturation and survival was accompanied by a selective increase in the levels of BDNF (brain-derived neurotrophic factor) vs those of VEGF (vascular endothelial factor) and IGF-1 (insulin-like growth factor 1), which were not affected. Agomelatine also activated several cellular signals (extracellular signal-regulated kinase1/2, protein kinase B, and glycogen synthase kinase 3β) known to be modulated by antidepressants and implicated in the control of proliferation/survival. Furthermore, as agomelatine possesses both melatonergic agonist and serotonergic (5-HT2C) antagonist properties, we determined whether melatonin and 5-HT2C receptor antagonists similarly influence cell proliferation and survival. Only the 5-HT2C receptor antagonists, SB243,213 or S32006, but not melatonin, mimicked the effects of agomelatine on cell proliferation in VH. The promoting effect of agomelatine on survival was not reproduced by the 5-HT2C receptor antagonists or melatonin alone. However, it was blocked by a melatonin antagonist, S22153. These results show that agomelatine treatment facilitates all stages of neurogenesis and suggest that a joint effect of melatonin agonism and 5HT2C antagonism may be involved in promotion by agomelatine of survival in the hippocampus.

Lesion Study of the Distribution of Serotonin 5-HT4 Receptors in Rat Basal Ganglia and Hippocampus

The regional distribution of 5‐hydroxytryptamine (5‐HT4) receptors labelled with [3H]GR113808 was examined in rat basal ganglia and hippocampus after specific lesions. Lesion of serotonin neurons induced by injections of 5,7‐dihydroxytryptamine into the dorsal and medial raphe nuclei resulted in increased 5‐HT4 receptor binding in most regions examined, compared with controls. More precisely, there was a 78% increase in the rostral but no change in the caudal part of caudate‐putamen, and 83% and 54% increases in the shell and core of the nucleus accumbens respectively. In the substantia nigra, the increase in 5‐HT4 binding was larger (72%) than that in the globus pallidus (32%). In the hippocampus, 63%, 30% and 28% increases were measured in CA2, CA1 and CA3 respectively. Following lesion of dopamine neurons by intranigral injection of 6‐hydroxydopamine, increased 5‐HT4 receptor binding was observed in the caudal (59%), but not the rostral part of caudate‐putamen, as well as in the globus pallidus (93%). Since no decreases in 5‐HT4 receptor density were detected after the dopamine lesion, it was concluded that these receptors are not expressed in dopamine neurons. Kainic acid lesions of the caudate‐putamen were associated with dramatic local decreases in 5‐HT4 receptor binding on the injected side (‐89%), which suggested that striatal neurons express 5‐HT4 receptors. Corresponding decreases of 72 and 20% in receptor density were detected in globus pallidus and substantia nigra, consistent with a presumed localization of 5‐HT4 receptors on striatal GABA neurons projecting to these regions. In the substantia nigra, the decrease in [3H]GR113808 binding was localized to the pars lateralis, indicating that striatal neurons belonging to the cortico‐striato‐nigrotectal pathway, and containing GABA and dynorphin, express 5‐HT4 receptors.

Adaptive changes in serotonin neurons of the raphe nuclei in 5-HT(4) receptor knock-out mouse.

Decreased serotonin (5‐HT) transmission is thought to underlie several mental diseases, including depression and feeding disorders. However, whether deficits in genes encoding G protein‐coupled receptors may down‐regulate the activity of 5‐HT neurons is unknown currently. Based on recent evidence that stress‐induced anorexia may involve 5‐HT4receptors (5‐HT4R), we measured various aspects of 5‐HT function in 5‐HT4R knock‐out (KO) mice. When compared to dorsal raphe nucleus (DRN) 5‐HT neurons from wild‐type mice, those from 5‐HT4R KO mice exhibited reduced spontaneous electrical activity. This reduced activity was associated with diminished tissue levels of 5‐HT and its main metabolite, 5‐hydroxyindole acetic acid (5‐HIAA). Cumulative, systemic doses of the 5‐HT uptake blocker citalopram, that reduced 5‐HT cell firing by 30% in wild‐type animals, completely inhibited 5‐HT neuron firing in the KO mice. This effect was reversed by administration of the 5‐HT1A receptor (5‐HT1AR) antagonist, WAY100635, in mice of both genotypes. Other changes in DRN of the KO mice included increases in the levels of 5‐HT plasma membrane transporter sites and mRNA, as well as a decrease in the density of 5‐HT1AR sites without any change in 5‐HT1A mRNA content. With the exception of increased 5‐HT turnover index in the hypothalamus and nucleus accumbens and a decreased density of 5‐HT1AR sites in the dorsal hippocampus (CA1) and septum, no major changes were detected in 5‐HT territories of projection, suggesting region‐specific adaptive changes. The mechanisms whereby 5‐HT4R mediate a tonic positive influence on the firing activity of DRN 5‐HT neurons and 5‐HT content remain to be determined.

Selective increases in serotonin 5-HT1B/1D and 5-HT2A/2C binding sites in adult rat basal ganglia following lesions of serotonergic neurons

Quantitative autoradiography was used to examine possible adaptive changes in serotonin 5-HT1B/1D and 5-HT2A/2C receptor binding sites in adult rat basal ganglia, after partial or severe lesions of serotonergic neurons produced by intraraphe injections of variable amounts of 5,7-dihydroxytryptamine. In controls, the 5-HT1B/1D sites labeled with S-CM-G[125I]TNH2 were evenly distributed in the core and the shell of the nucleus accumbens. The density of 5-HT1B/1D sites was higher in the ventral than dorsal part of the striatum and no regional differences were detected along the rostrocaudal axis of the structure. The 5-HT2A/2C sites labeled with [125I]DOI were preferentially distributed in the mediodorsal striatum and higher densities were detected in the shell than core of the nucleus accumbens. Following 5,7-dihydroxytryptamine injections, there were no changes in binding of either receptor subtype after partial lesions entailing 80-90% 5-HT depletions. After severe 5-HT depletions (over 95%), large increases in 5-HT1B/1D binding were observed in the substantia nigra (78%), but no changes took place in the globus pallidus. Increases in 5-HT1B/1D binding were also detected in the shell of the nucleus accumbens (27%). Similar sized increases in 5-HT2A/2C binding (22%) were restricted to the medial striatum. The present results suggest a preferential association between 5-HT1B/1D receptors and the striatonigral neurons containing substance P, as indicated by the striatal distribution of these receptors and their selective increases in the substantia nigra after severe 5-HT deprivation. We recently proposed a similar relationship between the 5-HT4 receptors and the striatopallidal neurons containing met-enkephalin. Moreover, the increases in 5-HT1B/1D binding in the substantia nigra and in the shell of the nucleus accumbens reinforce the view of an implication of this receptor subtype in motor functions. In contrast, the prominent increases in 5-HT2A/2C binding after severe 5-HT deprivation as restricted to the medial region of the striatum and suggest up-regulation of most probably 5-HT2C receptors in a region implicated in cognitive functions.

Region- and phase-dependent effects of 5-HT1A and 5-HT2C receptor activation on adult neurogenesis

Adult neurogenesis and serotoninergic transmission are associated to mood disorders and their treatments. The present study focused on the effects of chronic activation of 5-HT(1A) and 5-HT(2C) receptors on newborn cell survival in the dentate gyrus (DG) and olfactory bulb (OB), and examined whether potential neurogenic zones as the prefrontal cortex (PFC) and striatum (ST) are reactive to these treatments. Administration of 8-OH-DPAT, but not RO600,175 increases neurogenesis and survival of late differentiating cells (15-21days) in the DG. Both 8-OH-DPAT and RO600,175 increase neurogenesis in the OB, but only 8-OH-DPAT affected cell survival, inducing a parallel decrease in the number of BrdU cells in the OB and increase in the SVZ, which suggests an impaired migration. In the PFC and ST, 8-OH-DPAT and R0600,175 increase gliogenesis (NG2-labeled cells). This study provides new insights on the serotonergic regulation of critical phases of neurogenesis helpful to understand the neurogenic and gliogenic effects of antidepressant treatments in different brain regions.