Nicolas Narboux-Nême

cAMP oscillations and retinal activity are permissive for ephrin signaling during the establishment of the retinotopic map

Spontaneous activity generated in the retina is necessary to establish a precise retinotopic map, but the underlying mechanisms are poorly understood. We demonstrate here that neural activity controls ephrin-A–mediated responses. In the mouse retinotectal system, we show that spontaneous activity of the retinal ganglion cells (RGCs) is needed, independently of synaptic transmission, for the ordering of the retinotopic map and the elimination of exuberant retinal axons. Activity blockade suppressed the repellent action of ephrin-A on RGC growth cones by cyclic AMP (cAMP)-dependent pathways. Unexpectedly, the ephrin-A5–induced retraction required cAMP oscillations rather than sustained increases in intracellular cAMP concentrations. Periodic photo-induced release of caged cAMP in growth cones rescued the response to ephrin-A5 when activity was blocked. These results provide a direct molecular link between spontaneous neural activity and axon guidance mechanisms during the refinement of neural maps.

5-HT(2B) receptors are required for serotonin-selective antidepressant actions.

The therapeutic effects induced by serotonin-selective reuptake inhibitor (SSRI) antidepressants are initially triggered by blocking the serotonin transporter and rely on long-term adaptations of pre- and post-synaptic receptors. We show here that long-term behavioral and neurogenic SSRI effects are abolished after either genetic or pharmacological inactivation of 5-HT2B receptors. Conversely, direct agonist stimulation of 5-HT2B receptors induces an SSRI-like response in behavioral and neurogenic assays. Moreover, the observation that (i) this receptor is expressed by raphe serotonergic neurons, (ii) the SSRI-induced increase in hippocampal extracellular serotonin concentration is strongly reduced in the absence of functional 5-HT2B receptors and (iii) a selective 5-HT2B agonist mimics SSRI responses, supports a positive regulation of serotonergic neurons by 5-HT2B receptors. The 5-HT2B receptor appears, therefore, to positively modulate serotonergic activity and to be required for the therapeutic actions of SSRIs. Consequently, the 5-HT2B receptor should be considered as a new tractable target in the combat against depression.

Serotonin transporter transgenic (SERTcre) mouse line reveals developmental targets of serotonin specific reuptake inhibitors (SSRIs)

The serotonin transporter gene (SLC6A4; synonyms, SERT, 5-HTT) is expressed much more broadly during development than in adulthood. To obtain a full picture of all sites of SERT expression during development we used a new mouse model where Cre recombinase was inserted into the gene encoding the serotonin transporter. Two reporter mouse lines, ROSA26R and the Tau(mGFP), allowed to map all the cells that express SERT at any point during development. Combined LacZ histochemistry and GFP immunolabelling showed neuronal cell bodies and axon fiber tracts. Earliest recombination in embryos was visible in the periphery in the heart and liver by E10.5 followed by recombination in the brain in raphe serotonergic neurons by E12.5. Further, recombination in non-serotonin neurons was visible in the choroid plexus, roof plate, and neural crest derivatives; by E15.5, recombination was found in the dorsal thalamus, cingulate cortex, CA3 field of the hippocampus, retinal ganglion cells, superior olivary nucleus and cochlear nucleus. Postnatally, SERT mediated recombination was visible in the medial prefrontal cortex and layer VI neurons in the isocortex. Recombined cells were co-labelled with Neu-N, but not with GAD67, and were characterized by long range projections (corpus callosum, fornix, thalamocortical). This fate map of serotonin transporter expressing cells emphasizes the broad expression of SERT in non-serotonin neurons during development and clarifies the localization of SERT expression in the hippocampus and limbic cortex. The identification of targets of SSRIs and serotonin releasers during embryonic and early postnatal life helps understanding the very diverse physiological consequences of administration of these drugs during development.

Neurotransmitter release at the thalamocortical synapse instructs barrel formation but not axon patterning in the somatosensory cortex.

To assess the impact of synaptic neurotransmitter release on neural circuit development, we analyzed barrel cortex formation after thalamic or cortical ablation of RIM1 and RIM2 proteins, which control synaptic vesicle fusion. Thalamus-specific deletion of RIMs reduced neurotransmission efficacy by 67%. A barrelless phenotype was found with a dissociation of effects on the presynaptic and postsynaptic cellular elements of the barrel. Presynaptically, thalamocortical axons formed a normal whisker map, whereas postsynaptically the cytoarchitecture of layer IV neurons was altered as spiny stellate neurons were evenly distributed and their dendritic trees were symmetric. Strikingly, cortex-specific deletion of the RIM genes did not modify barrel development. Adult mice with thalamic-specific RIM deletion showed a lack of activity-triggered immediate early gene expression and altered sensory-related behaviors. Thus, efficient synaptic release is required at thalamocortical but not at corticocortical synapses for building the whisker to barrel map and for efficient sensory function.

Severe Serotonin Depletion after Conditional Deletion of the Vesicular Monoamine Transporter 2 Gene in Serotonin Neurons: Neural and Behavioral Consequences

The vesicular monoamine transporter type 2 gene (VMAT2) has a crucial role in the storage and synaptic release of all monoamines, including serotonin (5-HT). To evaluate the specific role of VMAT2 in 5-HT neurons, we produced a conditional ablation of VMAT2 under control of the serotonin transporter (slc6a4) promoter. VMAT2sert−cre mice showed a major (−95%) depletion of 5-HT levels in the brain with no major alterations in other monoamines. Raphe neurons contained no 5-HT immunoreactivity in VMAT2sert−cre mice but developed normal innervations, as assessed by both tryptophan hydroxylase 2 and 5-HT transporter labeling. Increased 5-HT1A autoreceptor coupling to G protein, as assessed with agonist-stimulated [35S]GTP-γ-S binding, was observed in the raphe area, indicating an adaptive change to reduced 5-HT transmission. Behavioral evaluation in adult VMAT2sert−cre mice showed an increase in escape-like reactions in response to tail suspension and anxiolytic-like response in the novelty-suppressed feeding test. In an aversive ultrasound-induced defense paradigm, VMAT2sert−cre mice displayed a major increase in escape-like behaviors. Wild-type-like defense phenotype could be rescued by replenishing intracellular 5-HT stores with chronic pargyline (a monoamine oxidase inhibitor) treatment. Pargyline also allowed some form of 5-HT release, although in reduced amounts, in synaptosomes from VMAT2sert−cre mouse brain. These findings are coherent with the notion that 5-HT has an important role in anxiety, and provide new insights into the role of endogenous 5-HT in defense behaviors.

Postnatal Growth Defects in Mice with Constitutive Depletion of Central Serotonin

Although the trophic actions of serotonin (5-HT) are well established, only few developmental defects have been reported in mouse strains with constitutive hyposerotonergia. We analyzed postnatal growth and cortical development in three different mutant mouse strains with constitutive reductions in central 5-HT levels. We compared two previously published mouse strains with severe (-95%) depletions of 5-HT, the tryptophan hydroxylase (Tph) 2(-/-) mouse line and VMAT2(sert-cre) mice, with a new strain, in which VMAT2 deletion is driven by Pet1 (VMAT2(pet1-cre)) in 5-HT raphe neurons leading to partial (-75%) reduction in brain 5-HT levels. We find that normal embryonic growth and postnatal growth retardation are common features of all these mouse strains. Postnatal growth retardation varied from mild to severe according to the extent of the brain 5-HT reduction and gender. Normal growth was reinstated in VMAT2(sert-cre) mice by reconstituting central 5-HT stores. Growth abnormalities could not be linked to altered food intake or temperature control. Morphological study of the cerebral cortex over postnatal development showed a delayed maturation of the upper cortical layers in the VMAT2(sert-cre) and Tph2(-/-) mice, but not in the VMAT2(pet1-cre) mice. No changes in layer-specific gene expression or morphological alterations of barrel cortex development were found. Overall, these observations sustain the notion that central 5-HT signaling is required for the preweaning growth spurt of mouse pups. Brain development appeared to be immune to severe central 5-HT depletion for its overall growth during prenatal life, whereas reduced brain growth and delayed cortical maturation development occurred during postnatal life. Reduced developmental 5-HT signaling during postnatal development might modulate the function and fine structure of neural circuits in ways that affect adult behavior.

Paradoxical increase in survival of newborn neurons in the dentate gyrus of mice with constitutive depletion of serotonin

Increased adult neurogenesis is a major neurobiological correlate of the beneficial effects of antidepressants. Indeed, selective serotonin (5‐HT) re‐uptake inhibitors, which increase 5‐HT transmission, enhance adult neurogenesis in the dentate gyrus (DG) of the hippocampus. However, the consequences of 5‐HT depletion are still unclear as studies using neurotoxins that target serotonergic neurons reached contradictory conclusions on the role of 5‐HT on DG cell proliferation. Here, we analysed two genetic models of 5‐HT depletion, the Pet1−/− and the VMAT2f/f; SERTcre/+ mice, which have, respectively, 80 and 95% reductions in hippocampal 5‐HT. In both models, we found unchanged cell proliferation of the neural precursors in the DG subgranular zone, whereas a significant increase in the survival of newborn neurons was noted 1 and 4 weeks after BrdU injections. This pro‐survival trait was phenocopied pharmacologically with 5‐HT synthesis inhibitor PCPA treatment in adults, indicating that this effect was not developmental. Furthermore, a 1‐week administration of the 5‐HT1A receptor agonist 8‐OH‐DPAT in Pet1−/− and PCPA‐treated mice normalised hippocampal cell survival. Overall, our results indicate that constitutive 5‐HT depletion does not alter the proliferation of neural precursors in the DG but promotes the survival of newborn cells, an effect which involves activation of postsynaptic 5‐HT1A receptors. The role of 5‐HT in selective neuronal elimination points to a new facet in its multiple effects in controlling neural circuit maturation.

Sensory Map Transfer to the Neocortex Relies on Pretarget Ordering of Thalamic Axons

Sensory maps, such as the representation of mouse facial whiskers, are conveyed throughout the nervous system by topographic axonal projections that preserve neighboring relationships between adjacent neurons. In particular, the map transfer to the neocortex is ensured by thalamocortical axons (TCAs), whose terminals are topographically organized in response to intrinsic cortical signals. However, TCAs already show a topographic order early in development, as they navigate toward their target. Here, we show that this preordering of TCAs is required for the transfer of the whisker map to the neocortex. Using Ebf1 conditional inactivation that specifically perturbs the development of an intermediate target, the basal ganglia, we scrambled TCA topography en route to the neocortex without affecting the thalamus or neocortex. Notably, embryonic somatosensory TCAs were shifted toward the visual cortex and showed a substantial intermixing along their trajectory. Somatosensory TCAs rewired postnatally to reach the somatosensory cortex but failed to form a topographic anatomical or functional map. Our study reveals that sensory map transfer relies not only on positional information in the projecting and target structures but also on preordering of axons along their trajectory, thereby opening novel perspectives on brain wiring.

Genetic Models of Serotonin (5-HT) Depletion: What do They Tell Us About the Developmental Role of 5-HT?

A large number of hyposerotonergic genetic models have been generated over the past few years. Serotonin (5‐HT) depletion has been obtained via targeting of genes involved in 5‐HT synthesis (Tph1 and Tph2), specification and determination of the 5‐HT phenotype during development (GATA3, Pet1, and Lmx1b), and 5‐HT storage or clearance (Vmat2 and SERT). Here we review these various models from a developmental perspective, beginning with a description of the sources of 5‐HT during development. We then summarize the neurological and behavioral alterations that have been observed in the genetic hyposerotonergic models. Although these models appear to have normal brain development and do not exhibit any gross morphological defects, problems in somatic growth and physiological functions have been observed. Abnormal adult behavior is also seen, although whether it results from depletion of 5‐HT during development or functional 5‐HT deficiencies in adult life remains unclear. Evidence from these hyposerotonergic models suggests that the developing brain may not need 5‐HT for the establishment of general organization and structure. However, central 5‐HT appears to be necessary for postnatal body growth, maturation of respiratory and vegetative control, and possibly for the development of normal adult behavior. Anat Rec, 2011.

Mice lacking the serotonin 5-HT2B receptor as an animal model of resistance to selective serotonin reuptake inhibitors antidepressants

Depressive disorders are among the most prevalent neuropsychiatric dysfunctions worldwide, with high rates of resistance to antidepressant treatment. Genetic factors clearly contribute to the manifestation of depression as well as to the response to antidepressants. Transgenic mouse models appear as seminal tools to disentangle this complex disorder. Here, we analyzed new key aspects of the phenotype of knock-out mice for the gene encoding the serotonin 2B receptor (Htr(2B)(-/-)), including basal phenotype, ability to develop a depressive-like phenotype upon chronic isolation, and effect of chronic exposure to fluoxetine on chronically stressed Htr(2B)(-/-) mice. We find, here, that Htr(2B)(-/-) mice display an antidepressant-like phenotype, which includes reduced latency to feed in the novelty suppressed feeding test, basal increase in hippocampal BDNF levels, no change in TrkB and p75 protein levels, and an increased preference for sucrose consumption compared to wild type (Htr(2B)(+/+)) mice. Nevertheless, we show that these mice can develop depressive-like behaviors when socially isolated during four weeks. Selective serotonin reuptake inhibitors (SSRI) have been previously shown to be ineffective in non-stressed Htr(2B)(-/-) mice. We evaluated, here, the effects of the SSRI fluoxetine in chronically stressed Htr(2B)(-/-) mice and similarly no behavioral or plastic effect was induced by this antidepressant. All together, these results highlight the suitability to study resistance to SSRI antidepressants of this mouse model displaying panoply of conditions among which behavioral, neurotrophic and plastic causative factors can be analyzed.