Cajal-Retzius cells are critical in cortical lamination, but very little is known about their origin and development. The homeodomain transcription factor Dbx1 is expressed in restricted progenitor domains of the developing pallium: the ventral pallium (VP) and the septum. Using genetic tracing and ablation experiments in mice, we show that two subpopulations of Reelin(+) Cajal-Retzius cells are generated from Dbx1-expressing progenitors. VP- and septum-derived Reelin(+) neurons differ in their onset of appearance, migration routes, destination and expression of molecular markers. Together with reported data supporting the generation of Reelin(+) cells in the cortical hem, our results show that Cajal-Retzius cells are generated at least at three focal sites at the borders of the developing pallium and are redistributed by tangential migration. Our data also strongly suggest that distinct Cajal-Retzius subtypes exist and that their presence in different territories of the developing cortex might contribute to region-specific properties.

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-HT(2B) receptors. Conversely, direct agonist stimulation of 5-HT(2B) 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-HT(2B) receptors and (iii) a selective 5-HT(2B) agonist mimics SSRI responses, supports a positive regulation of serotonergic neurons by 5-HT(2B) receptors. The 5-HT(2B) receptor appears, therefore, to positively modulate serotonergic activity and to be required for the therapeutic actions of SSRIs. Consequently, the 5-HT(2B) receptor should be considered as a new tractable target in the combat against depression.

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.

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.

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. VMAT2 sertÀcre mice showed a major (À95%) depletion of 5-HT levels in the brain with no major alterations in other monoamines. Raphe neurons contained no 5-HT immunoreactivity in VMAT2 sertÀcre mice but developed normal innervations, as assessed by both tryptophan hydroxylase 2 and 5-HT transporter labeling. Increased 5-HT 1A autoreceptor coupling to G protein, as assessed with agonist-stimulated [ 35 S]GTP-g-S binding, was observed in the raphe area, indicating an adaptive change to reduced 5-HT transmission. Behavioral evaluation in adult VMAT2 sertÀcre 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 ultrasoundinduced defense paradigm, VMAT2 sertÀcre 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 VMAT2 sertÀcre 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.

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 VMAT2(f/f) ; SERT(cre/+) 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.

The amniote middle ear is a classical example of the evolutionary novelty. Although paleontological evidence supports the view that mammals and diapsids (modern reptiles and birds) independently acquired the middle ear after divergence from their common ancestor, the developmental bases of these transformations remain unknown. Here we show that lower-to-upper jaw transformation induced by inactivation of the Endothelin1-Dlx5/6 cascade involving Goosecoid results in loss of the tympanic membrane in mouse, but causes duplication of the tympanic membrane in chicken. Detailed anatomical analysis indicates that the relative positions of the primary jaw joint and first pharyngeal pouch led to the coupling of tympanic membrane formation with the lower jaw in mammals, but with the upper jaw in diapsids. We propose that differences in connection and release by various pharyngeal skeletal elements resulted in structural diversity, leading to the acquisition of the tympanic membrane in two distinct manners during amniote evolution.

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.