The cerebral cortex of adult mammals is a laminated structure comprised of six layers that each contain a complement of pyramidal (glutamatergic) and non-pyramidal (GABAergic) neurons (Peters and Jones, 1984). Pyramidal neurons make up ~80% of all adult cortical neurons, sending excitatory output axons to other cortical areas and to distant parts of the brain (Peters and Kara, 1985a; Thomson and Lamy, 2007; Spruston, 2008). The vast majority of cortical GABAergic cells are interneurons that only make local connections. GABAergic interneurons are extremely diverse, differing in shape, electrophysiological properties, and the combination of neuropeptides and calcium-binding proteins that they express (Peters and Kara, 1985b; Cavanagh and Parnavelas, 1988; DeFelipe, 1993; Kawaguchi and Kondo, 2002; Blatow et al., 2005; Tomson and Lamy, 2007; PING et al., 2008; Karagiannis et al., 2009; Xu et al., 2010; Vitalis and Rossier, 2011; Tricoire and Vitalis, 2012; DeFelipe et al., 2013). Using these differentiating characteristics, one can at a first approximation distinguish four main classes of interneurons populating the somatosensory cortex (PING et al., 2008; DeFelipe et al., 2013). First, fast-spiking interneurons that express parvalbumin (Parv), and act as an inhibitory gate for incoming sensory information (Inoue and Imoto, 2006; Sun et al., 2006). Second, adapting martinotti cells that express somatostatin (SOM), and are thought to control dendritic information through local feedback inhibition (Karube et al., 2004). Third, adapting bipolar interneurons that express vasoactive intestinal peptide (VIP) and calretinin (CR), and preferentially target other interneurons and receive direct input from the thalamus (Férézou et al., 2007; Vitalis and Rossier, 2011). Fourth, adapting neurogliaform interneurons that express neuropeptide Y (NPY) and/or nitric oxide (NO), and that are responsible for the slow GABAergic inhibition of pyramidal cells and interneurons (Karagiannis et al., 2009; Oláh et al., 2009; Perrenoud et al., 2012a,b; Tricoire and Vitalis, 2012).

5-HT is synthesized from the essential amino-acid tryptophan. In the blood stream, 90% of tryptophan is linked to serum-albumin. A proportion reaching ~10% when the blood-brain barrier becomes fully functional (postnatal day 12) and decreasing with age is free to cross the developing blood-brain barrier (Ribatti et al., 2006). Tryptophan is accumulated in 5-HT producing cells by a non-specific transporter with high affinity to several uncharged aromatic amino-acids. Tryptophan is then hydroxylated in these cells into 5-hydroxytryptophan by the tryptophan hydroxylase. Tryptophan hydroxylase type 2 (Tph2) is expressed in serotonergic neurons of the raphe nuclei (Côté et al., 2003; Walther et al., 2003), while peripheral tissues mostly express tryptophan hydroxylase type 1 (Tph1). 5-hydroxytryptophan is then further decarboxylated into 5-HT by the aromatic amino-acid decarboxylase (AADC). 5-HT is catabolized in the cytoplasm of 5-HT transporter (SERT) expressing cells by monoamine oxidase A or B (MAOA or MAOB). MAOA has higher affinity to 5-HT than MAOB, but both enzymes are co-expressed in rodent serotonergic neurons between E12 and P7 (Vitalis et al., 2002a). After P7, the expression of MAOB becomes predominant, and MAOA deficiency could be partially compensated for by the increased expression of MAOB in serotonergic neurons (Cases et al., 1996; Vitalis et al., 2002a; Cheng et al., 2010).

5-HT of the embryonic telencephalon is not only produced locally by serotonergic neurons of the raphe nuclei, but also originates from extra-CNS (embryonic periphery, placental) as well as extra-embryonic (maternal) sources. In the two following sections, we briefly recapitulate the development of the serotonergic system and review the various sources of telencephalic 5-HT during embryonic and early postnatal life.

Serotonergic neurons of the brainstem are subdivided into 9 groups forming two clusters: the caudal division (B1–B4; including the raphe pallidus, obscurus, magnus, and pontis) projecting to the spinal cord and the cerebellum, and the rostral division [B5–B9; including the dorsal (B6, B7) and median raphe nuclei (B5, B8)] projecting to the forebrain (Lidov and Molliver, 1982; Steinbusch and Nieuwenhuys, 1983; Törk, 1990; Figure 1A). Recent genetic and developmental approaches revealed differential rhombomeric identities of raphe 5-HT neurons, which introduce a new layer of functional classification (Jensen et al., 2008; Kiyasova and Gaspar, 2011). Together with genetic tracing and topographic projection mapping, we will soon have a much better understanding of the anatomical organization of the 5-HT systems.

In mice, dorsal raphe neurons differentiate in the brainstem by E10–E11 (E12–E15 in rats). This period coincides with the beginning of telencephalic vesicle formation (Wallace and Lauder, 1983; Aitken and Törk, 1988). Serotonergic neurons generated rostral to the isthmus (B6–B9 groups; dorsal and median raphe) send axons only one day after their genesis. These axons reach the cortico-striatal junction by E14 in mice (by E16 in rats; Wallace and Lauder, 1983; Figure 1B), during the peak of migration of cortical GABAergic interneurons generated in the MGE. 5-HT-containing axons enter the cortical anlage as two tangential streams, one above and the other below the CP (Wallace and Lauder, 1983; Aitken and Törk, 1988). The former is distributed in the MZ where pioneering C-R cells are located and with which they are in close appositions, making transient synaptic contacts (Radnikow et al., 2002; Janusonis et al., 2004).

Below the CP, 5-HT afferents are mainly restricted to the IZ and the SP (Wallace and Lauder, 1983). At E14, the developing cerebral cortex (Bayer and Altman, 1991) and the ganglionic eminences produce deeper-layer neurons (glutamatergic and GABAergic, respectively) that are in the process of migration to their final positions. By E16–E17 in mice, thalamocortical axons (TCAs) penetrate the cortical anlage and are in close apposition with 5-HT axons running in the IZ. In parallel, cortical neurons begin to establish their polarity, sending their axons toward their respective targets and developing numerous dendritic processes. At the end of corticogenesis, 5-HT axons gradually arborize sending numerous branches into the CP (Wallace and Lauder, 1983). During this period a large proportion of GABAergic interneurons enter the CP where they radially migrate to reach their final positions (see above). Progressively, serotonergic axons become evenly distributed in the different cortical territories and show their mature pattern of innervation by P21 (Steinbusch, 1981). However, dorsal raphe and median raphe projections differ anatomically. The dorsal raphe projections have been described as generally thin, displaying numerous branches with pleiotropic varicosities and preferentially arborize in cortical layers IV and V that receive thalamic inputs. By contrast, median raphe projections are characterized by large spherical varicosities that can form true chemical synapses (Törk, 1990). They preferentially arborize in layer I and lower white matter, give collaterals that could surround neuronal cell bodies and proximal dendrites, and preferentially contact interneurons containing VIP- and cholecystokinin (CCK) in various species (Törk, 1990; Hornung and Celio, 1992; Férézou et al., 2007). Interestingly, in the mature brain, a dense plexus of 5-HT-positive fibers is present in the SVZ, in close apposition with progenitor cells of this region (Jahanshahi et al., 2011). At all stages 5-HT could be released along the entire axonal network thus diffusing into the entire extracellular fluid. It is still not clear whether subsets of serotonergic axons preferentially release 5-HT in synaptic clefts vs. volume transmission.

Although 5-HT is likely to act as a trophic or instructive factor during early periods of cortical development, its sources have remained elusive. Evidence indicates that 5-HT is supplied to the developing cerebral cortex before 5-HT axons reach their targets or even before serotonergic neurons are generated. In line with this observation, 5-HT receptors are expressed in the rostral forebrain, craniofacial region, and peripheral region days before serotonergic axons enter these regions (Buznikov et al., 2001). Furthermore, ex vivo application of 5-HT or alteration of 5-HT levels during early embryonic stages can alter normal development of various embryonic structures before serotonergic neurons have innervated these structures (Lauder, 1988; Shuey et al., 1992; Moiseiwitsch and Lauder, 1995; Whitaker-Azmitia et al., 1996; Buznikov et al., 2001; Witaker-Azmitia, 2001). Recently, the placenta (that is of embryonic origin) has been identified as an important source of 5-HT for the developing embryo (Bonnin et al., 2011; Figure 2). Syncytiotrophoblastic cells of the placenta contain Tph1, AADC, and MAO (Grimsby et al., 1990; Shih et al., 1990), and convert tryptophan of maternal origin into 5-HT as soon as E10–E11 (Bonnin et al., 2011). Genetically modified mice in which 5-HT neurons fail to fully differentiate or to produce normal amounts of 5-HT levels do not display severe cortical defects when gestating in heterozygous dams with an almost unaltered serotonergic system, suggesting that sources of 5-HT independent of embryonic serotonergic neurons could be sufficient to permit normal cortical development. Examples include mice lacking the transcription factors Lmx1b (Smidt et al., 2000) or Pet-1 (Hendricks et al., 1999), in which all or 70–80% of 5-HT raphe neurons fail to develop, respectively, and in mice lacking Tph2 Alenina et al., 2009; Gutknecht et al., 2012; Migliarini et al., 2012. Further analysis revealed that Pet-1 knockout embryos developing in heterozygous dams have normal 5-HT levels before the closure of the brain-blood barrier (before E15; Daneman et al., 2010). In addition, SERT^+/− embryos developing in SERT^−/− or wild type dams had similar levels of 5-HT before E15 (Bonnin et al., 2011). Together, these results revealed that the placenta is an important source of 5-HT for the embryonic CNS before E15 but questioned the contribution of maternal 5-HT that was suspected in earlier studies (Shuey et al., 1992; Yavarone et al., 1993; Côté et al., 2003, 2007).

Outside the CNS, 5-HT is also synthesized in the periphery of the developing embryo. In particular, high levels of 5-HT are produced in the myenteric plexus (from E15 to E16), by enterochromaffin cells of the lining lumen of the digestive tract (from E18), by neuroepithelial cells of the respiratory tracts, by pinealocytes (from E11 to E12) and by parafollicular cells of the thyroid. After being released from 5-HT producing cells, 5-HT could be taken up by SERT expressing cells including platelets and mast cells (Jankovic, 1989; Zhuang et al., 1996) that become numerous around E12 in mice. These cells could cross the blood-brain barrier and transit across blood vessels that start to invade the developing cortex by E10–E11 in mice (Daneman et al., 2010). However, overall peripheral structures are thought to contribute only to a small proportion of cortical 5-HT during development.

In addition, sensory thalamic neurons projecting to primary sensory cortices (i.e., somatosensory, visual, auditory) transiently express SERT (E15–P15) and the vesicular monoamine transporter type 2 (VMAT2) that are respectively responsible for the uptake and packaging of 5-HT into synaptic vesicles (Cases et al., 1996, 1998; Vitalis et al., 1998; Lebrand et al., 1996, 1998; Gaspar et al., 2003; Vitalis and Parnavelas, 2003; Figure 2). While equipped with these transporters, thalamic neurons may release 5-HT in an activity-dependent fashion by transiently adopting a serotonergic phenotype even without expressing TPH or MAOs (Vitalis et al., 2002a). Interestingly, it has been suggested that TCAs could be implicated in the proliferation and migration of glutamatergic neurons, and it is thus possible that release of 5-HT by TCAs could contribute to the regulation of these processes (Kennedy and Dehay, 1997; Edgar and Price, 2001). Fate mapping of SERT-expressing cells in mice revealed that in addition to the thalamus, also the cortex, hippocampus, hypothalamus, and brainstem harbor neurons that transiently adopt a serotonergic phenotype (Narboux-Nême et al., 2008). Within the cortex, transient SERT expression starts between E15 and P0 and is confined to layers V and VI (infralimbic, prelimbic, and anterior cingulate cortex) or layers II, V, and VI (posterior cingulate and retrosplenial cortex). The role of 5-HT signaling by these neurons remains to be elucidated. However, because of the spatial and temporal aspects of this phenomenon, it is tempting to speculate that transient serotonergic neurons might influence cortical maturation and circuit formation.

At least 14 genes that encode for 5-HT receptors have been identified and cloned in the mammalian brain (Hoyer et al., 1994, 2002; Raymond et al., 2001; Hannon and Hoyer, 2008; Millan et al., 2008). In addition, alternative splicing and RNA editing add to the diversity of 5-HT receptors. With the exception of the 5-HT₃ receptors, all 5-HT receptors are coupled to G-proteins, leading to a categorization into four groups according to their second messenger coupling pathways. The 5-HT₁ and 5-HT₅ receptors are coupled to Gi/Go proteins and exert their inhibitory effects on adenylate cyclase inhibiting cAMP formation. The 5-HT₂ receptors are coupled to Gq proteins and stimulate phospholipase C to increase the hydrolysis of inositol phosphates and elevate intracellular Ca²⁺. The 5-HT_4,6,7 receptors are coupled to Gs proteins and are positively linked to adenylate cyclase and increase cAMP formation. 5-HT₃ (5-HT_3A and 5-HT_3B) receptors belong to a family of ligand-gated ion channel receptors that include nicotinic acetylcholine receptors, GABA_A receptors, and glycine receptors and that are modulated by intracellular cyclic AMP (Hoyer et al., 1994). The 5-HT₃ receptors respond to neurotransmitter release via direct (through the 5-HT₃ receptor itself) or indirect (via the activation of the voltage-gated Ca²⁺ channels) increase of Ca²⁺ entry into the cell (reviewed in Chameau and van Hooft, 2006). 5-HT₃ receptors are composed of five subunits, with the majority being homomers of 5-HT_3A receptors. Heteromeric 5-HT_3AB receptors have been observed in specific brain regions and display lower Ca²⁺ permeability than the homomeric 5-HT_3A receptors (Tecott et al., 1993; Morales and Bloom, 1997; Davies et al., 1999; Morales and Wang, 2002). Furthermore, the co-assembly of the 5-HT₃ with the alpha4 subunit of the nicotinic acetylcholine has been reported to confer increased permeability to Ca²⁺ (Kriegler et al., 1999; Chameau and van Hooft, 2006).

The expression of 5-HT receptors during cortical development is not yet fully characterized. However, the recent use of transgenic animals (i.e., carrying the GFP/YFP under the control of a specific 5-HTR promoter) and open in situ hybridization databases (i.e., Allen Brain Atlas) have started to provide valuable insights. For example, 5-HT_1A,F are expressed in neocortical proliferative zones in E14.5 rodent brain (Hillion et al., 1994; Bonnin et al., 2006) and the 5-HT_2B are expressed in the proliferative zones of the human occipital cortex (Lidov and Rakic, 1995). The 5-HT_1A,B,D, 5-HT_2A, 5-HT_2C, and 5-HT_3A, are expressed in specific subpopulations of postmitotic neurons (Hillion et al., 1994; Johnson and Heinemann, 1995; Tecott et al., 1993; Morales and Bloom, 1997; Bonnin et al., 2006; Chameau et al., 2009; Vucurovic et al., 2010; Tanaka et al., 2012), whereas the 5-HT₆ is expressed in both migrating interneurons and pyramidal neurons (Riccio et al., 2011; Figure 3). Although a complete developmental time-course of 5-HT₆ expression in the dorsal pallium is not available, 5-HT₆ expression is detected in the developing rat brain as early as E12 and is maintained stable until adult age (Grimaldi et al., 1998). In adulthood, 5-HT₆ receptors are expressed in layers II–VI of the rodent postnatal and mature cerebral cortex (Ward et al., 1995; Hamon et al., 1999; Gerard et al., 1997), and pyramidal neurons and glial cells of the human prefrontal cortex (Marazziti et al., 2013). Interestingly, human prefrontal cortex expression of the 5-HT₆ receptor peaks in toddlers (Lambe et al., 2011).

The dynamic expression pattern of the 5-HT_3A receptor is recapitulated in Figure 4. In the mouse cortical anlage, 5-HT_3A is expressed as early as E12 in PP neurons expressing reelin (C-R cells) and/or GABA (Chameau et al., 2009; Vucurovic et al., 2010). During the period of intense production of GABAergic neurons, the 5-HT_3A is expressed by newly postmitotic (Tuj-1+) neurons located in the CGE and AEP/PO, where about 30% of cortical GABAergic neurons are generated (Lee et al., 2010; Vucurovic et al., 2010). Using homochronic in utero grafting in combination with a transgenic mouse line expressing GFP under the control of the 5-HT_3A promoter (5-HT_3A:GFP animals) we have shown that this expression was protracted in two large subpopulations of cortical GABAergic neurons that could be distinguished based on their electrophysiological properties, molecular contents, and morphologies. The first one corresponded to multipolar interneurons expressing NPY and displaying late spiking and accommodating properties while the second one corresponded to small bipolar and doublet bouquet interneurons expressing VIP and displaying adapting and bursting properties (Vucurovic et al., 2010; Lee et al., 2010; Rudy et al., 2011; Vitalis and Rossier, 2011). During postnatal stages and decreasing with age 5-HT_3A receptors are also expressed in young neurons (doublecortin+ and/or CR+), which are generated in the SVZ and migrate toward the olfactory bulb and various cortical and subcortical regions (Inta et al., 2008; Riccio et al., 2012). In addition, we recently found that 5-HT_3A receptors are expressed during postnatal development (P0–21) in a pool of migrating interneurons, which are probably generated from local transient amplifying precursors within the white matter, ventral to the anterior cingulate cortex (Riccio et al., 2012).

It has been postulated for some time that 5-HT regulates the proliferation of a wide variety of cell types including cortical neurons. Indeed, studies that pharmacologically or genetically deplete maternal and embryonic brain 5-HT levels or restrict tryptophan availability have found reduced embryonic brain size as a major consequence. Chronic pCholophenylalanin (PCPA)-treatment, which inhibits 5-HT synthesis, alters the proliferation of serotonergic target cells (i.e., the hippocampal field and cerebral cortex) when administrated daily to pregnant dams from E8 to E12 (Lauder and Krebs, 1978) or from E12 to E17 (Vitalis et al., 2007). Similar observations were made after reserpine-treatments that deplete 5-HT (Holson et al., 1994), or after lesions of serotonergic fibers such as those observed after high cocaine administration (Clarke et al., 1996). However, there are several drawbacks in these initial studies. For example, chronic treatments are likely to induce secondary alterations, which might be ultimately responsible for the effects observed. Another major problem is the selectivity of the neurotransmitter system affected. This is particularly problematic for reserpine-treatments that deplete all monoamines. Recently, the generation of transgenic models selectively targeting specific 5-HT-related genes in different neuronal populations have started to provide more specific insights. For instance mice deficient for tph1 or tph2 showed body weight reduction and delayed maturation of upper cortical layers (Côté et al., 2007; Alenina et al., 2009; Narboux-Neme et al., 2013). A 2 h pulse labeling experiment revealed that heterozygous embryos growing in null mutant tph1^−/− mice showed an ~30% reduction of BrdU-positive cells in the VZ when compared to tph1^−/− embryos growing in heterozygous mice (Côté et al., 2007). Together these studies suggest that 5-HT regulates the proliferation of neuronal precursors, but additional studies are needed to refine these initial observations and confirm this conclusion.

Initial in vitro studies have failed to show that 5-HT could modulate the proliferation of cortical progenitors (Dooley et al., 1997; Lavdas et al., 1997), as the proportion of cells that integrated BrdU was similar in untreated and treated cultures. However, since 5-HT had an anti-apoptotic effect the dilution of BrdU+ cells may have masked this proliferative effect. Furthermore, it was demonstrated that stimulation of the 5-HT₂ and 5-HT₃ had no effect on cortical neurogenesis (Dooley et al., 1997; Vitalis and Parnavelas, 2003). This is consistent with the fact that 5-HT_3A is not expressed in pallial and subpallial proliferative zones (Vucurovic et al., 2010). In contrast, the 5-HT_1A appears to mediate such a role. In vivo, PCPA-induced microcephaly is reversed after treatment with a 5-HT_1A agonist. Furthermore, in the adult rodent brain, 5-HT_1A promotes neurogenesis in the subgranular zone of the dentate gyrus (Brezum and Daszuta, 1999, 2000; Gould, 1999; Haring and Yan, 1999) and such a role has been postulated to be a key feature of antidepressant therapies (Guthrie and Gilula, 1989; Santarelli et al., 2003). Recently, the analysis of mice lacking MAOA and B, which display high 5-HT levels but normal dopamine and norepinephrine levels during embryonic and early postnatal development, revealed a specific reduction of symmetric divisions of intermediate precursor cells (Corbin et al., 2008) in the SVZ during late corticogenesis (E17.5; Cheng et al., 2010). This unexpected alteration was reverted after E14.5–E19.5 PCPA-treatment. In addition, neurosphere formation was modulated by 5-HT in a dose-dependent manner in vitro, with proliferative effects observed for concentration ranging from 10 to 100 ng/ml and inhibitory effects observed for higher concentration (1000 ng/ml). Interestingly, these inhibitory effects were associated with decreased 5-HT_1A labeling of neuronal precursors (Cheng et al., 2010). Together, these studies identified 5-HT_1A as a largely positive regulator of neuronal proliferation in embryonic and postnatal life. Hence, 5-HT might modulate cortical density through its proliferation-inducing action on progenitors.

Additional mechanisms exist through which 5-HT could potentially modulate proliferation and cortical density. 5-HT could be involved in modulating the length of the cell cycle or participate in progenitor cell death regulation. Interestingly, E12–E17 PCPA-treatment reduces the number of cells expressing Ki67 (a proliferation marker), promotes early GFAP expression, and impairs the normal development/organization of radial glial processes (Vitalis et al., 2007). In turn, early differentiation of RGCs could reduce cortical neurogenesis. Alternatively, hypo-5-HT induced microcephaly could be due to increased death of postmitotic neurons or neuronal progenitors. Indeed, 5-HT₂ stimulation promotes the survival of glutamatergic neurons in vitro with a maximal effect observed for stages E16 and E18 in rats (Dooley et al., 1997), and 5-HT_1A stimulation increases neuroprotection in models of ischemia and protects neuronal cultures against serum withdrawal (Bielenberg and Burkhardt, 1990; Azmitia et al., 1995; Ahlemeyer et al., 2000). Furthermore, activation of 5-HT₂ reverts increased apoptosis observed in VMAT2:KO mice, in which dopamine, norepinephrine, and serotonin are depleted (Stankovski et al., 2007). Such a role was also observed in mice lacking TrkB, the high affinity receptor for the brain-derived neurotrophins factor (BDNF) and neurotrophin 4, and in both cases 5-HT₂ activation was able to normalize the caspase 3–9 cascades (Vitalis et al., 2002b; Stankovski et al., 2007).

During early development, 5-HT could also influence cortical proliferation through the modulation of gap junctions that coordinate cell-cell assembly (Guthrie and Gilula, 1989; Lo Turco and Kriegstein, 1995; Bittman et al., 1997). Interestingly, monoaminergic receptor activation modulates postnatal gap junction coupling in various brain regions including the developing neocortex, where regulation appears to occur at the level of connexin subunit phosphorylation (Röerig and Feller, 2000). Pharmacologic evidence suggests that 5-HT promotes uncoupling of gap junctions through 5-HT_2R stimulation (Röerig and Feller, 2000). However, to our knowledge, no study has investigated the action of 5-HT receptor modulation on gap junction coupling in the embryonic cortex.

5-HT modulates the migration of various cell types and this effect is maintained across most phyla. For example, 5-HT acts as a permissive signal that triggers cell motility of mature lymphocytes in the vertebrate immune system (chick, fish, rodent; Khan and Deschaux, 1997; Boehme et al., 2004) and of microglial cells toward the central nervous system (Krabbe et al., 2012). In the non-vertebrate developing CNS a role for 5-HT in promoting-directed neuronal migration has been reported for Caenorhabditis elegans (Kindt et al., 2002). In the mammalian cortex, a role for 5-HT in regulating the migration of cortical neurons has emerged recently with studies focused on the late phase of corticogenesis. Using a pharmacological approach and cortical slices, high 5-HT levels have been shown to decrease the migratory speed of non-GABAergic and GABAergic neurons (Riccio et al., 2009, 2011; Figure 5). In cortical explants of E17.5 or P0 mouse brain, in which pyramidal neurons were labeled by in utero electroporation at E14.5 or E16.5 respectively, neuronal migration was analysed using video-microscopy in control condition or after acute bath application of 5-HT. This study revealed that acute application of high 5-HT concentration leads to a reversible decrease in the migration speed of glutamatergic neurons running in the IZ. Interestingly, SERT^−/− mice exhibit an abnormal distribution of pyramidal neurons in the most superficial regions of the CP at E19 (presumptive layers II–III) suggesting that 5-HT excess could lead to a delay in the migration of cortical pyramidal neurons in vivo. Furthermore, activation of the 5-HT₆ receptor recapitulates these events: application of a specific 5-HT₆ agonist to E17.5 or P0.5 cortical explants reduced the migratory speed of pyramidal neurons labeled at E14.5 or E16.5 respectively, suggesting that the 5-HT₆ receptor is involved in regulating neuronal migration (Riccio et al., 2011). Similarly to non-GABAergic neurons, GABAergic neurons expressing GAD₆₅ reversibly and in a dose-dependent manner decrease their migratory speed following acute high levels of 5-HT application ex vivo (Riccio et al., 2009). 5-HT also induced a retraction of the leading processes of GABAergic neurons migrating into the IZ and CP. RT-PCR performed on cells sorted by flow cytometry and obtained from E18.5 cortical slices of GAD₆₅:GFP mice, revealed that these cells expressed the 5-HT_3A and the 5-HT₆ receptors. Again, 5-HT₆ agonist application mimicked 5-HT-induced effects on GABAergic neurons. Furthermore pharmacological manipulation of the cAMP-signaling pathway partially modulates the 5-HT₆ mediated effects on cortical interneuron migration (Riccio et al., 2009). Interestingly, recent large-scale proteomic strategies have revealed that the 5-HT₆ receptor binds to a large variety of signaling molecules that play a critical role during brain development including the mTOR pathway (Meffre et al., 2012). It is thus likely that the effects on migration elicited by the pharmacological manipulation of 5-HT₆ receptors also involve these signaling pathways. Studies are currently underway to test this hypothesis.

It must be noted that the impact of 5-HT on the migration of cortical neurons was revealed using high doses of 5-HT. As in other cell types (Moiseiwitsch and Lauder, 1995), it is possible that 5-HT produces opposite effects on neuronal migration depending on the levels of extracellular 5-HT. In cortical explants maintained in a serum-free medium containing lower concentration of 5-HT than those used in experiments described above (5 uM), glutamatergic neurons reach their laminar location faster than in explants maintained in serum-free medium alone, suggesting that 5-HT may enhance the radial migration of these neurons (Lepore et al., 2001). Furthermore, decreasing 5-HT levels during development delayed or disrupted cortical migration suggesting 5-HT could also act as a positive drive on cortical migration (Stankovski et al., 2007; Vitalis et al., 2007). In animals treated with PCPA during the peak of migration (E12/E13 to E17 in rats), GABAergic neurons accumulated at the level of the SP and showed a marked deficit to integrate in the developing CP (Vitalis et al., 2007). Long-lasting consequences of E12–E17 PCPA-treatment lead to a marked reduction of CR- and CCK/VIP-positive GABAergic neurons, two neuronal populations that express the 5-HT_3A receptor (Férézou et al., 2007). Interestingly, mice lacking Tph2 also display reductions of selective GABAergic populations in limbic structures (Waider et al., 2013). 5-HT_3A is protractedly expressed by 30% of GABAergic neurons and it could be that this population is particularly sensitive to 5-HT depletion during corticogenesis. 5-HT_3A is associated with F-actin that decorates the tips of the of dendritic and axonal growth cones. Interestingly, pharmacological alteration of F-actin induced a modification in the distribution of 5-HT_3A (Emerit et al., 2002). In addition, 5-HT_3A mediates calcium entry into the cell (see above). Together these results suggest that 5-HT_3A activation could play a role in promoting the migration of cortical interneurons. Such a role is under investigation.

Lauder and Krebs were the first to report that 5-HT depletion delays neuronal maturation in areas normally receiving 5-HT afferents (Lauder and Krebs, 1978; Lauder, 1993). These investigators defined differentiation as the cessation of cell division measured by incorporation of ³H-thymidine. After these pioneering studies, numerous groups have shown that 5-HT can influence neuritic outgrowth in various phyla (such a role was intensively investigated in Aplysia) and in various regions of the CNS (Haydon et al., 1984, 1987; Whitaker-Azmitia et al., 1996; Lieske et al., 1999; Lotto et al., 1999; Kondoh et al., 2004; Fricker et al., 2005 and see below). Here we review the role for 5-HT on dendritic and axonal morphogenesis during cortical development.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.