In a transgenic mouse line (Tg8) deficient for the gene encoding monoamine oxidase A (MAOA), we show that the primary somatosensory cortex (S1) lacks the characteristic barrel-like clustering of layer IV neurons, whereas normal pattern formation exists in the thalamus and the trigeminal nuclei. No barrel-like patterns were visible with tenascin or serotonin immunostaining or with labeling of thalamocortical axons. An excess of brain serotonin during the critical period of barrel formation appears to have a causal role in these cortical abnormalities, since early administration of parachlorophenylalanine, an inhibitor of serotonin synthesis, in Tg8 pups restored the formation of barrels in S1, whereas inhibition of catecholamine synthesis did not. Transient inactivation of MAOA in normal newborns reproduced a barrelless phenotype in parts of S1.

To identify neocortical neurons expressing the type 3 serotonergic receptor, here we used transgenic mice expressing the enhanced green fluorescent protein (GFP) under the control of the 5-HT3A promoter (5-HT3A:GFP mice). By means of whole-cell patch-clamp recordings, biocytin labeling, and single-cell reversed-transcriptase polymerase chain reaction on acute brain slices of 5-HT3A:GFP mice, we identified 2 populations of 5-HT3A-expressing interneurons within the somatosensory cortex. The first population was characterized by the frequent expression of the vasoactive intestinal peptide and a typical bipolar/bitufted morphology, whereas the second population expressed predominantly the neuropeptide Y and exhibited more complex dendritic arborizations. Most interneurons of this second group appeared very similar to neurogliaform cells according to their electrophysiological, molecular, and morphological properties. The combination of 5-bromo-2-deoxyuridine injections with 5-HT3A mRNA detection showed that cortical 5-HT3A interneurons are generated around embryonic day 14.5. Although at this stage the 5-HT3A receptor subunit is expressed in both the caudal ganglionic eminence and the entopeduncular area, homochronic in utero grafts experiments revealed that cortical 5-HT3A interneurons are mainly generated in the caudal ganglionic eminence. This protracted expression of the 5-HT3A subunit allowed us to study specific cortical interneuron populations from their birth to their final functional phenotype.

During development of the cerebral cortex, the invasion of thalamic axons and subsequent differentiation of cortical neurons are tightly coordinated. Here we provide evidence that glutamate neurotransmission triggers a critical signaling mechanism involving the activation of phospholipase C-beta1 (PLC-beta1) by metabotropic glutamate receptors (mGluRs). Homozygous null mutation of either PLC-beta1 or mGluR5 dramatically disrupts the cytoarchitectural differentiation of 'barrels' in the mouse somatosensory cortex, despite segregation in the pattern of thalamic innervation. Furthermore, group 1 mGluR-stimulated phosphoinositide hydrolysis is dramatically reduced in PLC-beta1-/- mice during barrel development. Our data indicate that PLC-beta1 activation via mGluR5 is critical for the coordinated development of the neocortex, and that presynaptic and postsynaptic components of cortical differentiation can be genetically dissociated.

In the neocortex, neuronal nitric oxide (NO) synthase (nNOS) is essentially expressed in two classes of GABAergic neurons: type I neurons displaying high levels of expression and type II neurons displaying weaker expression. Using immunocytochemistry in mice expressing GFP under the control of the glutamic acid decarboxylase 67k (GAD67) promoter, we studied the distribution of type I and type II neurons in the barrel cortex and their expression of parvalbumin (PV), somatostatin (SOM), and vasoactive intestinal peptide (VIP). We found that type I neurons were predominantly located in deeper layers and expressed SOM (91.5%) while type II neurons were concentrated in layer II/III and VI and expressed PV (17.7%), SOM (18.7%), and VIP (10.2%). We then characterized neurons expressing nNOS mRNA (n = 42 cells) ex vivo, using whole-cell recordings coupled to single-cell reverse transcription-PCR and biocytin labeling. Unsupervised cluster analysis of this sample disclosed four classes. One cluster (n = 7) corresponded to large, deep layer neurons, displaying a high expression of SOM (85.7%) and was thus very likely to correspond to type I neurons. The three other clusters were identified as putative type II cells and corresponded to neurogliaform-like interneurons (n = 19), deep layer neurons expressing PV or SOM (n = 9), and neurons expressing VIP (n = 7). Finally, we performed nNOS immunohistochemistry on mouse lines in which GFP labeling revealed the expression of two specific developmental genes (Lhx6 and 5-HT3A). We found that type I neurons expressed Lhx6 but never 5-HT3A, indicating that they originate in the medial ganglionic eminence (MGE). Type II neurons expressed Lhx6 (63%) and 5-HT3A (34.4%) supporting their derivation either from the MGE or from the caudal ganglionic eminence (CGE) and the entopeduncular and dorsal preoptic areas. Together, our results in the barrel cortex of mouse support the view that type I neurons form a specific class of SOM-expressing neurons while type II neurons comprise at least three classes.

No abstract

Genetic loss or pharmacological inhibition of monoamine oxidase A (MAOA) in mice leads to a large increase in whole-brain levels of serotonin (5-HT). Excess 5-HT in mouse neonates prevents the normal barrel-like clustering of thalamic axons in the somatosensory cortex. Projection fields of other neuron populations may develop abnormally. In the present study, we have analyzed the localization of 5-HT immunolabeling in the developing brain of MAOA knock-out mice. We show numerous atypical locations of 5-HT during embryonic and postnatal development. Catecholaminergic cells of the substantia nigra, ventral tegmental area, hypothalamus, and locus ceruleus display transient 5-HT immunoreactivity. Pharmacological treatments inhibiting specific monoamine plasma membrane transporters and genetic crosses with mice lacking the dopamine plasma membrane transporter show that the accumulation of 5-HT in these catecholaminergic cells is attributable to 5-HT uptake via the dopamine or the norepinephrine plasma membrane transporter. In the telencephalon, transient 5-HT immunolabeling is observed in neurons in the CA1 and CA3 fields of the hippocampus, the central amygdala, the indusium griseum, and the deep layers of the anterior cingulate and retrosplenial cortices. In the diencephalon, primary sensory nuclei, as well as the mediodorsal, centrolateral, oval paracentral, submedial, posterior, and lateral posterior thalamic nuclei, are transiently 5-HT immunolabeled. The cortical projections of these thalamic nuclei are also labeled. In the brainstem, neurons in the lateral superior olivary nucleus and the anteroventral cochlear nucleus are transiently 5-HT immunolabeled. None of these structures appear to express the monoamine biosynthetic enzyme L-aromatic amino acid decarboxylase. The administration of monoamine plasma membrane transporter inhibitors indicates that the 5-HT immunolabeling in these structures is attributable to an uptake of 5-HT by the 5-HT plasma membrane transporter. This points to neuron populations that form highly precise projection maps that could be affected by 5-HT during specific developmental stages.

The cerebral cortex is widely innervated by serotonin (5-HT)-containing axons originating from neurons in the raphe nuclei. The early development of this monoamine system in the cortex prompted speculation long ago that it has important functions in cortical maturation and plasticity. Here we review evidence, derived from a plethora of studies and from our recent unpublished work, that supports an important role for 5-HT in a number of major events in the developing cortex, especially at the early stages. This evidence points to a regulatory role for 5-HT in neuronal proliferation, migration and differentiation, and in preventing apoptotic cell death.

In the rodent and human embryonic brains, the cerebral cortex and hippocampus transiently express high levels of type 1 cannabinoid receptors (CB(1)Rs), at a developmental stage when these areas are composed mainly of glutamatergic neurons. However, the precise cellular and subcellular localization of CB(1)R expression as well as effects of CB(1)R modulation in this cell population remain largely unknown. We report that, starting from embryonic day 12.5, CB(1)Rs are strongly expressed in both reelin-expressing Cajal-Retzius cells and newly differentiated postmitotic glutamatergic neurons of the mouse telencephalon. CB(1)R protein is localized first to somato-dendritic endosomes and at later developmental stages it localizes mostly to developing axons. In young axons, CB(1)Rs are localized both to the axolemma and to large, often multivesicular endosomes. Acute maternal injection of agonist CP-55940 results in the relocation of receptors from axons to somato-dendritic endosomes, indicating the functional competence of embryonic CB(1)Rs. The adult phenotype of CB(1)R expression is established around postnatal day 5. By using pharmacological and mutational modulation of CB(1)R activity in isolated cultured rat hippocampal neurons, we also show that basal activation of CB(1)R acts as a negative regulatory signal for dendritogenesis, dendritic and axonal outgrowth, and branching. Together, the overall negative regulatory role in neurite development suggests that embryonic CB(1)R signaling may participate in the correct establishment of neuronal connectivity and suggests a possible mechanism for the development of reported glutamatergic dysfunction in the offspring following maternal cannabis consumption.