Gestational exposure to valproic acid (VPA) is known to cause behavioral deficits of sociability, matching similar alterations in human autism spectrum disorder (ASD). Available data are scarce on the neuromorphological changes in VPA-exposed animals. Here, we focused on alterations of the dopaminergic system, which is implicated in motivation and reward, with relevance to social cohesion. Whole brains from 7-day-old mice born to mothers given a single injection of VPA (400 mg/kg b.wt.) on E13.5 were immunostained against tyrosine hydroxylase (TH). They were scanned using the iDISCO method with a laser light-sheet microscope, and the reconstructed images were analyzed in 3D for quantitative morphometry. A marked reduction of mesotelencephalic (MT) axonal fascicles together with a widening of the MT tract were observed in VPA treated mice, while other major brain tracts appeared anatomically intact. We also found a reduction in the abundance of dopaminergic ventral tegmental (VTA) neurons, accompanied by diminished tissue level of DA in ventrobasal telencephalic regions (including the nucleus accumbens (NAc), olfactory tubercle, BST, substantia innominata). Such a reduction of DA was not observed in the non-limbic caudateputamen. Conversely, the abundance of TH+ cells in the substantia nigra (SN) was increased, presumably due to a compensatory mechanism or to an altered distribution of TH+ neurons occupying the SN and the VTA. The findings suggest that defasciculation of the MT tract and neuronal loss in VTA, followed by diminished dopaminergic input to the ventrobasal telencephalon at a critical time point of embryonic development (E13-E14) may hinder the patterning of certain brain centers underlying decision making and sociability.

During the development of the central nervous system, the immature neurons suffer different migration processes. It is well known that Nkx2.1-positive ventricular layer give rise to critical tangential migrations into different regions of the developing forebrain. Our aim was to study this phenomenon in the hypothalamic region. With this purpose, we used a transgenic mouse line that expresses the tdTomato reporter driven by the promotor of Nkx2.1. Analysing the Nkx2.1-positive derivatives at E18.5, we found neural contributions to the prethalamic region, mainly in the zona incerta and in the mes-diencephalic tegmental region. We studied the developing hypothalamus along the embryonic period. From E10.5 we detected that the Nkx2.1 expression domain was narrower than the reporter distribution. Therefore, the Nkx2.1 expression fades in a great number of the early-born neurons from the Nkx2.1-positive territory. At the most caudal positive part, we detected a thin stream of positive neurons migrating caudally into the mes-diencephalic tegmental region using time-lapse experiments on open neural tube explants. Late in development, we found a second migratory stream into the prethalamic territory. All these tangentially migrated neurons developed a gabaergic phenotype. In summary, we have described the contribution of interneurons from the Nkx2.1-positive hypothalamic territory into two different rostrocaudal territories: the mes-diencephalic reticular formation through a caudal tangential migration and the prethalamic zona incerta complex through a dorsocaudal tangential migration.

Background The fasciculus retroflexus is the prominent efferent pathway from the habenular complex. Medial habenular axons form a core packet whereas lateral habenular axons course in a surrounding shell. Both groups of fibers share the same initial pathway but differ in the final segment of the tract, supposedly regulated by surface molecules. The gene Amigo2 codes for a membrane adhesion molecule with an immunoglobulin‐like domain 2 and is selectively expressed in the medial habenula. We present it as a candidate for controlling the fasciculation behavior of medial habenula axons. Results First, we studied the development of the habenular efferents in an Amigo2 lack of function mouse model. The fasciculus retroflexus showed a variable defasciculation phenotype. Gain of function experiments allowed us to generate a more condensed tract and rescued the Amigo2 knock‐out phenotype. Changes in Amigo2 function did not alter the course of habenular fibers. Conclusion We have demonstrated that Amigo2 plays a subtle role in the fasciculation of the fasciculus retroflexus.

Wnt1 is one of the morphogenes that controls the specification and differentiation of neuronal populations in the developing central nervous system. The habenula is a diencephalic neuronal complex located in the most dorsal aspect of the thalamic prosomere. This diencephalic neuronal population is involved in the limbic system and its malfunction is related with several psychiatric disorders. Our aim is to elucidate the Wnt1 role in the habenula and its main efferent tract, the fasciculus retroflexus, development. In order to achieve these objectives, we analyzed these structures development in a Wnt1 lack of function mouse model. The habenula was generated in our model, but it presented an enlarged volume. This alteration was due to an increment in habenular neuroblasts proliferation rate. The fasciculus retroflexus also presented a wider and disorganized distribution and a disturbed final trajectory toward its target. The mid-hindbrain territories that the tract must cross were miss-differentiated in our model. The specification of the habenula is Wnt1 independent. Nevertheless, it controls its precursors proliferation rate. Wnt1 expressed in the isthmic organizer is vital to induce the midbrain and rostral hindbrain territories. The alteration of these areas is responsible for the fasciculus retroflexus axons misroute.

Dysfunction of the LIS1 gene causes lissencephaly, a drastic neurological disorder characterized by a deep disruption of the cortical structure. We aim to uncover alterations of the cortical neuronal networks related with the propagation of epileptiform activity in the Lis1/sLis1 mouse, a model lacking the LisH domain in heterozygosis. We did extracellular field-potential and intracellular recordings in brain slices of the anterior cingulate cortex (ACC) or the retrosplenial cortex (RSC) to study epileptiform activity evoked in the presence of bicuculline (10 µM), a blocker of GABAA receptors. The sensitivity to bicuculline of the generation of epileptiform discharges was similar in wild type (WT) and Lis1/sLis1 cortex (EC50 1.99 and 2.24 µM, respectively). In the Lis1/sLis1 cortex, we observed a decreased frequency of the oscillatory post-discharges of the epileptiform events; also, the propagation of epileptiform events along layer 2/3 was slower in the Lis1/sLis1 cortex (WT 47.69 ± 2.16 mm/s, n = 25; Lis1/sLis1 37.34 ± 2.43 mm/s, n = 15; p = 0.004). The intrinsic electrophysiological properties of layer 2/3 pyramidal neurons were similar in WT and Lis1/sLis1 cortex, but the frequency of the spontaneous EPSCs was lower and their peak amplitude higher in Lis1/sLis1 pyramidal neurons. Finally, the propagation of epileptiform activity was differently affected by AMPA receptor blockers: CNQX had a larger effect in both ACC and RSC while GYKI53655 had a larger effect only in the ACC in the WT and Lis1/sLis1 cortex. All these changes indicate that the dysfunction of the LIS1 gene causes abnormalities in the properties of epileptiform discharges and in their propagation along the layer 2/3 in the anterior cingulate cortex and in the restrosplenial cortex.