In utero exposure to valproic acid (VPA) may cause symptoms related to autism spectrum disorder (ASD). An abnormal serotonergic (5-HT) system has been implicated in the etiology of ASD. In the present study, we have examined the expression and distribution of two early inducers of 5-HT neurons in rat embryos, to elucidate the prenatal development of 5-HT neurons after VPA exposure at embryonic day (E) 9.5. Whole-embryo in situ hybridization at E11.5 showed that the expression of sonic hedgehog, one of the early inducers of 5-HT neurons, was reduced around the isthmus in the VPA-exposed group. Furthermore, whole-mount immunohistochemistry of the hindbrain and quantitative analysis of 5-HT neurons in the rostral raphe nucleus (rRN) revealed that neuronal distribution in the caudal part of the rRN was narrower at E15.5 in the VPA-exposed group than in controls. Thus, the early development of 5-HT neurons was altered after VPA exposure in utero. The observed prenatal alteration may be significant in the etiology of autism.

Autism is often associated with multiple developmental anomalies including asymmetric facial palsy. In order to establish the etiology of autism with facial palsy, research into developmental abnormalities of the peripheral facial nerves is necessary. In the present study, to investigate the development of peripheral cranial nerves for use in an animal model of autism, rat embryos were treated with valproic acid (VPA) in utero and their cranial nerves were visualized by immunostaining. Treatment with VPA after embryonic day 9 had a significant effect on the peripheral fibers of several cranial nerves. Following VPA treatment, immunoreactivity within the trigeminal, facial, glossopharyngeal and vagus nerves was significantly reduced. Additionally, abnormal axonal pathways were observed in the peripheral facial nerves. Thus, the morphology of several cranial nerves, including the facial nerve, can be affected by prenatal VPA exposure as early as E13. Our findings indicate that disruption of early facial nerve development is involved in the etiology of asymmetric facial palsy, and may suggest a link to the etiology of autism.

No abstract

Astrocytes are thought to be active participants in synaptic plasticity in the developing nervous system. Previous studies suggested that axosomatic synapses decreased in number on the small cells of the rat caudal nucleus of tractus solitarius (cNTS) toward the end of the first postnatal week. Astrocytes might be involved in this phenomenon. We examined the morphological development of astrocytic processes around the small cell soma in the rat cNTS using light and electron microscopy. Glial fibrillary acidic protein (GFAP), glutamate-aspartate transporter (GLAST), and glutamate transporter-1 (GLT-1)-positive structures within the cNTS became more intensely stained as development proceeded. GLAST-positive structures encompassed calbindinpositive small cell somata after postnatal day 10. Electron microscopic observations indicated that astrocytic processes encompass the small cell soma, while the number of axosomatic synapses decreases as development proceeds. The timing for glial coverage of the small cell soma appears to be consistent with the decrease in axosomatic synapses on the small cells. These observations imply that astrocytes may participate actively in regulating the decrease of axosomatic synapses on small cells in the cNTS during postnatal development. V V C 2007 Wiley-Liss, Inc.

No abstract

The development of facial nuclei in animal models of disease is poorly understood, but autism is sometimes associated with facial palsy. In the present study, to investigate migration of facial neurons and initial facial nucleus formation in an animal model of autism, rat embryos were treated with valproic acid (VPA) in utero at embryonic day (E) 9.5 and their facial nuclei were analyzed by in situ hybridization at E13.5, E14.5 and E15.5. Signals for Tbx20, which is expressed in early motor neurons, appeared near the floor plate at the level of the vestibular ganglion and extended caudolaterally, where they became ovoid in shape. This pattern of development was similar between control and VPA-exposed embryos. However, measurements of the migratory pathway and the size of the facial nuclei revealed that exposure to VPA hindered the caudal migration of neurons to the facial nuclei. Signals for cadherin 8, which is expressed in mature facial nuclei, revealed that exposure to VPA caused a significant reduction in the size of the facial nuclei. Our findings provide the first quantitative description of tangential migration and nucleus formation in the developing hindbrain in a rat model of autism.

No abstract

In vertebrates, sensory neurons interconnect a variety of peripheral tissues and central targets, conveying sensory information from different types of sensory receptors to appropriate second‐order neurons in the central nervous system (CNS). To explore the possibility that the different rhombomere environments where sensory neurons enter into the hindbrain affect the pathfinding capability of growth cones, we studied the development of the VIIIth ganglion afferent both in vivo and in vitro. We focused on the vestibular nerve because it is the only cranial nerve projecting to the cerebellum, allowing for ready identification from its pattern of projection. Embryonic rat brain was cut along the dorsal midline and, with the VIIIth and Vth ganglia still attached, flat mounted and visualized with antibodies specific for sensory ganglia. Axons reached the cerebellar primordium at embryonic day (E) 13, then splayed out towards the edges of the rhombic lip of rostral hindbrain. In vitro, the VIIIth ganglion showed development similar to that in vivo and innervated the cerebellum, an appropriate target, indicating that mechanisms for axon guidance and target recognition are preserved in vitro. When the VIIIth ganglion was transplanted to the position of the Vth ganglion, axons from the transplanted ganglion entered the cerebellar primordium with a trajectory characteristic of the VIIIth nerve. These results indicate that the central projection pattern of the VIIIth nerve is not affected by the environment of nerve entry into the brainstem, suggesting that axons of sensory cranial ganglion intrinsically possess the capacity to find their target correctly. J. Comp. Neurol. 417:491–500, 2000. © 2000 Wiley‐Liss, Inc.