Site‐specific interactions of neurotrophin‐3 and fibroblast growth factor (FGF2) in the embryonic development of the mouse cochlear nucleus

Neurotrophins and FGF2 contribute to formation of the cochlea, but their roles in cochlear nucleus development are unknown. The effects of these factors may differ in the cochlea and cochlear nucleus, which may influence each other's development. It is important to analyze the effects of these factors on cellular structures at well‐defined steps in the normal morphogenetic sequence. The present study used immunohistochemistry to localize factors in situ and to test hypotheses about their roles in an in vitro model. Specific antibody staining revealed that TrkC, the NT3 receptor, is present in neural precursors prior to embryonic day E11 until after birth. NT3 appeared in precursor cells during migration (E13–E15) and disappeared at birth. TrkC and NT3 occurred in the same structures, including growing axons, terminals, and their synaptic targets. Thus, NT3 tracks the migration routes and the morphogenetic sequences within a window defined by TrkC. In vitro, the cochlear nucleus anlage was explanted from E11 embryos. Cultures were divided into groups fed with defined medium, with or without FGF2, BDNF, and NT3 supplements, alone or in combinations, for 7 days. When neuroblasts migrated and differentiated, immunostaining was used for locating NT3 and TrkC in the morphogenetic sequence, bromodeoxyuridine for proliferation, and synaptic vesicle protein for synaptogenesis. By time‐lapse imaging and quantitative measures, the results support the hypothesis that FGF2 promotes proliferation and migration. NT3 interacts with FGF2 and BDNF to promote neurite outgrowth, fasciculation, and synapse formation. Factors and receptors localize to the structural sites undergoing critical changes. © 2006 Wiley Periodicals, Inc. J Neurobiol, 2006     

Site-specific interactions of neurotrophin-3 and fibroblast growth factor (FGF2) in the embryonic development of the mouse cochlear nucleus

Neurotrophins and FGF2 contribute to formation of the cochlea, but their roles in cochlear nucleus development are unknown. The effects of these factors may differ in the cochlea and cochlear nucleus, which may influence each other's development. It is important to analyze the effects of these factors on cellular structures at well-defined steps in the normal morphogenetic sequence. The present study used immunohistochemistry to localize factors in situ and to test hypotheses about their roles in an in vitro model. Specific antibody staining revealed that TrkC, the NT3 receptor, is present in neural precursors prior to embryonic day E11 until after birth. NT3 appeared in precursor cells during migration (E13-E15) and disappeared at birth. TrkC and NT3 occurred in the same structures, including growing axons, terminals, and their synaptic targets. Thus, NT3 tracks the migration routes and the morphogenetic sequences within a window defined by TrkC. In vitro, the cochlear nucleus anlage was explanted from E11 embryos. Cultures were divided into groups fed with defined medium, with or without FGF2, BDNF, and NT3 supplements, alone or in combinations, for 7 days. When neuroblasts migrated and differentiated, immunostaining was used for locating NT3 and TrkC in the morphogenetic sequence, bromodeoxyuridine for proliferation, and synaptic vesicle protein for synaptogenesis. By time-lapse imaging and quantitative measures, the results support the hypothesis that FGF2 promotes proliferation and migration. NT3 interacts with FGF2 and BDNF to promote neurite outgrowth, fasciculation, and synapse formation. Factors and receptors localize to the structural sites undergoing critical changes.     

Uptake and Release of γ-Aminobutyric Acid in the Guinea Pig Cochlear Nucleus After Axotomy of Cochlear and Centrifugal Fibers

Abstract: This study attempts to determine if γ-aminobutyric acid (GABA) may be a transmitter of cochlear nerve fibers projecting from the cochlea to the cochlear nucleus, and of centrifugal fibers projecting to the cochlear nucleus via the trapezoid body and the acoustic striae of the medulla. The uptake and the electrically evoked release of exogenous [¹⁴C]GABA were measured, in vitro, in the three major subdivisions of the guinea pig cochlear nucleus: the anteroventral, posteroventral, and dorsal cochlear nuclei. These activities were compared using unlesioned animals, animals with bilateral cochlear ablations, and animals whose trapezoid body and acoustic striae were interrupted on the right side of the medulla. Subdivisions from unlesioned animals took up [¹⁴C]GABA, achieving concentrations in the tissues that were 11–19 times that in the medium. Electrical stimulation evoked a Ca²⁺-dependent release of [¹⁴C]GABA from each subdivision. Bilateral cochlear ablation, which presumably destroyed the cochlear nerve fibers, had no effect on [¹⁴C]GABA uptake and release. Section of the trapezoid body and the acoustic striae on the right side of the medulla typically severed all known connections of the right posteroventral and dorsal cochlear nuclei with the rest of the brain, but left intact many connections involved with the right anteroventral cochlear nucleus. This lesion partially depressed [¹⁴C]GABA uptake and release in the right posteroventral and dorsal cochlear nuclei, but not in the right anteroventral cochlear nucleus. These findings suggest that one or more of the centrifugal tracts projecting to the cochlear nucleus may be GABAergic, 88% or more of the cochlear nerve fibers probably are not GABAergic, and some neurons of the cochlear nucleus are probably GABAergic.     

A quantitative profile of the synapses on the stellate cell body and axon in the cochlear nucleus of the chinchilla

Summary. One of the most numerous neurons in the cochlear nucleus is the type I stellate cell. Previous attempts to understand the structural basis for its signal coding assumed that integration of synaptic potentials arising from axodendritic synapses should account for the generation of its response properties. However, the present study documents the importance of excitatory and inhibitory types of synapses on the soma and axon. Retrograde transport of cholera toxin B subunit, injected in the inferior colliculus of chinchillas, was used to label exclusively type I stellate cells in the anteroventral cochlear nucleus. The relative distribution of terminal types by vesicle morphology was pleomorphic < large spherical < flattened < smaller spherical. The somatic perimeter covered by endings ranged from almost none to nearly half. More flattened-vesicle terminals contacted somata in the high-frequency than in the low-frequency region. Eight of twenty axons received endings that contained large spherical vesicles and made asymmetric junctions; half of these extensively apposed the initial segment, forming a collar of presumed excitatory input. Thus, type I stellate cells are a heterogeneous group. Inhibitory synapses probably compose the majority of terminals. Some cells receive mostly inhibitory synapses near the presumed site of the spike generator, but others also have a prominent excitatory input. These findings call for a new look at the mechanisms for signal coding in stellate cells in the auditory system in particular and raise issues concerning the stochastic nature of information processing in sensory systems in general.