Structure-function studies of second-order vestibular nucleus neurons in the developing chicken
Open AccessVestibular nucleus neurons are essential for the function of the three-neuron reflex pathways which maintain posture, balance, and clear vision during movements. In these studies, the morphological and electrophysiological properties of second-order vestibular nucleus neurons were investigated in the embryonic and hatchling chicken. First, coupling via gap junction channels was investigated in principal cells of the tangential nucleus (TN), an identified class of vestibular neurons that are part of the horizontal vestibuloocular reflex (VOR) pathway. Using whole-cell patch-clamp electrophysiology in brain slices, it was found that gap junctions between both neurons and glia are present at embryonic ages E13 and E16, but are rarely found after hatching. This supports the hypothesis that gap junctions play an important role during development of the vestibular reflex circuitry, but are not essential for mature function. Second, retrograde dye tracing was used to examine the morphology and topographic organization of VOR neurons in the medial vestibular nucleus (MVN). Biocytin was injected into the oculomotor, trochlear, or abducens nucleus on one side using isolated chicken brainstem or brain slices to identify MVN neurons that project to these targets. The fundamental pattern of vestibuloocular projecting neurons was similar at embryonic ages E13 and E16. In contrast to mammals, where most vestibuloocular projection neurons reside within the MVN, the majority of retrogradely labeled neurons in chicken resided within the ventrolateral, descending, and tangential vestibular nuclei. This morphological identification and mapping of vestibuloocular projection neurons provides the first step in evaluating the relationship between avian vestibuloocular neuron structure and function. Finally, whole-cell patch-clamp recordings were made on abducens-projecting neurons of the MVN identified via anterograde and retrograde fluorescent labeling. Passive and active membrane properties and the properties of synaptic inputs were characterized. Compared to other MVN neuron groups studied, MVN neurons which project to the abducens nucleus (MVN/ABi and MVN/ABc) showed distinctive electrophysiological properties. These findings demonstrate that vestibular nucleus neurons are not uniform in their physiological properties and there are sub-populations with distinctive properties that depend on the output connectivity of the cells. These data underscore the case for studying identified neuron classes to better understand their roles in the vestibular reflex pathways.
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GottesmanDavis_gwu_0075A_10904.pdf | 2018-01-16 | Open Access |
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