Mitral Cell Dendritic Morphology in the Adult Zebrafish Olfactory Bulb following Growth, Injury and Recovery.

The role of afferent target interactions in dendritic plasticity within the adult brain remains poorly understood. There is a paucity of data regarding the effects of deafferentation and subsequent dendritic recovery in adult brain structures. Moreover, although adult zebrafish demonstrate ongoing growth, investigations into the impact of growth on mitral cell (MC) dendritic arbor structure and complexity are lacking. Leveraging the regenerative capabilities of the zebrafish olfactory system, we conducted a comprehensive study to address these gaps. Employing an eight-week reversible deafferentation injury model followed by retrograde labeling, we observed substantial morphological alterations in MC dendrites. Our hypothesis posited that cessation of injury would facilitate recovery of MC dendritic arbor structure and complexity, potentially influenced by growth dynamics. Statistical analyses revealed significant changes in MC dendritic morphology following growth and recovery periods, indicating that MC total dendritic branch length retained significance after 8 weeks of deafferentation injury when normalized to individual fish physical characteristics. This suggests that regeneration of branch length could potentially function relatively independently of growth-related changes. These findings underscore the remarkable plasticity of adult dendritic arbor structures in a sophisticated model organism and highlight the efficacy of zebrafish as a vital implement for studying neuroregenerative processes.

Deafferentation-induced alterations in mitral cell dendritic morphology in the adult zebrafish olfactory bulb.

The removal of afferent input to the olfactory bulb by both cautery and chemical olfactory organ ablation in adult zebrafish results in a significant decrease in volume of the ipsilateral olfactory bulb. To examine the effects of deafferentation at a cellular level, primary output neurons of the olfactory bulb, the mitral cells, were investigated using retrograde tract tracing with fluorescent dextran using ex vivo brain cultures. Morphological characteristics including the number of major dendritic branches, total length of dendritic branches, area of the dendritic arbor, overall dendritic complexity, and optical density of the arbor were used to determine the effects of deafferentation on mitral cell dendrites. Following 8 weeks of permanent deafferentation there were significant reductions in the total length of dendritic branches, the area of the dendritic arbor, and the density of fine processes in the dendritic tuft. With 8 weeks of chronic, partial deafferentation there were significant reductions in all parameters examined, including a modified Sholl analysis that showed significant decreases in overall dendritic complexity. These results show the plasticity of mitral cell dendritic structures in the adult brain and provide information about the response of these output neurons following the loss of sensory input in this key model system.

Reversible deafferentation of the zebrafish olfactory bulb with wax plug insertion.

BACKGROUND: Deafferentation of the zebrafish olfactory bulb allows investigation of neuroplasticity in a particularly dynamic brain region of a popular model animal known for its regenerative abilities. Current methods to remove sensory input to the zebrafish olfactory bulb differ in the extent of deafferentation and potential for recovery. NEW METHOD: We present a novel method of olfactory bulb deafferentation using continuous wax plug insertions into the nasal cavity of zebrafish. Wax plugs were placed in the nasal cavity and replaced if needed over 1wk or 3wk survival periods. Wax plugs were removed from fish after 1wk of occlusion to analyze the potential recovery of the olfactory organ and bulb. RESULTS: Wax plug insertions caused a dramatic reduction in olfactory organ size and structure and significantly reduced afferent input to the olfactory bulb after 1wk and 3wk. Removal of the wax plugs after 1wk allowed for recovery of the olfactory organ and subsequent reinnervation of the olfactory bulb. COMPARISONS WITH EXISTING METHODS: Chemical ablation with detergent causes partial, temporary deafferentation of the olfactory bulb. Cautery ablation causes complete, permanent deafferentation of the olfactory bulb. Wax plug insertions cause nearly complete, temporary deafferentation, allowing both significant deafferentation and the potential for reinnervation of the olfactory bulb. CONCLUSIONS: The wax plug insertion method of deafferentation described here is unique in that it destroys almost completely the structure of the olfactory organ and removes almost completely sensory input to the olfactory bulb, yet the organ returns to its typical morphology and afferent innervation returns.