Microglial response patterns following damage to the zebrafish olfactory bulb.

The inherent plasticity of the zebrafish olfactory system serves as a useful model for examining immune cell responses after injury. Microglia are the resident immune cells of the CNS that respond to damage by migrating to the site of injury and phagocytizing neuronal debris. While the olfactory system is renowned for its ability to recover from damage, the specific mechanisms of microglial involvement in olfactory system plasticity are unknown. To approach the potentially time-dependent effects of microglial activation after injury, we performed a time course analysis of microglial response profiles and patterns following different forms of damage: deafferentation by cautery ablation of the olfactory organ, deafferentation by chemical ablation of the olfactory epithelium, and direct lesioning of the olfactory bulb. Our aim was to demonstrate that immunocytochemistry and microscopy methods in zebrafish can be used to determine the timing of distinct microglial response patterns following various forms of injury. We found that permanent and temporary forms of damage to the olfactory bulb resulted in different microglial response profiles from 1 to 72 h after injury, suggesting that there may be critical timepoints in which microglia are activated that contribute to tissue and neuronal repair with a regenerative outcome versus a degenerative outcome. These distinctions between the different forms of damage suggest temporal changes relative to the potential for regeneration, since cautery deafferentation is permanent and unrecoverable while chemical ablation deafferentation and direct lesioning is reversible and can be used to observe the microglial relationship in neural regeneration and functional recovery in future studies.