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

Abstract

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.

The Olfactory System of Zebrafish as a Model for the Study of Neurotoxicity and Injury: Implications for Neuroplasticity and Disease

“…Medial and dorsal glomeruli morphology was severely affected or reduced altogether, whereas lateral glomeruli were more resistant, consistent with previous reports that suggest that Triton X-100 treatment affects preferentially ciliated OSNs [21,22]. Additionally, this chemical treatment leads to profound changes in mitral cell structure and dendritic complexity [32,137].…”

“…OSNs extend axonal projections to the olfactory bulbs, forming a fasciculated connecting fiber known as the olfactory nerve [7,29]. OSN axons reaching the olfactory bulbs form discrete structures known as glomeruli, where they form synapses with bulbar mitral cells [30,31,32,33]. These neurons extend axons to the forebrain in bundles known as olfactory tracts, where they relay signals to telencephalic olfactory-processing areas: the posterior zone of the dorsal telencephalon (Dp); the ventral nucleus of the central telencephalon (Vv); the posterior tuberculum (PT); and the right habenula (rHb).…”

“…Axons of OSNs that express the same ORs, and thus respond to the same subset of odorants, converge on an individual ipsilateral glomerulus where they form glutamatergic synapses with the mitral/tufted projection neurons. Additionally, periglomerular and granular interneurons mediate feedback inhibition and modulate the activity of mitral cells [31,32,33,55,56,57,81]. Mitral and tufted cells relay output signals to olfactory-processing regions in the telencephalon and the habenular nuclei in parallel axonal bundles that form the medial (MOT) and lateral olfactory tracts (LOT) [10,31,35].…”

“…Deafferentation of the olfactory bulb by physical injury to the olfactory organ is a common method of examining afferent–target interactions [32]. Physical damage to the zebrafish rosettes has been achieved experimentally by removing the rosettes entirely using a cautery iron or by progressively damaging the organ with a wax plug.…”

“…Complete removal of the olfactory organ by cautery produces irreversible and complete deafferentation to the olfactory bulb, followed by severe bulb alterations. These include a significantly reduced bulbar size and activity; vast apoptosis; a reduction in survival and differentiation of newly generated bulbar neurons; complete degeneration of the olfactory nerve and the glomerular layer; and reduction in mitral cell dendritic complexity [32,41,42,137,141,142]. The olfactory bulbs of fish that receive nasal plug insertions show extensive deafferentation along with defasciculation and disruption of several glomeruli of the ventromedial cluster [140].…”