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Examining the effects of early-life seizures on neuronal dendrite growth in vivo using a novel experimental model system Hewapathirane, Daminda Sesath

Abstract

The effects of highly prevalent early-life seizures on neuronal activity-dependent developmental programs within the immature brain remain unclear. To address this issue, the present work examined the acute and persistent effects of early-life seizures on neuronal dendritogenesis, a key activity-dependent component of neural circuit development. A novel experimental model system of early-life seizures, based on the albino Xenopus laevis tadpole, was developed for these studies. The transparency of this organism allows in vivo imaging of neuronal growth and activity within the intact developing brain. Additionally, immobilization of tadpoles using reversible paralytics and immersion in agar, for electrophysiological or imaging experiments, allows examination of seizure activity and seizure-induced effects on neuronal growth for the first time within the unanaesthetized and awake brain. Chemoconvulsant-induced seizures in tadpoles were extensively characterized using behavioural assessment, measures of cell death, and in vivo examination of neural activity during seizures through electrophysiological recordings and imaging of intracellular calcium dynamics. Rapid and long-interval time-lapse in vivo two-photon imaging of individual fluorescently labelled growing optic tectal neurons within the intact tadpole brain revealed that seizures inhibit dendritic arbor growth, that these effects are mediated cell-autonomously by excessive AMPA-receptor mediated excitatory activity, and that a single seizure episode persistently stunts subsequent arbor growth. Reduced dendritic growth is a result of decreased branch elongation, increased branch elimination, and loss of dendritic filopodia. Seizures also persistently reduced the density of immunostained excitatory synaptic markers within the tectal neuropil. Rapid time lapse imaging at 5 minute intervals for 5 hours reveals selective effects on filopodial growth dynamics, characterized by rapid increase in the rate of elimination of pre-existing filopodia within minutes of seizure onset, followed by hyper-stabilization of filopodia generated during seizures. These data suggest that seizures interfere with neural circuit development by acutely destabilizing filopodia present prior to seizure induction and hyper-stabilizing filopodia formed during seizures, leading to a persistent inhibition of continued arbor elaboration and growth. This is the first examination of the effects of common early-life seizures on dendritic morphogenesis within the intact and awake brain, and these findings identify a potential morphological correlate of persistent seizure-induced neural dysfunction.

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