Early-life seizures persistently alter visual brain circuit development via neural activitydependent synaptotropic mechanisms. D Sesath Hewapathirane, Xuefeng Liu, Simon Chen, Wesley Yen, Parisa Karimi Tari, Shay Neufeld and Kurt Haas. Brain Research Centre and the Department of Cellular and Physiological Sciences, Faculty of Medicine, University of British Columbia, Vancouver, Canada. Rationale: Early-life seizures are highly prevalent, occurring mainly during the first years of life. Unfortunately, this is a critical period of brain maturation where precisely patterned neural activity directs the formation and refinement of complex circuits. This work examined whether abnormal and excessive neuronal activity associated with seizures interferes with normal neural circuit development. Methods: For these experiments we developed and characterized a unique model of developmental seizures based on the albino Xenopus laevis tadpole. Due to this organism’s transparency, when combined with two-photon in vivo time-lapse fluorescence microscopy, this system confers the unprecedented ability to examine the effects of seizures on neuronal growth and circuit function—during a seizure event—within the intact unanaesthetized developing brain. Single-cell electroporation was employed to fluorescently label and functionally manipulate individual optic tectal neurons within the otherwise unaltered brain. Results: We find that developmental seizures persistently alter retino-tectal circuitry complexity, producing a lasting inhibition of dendritic arborization and reduction in excitatory synaptogenesis. These effects are downstream of excessive excitatory glutamatergic input, since individual neurons with reduced AMPA receptor-mediated neurotransmission are protected from seizure-induced inhibition of arbor growth. Examination of dynamic dendritic growth reveals two distinct yet opposing effects of seizures, the rapid destabilization and retraction of dendritic elements generated prior to seizure onset, and the hyperstabilization of dendritic elements generated during seizure episodes—suggesting that seizures destabilize pre-existing endogenous circuitry and promote the formation of aberrant circuitry. Strikingly, we find that seizure-induced hyperstabilization of dendritic elements could be blocked by reducing the expression of the protein kinase PKMzeta, a kinase implicated in late-phase glutamatergic synapse long-term potentiation and synaptotropic dendritogenesis. Conclusions: These experiments are the first to examine seizure-induced effects on neural network maturation within the intact awake developing brain. Our findings identify morphological substrates potentially underlying neurological dysfunction associated with early-life seizures and, importantly, identify a molecular pathway which can be targeted to protect the brain from the observed deleterious changes. Ongoing experiments are investigating circuit function by recording neural calcium responses from the intact optic tectum in response to visual stimulation, and using visual-cue based avoidance learning assays in freely-swimming tadpoles.
Funding: CIHR, Savoy, BCIC, SRCF, UBC. Presenter can be contacted at sesath@alumni.ubc.ca 21