Early Seizures Prematurely Unsilence Auditory Synapses to Disrupt Thalamocortical Critical Period Plasticity

Heightened neural excitability in infancy and childhood results in increased susceptibility to seizures. Such early-life seizures are associated with language deficits and autism that can result from aberrant development of the auditory cortex. Here, we show that early-life seizures disrupt a critical period (CP) for tonotopic map plasticity in primary auditory cortex (A1). We show that this CP is characterized by a prevalence of “silent,” NMDA-receptor (NMDAR)-only, glutamate receptor synapses in auditory cortex that become “unsilenced” due to activity-dependent AMPA receptor (AMPAR) insertion. Induction of seizures prior to this CP occludes tonotopic map plasticity by prematurely unsilencing NMDAR-only synapses. Further, brief treatment with the AMPAR antagonist NBQX following seizures, prior to the CP, pre

Dendritic spine morphogenesis and plasticity

Dendritic spines are small protrusions off the dendrite that receive excitatory synaptic input. Spines vary in size, likely correlating with the strength of the synapses they form. In the developing brain, spines show highly dynamic behavior thought to facilitate the formation of new synaptic contacts. Recent studies have illuminated the numerous molecules regulating spine development, many of which converge on the regulation of actin filaments. In addition, interactions with glial cells are emerging as important regulators of spine morphology. In many cases, spine morphogenesis, plasticity, and maintenance also depend on synaptic activity, as shown by recent studies demonstrating changes in spine dynamics and maintenance with altered sensory experience

Bergmann glial ensheathment of dendritic spines regulates synapse number without affecting spine motility

In the cerebellum, lamellar Bergmann glial (BG) appendages wrap tightly around almost every Purkinje cell dendritic spine. The function of this glial ensheathment of spines is not entirely understood. The development of ensheathment begins near the onset of synaptogenesis, when motility of both BG processes and dendritic spines are high. By the end of the synaptogenic period, ensheathment is complete and motility of the BG processes decreases, correlating with the decreased motility of dendritic spines. We therefore have hypothesized that ensheathment is intimately involved in capping synaptogenesis, possibly by stabilizing synapses. To test this hypothesis, we misexpressed GluR2 in an adenoviral vector in BG towards the end of the synaptogenic period, rendering the BG a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) Ca2+-impermeable and causing glial sheath retraction. We then measured the resulting spine motility, spine density and synapse number. Although we found that decreasing ensheathment at this time does not alter spine motility, we did find a significant increase in both synaptic pucta and dendritic spine density. These results indicate that consistent spine coverage by BG in the cerebellum is not necessary for stabilization of spine dynamics, but is very important in the regulation of synapse number

Seizure susceptibility following PTZ seizures in larval zebrafish

Early-life seizures (ELS) often leads to the development of epilepsy (epileptogenesis) in humans, but the underlying mechanisms are unclear. Zebrafish have emerged as a valuable model to study ELS, but whether ELS leads to epileptogenesis in zebrafish is unknown. To address this, we investigated whether ELS in larval zebrafish increases seizure susceptibility later in life. We induced ELS using the chemoconvulsant pentylenetetrazole (PTZ) daily from 5-7 dpf (days post-fertilization) for 40 minutes. Based on human and rodent ELS studies, we hypothesized that seizure susceptibility would increase compared to controls two weeks after initial seizure induction. Thus, at 21 dpf, we challenged fish with a second dose of PTZ (1, 2.5, or 5 mM) for 20 minutes. We compared the PTZ response in ELS PTZ-exposed (PE) fish to that of handled (HC) and unhandled controls (UC) that did not experience ELS. Outcome measures included latency to first seizure, number of seizures, distance moved, and frequency of high-speed movements. We found no significant changes between experimental groups possibly due to high variability and low n. However, zebrafish had more seizures as PTZ concentration increased, as expected. The lowest PTZ concentration (1mM) showed the most potential, as the UC fish did not have seizures, but the PE and HC groups did. There was no significant difference in number of seizures or latency to seizures within groups, although it was often lower in PE fish compared to HC fish. At 1mM, the PE group traveled slightly more than the UC and HC groups with more high-speed movement, but again did not reach statistical significance. Despite the lack of statistical significance, the data is promising. Future experiments will increase the sample size to provide enough statistical power to determine if the zebrafish ELS model can be used to study mechanisms of epileptogenesis

Acute And Chronic Learning Impairment Following Seizures In Larval Zebrafish

Early-life seizures (ELS) can disrupt brain development, often leading to cognitive impairments in humans and rodents. Here, we investigated whether seizures in larval zebrafish (Danio rerio) lead to acute and chronic learning deficits. We hypothesized that seizures in larval zebrafish would impair their performance in declarative memory tasks. To test this, we induced seizures in larval zebrafish using the chemoconvulsant pentylenetetrazole (PTZ). We then assessed later-life learning using short-term memory tasks that focused on conscious recognition and interest in novel stimuli. Specifically, for acute memory (5 days after seizure induction), we used the visual lateralization novel object recognition (VLNOR) test, based on the novel object recognition test frequently used in rodents. Zebrafish show asymmetries in the use of left (LES) and right eye systems (RES), and will initially use LES to observe novel objects and RES to view familiar objects. We found that PTZ fish never reached 50% RES use during the observation phase whereas the UC fish reached 50% RES by 4 minutes. Interestingly, RES behavior patterns of PTZ fish during the recall phase were similar to that of naïve fish during the observation phase. To assess chronic memory deficits, we tested fish in a y-maze spatial learning task 3 months after seizures. In this task, fish had to recall the novel arm of the maze 90 minutes after being removed from the maze. The PTZ group spent significantly less time in the novel arm of the y-maze during the recall test compared to their UC clutchmates. Finally, we have begun preliminary molecular studies which show distinct expression profiles of synaptic plasticity genes between PTZ and control groups by qPCR. These findings expand the behavioral characterization of the larval zebrafish seizure model, strengthening the power of this model for researching the cognitive consequences of ELS

The effects of in vitro seizure-like activity on actin capping in dendritic spines

Introduction: Neonatal seizures can cause dysregulation of excitatory synaptic activity that may contribute to the development of cognitive deficits later in life. While the mechanisms underlying this association are not known, one possibility is the regulation of dendritic spines, the postsynaptic structures of most of the excitatory synapses in the mammalian brain. These membranous protrusions function as chemical and electrical micro-compartments, transducing individual signals of excitatory neurotransmitters. Structural plasticity, the ability of spines to change their shape and thus their function in response to synaptic activity, underlies learning and memory, and many neurological diseases defined by altered cognitive abilities display dendritic spine dysgenesis. This shape change requires regulation of actin mesh- works by actin binding proteins (ABPs) within dendritic spines. Decreased concentrations of functional ABPs induce abnormalities in the density, morphology, and dynamic nature of dendritic spines, resulting in changes in synaptic function such as hindered synaptic signal amplitude. Based upon these studies, we hypothesized that seizure activity causes deficits in the structural plasticity of dendritic spines via depletion of functional ABPs. Methods: To investigate the molecular physiological consequences of uncontrolled, synaptic excitability on intracellular actin regulation, we used a zero magnesium model to induce in vitro seizure-like activity in rat hippocampal neurons. At multiple time points after zero Mg2+ exposure, we assessed localization of ABPs in dendritic spines using immunocytochemistry and analyzed the motility of dendritic spines in response to excitatory stimulation using live-cell imaging of GFP-actin transfected neurons. Results: Preliminary results show a neuronal age and time point dependent decrease in ABP localization and spine motility in dendritic spines after in vitro seizure-like activity. Conclusions: Taken together, these data indicate that seizure-like activity can alter ABPs and baseline spine dynamics. Ongoing work will examine whether seizure-induced ABPs could affect structural plasticity of dendritic spines to determine the feasibility of ABPs as a therapeutic target to decrease cognitive deficits that can result from early-life seizures