ABSTRACT
Temporal lobe epilepsy (TLE) causes pervasive and progressive memory impairments, yet the specific circuit changes that drive these deficits remain unclear. To investigate how hippocampal-entorhinal dysfunction contributes to progressive memory deficits in epilepsy, we performed simultaneous in vivo electrophysiology in hippocampus (HPC) and medial entorhinal cortex (MEC) of control and epileptic mice 3 or 8 weeks after pilocarpine-induced status epilepticus (Pilo-SE). We found that HPC synchronization deficits (including reduced theta power, coherence, and altered interneuron spike timing) emerged within 3 weeks of Pilo-SE, aligning with early-onset, relatively subtle memory deficits. In contrast, abnormal synchronization within MEC and between HPC-MEC emerged later, by 8 weeks after Pilo-SE, when spatial memory impairment was more severe. Furthermore, a distinct subpopulation of MEC layer 3 excitatory neurons (active at theta troughs) was specifically impaired in epileptic mice. Together, these findings suggest that hippocampal-entorhinal circuit dysfunction accumulates and shifts as cognitive impairment progresses in TLE.
ABSTRACT
The precise timing of neuronal spiking relative to the brain's endogenous oscillations (i.e., phase-locking or spike-phase coupling) has long been hypothesized to coordinate cognitive processes and maintain excitatory-inhibitory homeostasis. Indeed, disruptions in theta phase-locking have been described in models of neurological diseases with associated cognitive deficits and seizures, such as Alzheimer's disease, temporal lobe epilepsy, and autism spectrum disorders. However, due to technical limitations, determining if phase-locking causally contributes to these disease phenotypes has not been possible until recently. To fill this gap and allow for the flexible manipulation of single-unit phase-locking to on-going endogenous oscillations, we developed PhaSER, an open-source tool that allows for phase-specific manipulations. PhaSER can deliver optogenetic stimulation at defined phases of theta in order to shift the preferred firing phase of neurons relative to theta in real-time. Here, we describe and validate this tool in a subpopulation of inhibitory neurons that express somatostatin (SOM) in the CA1 and dentate gyrus (DG) regions of the dorsal hippocampus. We show that PhaSER is able to accurately deliver a photo-manipulation that activates opsin+ SOM neurons at specified phases of theta in real-time in awake, behaving mice. Further, we show that this manipulation is sufficient to alter the preferred firing phase of opsin+ SOM neurons without altering the referenced theta power or phase. All software and hardware requirements to implement real-time phase manipulations during behavior are available online (https://github.com/ShumanLab/PhaSER).
ABSTRACT
Tracking animal behavior by video is one of the most common tasks in neuroscience. Previously, we have validated ezTrack, a free, flexible, and easy-to-use software for the analysis of animal behavior. ezTrack's Location Tracking Module can be used for the positional analysis of an individual animal and is applicable to a wide range of behavioral tasks. Separately, ezTrack's Freeze Analysis Module is designed for the analysis of defensive freezing behavior. ezTrack supports a range of desirable tools, including options for cropping and masking portions of the field of view, defining regions of interest, producing summary data for specified portions of time, algorithms to remove the influence of electrophysiology cables and other tethers, batch processing of multiple videos, and video down-sampling. Moreover, ezTrack produces a range of interactive plots and visualizations to promote users' confidence in their results. In this protocols paper, we provide step-by-step instructions for the use of ezTrack, from tips for recording behavior to instructions for using the software for video analysis. © 2021 Wiley Periodicals LLC. Basic Protocol 1: Software environment installation Basic Protocol 2: Using the Location Tracking Module Basic Protocol 3: Using the Freeze Analysis Module.
