Role of Neocortical-Hippocampal Interactions

Principal Investigator: KOCH, PAUL

Proposal Number: EP220027

Award Number: HT9425-23-1-0363

Period of Performance: 6/1/2023 - 5/31/2026

PUBLIC ABSTRACT

After severe traumatic brain injury (TBI), up to 20%-50% of patients may develop delayed seizures, or post-traumatic epilepsy (PTE), several months to several years after injury in a process called epileptogenesis. We have no effective therapies to prevent it. There is evidence that injury to a part of the brain called the temporal lobe increases one’s risk of PTE, yet most seizures that occur in PTE do not seem to originate from the temporal lobe, which is a common location for non-traumatic epileptic seizures to originate. Given the apparent discrepancy, this research proposal seeks to understand whether temporal lobe injury after TBI plays a role in the development of PTE even if few temporal lobe seizures result. The lateral fluid percussion injury (FPI) rat model of TBI parallels this seeming paradox in humans: FPI causes characteristic damage to a structure in the temporal lobe called the hippocampus, but leads to seizures coming predominantly from a different area of the brain, the overlying neocortex. Damage after FPI makes the hippocampus more excitable, exhibiting exaggerated responses to inputs, but whether this contributes to PTE is unknown. The hippocampus receives information from nearly all of the neocortex and is responsible for forming memories from that information. The hippocampus, in turn, communicates encoded memories back to the neocortex for long-term storage. The mechanisms by which this bidirectional communication occurs may be corrupted by the epileptogenic process. This study seeks to understand how the circuitry within the hippocampus may change over time after its initial injury following FPI and whether those changes can contribute to abnormal activity in the neocortex that ultimately leads to PTE. To do this, we will use the FPI rat model of TBI. In the first set of experiments, rats will undergo FPI and then will be implanted with electrodes in the hippocampus and the neocortex. We will monitor the rats for up to 24 weeks after FPI, looking for the evolution of abnormal signals recorded from the hippocampus and neocortex that suggest progressive hippocampal hyperexcitability and abnormal communication between these structures, as well as whether these abnormalities predict the development of PTE. In the second set of experiments, we will attempt to reverse the hyperexcitability of the hippocampus after injury by activating a particular set of cells called interneurons within the hippocampus (some of which are damaged after FPI) via delivery of light using a technique called optogenetics. We will test whether reducing the injury-induced hyperexcitability in the hippocampus can prevent, or attenuate, the development of PTE.

The military population bears a disproportionate burden of PTE, occurring in up to 53% of cases of severe TBI, compared to 10%-20% in civilians. Delayed onset of PTE and associated cognitive and memory impairment after TBI leads to increased risk of death as well as further loss of functional independence, reduced quality of life, and increased economic burden. Effective preventative therapies (of which there are currently none) are urgently needed to improve function and quality of life after TBI, particularly in the military population. The immediate goal of this proposed research is to identify key changes in the circuitry of the hippocampus that contribute to the development of PTE. As an intermediate step for subsequent preclinical studies, strategies for identifying and reversing these key changes would then be developed and tested for effectiveness in reducing PTE. The long-term goal is two-fold: to define key hippocampus circuitry changes that both (1) predict development of PTE and thus help define the population of TBI patients that should receive preventative therapy, and (2) can be intervened upon as an effective therapy to prevent, or attenuate, the development of PTE. Modifying the function of brain circuits to treat disease using implantable stimulating electrodes has had great success in diseases such as Parkinson’s disease and essential tremor. Thus, applying such existing technology to new brain targets makes the development of a novel “anti-PTE” therapy achievable within 10 years. Use of more experimental therapeutic technologies, such as the targeted delivery of gene therapy products to parts of the brain, would likely increase that time.

Achievement of this project’s goals will directly enable the Principal Investigator’s (PI’s) career goal in PTE research, which is to understand how brain circuits become progressively dysfunctional after TBI leading to PTE, identifying novel circuit targets for intervention to reverse that dysfunction and prevent the development of PTE. The PI’s participation in the Virtual P-TERC is an ideal environment to find PTE-specific mentorship that will complement his Virginia Commonwealth University (VCU) team of collaborators in neural data analysis and optogenetics, to engage national collaborators with complementary approaches to the same, or similar problems that strengthen the scientific foundation and therapeutic potential of his strategy, and gain feedback from the national PTE scientific community. Similarly, the PI’s Career Development and Sustainment Plan includes specific research and writing skills to acquire, as well as milestones for the local, regional, and national dissemination of findings and establishment of collaborative projects, culminating in the application for independent funding and publication of novel findings, which will support the PI’s career goal by giving him the skills to maintain a sustainable and relevant research program.

TECHNICAL ABSTRACT

Early risk factors for the development of post-traumatic epilepsy (PTE) have been established, including the presence of cortical contusions, but whether contusion location is an important factor remains uncertain. Several large population studies have identified frontal and temporal contusions as PTE risk factors while a recent single-institution study finds temporal lobe injury to be key. Despite these data, focal temporal lobe epilepsy is likely a minority of PTE cases. This apparent discrepancy raises the question of whether temporal lobe injury plays an important role in post-traumatic epileptogenesis, even in the absence of focal temporal lobe PTE. The lateral fluid percussion injury (FPI) rodent model of traumatic brain injury (TBI) approximates the human 30%-50% PTE rate after severe TBI. Lateral FPI results in neocortical damage as well as cell loss within the dentate hilus of the hippocampus (HC), parallel to the HC damage seen in human and rodent mesial temporal lobe epilepsy (TLE). The dentate gate theory postulates that HC seizures occur when the natural local inhibitory network within the dentate gyrus (DG) is impaired after hilar neuron loss, allowing excess excitatory activity to form an intra-hippocampal excitatory loop. However, although hilar neuron loss after FPI is associated with early DG hyperexcitability, the majority of PTE cases after FPI appear to be neocortical in origin, just as in humans. Whether DG hyperexcitability can persist, or even progress after FPI, and whether an impaired “dentate gate” can play a role in post-traumatic epileptogenesis are unknown. Intriguingly, normal mechanisms of neocortical-HC communication have been shown to be co-opted by pathological mechanisms during epileptogenesis in the kindling model of TLE: over time, neocortical sleep spindles progress from being triggered by HC ripples during sleep to being triggered by pathological spiking from a damaged HC. Injured neocortex after FPI can exhibit trains of pathological spiking activity and high frequency oscillations (HFO) at sleep spindle frequencies, predicting PTE. These data suggest that an impaired dentate gate after FPI, in addition to allowing pathological intra-hippocampal propagation of excitatory inputs, may also facilitate its propagation back to the injured neocortex, contributing to the epileptogenic process. We hypothesize that persistent dentate gate impairment after injury contributes to pathological neocortical activity after FPI and leads to development of PTE. We will test this hypothesis through the following specific aims:

Aim 1: Identify the progressive changes in DG and downstream CA1 (cornu ammonis subfield 1) circuitry and their relationship to pathological neocortical activity after FPI in rats that do and do not develop PTE. Using a 32-channel depth electrode spanning CA1 and DG as well as neocortical bipolar electrodes, we will chronically video-EEG (electroencephalogram) monitor FPI and control behaving rats up to 24 weeks after injury. We will measure longitudinal alterations in characteristic LFP (local field potential) laminar and neuronal firing patterns in DG and CA1 in response to naturally occurring afferent input to the DG (dentate spikes), and temporally correlate them with neocortical pathological HFOs (pHFOs), loss of hilar neurons, and development of PTE.

Aim 2: Determine the role of DG disinhibition in the generation of pHFOs and development of PTE after FPI. We will use optogenetic activation of parvalbumin (PV+) interneurons in the DG to restore local inhibition (restore the dentate gate). Using depth electrodes in both peri-lesional neocortex and HC, we will chronically video-EEG monitor FPI and control rats up to 24 weeks after injury. We will develop a closed-loop stimulation paradigm that triggers DG PV+ interneuron activation off of detected neocortical pHFOs, comparing this to sham stimulation. We will compare rates of PTE and seizure frequency between stimulation groups.

Through these aims, we expect to define electrophysiological mechanisms by which post-TBI neuronal loss in the dentate hilus leads to progressive impairment in the dentate gate and to what extent that impairment contributes to neocortical pathological activity and the development of PTE. In the short term, these results are expected to improve our understanding of the mechanisms of post-traumatic epileptogenesis and identify novel circuit biomarkers of PTE. In the long term, these results may define key circuit changes leading to PTE that serve as targets for novel neuromodulation treatment strategies in order to prevent or attenuate the development of PTE. Given the disproportionate burden of PTE on military Service Members and Veterans compared to the civilian population, they have the potential to benefit greatly from the development of an effective preventative PTE therapy, as there is currently none available.

To complement execution of these aims, we have outlined milestones for research skill acquisition, training in grant writing, the oral and written presentation of research progress at the regional and national levels, and the development of collaborations with PTE researchers outside Virginia Commonwealth University (VCU) in Dr. Koch’s Career Development and Sustainment Plan. Together with and enhanced by the networking and training provided by the Virtual P-TERC and the mentorship from Dr. Engel (Career Guide) at University of California Los Angeles (UCLA), achievement of these milestones is designed to ultimately enable Dr. Koch’s successful application for independent PTE funding and publication of novel research in the PTE literature that is clinically relevant and mechanistically driven.