Targeted hippocampal GABA neuron ablation produces hippocampal sclerosis, epilepsy, and dissociable effects on the Morris Water Maze and Object-Place Paired Association tasks
by Argyle V. Bumanglag1, Leah M. Truckenbrod1, Eugene Chun2, Abbi R. Hernandez1, Quinten P. Federico1, Andrew P. Maurer1, Robert S. Sloviter3,4, Sara N. Burke1
1Neuroscience, McKnight Brain Institute, University of Florida, Gainesville, FL, 2Graduate Education Biomedical Sciences Program, Morehouse SOM, Atlanta, G, 3Neurobiology, Morehouse SOMe, Atlanta, GA, 4Pharmacology and Toxicology, Morehouse SOM, Atlanta, GA.
Temporal lobe epilepsy with hippocampal sclerosis is a common neurological disorder characterized by seizures that arise from the hippocampus and related structures, often as a consequence of brain injury.1 Perforant path stimulation-induced hippocampal injury in rats is associated with immediate dentate granule cell hyperexcitability and a clinically subtle, focal epileptic brain state without significant delay.2 Thus, an immediate disruption in inhibition from injury-induced neuron loss or GABA neuron dysfunction may be a sufficient cause of temporal lobe epilepsy with hippocampal sclerosis.3-4 We recently tested this hypothesis by determining whether the selective ablation of hippocampal GABA neurons along a longitudinal expanse of the hippocampus, in the absence of any other induced brain injury, is sufficient to initiate hippocampal epileptogenesis.5 The results of this experiment suggested that targeted hippocampal GABA neuron ablation by Stable Substance P-Saporin (SSP-SAP; Cat. #IT-11) initiated a subclinical state of non-convulsive status epilepticus, which produced hippocampal sclerosis and dentate granule cell-onset epilepsy, without involving convulsive status epilepticus or any lethality.5 If correct, GABA neuron loss or GABAergic dysfunction alone may be a primary epileptogenic mechanism.2-5
In this study, we determined whether selective hippocampal GABA inhibitory interneuron loss produced a chronic epileptic state, and assessed cognitive function in chronically epileptic SSP-SAP-treated rats and vehicle-injected controls to identify behavioral co-morbidities associated with GABA neuron ablation. Male Sprague Dawley rats (350-450 g) were injected bilaterally with SSP-SAP (0.4 ng/10 nL) or PBS (vehicle control) into 4 sites along the longitudinal axis of each hippocampus. Receptor-mediated lesioning with SSP-SAP is highly selective because the neurotoxin Saporin enters GABA neurons via the NK-1 receptor, which all hippocampal GABA neurons constitutively and selectively express.6 Cognitive function was assessed in chronically epileptic SSP-SAP-treated rats and their vehicle-injected controls ~8 months post-injection, when treated rats were observed to exhibit spontaneous clinical focal motor seizures.
Spatial learning and memory were initially assessed with the Morris Water Maze.7 No significant differences were detected in Morris Water Maze performance between SSP-SAP-treated rats and the control animals. Controls and epileptic rats with dorsal hippocampal sclerosis were able to learn the location of the hidden platform in the Morris Water Maze task, suggesting that entorhinal regions and associated areas involved in spatial navigation may have retained their functional integrity (Fig. 2).
The same animals were subsequently tested with the object-place paired association (OPPA) task, which requires animals to integrate spatial location memory with a correct object choice, and is a more sensitive measure of dysfunction within hippocampal-cortical circuits than the Morris Water Maze.8-10 SSP-SAP-treated rats exhibited significantly impaired performance on the OPPA task, and made significantly more errors before reaching criterion compared to control animals (Fig. 3A). Thus, cognitive functions that require functional connectivity between the hippocampus and cortical areas may be selectively affected by hippocampal GABA neuron ablation. No significant differences were detected on a control simple object discrimination task (Fig. 3C), suggesting that observed impairments on the OPPA task were not simply due to differences in motivation, procedural or sensorimotor deficits.
Histological analysis was performed ~12 months post-injection to determine the extent of hippocampal sclerosis in chronically epileptic rats. Targeted hippocampal GABA neuron ablation via SSP-SAP consistently caused hippocampal sclerosis (Fig. 1). Varying extents of neuron loss were observed in different rostrocaudal locations along the same hippocampus. Rats corresponding to panels 2-4 in Fig. 3 were observed to exhibit a minimum of two clinically-obvious seizures that were detected by intermittent observation. Clinically-obvious seizures were observed during feeding and cage changes, or on behavioral task trials, indicating that animals continue to be epileptic for more than a year after SSP-SAP microinjection.
These data suggest that, similar to humans with mesial temporal lobe epilepsy, hippocampal sclerosis and epilepsy in this model do not result in global cognitive decline. Rather, cognitive functions that require functional connectivity between the hippocampus and cortical areas are selectively affected. Future studies will examine the effects of selective GABA neuron ablation on hippocampal network dynamics across the lifespan, and identify cognitive co-morbidities associated with temporal lobe epilepsy and aging. The relationship between different extents of hippocampal neuron loss and performance on cognitive tasks also remains to be determined.
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