Effect of hyperthermia on hippocampal synaptic transmission and CA3 kindling in developing rats

Febrile Seizures, Ongoing Epileptiform Activity, and the Resulting Long-Term Consequences: Lessons From Animal Models

Febrile seizures affect 2% to 14% of children. Prospective studies indicate that following a relatively prolonged febrile seizure there are long-term consequences. Although controlled experiments in children have ethical limitations, nonhuman animal models give us the ability to discover new phenomena, determine their mechanisms, and test treatments that can potentially translate to the human clinical population. Rat models of febrile seizures show two temporally distinct phases: (1); behavioral seizures and (2); ongoing epileptiform activity associated with hyperoxia. The behavioral seizures mimic those displayed by children including tonic-clonic convulsions and loss of postural control. Recordings show classic spiking discharges from cortical regions during the behavioral seizures. Following behavioral seizure termination electrical recordings in rodent models reveal that there is ongoing epileptiform activity that lasts longer than the duration of the behavioral seizures themselves. This ongoing epileptiform activity is also associated with hyperoxia-levels of brain tissue oxygen well above the normoxic zone (typical oxygen levels)-and can last more than an hour. When this hyperoxia, but not the epileptiform activity, is prevented in febrile rat pups the long-term learning impairments are also prevented. This leaves important questions unanswered, "Do children also have ongoing and long-lasting epileptiform activity and associated hyperoxia following termination of their febrile behavioral seizures and does this second phase have long-term consequences"? Here we discuss appropriate animal models of febrile seizures that replicate much of the human condition with special attention to the long-term effects of occult epileptiform activity following termination of a behavioral febrile seizure.

Dysfunctional hippocampal-prefrontal network underlies a multidimensional neuropsychiatric phenotype following early-life seizure

Brain disturbances during development can have a lasting impact on neural function and behavior. Seizures during this critical period are linked to significant long-term consequences such as neurodevelopmental disorders, cognitive impairments, and psychiatric symptoms, resulting in a complex spectrum of multimorbidity. The hippocampus-prefrontal cortex (HPC-PFC) circuit emerges as a potential common link between such disorders. However, the mechanisms underlying these outcomes and how they relate to specific behavioral alterations are unclear. We hypothesized that specific dysfunctions of hippocampal-cortical communication due to early-life seizure would be associated with distinct behavioral alterations observed in adulthood. Here, we performed a multilevel study to investigate behavioral, electrophysiological, histopathological, and neurochemical long-term consequences of early-life Status epilepticus in male rats. We show that adult animals submitted to early-life seizure (ELS) present working memory impairments and sensorimotor disturbances, such as hyperlocomotion, poor sensorimotor gating, and sensitivity to psychostimulants despite not exhibiting neuronal loss. Surprisingly, cognitive deficits were linked to an aberrant increase in the HPC-PFC long-term potentiation (LTP) in a U-shaped manner, while sensorimotor alterations were associated with heightened neuroinflammation, as verified by glial fibrillary acidic protein (GFAP) expression, and altered dopamine neurotransmission. Furthermore, ELS rats displayed impaired HPC-PFC theta-gamma coordination and an abnormal brain state during active behavior resembling rapid eye movement (REM) sleep oscillatory dynamics. Our results point to impaired HPC-PFC functional connectivity as a possible pathophysiological mechanism by which ELS can cause cognitive deficits and psychiatric-like manifestations even without neuronal loss, bearing translational implications for understanding the spectrum of multidimensional developmental disorders linked to early-life seizures.

Cognitive Sequelae and Hippocampal Dysfunction in Chronic Kidney Disease following 5/6 Nephrectomy

Neurological disorders are prevalent in patients with chronic kidney disease (CKD). Vascular factors and uremic toxins are involved with cognitive impairment in CKD. In addition, vascular dementia-induced alterations in the structure and function of the hippocampus can lead to deficits in hippocampal synaptic plasticity and cognitive function. However, regardless of this clinical evidence, the pathophysiology of cognitive impairment in patients with CKD is not fully understood. We used male Sprague Dawley rats and performed 5/6 nephrectomy to observe the changes in behavior, field excitatory postsynaptic potential, and immunostaining of the hippocampus following CKD progression. We measured the hippocampus volume on magnetic resonance imaging scans in the controls (n = 34) and end-stage renal disease (ESRD) hemodialysis patients (n = 42). In four cognition-related behavior assays, including novel object recognition, Y-maze, Barnes maze, and classical contextual fear conditioning, we identified deficits in spatial working memory, learning and memory, and contextual memory, as well as the ability to distinguish familiar and new objects, in the rats with CKD. Immunohistochemical staining of Na⁺/H⁺ exchanger1 was increased in the hippocampus of the CKD rat models. We performed double immunofluorescent staining for aquaporin-4 and glial fibrillary acidic protein and then verified the high coexpression in the hippocampus of the CKD rat model. Furthermore, results from recoding of the field excitatory postsynaptic potential (fEPSP) in the hippocampus showed the reduced amplitude and slope of fEPSP in the CKD rats. ESRD patients with cognitive impairment showed a significant decrease in the hippocampus volume compared with ESRD patients without cognitive impairment or the controls. Our findings suggest that uremia resulting from decreased kidney function may cause the destruction of the blood–brain barrier and hippocampus-related cognitive impairment in CKD.

Long-lasting modulation of synaptic plasticity in rat hippocampus after early-life complex febrile seizures

A small fraction of children with febrile seizures appears to develop cognitive impairments. Recent studies in a rat model of hyperthermia-induced febrile seizures indicate that prolonged febrile seizures early in life have long-lasting effects on the hippocampus and induce cognitive deficits. However, data on network plasticity and the nature of cognitive deficits are conflicting. We examined three specific measures of hippocampal plasticity in adult rats with a prior history of experimental febrile seizures: (i) activity-dependent synaptic plasticity (long-term potentiation and depression) by electrophysiological recordings of Schaffer collateral/commissural-evoked field excitatory synaptic potentials in CA1 of acute hippocampal slices; (ii) Morris water maze spatial learning and memory; and (iii) hippocampal mossy fiber plasticity by Timm histochemistry and quantification of terminal sprouting in CA3 and the dentate gyrus. We found enhanced hippocampal CA1 long-term potentiation and reduced long-term depression but normal spatial learning and memory in adult rats that were subjected to experimental febrile seizures on postnatal day 10. Furthermore, rats with experimental febrile seizures showed modest but significant sprouting of mossy fiber collaterals into the inner molecular layer of the dentate gyrus in adulthood. We conclude that enhanced CA1 long-term potentiation and mild mossy fiber sprouting occur after experimental febrile seizures, without affecting spatial learning and memory in the Morris water maze. These long-term functional and structural alterations in hippocampal plasticity are likely to play a role in the enhanced seizure susceptibility in this model of prolonged human febrile seizures but do not correlate with overt cognitive deficits.