Control of in vivo ictogenesis via endogenous synaptic pathways

Selective Medial Septum Lesions in Healthy Rats Induce Longitudinal Changes in Microstructure of Limbic Regions, Behavioral Alterations, and Increased Susceptibility to Status Epilepticus

Septo-hippocampal pathway, crucial for physiological functions and involved in epilepsy. Clinical monitoring during epileptogenesis is complicated. We aim to evaluate tissue changes after lesioning the medial septum (MS) of normal rats and assess how the depletion of specific neuronal populations alters the animals' behavior and susceptibility to establishing a pilocarpine-induced status epilepticus. Male Sprague-Dawley rats were injected into the MS with vehicle or saporins (to deplete GABAergic or cholinergic neurons; n = 16 per group). Thirty-two animals were used for diffusion tensor imaging (DTI); scanned before surgery and 14 and 49 days post-injection. Fractional anisotropy and apparent diffusion coefficient were evaluated in the fimbria, dorsal hippocampus, ventral hippocampus, dorso-medial thalamus, and amygdala. Between scans 2 and 3, animals were submitted to diverse behavioral tasks. Stainings were used to analyze tissue alterations. Twenty-four different animals received pilocarpine to evaluate the latency and severity of the status epilepticus 2 weeks after surgery. Additionally, eight different animals were only used to evaluate the neuronal damage inflicted on the MS 1 week after the molecular surgery. Progressive changes in DTI parameters in both white and gray matter structures of the four evaluated groups were observed. Behaviorally, the GAT1-saporin injection impacted spatial memory formation, while 192-IgG-saporin triggered anxiety-like behaviors. Histologically, the GABAergic toxin also induced aberrant mossy fiber sprouting, tissue damage, and neuronal death. Regarding the pilocarpine-induced status epilepticus, this agent provoked an increased mortality rate. Selective septo-hippocampal modulation impacts the integrity of limbic regions crucial for certain behavioral skills and could represent a precursor for epilepsy development.

The thalamic midline nucleus reuniens: potential relevance for schizophrenia and epilepsy

Anatomical, electrophysiological and behavioral studies in rodents have shown that the thalamic midline nucleus reuniens (RE) is a crucial link in the communication between hippocampal formation (HIP, i.e., CA1, subiculum) and medial prefrontal cortex (mPFC), important structures for cognitive and executive functions. A common feature in neurodevelopmental and neurodegenerative brain diseases is a dysfunctional connectivity/communication between HIP and mPFC, and disturbances in the cognitive domain. Therefore, it is assumed that aberrant functioning of RE may contribute to behavioral/cognitive impairments in brain diseases characterized by cortico-thalamo-hippocampal circuit dysfunctions. In the human brain the connections of RE are largely unknown. Yet, recent studies have found important similarities in the functional connectivity of HIP-mPFC-RE in humans and rodents, making cautious extrapolating experimental findings from animal models to humans justifiable. The focus of this review is on a potential involvement of RE in schizophrenia and epilepsy.

Neurophysiological alterations in the nucleus reuniens of a mouse model of Alzheimer's disease

Recently, increased neuronal activity in nucleus reuniens (Re) has been linked to hyperexcitability within hippocampal-thalamo-cortical networks in the J20 mouse model of amyloidopathy. Here in vitro whole-cell patch clamp recordings were used to compare old pathology-bearing J20 mice and wild-type controls to examine whether altered intrinsic electrophysiological properties could contribute to the amyloidopathy-associated Re hyperactivity. A greater proportion of Re neurons display hyperpolarized membrane potentials in J20 mice without changes to the incidence or frequency of spontaneous action potentials. Re neurons recorded from J20 mice did not exhibit increased action potential generation in response to depolarizing current stimuli but an increased propensity to rebound burst following hyperpolarizing current stimuli. Increased rebound firing did not appear to result from alterations to T-type Ca²⁺ channels. Finally, in J20 mice, there was an ~8% reduction in spike width, similar to what has been reported in CA1 pyramidal neurons from multiple amyloidopathy mice. We conclude that alterations to the intrinsic properties of Re neurons may contribute to hippocampal-thalmo-cortical hyperexcitability observed under pathological beta-amyloid load.

Use and Future Prospects of in Vivo Microdialysis for Epilepsy Studies

Epilepsy is a common neurological disease characterized by recurrent unpredictable seizures. For the last 30 years, microdialysis sampling has been used to measure changes in excitatory and inhibitory neurotransmitter concentrations before, during, and after seizures. These advances have fostered breakthroughs in epilepsy research by identifying neurochemical changes associated with seizures and correlating them to electrophysiological data. Recent advances in methodology may be useful in further delineating the chemical underpinnings of seizures. A new model of ictogenesis has been developed that allows greater control over the timing of seizures that are similar to spontaneous seizures. This model will facilitate making chemical measurements before and during a seizure. Recent advancements in microdialysis sampling, including the use of segmented flow, "fast" liquid chromatography (LC), and capillary electrophoresis with laser-induced fluorescence (CE-LIF) have significantly improved temporal resolution to better than 1 min, which could be used to measure transient, spontaneous neurochemical changes associated with seizures. Microfabricated sampling probes that are markedly smaller than conventional probes and allow for a much greater spatial resolution have been developed. They may allow the targeting of specific brain regions important to epilepsy studies. Coupling microdialysis sampling to optogenetics and light-stimulated release of neurotransmitters may also prove useful for studying epileptic seizures.

Chemical biomarkers of epileptogenesis and ictogenesis in experimental epilepsy

Epilepsy produces chronic chemical changes induced by altered cellular structures, and acute ones produced by conditions leading into individual seizures. Here, we aim to quantify 24 molecules simultaneously at baseline and during periods of lowered seizure threshold in rats. Using serial hippocampal microdialysis collections starting two weeks after the pilocarpine-induced status epilepticus, we evaluated how this chronic epilepsy model affects molecule levels and their interactions. Then, we quantified the changes occurring when the brain moves into a pro-seizure state using a novel model of physiological ictogenesis. Compared with controls, pilocarpine animals had significantly decreased baseline levels of adenosine, homovanillic acid, and serotonin, but significantly increased levels of choline, glutamate, phenylalanine, and tyrosine. Step-wise linear regression identified that choline, homovanillic acid, adenosine, and serotonin are the most important features to characterize the difference in the extracellular milieu between pilocarpine and control animals. When increasing the hippocampal seizure risk, the concentrations of normetanephrine, serine, aspartate, and 5-hydroxyindoleacetic acid were the most prominent; however, there were no specific, consistent changes prior to individual seizures.