Synaptic plasticity in area CA1 of rat hippocampal slices following intraventricular application of albumin.

Epileptogenesis following insults to the brain may be triggered by a dysfunctional blood-brain barrier (BBB) associated with albumin extravasation and activation of astrocytes. Using ex vivo recordings from the BBB-disrupted hippocampus after neocortical photothrombotic stroke, we previously demonstrated abnormal activity-dependent accumulation of extracellular potassium with facilitated generation of seizure like events and spreading depolarizations. Similar changes could be observed after intracerebroventricular (icv) application of albumin. We hypothesized that alterations in extracellular potassium and glutamate homeostasis might lead to alterations in synaptic interactions. We therefore assessed the effects of icv albumin on homo- and heterosynaptic plasticity in hippocampal CA1, 24h after a single injection or 7days after continuous infusion of icv albumin. We demonstrate alterations in both homo- and heterosynaptic plasticity compared to control conditions in ex vivo slice studies. Albumin-treated tissue reveals (1) reduced long-term depression following low-frequency stimulation; (2) increased long-term potentiation of population spikes in response to 20Hz stimulation; (3) potentiated responses to Schaffer collateral stimulation following high-frequency stimulation of the direct cortical input and low-frequency stimulation of alveus and finally, (4) TGFß receptor II (TGFßR-II) involvement in albumin-induced homosynaptic plasticity changes. We conclude that albumin-induced network hyperexcitability is associated with abnormal homo- and heterosynaptic plasticity that could partly be reversed by interference with TGFßR-II-mediated signaling and therefore it might be an important factor in the process of epileptogenesis.

Increased susceptibility to acetylcholine in the entorhinal cortex of pilocarpine-treated rats involves alterations in KCNQ channels.

In models of temporal lobe epilepsy, in-vitro exposure of the entorhinal cortex (EC) to low concentrations of acetylcholine (ACh) induces muscarinic-dependent seizure-like events. Potassium channels from the KCNQ/Kv7 family, which close upon activation of muscarinic receptors, are mutated in several epileptic syndromes such as benign familial neonatal convulsions (KCNQ2/KCNQ3) and sudden unexplained death in epilepsy (KCNQ1). Therefore, we tested the hypothesis whether the ictogenic effect of ACh involves alterations of KCNQ channels. In horizontal temporo-hippocampal slices from pilocarpine-treated chronically epileptic rats, field potential recordings of epileptiform activity were performed in response to the application of ACh, the KCNQ blocker linopirdine, and KCNQ agonists. In the EC of control rats, ACh (20 and 50 µM) induced nested fast activity in the range of 15-20 Hz riding on <1 Hz slow oscillations. By contrast, in slices from pilocarpine-treated rats, 5 µM ACh was sufficient to induce interictal discharges that frequently transformed to epileptiform events at 20 µM ACh. While the non-specific KCNQ/Kv7 channel blocker linopirdine (20 and 50 µM) had no effect in control animals, in slices from epileptic rats it induced interictal discharges or seizure-like events. These could be blocked by the unspecific KCNQ/Kv7 agonist retigabine and attenuated by the Kv7.1 agonist L364-373. Immunohistochemistry revealed reduced expression of KCNQ2 and KCNQ3 in the EC and of KCNQ3-positive dendrites in the subiculum of epileptic rats. These results indicate that channels of the KCNQ family are key regulators of seizure susceptibility and their decreased availability in the epileptic tissue may reduce seizure threshold and contribute to ictogenesis.

Chronically epileptic human and rat neocortex display a similar resistance against spreading depolarization in vitro.

BACKGROUND AND PURPOSE: Experimental and clinical evidence suggests that prolonged spreading depolarizations (SDs) are a promising target for therapeutic intervention in stroke because they recruit tissue at risk into necrosis by protracted intracellular calcium surge and massive glutamate release. Unfortunately, unlike SDs in healthy tissue, they are resistant to drugs such as N-methyl-d-aspartate-receptor antagonists. This drug resistance of SD in low perfusion areas may be due to the gradual rise of extracellular potassium before SD onset. Brain slices from patients undergoing surgery for intractable epilepsy allow for screening of drugs, targeting pharmacoresistant SDs under elevated potassium in human tissue. However, network changes associated with epilepsy may interfere with tissue susceptibility to SD. This could distort the results of pharmacological tests. METHODS: We investigated the threshold for SD, induced by a gradual rise of potassium, in neocortex slices of patients with intractable epilepsy and of chronically epileptic rats as well as age-matched and younger control rats using combined extracellular potassium/field recordings and intrinsic optical imaging. RESULTS: Both age and epilepsy significantly increased the potassium threshold, which was similarly high in epileptic rat and human slices (23.6±2.4 mmol/L versus 22.3±2.8 mmol/L). CONCLUSIONS: Our results suggest that chronic epilepsy confers resistance against SD. This should be considered when human tissue is used for screening of neuroprotective drugs. The finding of similar potassium thresholds for SD in epileptic human and rat neocortex challenges previous speculations that the resistance of the human brain against SD is markedly higher than that of the rodent brain.

Astrocytic dysfunction in epileptogenesis: consequence of altered potassium and glutamate homeostasis?

Focal epilepsy often develops following traumatic, ischemic, or infectious brain injury. While the electrical activity of the epileptic brain is well characterized, the mechanisms underlying epileptogenesis are poorly understood. We have recently shown that in the rat neocortex, long-lasting breakdown of the blood-brain barrier (BBB) or direct exposure of the neocortex to serum-derived albumin leads to rapid upregulation of the astrocytic marker GFAP (glial fibrillary acidic protein), followed by delayed (within 4-7 d) development of an epileptic focus. We investigated the role of astrocytes in epileptogenesis in the BBB-breakdown and albumin models of epileptogenesis. We found similar, robust changes in astrocytic gene expression in the neocortex within hours following treatment with deoxycholic acid (BBB breakdown) or albumin. These changes predict reduced clearance capacity for both extracellular glutamate and potassium. Electrophysiological recordings in vitro confirmed the reduced clearance of activity-dependent accumulation of both potassium and glutamate 24 h following exposure to albumin. We used a NEURON model to simulate the consequences of reduced astrocytic uptake of potassium and glutamate on EPSPs. The model predicted that the accumulation of glutamate is associated with frequency-dependent (>100 Hz) decreased facilitation of EPSPs, while potassium accumulation leads to frequency-dependent (10-50 Hz) and NMDA-dependent synaptic facilitation. In vitro electrophysiological recordings during epileptogenesis confirmed frequency-dependent synaptic facilitation leading to seizure-like activity. Our data indicate a transcription-mediated astrocytic transformation early during epileptogenesis. We suggest that the resulting reduction in the clearance of extracellular potassium underlies frequency-dependent neuronal hyperexcitability and network synchronization.