Putative mechanisms of induction and maintenance of seizure-like activity (SLA) in the low Mg²⁺model of seizures are: facilitation of NMDA receptors and decreased surface charge screening near voltage-gated channels. We have estimated the role of such screening in the early stages of SLA development at both physiological and room temperatures. External Ca²⁺and Mg²⁺promote a depolarization shift of the sodium channel voltage sensitivity; when examined in hippocampal pyramidal neurons, the effect of Ca²⁺was 1.4 times stronger than of Mg²⁺. Removing Mg²⁺from the extracellular solution containing 2 mM Ca²⁺induced recurrent SLA in hippocampal CA1 pyramidal layer in 67% of slices. Reduction of [Ca²⁺]_oto 1 mM resulted in 100% appearance of recurrent SLA or continuous SLA. Both delay before seizure activity and the inter-SLA time were significantly reduced. Characteristics of seizures evoked in low Mg²⁺/1 mM Ca²⁺/3.5 K⁺were similar to those obtained in low Mg²⁺/2 Ca²⁺/5mM K⁺, suggesting that reduction of [Ca²⁺]_oto 1 mM is identical to the increase in [K⁺]_oto 5 mM in terms of changes in cellular excitability and seizure threshold. An increase of [Ca²⁺]_oto 3 mM completely abolished SLA generation even in the presence of 5 mM [K⁺]_o. A large variation in the ability of [Ca²⁺]_oto stop epileptic discharges in initial stage of SLA was found. Our results indicate that surface charge of the neuronal membrane plays a crucial role in the initiation of low Mg²⁺-induced seizures. Furthermore, our study suggests that Ca²⁺and Mg²⁺, through screening of surface charge, have important anti-seizure and antiepileptic properties.

Neonatal hyperbilirubinemia is a common clinical condition that can lead to brain encephalopathy, particularly when concurrent with acidosis due to infection, ischemia, and hypoxia. The prevailing view is that acidosis increases the permeability of the blood-brain barrier to bilirubin and exacerbates its neurotoxicity. In this study, we found that the concentration of the cell death marker, lactate dehydrogenase (LDH) in cerebrospinal fluid (CSF), is elevated in infants with both hyperbilirubinemia and acidosis and showed stronger correlation with the severity of acidosis rather than increased bilirubin concentration. In mouse neonatal neurons, bilirubin exhibits limited toxicity but robustly potentiates the activity of acid-sensing ion channels (ASICs), resulting in increases in intracellular Ca²⁺ concentration, spike firings, and cell death. Furthermore, neonatal conditioning with concurrent hyperbilirubinemia and hypoxia-induced acidosis promoted long-term impairments in learning and memory and complex sensorimotor functions in vivo, which are largely attenuated in ASIC1a null mice. These findings suggest that targeting acidosis and ASICs may attenuate neonatal hyperbilirubinemia complications.

The exact roles of acid-sensing ion channels (ASICs) in synaptic plasticity remain elusive. Here, we address the contribution of ASIC1a to five forms of synaptic plasticity in the mouse hippocampus using an in vitro multi-electrode array recording system. We found that genetic deletion or pharmacological blockade of ASIC1a greatly reduced, but did not fully abolish, the probability of long-term potentiation (LTP) induction by either single or repeated high frequency stimulation or theta burst stimulation in the CA1 region. However, these treatments did not affect hippocampal long-term depression induced by low frequency electrical stimulation or (RS)-3,5-dihydroxyphenylglycine. We also show that ASIC1a exerts its action in hippocampal LTP through multiple mechanisms that include but are not limited to augmentation of NMDA receptor function. Taken together, these results reveal new insights into the role of ASIC1a in hippocampal synaptic plasticity and the underlying mechanisms. This unbiased study also demonstrates a novel and objective way to assay synaptic plasticity mechanisms in the brain.