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Rapid Changes in Neuronal Excitability During Osmotic Edema in Juvenile and Adult Hippocampus Through NMDA Receptor Activation


Cerebral edema affects millions of people worldwide and is associated with a plethora of diseases, disorders, and conditions such as traumatic brain injury, stroke, cardiac arrest, autism and epilepsy. Cellular edema has been known to increase epileptiform activity and seizure susceptibility in vitro and in vivo. However, the identity of the cell types undergoing volume increases and the types of excitability changes that occur in neurons remain unclear. Electrophysiological whole-cell patch clamp techniques were used to record currents and potentials from CA1 pyramidal neurons during the application of hypoosmolar ACSF (hACSF) in acutely isolated hippocampal slices from mice. Hypoosmolar ACSF evoked slow inward currents (SICs) in neurons, which initiated after ~1 minute of hACSF application. Neuronal excitability increased as osmolarity decreased in a dose-dependent manner. Even 5% reductions in osmolarity were sufficient to significantly increase neuronal excitability. In addition, hACSF induced neuronal firing of action potentials (APs), independent of AMPA receptor activation. Neuronal excitability was also increased during application of hACSF while blocking both APs and AMPA receptors. Increased sub-threshold EPSPs, neuronal APs, and bursting activity were also evoked in the presence of Mg2+, suggesting that hypoosmolar insults increase neuronal excitability under more physiological conditions. Hypoosmolar insults increased neuronal excitability in both juvenile (P15-P21) and adult 2- to 5-month-old) mice. Bursting activity in adult mice during osmotic insult was elevated compared to juvenile mice. During hypoosmolar insults the frequency of SICs recorded at physiological temperature were significantly elevated from SICs recorded at room temperature. SICs were potentiated by D-serine, and blocked by both DL-AP5 and the NR2B specific compound Ro25-6981. Together, these results indicate that osmotic insults produce cellular edema in both neurons and astrocytes, and increase neuronal excitability within minutes through a combination of synaptic and non-synaptic activation of glutamate receptors.

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