Ionized calcium in human cerebrospinal fluid and its influence on intrinsic and synaptic excitability of hippocampal pyramidal neurons in the rat

It is well‐known that the extracellular concentration of calcium affects neuronal excitability and synaptic transmission. Less is known about the physiological concentration of extracellular calcium in the brain. In electrophysiological brain slice experiments, the artificial cerebrospinal fluid traditionally contains relatively high concentrations of calcium (2‐4 mM) to support synaptic transmission and suppress neuronal excitability. Using an ion‐selective electrode, we determined the fraction of ionized calcium in healthy human cerebrospinal fluid to 1.0 mM of a total concentration of 1.2 mM (86%). Using patch‐clamp and extracellular recordings in the CA1 region in acute slices of rat hippocampus, we then compared the effects of this physiological concentration of calcium with the commonly used 2 mM on neuronal excitability, synaptic transmission, and long‐term potentiation (LTP) to examine the magnitude of changes in this range of extracellular calcium. Increasing the total extracellular calcium concentration from 1.2 to 2 mM decreased spontaneous action potential firing, induced a depolarization of the threshold, and increased the rate of both de‐ and repolarization of the action potential. Evoked synaptic transmission was approximately doubled, with a balanced effect between inhibition and excitation. In 1.2 mM calcium high‐frequency stimulation did not result in any LTP, whereas a prominent LTP was observed at 2 or 4 mM calcium. Surprisingly, this inability to induce LTP persisted during blockade of GABAergic inhibition. In conclusion, an increase from the physiological 1.2 mM to 2 mM calcium in the artificial cerebrospinal fluid has striking effects on neuronal excitability, synaptic transmission, and the induction of LTP. Open science badges This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/. Read the Editorial Highlight for this article on page 435. Extracellular calcium greatly affects both intrinsic excitability and synaptic transmission. We measured the concentration of ionized calcium in healthy human cerebrospinal fluid (hCSF) and compared this with the high concentration commonly used in artificial cerebrospinal fluid (aCSF) in regard to intr