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

Abstract

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.

What is the extracellular calcium concentration within brain synapses?

“…The question addressed by Forsberg et al . () of defining the synaptic Ca 2+ concentration is of critical interest, since small changes of Ca 2+ levels can lead to a profound change in the propensity of neurons to fire action potentials in bursts, which are a main trigger and modulator of long‐term changes of synaptic efficiency (Wang et al . ).…”

“…In conclusion, the data reported in the study by Forsberg et al . () provide a new impetus in this long quest of assessing the extracellular Ca 2+ concentration in synapses, which is essential in the design of neurophysiology experiments. It is hoped that future studies will detail some generalizations made by Forsberg et al .…”

“…It is hoped that future studies will detail some generalizations made by Forsberg et al . () to consider the role of the glymphatic system, of astrocytes, and of transmembrane proteins, to define the impact of variations of extracellular Ca 2+ in different forms of plasticity expressed by synapses (Fig. ).…”

“…In fact, the study of Forsberg et al (2019) explicitly assumes that the brain extracellular fluid communicates freely with the CSF. Such an assumption might be questioned in light of Nedergaard's work, which provided data supporting the existence of a glymphatic system (Rasmussen et al 2018).…”

“…The question addressed by Forsberg et al (2019) of defining the synaptic Ca 2+ concentration is of critical interest, since small changes of Ca 2+ levels can lead to a profound change in the propensity of neurons to fire action potentials in bursts, which are a main trigger and modulator of longterm changes of synaptic efficiency (Wang et al 2004). In fact, one of the most striking observations in this study is that LTP was negligible at a Ca 2+ concentration matching that of human CSF (1.2 mM), whereas a prominent LTP was observed when slices were superfused with artificial CSF containing Ca 2+ concentrations normally used in electrophysiology studies (2 mM).…”