Spontaneous Ultraslow Na+ Fluctuations in the Neonatal Mouse Brain

Abstract

In the neonate forebrain, network formation is driven by the spontaneous synchronized activity of pyramidal cells and interneurons, consisting of bursts of electrical activity and intracellular Ca2+ oscillations. By employing ratiometric Na+ imaging in tissue slices obtained from animals at postnatal day 2–4 (P2–4), we found that 20% of pyramidal neurons and 44% of astrocytes in neonatal mouse hippocampus also exhibit transient fluctuations in intracellular Na+. These occurred at very low frequencies (~2/h), were exceptionally long (~8 min), and strongly declined after the first postnatal week. Similar Na+ fluctuations were also observed in the neonate neocortex. In the hippocampus, Na+ elevations in both cell types were diminished when blocking action potential generation with tetrodotoxin. Neuronal Na+ fluctuations were significantly reduced by bicuculline, suggesting the involvement of GABAA-receptors in their generation. Astrocytic signals, by contrast, were neither blocked by inhibition of receptors and/or transporters for different transmitters including GABA and glutamate, nor of various Na+-dependent transporters or Na+-permeable channels. In summary, our results demonstrate for the first time that neonatal astrocytes and neurons display spontaneous ultraslow Na+ fluctuations. While neuronal Na+ signals apparently largely rely on suprathreshold GABAergic excitation, astrocytic Na+ signals, albeit being dependent on neuronal action potentials, appear to have a separate trigger and mechanism, the source of which remains unclear at present.

Sodium Fluctuations in Astroglia and Their Potential Impact on Astrocyte Function

“… Different forms of Na + signaling in astrocytes. (A) Spontaneous Na + fluctuations as recently reported from neonate hippocampus and cortex ( Felix et al, 2020 ). (B) More rapid, global Na + oscillations, which are synchronized between cells and accompany recurrent network activity in mouse hippocampus ( Karus et al, 2015 , 2017 ).…”

“…A recent study showed that neonatal astrocytes of mouse hippocampus and cortex display extremely long (~8 min), asynchronous and irregular changes in their somatic [Na + ], termed "ultraslow Na + fluctuations" (Felix et al, 2020; Figure 2). These events had amplitudes of ~2 mM and were largely limited to the first 2 postnatal weeks.…”

“…Astrocytic signals were dampened by the application of tetrodotoxin to block voltage-gated Na + channels. While this linked them to neuronal activity, it remained unclear which pathways are involved in their generation (Felix et al, 2020). Independent of the -as yet -unknown origin of the neonate ultraslow Na + fluctuations, the timing of their appearance makes it tempting to assign them a developmental function, as has been shown to be the case for other spontaneous activity patterns in the young brain (Ben-Ari, 2001).…”

“…In layer 2/3 astrocytes, application of glutamate or synaptic stimulation produces Na + transients with an amplitude twice as large as those induced by the same protocol in hippocampal astrocytes (Ziemens et al, 2019). Large [Na + ] i increases of ~13 mM were induced specifically in processes and were (Felix et al, 2020). (B) More rapid, global Na + oscillations, which are synchronized between cells and accompany recurrent network activity in mouse hippocampus (Karus et al, 2015(Karus et al, , 2017.…”

“…Astrocytes rely on a steeply inwardly directed Na + gradient to drive a multitude of membrane transport processes. The gradient is based on their low [Na + ] i reported values, for which vary between 10 and 17 mM ( Kirischuk et al, 2012 ; Rose and Karus, 2013 ; Felix et al, 2020 ), coupled with an extracellular Na + concentration ([Na + ] o ) of 140–150 mM ( Rose and Karus, 2013 ). The primary mechanism for its installation is the sodium-potassium-ATPase (NKA), which uses energy obtained via ATP breakdown in order to transport 3 Na + and 2 K + ions against their concentration gradients (out of and into the cell respectively; Jorgensen et al, 2003 ; Aperia, 2007 ; Hertz et al, 2015 ; Figure 1 ).…”