On the Origin of Ultraslow Spontaneous Na+ Fluctuations in Neurons of the Neonatal Forebrain

Abstract

Spontaneous neuronal and astrocytic activity in the neonate forebrain is believed to drive the maturation of individual cells and their integration into complex brain-region-specific networks. The previously reported forms include bursts of electrical activity and oscillations in intracellular Ca2+ concentration. Here, we use ratiometric Na+ imaging to demonstrate spontaneous fluctuations in the intracellular Na+ concentration of CA1 pyramidal neurons and astrocytes in tissue slices obtained from the hippocampus of mice at postnatal days 2-4 (P2-4). These occur at very low frequency ([~]2/h), can last minutes with amplitudes up to several mM, and mostly disappear after the first postnatal week. To further study the mechanisms that may generate such spontaneous fluctuations in neurons, we model a network consisting of pyramidal neurons and interneurons. Experimentally observed Na+ fluctuations are mimicked when GABAergic inhibition in the simulated network is inverted. Both our experiments and computational model show that the application of tetrodotoxin to block voltage-gated Na+ channels or of inhibitors targeting GABAergic signaling respectively, significantly diminish the neuronal Na+ fluctuations. On the other hand, blocking a variety of other ion channels, receptors, or transporters including glutamatergic pathways, does not have significant effects. In addition, our model shows that the amplitude and duration of Na+ fluctuations decrease as we increase the strength of glial K+ uptake. Furthermore, neurons with smaller somatic volumes exhibit fluctuations with higher frequency and amplitude. As opposed to this, the larger relative size of the extracellular with respect to intracellular space observed in neonatal brain exerts a dampening effect. Finally, our model also predicts that these periods of spontaneous Na+ influx leave neonatal neuronal networks more vulnerable to hyperactivity when compared to mature brain. Taken together, our model thus confirms the experimental observations, and offers additional insight into how the neonatal environment shapes early signaling in the brain. Author SummarySpontaneous neuronal and astrocytic activity during the early postnatal period is crucial to the development and physiology of the neonate forebrain. Elucidating the origin of this activity is key to our understanding of the cell maturation and formation of brain-region-specific networks. This study reports spontaneous, ultraslow, large-amplitude, long-lasting fluctuations in the intracellular Na+ concentration of neurons and astrocytes in the hippocampus of mice at postnatal days 2-4 that mostly disappear after the first postnatal week. We combine ratiometric Na+ imaging and pharmacological manipulations with a detailed computational model of neuronal networks in the neonatal and adult brain to provide key insights into the origin of these Na+ fluctuations. Furthermore, our model predicts that these periods of spontaneous Na+ influx leave neonatal neuronal networks more vulnerable to hyperactivity when compared to mature brain.