Developmental Decrease of Neuronal Chloride Concentration Is Independent of Trauma in Thalamocortical Brain Slices

The intraneuronal chloride concentration ([Cl - ] i ) is paramount for determining the polarity of signaling at GABA A synapses in the central nervous system. Sectioning hippocampal brain slices increases [Cl - ] i in the superficial layers. It is not known whether cutting trauma also increases [Cl - ] i in the neocortex and thalamus, and whether the effects of trauma change during development. We used Cl - imaging to study the [Cl - ] i vs. the distance from the cut surface in acute thalamocortical slices from mice at developmental ages ranging from post-natal day 5 (P5) to P20. We demonstrate: 1) [Cl - ] i is higher in the most superficial areas in both neocortical and thalamic brain slices at all ages tested and, 2) there is a developmental decrease in [Cl - ] i that is independent of acute trauma caused by brain slicing. We conclude that [Cl - ] i has a developmental progression during P5-20 in both the neocortex and thalamus. However, in both brain regions and during development the neurons closest to the slicing trauma have an elevated [Cl - ] i .