Electrophysiological behavior of neonatal astrocytes in hippocampal stratum radiatum

Background: Neonatal astrocytes are diverse in origin, and undergo dramatic change in gene expression, morphological differentiation and syncytial networking throughout development. Neonatal astrocytes also play multifaceted roles in neuronal circuitry establishment. However, the extent to which neonatal astrocytes differ from their counterparts in the adult brain remains unknown. Results: Based on ALDH1L1-eGFP expression or sulforhodamine 101 staining, neonatal astrocytes at postnatal day 1-3 can be reliably identified in hippocampal stratum radiatum. They exhibit a more negative resting membrane potential (V-M), -85 mV, than mature astrocytes, -80 mV and a variably rectifying whole-cell current profile due to complex expression of voltage-gated outward transient K+ (/K-a), delayed rectifying K+ (/K-d) and inward K+ (/K-in) conductances. Differing from NG2 glia, depolarization-induced inward Na+ currents (/Na) could not be detected in neonatal astrocytes. A quasi-physiological V-M of -69 mV was retained when inwardly rectifying K(ir)4.1 was inhibited by 100 mu M Ba2+ in both wild type and TWIK-1/TREK-1 double gene knockout astrocytes, indicating expression of additional leak K+ channels yet unknown. In dual patch recording, electrical coupling was detected in 74 % (14/19 pairs) of neonatal astrocytes with largely variable coupling coefficients. The increasing gap junction coupling progressively masked the rectifying K+ conductances to account for an increasing number of linear voltage-to-current relationship passive astrocytes (PAs). Gap junction inhibition, by 100 mu M meclofenamic acid, substantially reduced membrane conductance and converted all the neonatal PAs to variably rectifying astrocytes. The low density expression of leak K+ conductance in neonatal astrocytes corresponded to a similar to 50 % less K+ uptake capacity compared to adult astrocytes. Conclusions: Neonatal astrocytes predominantly express a variety of rectifying K+ conductances, form discrete cell-to-cell gap junction coupling and are deficient in K+ homeostatic capacity.