Abstracts

RATIONALE: Electrophysiological properties of astrocytes in the adult hippocampus and their alterations in response to injury are well characterized in Ammon[ssquote]s horn (D[ssquote]Ambrosio et al. 1999. J. Neurosci. 19:8125) and the dentate gyrus (Schroder et al. 1999. Glia 28:166). These studies sampled astrocytes from dendritic layers where the astrocytic population is abundant, and suggested that, following a variety of insults including seizures, astrocytes dedifferentiated and displayed an immature current pattern, i.e. a reduction of inward rectifier (K[sub]IR[/sub]) K⁺ currents. In the dentate gyrus, however, both granule cell genesis and gliogenesis occur in adulthood. Thus, we hypothesized that astrocytes in a region of immature neurons may maintain an immature electrophysiological profile.
METHODS: Patch-clamp recordings of passive and voltage-gated potassium (K⁺) currents were obtained from astrocytes in the dentate granule cell layer in acute slices prepared from young adult rat hippocampus. Astrocytes were visualized and sampled from inner, middle, and outer portions of the granule cell layer. Astrocytic identification was based on electrophysiological characterization and cell morphology following Lucifer yellow injection.
RESULTS: We found two types of astrocytes:Passive and Complex. Passive astrocytes almost exclusively possessed a time- and voltage-independent K⁺ conductance, while complex astrocytes showed transient and sustained outward K⁺ conductances above -40 mV and +30 mV, respectively. Passive astrocytes were the predominant type, with properties similar to those reported for passive glia in the molecular layer of the dentate gyrus and the radiatum/Lucidum of Ammon[ssquote]s horn. They exhibited an inward current in response to an elevated concentration of extracellular K⁺, which was largely insensitive to extracellular cesium (Cs). On the other hand, complex astrocytes were less frequently detected. In comparison to complex glia reported in CA1, CA3 and the molecular layer, complex astrocytes in the granule cell layer exhibited less K[sub]IR[/sub].
CONCLUSIONS: K[sub]IR[/sub] increases during maturation and is prominent in adult hippocampal astrocytes. However, in the dentate granule cell layer where neuro- and gliogenesis continue, a subpopulation of astrocytes may retain immature pattern of K⁺ conductances to generate prolonged depolarization of glia and neurons. Whether the low level of dentate granule cell layer astrocytic K[sub]IR[/sub] expression relates to the increase in neurogenesis that accompanies seizures remains to be determined.
Support: Klingenstein Foundation Award to GMM.