Passive Conductance And Electrical Coupling Of Astrocytes In Rat Hippocampal Slices

Astrocytes and NG2 glia are two major glial types in the mammalian central nervoussystem (CNS). NG2+ glial cells, previously known as oligodendrocyte precursor cells(OPCs), are distinct by their proteoglycan NG2 expression and have now been classified asthe fourth members in the glial family. They have unique electrophysiological propertiesand extensively distribute in adult brain. Here we systematically studied the difference between astrocytes and NG2+glial cells in basal electrophysiological properties andmorphology. We also described a frequently observed space arrangement with somataphysically attached between astrocytes and NG+ gial cells in rat hippocampal CA1 regionwhich may indicate a lineage relationship between these two types of cells. Over the last twenty years, the somatic whole-cell voltage clamp technique has beenwidely used to investigate the electrophysiology of astrocyte in situ. Hippocampalastrocytes showing a linear current-voltage (I-V) relationship, or electrophysiologicallypassive K~+ membrane conductance and a very low membrane resistance have been reportedby employing this approach. However, the somatic votage clamp will poorly control the membrane potential due to a very low membrane resistance. To quantify this measurementerror is crucial to help us to understand the nature of passive conductance. Here, we directlyquantify the error in the voltage clamp measurement of astrocytes in situ using dual patchwhole-cell recordings from single astrocytes in hippocampal slices. The average voltageescape was 73.1%. However, the compensating for access resistance by 80% failed todecrease the deviation between the recorded membrane potential and the voltage commandsignificantly and the average voltage escape was still as high as 45.7%. Thus, wedemonstrate for the first time that the passive conductance is a resting conductance which isinduced over a narrow range of membrane potentials around the astrocyte restingmembrane potentials. Furthermore, this measurement error indicates that the limitations ofvoltage clamp study on astrocytes in situ should be paid more attention to. Mammalian protoplasmic astrocytes are coupled extensively through gap junctionchannels in vivo. However, the biophysical characterizations of gap junction channelsunder physiological and ischemic conditions are not fully understood. Starting with amorphometrical analysis of astrocytic syncytia in rat hippocampal CA1 stratum radiatumusing intracellular loading of biocytin, we show that on average each astrocyte directlycoupled to another 11 astrocytes 45μm apart. In dual voltage clamp recording,voltage-independent and bidirectional transjunctional currents were always measured fromthe directly coupled astrocytes, but not from astrocyte-NG2 gila or astrocyte-interneuronpairs. The electrical coupling ratio varied significantly among astrocytes in the developingpostnatal day 14 (P14) rats (0.5%-12.4%, mean=3.6%), but became more constant in themature P21 rats (0.18%-3.9%, mean=1.6%). Only in the mature rats, the coupling ratiodeclined exponentially with the increasing pair distance. Electrical coupling was notaffected by a short-term oxygen-glucose deprivation (OGD) treatment, but inhibited in adelayed fashion by the acidic extracellular pH of 6.4. Strikingly. acidic OGD (pH 6.4), acondition that better represents the cerebral ischemia in vivo, accelerated the inhibition ofelectrical coupling markedly. Altogether, a rather low effective electrical coupling ratiosuggests that astrocytic gap junctions conduct little K~+ spatial buffering currents underphysiological condition, and astrocyte gap junctional communication should be severelyaffected as the consequence ofa synergy effect of OGD and acidosis in ischemic brain.