UP-REGULATION OF GABA(A) CURRENT BY ASTROCYTES IN CULTURED EMBRYONIC RAT HIPPOCAMPAL-NEURONS

Embryonic rat hippocampal neurons were cultured on poly-D-lysine (PDL) or a monolayer of postnatal cortical astrocytes to reveal putative ch anges in neuronal physiology that involve astrocyte-derived signals du ring the first 4 d of culture. GABA-induced Cl- current (I-GABA) was q uantified using outside-out and whole-cell patch-clamp recordings begi nning at 30 min, when cells had become adherent. The amplitude and den sity (current normalized to membrane capacitance) of I-GABA in neurons grown on astrocytes became statistically greater than that recorded i n neurons grown on PDL after 2 hr in culture (HIC). Although the curre nt density remained unchanged in neurons on astrocytes, that in neuron s on PDL decreased and became statistically lower beginning after 2 HI C. The differences in amplitude and density of I-GABA in the two group s of neurons were maintained during the 4 d experiment. The upregulati on effect of astrocytes on neuronal I-GABA required intimate contact b etween the neuronal cell body and underlying astrocytes. Suppression o f spontaneous Ca-c(2+) elevations in astrocytes by bis(2-aminophenoxy) ethane-N,N,N',N'-tetra-acetic acid that was loaded intracellularly dec reased their modulatory effects on I-GABA. I-GABA in all cells was blo cked completely by bicuculline and exhibited virtually identical affin ity constants, Hill coefficients, and potentiation by diazepam in the two groups. Outside-out patch recordings revealed identical unitary pr operties of I-GABA in the two groups. More channels per unit of membra ne area could explain the astrocyte enhancement of I-GABA. The results reveal that cortical astrocytes potentiate I-GABA in hippocampal neur ons in a contact-dependent manner via a mechanism involving astrocyte Ca-c(2+) elevation.