Figure 7.
Synaptic currents mediated by NMDA receptors in astrocytes. Astrocytes in layer II of the slice were identified by EGFP fluorescence; electrical stimulation was in layer IV. A, Synaptically evoked currents are inhibited by MK-801 (10 μm), and the residual current is partially blocked by dl-TBOA (100 μm). The dl-TBOA-sensitive component obtained by subtraction is shown in the inset. B, In the presence of dl-TBOA (100 μm), NBQX (30 μm) has no effect on the peak synaptic current, which is then almost completely blocked by MK-801 (10 μm). Each point on the time graphs represents the mean ± SEM for five EPSCs; illustrative EPSCs are shown below. Note that the tail current was mostly blocked by dl-TBOA, whereas the initial peak current was unaffected by NBQX but blocked by MK-801. C, Current–voltage relationship for a typical astrocyte (voltage steps of 20 mV from −100 to +60 mV; holding potential, −80 mV). Amplitudes were normalized to the value measured at 0 mV (Inorm). Each point represents the mean ± SD for seven cells. D, Selective inhibition of peak and tail current by MK-801 and dl-TBOA. MK-801 (10 μm) and dl-TBOA (100 μm) were applied separately to different astrocytes as illustrated in A and B. The peak (Ipeak) and tail (Itail) of the current were averaged within the 20 ms time window as indicated in the bottom panel of B; error bars are mean ± SEM. The amplitudes were normalized to the values of Ipeak, Itail, and Ipeak − Itail before drug application. E, Voltage dependence of synaptic currents in astrocytes. Note the reversal close to 0 mV and the almost linear relationship between current and voltage. Currents were recorded in the presence of NBQX (30 mm) and dl-TBOA (100 mm). Im, Amplitude of membrane current.