Currents evoked in Bergmann glial cells by parallel fibre stimulation in rat cerebellar slices

Abstract

1 Whole‐cell recordings were obtained from Bergmann glial cells in rat cerebellar slices. 2 The cells had low input resistances (70 ± 38 mV; n= 13) and a mean resting potential of −82 ± 6 mV (n= 12) with a potassium‐based internal solution. Electrical and dye coupling between Bergmann glia were confirmed. 3 Stimulation of parallel fibres induced a complex, mostly inward current which could be decomposed pharmacologically. 4 The ionotropic glutamate receptor antagonist, 6‐cyano‐7‐nitroqumoxalme‐2,3‐dione (CNQX; 10 μM), but not DL‐2‐amino‐5‐phosphonopentanoic acid (DL‐APV; 100μM) consistently blocked an early inward current component that may reflect synaptic activation of AMPA/kainate receptors in Bergmann glia. 5 Addition of cadmium ions (100 μM) to inhibit transmitter release blocked most of the CNQX‐APV‐insensitive current. This component probably reflects electrogenic uptake of the synaptically released glutamate. 6 Tetrodotoxin (TTX; 1 μM) blocked the remaining inward current: a slow component, possibly produced by the potassium ion efflux during action potential propagation in parallel fibres. An initial triphasic component of the response was also TTX sensitive and reflected passage of the parallel fibre action potential volley. 7 The putative glutamate uptake current was further characterized; it was blocked by the competitive uptake blockers D‐aspartate (0.5 mM) and L‐trans‐pyrrolidine‐2,4‐dicarboxylic acid (PDC; 0.5 mM), and by replacement of sodium with lithium. Monitoring the triphasic TTX‐sensitive component showed that this inhibition did not result from changes of action potential excitation and propagation. 8 Intracellular nitrate ions increased the putative uptake current, consistent with the effect of this anion on glutamate transporters. 9 The putative uptake current was reduced by depolarization, consistent with the voltage dependence of glutamate uptake. 10 It is concluded that a large fraction of the current induced by parallel fibre stimulation reflects the uptake of synaptically released glutamate. The uptake current activated rapidly, with a 20–80% rise time of 2.3 ± 0.7 ms (n= 10), and decayed with a principal time constant of 25 ± 6 ms (n= 10).