INHIBITORY SYNAPTIC CURRENTS IN STELLATE CELLS OF RAT CEREBELLAR SLICES

1. In thin cerebellar slices of rats aged 14-21 days, voltage-gated currents, synaptic currents and GABA responses were studied with the tight-seal whole-cell recording technique from stellate cells (8-9 mum soma diameter) located in the outer two-thirds of the molecular layer. 2. In symmetrical Cl- conditions, stellate cells voltage-clamped at -60 mV showed spontaneous inhibitory postsynaptic currents (IPSCs). As were the GABA(A) responses of the same cells, the IPSCs were blocked by bicuculline. The frequency of occurrence of IPSCs ranged from 0.2 to 1.9 events per second (21 cells). The mean amplitude of the events ranged from 61 to 226 pA (mean +/- S.E.M.: 132 +/-11; n = 21). 3. The temporal course of IPSCs was characterized by a rapid rise (mean + S. E. M. of the time to peak: 1.1 +/- 0.1 ms, n = 7) and a slow decay. The decay phase was described by a double exponential function with time constants of 8.7 +/- 0-6 ms, and 40.9 + 3.7 ms respectively (means + S.E.M.; n = 7). 4. A minor fraction (15 to 20%) of the spontaneous synaptic events recorded in control saline had a faster onset than that of the IPSCs and decayed with a rapid mono-exponential decay (time constant of 1.0-1.3 ms). These were excitatory postsynaptic currents (EPSCs) unaffected by bicuculline and blocked by the glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). 5. Bath application of TTX (0.5-1 mum), which blocked voltage-gated Na+ currents in stellate cells, induced a variable decrease in the frequency of IPSCs (mean + S.E.M.; of the frequency ratio in TTX over control: 0.47 +/- 0.09; n = 12). However, the toxin had no significant effect either on the mean amplitude or on the kinetics of the 1PSCs. The mean amplitude of the miniature IPSCs was 141 +/- 13pA (mean +/- S.E.M.; n = 22). 6. In TTX-containing solutions, the frequency of the IPSCs was unaffected when Ca'' currents were eliminated either by removal of extracellular Ca2+ and addition of EGTA, or by addition of Cd2+. Miniature IPSCs of 200-300 pA were still observed. 7. In symmetrical Cl- conditions, local application of GABA to stellate cells induced an inward current and an increase in membrane noise. Responses to prolonged applications of GABA showed desensitization in both whole-cell mode and somatic outside-out patches. The chord conductance estimated from recording single GABA channel events in somatic outside-out patches was 28 pS. 8. The present results suggest that in stellate cells the release of a single quantum of transmitter activates a large number of postsynaptic GABA(A) receptor-gated channels.