INHIBITORY SYNAPTIC CURRENTS IN STELLATE CELLS OF RAT CEREBELLAR SLICES

1. In thin cerebellar slices of rats aged 14-21 days, voltage-gated cu rrents, synaptic currents and GABA responses were studied with the tig ht-seal whole-cell recording technique from stellate cells (8-9 mum so ma 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 we re 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 rang ed from 61 to 226 pA (mean +/- S.E.M.: 132 +/-11; n = 21). 3. The temp oral 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 de cay phase was described by a double exponential function with time con stants 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 synapti c events recorded in control saline had a faster onset than that of th e IPSCs and decayed with a rapid mono-exponential decay (time constant of 1.0-1.3 ms). These were excitatory postsynaptic currents (EPSCs) u naffected by bicuculline and blocked by the glutamate receptor antagon ist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). 5. Bath application o f TTX (0.5-1 mum), which blocked voltage-gated Na+ currents in stellat e 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 C a'' currents were eliminated either by removal of extracellular Ca2+ a nd addition of EGTA, or by addition of Cd2+. Miniature IPSCs of 200-30 0 pA were still observed. 7. In symmetrical Cl- conditions, local appl ication of GABA to stellate cells induced an inward current and an inc rease in membrane noise. Responses to prolonged applications of GABA s howed desensitization in both whole-cell mode and somatic outside-out patches. The chord conductance estimated from recording single GABA ch annel 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) recep tor-gated channels.