The patch-clamp technique was used to study whole cell currents evoked by γ-aminobutyric acid (GABA) in rat cerebellar granule cells cultured in 5 mM potassium, a condition that favors the development of functional GABAergic synapses. GABA activated both high- and low-sensitivity receptors. The high- sensitivity receptor had an effective concentration producing half the maximum response (EC50) of 13 μM, whereas the low-sensitivity one had an EC50 of 255 μM. The GABA(A) receptor agonist isoguvacine activated only the high-sensitivity receptor with an EC50 of 16 μM. When GABA was applied during the desensitized phase of the response elicited by a saturating concentration of isoguvacine, it was still able to induce a small response, whereas when isoguvacine was applied during the desensitizing phase of GABA- evoked current no response was detected. GABA responses were highly heterogeneous regarding their sensitivity to bicuculline. In a small number of cells (3 of 25), bicuculline (10 μM) completely abolished GABA-evoked currents. In the majority of the neurons (22 of 25) the blocking effect of bicuculline (100 μM) was 64 ± 4% (mean ± SE). The bicuculline-resistant component was abolished by picrotoxin (100 μM). In bicuculline, the dose- response curve for GABA was fitted with a sigmoidal curve with an EC50 value of 209 μM. These data indicate that functional new GABA receptor types with unusual pharmacology could be switched on by conditions that maintain cells in their undifferentiated state.
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