Inhibition of constitutive inward rectifier currents in cerebellar granule cells by pharmacological and synaptic activation of GABAB receptors

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Abstract
Gamma-Aminobutyric acid (GABA)(B) receptors are known to enhance activation of Kir3 channels generating G-protein-dependent inward rectifier K(+)-currents (GIRK). In some neurons, GABA(B) receptors either cause a tonic GIRK activation or generate a late K(+)-dependent inhibitory postsynaptic current component. However, other neurons express Kir2 channels, which generate a constitutive inward rectifier K(+)-current (CIRK) without requiring G-protein activation. The functional coupling of CIRK with GABA(B) receptors remained unexplored so far. About 50% of rat cerebellar granule cells in the internal granular layer of P19-26 rats showed a sizeable CIRK current. Here, we have investigated CIRK current regulation by GABA(B) receptors in cerebellar granule cells, which undergo GABAergic inhibition through Golgi cells. By using patch-clamp recording techniques and single-cell reverse transcriptase-polymerase chain reaction in acute cerebellar slices, we show that granule cells co-express Kir2 channels and GABA(B) receptors. CIRK current biophysical properties were compatible with Kir2 but not Kir3 channels, and could be inhibited by the GABA(B) receptor agonist baclofen. The action of baclofen was prevented by the GABA(B) receptor blocker CGP35348, involved a pertussis toxin-insensitive G-protein-mediated pathway, and required protein phosphatases inhibited by okadaic acid. GABA(B) receptor-dependent CIRK current inhibition could also be induced by repetitive GABAergic transmission at frequencies higher than the basal autorhythmic discharge of Golgi cells. These results suggest therefore that GABA(B) receptors can exert an inhibitory control over CIRK currents mediated by Kir2 channels. CIRK inhibition was associated with an increased input resistance around rest and caused a approximately 5 mV membrane depolarization. The pro-excitatory action of these effects at an inhibitory synapse may have an homeostatic role re-establishing granule cell readiness under conditions of strong inhibition.