Postsynaptic GABAB receptor signalling enhances LTD in mouse cerebellar Purkinje cells

Long-term depression (LTD) of excitatory transmission at cerebellar parallel fibre-Purkinje cell synapses is a form of synaptic plasticity crucial for cerebellar motor learning. Around the postsynaptic membrane of these synapses, B-type gamma-aminobutyric acid receptor (GABA(B)R), a G(i/o) protein-coupled receptor for the inhibitory transmitter GABA is concentrated and closely associated with type-1 metabotropic glutamate receptors (mGluR1) whose signalling is a key factor for inducing LTD. We found that in cultured Purkinje cells, GABA(B)R activation enhanced LTD of a glutamate-evoked current (LTDglu), increasing the magnitude of depression. It has been reported that parallel fibre-Purkinje cell synapses receive a micromolar level of GABA spilt over from the synaptic terminals of the neighbouring GABAergic interneurons. This level of GABA was able to enhance LTDglu. Our pharmacological analyses revealed that the beta gamma subunits but not the alpha subunit of G(i/o) protein mediated GABA(B)R-mediated LTDglu enhancement. G(i/o) protein activation was sufficient to enhance LTDglu. In this respect, LTDglu enhancement is clearly distinguished from the previously reported GABA(B)R-mediated augmentation of an mGluR1-coupled slow excitatory postsynaptic potential. Baclofen application for only the induction period of LTDglu was sufficient to enhance LTDglu, suggesting that GABA(B)R signalling may modulate mechanisms underlying LTDglu induction. Baclofen augmented mGluR1-coupled Ca2+ release from the intracellular stores in a G(i/o) protein-dependent manner. Therefore, GABA(B)R-mediated LTDglu enhancement is likely to result from augmentation of mGluR1 signalling. Furthermore, pharmacological inhibition of GABA(B)R reduced the magnitude of LTD at parallel fibre-Purkinje cell synapses in cerebellar slices. These findings demonstrate a novel mechanism that would facilitate cerebellar motor learning.