Climbing fiber-evoked Purkinje cell discharge reduces expression of GABA_A receptor-associated protein and decreases its interaction with GABA_A receptors

Sustained neuronal activity induces synaptic remodeling, in part, by altering gene expression. We have used a major climbing fiber pathway onto cerebellar Purkinje cells to investigate the effects of sustained climbing fiber-evoked glutamatergic synaptic transmission on transcription, expression and phosphorylation of proteins related to the regulation of inhibitory GABA_A receptor function. Binocular horizontal optokinetic stimulation was used to modulate climbing fiber signals to Purkinje cells in the flocculus and nodulus of rabbits and mice. Purkinje cells in the flocculus and nodulus ipsilateral to the eye stimulated in the Posterior → Anterior direction received increased climbing fiber activity. Purkinje cells in flocculus and nodulus ipsilateral to the eye stimulated in the Anterior → Posterior direction received decreased climbing fiber activity. We identified changes in levels of gene transcripts in floccular and nodular Purkinje cells with the technique of differential display RT-PCR. Increased climbing fiber input reduced transcript levels and expression of GABA receptor-associated protein (GABARAP). Using a protein 'pull down' technique, we showed that GABARAP interacts with serine phosphorylated GABA_Aγ2, gephyrin and β-tubulin. Serine de-phosphorylation of GABA_Aγ2 reduced association of GABARAP with GABA_Aγ2. Climbing fiber activity did not influence the expression of GABA_Aγ2. Rather, it decreased its serine phosphorylation. Climbing fiber discharge decreased both expression of GABARAP and serine phosphorylation of GABA_Aγ2. Consequently, climbing fiber activity may reduce the surface expression of GABA_A receptors in Purkinje cells rendering Purkinje cells less susceptible to interneuronal GABAergic inhibition.