Reviewer #2 (Public review): Increased reluctant vesicles underlie synaptic depression by GPR55 in axon terminals of cerebellar Purkinje cells

This is an important study reporting that activation of the presynaptic GPR55 receptor suppresses synaptic transmission by modulating GABA release through the reduction of the readily releasable pool without affecting the presynaptic AP waveform and calcium influx.The evidence supporting this claim is compelling and based on an impressive array of techniques including patch-clamp recordings from the axon terminals of cerebellar Purkinje cells and fluorescent imaging of vesicular exocytosis.While the authors have strengthened their conclusions on several technical fronts in the revised version, further investigation is needed into the mechanism by which GPR55 activation might make vesicles insensitive to the rise in presynaptic [Ca²⁺] mediated by VGCCs, and the nature of the endogenous process that would activate this pathway in vivo.