Depolarization-induced suppression of inhibition (DSI) is a form of short-term plasticity of GABAergic synaptic transmission that is found in cerebellar Purkinje cells and hippocampal CA1 pyramidal cells. DSI involves the release of a calcium-dependent retrograde messenger by the somatodendritic compartment of the postsynaptic cell. Both glutamate and endogenous cannabinoids have been proposed as retrograde messenger. Here we show that, in cerebellar parasagittal slices, type 1 cannabinoid receptors (CB1Rs) are expressed at high levels in axons of GABAergic interneurons and in presynaptic terminals onto Purkinje cells. Application of the cannabinoid antagonist AM-251 (500 nm) leads to the abolition of the DSI of evoked currents (eIPSCs) recorded in paired recordings and to a strong reduction of the DSI of TTX-insensitive miniature events (mIPSCs) recorded from Purkinje cells. Furthermore, the CB1R agonist WIN 55-212,2 (5 microm) induces a presynaptic inhibition of synaptic currents similar to that occurring during DSI, as well as an occlusion of DSI after stimulation of Purkinje cells. Moreover, WIN 55-212,2 reduces the calcium transients evoked in presumed presynaptic varicosities by short trains of action potentials. Our results indicate that DSI is mediated by the activation of presynaptic CB1Rs and that an endogenous cannabinoid is a likely candidate retrograde messenger in this preparation. They further suggest that DSI involves distinct presynaptic modifications for eIPSCs and mIPSCs, including an inhibition of action potential-evoked calcium rises.

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The orphan GluD2 receptor belongs to the ionotropic glutamate receptor family but does not bind glutamate. Ligand-gated GluD2 currents have never been evidenced, and whether GluD2 operates as an ion channel has been a long-standing question. Here, we show that GluD2 gating is triggered by type 1 metabotropic glutamate receptors, both in a heterologous expression system and in Purkinje cells. Thus, GluD2 is not only an adhesion molecule at synapses but also works as a channel. This gating mechanism reveals new properties of glutamate receptors that emerge from their interaction and opens unexpected perspectives regarding synaptic transmission and plasticity.