Rationale: CB1 localized in mitochondria (mtCB1) regulates mitochondrial respiration: using purified mitochondria from wild-type (WT) or CB1−/− brains it is shown that treatment with CB1 receptor agonists WIN (Figure 3) or THC (Figure 4a-c) reduces respiration, cAMP levels and PKA activity in neuronal mitochondria. Depolarization of CA1 postsynaptic pyramidal neurons mobilizes endocannabinoids, which retrogradely activate presynaptic CB1 receptors, transiently decreasing GABAergic inhibitory neurotransmission in a process known as depolarization-induced suppression of inhibition (DSI), an endocannabinoid-dependent form of short-term plasticity of inhibitory neurotransmission in the hippocampus. DSI is fully blocked by cell-permeant CB1 antagonists (AM251) but only partially blocked by cell-impermeant CB1 antagonists (hemopressin) (Figure 6A), indicating a contribution of mtCB1 activation in DSI. In addition, suppression similar to the one obtained by DSI is found only with cell-permeant agonists of CB1 (HU210) but not with cell-impermeant versions (biotinylated-HU210). Also, HU210 occludes DSI completely, whereas biotinylated-HU210 only occludes it partially, suggesting again a contribution of mtCB1 activation for DSI (Figure 6C). mtCB1 activation also results in reduced mitochondrial respiration and, additionally, direct inhibition of complex I activity using rotenone also mimics DSI, suggesting that endocannabinoid-induced DSI depends on mitochondrial respiration, which would be controlled by mtCB1 (Figure 7).. Experimental description: Female mice (2–4 months old) were used for anatomy and biochemistry. Mice 20–30 d old of both sexes were used for electrophysiology.