The modifiability of neuronal response plasticity is called ''metaplasticity.'' In suppressing synaptic inhibition and facilitating induction of long-term excitatory synaptic plasticity, endocannabinoids (eCBs) act as agents of metaplasticity. We now report the discovery of a calcium-dependent mechanism that regulates eCB mobilization by metabotropic glutamate receptor (mGluR) activation. The switch-like mechanism primes cells to release eCBs and requires a transient rise in intracellular Ca 2؉ concentration ([Ca 2؉ ]i) but not concurrent activation of mGluRs. Conversely, short-term, [Ca 2؉ ]i-dependent eCB release can be persistently enhanced by mGluR activation. Hence, eCBs are also objects of metaplasticity, subject to higher levels of physiological control.2-arachidonyl glycerol ͉ calcium ͉ GABA ͉ metabotropic glutamate ͉ muscarinic E ndocannabinoids (eCBs) are important signaling molecules that modulate synaptic strength throughout the CNS (for reviews, see refs. 1-4). They control both short-and long-term forms of synaptic plasticity and are implicated in many animal behaviors. eCBs, which are fatty acid derivatives, are synthesized directly by enzymatic action on cellular plasma membrane phospholipids. Because they are not stored before use, eCBs are said to be available ''on-demand.'' Although the first identified eCB was anandamide, 2-arachidonyl glycerol (2-AG) is probably the principal ligand for cannabinoid receptors (CB1Rs) in much of the brain (5, 6). 2-AG synthesis is thought to proceed via phospholipase C (PLC) and the action of diacylglycerol lipase (DGL) on the PLC product, diacyglycerol (DAG). eCBs are retrograde messengers, and their release from postsynaptic cells may also be regulated. Physiological techniques cannot distinguish among synthesis, release, and transport processes, and we use the term ''mobilization'' to encompass all steps between stimulation of the eCB system and activation of CB1R. eCB mobilization occurs after either a rise in the intracellular Ca 2ϩ concentration ([Ca 2ϩ ] i ) (7-9) or activation of G protein-coupled receptors (GPCRs), including dopaminergic (10, 11), metabotropic glutamatergic (12, 13), and muscarinic cholinergic (14) receptors.In the hippocampus, CB1R is present in high concentrations on the synaptic terminals of certain GABAergic interneurons (15). Ca 2ϩ -dependent eCB mobilization transiently suppresses CA1 pyramidal cell evoked inhibitory postsynaptic currents (eIPSCs) (7, 8), a retrograde signal process called DSI (1). Group I metabotropic glutamate receptor (mGluR) activation potently stimulates eCB mobilization, thereby depressing eIPSCs (12, 13) or excitatory postsynaptic currents (EPSCs) for short (seconds to minutes) or long (minutes to hours) (16, 17) periods of time, depending on the stimulation conditions.Various stimuli use different biochemical pathways for eCB mobilization, which can be designated eCB mGluR , eCB mAChR , and eCB Ca . Pharmacological inhibitors of PLC abolish hippocampal long-term IPSC suppression (eCB-iLTD) initiated by...