Multiple mechanisms of endocannabinoid response initiation in hippocampus

Endocannabinoids (eCBs) act as retrograde messengers at inhibitory synapses of the hippocampal CA1 region. Current models place eCB synthesis in the postsynaptic pyramidal cell and the site of eCB action at cannabinoid receptors located on presynaptic interneuron terminals. Four responses at the CA1-interneuron synapse are attributed to eCBs: depolarization-induced suppression of inhibition (DSI), G-protein-coupled receptor-mediated enhancement of DSI (DeltaDSI), persistent suppression of evoked inhibitory postsynaptic currents (eIPSCs), and finally, mGluR-dependent long-term depression (iLTD). It has been proposed that all are mediated by the eCB, 2-arachidonoyl glycerol, yet there is evidence that DSI does not arise from the same underlying biochemical processes as the other responses. In view of the increasing importance of eCB effects in the brain, it will be essential to understand the mechanisms by which eCB effects are produced. Our results reveal new differences in the biochemical pathways by which the eCB-dependent responses are initiated. Both U73122, a phospholipase C antagonist, and RHC-80267, a diacylglycerol (DAG) lipase antagonist, prevented eCB-dependent iLTD induction by 3,5-dihydroxyphenylglycine (DHPG). However, mAChR activation does not cause eCB-dependent iLTD. Neither enzyme inhibitor affects DSI, and persistent eCB-dependent eIPSC suppression induced by either mGluRs or mAChRs is unaffected by U73122. On the other hand, inhibition of DAG lipase prevents persistent eCB-dependent eIPSC suppression triggered by mAChRs. The results show that the biochemical pathways for the various eCB-dependent responses differ and might therefore be independently manipulated.