Cannabinoid CB1 receptor-dependent long-term depression in autaptic excitatory neurons

Long-term depression (LTD) of synaptic signaling-lasting from tens of minutes to hours or longer-is a widespread form of synaptic plasticity in the brain. Neurons express diverse forms of LTD, including autaptic LTD (autLTD) observed in cultured hippocampal neurons, the mechanism of which remains unknown. We have recently reported that autaptic neurons express both endocannabinoid-mediated depolarization-induced suppression of excitation (DSE) and metabotropic suppression of excitation (MSE). We now report that activating cannabinoid CB(1) receptors is necessary for the induction of autLTD. Most surprisingly, CB(1) does not induce autLTD via the G(i/o) proteins typically activated by this receptor nor with G(s). Rather, the requirements of presynaptic phospholipase C and filled calcium stores suggest G(q). In autLTD, a 3- to 4-min activation of the receptor by the endocannabinoid 2-arachidonoyl glycerol leads to prolonged inhibition while leaving short-term inhibition (e.g., DSE) intact. autLTD requires activation of both metabo- and ionotropic glutamate receptors. autLTD also requires MEK/ERK activation. Under certain conditions, one or more DSE stimuli will elicit autLTD. It is becoming evident that cannabinoids mediate multiple forms of plasticity at a single synapse, stretching temporally from tens of seconds (DSE/MSE) to tens of minutes (autLTD) to hours (CB(1) desensitization). Our findings imply a remarkable flexibility for the cannabinoid signaling system whereby discrete mechanisms of CB(1) activation within a single neuron yield temporally and mechanistically distinct forms of plasticity.