Cooperative endocannabinoid production by neuronal depolarization and group I metabotropic glutamate receptor activation

Endocannabinoids are retrograde messengers that are released from central neurons by depolarization-induced elevation of intracellular Ca2+ concentration [Ca2+ ](I) or by activation of a group I metabotropic glutamate receptor (mGluR). We studied the interaction between these two pathways for endocannabinoid production in rat hippocampal neurons. We made a paired whole-cell recording from cultured hippocampal neurons with inhibitory synaptic connections. Activation of group I mGluRs, mainly mGluR5, by the specific agonist (RS)-3,5-dihydroxyphenylglycine (DHPG), suppressed inhibitory postsynaptic currents (IPSCs) in about half of the neuron pairs. A cannabinoid agonist, WIN55,212-2, suppressed IPSCs in all DHPG-sensitive pairs but not in most of DHPG-insensitive pairs. The effects of both DHPG and WIN55,212-2 were abolished by the cannabinoid antagonists, AM281 and SR141716A, indicating that activation of group I mGluR releases endocannabinoids and suppress inhibitory neurotransmitter release through activation of presynaptic cannabinoid receptors. Depolarization of the postsynaptic neurons caused a transient suppression of IPSCs, a phemomenon termed depolarization-induced suppression of inhibition (DSI) that was also abolished by cannabinoid antagonists. Importantly, DSI was enhanced significantly when group I mGluRs were activated simultaneously by DHPG. This enhancement was much more prominent than expected from the simple summation of depolarization-induced and group I mGluR-induced endocannabinoid release. DHPG caused no change in depolarization-induced Ca2+ transients, indicating that the enhanced DSI by DHPG was not due to the augmentation of Ca2+ influx. Enhancement of DSI by DHPG was also observed in hippocampal slices. These results suggest that two pathways work in a cooperative manner to release endocannabinoids via a common intracellular cascade.