Endocannabinoid signalling triggered by NMDA receptor‐mediated calcium entry into rat hippocampal neurons

Endocannabinoids are released from neurons in activity‐dependent manners, act retrogradely on presynaptic CB1 cannabinoid receptors, and induce short‐term or long‐term suppression of transmitter release. The endocannabinoid release is triggered by postsynaptic activation of voltage‐gated Ca2+ channels and/or Gq‐coupled receptors such as group I metabotropic glutamate receptors (I‐mGluRs) and M1/M3 muscarinic receptors. However, the roles of NMDA receptors, which provide another pathway for Ca2+ entry into neurons, in endocannabinoid signalling have been poorly understood. In the present study, we investigated the possible contribution of NMDA receptors in endocannabinoid production by recording IPSCs in cultured hippocampal neurons. Under the conditions minimizing the activation of voltage‐gated Ca2+ channels, local application of NMDA (200 μm) transiently suppressed cannabinoid‐sensitive IPSCs, but not cannabinoid‐insensitive IPSCs. This NMDA‐induced suppression was abolished by blocking NMDA receptors, CB1 receptors and diacylglycerol lipase, but not by inhibiting voltage‐gated Ca2+ channels. When the postsynaptic neuron was dialysed with 30 mm BAPTA, the NMDA‐induced suppression was reduced significantly. A lower dose of NMDA (20 μm) exerted little effect when applied alone, but markedly enhanced the cannabinoid‐dependent suppression driven by muscarinic receptors or I‐mGluRs. These data clearly indicate that the activation of NMDA receptors facilitates the endocannabinoid release either alone or in concert with the Gq‐coupled receptors.