Endogenous cannabinoids (endocannabinoids) mediate retrograde signals at various regions of the brain. Endocannabinoids are produced on demand in activity-dependent manners and released from postsynaptic neurons. The released endocannabinoids travel backward across the synapse, activate presynaptic CB1 cannabinoid receptors, and cause transient or long-lasting suppression of neurotransmitter release. Endocannabinoid release can be triggered by membrane depolarization that elevates intracellular calcium concentration ([Ca²⁺]_i) to a micromolar range or by activation of the G_q/11 protein-coupled receptors such as group I metabotropic glutamate receptors (mGluRs) and M₁/M₃ muscarinic acetylcholine receptors. Furthermore, coincidence of [Ca²⁺]_i elevation and G_q/11 protein-coupled receptor activation cooperatively induces endocannabinoid release. G_q/11 protein-coupled receptors activate phospholipase Cβ (PLCβ) leading to diacylglycerol (DAG) production. DAG is then broken down to the major endocannabinoid 2-arachidonoylglycerol (2-AG) by the action of DAG lipase. To determine the role of PLCβ in endocannabinoid release, we used cultured hippocampal neurons and monitored the endocannabinoid release by measuring cannabinoid-sensitive synaptic currents (Reference 1). The PLCβ family consists of 4 isoforms (PLCβ1-4) of which PLCβ1 is the predominant isoform in the forebrain including hippocampus. We found that the receptor-driven endocannabinoid release was absent in mutant mice lacking PLCβ1. This PLCβ1-mediated endocannabinoid release was dependent on physiological levels of [Ca²⁺]_i. We measured PLCβ1 activity in intact neurons by using exogenous TRPC6 channel as a biosensor for the PLC product DAG. The TRPC6 channel is a member of canonical transient receptor potential family and is activated by intracellular DAG. In hippocampal neurons expressing exogenous TRPC6 channels, large inward currents were induced by application of a membrane-permeable DAG analogue or by activation of M₁/M₃ muscarinic acetylcholine receptors or group I mGluRs. These currents were negligible in control neurons, indicating that exogenous TRPC6 channels mediate these currents. In hippocampal neurons from PLCβ1-knockout mice expressing exogenous TRPC6 channels, activation of G_q/11 protein-coupled receptors caused negligible inward currents. The receptor-driven TRPC6-mediated currents showed a similar [Ca²⁺]_i dependence to that of the receptor-driven endocannabinoid release. These results indicate that PLCβ1 serves as a coincidence detector for triggering endocannabinoid release in the hippocampus. We then examined the roles of PLCβ in endocannabinoid release triggered by synaptic activity. For this purpose, we made whole-cell recordings from Purkinje cells in mouse cerebellar slices and examined their excitatory synapses arising from climbing fibers and parallel fibers (Reference 2). We sampled Purkinje cells from the rostral half of the cerebellum where PLCβ4 is the predominant isoform in Purkinje cells. We first characterized three distinct modes to induce endocannabinoid release by analyzing climbing fiber to Purkinje cell synapses. The first mode is strong activation of mGluR subtype 1 (mGluR1) – PLCβ4 cascade without detectable Ca²⁺ elevation. The second mode is Ca²⁺ elevation to a micromolar range without activation of mGluR1 – PLCβ4 cascade. The third mode is Ca²⁺-assisted mGluR1 – PLCβ4 cascade that requires weak mGluR1 activation and Ca²⁺ elevation to a sub-micromolar range. By analyzing parallel fiber to Purkinje cell synapses, we found that the third mode is essential for effective endocannabinoid release from Purkinje cells by excitatory synaptic activity. Furthermore, we demonstrated by biochemical analysis that combined weak mGluR1 activation and mild depolarization in Purkinje cells effectively produces 2-AG, whereas either stimulus alone does not produce detectable 2-AG. Our results strongly suggest that under physiological conditions, excitatory synaptic inputs to Purkinje cells activate the Ca²⁺-assisted mGluR1-PLCβ4 cascade, and thereby produces 2-AG that retrogradely modulates synaptic transmission to Purkinje cells. Our results obtained from hippocampal neurons and cerebellar Purkinje cells indicate that PLCβ functions as a neuronal coincidence detector through its Ca²⁺ dependency and may play important roles in various synaptic modulations and plasticity.