Unique inhibitory synapse with particularly rich endocannabinoid signaling machinery on pyramidal neurons in basal amygdaloid nucleus

2-Arachidonoylglycerol (2-AG) is the endocannabinoid that mediates retrograde suppression of synaptic transmission in the brain. 2-AG is synthesized in activated postsynaptic neurons by sn-1-specific diacylglycerol lipase (DGL), binds to presynaptic cannabinoid CB1 receptors, suppresses neurotransmitter release, and is degraded mainly by monoacylglycerol lipase (MGL). In the basolateral amygdala complex, it has been demonstrated that CB1 is particularly enriched in axon terminals of cholecystokinin (CCK)-positive GABAergic interneurons, induces short-and long-term depression at inhibitory synapses, and is involved in extinction of fear memory. Here, we clarified a unique molecular convergence of DGL alpha, CB1, and MGL at specific inhibitory synapses in the basal nucleus (BA), but not lateral nucleus, of the basolateral amygdala. The synapses, termed invaginating synapses, consisted of conventional symmetrical contact and unique perisynaptic invagination of nerve terminals into perikarya. At invaginating synapses, DGL alpha was preferentially recruited to concave somatic membrane of postsynaptic pyramidal neurons, whereas invaginating presynaptic terminals highly expressed CB1, MGL, and CCK. No such molecular convergence was seen for flat perisomatic synapses made by parvalbumin-positive interneurons. On the other hand, DGL alpha and CB1 were expressed weakly at axospinous excitatory synapses. Consistent with these morphological data, thresholds for DGL alpha-mediated depolarization-induced retrograde suppression were much lower for inhibitory synapses than for excitatory synapses in BA pyramidal neurons. Moreover, depolarization-induced suppression was readily saturated for inhibition, but never for excitation. These findings suggest that perisomatic inhibition by invaginating synapses is a key target of 2-AG-mediated control of the excitability of BA pyramidal neurons.