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

Abstract

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 CB 1 receptors, suppresses neurotransmitter release, and is degraded mainly by monoacylglycerol lipase (MGL). In the basolateral amygdala complex, it has been demonstrated that CB 1 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 DGLα, CB 1 , 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, DGLα was preferentially recruited to concave somatic membrane of postsynaptic pyramidal neurons, whereas invaginating presynaptic terminals highly expressed CB 1 , MGL, and CCK. No such molecular convergence was seen for flat perisomatic synapses made by parvalbumin-positive interneurons. On the other hand, DGLα and CB 1 were expressed weakly at axospinous excitatory synapses. Consistent with these morphological data, thresholds for DGLα-mediated depolarizationinduced 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-AGmediated control of the excitability of BA pyramidal neurons.retrograde signaling | metabotropic glutamate receptor | muscarinic acetylcholine receptor | depolarization-induced suppression of inhibition | depolarization-induced suppression of excitation M arijuana and other derivatives of the plant Cannabis sativa exert diverse neuropsychopharmacological actions, such as hallucination, euphoria, sedation, memory impairment, anxiolytic effect, appetite stimulation, motor deficit, and pain relief. These actions are mediated by Δ9-tetrahydrocannabinol (Δ9-THC), an active component of cannabis, through its binding to cannabinoid CB 1 receptors (1). CB 1 is particularly abundant in specific types of neurons, is selective to presynaptic elements, and induces shortand long-term forms of retrograde suppression of neurotransmitter release (1). In the brain, 2-arachidonoylglycerol (2-AG) is the major endogenous cannabinoid, synthesized in postsynaptic neurons by sn-1-specific diacylglycerol lipase (DGL) (2), and degraded mainly by monoacylglycerol lipase (MGL) (3, 4). 2-AG synthesis is facilitated when depolarization of postsynaptic neurons is combined with G q/11 -coupled receptor activation (1). Selective postsynaptic expression...

Enhanced Endocannabinoid-Mediated Modulation of Rostromedial Tegmental Nucleus Drive onto Dopamine Neurons in Sardinian Alcohol-Preferring Rats

“…Remarkably, the observation that 2-AG signal is enhanced in either time and/or space at these inhibitory afferents impinging upon VTA DA neurons in sP rats is consistent with studies reporting that both short-and longterm exposure to alcohol increases endocannabinoid levels in many brain regions (Basavarajappa et al, 2000;Vinod et al, 2006;Rubio et al, 2007) including the midbrain (González et al, 2004). Similar to other reports (Pan et al, 2009;Yoshida et al, 2011), 2-AG degradation by MAGL determined the strength of retrograde synaptic depression. Particularly, pharmacological blockade of MAGL resulted in a similar DSI expressed by sNP and sP rat DA cells.…”

Synapse type-independent degradation of the endocannabinoid 2-arachidonoylglycerol after retrograde synaptic suppression

“…1). The specificity of MGL antibodies has been verified by blank labeling in global MGL-KO brains in our previous studies (22,35). By double immunofluorescence for MGL and carbonic anhydrase 8 (Car8), a marker of PCs (38), punctate MGL labeling was prominent in the neuropil of the molecular layer (ML), but was absent in Car8-labeled PC dendrites and somata (Fig.…”

“…Thus 2-AG, which is released from PCs and causes retrograde suppression of PF-, CF-, and SC-PC synapses, is degraded in a synapse-type independent manner by MGL present in the cytoplasm of PF terminals and the BG. The 2-AG that acts as a synaptic retrograde messenger is produced exclusively by DGLα, (7,8) which is essentially targeted to dendritic spines (18)(19)(20)(21)(22). In PCs, DGLα is expressed densely at the base of the spine neck and sparsely on the somatodendritic membrane, but is excluded from the main body of the spine neck and head (21).…”

“…However, CB 1 receptor expression is generally higher at inhibitory synapses than at excitatory synapses in various brain regions (18,24). Thus, specificity and efficiency of 2-AG-mediated retrograde synaptic suppression are thought to depend not only on the expression level of CB 1 receptors at presynaptic terminals but also on the distance from the postsynaptic 2-AG production sites to the CB 1 receptors on presynaptic terminals (18)(19)(20)(21)(22).Retrograde signaling mediated by 2-AG is generally determined by the balance between the production and clearance of 2-AG. About 85% of 2-AG is reported to be degraded by a serine hydrolase, monoacylglycerol lipase (MGL) (25).…”

“…Previous studies have clarified detailed subcellular localizations of 2-AG-producing molecules, including group I metabotropic glutamate receptors (mGluRs) (15), G q/11 (16), phospholipase Cβs (PLCβs) (17), and DGLα (18)(19)(20)(21)(22). These molecules are essentially targeted to dendritic spines on which glutamatergic excitatory synapses are formed, suggesting that 2-AG is efficiently produced by excitatory synaptic activity (23).…”