Spontaneous acetylcholine release potentiation induced by 2-arachidonoylglycerol and anandamide in mouse motor synapses

We compared the changes in miniature endplate potential (MEPP) parameters of the mouse diaphragm caused by exogenous application of two classical endocannabinoids – 2-arachidonoylglycerol (2-AG) (1 μM) and anandamide (AEA) (30 μM). 2-AG caused a slowly developing stable increase in MEPP amplitude by 50%, without affecting the frequency of the MEPPs. This effect was prevented by AM-251, an inverse agonist of CB1-receptors, as well as by vesamicol, a blocker of the vesicular ACh transporter. On the contrary, AEA did not cause significant changes in the MEPP amplitude, but induced a slowly developing (within 2 hours) increase in MEPP frequency by on average 75%. The effect of AEA was prevented by AM-251 (1 μM) as well as by blocking of L-type Ca²⁺-channels with nitrendipine (1 μM) and inhibition of PKA activity by H89 (1 μM). We concluded that both 2-AG and AEA are able to exert a noncanonical facilitating presynaptic effect on spontaneous ACh release. Even though these endocannabinoids activate the same type of CB-receptors, their facilitating effects do not overlap and are strictly aimed either at potentiating the size of ACh quanta (in case of 2-AG) or increasing the frequency of MEPP (in case of AEA). We assume that different intracellular targets and signaling pathways may be involved in the differentiated facilitating effects of 2-AG and AEA in mouse neuromuscular junctions.

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