Anandamide and NADA bi-directionally modulate presynaptic Ca2+ levels and transmitter release in the hippocampus

Background and purpose: Inhibitory CB1 cannabinoid receptors and excitatory TRPV1 vanilloid receptors are abundant in the hippocampus. We tested if two known hybrid endocannabinoid/endovanilloid substances, N-arachidonoyl-dopamine ( NADA) and anandamide (AEA), presynapticaly increased or decreased intracellular calcium level ([Ca2+](i)) and GABA and glutamate release in the hippocampus. Experimental approach: Resting and K+-evoked levels of [Ca2+](i) and the release of [H-3] GABA and [H-3] glutamate were measured in rat hippocampal nerve terminals. Key results: NADA and AEA per se triggered a rise of [Ca2+](i) and the release of both transmitters in a concentration- and external Ca2+-dependent fashion, but independently of TRPV1, CB1, CB2, or dopamine receptors, arachidonate-regulated Ca2+-currents, intracellular Ca2+ stores, and fatty acid metabolism. AEA was recently reported to block TASK- 3 potassium channels thereby depolarizing membranes. Common inhibitors of TASK- 3, Zn2+, Ruthenium Red, and low pH mimicked the excitatory effects of AEA and NADA, suggesting that their effects on [Ca2+](i) and transmitter levels may be attributable to membrane depolarization upon TASK- 3 blockade. The K+-evoked Ca2+ entry and Ca2+- dependent transmitter release were inhibited by nanomolar concentrations of the CB1 receptor agonist WIN55212-2; this action was sensitive to the selective CB1 receptor antagonist AM251. However, in the low micromolar range, WIN55212-2, NADA and AEA inhibited the K+-evoked Ca2+ entry and transmitter release independently of CB1 receptors, possibly through direct Ca2+ channel blockade. Conclusions and implications: We report here for hybrid endocannabinoid/endovanilloid ligands novel dual functions which were qualitatively similar to activation of CB1 or TRPV1 receptors, but were mediated through interactions with different targets.