Anandamide and NADA bi-directionally modulate presynaptic Ca 2+ levels and transmitter release in the hippocampus

Abstract

Background and purpose: Inhibitory CB 1 cannabinoid receptors and excitatory TRPV 1 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 ([Ca 2+] i) and GABA and glutamate release in the hippocampus. Experimental approach: Resting and K +-evoked levels of [Ca 2+] i and the release of [ 3H]GABA and [ 3H]glutamate were measured in rat hippocampal nerve terminals. Key results: NADA and AEA per se triggered a rise of [Ca 2+] i and the release of both transmitters in a concentration- and external Ca 2+-dependent fashion, but independently of TRPV 1, CB 1, CB 2, or dopamine receptors, arachidonate-regulated Ca 2+-currents, intracellular Ca 2+ stores, and fatty acid metabolism. AEA was recently reported to block TASK-3 potassium channels thereby depolarizing membranes. Common inhibitors of TASK-3, Zn 2+, Ruthenium Red, and low pH mimicked the excitatory effects of AEA and NADA, suggesting that their effects on [Ca 2+] i and transmitter levels may be attributable to membrane depolarization upon TASK-3 blockade. The K +-evoked Ca 2+ entry and Ca 2+-dependent transmitter release were inhibited by nanomolar concentrations of the CB 1 receptor agonist WIN55212-2; this action was sensitive to the selective CB 1 receptor antagonist AM251. However, in the low micromolar range, WIN55212-2, NADA and AEA inhibited the K +-evoked Ca 2+ entry and transmitter release independently of CB 1 receptors, possibly through direct Ca 2+ channel blockade. Conclusions and implications: We report here for hybrid endocannabinoid/endovanilloid ligands novel dual functions which were qualitatively similar to activation of CB 1 or TRPV 1 receptors, but were mediated through interactions with different targets. © 2007 Nature Publishing Group All rights reserved.