Concentration-Dependent Effects of the Endocannabinoid 2-Arachidonoylglycerol on Nociceptive and Non-Nociceptive Synapses

The endogenous cannabinoid (endocannabinoid) system produces both pro- and anti-nociceptive effects at the level of primary afferent synapses, depressing nociceptive synapses and enhancing non-nociceptive synapses. This complicates efforts to develop cannabinoid-based therapies to treat pain because it is not known how to separate pro- versus anti-nociceptive neuromodulation. This study examines if these opposing effects can be separated by concentration, i.e., low levels of endocannabinoids may selectively elicit anti-nociceptive effects without engaging pro-nociceptive effects. Using Hirudo verbana (the medicinal leech) the effects of 0.001 to 200 µM 2-arachydonoylglycerol (2-AG) were compared in synapses made by nociceptive (N) and non-nociceptive pressure-sensitive (P) afferents. 2-AG elicited depression of N synapses and potentiation of P synapses over approximately the same concentration ranges. Surprisingly, 2-AG produced a biphasic concentration profile in both synapse types, a low concentration range centered around 0.01 µM and a higher concentration range centered around 100 µM. In between was an intermediate range centered around 1 µM where no change in N or P synapses was observed. Low 2-AG concentration effects were blocked by the CB1 reverse agonist AM251. High 2-AG concentration effects were blocked by AM251 and the TRPV1 inhibitor SB366792. The effects of 2-AG were also examined using MJN110, a drug that inhibits the 2-AG-metabolizing enzyme, monoacylglycerol lipase (MAGL). Increasing endogenous 2-AG in this manner also led to potentiation of P synapses and depression of N synapses over the same concentrations of MJN110 and these effects were blocked by the CB1 receptor inhibitor, but not the TRPV1 blocker.