Leech synapses show opposing 2-AG effects across the same concentration ranges

The endogenous cannabinoid (endocannabinoid) system produces both pro- and antinociceptive 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 antinociceptive neuromodulation. This study examines whether these opposing effects can be separated by concentration, i.e., low levels of endocannabinoids may selectively elicit antinociceptive effects without engaging pronociceptive effects. Using Hirudo verbana (the medicinal leech), the effects of 0.001-200 µM 2-arachidonoylglycerol (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.NEW & NOTEWORTHY Efforts to develop cannabinoid-based therapies to treat pain are complicated by the fact that the endocannabinoid system can produce both anti- and pronociceptive effects. The study examined whether these opposing effects can be separated by the concentration of the endocannabinoid, 2-AG. Surprisingly, it was found that 2-AG depressed nociceptive and potentiated non-nociceptive synapses over two distinct concentration ranges that were the same for both synapse types.