Anandamide produced by Ca2+-insensitive enzymes induces excitation in primary sensory neurons

The endogenous lipid agent N-arachidonoylethanolamine (anandamide), among other effects, has been shown to be involved in nociceptive processing both in the central and peripheral nervous systems. Anandamide is thought to be synthesised by several enzymatic pathways both in a Ca²⁺-sensitive and Ca²⁺-insensitive manner, and rat primary sensory neurons produce anandamide. Here, we show for the first time, that cultured rat primary sensory neurons express at least four of the five known Ca²⁺-insensitive enzymes implicated in the synthesis of anandamide, and that application of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-arachidonoyl, the common substrate of the anandamide-synthesising pathways, results in anandamide production which is not changed by the removal of extracellular Ca²⁺. We also show that anandamide, which has been synthesised in primary sensory neurons following the application of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-arachidonoyl induces a transient receptor potential vanilloid type 1 ion channel-mediated excitatory effect that is not inhibited by concomitant activation of the cannabinoid type 1 receptor. Finally, we show that sub-populations of transient receptor potential vanilloid type 1 ion channel-expressing primary sensory neurons also express some of the putative Ca²⁺-insensitive anandamide-synthesising enzymes. Together, these findings indicate that anandamide synthesised by primary sensory neuron via a Ca²⁺-insensitive manner has an excitatory rather than an inhibitory role in primary sensory neurons and that excitation is mediated predominantly through autocrine signalling. Regulation of the activity of the Ca²⁺-insensitive anandamide-synthesising enzymes in these neurons may be capable of regulating the activity of these cells, with potential relevance to controlling nociceptive processing.