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

Abstract

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 Ca 2+ -sensitive and Ca 2+ -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 Ca 2+ -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 Ca 2+ . 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 transient receptor potential vanilloid type 1 ion channel-mediated excitatory effect that is not inhibited by concomitant activation of the cannabinoid type 1receptor. 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 Ca 2+ -insensitive anandamide-synthesising enzymes. Together, these findings indicate that anandamide synthesised by primary sensory neuron via a Ca 2+ -insensitive manner has an excitatory rather than an inhibitory role in primary sensory neurons and that excitation is mediated predominantly through autocrine signalling.

Alterations in the Anandamide Metabolism in the Development of Neuropathic Pain

“…It was reported that tissues from NAPE-PLD knockout mice exhibited enzymatic activity converting NAPE to AEA in a calcium-independent manner [41], suggesting the involvement of parallel biosynthetic pathways and supporting the theory that NAPE-PLD only makes a partial contribution to the biosynthesis of AEA [42]. Although some data showed the expression of enzymes of alternative pathways in neuronal tissues [43], comparison of expression profiles between control and neuropathic pain animals has never been performed. The present investigation has supplemented these observations by showing the upregulation of AEA synthetic enzymes in parallel pathways in a rat model of neuropathic pain (Figure 5).…”

Anandamide in primary sensory neurons: too much of a good thing?

“…() have reported that PKA and/or PKC activation also induces anandamide synthesis in cultured PSNs (Vellani et al ., ). In agreement with these findings, we have found that both the Ca 2+ ‐sensitive putative anandamide‐synthesising enzyme N ‐acylphosphatidylethanolamine phospholipase D as well as at least four of the five Ca 2+ ‐insensitive enzymes, which have been implicated in anandamide synthesis and have known molecular identity, are expressed in subpopulations of PSNs (Nagy et al ., ; Valente et al ., ; Varga et al ., , ). We have also shown that all the putative anandamide‐synthesising enzymes we found to be expressed in PSNs exhibit various levels of co‐expression with TRPV1 (Nagy et al ., ; Valente et al ., ; Varga et al ., , ), and that application of the common substrate of all the putative anandamide‐synthesising enzymatic pathways results in a Ca 2+ ‐insensitive increase in the anandamide levels of the superfusate and cells (Varga et al ., ).…”

“…However, it appears that this autocrine signalling, at least in the majority of cases, may produce signal amplification rather than signal attenuation because anandamide synthesised by PSN, either in a Ca 2+ -sensitive or Ca 2+ -insensitive manner, activates TRPV1 (van der Stelt et al, 2005;Varga et al, 2013). Importantly the TRPV1-mediated excitatory effects by anandamide synthesised in a Ca 2+ -insensitive manner are not accompanied by a concomitant CB1 receptor-mediated inhibitory effect (Varga et al, 2013). In agreement with the excitatory effect of anandamide synthesised in PSNs either in a Ca 2+ -sensitive or Ca 2+ -insensitive manner, the FAAH inhibitor URB597 also induces TRPV1-mediated cationic influx into cultured PSNs (Lever et al, 2009).…”

Peripheral inflammation affects modulation of nociceptive synaptic transmission in the spinal cord induced by N‐arachidonoylphosphatidylethanolamine

“…Several endogenous agents including N‐arachidonoylethanolamine (anandamide) activate both TRPV1 channels and CB 1 receptors (Devane et al, ; Zygmunt et al, ; Tognetto et al, ; Ahluwalia et al, ). Importantly, sub‐populations of DRG neurons as well as spinal cord cells are able to synthesize or degrade anandamide (Carrier et al, ; van der Stelt et al, ; Vellani et al, ; Varga et al, ). Anandamide synthesis occurs through many metabolic pathways either in a Ca 2+ ‐insensitive or Ca 2+ ‐sensitive manner (Ueda et al, ).…”

“…N‐arachidonoylphosphatidylethanolamine (20:4‐NAPE) constitutes the precursor for anandamide synthesis in all pathways (Wang and Ueda, ; Snider et al, ; Ueda et al, ). We have shown recently that application of 20:4‐NAPE to cultured DRG neurons results in anandamide production in a concentration and temperature‐dependent manner (Varga et al, ).…”