Attenuation of fear‐conditioned analgesia in rats by monoacylglycerol lipase inhibition in the anterior cingulate cortex: Potential role for CB₂ receptors

Abstract

Background and Purpose: Improved understanding of brain mechanisms regulating endogenous analgesia is important from a fundamental physiological perspective and for identification of novel therapeutic strategies for pain. The endocannabinoid system plays a key role in stress-induced analgesia, including fear-conditioned analgesia (FCA), a potent form of endogenous analgesia. Here, we studied the role of the endocannabinoid 2-arachidonoyl glycerol (2-AG) within the anterior cingulate cortex (ACC; a brain region implicated in the affective component of pain) in FCA in rats.Experimental Approach: FCA was modelled in male Lister-hooded rats by assessing formalin-evoked nociceptive behaviour in an arena previously paired with footshock.The effects of intra-ACC administration of MJN110 (inhibitor of monoacylglycerol lipase [MGL], the primary enzyme catabolizing 2-AG), AM630 (CB 2 receptor antagonist), AM251 (CB 1 receptor antagonist) or MJN110 + AM630 on FCA were assessed.Key Results: MJN110 attenuated FCA when microinjected into the ACC, an effect associated with increased levels of 2-AG in the ACC. This effect of MJN110 on FCA was unaltered by co-administration of AM251 but was blocked by AM630, which alone reduced nociceptive behaviour in non-fear-conditioned rats. RT-qPCR confirmed that mRNA encoding CB 1 and CB 2 receptors was detectable in the ACC of formalin-injected rats and unchanged in those expressing FCA. Conclusion and Implications:These results suggest that an MGL substrate in the ACC, likely 2-AG, modulates FCA and that within the ACC, 2-AG-CB 2 receptor signalling may suppress this form of endogenous analgesia. These results may facilitate increased understanding and improved treatment of pain-and fear-related disorders and their co-morbidity. analgesia is a potent form of endogenous analgesia and a useful model with which to study the neurobiological mechanisms underpinning endogenous pain suppression and fear-pain interactions.The endocannabinoid system consists of cannabinoid 1 (CB 1 ) and cannabinoid 2 (CB 2 ) receptors, their endogenous ligands, anandamide and 2-arachidonoylglycerol (2-AG), and their key catabolizing enzymes, fatty acid amide hydrolase and monoacylglycerol lipase (MGL). The endocannabinoid system has been shown to play a key role in the expression of fear-conditioned analgesia (Finn et al.

Sex Differences in a Rat Model of Peripheral Neuropathic Pain and Associated Levels of Endogenous Cannabinoid Ligands

“…Quantification of endocannabinoids (AEA, 2-AG) and related N -acylethanolamines (PEA, OEA) levels in brain and spinal cord tissue was carried out following a lipid extraction method described previously ( 40 , 41 ). Two hundred microlitres of 100% acetonitrile containing deuterated internal standard for endogenous cannabinoid ligands (2.5 ng d8-AEA, 50 ng d8-2-AG, 2.5 ng d4-PEA, 2.5ng d4-OEA; Cayman Chemicals, Biosciences, UK) and 75 μl of 100% pure acetonitrile were added to the samples.…”

Pathology of pain and its implications for therapeutic interventions

“…2-AG can bind to CB2 receptor in the ACC and promotes fear-conditioned analgesia. 576 Additionally, CB1 and CB2 receptors have overlaps in mechanisms, like interactions with Nav1.7 and Nav1.8 to inhibit neuronal activity. 577 , 578 However, the endocannabinoid system can activate GABAergic neurons in the mPFC.…”

Time-Course of Alterations in the Endocannabinoid System after Viral-Mediated Overexpression of α-Synuclein in the Rat Brain

“…Quantification of the concentration of endocannabinoids and related N-acylethanolamines was carried out in the NUIG Biological Mass Spectrometry Core Facility (National University of Ireland Galway, Galway, Ireland) according to a standardised protocol (unpublished). Samples and standards were prepared essentially as previously described [76,77]. Specifically, levels of AEA, 2-AG, PEA and OEA were analysed.…”