Endocannabinoid modulation of nociceptive processing

Norris, Leonie (2010) Endocannabinoid modulation of nociceptive processing. PhD thesis, University of Nottingham.

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The analgesic effects of cannabis-based medicines are widely described and elevation of the endocannabinoids (ECLs), by inhibition of endocannabinoid metabolism, also produces analgesia. Measurement of endocannabinoids (ECLs) and metabolites, in low weight biological tissue, provides an important tool for investigating the potential therapeutic effects of this receptor system.

The main aims of this thesis were to develop an analytical method to measure ECLs, and cyclooxgenase-2 (COX-2) metabolites of anandamide (AEA) and 2-arachidonoylglycerol (2-AG), and to apply the method to determine their role in nociceptive processing in models of acute and chronic inflammatory pain.

A liquid chromatography-tandem mass spectrometry (LC-MS/MS) method, which allowed for the simultaneous measurement of AEA, oleoylethanolamide (OEA), palmitoylethanolamine (PEA) and 2-AG, as well as COX-2 metabolites of AEA and 2-AG was developed and validated. This method was demonstrated to be able to quantify ECLs and related compounds in rat hindpaw, brain, spinal cord and cell samples.

The effects of TRPV1 activation on levels of intracellular calcium [Ca2+] and ECLs in sensory nerves were investigated. The TRPV1 agonist, capsaicin, dose dependently increased [Ca2+], which was blocked by the TRPV1 antagonist iodoresiniferatoxin (IRTX). Although synthesis of ECLs is activity and calcium dependent, there was no relationship between the TRPV1-mediated increases in [Ca2+], and levels of intracellular or extracellular ECLs. The mechanism of sensitization of primary afferent nociceptors by the proinflammatory cytokine tumor necrosis factor-α (TNFα), which is heavily implicated in inflammatory responses, was also studied. TNFα facilitated capsaicin-evoked calcium responses, an effect which was mediated by p38MAPK-induced phosphorylation.

The ability of inhibition of fatty acid amide hydrolase (FAAH) to modulate inflammatory pain responses, and the sites at which ECLs were elevated, was studied in the carrageenan model of inflammatory pain. This model was considered ideal for the investigation of the role of alternative metabolism of ECLs by COX-2 as inflammatory pain is associated with an up-regulation of COX-2. The effects of acute and repeated administration of URB597 (0.3 mgkg-1), a selective inhibitor of FAAH metabolism of ECLs, on carrageenan-evoked changes in weight bearing and hindpaw oedema were determined. Behavioural effects were considered in parallel with the changes in levels of ECLs and COX-2 metabolites of AEA and 2-AG in the hindpaw and spinal cord. Acute administration of URB597 delayed the onset and attenuated carrageenan-induced hyperalgesia, an effect which was associated with increased levels of AEA, OEA and PEA in the spinal cord. By contrast, carrageenan-induced hyperalgesia was not altered by repeated administration of URB597 and levels of AEA in the spinal cord were not significantly increased by this treatment. There was no evidence for the metabolism of ECLs via COX-2 in the hindpaw, or spinal cord, of carrageenan-treated rats, under basal conditions or following inhibition of FAAH.

The role of the endocannabinoid system in a model of osteoarthritis (OA) pain was also investigated. Intra-articluar injection of the chondrocyte inhibitor monosodium iodoacetate (MIA, 1 mg/50µl) produced histological changes reminiscent of joint damage in OA patients, and a time dependant decrease in paw withdrawal thresholds and weight bearing on the injured (ipsilateral) side. These behavioural pain responses were accompanied with an increase in ECLs in the spinal cord, suggestive of a role of ECLs in modulation of OA-induced pain.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Barrett, D.A.
Chapman, V.
Kendall, D.A.
Keywords: Nociceptors, Nociceptive processing, Endocannabinoids, Cyclooxgenase-2, Metabolites of anandamide, 2-arachidonoylglycerol, Pain, Analgesia
Subjects: QS-QZ Preclinical sciences (NLM Classification) > QV Pharmacology
Faculties/Schools: UK Campuses > Faculty of Medicine and Health Sciences > School of Biomedical Sciences
Item ID: 28701
Depositing User: Blore, Mrs Kathryn
Date Deposited: 21 Apr 2015 07:57
Last Modified: 26 Oct 2016 13:42
URI: http://eprints.nottingham.ac.uk/id/eprint/28701

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