Effects of the GPR55 agonist L-α-lysophosphatidylinositol on mechanosensitive knee joint afferents

Miriam Mohamed1, John Harris1, David A Kendall2, Sara Kelly1. 1School of Biosciences, University of Nottingham, Nottingham, United Kingdom, 2School of Biomedical Sciences, University of Nottingham, Nottingham, United Kingdom.

The endocannabinoid (EC) system has a role in the modulation of pain. Recent evidence suggests that the orphan receptor, GPR55 is part of the EC system and is activated by cannabinoid ligands and the bioactive lipid L-α-lysophosphatidylinositol (LPI) (Ross 2009). Initial studies indicate that GPR55 has a pro-nociceptive role in vivo (Staton et al., 2006). The aim of this work was to examine the role of GPR55, as a pro-nociceptive influence on knee joint primary afferent neurones.

Male wistar rats (250-350g) (n=26) were deeply anaesthetized with sodium pentobarbital (60mg/kg, i.p.) and the external jugular vein and trachea cannulated. Extracellular recordings were made from fine filaments of the saphenous nerve. Receptive fields (RFs) overlying the knee joint were stimulated with von Frey monofilaments (0.4-15g, 5s each / 5mins). Once stable control mechanically-evoked responses were obtained, 100μl of LPI (150, 250μM) (n=15) or vehicle (0.7% dH2O, 99.3% saline) (n=11) was injected peripherally (femoral artery) towards the knee, and effects followed for 60mins at 5 min intervals. Conduction velocities were estimated following RF electrical stimulation (range = 0.25-38ms-1; A- and C-fibres).

Vehicle had non-significant effects on mechanically-evoked responses of knee joint afferents. In contrast, LPI had dose related inhibitory effects on mechanically-evoked responses of knee joint afferents (p<0.05, Kruskall-Wallis). Maximal inhibitory effects of LPI (250μM) were observed 37 ± 3 minutes post injection (mean ± s.e.m). Control responses of knee joint afferents studied in LPI and vehicle experiments were not statistically different (p>0.05, Mann Whitney).

A genetic knock out study suggests that GPR55 may mediate some of the pro-nociceptive effects of cannabinoids (Staton et al., 2008). However, in our pharmacological study we saw an inhibition of mechanically-evoked responses of A- and C-fibre knee joint primary afferent neurones. It is possible that in the present study, LPI is leading to a desensitization of GPR55 or an increase in EC synthesis and release, via an increase in intracellular calcium (Ross, 2009). Further studies are required to clarify the role of GPR55 in sensory neurones.

Ross, R. 2009. Trends in Pharmacol. Sci. 30, 156-63.
Staton, P.C., 2008. Pain 139, 225-236.

MM is supported by a BBSRC studentship.

All procedures accord with current UK legislation.