Intrathecal administration of A1 receptor agonist GR242468x, P38 inhibitor SB239063A, and μ opiate receptor agonist morphine, but not COX-2 inhibitor rofecoxib, reverse the FCA induced hypersensitivity to pain

L.A. Winyard, N.M. Clayton, A.W. Wilson, A. Billinton, I..P. Chessell, Neurology CEDD, GlaxoSmithkline, New Frontiers Science Park, Harlow, Essex, CM19 5AW, UK.

Introduction: A1 receptor agonists, µ opiate agonists, COX-2 inhibitors and P38 inhibitors, when dosed peripherally, have all been shown to reverse the hypersensitivity to pain produced by intraplantar (i.pl.) administration of Freund’s Complete Adjuvant (FCA) into the paw (Clayton et al., 2003, 2004).

Aim of investigation: To investigate the central effects of the A1 receptor agonist GR242468X (5'-deoxy-5'-fluoro-N- (tetrahydro-pyran-4-yl)-adenosine), P38 inhibitor SB239063A (trans-1-(4-hydroxycyclohexyl)-4-(4-fluorophenyl)-5-[(2-methoxy)pyrimidin-4-yl]imidazole), µ opiate agonist, morphine, and the COX-2 inhibitor, rofecoxib, in the established FCA model of hypersensitivity to pain following intrathecal (i.t.) administration.

Methods: Male Random Hooded Rats (180-200g) received 100µl of 1mg/ml FCA by i.pl. injection into the left hind paw. Animals were dosed intrathecally with GR242468X (0.01, 0.1 or 1µg), morphine (0.2, 0.6, 2µg), rofecoxib (2, 6, 20µg) or SB239063A (3, 6, 20μg) 23 hours later (n=7 per dose group). All animals were anesthetised by isoflourane inhalation and using a 50μl gas tight Hamilton syringe with a 25 gauge needle, dosing (10μl) was performed by acute percutaneous lumbar puncture (Hylden et al., 1980). Animals were allowed to fully recover before behavioural assessments were carried out. Behavioural readings were recorded as amount of weight bearing on the inflamed left hind paw, using a Dual Channel Weight Averager, before FCA administration (naïve readings), 23 hours after FCA (pre-dose readings) and 30 minutes after i.t. dosing (post-dose readings) (Clayton et al., 1997). Results were calculated as % reversal of hypersensitivity. Data are presented as mean ± s.e.m. Statistical analysis was carried out to determine whether there was a significant difference (p<0.05) between the vehicle treated and drug treated groups using ANOVA and post-hoc Fischer LSD test.

Results: The A1 agonist GR242468X produced a significant dose related reversal of the FCA induced hypersensitivity, maximal at 83.0+/-14.2% at 1μg (p<0.05). Morphine significantly reversed the hypersensitivity in a dose dependant manner, maximal at 71.0+/-21.1% (p<0.05) at 2μg i.t. The P38 inhibitor SB239063A displayed a partial reversal when dosed intrathecally, maximal at 49.8+/-6.8% at 20μg i.t. (p<0.05) and the COX-2 inhibitor, rofecoxib, failed to have any significant effect on the hypersensitivity at 2, 6, or 20μg (p>0.05) when administered i.t.

Conclusions: The A1 receptor agonist, GR242468X, and morphine both produced a full reversal of the hypersensitivity to pain after intrathecal administration. However the P38 inhibitor and COX-2 inhibitor produced only partial and non-significant effects, respectively. Given that all of these agents are able to produce full reversal when administered systemically, these data suggest a significant central role for both A1 receptor agonists and morphine, and a predominantly peripheral site of action for P38 inhibitors and COX-2 inhibitors.

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