Investigating changes in GABAergic signalling in the spinal cord in an experimental model of osteoarthritis pain

L. Sinnett-Smith^1,2, S. G. Woodhams^1,2, G. Hathway², V. Chapman^1,2. ¹Arthritis Research UK Pain Centre, University of Nottingham, Nottingham, United Kingdom, ²School of Life Sciences, University of Nottingham, Nottingham, United Kingdom.

Introduction: Osteoarthritis (OA) is a common cause of chronic joint pain. Previous work (1) shows decreased GABAergic inhibition in chronic nerve injury pain models; this has not been investigated in OA pain. This study explored the changes to GABAergic signalling at a spinal cord level in a rat OA pain model.

Methods: Procedures were carried out in accordance with ASPA. Male Sprague-Dawley rats (150-200g) were anesthetised (3% isoflurane, 1l/min O2) and received an intra-articular injection of monosodium iodoacetate (MIA) (1mg/50μL 0.9%w/v saline, N=16) or vehicle (50μL 0.9%w/v saline, N=16). A naïve group did not undergo anaesthesia (N=12). Weight-bearing asymmetry and paw withdrawal threshold (PWT) were assessed twice weekly post-injection until day 21, after which spinal single-unit electrophysiology was performed. Rats were anesthetised (1.5-3% isoflurane, 40% O2/60% NO₂) and a laminectomy performed, exposing lumbar 4 and 5 regions of the spinal cord. Recordings were made from wide-dynamic range (WDR) neurons. WDR neuron firing rate was measured in response to Von Frey hair stimulation of the toes at 8, 10, 15 and 26g (baseline) and after three intrathecal doses of GABA_A receptor agonist (muscimol: 1μg, 3μg and 10μg) or antagonist (bicuculline: 10μ g, 30μg, and 60μg) both in 50μL saline.

Results: By day 21 there was a significant decrease in weight borne on the ipsilateral hind paw in MIA rats (38±2%) compared to saline rats (50±0.6%) (P<0.0001 N=16 each MIA/saline) and decrease in ipsilateral PWT (percentage of baseline): 46±7.7% in MIA rats vs 91±10% in saline rats (P<0.0001, N=12 each MIA/saline). Muscimol produced a significant reduction in firing rate (P<0.0001, N=21, groups combined). Firing as percentage of baseline: 1μg=50.12±8%; 3μg=22.7±7.5%; 10μg=4.7±2.4%. There were no significant differences in the effects of spinal muscimol on evoked responses of spinal neurons between MIA (N=7), saline (N=8) or naïve (N=6) rats. In naïve rats (N=6) 10μ g bicuculline produced a significant increase in firing in response to 8g and 10g stimulation (P<0.01); 30μg produced a significant increase at 8g, 10g (P<0.01) and 15g (P<0.05).

Conclusions: Muscimol inhibited neuronal firing in all groups, indicating functional GABA_A receptors in the spinal cord in the OA pain model but suggesting no change in receptor expression and function. Bicuculline response suggests a greater role of GABA tone in controlling response to innocuous stimuli; this will be repeated in the MIA model.

References:

1. Zeilhofer H (2008) International Immunopharmacology 8: 182-187