Investigation of diffuse noxious inhibitory controls in the rat monosodium iodoacetate model of osteoarthritis pain

Osteoarthritis (OA) is the most common form of arthritis, with chronic pain being the most frequent and debilitating consequence. Current analgesics are inadequate in treating OA pain due to poor efficacy or serious side effects. Mechanistic studies in humans suggest dysfunction of descending inhibitory pathways in symptomatic knee OA is a key contributor in the development and maintenance of chronic pain. Such inhibitory pathways are recruited by diffuse noxious inhibitory controls (DNIC), whereby inhibition of spinal dorsal horn neuronal excitability to a ‘test’ stimulus can be induced by applying a noxious ‘conditioning’ stimulus to widespread areas of the body outside the peripheral excitatory receptive field (RF) of the neuron (heterotopic stimulation); hence DNIC can be used to study whether inhibition of spinal neural transmission has been compromised. In this respect spinally organised nociceptive withdrawal reflexes (NWRs) are functionally characterised population responses, modularly organised to most effectively move a limb away from a source of pain, with descending inhibitory pathways helping to set an appropriate activation threshold. The aim of this thesis was therefore to investigate whether descending inhibition was altered in an experimental model of OA pain by electrophysiologically measuring DNIC of reflexes in hindlimb muscles. In alfaxalone-anaesthetised rats, electromyograms (EMGs) to noxious stimulation of the heel or toes were recorded in the ankle extensor medial gastrocnemius (MG), the knee flexor biceps femoris (BF) and the ankle flexor tibialis anterior (TA) of the left hindlimb before and after a noxious ‘conditioning’ capsaicin injection into the contralateral hind or forelimb. In naïve animals, depending on the reflex, responses were inhibited by capsaicin (50 µg, 500 µg or 5 mg) in a dose-related manner to a median of between 14 – 76% of controls for 29 to 63 min. For all reflexes this inhibition was abolished by spinalization and reduced by decerebration for heel-MG and toes-BF when capsaicin was applied to the contralateral forelimb. For the same reflexes and conditioning site, reduced alfaxalone levels led to significantly greater inhibition. Reflexes were inhibited to a similar level irrespective of whether electrical (up to 10 mA, 8 x 2 ms pulses at 1 Hz) or graded mechanical (10 – 300 g von Frey (vF) monofilaments) stimuli were used although the latter appeared to offer a better means by which to detect changes in DNIC as both A- and C-fibre responses were measured along with the range of stimulus intensities. Subsequent experiments investigated DNIC of reflex responses in a model of OA pain induced by knee joint injection of monosodium iodoacetate (MIA). At 28-35 days post-injection, MIA-injected animals had severe cartilage damage and established pain behaviour. However, DNIC of reflex responses was to a similar degree for all three reflexes after injection of capsaicin into the contralateral hind or forelimb. In contrast, 3-4 days post-injection, MIA animals had very little to no cartilage damage accompanied by pain behaviour. Although DNIC of reflex responses was similar following injection of capsaicin into the contralateral forelimb, a reduced efficacy in DNIC inhibition was found after hindlimb injection. These data show that stimulus-evoked inhibition (DNIC) of spinal reflexes originates from regions caudal to the colliculi (likely within the pons and medulla) but higher brain areas may modify this inhibition. Similar to other general anaesthetics, alfaxalone levels affect the degree of DNIC. A reduction in DNIC efficacy is present in early stages of OA, but there was no evidence of compromised DNIC in established OA. Mechanical test stimuli provided a more sensitive method for detecting changes in DNIC whilst findings were also dependent on choice of conditioning site. Similarly, differential sensitivities to DNIC highlights the importance of studying multiple reflex responses in mechanistic studies of chronic pain