The role of nitric oxide and prostaglandin signaling pathways in spinal nociceptive processing in chronic inflammation

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Abstract
Both nitric oxide (NO) and prostaglandins (PG) and their associated enzymes nitric oxide synthases (NOS) and cyclooxygenases (COX) (specifically COX-2) have been implicated in the development of hyperalgesia. This study examined the effects of naturally occurring chronic inflammation, chronic mastitis, on spinal nociceptive processing in sheep and focused on potential alterations in spinal PG and NO signaling pathways. Mechanical withdrawal thresholds were significantly lower in animals suffering from chronic inflammation (n=6) compared to control animals (n=6). Hyperalgesia was restricted to the side contralateral to the inflammation (decrease from ipsilateral side: hindlimb 33.2±5%, forelimb 19.4±5%). Neuronal NOS-immunoreactivity was significantly reduced bilaterally in lumbar and cervical spinal cord throughout laminae I–III (decrease 18.4±5% and 16.9±4%, respectively) and in lamina X (decrease 29.1±6% and 17.1±4%, respectively) in mastitic animals relative to control animals. No difference was detected in eNOS or iNOS-immunoreactivity or in NADPH-diaphorase staining, a marker of dynamically active NOS. RT-PCR failed to detect any change in levels of nNOS, eNOS, iNOS, COX-1 or COX-2 mRNAs. However, a marked increase in the PGE receptor, EP3 (but not EP2) mRNA was detected in ipsilateral spinal cord tissue from animals with chronic inflammation. This increase in EP3 receptor expression indicates that spinal PGs are important in the spinal response to chronic peripheral inflammation. Contralateral mechanical hyperalgesia may not be directly linked to changes in spinal EP3 receptor mRNA expression, however, the bilateral changes in nNOS suggest that this pathway may contribute to the adaptive behavioural response observed.