A study of the role of spinal prostaglandins and nitric oxide early after nerve injury

Allodynia is an abnormal state in which pain is triggered by innocuous sensory stimuli. Previous work in our laboratory has shown that L5/L6 spinal nerve ligation (SNL) induces robust mechanical allodynia in the rat which consists of two distinct phases: (a) an early spinal prostaglandin (PG)-dependent phase lasting approximately seven to ten days; and (b) a delayed PG-independent phase lasting at least seventy days. Importantly, the former is a prerequisite to and a trigger for the more complex and time-dependent changes underlying long-term, irreversible allodynia. Interfering with critical signaling events early after nerve injury is a logical strategy by which chronic neuropathic pain might be prevented. Such an approach requires a clear understanding of the sequence, time-course and pharmacology of these early signals. -- In light of the apparent pathogenic role of spinal PG early after nerve injury, the present research investigated the SNL-induced changes in spinal PG synthesis and signaling, their relevance to the development of spinal hyperexcitability and allodynia, and the mechanisms underlying the changes in PG synthesis and signaling in the spinal PG-dependent phase. The specific objectives were: -- (1) To characterize the effect of SNL on A- and C-fiber mediated reflex responses (AFRR and CFRR, respectively) in the affected hind limb, and to determine the temporal and spatial relationship of these changes to SNL-induced allodynia. -- (2) To determine if the SNL-induced changes in the AFRR and CFRR (i.e. spinal hyperexcitability) are spinal PG-dependent, and if so, to determine the relevant cyclooxygenase (COX) isoform(s). -- (3) To determine if SNL triggers the activation of nuclear factor kappa-B (NFκB) in the affected spinal cord which initiate the delayed induction of spinal COX-2. -- (4) To determine if the sensitivity to PG E type (PGE₂) is exaggerated during spinal PG-dependent allodynia, and if this abnormal state is limited to the affected spinal cord three days after SNL. -- (5) To determine if PG-dependent spinal hyperexcitability and mechanical allodynia are mediated by E-type prostaglandin (EP) receptors, and to investigate the changes in the expression of spinal EP₁₋₃ receptor subtypes and the glycine-α3 receptor subunit (GLY-α3R) three days after SNL. -- (6) To determine if disrupting spinal PG synthesis/signaling immediately after SNL (i.e. pre-emptive treatment) prevents the development of PG-dependent spinal hyperexcitability and mechanical allodynia. -- (7) To determine if spinal PG-dependent hyperexcitability is affected by the SNL-induced generation of spinal nitric oxide (NO), and if so, to investigate relevant NO synthase (NOS) isoform(s). -- 8. To determine if, and how, spinal NO-mediated activity affects spinal PG-dependent spinal hyperexcitability and mechanical allodynia. Male Sprague-Dawley rats were initially fitted with intrathecal (i.t.) catheters for drug delivery near the dorsal L1-L3 segments. Three days later, animals were anesthetized with halothane and the left L4 and L5 spinal nerves separated. In the SNL group, the L5 and L6 spinal nerves were tightly ligated with 6-0 silk thread. In sham-controls, the L5 and L6 spinal nerves were isolated but not ligated. Allodynia, defined as a paw withdrawal threshold (PWT) of ≤4g, was confirmed using von Frey filaments. The decrease in PWT from baseline (>15g) was evident in the ipsilateral hind paw 24h after SNL, and remained stable for at least twenty days. It was also accompanied by the protective posturing (i.e. elevation above the cage floor and cupping) of the affected hind paw. Allodynic animals were otherwise healthy and exhibited normal behaviour and weight gain. -- [Please see thesis for remainder of abstract.]

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