Chronic pain is associated with N-methyl-D-aspartate (NMDA) receptor activation and downstream production of nitric oxide, which has a pivotal role in multisynaptic local circuit nociceptive processing in the spinal cord. The formation of nitric oxide is catalyzed by three major nitric oxide synthase (NOS) isoforms (neuronal, nNOS; inducible, iNOS; endothelial, eNOS), which are increased in the spinal cord of rodents subjected to some tonic and chronic forms of experimental pain. Despite the important role of NOS in spinal cord nociceptive transmission, there have been no studies exploring the effect of NMDA receptor blockade on NOS expression in the dorsal horn during chronic pain. Furthermore, NOS isoforms have not been fully characterized in the dorsal horn of animals subjected to arthritic pain. The aim of this work was therefore to study the expression of nNOS, iNOS and eNOS in the dorsal horns of monoarthritic rats, and the modifications in NOS expression induced by pharmacological blockade of spinal cord NMDA receptors. Monoarthritis was produced by intra-articular injection of complete Freund's adjuvant into the right tibio-tarsal joint. At week 4, monoarthritic rats were given either the competitive NMDA antagonist (±)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) or the uncompetitive NMDA antagonist ketamine. After 6 and 24 hours, animals were killed and posterior quadrants of the lumbar spinal cord were dissected. Sample tissues were homogenized and subjected to immunoblotting with anti-nNOS, anti-iNOS or anti-eNOS monoclonal antibodies. The nNOS isoform, but not the iNOS and eNOS isoforms, were detected in the dorsal horns of control rats. Monoarthritis increased the expression of nNOS, iNOS and eNOS in the dorsal horns ipsilateral and contralateral to the inflamed hindpaw. Intrathecal administration of CPP and ketamine reduced nNOS expression in monoarthritic rats but increased the expression of iNOS and eNOS. Results suggest that blockade of spinal cord NMDA receptors produces complex regulatory changes in the expression of NOS isoforms in monoarthritic rats that may be relevant for nitridergic neuronal/ glial mechanisms involved in the pathophysiology of monoarthritis and in the pharmacological response to drugs interacting with NMDA receptors.

We investigated the antinociceptive properties of dexketoprofen trometamol (S(+)-ketoprofen tromethamine salt; SKP), a new analgesic, antiinflammatory drug, using the pain-induced functional impairment model in the rat (PIFIR), an animal model of arthritic pain. SKP was compared with racemic ketoprofen tromethamine salt (rac-KP), R(-)-ketoprofen tromethamine salt (RKP), ketorolac (KET), and morphine (MOR). We also assessed the effects of flurbiprofen (rac-FB) and its enantiomers (SFB and RFB) in the same model. Groups of six rats received either vehicle or analgesic drug and antinociception was evaluated by evaluating the dose-response curves over time. SKP was an effective antinociceptive drug in this model and was almost equally potent by either oral or intracerebroventricular administration. The oral potency of SKP was similar to that of oral KET and greater than that of oral MOR. No significant differences were observed between racemic ketoprofen and its enantiomers when administered orally. In the rat, significant bioinversion of RKP to SKP occurs when RKP is given orally. After oral administration of RKP, SKP was detectable in 30 min and surpassed the concentration of RKP after 3 h. Nevertheless, when the compounds were given intracerebroventricularly, some stereoselectivity in favor of SKP was observed. Stereoselectivity was observed with flurbiprofen, an analogue of ketoprofen that does not undergo significant metabolic inversion. Whereas SFB was an effective antinociceptive, RFB had no antinociceptive effect at the doses tested when given either orally or intracerebroventricularly.

An appropriate animal model for investigation of antiinflammatory analgesic effects of NSAIDs is the pain-induced functional impairment model in the rat (PIFIR model). The PIFIR model provides a model of inflammatory and chronic pain similar to that of clinical gout. We investigated the possible role of peripheral prostaglandins synthesized by COX-1 and COX-2 in arthritic pain produced by uric acid in the rat PIFIR model. For this purpose, the antinociceptive effects of indomethacin, a nonselective COX-1/COX-2 inhibitor, and those of SC-560 and MK-966 (Vioxx), selective inhibitors of COX-1 and COX-2, respectively, on the functional impairment induced by intraarticular (i.a.) injection of uric acid into the knee joint of the right hindlimb were compared. The antinociceptive efficacy was determined from the area under the curve (AUC) values obtained from the time course of the antinociceptive effect. Animals received vehicle, 100 µg indomethacin, 100 µg SC-560, or 50 µg of MK-966 (these drug doses were selected from dose-response curves of each drug in previous experiments) 20 min before uric acid and the ability of the rat to use the injured hindlimb was recorded with a computerized system. The effect produced by each treatment was compared with that observed after the compounds were administered in the left hindlimb, i.e., contralateral administration. The results showed that all three compounds significantly inhibited uric acid-induced dysfunction of the right hindlimb (control = 64 ± 8 au; indomethacin = 391 ± 30 au; MK-966 = 368 ± 17 au; SC-560 = 445 ± 25 au, ANOVA, Dunnett's test, P < 0.01). In addition, the results using contralateral administration suggest that the analgesic effect was due to inhibition of locally produced COX-1 and COX-2 and, therefore, that no central mechanisms were involved. Taken together, the present data using the pain-induced functional impairment model in rats support the idea that both COX isoforms (1 and 2) contribute to the local inflammatory response in the model and that they may have a role in the maintenance of physiological homeostasis. They might also suggest that the therapeutic benefits of NSAIDs are mainly due to inhibition of both COX isoforms. Drug Dev. Res. 51:253-259, 2000.