Changes in nitric oxide synthase isoforms in the spinal cord of rat following induction of chronic arthritis

Nitric oxide (NO) possibly plays an important role in the events resulting in hyperalgesia. Nitric oxide synthase (NOS) is a key enzyme in the production of NO. In this study, the relationship between NOS and hyperalgesia in a rat chronic arthritis model was tested. Chronic arthritis was induced by injection of incomplete Freund's adjuvant into the knee joint cavity unilaterally. The paw withdrawal latency (PWL) to radiant heat was used to detect secondary thermal hyperalgesia induced by the arthritis. After 1 day the PWL of the arthritic hind-paw decreased and it reached its nadir at 3 days after induction of arthritis. The lumbar and cervical enlargement of the spinal cord were removed in different groups of animals 3, 7, 14, or 21 days after induction of arthritis, and frozen tissue sections were cut. Two series of sections were incubated with polyclonal antibodies to neuronal NOS (nNOS) or to inducible NOS (iNOS). nNOS was found to increase gradually in laminae I-III in the lumbar but not in the cervical enlargement. The change became most obvious 14 days after induction of arthritis as compared to the control animals. Ependymal cells around the central canal of the lumbar enlargement were more densely stained by anti-iNOS after arthritis. A corresponding change was also found in the cervical enlargement. Computer-assisted image analysis revealed that the mean density of the affected areas in the treated group increased significantly compared with the control animals. This study suggests that the expression of both nNOS and iNOS increase following induction of chronic arthritis, which in turn would presumably lead to an increase in the production of NO. This process could be involved in mediation of the secondary thermal hyperalgesia induced by chronic arthritis.