Figure 4From: Nitric oxide is negatively correlated to pain during acute inflammation Tissue injury increases NO production possibly via eNOS or nNOS in the early phase following surgery. This causes a small increase in NO level that lead to an analgesic effect. However, in the late phase iNOS isoform is up-regulated, producing a marked increase in NO level that would have a hyperalgesic effect. cNOS, constitutive NOS.Back to article page