Abstract
Phosphodiesterase 4 (PDE4) is an adenosine cyclic 3,5-monophosphate-specific degradative enzyme, which is closely associated with the inflammatory response. Among its four subtypes (A-D), it remains unclear which one exerts suppressive effects on inflammation and reduces neuropathic pain. The present study aimed to examine the modulation of neuroinflammation by PDE4 subtypes in the spinal cord of a rat model of L5 spinal nerve ligation (SNL)-induced neuropathic pain. The expression levels of PDE4A-D were measured in the lumbar spinal cords of naïve rats. The rats were then divided into seven groups: The sham group (sham surgery + saline), the saline group (SNL + saline), the vehicle group (SNL + Lipofectamine® RNAiMAX), the mismatch small interfering (si)RNA group (SNL + mismatch siRNA), the PDE4A-siRNA group (SNL + PDE4A-siRNA), the PDE4B-siRNA group (SNL + PDE4B-siRNA) and the PDE4D-siRNA group (SNL + PDE4D-siRNA). In order to determine behavioral changes, mechanical withdrawal threshold (MWT) and thermal withdrawal latency (TWL) were recorded. The mRNA and protein expression levels of PDE4s were also detected. Furthermore, the association between behavioral changes and individual subtypes of PDE4 were studied following intrathecal administration of PDE4A, B and D-specific siRNA. The expression levels of protein kinases, including phosphorylated-extracellular signal-regulated kinases (p-ERK), and inflammatory cytokines were measured, in order to explore the underlying mechanisms. Subtypes A, B and D, but not C, were detected in the naïve rats. After SNL, both MWT and TWL were reduced. The mRNA and protein expression levels of PDE4A, B and D were significantly upregulated after 2, 4, 6 and 8 days of SNL. Subtype-specific siRNA significantly suppressed the elevated expression levels; however, only rats treated with PDE4B siRNA exhibited improved MWT and TWL. Further analysis of the PDE4B siRNA-treated rats demonstrated that 8 days after SNL, the intensity of p-ERK was reduced, the expression levels of CD11b and glial fibrillary acidic protein GFAP were reduced, as well as the expression levels of proinflammatory cytokines such as tumor necrosis factor-α, interleukin (IL)-1β and IL-6. These results suggested that selective inhibition of PDE4B may relieve neuropathic pain, potentially via the suppression of glial activation and the release of cytokines in the spinal cord.