Clonidine reduces hypersensitivity and alters the balance of pro- and anti-inflammatory leukocytes after local injection at the site of inflammatory neuritis

Perineural α2-adrenoceptor activation relieves hypersensitivity induced by peripheral nerve injury or sciatic inflammatory neuritis. This effect is associated with a reduction in pro-inflammatory cytokines, as well as a reduction in local leukocyte number and their capacity to produce pro-inflammatory cytokines. Curiously, clonidine’s antinociceptive effect appears with a 2–3-day delay after injection. Previous observations have shown that α-adrenoceptor activation induces apoptosis in leukocytes, which would reduce leukocyte number. Additionally, macrophage scavenging of apoptotic cells results in a shift to an anti-inflammatory phenotype, with expression of transforming growth factor (TGF)-β1. We therefore examined the effects of perineural clonidine 24 h and 3 days after its injection on apoptosis, TGF-β1 expression and lymphocyte and macrophage phenotype in acute sciatic inflammatory neuritis. Perineural clonidine reduced ipsilateral neuritis-induced hypersensitivity in a delayed manner (3 days after treatment), along with a reduction at this time in lymphocyte number and an increase in caspase-3 and TGF-β1 expressing cells and macrophages co-expressing TGF-β1 in the sciatic nerve. One day after injection clonidine treatment was associated with a reduction in lymphocytes and pro-inflammatory Th-1 cells as well as increased numbers of caspase-3 and TGF-β1 expressing cells and macrophages co-expressing TGF-β1 in sciatic nerve. Clonidine’s effects were prevented by co-administration of an α2-adrenoceptor antagonist. These data suggest that α2-adrenoceptor activation in sciatic inflammatory neuritis increases local apoptosis and anti-inflammatory products early after treatment. This early effect likely underlies the delayed anti-inflammatory and anti-hypersensitivity effects of perineural clonidine in this setting.