Peripheral inflammation induces long-term changes in tyrosine hydroxylase activation in the substantia nigra

Inflammation plays a role in neuropathology. We hypothesised that inflammation, induced by a single intraperitoneal injection of lipopolysaccharide (LPS), would induce long-term changes in the regulation of tyrosine hydroxylase (TH) in the rat midbrain. The level of 12 cytokines was initially analysed from one day to six months after LPS injection to confirm that peripheral inflammation led to neuroinflammatory changes in the midbrain. In the substantia nigra (SN), the levels of 8 of the 12 measured cytokines was significantly increased at one day. Granulocyte-macrophage colony-stimulating factor showed a threefold increased level at 6 months. The ventral tegmental area (VTA) showed a completely different pattern, with no increases in the levels of the 12 cytokines at one day and no changes beyond one week. TH activity was determined using a tritiated water release assay, TH protein and phosphorylation levels (Ser19, Ser31 and Ser40) were determined using western blotting. TH-specific activity in the SN was unchanged at one day but was substantially increased at one week and one month with no concomitant increase in TH phosphorylation. Substantial changes in TH activation without changes in TH phosphorylation have not previously been observed in the brain in response to a range of stressors. TH-specific activity was increased in the SN, and in the caudate putamen, at 6 months and was associated with increased TH phosphorylation at Ser19 and Ser40 at both locations. TH-specific activity in the VTA showed only a transient increase at day one associated with increased phosphorylation at Ser19 and Ser31 but thereafter showed no changes. This study shows that inflammation induced by LPS generated two distinct long-term changes in TH activity in the SN that are caused by different mechanisms, but there were no long-term changes in the adjacent VTA.