Depletion of noradrenaline by DSP-4 alters the nigral inflammatory response following supranigral administration of lipopolysaccharide in the rat

Mahmoud Iravani, Mona Sadeghian, Sarah Salvage, Peter Jenner. King’s College London, London, United Kingdom.

In Parkinson’s disease (PD), the loss of noradrenergic neurons might precede the loss of dopaminergic cells (DA). This may have important consequences for the sequence of pathological change occurring in PD. Noradrenaline (NA) depletion by the specific NAergic neurotoxin DSP-4, results in the enhancement of MPTP induced neurotoxicity in mice. We now investigate the effect of DSP-4 mediated NA depletion in an inflammation by lipopolysaccharide (LPS) induced DAergic cell death in the substantia nigra (SN).

Four groups of male rats were treated with DSP-4 (50 mg/kg, i.p.) or saline and after 7 days, LPS (4 µg in 2 µl) or saline was administered over the SN. After 24h, rats were anaesthetized; their brains perfusion fixed with buffered 4% paraformaldehyde. Using tyrosine hydroxylase (TH) as a marker for either NA or DAergic neurons in the locus coeruleus (LC) or SN; GFAP for astrocytes; OX-42 and OX-6 for microglia and p47phox for activated NADPH-oxidase, the effect of NA depletion on DA cell death and the markers of inflammation was assessed. All cell types were counted within a grid area of 2.5 mm2 except for the TH+ve neurons which were counted in total.

Compared with the saline treated controls, rats treated with DSP-4 had 64% fewer NAergic TH+ve neurons in the LC. DSP-4 treatment also led to a massive astrocytosis in the SN (control: 138 ± 18; DSP-4: 417 ± 75; P<0.001). Although LPS increased astrocytosis this was not further enhanced in the LPS + DSP-4 group compared to DSP-4 alone (LPS, 268 ± 27; DSP-4 + LPS, 338 ± 24; ns). In confirmation of previous studies, LPS treatment also induced reactive microgliosis and iNOS induction. Rather surprisingly, in the DSP-4 + LPS group, OX-6 was markedly reduced (LPS, 178 ± 35; LPS + DSP-4, 65 ± 12, P<0.001) as was the expression of iNOS compared to LPS alone (LPS, 170 ± 46, DSP-4, 52 ± 4, P<0.001). In the SN of untreated rats, there were a few faint p47phox positive neurons constitutively present following LPS treatment, but p47phox immunoreactivity increased in the OX-42 and some OX-6 positive microglia. However in the LPS + DSP-4 group, p47phox was markedly diminished in microglia but was preferentially upregulated within the TH+ve neurons. Interestingly, there was no significant difference between the number of TH positive neurons in the LPS and LPS + DSP-4 groups in the SN.

This study suggests that NA depletion markedly affects the inflammatory response to LPS. This study further points to the complex role of inflammation in the neurodegenerative process.