In this study, we have further characterized our previously reported neurodegeneration model after a systemic injection of LPS in mice (Qin et al., 2007
). A clear sex difference was found: female mice were more resistant to LPS than male mice. Repeated LPS injections at monthly, but not weekly intervals were required to cause both motor behavioral deficits and DA neuronal loss. Time course studies revealed the time-dependent progressive nature of both rotarod ability and loss of DA neurons. Pharmacological challenge with l
-dopa/carbidopa was able to restore the motor function deficits. Markers for α-synuclein, CD45, and IL-6 revealed that the LPS injections gave rise to more wide-spread effects on the nigral region, including both neuronal and glial correlates, and further suggested that the injections gave rise to a chronic increase in neuroinflammation to the nigral region.
A delayed and time-dependent loss of nigral dopaminergic neurons was found in male (Qin et al., 2007
), but not in female mice (), after a single systemic injection of LPS. This finding in sex differences is similar to the previous report indicating that male mice are more sensitive than female mice to MPTP-induced DA neuron degeneration (Miller et al., 1998
). These findings from animal studies are in concert with the fact that males outnumber females in PD patients (Shulman, 2007
). One of the factors underlying the sex difference in response to LPS is estrogen. It is well known that estrogen protects neurons against a vast variety of toxic insults (Maggi et al., 2004
; Wise, 2002
). Estrogen has been shown to suppress the LPS-induced activation of microglia by down-regulating inducible nitric oxide synthase (iNOS), nitric oxide (NO), prostaglandin-E2 (PGE2
) and matrix metalloproteinase (MMP)-9 activation (Baker et al., 2004
; Bruce-Keller et al., 2000
; Drew and Chavis, 2000
; Vegeto et al., 2000
). Recently, Tenenbaum et al. (2007)
reported that 17 beta-estradiol (E2) significantly reduced LPS-induced increase in NO and TNFα (but not PGE2
) production in glial cells. Marotta et al. (2006)
found that phytoestrogen treatment lowered levels of proinflammatory cytokines including TNFα, IL-β and IL-6 and a higher level of TGF-β. Since female mice are more resistant to LPS exposure, repeated injections of LPS are required to show behavioral deficits and neuronal damage, possibly implying a protective effect of estrogen or other female-specific factors on LPS-induced damages in vivo as well.
It is interesting to note the different responses in behavioral deficits and DA damage in weekly vs. monthly injected paradigms (). These differences can be related to endotoxin tolerance, a phenomenon whereby pre-exposure to endotoxin causes a reduced response to subsequent challenge with endotoxin (Buckley et al., 2006
; Munoz et al., 1991
; Setrakian et al., 1994
). Tolerant animals dramatically reduced cytokine production, including TNFα, IL-1β following an LPS challenge (Cavaillon and Adib-Conquy, 2006
). It is likely that LPS injections at weekly interval could elicit tolerance. Thus, in spite of repeated injections, nevertheless, the net result is equivalent to a single injection.
Consistent with our previous report that a single systemic injection of LPS elicited a time-dependent progressive loss of DA neurons in the SN (Qin et al., 2007
), five monthly injection of LPS resulted in similar decrease in DA neurons. In this study, we demonstrated that the progressive loss of DA neurons was accompanied by a parallel decrease in rotarod ability in LPS-injected mice. These results suggested that the neurodegeneration of DA neurons is closely associated with the behavioral deficits. This notion was further supported by a pharmacological challenge study using l
-dopa/carbidopa. The rotarod ability was restored in LPS-injected mice 2 h, but not 1 week, after l
-dopa/carbidopa treatment (). Further, we found that the LPS injection gave rise to a long-term increase in α-synuclein immunoreactivity in the substantia nigra, further demonstrating a wide-spread reduction in DA function in these neurons. Thus, peripheral LPS injection produced a rodent PD model, which reproduced several important features in PD patients, such as a delayed progressive decrease in motor function, loss of DA neurons in the SN and temporal relief of motor deficits after l
-dopa/carbidipa replacement therapy.
One of the salient features of systemic LPS PD model is the long-lasting effects of the motor deficits and permanent loss of DA neurons in the SN. Most of the above-mentioned phenomena were still observed 20 months after the first injection of LPS. These long-lasting effects in this model are different from MPTP model in mice, which is known to be reversible 9–12 months after the treatment (Hallman et al., 1985
; Mitsumoto et al., 1998
). As we previously reported (Qin et al., 2004
), inflammation played a key role in LPS-induced neurodegeneration, which was mediated through cytokines, such as TNFα, produced by macrophages. The entry of these pro-inflammatory cytokines into brain further activates microglia to produce more pro-inflammatory factors, which continue to activate the same or neighboring microglia. Thus, we postulate that once brain inflammation reaches a critical point, due to the dysregulated over-activation of microglia either through the direct action of pro-inflammatory factors or indirect action of reactive microgliosis results in the damage of neurons, a vicious, self-propelling cycle occurs, which perpetuates the progression of neurodegeneration. Consistent with this hypothesis, we observed increase in immunoreactivity of α-synuclein, CD45 and IL-6 even at 20 months after the first LPS injection. In view of the increasing evidence indicating a critical role of inflammation in the pathogenesis of PD (Hong, 2005
), results presented in this study suggest that the systemic LPS is a viable rodent PD model for studying the pathogenesis and therapy of neurodegenerative diseases.