Using an animal model of global brain inflammation, we found that injection of LPS into the cisterna magna resulted in a sustained memory deficit measured by the novel object recognition test and relatively minor and transient motor deficits as detected by the NSS.
The NSS was originally developed for head-injured mice by Beni-Adani et al. (Beni-Adani et al., 2001
). This 10-point scale was later found to be a reliable measure of injury severity and a predictor of outcome after experimental closed head injury (Tsenter et al., 2008
). Intracisternal injection of LPS as well as vehicle resulted in motor deficits equivalent to those seen after mild traumatic brain injury when measured in the first 2 days after injection, probably due to a volume/pressure effect on the spinal cord. Severe deficits were not expected and indeed hemiparesis was not found in any of the mice treated with i.c saline or LPS. The early NSS was higher in the LPS injected mice compared to saline injected controls, but both groups showed rapid spontaneous improvement over time and the difference between the groups was not significant beyond the first day after LPS administration..
Functional damage due to neuroinflammation has been investigated in several studies in mice and rats. Administration of LPS via different routes was shown to produce short-term changes in behavior (“sickness behavior”) including reduced food and water intake and decreased exploration and social interactions (Boje, 1995
; Sparkman et al., 2005a
; Sparkman et al., 2005b
). These behavioral changes can be accompanied by transient motor deficits as measured by decreased swim speed in the Morris water maze (Sparkman et al., 2005b
In contrast with the transient nature of the motor deficits, the performance of LPS injected mice on the ORT was significantly impaired 2 and 7 days after injection. Unlike saline injected mice, the LPS group showed no significant preference for the novel object on both test days. To our knowledge, this is the first observation of cognitive deficits in this relatively pure and non-invasive model of neuroinflammation, since previous studies of intra cisternal LPS in mice and rats did not employ cognitive or behavioral endpoints (Biegon et al., 2002
; Boje, 1995
). Cognitive deficits were reported following intraventricular injection of LPS, (Hauss-Wegrzyniak et al., 2000
) which involves intraparenchymal damage, and with intraperitoneal injection, which is complicated by peripheral organ involvement and sepsis (Noble et al., 2007
; Semmler et al., 2007
; Sparkman et al., 2005a
; Sparkman et al., 2005b
). Our findings are similar to those reported in human brain inflammation, as it is well documented that meningitis and encephalitis in children and adults is accompanied by long lasting deficits in learning ability, memory, language, attention and executive function (Anderson et al., 1997
; Carter et al., 2003
; Hoogman et al., 2007
Those higher cognitive functions are intimately associated with areas such as the hippocampus, frontal, entorhinal and temporal cortices in both humans and rodents. Therefore, we have focused next on exploring the regional distribution of neuroinflammation in the same animals. Neuroinflammation was measured eight days post injection since this time point was shown to coincide with peak microglial activation and macrophage infiltration in acute-onset models (Miyazawa et al., 1995
). As expected in a global neuroinflammation model, we found evidence of microgliosis (increased binding of [3H]PK11195) throughout the brain of LPS injected mice. However, the size of the increase in PK11195 bindings was region-dependant, with the largest increases observed in hippocampus and cortex and the smallest increase, which did not reach statistical significance in this sample, found in the striatum.
This enhanced sensitivity of hippocampal and cortical regions and relative resistance of the striatum, is similar to our published observations in rats subjected to the same treatment (Biegon et al., 2002
) suggesting that this phenomenon is not species specific and may be common in mammalian brain including humans.
Regional sensitivity to neuroinflammation was also reported by Kim et al (2000)
, who investigated a small number of regions and showed regional differences in the number of resident microglia cells and their activational state. Hauss et al. (2000)
reported large increases in microglial number and reactivity in dorsal hippocampus, subiculum, entorhinal, and piriform cortices following chronic intracerebroventricular infusion of LPS. Indeed, in the present work, we, like others (Dubois et al., 1988
; Raghavendra Rao et al., 2000
) found that the distribution of PBR in normal mouse brain is heterogeneous, although the regional distribution of the response to LPS could not be explained by this difference alone (Biegon et al., 2002
Several putative explanations are compatible with such regional sensitivity. Since the inflammogen is delivered through the cisterna magna, extraparenchymal factors such as proximity to CSF, the regional density of the vascular network or the effectiveness of the blood brain barrier may contribute to the intensity of neuroinflammation and ensuing neuronal damage. Conversely, the differences may be due to intrinsic properties of the brain cells comprising various brain regions; which may be established in early brain development. To address this possibility, we chose to examine the sensitivity of striatal and cortical neurons to neuroinflammatory mediators in primary culture, where extraparenchymal factors do not play a role. Activated glia are known to secrete glutamate and proinflammatory cytokines, most notably TNFalpha (Ghoshal et al., 2007
; Pickering et al., 2005
; Qin et al., 2007
); so these two agents were chosen for investigation. In this experimental setup, we found that neurons of striatal origin were significantly protected from glutamate and TNFalpha toxicity compared to cortical neurons. Furthermore, TNFalpha concentrations which were toxic to cortical neurons, exerted neuroprotective effects against glutamate excitotoxicity and spontaneous neuronal death in striatal neurons in culture. While enhanced sensitivity of cortical, relative to striatal, neurons to excitotoxicity could be expected based on the higher concentration of glutamate receptors in the cortex relative to striatum (Betarbet et al., 2000
; Biegon et al., 2002
), the qualitative difference in the response to TNFalpha is novel and unexpected. Since TNFalpha is known to interact with two receptor types, one which mediates cytotoxicity and apoptosis and one that can be cytoprotective (Pickering et al., 2005
; Quintana et al., 2005
), it is possible that the relative density of these receptor subtypes also varies among brain regions. This possibility as well as alternative or additional explanations needs to be explored in future studies, including investigation of the fate of the other cell types (astrocytes, microglia) present in these regions.
In summary, neuroinflammation is a common pathway in many different acute and chronic human neuropathologies. A better understanding of the regional determinants governing sensitivity or resistance to neuroinflammation may help to identify new targets for treatment or prevention of neuroinflammatory damage in human disease.