Co-morbid beta-amyloid toxicity and stroke produce impairments in an ambiguous context task in rats without any impairment in spatial working memory
Introduction
Alzheimer’s disease (AD) is a progressive neurodegenerative disease affecting approximately 40 million people worldwide (Sosa-Ortiz et al., 2012, Thies and Bleiler, 2013). It is characterized by progressive memory loss, cognitive dysfunction and verbal deficits (Minati, Edginton, Bruzzone, & Giaccone, 2009), as well as neurodegeneration and the accumulation of senile plaques in brain areas related to attention and memory, among others (Braak and Braak, 1991, Braak and Braak, 1997).
The most frequent and widely studied neuropathologies associated with AD are decreased acetylcholine (ACh) producing neurons (Bowen et al., 1982, Perry et al., 1978, Whitehouse et al., 1981), changes in ACh producing cells’ form and volume (Mesulam et al., 2004, Sassin et al., 2000), decreased cholinergic tone (Davies & Maloney, 1976) and increased presence of senile plaques in post-mortem samples (Joshi, Ringman, Lee, Juarez, & Mendez, 2012). The characteristic senile plaque found in AD is composed of the accumulation of beta-amyloid typically in the presence of hyperphosphorylated tau (Braak and Braak, 1991, Braak and Braak, 1997). Genetic mutations of the amyloid precursor protein (APP), from which beta-amyloid is cleaved, have been associated with increased risk and early onset of AD (Strittmatter et al., 1993, Van Broeckhoven et al., 1992), which is typically thought to represent the familial variant of AD. However, studies examining concordance rates indicate that AD is not purely a genetic disease (Gatz et al., 2006). Beta-amyloid accumulation is also associated with regular aging (Whitwell et al., 2013), although not to the extent seen in pathological AD, and accumulation of beta-amyloid alone does not necessarily result in loss of memory processes or cognitive function (Whitwell et al., 2013) but can decrease cholinergic tone (Auld et al., 2002, Pedersen et al., 1996, Tran et al., 2002). Furthermore, the characteristic changes to the cholinergic system in AD may be a result of aging as cholinergic tone decreases with regular aging (Efange et al., 1997, Kuhl et al., 1996, Perry et al., 1978), and drastic changes to the cholinergic system in AD may only occur in late stage AD (Davis et al., 1999). There is evidence of compensatory mechanisms in early stage AD to combat age-related as well as AD-related cholinergic decline (DeKosky et al., 2002, Mufson et al., 2007). Indeed, the neuropathology of AD is likely to occur with an initial age-related decline in cholinergic function (as discussed in Craig, Hong, & McDonald, 2011).
Animal models have investigated environmental risk factors for the development of sporadic AD, including stroke, head trauma, stress, diabetes and chronic circadian disruption (Craig et al., 2011, McDonald, 2002, McDonald et al., 2010). These studies observed worse cognitive functioning and increased pathology when these factors were combined than when they were presented alone. Furthermore, the location and severity of neuronal pathology was dependent on the factors that were combined. These models suggest that AD results from a combination of passive and active factors, whereby passive factors (such as beta-amyloid accumulation) provide an environment in which active factors (such as stroke) will cause more brain damage and/or cognitive dysfunction than if they were presented alone (McDonald, 2002). The hippocampus, which is integral for episodic learning and memory, is especially vulnerable (Arnold et al., 1994, Phillips et al., 1991). Indeed, in the aging population, stroke alone has been associated with decreased cognitive functioning (Hankey, 2003). However, AD patients are at a high risk for stroke (Tolppanen et al., 2013), and in the AD population, increased incidence of stroke is associated with the worst disease prognosis (Doraiswamy et al., 2002, Hachinski, 1979, Hachinski, 2011). Therefore, stroke itself is damaging (Small, Morley, & Buchan, 1999), but when it occurs in combination with AD pathology, it will result in more damage and decreased cognitive function. To explore this theory in a rat model, stroke, specifically in the form of mild or mini-strokes, conducted in the presence of high levels of beta-amyloid produced larger infarct volumes, increased inflammation and memory loss more so than if only stroke or beta-amyloid were presented alone (Amtul et al., 2014a, Cechetto et al., 2008, Whitehead et al., 2005, Whitehead et al., 2007, Whitehead et al., 2005). This model of a combination of factors that increases AD-like symptoms and pathology provides insight into the pathology associated with AD as well as the cognitive disruptions associated with AD and potential therapeutic targets and treatment options specific to the co-factors involved.
In this study, we examined the combined effects of elevated beta-amyloid, using the toxic Aβ25–35 fragment, and stroke, induced by small bilateral injection of endothelin-1 into the right striatum, as a rat model of sporadic AD on cognitive function in a modified version of the Morris water task (MWT) and in a discriminative fear-conditioning to context paradigm (DFCTC). The modified MWT consisted of learning a new spatial location every single day, thus taxing hippocampal and prefrontal cortex functions (McDonald, King, Foong, Rizos, & Hong, 2008). The DFCTC task, through context-shock pairings, assessed both hippocampal pattern separation functions and orbital frontal cortical contributions to the control of generalized fear (Antoniadis and McDonald, 1999, Antoniadis and McDonald, 2000, Zelinski et al., 2010). It was predicted that deficits would be observed in both tasks, indicating disruptions to both hippocampal and prefrontal cortex contributions to behaviour. Immunohistochemical staining for choline acetyltransferase (ChAT) was used to assess the status of the cholinergic system in this animal model of AD, and it was predicted that cholinergic status would be diminished in the rats who had received the combination of factors, based on findings describing the negative feedback loop between beta-amyloid and cholinergic tone (Kar and Quirion, 2004, Poirier et al., 1995).
Section snippets
Animals
32 male Wistar rats (∼250 g) were obtained from Charles River laboratories (Quebec, Canada). Animals were housed in pairs in standard shoebox style cages with ad libitum access to food and water and were maintained on a standard 12:12 light:dark cycle. All procedures were done in accordance to the University of Lethbridge Animal Welfare committee and the Canadian Council on Animal Care guidelines.
Surgery
Surgeries were conducted as previously described (Amtul et al., 2014a, Amtul et al., 2014b, Cheng et
Visible platform
For the visible platform days, only latency to reach the platform was examined due to tracking errors when the visible platform was present.
There was a significant effect of trial (F(3,81) = 7.648, p < 0.000) and a trial by group interaction (F(9,81) = 1.755, p = 0.090; data not shown), where overall, rats decreased their latency to reach the platform over the course of trials. There was no significant main effect of group. Post hoc analyses using planned comparisons revealed a significant effect of
Discussion
Sporadic AD has been theorized to arise from a combination of factors (McDonald, 2002, McDonald et al., 2010). One posited factor is beta-amyloid, due to the large amounts of aggregated beta-amyloid found in post-mortem AD (Braak and Braak, 1991, Braak and Braak, 1997), as well as the cognitive disruptions caused by both genetically (Glenner and Wong, 1984, Joshi et al., 2012, Lesne et al., 2006, Masters et al., 1985, Strittmatter et al., 1993, Van Broeckhoven et al., 1992) and artificially (
Conclusions
This animal model of AD has been posited as an explanation for the development of sporadic AD, such that multiple factors can predispose to the development of AD, and the combination of these factors can induce the presence, onset and severity of AD dependent on the number and types of factors presented in combination (McDonald, 2002, McDonald et al., 2010). Earlier research from our group has examined the effects of cholinergic depletions in combination with seizures, stress, circadian
Acknowledgments
This research was supported by a Grants awarded to RJM from the Alzheimer’s Society of Canada and the CIHR Emerging Team program.
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