Maternal experience and Alzheimer's disease : degenerative differences in the female rat

Alzheimer’s disease is a degenerative disease found in many aging adults. The presence of amyloid precursor protein (APP) is an early indicator of the onset of Alzheimer’s, primarily in memory-related brain regions like the hippocampus. Hormones accompanying pregnancy, such as estrogen, may provide the female brain with protection against neurodegeneration and deposits of APP. The present study will compare concentrations of APP in the brains of parous and nulliparous animals and examine the interaction of APP with estrogen receptor beta (ER ). Young and aged animals will also be compared to determine any early effects of APP or ER . It is proposed that because of the neuroprotective effects of preganacy, the parous animals will have lower concentrations of APP and higher concentrations of ER . Maternal Experience 3 Maternal Experience and Alzheimer’s Disease: Degenerative Differences in the Female Rat Alzheimer’s disease is quickly becoming one of the most prevalent and devastating afflictions of today’s aging population. Alzheimer’s disease is the fourth leading cause of death in Western nations, and it is estimated that approximately 24 million people worldwide suffer from Alzheimer’s-related dementia (Miller, McLoughlin, Kwok-Fai, Tennant, & Rogelj, 2006). There is no cure or effective treatment currently available for Alzheimer’s disease, but given its frequent occurrence in older adults, the need for treatment is becoming more and more pronounced. In addition to seeking treatment for Alzheimer’s, scientists are still working to understand the intricate details of the complex pathology of this disease (Kar, Slowikowski, Westaway, & Mount, 2004). Alzheimer’s disease is most distinctly characterized by neural plaques and neurofibrillary tangles, which both lead to eventual lesions in the brain, as well as a decrease in number of neurons and functional synaptic connections. Therefore, neurons are unable to efficiently pass on messages, and cognitive functioning begins to decline (Stephan, Laroche, & Davis, 2001). The development of these degenerative features is linked to deposits of amyloid precursor protein (APP) in the brain, especially in regions associated with memory such as the hippocampus. High concentrations of APP deposits are often found extremely early in the pathogenesis of the disease, even before specific Alzheimer’s symptoms emerge (Stephan, Laroche, & Davis, 2001). Most notably, -amyloid peptide (A ) is derived from the APP deposits in the brain. The A acts as the core for the formation of neural plaques (Kar et al., 2004). The role of A and APP in the pathogenesis of Alzheimer’s disease has been evaluated in many studies of animal models. Stephan, Laroche, and Davis (2001) found that rats injected Maternal Experience 4 with small amounts of A experienced a significantly accelerated development of neural plaques. The injected animals also performed poorly on cognitive tasks, made more behavioral mistakes in navigation, and made a greater number of errors on a working memory task than the non-injected control group. These decreases in memory performance indicate that the presence of Alzheimer’s-related proteins in the brain lead to both negative behavioral modifications and disruptions in underlying neuroanatomy. APP can have detrimental effects on the strength and speed of neuronal firing. In a mouse model of Alzheimer’s, results indicated that hippocampal neurons lacking in APP had a higher frequency and strength of synaptic transmissions, and there were more functional neurons in the areas of the brain without APP (Priller, Bauer, Mitteregger, Krebs, Kretzschmar, & Herms, 2006). Once APP and A effectively deposit the neural tangles and plaques, the target regions of the brain start to lose neurons and functional synapses, resulting in the beginnings of the slow cognitive degeneration associated with Alzheimer’s disease (Kar et al., 2004). Sex Differences and Hormones Alzheimer’s disease is more prevalent in women than in men. This sex difference can be attributed to an overall longer female lifespan and differences in the duration of the disease; women with Alzheimer’s tend to live longer with the disease than men (Baum, 2005). The body may also provide some natural defenses against the effects of Alzheimer’s. Specifically, hormones may play a mediating role in the development and progression of the disease. It has been suggested that testosterone may protect men from Alzheimer’s disease, while estrogen may have similar protective effects for women (Baum, 2005). Studies on hormone replacement therapy have confirmed the beneficial effects of female hormones like estrogen on the pathogenesis of Alzheimer’s disease. Henderson, Benke, and Greene (2005) found that a hormone therapy regimen including estrogen might reduce the risk of Maternal Experience 5 developing early onset Alzheimer’s. It was also concluded that the hormone therapy must be administered at a certain critical time period, during the early stages of postmenopausal symptoms, for protective effects against Alzheimer’s to be executed. Beginning a hormone therapy regimen years after the onset of menopause and the loss of endogenous estrogen is too late, and the beneficial effects that result from earlier hormone administration will not occur (Baum, 2005). Generally, hormone replacement therapy exhibits a ‘healthy cell bias,’ in which neurons that are healthy during administration will experience the benefits of therapy, while hormones will cause further damage in unhealthy neurons (Brann, Dhandapani, Wakade, Mahesh, & Khan, 2007). It is also important to consider dosage when administering hormone replacement therapy. Baum (2005) reported an inverted U-shaped pattern in the effectiveness of hormone dosages; a small dosage did not show any effects and a large dosage could have toxic and harmful effects. An excessive administration of exogenous estrogen can disrupt the body’s natural fluctuations of hormones, which can actually have detrimental effects on the functioning of the brain (Marriott & Wenk, 2004). Moderate amounts of hormone replacement therapy that more closely mimic the body’s natural cycle may provide the best protection against the degeneration of Alzheimer’s disease (Marriott & Wenk, 2004). The Role of Estrogen Receptors Estrogen is a female reproductive hormone, traditionally associated with gender differentiation during the developmental period and increased synaptic plasticity during puberty and the ovarian cycle. More recently, estrogen has been implicated in playing a role in vital actions of the brain, including the release of neurotransmitters, electrical excitability, and synaptic function (Genazzani, Pluchino, Luisi, & Luisi, 2007). The positive role of estrogen in neuroaction has made it a focus of recent research on the prevention of neurodegenerative diseases. Estrogen depletion in the aging brain may be an early risk factor for Alzheimer’s Maternal Experience 6 disease, and a continued presence of estrogen in the brain may preserve specific areas in the brain targeted by disease pathology (Brann et al., 2007). Although estrogen-based hormone replacement therapy has been linked to reduced risk of Alzheimer’s onset, the side effects have made the therapy controversial. The actions of the therapy are beneficial in the brain, but can have detrimental impacts on peripheral tissues, including an increased risk of breast and uterine cancer (Carroll & Pike, in press). Because of these harmful effects, research on estrogen therapies has focused on making treatments more specific to the areas of the brain associated with neurodegeneration. One target-specific focus of research has involved the differentiation between two different estrogen receptors: estrogen receptor alpha (ER ) and estrogen receptor beta (ER ). Both receptors have been linked to neuroprotection in the brain, especially in learning and memory-related areas (Genazzani et al., 2007). ER and ER are both present in the hippocampus, and deposits of Alzheimer’s-related proteins are regulated through each of these receptors (Brann et al., 2007). However, the different mechanisms of ER and ER are not fully understood. Overall, the actions of estraidol, a clinical form of estrogen, are found to be predominantly dependent on ER , indicating that it is the primary estrogen receptor (Carroll & Pike, in press). Research has shown that ER , a more recently discovered receptor, might also play a primary role in estrogen regulation in the brain, especially in association with Alzheimer’s pathology. ER is closely associated with learning and memory because it is the predominant type of estrogen receptor found in the hippocampus (Genazzani et al., 2007). Because of its increased presence in the hippocampus, hormone therapy that is ER specific might have especially beneficial effects in the brain (Wang, Irwin, & Brinton, 2006). Research by Forsell, Maternal Experience 7 Enmark, Axelman, Blomberg, Wahlund, Gustafsson, et al. (2001) further linked ER and Alzheimer’s by confirming that a mutation in the ER gene might increase susceptibility to Alzheimer’s disease. Given this recent research on its presence in the hippocampus and association with Alzheimer’s disease, the present study will focus on the expression of ER in the brain. Maternal Experience The benefits of the natural hormonal changes associated with the female reproductive experience are well established. Motherhood leads to improvements in learning and memory, both of which are functions of the hippocampus. Oxytocin, another hormone associated with reproduction, also enhances neural functioning and potentiation of neural messages (Pawluski, Walker, & Glaea, 2006). Pregnant female rats demonstrated better performance on tests of spatial memory than virgin rats, which can be attributed to the elevated and relatively stable levels of estrogen present in the body during pregnancy (Galea, Ormerod, Sampath, Kostaras, Wilkie, & Phelps, 2000). Galea et al. (2000) also established that pregnant rats spent more time in the target quadrant of the water maze and traveled shorter distances to the reward than nonpregnant animals, demonstrating the positive effects of pregnancy on learning and memory. In addition to hormonal changes, maternal animals must respond to a large number of novel stimuli as a result of exposure to pups. Suddenly, new mothers find that they are responsible for the survival of their offspring (Love, Torrey, Glasper, DeVries, Lambert, & Kinsley, 2005). They must quickly acquire new skills, such as improving their hunting abilities, so that they can obtain food for themselves as well as their new pups. New mothers must learn to forage and recall the spatial location of food in an effort to spend less time away from their offspring (Love et al., 2005). The first pregnancy is therefore critical, as the female brain begins to change in response to new hormone levels and a range of sensory stimuli from the pups. The Maternal Experience 8 maternal rat must make her own modifications and obtain new skills to ensure the survival of her offspring, without any previous experience or knowledge in this area (Pawluski et al., 2006). Subsequent pregnancies can increase the benefits of initial hormone and pup exposure. While primiparous (single reproduction) animals outperformed nulliparous (no reproduction) animals on tests of learning and memory, multiparous (more than one reproduction) animals often performed better than both groups (Gatewood, Morgan, Eaton, McNamara, Stevens, & Kinsley, 2005). Therefore, repeated exposure to reproductive hormones and offspring can lead to a further enhancement in neural functioning and memory. The present study looks to extend the findings of Gatewood et al. (2005) that the paired exposure to both pups and the natural hormonal changes of the female reproductive experience preserves the aging brain from degeneration and provides neuroprotection against memory declines. Parous animals have lower levels of amyloid precursor protein in their brains, and multiparous animals have significantly less of the protein deposited in their brains than primiparous animals (Gatewood, 2005). These results suggest that the protection provided by maternal experience can extend specifically to defense against the risks of Alzheimer’s disease. Given that the results of hormone replacement studies have supported the benefits of moderate dosages that mimic the actual female hormone cycle, it is possible that the natural exposure to female reproductive hormones provides an effective barrier against the onset of Alzheimer’s, even at a relatively young age (Marriott & Wenk, 2004). It is also possible that moderate dosages of hormone replacement therapy and natural reproductive hormone levels have equally beneficial effects (Love et al., 2005). A preliminary study in female rats did indicate higher concentrations of APP in nulliparous animals than primiparous animals, which were aligned with the findings of previous research (Gatewood et al., 2005). An immunofluorescent investigation of the expression of APP Maternal Experience 9 in the female hippocampus showed that primiparous animals had a lower concentration of APP in the hippocampal region of their brains than age-matched nulliparous animals. While it is expected that maternal experience will offer some protection against the onset of neurodegeneration, the present study looks to specifically examine the expression of ER in the female rat brain. An understanding of how the deposits of ER and APP interact and are expressed together can give some further indication to the specific protective mechanisms of pregnancy hormones in the brain. The current study will expand upon past findings investigating the early signs of neurodegeneration in younger rats. While past research has focused on the presence of Alzheimer’s-related declines in aged animals, more research in younger animal models will indicate the effects of APP and memory decline early in life. Priller et al. (2006) found that in animals lacking APP, the enhancements of neuron and synapse functioning were more pronounced in young mice, aged three weeks, than older mice, aged 11 months. Studies involving the administration of degenerative protein have also supported their quick and early action. In animals injected with A , collections of amyloid protein and neuron loss were seen around injection sites in as little as seven weeks (Stephan et al., 2001). Given the support of early administration of therapeutic remedies like hormone replacement therapy, it is important to identify the early progression of the disease to effectively stop the first signs of Alzheimer’s pathology. The present study will investigate the effects of maternal experience, its accompanying hormones, and its novel stimuli on the emergence of neuroanatomical changes related to Alzheimer’s disease in young and old female rats. It is expected that our findings will replicate past research that deposits of APP increase with age, while the presence of estrogen receptors decrease with age (Priller et al., 2006; Brann et al., 2007). It is expected that these neural Maternal Experience 10 differences will be seen predominantly in the hippocampus, a region of the brain essential for memory. Specifically, the current study will focus its investigation on the CA1 region of the hippocampus, an area of neural density and a high concentration of ER receptors (Carroll & Pike, in press). Hypotheses 1. It is hypothesized that primiparous animals will have less APP than nulliparous animals, as measured by the area of APP expression. 2. It is hypothesized that younger animals will have less APP than older animals, as measured by the area of APP expression. 2a. It is hypothesized that old primiparous animals will have less APP than old nulliparous animals. 2b. It is hypothesized that old primiparous animals will have more APP than young primiparous and young nulliparous animals. 2c. It is hypothesized that old nulliparous animals will have more APP than young primiparous and young nulliparous animals. 2d. It is hypothesized that young primiparous animals will have less APP than young nulliparous animals. 3. It is hypothesized that primiparous animals will have more ER than nulliparous animals, as measured by the area of ER expression. 4. It is hypothesized that younger animals will have more ER than older animals, as measured by the area of ER expression. 4a. It is hypothesized that old primiparous animals will have more ER than old nulliparous