The effect of standard laboratory diets on estrogen signaling and spatial memory in male and female rats
Introduction
Phytoestrogens are non-steroidal compounds that are present in many plants and plant products, including grains, legumes and soy [9]. They are highly lipophilic, absorbed by the gut and readily cross the blood brain barrier [21]. Once in the brain, they bind to the estrogen receptor (ER), with a higher affinity for ERβ than ERα, and affect estrogen-mediated signaling processes, including transcription of genes that are normally responsive to estradiol [29,36,37].
Phytoestrogens appear to confer health benefits in the form of protection against a number of disease conditions, such as cancers, cardiovascular disease and osteoporosis [34]. Supplements containing a sub-class of phytoestrogens, known as isoflavones, are also effective in alleviating some effects of aging in women, e.g., symptoms of menopause such as hot flushes [46]. In addition, these supplements are reported to slow normal age-related cognitive decline in women [7], although the outcomes for men are less clear [46].
Animal studies have confirmed that phytoestrogen intake influences cognition, specifically, spatial memory in ovariectomized female rats [24,25], but the effects of phytoestrogen intake on intact female and male rats are unclear. For example, studies that have examined the effects of high phytoestrogen intake in male rats have produced contradictory findings [22,25], including showing that dietary phytoestrogen enhanced visual spatial memory in females but impaired performance in males [25], whereas another study showed improved performance in older males [19].
The inconsistent effects of phytoestrogen exposure on cognition in animal studies may be due to the fact that rodents are inadvertently exposed to high levels of phytoestrogens in their laboratory diets. These phytoestrogens are present in the form of soy which is a primary ingredient of these diets (due to the fact that it is cheap and widely available), in combination with various additional grains including wheat, barley or legumes. One notable exception is the AIN-93G diet, which is low in soy but high in other carbohydrate (e.g., sucrose) and protein (e.g., casein) products [33]. Given the known effects of phytoestrogen exposure on estrogen-dependent signaling processes, laboratory diets which differ in phytoestrogen content may have distinct physiological, molecular and behavioural effects. However, a comprehensive assessment of any such effects remains to be conducted.
The current study examines the impact of laboratory diets with varying soy content on the brain and behavior of male and female rats. Rats received one of two standard laboratory diets (GOR: Gordon's Premium Rat and Mouse Pellets, Yanderra, Australia; and SF: Specialty Feeds Irradiated Rat and Mouse Cubes; Specialty Feeds, Perth, Australia) or a semi-pure, soy-free diet (AIN-93G; Specialty Feeds, Perth, Australia). We assessed the impact of these diets on weight gain across the study, aspects of estrogen and dopamine signaling across regions of the brain, and two tests of cognition. Specifically, to assess estrogen signaling, we measured plasma estradiol concentration, as well as estrogen and dopamine receptor mRNA expression in the prefrontal cortex, hippocampus, striatum and hypothalamus. Finally, to determine whether the different diets have varying effects on cognition, we tested male and female rats on two forms of recognition memory that differ in their dependence on the integrity of the hippocampus: object recognition, which occurs largely independently of activity in the hippocampus (see [8] for review), and place recognition, which is known to be hippocampal-dependent [4].
Section snippets
Subjects
Male and female Sprague-Dawley rats (7–8 weeks old; Animal Resources Centre, Perth, Western Australia) were housed by sex 3–4 rats/home cage on a reverse 12:12 light/dark cycle. Rats had ad libitum access to food and water throughout the study.
All procedures were approved by the Animal Care and Ethics Committee at the University of New South Wales and in accordance with the National Institute of Health Guidelines for the Care and Use of Animals, revised 1996.
Estrous cycle tracking
A sub-group of female rats (n = 15)
Estrous cycle
Diet had no detectable effect on the number of days spent in estrous or the number of complete cycles across a two-week period (Fs ≤ 1). All further data represent an aggregate across all stage of the estrous cycle in female rats.
Body weight
All rats gained weight across the two week treatment period (F(14,756) = 565.78, p<0.001, = 0.913). However the rate of increase differed among the three dietary conditions (day by diet interaction; F(28,756) = 3.760, p<0.001, = 0.122), with rats receiving
Discussion
This study has shown that three of the rodent diets commonly used in Australia have distinct effects on the brain, physiology and behavior of both male and female rats. As summarized in Table 4, most effects were evident in rats fed the soy-free diet (AIN-93G) compared to those fed either of the other two diets, suggesting that the critical factor could be soy content. Specifically, rats on the AIN-93G diet gained more weight, showed decreased DRD1 receptor gene expression in the striatum, and
Declaration of Competing Interest
The authors declare that they have no competing interests.
Acknowledgments
This research was supported by an Australian Government Research Training Program award to CF and a University of New South Wales Faculty of Science Research Grant to KC.
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