Research reportInfluence of gender on working and spatial memory in the novel object recognition task in the rat
Introduction
A wide range of behavioural tests are available to model human cognitive function and these tasks may be chosen for the particular aspect of cognition under study, for example, spatial and working memory or reversal learning. The novel object recognition (NOR) task for rodents is a non-spatial, non-aversive memory test [11] which involves the substitution of a familiar object with a novel object in a memory retention trial. However, the NOR task can be adapted for assessment of spatial memory, in this task the identical objects are not replaced with a novel object but are moved to a new position following a particular inter-trial interval [11]. These tasks are ethologically relevant as they rely on the animal's natural exploratory behavioural repertoire and avoid the confounding influence of reinforcement. Animals typically respond to environmental changes by preferential exploration of novel or moved objects over those that are familiar; it is these preferences that indicate the formation of memories regarding the location and identity of objects. Due to the relative simplicity and ethological relevance of this task, it is increasingly used as a powerful experimental tool to assess drug effects on cognition and to investigate the neural mechanisms underlying working and spatial memory [7], [20], [45].
Ovarian steroid action in the brain is not limited to reproductive neuroendocrinology and behaviour. Recent studies in rodents and humans have shown that the status of both the adrenal and gonadal axes (hypothalamic–pituitary–adrenal (HPA) and hypothalamic–pituitary–gonadal (HPG), respectively) can influence learning and memory strategies and capacities [3], [4], [8]. Research during the past decade confirms that gonadal steroids have the ability to influence the structural properties of the brain regions that sub-serve learning and memory. The majority of research has investigated the effect of oestrogen on cognitive function and oestrogen receptors have been found in several areas of the brain including the amygdala, cerebral cortex, cerebellum and hippocampus [41], [47]. Of particular interest to the current investigation is the hippocampus which is known to play a significant role in working and spatial memory [26] and which contains both forms of the oestrogen receptor (ER) -ERα and ERβ [3], [4], [17]. Other areas within the brain which are fast becoming associated with mnemonic processes include the frontal cortex and the striatum and it has become clear that there are strong interconnections between the hippocampus and frontal cortex [49]. In the frontal cortex of male and female rats, each ER isoform has been shown to display its own unique, selective distribution [32], [60] the most predominant being ERα with only a small number of ERβ containing cells [42]. Given the diverse actions of oestrogen, these differences will undoubtedly affect cognitive processes between the genders. Oestrogens, androgens and progestagens have been shown to alter hippocampal electrophysiology; studies suggest that oestrogen is excitatory whilst progesterone and its metabolites are inhibitory, the function of androgens is less clear [43]. Thus, the hippocampus has been shown to display different structural changes in response to different gonadal hormones. Hippocampal plasticity alters across the oestrous cycle of the rat in response to cyclic changes in oestrogen and progesterone [3], [26], [57], [58], [59]. During the pro-oestrous phase, when steroid levels are at their peak [56], the number and density of CA1 pyramidal cell spine synapses reach a maximum, in contrast, during vaginal oestrous when hormones are at low levels, a 30% decline in synaptic density and number is observed [3], [58], [59]. These cyclical changes could induce significant behavioural changes which may be detected using animal tests.
These mechanisms provide some biological foundation for the complex effects of ovarian steroids on cognitive function in various species, including humans [9], [23], [30], [31]. What is emerging from recent studies is that oestrogen, and perhaps progesterone and its metabolites such as allopregnanolone, can influence learning and memory in a task-dependent manner. In women, oestrogen levels peak twice during the menstrual cycle, the first peak (which is unopposed by progesterone) occurs just prior to ovulation and the second peak is in the mid-luteal phase when progesterone levels are also elevated. Kimura and Hampson [23], [29] have successfully demonstrated that women are negatively affected in verbal skills and muscular coordination tasks when their oestrogen levels are high compared to the early follicular phase (when all ovarian steroids are at a minimum) when male oriented spatial tasks are favoured. In contrast, female orientated tasks such as verbal fluency and motor skills have been shown to be enhanced prior to ovulation when oestrogen action is unopposed by progesterone [28]. Clinical research has been directed at the potential of ovarian steroids to facilitate the storage of new memories and to provide protection for neuronal circuits in debilitating disease states. Although controversial, some studies have shown that oestrogen supplementation/replacement may be beneficial, with respect to aspects of cognitive function, in Alzheimer's disease [1], menopausal women [3], [38], [40] and schizophrenia [5], [33].
Sex differences in cognition are often viewed as a product of evolution in which women and men's roles differ, hence their problem solving abilities became distinctive and complementary. Indeed, one evolutionary scenario suggests a ‘female foraging’ hypothesis; females should be better than males at remembering the identity of objects [10], [25] which is particularly relevant to the NOR paradigm. One of the few studies that has attempted to characterise gender differences and behavioural responses to spatial and object novelty used adult C57BL/6J mice [16]. It was found that males were superior to females in remembering the identity and location of objects in an open field after a 24 h and 7 day retention period, contradicting the ‘female foraging’ hypothesis but in agreement with the view that males excel in spatial tasks. However, it must be noted in this instance that the females will be in a different stage of oestrous during the retention period due to the short oestrous cycle in rodents and this may have had confounding effects on the findings. A more recent study [50] used the NOR paradigm to successfully demonstrate a positive effect of oestrogen administration on object recognition in ovariectomised rats and during behavioural oestrous (pro-oestrus) compared to other stages of the cycle in hormonally intact rats. The aim of the current study was to investigate the influence of gender and oestrous cycle phase on working and spatial memory in the NOR paradigm. Object recognition should be deceased as the inter-trial interval increases for both genders as essentially, the task gets harder. Females are expected to display novel object recognition over a longer inter-trial interval than males due to the presence of ovarian steroids and their effects on neurotransmitter systems, however, ovarian steroids have been proven to be detrimental to spatial memory tasks so, in the spatial NOR task it is assumed males will excel. It is further hypothesised that females will demonstrate variable performance in spatial and working memory tasks, which may be related to changes in the hippocampal structure in response to natural cyclic hormonal fluctuations.
Section snippets
Subjects
All experiments were carried out using mature male and female experimentally naïve, hooded Lister (hL) rats (Harlan, Bicester, UK) weighing 373 ± 17 g and 234 ± 17 g, respectively at the onset of the studies. Animals were housed in groups of five (in cages measuring 38 cm × 59 cm × 24 cm) with cardboard tubes (Datesand Ltd, Manchester, UK) for environmental enrichment as standard practice. Animals were maintained under standard laboratory conditions on a 12 h light:12 h dark cycle (lights on at 07:00 h) in a
Working memory
Overall two-way ANOVA revealed no significant effect of objects during the acquisition trial (F1,3 = 2.32, p = 0.14) or object versus stage of cycle (F3,23 = 0.79, p = 0.51) for naturally cycling female rats during the acquisition phase of the NOR paradigm (data not shown). During the retention trial, there was a significant difference in novel object exploration (F1,3 = 27.16, p < 0.001) but no effect of stage of the oestrous cycle (F3,23 = 0.29, p = 0.83). Paired sample's t-test showed that female rats
Discussion
The present study used the novelty of exploration to investigate the impact of gender differences and oestrous cycle phase in a single strain of laboratory rat, the hooded Lister, using a simple cognitive task. The hypothesis, that both gender and stage of oestrous cycle will affect cognitive performance in the rat, was based on neurophysiological and behavioural data suggesting that sex hormones will modify memory retention in rodents. All the tests took place in an open field and were based
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