Estrogen replacement in ovariectomized rats affects strategy selection in the Morris water maze

While estrogen enhances performance on some tasks of learning and memory, it has impairing or no effects on others. It has been proposed that estrogen differentially affects performance on various tasks of learning and memory by influencing the strategy used to solve a task. The goal of the present study was to determine if estrogen would influence strategy selection in the Morris water maze. Long-Evans rats were ovariectomized and implanted with Silastic capsules containing 25% estradiol diluted in cholesterol or 100% cholesterol. Rats were trained in a water maze task in which multiple strategies were available for use to locate a hidden escape platform that was moved to a new location for each set of four daily trials. During 10 days of acquisition trials, a visible floating landmark was always located in a static position relative to the hidden escape platform. Additionally, fixed extramaze cues visible to the animals surrounded the maze. Following acquisition, 2 days of probe trials were conducted in which the static landmark was removed. Estrogen replacement in ovariectomized rats resulted in impaired performance across 10 days of acquisition. Additionally, while removal of the visible landmark during the probe trials had no effect on the performance of the females receiving estrogen, it significantly disrupted performance of females receiving cholesterol treatment. These results indicate that estrogen replacement in ovariectomized rats biases an animal against using a landmark or static cue to aid in the location of a hidden escape platform in the water maze.     

Aggressive behavior in inbred strains of mice during pregnancy

Female aggressive behavior toward adult male ICR/JCl mice was compared for virgin and pregnant mice of five inbred strains (BALB/c, C3H/He, C57BL/6, DBA/2J, and AKR/J). Pregnant females from four strains except BALB exhibited intense aggressive behavior, whereas only virgin AKR females were aggressive. Aggressive behavior began in early pregnancy, was highest in midpregnancy, and declined slightly thereafter until the day of parturition. The level of aggressive behavior showed significant strain differences. The topography of aggressive behavior was also different among the four strains. DBA females showed marked contrast with the other three strains in the temporal changes of aggression in the first phase of encounter. Furthermore, strain specific behavioral pattern of aggression was demonstrated based on six behavioral acts ( Darting , Chasing, Attack, Biting, Wrestling, and Boxing). Virgin and pregnant AKR females showed the identical behavioral pattern of aggression.     

Fischer 344 and wistar rats differ in anxiety and habituation but not in water maze performance

The fact that various neuropharmacological substances have anxiolytic as well as amnesic effects suggests that neuronal mechanisms of anxiety and learning/memory closely interact. Hence, we hypothesized that differences in anxiety-related behavior could be accompanied with differences in cognition or habituation. Two rat strains with different levels of anxiety, more anxious Fischer 344 rats by Charles River (FC) and less anxious Wistar rats by Winkelmann (WW), were tested in the Morris water maze task and an open field test for habituation learning. Additionally, we investigated the effect of different light intensities on the performance in the Morris water maze and the elevated plus maze. The results of the water maze task indicate that differences in anxiety-related behavior do not go along with differences in this performance of learning/memory. Moreover, the test was not affected by different light intensities. In contrast, illumination did affect performance in the elevated plus maze test, wherein dim light provoked an anxiolytic effect in both rat strains. The findings that neither different baseline levels of anxiety nor fear modulating light conditions were accompanied by changes in the performance of rats in the Morris water maze led us to the suggestion that there is no connection between anxiety and learning/memory in this task. Contrarily, anxiety might be associated with habituation learning in the open field test, shown by the superior habituation of the anxious FC rats in comparison to the less anxious WW rats. In sum, these results indicate that anxiety and learning/memory seem to be independently regulated behaviors, whereas habituation might be more closely correlated with anxiety. Nevertheless, a general statement about the relation between emotionality and learning/memory mechanisms would be premature and the link between behaviors remains to be clarified.     

Emergence of an egocentric cue guiding and allocentric inferring strategy that mirrors hippocampal brain-derived neurotrophic factor (BDNF) expression in the Morris water maze

From insects to humans, successful navigation relies on retained representations of spatial relations. These representations are thought to depend on the hippocampal formation, particularly those that are independent of the navigator (allocentric representations). The Morris water maze is a simple and popular task often used to assess spatial navigation. But how animals navigate toward and retain information regarding the location of the goal in this task remains unclear. We provide a comprehensive account of how the water maze is accomplished behaviourally. Our findings suggest that animals solve the task using distal cues via an initial view-matching strategy that is supported by egocentric guidance. Through increased training, however, an emergence of an egocentric-guiding strategy combined with the animal’s greater ability to infer the hidden platform’s location (via allocentric extrapolation) emerges. We also demonstrate that behavioural changes, towards a more allocentric strategy, are reflected in increases in hippocampal brain-derived neurotrophic factor.     

Effects of ketamine on tunnel maze and water maze performance in the rat

The NMDA receptor, which has been implicated in memory formation, is noncompetitively blocked by ketamine. The present study examines the effect of ketamine (0, 3, 6, 12, and 25 mg/kg body wt; ip) on tunnel maze and water maze performance in Wistar rats. In the hexagonal tunnel maze (HTM) high doses of ketamine (12 and 25 mg/kg) decreased locomotor activity. Moreover, ketamine induced perimeter walking (6, 12, and 25 mg/kg) and attenuated exploratory efficiency (25 mg/kg). When the HTM was converted into a modified six-arm radial maze, ketamine impaired short-term but not long-term memory. In the Morris water maze, rats injected with ketamine (12 and 25 mg/kg) acquired a spatial navigation task more slowly than controls. When the escape platform was removed, the drug-treated rats did not preferentially search for it in the area where the platform had been during the acquisition phase. However, when the escape platform was visible, no differences in the performance of ketamine-treated and control rats could be found. In summary, ketamine seems to attenuate some but not all forms of learning in the tunnel maze and it impairs the acquisition of a spatial navigation task.     

Emergence of a cue strategy preference on the water maze task in aged C57B6 x SJL F1 hybrid mice

The effects of age on cue learning, spatial reference memory, and strategy preference were assessed in B6 × SJL F1 mice by using the Morris water maze. This mouse strain is of particular interest because it is the background strain for a common transgenic model of Alzheimer's disease, the Tg2576 mouse, which develops plaques and other neurobiological markers of pathology beginning at 8 mo and increasing in severity with advanced age. In the current study, 12- and 23-mo-old C57B6 × SJL F1 mice were serially trained in cue and place versions of the Morris water maze task. At the completion of training, mice received a strategy probe test in which place (hidden) and cue (visible) strategies were in competition. Cue and spatial learning ability was maintained between 12 and 23 mo of age; however, on the strategy preference probe test, the 23-mo-old mice exhibited a significant bias toward the selection of a cue strategy. There was no relationship between strategy preference in the probe test and spatial learning ability, but the 23-mo-old mice did exhibit a strong trend toward shorter latencies during visible platform training, possibly reflecting the enhanced function of striatal-based neural systems in aging. These data demonstrate that 23-mo-old C57B6 × SJL F1 mice are capable of effective place learning, but if a place strategy is pitted against the use of a cue strategy, the use of a cue strategy predominates in the aged mice. The strategy preference observed here may reflect an emergence of differential processing in underlying brain circuitry with age in the B6 × SJL F1 mouse strain.     

Search strategies used by APP transgenic mice during navigation in the Morris water maze

TgCRND8 mice represent a transgenic mouse model of Alzheimer's disease, with onset of cognitive impairment and increasing amyloid-beta plaques in their brains at 12 weeks of age. In this study, the spatial memory in 25- to 30-week-old TgCRND8 mice was analyzed in two reference and one working memory Morris water maze (MWM) tests. In reference memory tests, the mice were trained to escape to a hidden platform, which in one version of the test was marked by a visual cue. In the working memory test, the hidden platform was moved daily to different locations. The TgCRND8 mice were impaired in reference memory when trained in a hidden platform test. However, the mice developed spatial memory comparable to non-Tg littermates in a cued reference memory test. The mice showed also an impairment in spatial working memory. Analysis of search paths revealed that in contrast to non-Tg littermates, TgCRND8 mice did not use spatial strategies during their navigation. Instead, they learned to locate an escape platform using a nonspatial, chaining strategy. The study showed that (1) the impairment in the reference memory of TgCRND8 mice was reduced when a hidden platform was cued, and that (2) both working and reference memory systems of TgCRND8 mice, but not (3) the plasticity of choice between search strategies, are compromised by the transgene-induced pathology.