Research reportThe double-H maze test, a novel, simple, water-escape memory task: Acquisition, recall of recent and remote memory, and effects of systemic muscarinic or NMDA receptor blockade during training
Graphical abstract
Research highlights
▶ We validate a novel spatial memory test termed the double-H maze test. ▶ Post-acquisition trace degradation is weak, at least up to 18, and even 25 days. ▶ Appropriate protocols enable rats to be trained towards place- or response learning. ▶ Pre-acquisition muscarinic or NMDA receptor blockade alters memory formation. ▶ Place- and response-learning coexist; cues pilot the switch from one to the other.
Introduction
Spatial cognition, which relies upon declarative memory in humans, can be weakened in elderly (e.g. [1], [2]), and is markedly altered in Alzheimer's disease (AD) patients (e.g. [3], [4], [5], [6]). Similar alterations have been induced in laboratory rodents by lesions of selected brain regions or transmitter systems, or by the administration of drug treatments activating or blocking receptors of interest, among which cholinergic muscarinic receptors and glutamatergic NMDA receptors have awaked much interest (e.g. [7]). Spatial memory deficits have also been characterized in aged mice and rats, as well as in a variety of transgenic mice developed to reproduce one or more of the neurodegenerative features or histopathological signatures of various diseases (e.g. [8], [9]), or in which essential steps of learning-triggered intracellular signaling pathways have been knocked-out (e.g. [10], [11], [12]).
To characterize spatial memory in rodents, research usually assesses the effects of experimental treatments in a variety of maze tasks (e.g. [13], [14]). In these tasks, animals may achieve good performance by using strategies based on their acquired knowledge of the salient landmarks of their testing environment (a so-called “allocentric” strategy) or on bodily cues becoming central for the organization of displacements (a so-called “egocentric” strategy). These tasks can also be distinguished according to the degree of flexibility with which an animal may try to solve them. In some of them (e.g., the Stone maze), flexibility is weak: the task consists in acquiring the only correct route connecting a start point with a goal, leaving no space to alternative strategies or short cuts, reducing the spatial load on memory function in a training level-dependent manner, and facilitating the emergence of cognitive routines or motor response-based automatisms. In other tasks leaving more room to flexibility, such as Olton's radial maze [40] or the ziggurat maze [15] – formerly called the cone field task [16] – there are several goals, food (or other rewards) being provided at various locations. Good performance may be achieved by an allocentric or an egocentric strategy. In the largely used Morris water maze (e.g. [17]), a dry version of the latter (e.g. [18]) or the Barnes maze (e.g. [19]), animals have to learn a given location to which they have to navigate. Although the item to be learned and remembered is single, the search patterns and routes to this location are virtually infinite. Even with such tasks, animals do not necessarily use an allocentric strategy. Indeed, in e.g. the water maze, rats and mice can reach the escape point by swimming in circle along the pool border at about the distance from the border at which the platform is placed: they can know how to reach the platform without knowing precisely where it is immersed [20].
The problem with the tasks in which a specific route must be learned is that animals can solve them without having to use a spatial memory. In many if not all of the others, the problem is less that animals can solve them with alternative strategies, and thus without having to form a spatial memory, than the fact that the experimenter has no or relatively poor control over which spontaneous strategy an animal is going to develop during training. In addition, the allocentric solution to these tests, which are often used to screen the effects of cognition-enhancing drug candidates in preclinical approaches, requires relatively complex mental processing; if one goes back to the notion of model, especially of human memory systems, it is noteworthy that not all of our daily behaviours rely upon such complex operations. Under some instances, it might be interesting to know the effects of cognition-enhancing drug candidates on relatively simple behaviours. Regarding the aforementioned drawbacks on the use of alternative strategies, on the lack of control by the experimenter of an animal's strategy and on task complexity, the recently introduced starmaze (e.g. [21], [22]) appears an interesting compromise as, being a relatively simple navigation task preventing possible deviations from an ideal start-to-goal trajectory, it enables an extremely fine a posteriori analysis of an animal's spontaneous strategy during a retention test. However, as in the other tasks, the experimenter still has limited control over the strategy used by the animal to achieve good performance; for instance, mice can be forced into procedural routines, but the protocol is based on using a mobile goal, which is not very “ecological”. We therefore conceived a novel test device, which we call the double-H maze, and in which rodents have to learn to reach the location of an escape platform submerged in water, but the pathway possibilities from the start to the goal are limited to a reasonably low number and training may be adapted such as to shape an allocentric or an egocentric strategy.
Section snippets
Subjects
For the currently reported four experiments, we used a total of 219 adult, male Long-Evans rats weighing between 240 and 268 g at the start of each experiment. They were provided by the Centre d’Elevage R. Janvier, Le Genest St-Isle, France. All rats were kept in individual transparent Makrolon cages (42 cm × 26 cm × 15 cm) in temperature-controlled (23 ± 1 °C) rooms that were maintained on a 12: 12 h dark–light cycle (light on at 7:00 AM). All rats were housed with ad libitum access to food and water
Experiment 1: learning the place, retrieving the trace (post-acquisition delays of 1, 5 and 18 days)
The data for the acquisition period are shown in Fig. 4A–C. Concerning the number of initial and repetitive errors, a Delay (1d, 5d, 18d) × Day (1, 2, …,6) ANOVA showed no overall Delay effect (initial errors: F(2,24) = 0.28; repetitive errors: F(2,24) = 1.21) and no significant Delay × Day interaction (initial errors: F(10,120) = 0.37; repetitive errors: F(10,120) = 1.29), accounting for similar performances across groups. The overall Day effect (initial errors: F(5,120) = 87.72; repetitive errors: F(5,120) =
Discussion
The currently reported data validate the double-H maze test as a simple and rapidly acquired memory task requiring no prior motivation-inducing manipulations such as food or water deprivation. Our results show that (i) under conditions of sustained, drug-free training, performance accounting for a vivid remote memory (18 days post-acquisition) is not significantly degraded in comparison with recent memory (e.g., after 1 or 5 days post-acquisition); moreover (ii) only 2 days of training (2 × 3
Conclusions
The current series of experiments are the first ones which we carried out to validate a novel spatial memory test termed the double-H maze test. We believe that because this test enables the formation of a relatively stable memory trace and enables some control over an animal's strategy, it could be particularly adapted to study the neurobiological substrates of (and the dynamic interplay between) procedural, caudate-dependent and declarative-like, hippocampus-dependent memory functions.
Conflict of interest
The authors have no conflict of interest to declare.
Role of funding sources
Our funding sources had no involvement in study design, in the collection, analysis and interpretation of data, in the writing of the report, and in the decision to submit the paper for publication.
Acknowledgements
The authors are grateful to O. Bildstein, O. Egesi, G. Edomwony for care of animal colonies. They also acknowledge Zaepffel Manuel and Xavier Rezai for their outstanding help during the construction of the testing device, as well as Yanina Tsenkina for her help in the very first steps towards testing in the double-H maze. Finally, they thank Dr. A. Pereira de Vasconcelos for her critical reading of one of the last manuscript drafts. Research was supported by University of Strasbourg and CNRS.
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