2.5.1. Novel Object Recognition Test

The novel object recognition (NOR) test, used to assess learning and memory, is based on the innate preference for novelty that mice exhibit, where mice that remember the familiar object will take more time exploring a novel object. This test was accomplished by following the procedure described by Antunes and Biala [30]. The test took place over three consecutive days: habituation day, training day, and test day (days 20, 21, and 22, respectively). On habituation day, mice were placed individually into a wooden box (40 cm × 40 cm × 40 cm) and were allowed to explore the environment (without objects) for 10 min. On training day, mice were placed individually in the middle of the box and given 10 min to visually explore two identical objects placed along the diagonal of the box (one in the northwest corner and one in the southeast corner). On testing day, one of the previously explored objects was replaced by a novel object (the diagonal used on the training day was the same as that used on the testing day), and each mouse was given 3 min to explore the objects in the box. Objects that are simply discriminated by mice with the presence of protrusions/intrusions on the surface and have a comparable degree of complexity (texture, shape, color patterning, and brightness, etc.) were selected in order to diminish bias due to object preference [31]. Moreover, objects that are mouse-sized or only slightly larger to encourage exploration were used, and they were made of nonbreakable material, to avoid object damage during experimentation and hence interfere with the continuing testing and to avoid animals' injury. Great attention was directed to the object odors; thus, multiple copies of the sample objects were available and each copy was used once only for the same animal and all objects were cleaned with 70% ethanol after each test to eliminate possible bias due to odors left by previous animals [32]. Exploration was defined as directing the nose to the object at a distance of less than 2 cm, while sitting on the object was not considered exploration. The animals' behavior was video recorded, and the following parameters were calculated:

(1) Discrimination Index. Difference in time exploring the novel and familiar objects divided by the total time spent exploring both objects (this value varies between −1 and +1, with a negative score indicating that the mouse spends more time exploring the familiar object, a zero-score indicating that the mouse has no preference, and a positive score indicating that the mouse spends more time exploring the novel object).

(2) Recognition Index. Time spent by the animal exploring the novel object as a percentage of the total exploration time.