ANXIETY

The open field test is classically used to assess anxiety in rodent. This test is based on conflicting innate tendencies of avoidance of bright light and open spaces (that ethologically mimic a situation of predator risk) and of exploring novel environment. When placed into a brightly lit open field for the first time, rats and mice tend to remain in the periphery of the apparatus or against the walls (thigmotaxis). It had been shown that anxiolytics administration increases exploration time in the center of the open field while stressful stimuli decrease the number of center visits. Open field activity, therefore, represents a valid measure of marked changes in “anxiety-like” behaviors in drug-treated and genetically manipulated animals. Open-field procedure can also be used for general assessment of animal basal locomotor activity and exploration. In these cases, non-stressful conditions are needed (habituation, low lighting conditions) and the experiment duration can be longer.

The elevated plus-maze is a widely used animal model of anxiety that is based on two conflicting innate tendencies: exploring a novel environment and avoiding elevated and open spaces constituting situations of predator risk. The apparatus consists of two open (stressful) and two enclosed (protecting) elevated arms that form a “plus” or cross. Time spent in exploring enclosed versus open arms indicates then the anxiety level of the animal. When placed into this apparatus, naïve mice and rats will, by nature, tend to explore less the open arms due its natural fear of heights and open spaces. In this context, anxiolytics generally increase the time spent exploring the open arms and anxiogenics have opposite effect, increasing the time spent into the closed arms.

The elevated-O-maze is a modification of the plus-maze and displays the advantage of lacking the ambiguous central area of the elevated plus-maze. As in the elevated-plus maze, this test is based on two conflicting innate tendencies: exploring a novel environment and avoiding elevated and open spaces constituting situations of predator risk. The apparatus consists of two open (stressful) and two enclosed (protecting) elevated arms that form a zero or circle. Time spent in exploring enclosed versus open arms indicates then the anxiety level of the animal. When placed into this apparatus, naïve mice and rats will, by nature, tend to explore less the open arms due its natural fear of heights and open spaces. In this context, anxiolytics generally increase the time spent exploring the open arms and anxiogenics have opposite effect, increasing the time spent into the closed arms.

The Black and white test (also named light-dark test) is based on the conflict of natural tendencies of rodents to avoid lighted and open areas and to explore novel environments. The apparatus contains a white opened compartment and a small enclosed black compartment. Relative time spent in exploring each compartment indicates the anxiety level of the animal: avoidance of the brightly lit area is considered reflecting “anxiety-like” behaviors. When treated with anxiolytic drugs, rodents spend more time in this area, an effect purportedly due to a decrease in anxiety.

The holeboard test is mainly used for assessing exploratory behaviors in rodents. The animal is placed on an arena with regularly arranged holes on the floor. Both frequency and duration of spontaneous elicited hole-poking behavior are then measured during a short period of time. This test also provides a simple method for assessing anxious response of a rodent to an unfamiliar environment. The use of the hole-board in this perspective relies on the hypothesis that the behavior of animals exposed to a novel situation results from competition between an exploratory tendency and a withdrawal tendency. Thus, a high level of anxiety results in decreased head-dipping behavior and inversely, a low level of anxiety manifests as increased head-dipping behavior.
Other associated behaviors can be evaluated during the hole board test, such as grooming, rearing and locomotion.

The Aron test, or four-plate test, allows a quick characterization of putative anxiolytics compounds in naïve animals. The test consists basically of setting animals into a square chamber, the floor of which was composed of four metal plates, and monitoring their locomotor behavior by counting the number of crossings from one plate to another. Exploration of the chamber was markedly suppressed in response in subjects that received electrical shock at each crossing compared to control animals that are not shocked during their exploration.

The Vogel test paradigm is a popular conflict model in which water-deprived rats and mice first learn to lick from a water spout in an operant chamber. Then, usually after a period of unpunished licking, responses are punished with mild footshocks, inducing a significant reduction of drinking. In this context, administration of anxiolytics is shown to inhibit shock-induced drinking suppression.

The Geller-Seifter paradigm is a conflict model in which prior food-deprived rodents have to “choose” between consuming food and avoiding the punishment associated with this consumption. Basically, periods of responding by lever pressing to get sweetened condensed milk on a variable interval schedule are interrupted by periods where, in the presence of a signal, reward can be obtained more frequently but are accompanied by mild footshocks. Levels of punished responding are inversely proportional to the level of anxiety. Then, in this context, the shock-induced reduction of reward seeking can be reversed by administration of anxiolytics.

The social interaction test by pairs provides a popular and standard paradigm to study general social behavior. This test consists in allowing the experimental subject freely exploring an unfamiliar congener in its homecage or in a neutral environment. Social exploration is measured by the time spent by the experimental subject around the congener as well as the amount and duration of behaviors that compose social interaction (e.g. sniffing, following, allogrooming, biting, mounting, wrestling…). Social avoidance behavior is used in a wide variety of models, for instance for assessing neophobia anxiety and depression-like behaviors.