INSERM U930, Universite Francois Rabelais Sciences et Techniques, Tours, France
Open-Field Test. Fig. 1. A mouse in a circular open field - Dimensions of the arena: 40 cm diameter and
30 cm high.
animals have also been allowed free access to the arena from their familiar nest.
In this free confrontation situation, rodents will generally show avoidance for the brightly lit environment: before entering the arena, the animal will show risk assessment postures directed toward the open field. This is a first argument indicating that the open field may be a stressful situation. In case of forced confrontation, the animal is placed in the apparatus and the following behavioral items are recorded, usually during five min: locomotion, frequency of rearing or leaning (sometimes termed vertical activity), and grooming. Increase of time spent in the central part, higher ratio of central/total locomotion, or decrease of the latency to enter the central part are indications of reduced anxiety-like behavior. Therefore, experimenters are not measuring the effects of the factors they are interested in on exploration, as is sometimes claimed, but the effects on the reaction of the subjects to a stressful event. Additionally, other parameters may be measured, including physiological, neural, or endocrinological ones. For example, using telemetry, it has been shown that the forced confrontation to the open field induces increased heart rate, further indicating that the animal is in a stressful situation. Immunohistological experiments, allowing measuring of the activation of various brain areas via c-fos, have shown that exposure to the arena induces activation of several structures belonging to the limbic system, such as the ► hippocampus, the ► amygdala, or the bed nucleus of stria terminalis, further suggesting that the rodent is stressed. This is accompanied by an activation of some hypothalamic nuclei, including the paraventricular nucleus. Finally, open-field exposure also induces a rise of plasma corticosterone, which is the main stress hormone. Taken together, this indicates that forced exposure to the open field induces a complex response, characterized by a specific behavior associated with physiological, endocrine, and neural modifications all suggesting that the animal has been stressed.
The open-field test is now one of the most popular procedure in behavioral neuroscience (see Belzung 1999; Prut and Belzung 2003), and is used to assess the effects of different factors on anxiety-like behavior, including the action of ► anxiolytic drugs, brain lesions, or genetic invalidation. In fact, it has become a convenient procedure to measure not only anxiety-like behaviors but also activity, enabling to detect sedation (decrease of activity) or hyperactivity. It can also be used to assess learning and memory. Several versions of the apparatus have been designed, differing in shape of the environment (circular, square, or rectangular), lighting, presence of objects within the arena, and so on. Utilization of this apparatus has been extended to a great number ofspecies including not only farm animals such as calves, pigs, lambs, rabbits, but also other species including invertebrates such as honeybees and lobsters. In fact, stressful experience in the open field is triggered by two factors: individual testing (the animal is separated from its social group) and forced novelty (the animal is subjected to an unknown environment, with no ability to turn back to its home cage or to predict the outcome of this confrontation; further, the arena is very large relative to the animal's home cage or natural environment consisting in small galleries). These two factors may respectively be interpreted as stressful only in gregarious species and/or in species that show fear of open spaces into which they are forced. This is precisely the case with rodents that live in groups and in small underground tunnels. This is of course not the case in species such as lambs or cows that live in large fields.
Behavior of rodents in the open field depends on several sensorial modalities, with a main involvement of tactile factors. Indeed, mice without vibrissae no longer display thigmotaxis, as they lose tactile contact with the walls. One must thus emphasize the possibility of misinterpretation of data related to effects of some treatments on the sensorial characteristics of the animals. As already indicated, exploration can be increased by some factors including food or water deprivation: it is therefore very important to verify that a given treatment does not act on such variables, before concluding the possible anti-stress effects. Finally, open-field behavior also depends on lighting conditions and upon the light-dark cycle (because exploration and food patrolling is increased during the animal's high-activity period, i.e., at the beginning of the dark phase) so that it may be relevant to ensure that a treatment does not modify internal clock-related behaviors and test the treatment under different lighting conditions.
Assessment of Drug's Effects on Anxiety Behavior
Using this device, the effects of different treatments have been investigated, mainly in the field of behavioral genetics (18% of the studies investigating the effects of targeted mutation of a given gene on anxiety-like behavior have been conducted using this device, a percent rising to 26%
for mutants of the serotoninergic system) and in the one of psychopharmacology. Concerning this last aspect, one may notice that many different drugs have been investigated in this situation, including compounds not only with effective or potential anxiolytic-like effects (► ben-zodiazepines, ► serotonin ligands, ► neuropeptides) but also compounds with psychostimulant (► amphetamine, ► cocaine), sedative (► neuroleptic), preictal (the state characterizing the prostration state induced by some epi-leptogenic drugs) activity or amnesic effects. An increased locomotion or time spent in the central part of the device without any modification of total locomotion has been interpreted as an anxiolytic-like effect while the contrary, which is a decrease of these variables, is associated with ► anxiogenic-like effects. Increased total locomotion (i.e., an increase of central and peripheral locomotion, in the same proportions) can be interpreted as hyperactivity while decreased rearing and locomotion are related to sedation or to postictal prostration. Concerning anxiety-like activity, the effects of three categories of drugs have been investigated, including compounds acting on the ► GABAA pentamer (not only benzodiazepine receptor ligands but also GABAA receptor agonists, ► barbiturate and ► neurosteroid ligands), serotoninergic-acting drugs such as ligands of the different 5-HT receptors or inhibitors of the serotonin transporter, and the effects of neuropeptidergic ligands (CRF, corticotropin-releasing factor; CCK, ► cholecystokinin; NK, neurokinin; neuro-peptide Y). Interestingly, the open field is able to detect the anxiolytic-like effects produced by compounds effective in normal anxiety behavior, such as classical benzo-diazepines and 5-HT1A receptor agonists, while it lacks sensibility for the anxiolytic action of atypical benzodia-zepines such as ► alprazolam or for the anxiolytic action of chronic antidepressants such as ► Selective Serotonin Reuptake Inhibitors, that display efficacy in some anxiety disorders such as panic, obsessional-compulsive disorder, social phobias, and post-traumatic stress disorder. It is to be emphasized here that benzodiazepines and 5-HT1A receptor agonists do not increase exploration as it is sometimes claimed; they act by reducing the stress-induced inhibition of exploration. Further, this device detects the effects of these compounds on anxiety-like behavior, and not on anxiety. Indeed, anxiety-like behavior is only one component of the anxiety-like response, which also includes expressive reactions (e.g., the vocal expression such as ► ultrasonic distress calls), physiological alterations (modifications in body temperature, heart rate, arterial pressure), as well as cognitive/ subjective aspects.
Assessment of Drug's Action on Locomotion (Sedation and Hyperactivity)
Concerning the ability to detect sedation, for example, after administration with high doses of benzodiazepines, ethanol, or neuroleptics, an important point to consider is that the free exploration situation is much more sensitive to the sedative effects of a treatment when compared with the forced situation. Indeed, in order to detect sedative effects, two-fold higher doses should be used in the forced situation. This can be explained by the fact that the forced confrontation imposes the experimenter to take the animal out of its home cage, which may induce awaking of an animal that would have been sleeping in its home cage. One may also mention the fact that the effects of a sedative treatment on rearing appear at lower doses than the ones on locomotion, indicating that this parameter is more sensitive. Sedative-like effects have to be distinguished from the pre-ictal prostration that can be observed after administration of some proconvulsive treatments. The phenomenological features of these two behaviors may seem identical, as animals exhibit a reduced exploration in both cases. Therefore, when reduced exploration is seen, the experimenter has always to be very prudent in the interpretation of data. Further, reduced exploration can also be observed if the rodent is freezing; but in this case, the experimenter will observe a very different posture, the rodent showing a tonic immobility, while when sedated displays a nontonic immobility.
Hyperactivity can also be detected using this device. ► Hyperactivity never corresponds to increased exploration. Indeed, after treatment with ► psychostimulants, the animal will show elevated locomotion, sometimes close to stereotypy (as it may repeat the same locomotion pattern several times) with reduced exploratory items (no sniffing, few rearing).
Assessment of Drug's Effects on Learning and Memory
The open field has also been used to detect the effects of some treatments on learning and memory. Three different processes can be assessed in this case: ► habituation, object recognition, and ► spatial memory. To test habituation, one can compare the behavior of the rodent during the first 5 min with the ones of the last 5 min of a 15-min session, enabling to detect the effect of a treatment on short-term habituation. Alternatively, one can confront a rodent to several 5-min sessions, separated by inter-trial intervals of some hours or several days. In this case, one can distinguish the effects of a given treatment on encoding from its effects on consolidation or restitution, depending upon the injection schedule. When testing object recognition using an open field, the subject is usually first introduced in the arena during a 5-min session. After an inter-session delay, the animal is further subjected to the open field, which now contains two identical objects. During a third session, the rodent is again introduced in the device, which contains one object identical to the one it has been confronted before and another one that is different. Higher exploration of the new object indicates that the animal has been able to remember the object it had seen before and is therefore considered an index of recognition memory. Pharmacological treatments can be tested either to detect promne-sic/amnesic effects, or to counteract the stress-induced decline in learning and memory (e.g., ► antidepressants are able to restore a stress-induced decline in object recognition). Finally, this situation can also be used to detect the effects of drugs on spatial memory. In this case, the rodent is confronted to the arena containing three identical objects, placed in a precise spatial configuration; (e.g., a line) and after a delay, it will again be introduced in the device, with the same objects, placed, for example, in a triangular configuration. In this case, exploration of all objects that had been declining after habituation will start again. This process can be altered with pharmacological treatments such as, for example, anticholinergic drugs.
In conclusion, the open field is a very popular and useful device, not only enabling detection of effects of drugs on anxiety behavior, but also action of pharmacological compounds on sedation, hyperactivity, learning, and memory.
► Anxiety: Animal Models
► Anxiety Disorders
► Phenotyping of Behavioral Characteristics
► Translational Research
Belzung C (1999) Measuring exploratory behavior. In: Crusio WE, Gerlai RT (eds) Handbook of molecular genetics techniques for brain and behavior research (techniques in the behavioral and neural sciences). Elsevier Science, Amsterdam Hall CS (1934) Emotional behavior in the rat. I. Defecation and urination as measures of individual differences in emotionality. J Comp Psychol 18:385-403 Prut L, Belzung C (2003) The open field as a paradigm to measure the effects of drugs on anxiety-like behaviors: a review. Eur J Pharmacol 463:3-33
Walsh RN, Cummins RA (1976) The open field test: a critical review. Psychol Bull 83:481-504
Open-label is a method of research in which the identity of the treatment is known by both the researchers administering the treatment and the subjects involved in the study.
► Impulse Control Disorders
A class of activities, all of which produce a common effect on the environment. A typical example is pressing a lever. The different forms of behavior that result in lever depression are members of the operant class.
Was this article helpful?
Finally, Retired Clinical Counsellor Reveals the Secrets Successful Psychiatrists and Psychologists Don't Want You to Know. How to Stop Your Depression Now Reclaim Yourself and Live Again Get the Depression Busting Tools You Need To Win the War Against Depression. Depression is an illness that many people often sweep under a rug. However if depression is left untreated... Your life can become a living nightmare. Depression is a growing epidemic in the US, but it never gets the urgent attention it deserves. You need help and you need it now.