Rodent Models of Depression

TECHNOLOGY UPDATE Rodent Models of Depression GUY B. MULDER, DVM, MS, DIPLOMATE, ACLAM1 AND KATHLEEN PRITCHETT, DVM, DIPLOMATE, ACLAM2 Purpose. Clinical depression is an important social and economic problem. Depression can be characterized by three primary core symptoms: anhedonia (loss of pleasure or interest in most activities), depressed mood, and decreased energy levels or fatigue (1). How- ever, the symptoms of depression can take many forms and range in severity from mild to life-threatening. Other symptoms may include changes in sleep pattern, changes in food intake, pessimism, feelings of guilt, or thoughts of suicide. The wide array of symptoms associated with clinical depression, many of which are emotional in nature, makes it difficult to develop and assess animal models of the condition. More than 35 years ago, however, researchers developed a set of criteria to assist in the identification of animal models of depression (2). Their criteria for an animal model of depression were that the model should show symptomology reasonably analogous to the hu- man condition; that behavioral changes in the animal could be objectively monitored; that resultant behavioral changes could be reversed by therapies effective in humans; and that the model was reproducible. These criteria are still valid today, and in the interven- ing years, a number of animal models have met these criteria. Although the core symptoms of depression are associated with changes in higher emotional states such as mood and motivation, a number of rodent models exhibit behaviors associated with depression. These behaviors are often responsive to the effects of antidepressant therapy. Rodent models of depression are based on one of several paradigms; pharmacologic exposure, stress, learned helplessness, neurological lesion, or genetic manipulation (3). One of the earliest models of depression was a pharmacologic model using reserpine (2). Reserpine exposure produces sedation and decreases in brain levels of norepinephrine, serotonin, and dopamine. More recently, pharmacologic models based on drug withdrawal have been described such as amphetamine withdrawal (4). The most common rodent models of depression are the forced swim test, tail suspension test, olfactory bulbectomy, and learned helplessness. Genetically altered mice are also being developed with depressive and anti-depressive Figure 1. Plexiglass cylinder used in forced swim test of rodents. This device phenotypes and such strains are being assessed using the forced swim includes base and adjustable shelf for mounting of infrared photobeams for test and tail suspension test (3). automated data collection when connected to a personal computer with Methods. (i) Forced swim test. The forced swim test (FST), origi- appropriate software. (Photo courtesy of Hamilton Kinder, Poway, Calif.). nally described by Porsolt for use with rats, is commonly used in mice as well (5-7). The FST is widely used because of its ease of bility in each animal. Twenty-four hours after habituation, the ani- administration, repeatability between laboratories, and the ability of mal is placed in the water-filled cylinder and observed for 5 min. The the induced depressive-related behavior to respond to a wide array of time the animal spends immobile, swimming, and climbing is then antidepressant medications. The test is based on the observation that recorded. Administration of antidepressant medications before test- mice and rats placed in a cylinder of water from which escape is ing results in a decrease in the amount of time an animal spends impossible (Fig. 1) develop a characteristic immobile posture origi- immobile and an increase in the times spent swimming and climb- nally described as behavioral despair. Initially, a rat is placed in a ing. The mouse version of the FST is similar, but uses a single test water-filled transparent plastic cylinder approximately 20 cm in diam- period. Animals are placed into water-filled clear plastic cylinders eter and 40 cm high. The cylinder is filled to approximately 13 cm high (approximately 13 cm in diameter and 24 cm high) containing 10 cm of 22 C water. The behavior of the mice is assessed for the last 4 with 20°C water. The rat is habituated to the apparatus with an ° initial 15 min exposure (6). Habituation is required to acquaint the min of a 6 min test period. Fully automated systems are also avail- animal with the test apparatus and to produce a stable level of immo- able that employ infrared photoelectric beams and specialized software to collect data on an animals activity and immobility. (ii) Tail suspension test. The tail suspension test (TST) was devel- University Laboratory Animal Resources, University of CaliforniaIrvine, 147 BSA, Irvine, California 92612-13101; Charles River Laboratories, 251 Ballardvale St., Wilmington, oped for mice in the 1980s as an alternative to the FST described Massachusetts 01887-10002 above (6, 8). In the TST, mice are suspended by the tail for a period 52 CONTEMPORARY TOPICS © 2004 by the American Association for Laboratory Animal Science Volume 43, No. 6 / November 2004 with antidepressant medications. The primary behavioral change caused by olfactory bulbectomy is a hyperactivity response in a novel, brightly lit open field apparatus. This behavior response can only be reversed by chronic administration of antidepressant medications. The OB model is produced by bilateral removal of the olfactory bulbs and although it is most commonly applied to the rat, it has also been described in the mouse and hamster (3, 9). The olfactory bulbs are rostral (forward) extensions of the telencephalon and can constitute 4% of the total brain mass of the rat. The bulbectomy is performed following general anesthesia in the rat. The surgical procedure involves drilling 2 mm burr holes in the frontal bones overlying each olfactory bulb. The olfactory bulbs may be seen through the burr holes, and the tissue aspirated by suction. The incisions are then closed, and the animals are allowed two weeks for full recovery and development of the bulbectomy syndrome. Sham operated animals have a similar procedure performed without aspiration of the olfactory bulbs. A typical study with OB rats involves treating OB and sham operated rats for 14 days with an antidepressant, followed by behavioral testing in an open field apparatus. (iv) Learned helplessness. The learned helplessness model is based on the observation that a rat or mouse which has been exposed to repeated electrical shocks over which it has no control will subse- quently show a deficit in escape behavior when presented with a situation in which they can escape such shocks. The escape deficits exhibited by these animals conditioned to helplessness are reversible by treatment with various antidepressant medications (3, 6). The protocols required to produce the model are fairly involved. Initially, three animals (restraint, escape and yoked) are selected and placed in either individual restraint boxes for rats or a shuttle box for mice. For example, rats may be placed in a restraint box equipped with running wheels, termed a wheel turn box (6). Each rat has electrodes attached to its tail with adhesive tape. The electrodes attached to the restraint animal do not connect to a shock generator. The escape animal electrodes are connected to the shock generator, and when a shock is administered, the animal can escape or terminate the shock Figure 2. Tail suspension apparatus. A mouse is suspended by the tail from by turning the running wheel. The yoked animal is also attached to the hook projecting from the roof of the chamber. Note that the system the shock generator, but the shock is controlled by the activity of the shown can be automated by connecting it to a computer with appropriate escape animal. Consequently, the yoked animal cannot end the software to measure number and duration of escape events (struggling epi- shocks through its own actions. Shocks are administered repeatedly sodes), as well as strength of each event. (Photo courtesy of Hamilton Kinder, with various intensity (i.e. 1.0-2.0 mA) and delays (i.e. 5-120 sec) Poway, Calif.). between shocks for up to 80 trials. Animals are then removed from the apparatus and returned to regular housing for 24 h. of time until they display bouts of immobility. Similar to the FST, The rats are then tested for the acquisition of learned helplessness immobility induced by tail suspension has been shown to be sensi- behavior by the use of a two chambered shuttle box (Fig. 3). A shuttle tive to the effects of many antidepressants. Unlike the FST, however, box is a multi-chambered box with a steel grid floor for delivering the TST is not associated with the possible problem of hypothermia electric shocks. The chambers are divided by a partition with an open- and may be less stressful to the animals. The TST is performed as a ing, such as a door, that can be controlled (opened and closed) to single event. At test time, adhesive tape is wrapped around the distal limit access between the chambers. An animal is placed in the shuttle tail approximately three-quarters of the distance from the base of the box which is connected to a shock generator. After a brief habitua- tail. The animal is then suspended by attaching the tape on the tail to tion period, a series of trials is performed. A trial consists of the the suspension hook of the testing apparatus (Fig. 2). Care is taken production of a sound followed approximately 5 sec later by an elec- to insure the animal hangs with its tail as straight as possible. The tric shock delivered by the grid floor. The shock is terminated when animal is then observed for 6 min, and the amount of time spent the animal crosses from one chamber to the other, or after a set pe- either struggling to escape or immobile is recorded.

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