We evaluate antidepressants by a model called as "learned helplessness". Words that "work hard!" are usually said contraindication to depressive patients. The patients worked harder and harder and then became depression after they could not work more. Therefore, if we say "work hard" to the patients, it may make their depressive states worse. Thus, we will do nothing (helplessness) since we learn that cannot work hard and cannot overcome, which is called as learned helplessness. We can experimentally make leaned helplessness in rodents which is used as an animal model of depression.

3-1 Forced Swimming

A forced swimming test is an evaluation and screening method for antidepressants relatively easy and widely used. Rodents are put into water in a cylindrical container and measured their immobility time. The immobility is floating state without movements other than breath and minimum movement to float. It is considered that the rodents learned impossible escape from water and gave up swimming to escape. Antidepressants decrease the immobility time which is thought to mean mental recovery from helplessness. Antidepressants clinically elicit effects after a chronic intake and also show potent effects by chronic administrations in the forced swimming test. The immobility time is generally measured for a short time, e.g. 5 or 10 min, and can be measured by manual observation with a stopwatch but automatic measuring devices are also available.

Mouse Forced Swimming Test
A photo cited from Lafayette Instrument

The automatic devices are measured the immobility in rodents by infrared sensors (horizontal beam or pyroelectric sensors) or a video tracking including automatic PC analysis. We are relatively easy to set the devices by infrared sensors but must set a camera in consideration of lighting and reflection for automatic analysis by PC software in the video tracking. The video images are generally transformed to binary (monochrome) images and a rodent is detected by its differences from backgrounds in brightness. If we put white rats into a black measuring cage, the darker is the background and the brighter is the rodent. If we put black mice into white nesting materials, the darker is the rodent and the brighter is the background. Because there is reflection of light on water surface which is changeable by surface movement, the rodent in water is relatively difficult condition for correct detection by the video tracking. A tank should be a dark color at a bottom with less reflection for white mice and rats and have no direct light on the water surface. We must set experimental instruments including a camera to detect swimming (movements) by the rodents correctly before the forced swimming test by the video tracking. If the video tracking can detect correctly by such settings, it may detect immobility more strictly than the infrared sensors detect it. The video tracking makes us possible for re-analysis by recorded video images and manual analysis by observing the video film. Thus, the video tracking has several merits in spite of difficult settings.

We must lower water surface rodents not to go out the container in the forced swimming test because make the animals learn that they cannot escape it. Rats' tail may reach a bottom of the container by a height of water surface but it does not mean we cannot conduct the forced swimming test. The height of water surface that does not reach the bottom is considerably high and makes less easy the procedures of the test by its weight and size. Therefore, we may be not requested the height not to reach the bottom. A water temperature is very important because the lower makes less swimming. We must keep the same temperature during the measurement in all animals. Because general instruments for the forced swimming have no thermal retention systems, we actually keep a temperature in a measuring room and keep the water temperature same at the room temperature. It is easy that water in the container is left in the measuring room until the water temperature reaches same as the room before the measurement. Water change after each test in one rodent is ideal but not practical since we hardly prepare plenty water with the same temperature. Therefore, we usually pick up feces after one trial and conduct a next trial in another rodent.

If you conduct the forced swimming test, you will feel fairly heavy water and a bother to care for wet rodents (wipe water) and realize laborious experiments with water. A tail suspension test is the test measuring immobility time similar as the forced swimming test under conditions that mice cannot escape. Mice are suspended by their tails literally and measured the immobility time which is shortened by antidepressants. Although the immobility is considered as leaned helplessness same as the forced swimming test, mice but not rats are used in this test because the heavier is more difficult to suspend.

3-2 Learned Helplessness Model

The forced swimming test and the tail suspension test are explained as leaned helplessness described above but electrical foot shocks in rodents are used as inescapable stimuli in so-called "learned helplessness". We measure it in a "shuttle cage" which used in active avoidance learning such as a shuttle avoidance test. The shuttle cage generally consist of two same compartments connected each other and divided by a guillotine door between both. Its floor consists of steel rods placed with the same interval to add electric foot shocks. A rodent put into either compartment go to the next compartment to escape from the electric foot shock. When the guillotine door is opened, the rodent can escape to the next compartment. However, when the guillotine door is closed the electrical foot shocks become inescapable. Learned helplessness is induced in mice and rats by inescapable foot shocks and their escape behaviors (move to the next compartment) are measured with the door opened as their helplessness. Antidepressants improve the helplessness and increase escape behaviors. Although we need expensive devices such as the shuttle cage and shock generator, the escape behavior in the shuttle cage can be correctly counted. Therefore, it is considered as a highly-objective parameter compare to the immobility time in the forced swimming test.

3-3 Aggressive Behavior

Depression is one of mood disorders and DSM-IV categorizes the mood disorder into a depressive disorder ("monopolar depression"), a bipolar disorder, and two pathogenetic disorders, mood disorders caused by general physical illness and by substances. The depressive disorder is so-called depression and discriminated by presence or absence of major depressive episodes and the bipolar disorder is so-called manic-depressive psychosis. Almost all day-long and almost every day-continuing depressive moods shown by self-explanation such as feel sad or empty and observation by another person e.g. tear drops are described in a criteria of major depressive episodes A(1). It is told as a note to these criteria that annoying moods are possible in children and adolescents. Some depressive patients are annoyed and irritable or aggressive.

Antidepressants are evaluated by aggressive behaviors in rodents. Two individually reared mice are divided into an intruder and a resident. If we put the intruder into the resident cage, the intruder shows attacking behaviors such as biting the resident. Antidepressants decrease time spent in the attacking behaviors (1) and the time becomes a parameter evaluating antidepressants. This method is called as a resident-intruder model. Antidepressants also suppressed a mouse-killing behavior (muricide) by rats, which was one of animal models of depression but currently have not been used by humane reasons. Muricide was observed in thiamine (vitamin B1)-deficient rats or olfactory bulbectomized rats.

3-4 Closing

We often say that there are no appropriate animal models for depression. The forced swimming test can detect relatively specific effects to antidepressants and is a relatively simple procedure and good for screening of antidepressants. However, there are many discussions whether immobility shows a leaned helplessness state or not and the leaned helplessness reflect depression or not. Aggressive behaviors are clinically observed in the other patients than depression and are lack of specificity as the animal model of depression. Because a pathogenesis of depression is assumed as repetitive over stresses, we widely study by observation of behavioral changes in rodents after they are loaded various stresses. Because stresses possibly cause various illnesses as well as depression, evaluations of mental and physical changes after the stresses are very important. Although mental changes in rodents are evaluated by their behaviors, behaviors considered as depression such as immobility and escape failures cannot be discriminated from general behavioral suppressions (no movement), which make difficult the evaluation of depression. Although depressive disorders are clinically classed into mood disorders same as bipolar disorders, there are no rodent models of bipolar disorders. Because intakes of antidepressants have risks converting to manic states, mood stabilizers are used for therapy of depressive states in bipolar disorders. It is very interested that a typical drug, lithium (Li), shows positive effects in animal models of depression such as the forced swimming test, the tail suspension test, and a learned helplessness model and the aggressive behavioral model (2). The manic states in bipolar disorders are often evaluated as increases in exploratory behaviors (locomotion) but the increases are not specific to the manic states. What rodents we use to evaluate their manic states is very important and abnormal behaviors in knock-out mice of a specific gene may become a good animal model of bipolar disorders (3)(4). We hope that studies in the animal model of the bipolar disorders expressing depressive episodes (5)bring novel findings to the rodent model of depression.