Unpredictable Chronic Mild Stress

Research on depression and on the effects of antidepressant (AD) drugs is hampered by the lack of a relevant animal models. In our lab, we designed the Unpredictable Chronic Mild Stress (UCMS) model in mice to be used as a model of depression. This model had previously been developped mainly in rats by P Willner. Here, we give some methodological details on this procedure. Further information can be found in Surget and Belzung (2008). More informtions in this reference Models of depression: unpredictable chronic mild stress in mice, 2013, Nollet M, Le Guisquet AM, Belzung C.

Which stressors can be used?

The UCMS-exposed mice are faced with several different stressors such as housing on damp sawdust (about 200ml of water for 100 g of sawdust), sawdust changing (replacement of the soiled sawdust by an equivalent volume of new sawdust), placement in an empty cage (usually the home cage of the subject, but with no sawdust), placement in an empty cage with water (the mouse is placed in its empty cage, whose bottom has been filled up with 1 cm high water at 21°C), switching cages (also sometimes termed as social stress: the mouse from a cage A is placed in the soiled cage from mouse B, mouse B itself being absent in order to avoid aggressive interactions), cage tilting (45º), predator sounds, introduction of rat or cats faeces as well as fur in the mouse home cage, inversion of the light/dark cycle, lights on for a short time during the dark phase or light off during the light phase, confinement in small tubes (diameter: 4 cm; length: 5 cm). These stressors have been described in our previous papers ( Ducottet et BELZUNG 2004; 2005; Ducottet et al, 2003; . 2004; Pothion et al, 2004; . . Yalcin et al , Nollet et al)

How can we measure the stress-induced behavioural effects? Different behavioural tests have been used to assess the UCMS-induced effects. All of these effects are reversed by chronic ADs in mice from the BALB/c strain which is the most tested in this model in reason of its high stress sensitivity.

Coat state

The coat state is recorded on 7 different parts of the body: head, neck, dorsal coat (back), ventral coat (abdomen), tail, forepaws and hindpaws). In each body zone, the score is 0 if in a good state (the fur is smooth and shiny, with no tousled, spiky patches), 1 if in a bad state (fluffy on most of the body with slight staining of the fur.), 0.5 if intermediate between the 0 and 1 (slightly "fluffy" with some spiky patches). So, a given subject may have a score comprised between 0 and 7. For example, on the picture bellow, the mouse from the picture on the left has a score of 0 and the one from the picture on the right has a score of 3.  

Reward maze test (RMT)

This test allows the measure of the motivation to obtain a reward. It consists to assess the motivation for a palatable stimulus (a chocolate cookie) by measuring the latency to chew the cookie and the number of chewing bouts during four sessions. The RMT requires a device containing three aligned chambers with the same dimension (119x19x20cm). Only the colours of the walls and the floor are different between the chambers: white for the first one, grey for the second one and black for the third one. The three chambers communicate by two openings with a door whose opening is controlled by the experimenters. The device is illuminated by a 200-lux white light. Four weeks and half before the first session, a small portion of a chocolate cookie (2g±1) is placed in the home cage every two days and during 2.5 weeks in order to familiarize the mice with the palatable stimulus. Therefore, the last two weeks before the first session are cookie-free. One hour before testing, food is removed in order to avoid inter-individual differences in the drive for feeding (hunger). At the time of testing, a small amount of chocolate cookie (or of regular food for control) is positioned at the centre of the black room. The white room is the compartment of departure and the mouse is placed with the head facing the opposite side to the opening. The test lasts five minutes; the door of the first opening is closed after the transition of the mouse within a maximum time of 2 minutes (at 2 minutes mouse is gently guided to the second room). Latency to chew and number of chewing within 3 minutes after the passage of the first opening are recorded as well as latency to pass the first opening (2 minutes maximum), the second opening (5 minutes maximum), latency to chew and number of chewing within the total 5 minutes. Four sessions of testing are performed within 9 days, each session being separated from the previous one by 3 days.

Splash test

Animals are first islotated in home cages for 24 hrs. A 10% sucrose solution is squirted on the dorsal coat of mice; the latency to initiate a grooming behaviour as well as the frequency and the duration of grooming is recorded during five minutes after the vaporisation of sucrose solution. In this situation termed as Splash test, the sucrose solution is chosen as a releaser and a persisting factor of grooming due to its viscosity. One generally observes that the latency to groom is increased in the UCMS mice while the frequency of grooming is decreased. On the videoclip bellow, the chronometer will start after 13 sec (that is once the splash has been applied and the animal is no more maintained by the experimenter). After 37 sec, the mouse displays the first grooming episode. Thus, the latency to groom is 37 - 13= 24 sec.

Nest building

Mice are isolated in their home cages. This test recquires to introduce pieces of cotton in the cage of the tested animal. This is done at the beginning of the activity period of the mice. The quality of the nest is scored 5hr, and 24 hr later according to the scale proposed by Deacon (2006).


Novelty suppression of feeding

This test is conducted in an opendfield (30x30x30 cm) whose floor has been covered by sawdust. A piece of regular pellet is placed on a sheet of paper in the midle of the device. Animals are isolated and food deprived for 12hr in order to increase motivation for food. The test is conducted under low light conditions. The dimensions of the openfield and the lighting conditions have been chooosen to avoid ceiling effects in stressed groups. Indeed, in order to observe approach behaviour in UCMS mice, it is necessary to use conditions that are not very stressful. The latency to approach the food, to sniff and to eat is measured. Further, the quantity of food eaten is measured in the home cage after the test in order to assess food motivation.

Other tests

Other behavioural tests can be done, including bio-assays such as forced swimming test, tail suspension test, sucrose preference, resident-intruder test.. Further biochemical assays can be done. For example, in order to assess the glucocorticoid levels and feedback of the neuroendocrine stress axis, additional measures can be conducted, including the measure of plasmatic and fecal corticosterone, the circadian variations of fecal corticosterone and the response in the dexamethasone suppression test.