The Resident-intruder Paradigm: A Standardized Test for Aggression, Violence and Social Stress

The overall goal of this procedure is to reliably elicit offensive aggressive behavior in male rats. In a laboratory setting, the first step is to house an adult male rat together with a sterilized female for a period of at least one week. On the day of testing, video recording equipment is set up to record social interaction.

Next, the companion female is removed from the cage when ready, an unfamiliar male rat is introduced into the cage of the resident male, and they're allowed to interact for 10 minutes. Ultimately, the procedure is aimed at the establishment and maintenance of a territory by the resident's male that he is willing to defend against intruders. The strength of the method is that it allows the analysis of natural social behavior in a laboratory setting, not only in the resident male, but also in the intruder.

This method can help answering key questions about the neurobiology and neuropharmacology or biological functional aggression, as well as violence as the pathology Of aggression. The technique can be used to study the consequences of acute or chronic social stress in the intruder. Please notice that the resident intruder paradigm is not free from ethical concerns.

Serious suffering may occur in the intruder male potential. Animal suffering should always be in balance with the main scientific aims of the experiment. Offensive aggression is a highly active behavior, and therefore sufficient space is required for its full expression.

For optimal results, use resident cages that have at least a half a square meter of floor space to facilitate developing territoriality and reduce social isolation. House each resident with a female for at least one week before the start of the experiment. Use hormonally intact but sterile companion females.

This will ensure that the female will be receptive without becoming pregnant and developing maternal aggression. Since territoriality is strongly based on the presence of olfactory cues, do not change the cage bedding prior to or during testing. The intruder animal used in the following studies should be from a non-aggressive strain with a slightly lower body weight than the resident male.

Although aggression may occur at all times of day, it is best to test during the dark phase when the rat is most active. One hour before testing, remove the companion female from the resident cage. Set up a light sensitive video camera to record the resident's behavior for future analysis.

To begin the test, introduce an unfamiliar male intruder into the resident's home cage. Observe their behavioral interaction for 10 minutes. Offensive behaviors as well as social behaviors are recorded.

Shown here is rearing behavior and moving towards behavior. This is the lateral threat. This is the clinch attack, and this is a chase shown.

Here is a period of social exploration followed by a clinch attack. There is some anogenital sniffing. Now one rat moves towards the other.

This is the keep down behavior. Here is an upright posture. This behavior is the lateral threat.

Notice the raised hairs or pilo erection in the resident male, followed by the upright posture and some non-social exploration. Remove the intruder male from the cage at the end of testing and reunite the resident male with its companion female. The data are analyzed by playing back the video and scoring the behaviors with specialized software to measure violent behaviors.

Introduced an intruder as shown earlier and determine the attack latency of the resident male. A very short latency is a first indicator of violence. Bite attacks targeted at vulnerable body parts such as the throat, belly, and paw are also a sign of violence bites.

Targeted at the snout of the opponent indicate defensive behavior of the actor. In the case of a serious wound at vulnerable body parts, the experiment should be stopped. Depending on the severity.

This may be considered as a humanitarian endpoint or the intruder should be given appropriate medical care. In addition, violent behavior is demonstrated when a resident male attacks an anesthetized male intruder. The test with an anesthetized male can be stopped as soon as the resident attacks.

Aggressive behavior of the resident can also be tested in a novel environment. The absence of a change in aggression compared to the home cage is a sign of violence. To begin, select a well-trained, highly aggressive, non-violent resident male and follow the protocol demonstrated earlier.

Observe the intruder for various behaviors. After a period of non-social exploration, one rep moves away from the other, behaves submissively and takes flight. Shown here is a defensive upright posture.

After testing, return the intruder to their home cage and calculate the total defense score, which is the sum of the amount of time spent on flight. Defensive, upright posture, submission, and freezing. Social stress is achieved by following the same protocol for chronic social stress.

Keep the intruder in the resident's cage, but separated from the resident by a perforated screen. This figure shows the frequency distribution of the offensive aggression score in a laboratory bread, but originally feral wild type strain and a more common strain of laboratory rats in the wild type strain, about one third of the animals are extremely aggressive, whereas the highly aggressive phenotype is absent in wistar rats. This example shows the distribution of different behavioral categories in the resident intruder paradigm using the WTG strain as residents and the wistar rats as intruders.

Here, the resident intruder paradigm is used in conjunction with behavioral pharmacology. The selective serotonin one, A receptor agonist Alna spiron induces a dose dependent reduction in offensive aggression, which is accompanied by a dose dependent increase in social exploration. In some individuals, offensive aggression may escalate into a violent form of aggression.

The violent form of aggression is characterized by very short attack, latency attack of an anesthetized male or a female serious wounding, and a very low threat attack ratio. While using this procedure for social stress, it's important to use an aggressive strain of rats. As residents.

You have to select individuals that show offensive aggression during at least 30%of the test time. After watching this video, you should be able to correctly identify all the behaviors performed by both the resident and the intruder. Moreover, you should be able to recognize violence.

Don't forget that working with the resident intruder paradigm is not free from ethical concerns. Suffering of the intruder male should always have a clear relationship with the main aims of the experiment.