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3. Stress by type of stressor

3. Stress by type of stressor

The type of stressor influences the location of the neurophysiological changes in the brain.

Stress causes changes in the dopaminergic system. Different stressors lead to different changes in the dopaminergic system. The effects of different types of stress on the brain and the dopaminergic system in particular can be found at Dopamine and stress In the section Dopamine in the chapter Neurological aspects.

The different types of stress are called stress protocols in the laboratory application for researching stress in rodents. These include chronic immobilization stress, chronic social stress and chronic (unpredictable) mild stress.

3.1. Stress protocols

Most studies on stress examine stress in rodents under laboratory conditions. Different types of stress are caused by different treatments.

3.1.1. Chronic restraint stress (CRS)

According to the protocol for chronic immobilization stress, the animals are kept in a (ventilated and transparent) restraining device for 2 to 6 hours per day for 10 to 28 days. Chronic immobilization stress triggers rodent depression symptoms, which are measured by behavioral tests such as the sucrose preference test, the forced swim test and the tail suspension test.1

3.1.2. Chronic social defeat stress (CSDS)

The protocol for chronic social stress involves placing a single male (the intruder on which CSDS is tested) in the home cage of a larger male (aggressor, resident) for 5 to 10 minutes. There, the intruder is defeated by the resident animal. After this immediate physical interaction, the resident and intruder are kept in sensory contact for 24 hours. for 10 consecutive days, the intruders are exposed to the home cage of a new resident. After this CSDS exposure, the time of social interaction is measured.1
There are two phenotypes:

  • susceptible mice that develop depressive behaviors that manifest in reduced social interaction
  • resistant mice that do not develop depressive behaviors

3.1.3. Chronic (unpredictable) mild stress (CUMS, CMS)

The protocol for chronic (unpredictable) mild stress involves exposing rodents to a series of (only comparatively) mild unpredictable stressors in random order over 2 to 12 weeks, such as2

  • Night lighting
  • Tilting the cage
  • moist litter
  • unpleasant noises
  • Cage change
  • Food/water withdrawal
  • etc.

This stress protocol triggers persistent depressive behaviors and appears to mimic the stress-induced depression observed in depressed patients.
In another study on adolescent rats, it triggered ADHD and anxiety symptoms in adulthood.3

3.1.4. Tail suspension test (TST)

The tail of the mouse is suspended from a smooth Plexiglas plate and the mouse is hung from the roof of a soundproof box. The movements of the animals are recorded for several minutes with a camera.4

3.2. Homotypic vs. heterotypic stress

Heterotypical stress is made up of several stress experiences of different types of stressors. Homotypic stress, on the other hand, consists of repeated stress experiences of one type of stressor.
A sequence of different stress experiences causes different adaptation reactions in heterotypic stress and homotypic stress.
In the case of homotypic stress, a healthy adaptation reaction occurs far more frequently, in which the stress system reacts to the repetitive stressors with increasingly weaker stress reactions. In the case of heterotypic stress, on the other hand, the stress response often fails to adapt. This remains equally pronounced even with repeated stress experiences (maladaptation).5

3.3. Types of stress and ADHD subtypes

Different stressors correlated with different ADHD subtypes:

Neglect of supervisory duties that put the child at risk of physical or psychological harm correlated with ADHD-I (inattentive type, OR 1.6) as well as with ADHD-HI/ADHD-C (hyperactive type, OR 1.5).
In contrast, physical neglect only correlated significantly with ADHD-I (OR 2.1), but not with hyperactivity.6
ADHD-I correlated with:

  • sexual physical abuse (OR 2.6)
  • physical neglect (OR 2.1)
  • physical abuse (OR 1.6)
  • Neglect of the duty of supervision (OR 1.6)
    ADHD-HI/ADHD-C correlated with:
  • Neglect (OR 1.5)
  • physical abuse (OR 1.3)
  • not significantly against it:
    • physical neglect
    • sexual contact abuse