3. Stress by type of stressor

The type of stressor influences the location of 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 in particular on the dopaminergic system can be found under Dopamine and stress In the section Dopamine in the chapter Neurological aspects.

• 3.1. Stress Logs
  • 3.1.1. Chronic restraint stress (CRS)
  • 3.1.2. Chronic social defeat stress (CSDS)
  • 3.1.3. Chronic (unpredictable) mild stress (chronic (unpredictable) mild stress, CUMS, CMS)
  • 3.1.4. Tail Suspension Test (TST)
• 3.2. Homotypic vs. heterotypic stress

3.1. Stress Logs¶

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

3.1.1. Chronic restraint stress (CRS)¶

Under the chronic immobilization stress protocol, animals are kept in a (ventilated and transparent) restraint device for 2 to 6 hours per day for 10 to 28 days. Chronic immobilization stress triggers rodent depression symptoms as 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 each new resident. After this CSDS exposure, the time of social interaction is measured.¹
Two phenotypes emerge:

• susceptible mice that develop depressive behaviors as evidenced by decreased social interaction
• resistant mice that do not develop depressive behaviors

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

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

• Night lighting
• Tilting the cage
• damp litter
• unpleasant noises
• Cage change
• Feed/water deprivation
• etc.

This stress protocol elicits persistent depressive behaviors and appears to mimic the stress-induced depression observed in depressed patients.

3.1.4. Tail Suspension Test (TST)¶

The tail of the mouse is suspended from a smooth plexiglass plate and the mouse was hung on the roof of a soundproof box. The movements of the animals are recorded for a few minutes with a camera.³

3.2. Homotypic vs. heterotypic stress¶

Heterotypic stress is composed of multiple stress experiences of different stressor types. Homotypic stress, on the other hand, consists of repeated stress experiences of one stressor type.
A sequence of different stressful experiences causes different adaptive responses in heterotypic stress and homotypic stress.
In homotypic stress, a healthy adaptive response occurs far more frequently, in which the stress system responds to the repetitive stressors through progressively weaker stress responses. In heterotypic stress, on the other hand, adaptation of the stress response is more often absent. This remains equally pronounced even with repeated stressful experiences (maladaptation).⁴