Adverse early life experiences can have a negative impact on behavior later in life. This can be recapitulated in rodents using maternal separation (MS) of the pups in the first few days after birth, resulting in anxiety and depressive behavior in the offspring. Furthermore, MS followed by mild chronic stress in adolescence can induce a more pronounced depression and/or anxiety phenotype 1.
Seven timed-pregnant dams will arrive in the facility at embryonic day 4 and will be housed under standard laboratory conditions. After the dams give birth, pups will be separated daily from postnatal day 2 until postnatal day 14 for 3 hours/day in the morning. As part of this procedure, the mother is removed from the home cage, and the home cage with the pups will be placed on a heating pad to prevent hypothermia. Alternatively, infrared lights can be used to keep the pups warm (30-33 ℃). The control pups are left undisturbed with their mothers. All offspring will then be weaned at postnatal day 21 and housed as singletons. The pups will be weighed twice per week to monitor the effect of stress on body weight. A stronger depression phenotype can also be induced by adding a mild chronic stress (CS) paradigm for 5 consecutive days (postnatal day 85-89) to the maternal separation. This addition involves a 4-hour restraint in a plastic tube during the extra induction days (MS + CS) 1. Behavioral assays measuring depression and/or anxiety phenotypes will be performed at the age of 10-12 weeks old. Depending on the hypothesis, therapeutic interventions can start at gestation or just before behavioral analysis. The summary of the experimental design is laid out below:
Maternal separation and mild chronic stress induce developing animals to exhibit symptoms related to depression and anxiety. Depression can be measured in rats by using several behavioral assays. The Sucrose Preference Test (SPT) measures anhedonia, which is expressed as reduced motivation or reduced ability to experience pleasure. Accordingly, rats experiencing anhedonia will reduce their consumption of, and typical preference for, a 3% solution of sucrose in tap water2. The Forced Swim Test (FST) measures the rat’s response to the fear of drowning and is considered the gold standard for assessing depression-like behaviors. Animals that show reduced measures of mobility are interpreted as being susceptible to negative mood states3 or increased levels of “learned helplessness.” The Elevated Plus maze (EPM) is the gold standard for measuring anxiety-like behaviors. The EPM is based on the animal’s natural aversion to open spaces and has been validated to assess anti-anxiety effects of pharmacological agents (e.g. benzodiazepines) and the genes involved in anxiety-related behaviors4, 5. Finally, the Open Field Arena (OFA) measures both locomotor activity and anxiety. As with the EPM, rodents display a natural aversion to brightly lit open spaces, but they also have a drive to explore potentially threatening stimuli. Therefore, decreased levels of anxiety are correlated with increased exploratory behavior, especially the center of the arena.
Neuroinflammatory responses in the brain-
The behavioral assays OFA, EPM, SPT and FST can be used to measure depressive- and anxiety-like behaviors in rodents. Treatment of rats with specific drugs can reduce immobility in the FST, increase sucrose intake after stress induction or enhance exploration in the OFA and EPM. These data show that rats are a good model to study depression and anxiety.