Effects of Environmental Enrichment or Chronic Unpredictable Mild Stress on Motivated Behaviors In Male and Female Adolescent Rats

Environmental enrichment has positive influences on brain physiology and behavior, including enhanced plasticity and improved cognitive, emotional, and social performance. Conversely, stressful environments (especially when unpredictable) impair these functions. In this study, we use a 2 (sex: male, female) X 3 (housing: standard, enriched, stressful) factorial design. Enriched environments are larger than standard cages to allow for 4-6 rats per cage (social enrichment), and the addition of objects for exploration/play. The stressful environment occurs in standard caging and includes a series of mildly stressful, unpredictable, daily events. The housing manipulations occur from postnatal day 21-42. At the end of this three week period, all rats are pair-housed in standard cages for the remainder of the experiment. Behavioral assessments for anxiety, pain sensitivity, and fear are conducted during adolescence using an open field test, hot plate testing, and auditory fear conditioning/extinction. Preliminary results indicate that females demonstrate lower hot plate latencies independent of housing, suggesting increased sensitivity to pain. In the open field, enriched males spend less time in the thigmotaxic zone compared to all other males and females. This suggests that enrichment reduces anxiety in males but not females. Male rats, regardless of housing condition, demonstrate greater contextual fear conditioning. During the first tone extinction session, enriched animals trend toward greater freezing while rats from the stressful housing condition trend toward less freezing compared to standard housing. The rate of within-session extinction does not differ as a function of either sex or housing condition. When comparing freezing during the first tone on each of the four extinction days (between-session extinction), standard housing rats tend to freeze more compared to the enriched and stressful housing groups. Additional analyses are ongoing.

In this study, we useda 2 (sex: male vs. female) X 3 (environment: standard vs. enriched vs. stressful) factorial design. The pups were weaned at 21 days of age and immediately placed into standard caging, an enriched environment, or a stressful environment for 21 days (42 days of age). Enriched environments were be larger than standard cages to allow for more rats per cage (social enrichment), as well as the addition of objects for exploration/play (e.g., running wheels, tunnels, ladders, nesting material, etc.). The stressful environment occured in standard caging and included a series of mildly stressful events (e.g., cage tilt, wet bedding, disrupted light cycle, etc.) for 24-hour periods throughout the three weeks. At the end of the three week housing manipulation period (42 days of age), all rats were be pair-housed in standard cages for the remainder of the experiment. Behavioral assessments (anxious-like behavior, pain sensitivity, fear memory and learning) will be conducted from 45-60 days of age. 

The tests conducted were:

1. Open field test for anxious behavior

2. Hot plate for pain analgesia

3. Fear conditioning, context test, and tone test for fear memory and learning

4 animals excluded from Open Field data analysis due to bad tracking by the AnyMaze software.

With 21 animals analyzed, there appears to be a sex difference between the animals with females exhibiting increased anxious behavior, decreased pain tolerance, and decreased fear responses. There also appears to be a housing difference between enriched animals exhibited increased learning and fear memory and stressed animals exhibiting decreased learning and fear memory. We plan to include more animals in this study to see if we observe a more prominent sex x housing interaction.

Dandi Ε, Spandou E, Dalla C, Tata DA. (2023). Τhe neuroprotective role of environmental enrichment against behavioral, morphological, neuroendocrine and molecular changes following chronic unpredictable mild stress: A systematic review. Eur J Neurosci., 58(4):3003-3025.

Pryce, C. R., & Feldon, J. (2003). Long-term neurobehavioural impact of the postnatal environment in rats: manipulations, effects and mediating mechanisms. Neuroscience & Biobehavioral Reviews, 1–2, 57–71.

Willner, P. (1997). Validity, reliability and utility of the chronic mild stress model of depression: a 10-year review and evaluation. Psychopharmacology, 4, 319–329. 

This research was approved by the Miami University IACUC: Protocol 1112_2028_Jun.