Effects of Environmental Enrichment on Rat Behavior in the Open Field Test

Abstract

The open field test (OFT) is used to measure exploratory behavior and activity in rodents. Behaviors such as thigmotaxis (staying close to the walls of the field) and defecation are cues of anxiety or fear while time spent in the center of the field and high locomotor activity indicate exploratory behavior. Environmental enrichment (EE) is often used to attenuate behavioral deficits after brain injury; however, less is known about behavior changes caused by EE in normal animals. Conducting an OFT on normal and enriched adolescent rats may provide insight into the effects EE has on exploration of a novel environment. Male (n=4) and female (n=4) Long-Evans rats were exposed to EE with familiar and unfamiliar same-sex conspecifics for 1.5 hours on a two days on one day off schedule for 30 days starting on post-natal day (pnd) 34. The enrichment scheme consisted of variable placements of toys (e.g., shoestring, tennis ball, hanging objects) in an enclosure with four levels connected by ramps. On pnd 65, rats were placed into a forced-entry open field, which consisted of a dimly lit 1-m2, 8x8 grid of squares. Non-enriched males (n=4) and females (n=4) were also tested. To control for anxiety, data was collected from the last 3 minutes of the 5 minute OFT. OFT measurements included: time spent in outer squares, time spent in inner squares, rate of movement, time spent grooming, and number of rears and wall climbs. Enriched rats spent 56.6% more time in inner squares than non-enriched rats (p<0.05) and spent 18.7% less time in the outer squares than non-enriched rats (p<0.05). Additionally, enriched rats entered 137% more inner squares than non-enriched rats (p<0.05). These results indicate increased exploratory behavior in adolescent, enriched rats in a novel environment. It is likely that repeated EE through exposure to an often changing environment aided in inhibition of initial fear response when introduced to a new novel environment. EE has been shown to activate structures of the medial prefrontal cortex, which is involved in fear response inhibition.