On The Nature Of Neural Activity In The Hippocampal Formation: Effects Of Periodic And/Or Acute Environmental Enrichment On C-FOS Expression In Adolescent Rats

The interaction with novel objects and conspecifics, known as environmental enrichment (EE), provides a lasting impact on the brain and behavior of animals. EE alters networks associated with spatial learning and memory. The influence of periodic and/or acute EE exposure on activation in the hippocampal formation (HF) of adolescent rats was investigated. One group (EE-No; n=6) received twenty, 90-min EE exposures. A different group (EE-EE; n=6) received this same periodic EE, along with an acute 90-min exposure before perfusion. A third group (No-EE; n=6) solely received acute EE before perfusion. Controls (No-No; n=6) did not receive any EE. Tissue was processed using immunohistochemistry to visualize c-FOS expression; densities of c-FOS+ neurons were quantified via microscopy. Relative to controls, acute EE produced more c-FOS+ neurons in CA3 (+172%), CA2 (+136%), CA1 (+183%), and subiculum (+164%) (p<.03). Those that received periodic and acute EE (EE-EE) exhibited reductions in c-FOS expression among CA3 (-43%), CA2 (-47%), CA1 (-64%), and subiculum (-41%) (p<.03) compared to No-EE rats. The results suggest an EE history in adolescence suppresses activation as a signal moves sequentially throughout the HF. Acute EE may increase recruitment of neurons in sequence of the HF, without necessarily increasing input to this pathway.