Mice returned to their home cage get a cocaine-like dopamine boost, says study

A study conducted at Florida Atlantic University found that mice who were taken from the cage they lived in and later placed back in it caused a surge of dopamine "that mimics the response to a dose of cocaine."

From Science Daily:

The researchers questioned whether their findings reflected leaving an unappealing environment or if it truly was a response to the positive aspects of a known and safe environment.