They found this tickle-happiness connection seems to go through the region responsible for touch — and depending on how active that part of the brain is, it can affect how ticklish the rats are.
Tickling fired it up so much that rats jumped for joy after humans played with them. Jolting this area with electricity made them laugh even when they weren't getting tickled. But putting them in stressful situations made that part of the brain quiet down — and they didn't respond as happily to being tickled.
One other interesting detail: When you tickle a human or a rat, both of their brains light up in the same spot.