This article is from the WSSF 2015 AFRMA Rat & Mouse Tales news-magazine.

Did You Know?  There are 4 facial expressions to look for to see if rats are in pain—squinting eyes, nose/check flattening so there is no crease between the cheek and whisker bed, ear changes (fold, curl, angle forwards or outwards, resulting in a pointed shape), and whisker changes (whiskers move forward away from the face and look bunched).

You can read more in Rat Grimace Scale (RGS), The Manual.

Study The Rat Grimace Scale: a partially automated method for quantifying pain in the laboratory rat via facial expressions Mol Pain. 2011 Jul 29;7:55.