PE 22-28 is a heptapeptide with the sequence GVSWGLR or GlyâÂÂValâÂÂSerâÂÂTrpâÂÂGlyâÂÂLeuâÂÂArg. It is a fragment of the endogenous TREK-1 antagonist spadin, and is the smallest fragment identified which shows similar TREK-1 inhibitory activity to spadin itself. It has been investigated as a potential antidepressant.