As far as we know, DMT is NOT released before death. Dr. Strassman suggested that this MIGHT happen, and provided a mechanism by which is MIGHT occur, although no one has ever documented this happening.
What is more likely is that dying does similar things to your brain (depressing activity in certain areas of the brain) that DMT does.
PET scans of brains on psilocybin show that 4-HO-DMT depresses activity in areas associated with consciousness and the ego (the posterior cingulate cortex, to be exact), and it stands to reason that dying would depress similar areas (along with all the other ones, of course).
Blessings
~ND
"There are many paths up the same mountain."