I'm guessing it's the production of DMT (similar to psilocybin and LSD) by the pineal gland that is being referred to. IIRC there's still a debate about whether it can cross the blood-brain barrier, but it seems quite likely. Ostensibly it's produced at birth and at death.

Please excuse the lack of sources, but the book The Spirit Molecule kickstarted this line of thought if you're interested in learning more.