An endopsychosin is a theoretical endogenous neurotransmitter or ligand for the sigma and PCP1 receptors in the brain. It was formerly known as 'angeldustin', named after the slang term for PCP. Many researchers believe that the chemical in question is a neuropeptide. Substance P was believed to be a possible endopsychosin, but that theory has since been rejected. Currently, a peptide named alpha-endopsychosin is the closest candidate.
It is theorized that an endopsychosin is a substance that the body secretes which mimics the effects of phencyclidine (and other analogues such as ketamine). This substance would be released in times of extreme stress, creating a "detached, dreamy feeling". Endopsychosin could also be involved in a number of paranormal phenomena, such as alien abduction and astral projection. This neurotransmitter is also very likely to play a role in the near-death experience (NDE).
Sources:
http://www.erowid.org/chemicals/dxm/faq/dxm_neuropharm.shtml
http://www.planetk.org.uk/nrdeth.html