DMT has been found in the cerebral spinal fluid.  Furthermore Barker et al. have shown that it does exhibit properties of a classical neurotransmitter.  It has also been found in peripheral tissue fluids.  it is found in both normal and psychotic patients with no significant differences.   Work is currently underway to advance and clarify the findings and discover its true role.

Due to its properties and presence in the CNS it is very likely that DMT is an endogenous transmitter.  While it is not known what this endogenous DMT does, several hypothesis have been proposed.  Remember cannabinoids and opiates were both  known before their endogenous systems were discovered and understood. Furthermore their activities told us something about the role their endogenous counter-parts play in physiology.  Strassman's views are theories and lack any real evidence.  There are other theories that are more logical in my eyes.  Strassman is a very logical person and is open to these other theories.  His views are taken as true by the new age pop culture. 

There are no known larger molecules that contain the DMT skeleton. There are other tryptamines but not methylated ones.  DMT maybe a metabolite of tryptamine (tryptamine receptors are known) although it is generally not a good idea to make active metabolites (unless they are not true metabolites).  Even though it is found in low levels this does not mean it is not an active transmitter.  The site of release and its local concentrations are all that matters.  Endogenous DMT may only be used at a small select number of neural synapses (regions).  Thus the low levels.