The existence and endogenous production of NMDA receptors and their endogenous ligands (such as D-serine and D-aspartate) in mammalian bodies are well-established biological facts requiring no citation.
The claim refers to NMDA (or NMDA receptors and their endogenous agonists like D-serine/D-aspartate) being produced or occurring endogenously in the body. While papers like [1], [4], and [11] discuss endogenous D-aspartate and D-serine acting on NMDA receptors, the fundamental premise that the body possesses endogenous components of the NMDA system is a matter of basic biochemical and physiological fact (common knowledge) rather than a contestable hypothesis requiring empirical weighing against refutations.