Methods for increasing D-Serine concentration and reducing concentration of the toxic products of D-Serine oxidation, for enhancing learning, memory and/or cognition, or for treating schizophrenia, Alzheimer's disease, ataxia, or neuropathic pain, or preventing loss in neuronal function characteristic of neurodegenerative diseases involve administering to a subject in need of treatment a therapeutic amount of a compound of formula I, or a pharmaceutically acceptable salt or solvate thereof: ##STR00001## wherein Z.sup.1 is N or CR.sup.3; Z.sup.2 is N or CR.sup.4; Z.sup.3is O or S; A is hydrogen, alkyl or M.sup.+; M is aluminum, calcium, lithium, magnesium, potassium, sodium, zinc or a mixture thereof; R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are independently selected from hydrogen, alkyl, hydroxy alkoxy, aryl, acyl, halo, cyano, haloalkyl, NHCOOR.sup.5 and SO.sub.2NH.sub.2; R.sup.5 is aryl, arylalkyl, heteroaryl or heteroarylalkyl; at least one of R.sup.1, R.sup.2, R.sup.3 and R.sup.4 is other than hydrogen; and at least one of Z.sup.1 and Z.sup.2 is other than N.