Release of neuroactive substances: homocysteic acid as an endogenous agonist of the NMDA receptor.

Sulfur containing amino acids such as homocysteic acid (HCA), cysteinsulfinic acid, homocysteinsulfinic acid are released by depolarization of slices from various rat brain regions in a Ca++-dependent manner. L-HCA excites caudate neurons through their N-methyl-D-aspartic acid (NMDA) receptor and potentiates their cortically evoked excitatory postsynaptic potentials. 35S-methionine can label the releasable pool of HCA, and thus appears as a precursor of HCA. Thus HCA is a transmitter candidate which acts predominantly on the NMDA receptor.