D-aspartic acid is present in neurons and synapses in the brain, has a similar structure to the neurotransmitter N-methyl-D-aspartate (NMDA), and can bind to NMDA receptors. Therefore, it acts as a neurotransmitter/neuromodulator.[1][2][3] For example, D-aspartic acid directly affects neuroendocrine function in the hypothalamus and pituitary gland in the brain, causing the secretion of several hormones, including gonadotropin-releasing hormone, prolactin, luteinizing hormone, and growth hormone.[1][2][3] Additionally, D-aspartic acid directly affects cells in the testes, causing testosterone secretion.[1][2][3] However, many of these actions have only been detected in cell culture (in vitro) and/or animal studies, so their biological relevance in humans is not completely understood.