Endocrine activity of d-aspartate in nonmammalian animals

D-Aspartate is a natural D-amino acid endogenously present in animal tissues and able to bind to NMDA receptors. The biological actions of D-aspartate have been demonstrated in the central nervous system and in peripheral tissue where the amino acid is under the tight control of two enzymes responsible for its synthesis and degradation, D-aspartate racemase and D-aspartate oxidase, respectively. Furthermore, D-aspartate intervenes on aromatase enzyme functioning. In the last 20 years, many reports have pointed out this D-amino acid as mediators of critical signals involved together with sex hormones in various aspects of animal and human reproduction. Although mammalian models have been mainly used in the investigation of the role of D-aspartate in reproduction, this amino acid has also been identified in the gonads of nonmammalian animals, in particular in amphibians, reptiles, and birds. The identification of its occurrence in reproductive cells and organs of vertebrates highlights the key role played by these endogenous compounds along the evolutionary axis. In this review we have summarized the current knowledge of the main actions of D-aspartate on female and male reproductive processes along the evolutionary axis of nonmammalian animals, focusing on the endocrine activity and discussing the interplay between D-aspartate functions and steroid hormones in regulating the reproduct