d-aspartate affects NMDA receptor-extracellular signal-regulated kinase pathway and upregulates androgen receptor expression in the rat testis

Previous studies have demonstrated that d-aspartic acid (d-Asp) has a role in regulating the release and synthesis of testosterone in rats. In this study, we investigated the molecular pathway by which this amino acid triggers its action in the rat testis. We found expression of N-Methyl-D-Aspartic Acid (NMDA) receptor messenger RNAs for NR1, NR2A, and NR2D receptor subunits. After d-Asp administration, NR1 and NR2A messenger RNA levels were significantly higher than those of controls, whereas NR2D levels remained unchanged. Expression of extracellular signal-regulated kinase (ERK) 1 protein was higher than that of ERK2 protein in the testis of both d-Asp-treated rats and controls. d-Asp administration increased testis levels of both phosphorylated ERK (P-ERK) 1 and 2. Using immunohistochemical technique, NR1 and P-ERK 1 or 2 proteins were preferentially localized within the spermatogonia. Moreover, d-Asp administration increased both serum and testis testosterone levels but not estradiol levels. Finally, in d-Asp-treated rats, testicular androgen receptor protein levels were significantly increased, whereas both estrogen receptor α and P-450 aromatase levels were significantly decreased. Conclusively, our results, besides strengthening the evidence that d-Asp administration in rats induces testosterone synthesis, demonstrate for the first time that d-Asp (1) induces testicular NMDA receptor-ERK pathway, (2) upregulates androgen receptor expression, and (3) downregulates estrogen receptor expression.