Effects of D-Aspartate on sex hormone-dependent tissues in pelophylax esculentus

D-Aspartate (D-Asp) is an excitatory amino acid involved in the synthesis of sex steroid hormones. High concentrations of D-Asp were found in the gonads, in the brain and in the Harderian gland green frog, Pelophylax esculentus. This chapter reports the existing work on the D-Asp in gonads and in tissues target of the sex steroid hormones during seasonal reproductive cycle. Since D-Asp modulate the sex hormone synthesis, we correlate the local amino acid levels with serum and local sex hormone concentrations. A functional role of D-Asp in the activity of these organs is also assumed. D-Asp concentration in the gonads shows significant variations during the reproductive cycle. In the frog testis DAsp content is directly linked with gonadal and plasmatic levels of androgens showing the highest levels in sexually active males. Accordingly, star and steroidogenic enzyme genes (hsd 3b1, hsd 17b1, srd 5a1) are highly expressed in reproductive testis whereas cyp 19a is higher in post-reproductive one. Analysis of the proliferative pathway shows a major activity in reproductive phase, suggesting a higher spermatogenetic activity. Intra-peritoneal injection of D-Asp induced a rise of testosterone in post-reproductive phase; in reproductive males increased 17ß-estradiol levels. An inverse correlation has been demonstrated between D-Asp concentration in ovary and sex hormone levels. In vivo and in vitro experiments demonstrated that exogenous D-Asp accumulated in the ovary and induced a decrease of testosterone levels. D-Asp, chronically administered to female frogs, enhanced the maturation of ovaries and the accumulation of carbohydrate yolk plates in the ooplasm. There is also evidence that D-Asp modulates sex hormone levels in the brain of P. esculentus males. A direct correlation among brain levels of D-Asp and brain/serum 17β-estradiol has been demonstrated in both reproductive and post-reproductive male frogs, with highest levels being observed in the post-reproductive males. Accordingly, P450 aromatase protein is more expressed in post-reproductive period. D-Asp i.p. administration to frogs ready for reproduction, induced an increase of brain P450 aromatase expression with concomitant enhancement of both 17β-estradiol levels and estradiol receptor-a expression; at the same time, brain testosterone levels and androgen receptor expression decreased. Finally, a substantial amount of D-Asp is endogenously found within the P. esculentus Harderian gland, an androgen-dependent orbital gland that displays seasonal changes in secretory activity. D-Asp HG levels during two periods of differing glandular activity (high or low secretory activities) appear to be correlated with the androgen receptor expressions. Injection of D-Asp to frogs of both sexes collected during two periods of differing glandular activity, revealed that the amino acid may inhibit or activate the exocrine secretion through ERK1 pathway, depending on gland activity. In conclusion, the regulation of sex hormone production by D-Asp affects the androgen-dependent organs in P. esculentus. However, D-Asp can assume different roles, excitatory or inhibitory, depending on the organ and functional state.