Expression of Rate-Limiting Enzymes of Melatonin Synthesis in Several Extrapineal Organs During Pregnancy in Ewes

Melatonin is not only produced by the pineal gland but is also synthesized in extrapineal organs. It plays a key role in antioxidant defense and immune regulation during pregnancy. Two key rate-limiting enzymes are involved in the melatonin synthesis, aralkylamine N-acetyltransferase (AANAT), and acetylserotonin O-methyltransferase (ASMT). Nevertheless, it remained unclear whether pregnancy affects the expression of AANAT and ASMT in extrapineal organs. In this study, ovine maternal thymus, spleen, liver, lymph nodes, thyroid, duodenum, and endometrium were collected on day 16 of the estrous cycle (N16), and on days 13, 16, 25, and 70 (G70) of gestation (n = 6 per group). The expression of AANAT and ASMT in these extrapineal organs was analyzed using RT-qPCR, Western blot, and immunohistochemistry. The results revealed that pregnancy upregulated the expression of AANAT in the liver and ASMT in the thymus and duodenum, but downregulated AANAT expression in the lymph nodes and duodenum. Moreover, AANAT expression was elevated in the thymus and spleen but reduced in the thyroid and endometrium at G70 compared with N16. ASMT expression was increased in the lymph nodes yet decreased in the spleen, liver, and thyroid at G70 compared with N16. Notably, endometrial ASMT expression showed a pregnancy-stage-specific manner with no significant difference between N16 and G70. In summary, this paper reports, for the first time in sheep, that pregnancy modulates the expression of AANAT and ASMT in these extrapineal organs in a pregnancy-stage- and tissue-specific manner.