The effect of orexin B receptor antagonist and steroid hormones on milk lactose synthesis in the lactating rats

Prolactin is a necessary factor for lactation and synthesis of milk constituent. Progesterone (P4) and 17β-estradiol (E2) are inhibitory factors for lactation. Orexin is involved in regulating the metabolism and lipid synthesis. Present study investigated the orexin B receptor antagonist and steroid hormone effects on milk lactose synthesis. Thirty Wistar lactating rats were used. Lactating animals in the group 1-4 received saline, 1, 2 or 4 µg of orexin antagonist. Lactating animals of the groups 5 and 6 received 4 µg of orexin antagonist plus 1 µg of 17-β estradiol (E2) or 4 mg of progesterone (p4). Blood and tissue samples were collected at 60 and 180 minutes of injections. Blood samples were measured for prolactin concentrations and tissue samples were examined for alpha lactalbumin (Lalba) and beta-1,4-galactosyltransferase 1 (B4galt1) gene expression in the mammary gland by RT-PCR technique. Injection of orexin antagonist significantly increased the percentage of milk lactose, plasma prolactin and Lalba gene expression in comparison to control group. Injections of E2 or P4 inhibited the increased effects of orexin antagonist on mean milk lactose percentage, prolactin and Lalba gene expression in comparison to orexin antagonist group. Injections of all drugs did not alter the mean B4galt1 gene expression. Stimulatory effects of orexin antagonist on milk production may be partly due to the increased prolactin concentration and Lalba gene expression. The mechanism by which the steroid hormones supress the orexin antagonist-induced lactose synthesis may be mediated partly via inhibiting the prolactin production.