Effect of melatonin on hormone secretion and tissue ultrastructure of hypothalamus-pituitary-thyroid axis in rats

Abstract

Objective:To investigate the effect of melatonin(Mel) on hormone secretion and tissue ultrastructure of hypothalamus-pituitary-thyroid axis(HPTA) in rats.Methods:A total of 60 male Sprague-Dawley rats were evenly randomized into four groups:control,low-dose Mel(0.5 mg·kg-1·d-1),middle-dose Mel(10 mg·kg-1·d-1),and high-dose Mel(50 mg·kg-1·d-1) group.The rats in the latter 3 groups received intraperitoneal injection of different doses of Mel daily.At 1st,2nd and 3rd weeks after treatment,rats were sacrificed and blood serum samples,hypothalamus,pituitary and thyroid tissues were obtained.Serum T3,T4,FT3,FT4 and TSH levels were estimated by radioimmunoassay(RIA).Expression of proTRH mRNA in hypothalamus tissues were detected by real time RT-PCR.The ultrastructures of hypothalamus,pituitary and thyroid were observed under transmission electronic microscope.Results:The serum levels of T4,FT3,FT4 and TSH in the middle-dose and high-dose Mel groups were significantly decreased compared with those in the control and low-dose Mel group at 1st,2nd and 3rd weeks(P0.01).T4 levels decreased in the middle-dose and high-dose Mel groups at a weekly basis,with significant difference found between different weeks(P0.01).TSH level was deceased at the 2nd week,and that at the 3rd week was significantly lower than those at the first two weeks(P0.01).The expression of proTRH mRNA was lower in high-dose Mel group compared to that in the control group at 1st,2nd and 3rd weeks.With the prolongation of Mel treatment,expression of proTRH mRNA had a decreasing tendency first and an increasing tendency later.No obvious pathological ultrastructure changes were found in the hypothalamus,pituitary and thyroid tissues.Conclusion:Melatonin may inhibit Hypothalamus-Pituitary-Thyroid axis in a time-and dose-dependent manner in rats.