Melatonin And Melatonin Receptor In The Sexual Development Of The Female Rats

Puberty is the transition period between childhood and adulthood, resulted fromthe activation of Hypothalamic-Pituitary-Gonadal Axis. The high frequency andamplitude of GnRH is released from GnRH neuron, leading to precocious puberty.An increase of GnRH with pulsatile release is essential for the onset of puberty.The regulation mechanism of the pulsatilel increase in GnRH release is not yet clear.Various neurotransmitter and multiple signals are involved in the onset of puberty.They are intergrated with each other and form “permissive signals” to permit pubertyoccur or progress.Melatonin, as an indole hormone, is produced and secreted by pineal gland. Thesecretion of melatonin is related to the length of night with a circadian rhythmcharacterized by low levels during the day and peak values at night. It has beenknown that melatonin and its receptors are related to sexual development, andMelatonin is shown to have effects on HPGA and be involved in the onset of pubertyand the precocious puberty. The pineal gland has been suggested to conveyphotoperiodic information by its melatonin secretion to regulate endocrinalreproductive function. In animals, melatonin binding to its receptors exerts aninhibitory role in puberty onset.As non-seasonal breeders, the development of HPGA in rats is similar to human.The sign of sexual development in female rats is easily recognizable andcomparatively definite, so the female rat modle is major and common in animalexperimental study on sexual development.The nocturnal plasma melatonin, luteinizing hormone level and theexpression levels of melatonin receptors(MT1, MT2) and GnRH mRNA in hypothalamus and pituitary of female rats in the different stages would beexplored in sexual development, including juvenile(postnatal day15), prepuberty(postnatal day25), early puberty(postnatal day35) and adult stage(postnatalday45). In order to study the role of melatonin and melatonin receptors(MT1,MT2) in the precocious puberty, the precocious puberty rat model would becreated, which induced by N-Methyl-DL-Aspartate (NMA) and usedGonadotrophin releasing hormone agonist (GnRHa, triptorelin) to treated. Sowe would conduct further study in the role of melatonin and its receptors inonset of puberty and precocious puberty. Part I：Level of plasma melatonin and expression ofmelatonin receptor in the hypothalamus and pituitary inthe female ratsObjective：To explore the effect of melatonin and melatonin receptors on theonset of the puberty and the reproduction.Method：SD rats(n=40) were divided into four groups: juvenile group(GroupA，n=10), prepubertal group (Group B，n=10), early pubertal group(Group C，n=10) and adult stage group(Group D，n=10). The levels of nocturnal plasmamelatonin and luteinizing hormone were assayed by Enzyme linkedImmunosorbent Assay (ELISA) and the expression levels of melatoninreceptors (MT1、MT2) and GnRH mRNA were conducted in hypothalamus andpituitary at the different sexual development stages with Real time-PCR.Results：The level of plasma melatonin was the highest in Group A and thelowest in Group D, which decreased gradually during the sexual development(P<0.05). The level of plasma luteinizing hormone was lower in Group A and Band increased obviously in Group C (P<0.05). The expression level of GnRHmRNA was increased gradually, which was the lowest in Group A and reachedto the highest level in Group C (P<0.05). The expression levels of MT1mRNAin the hypothalamus and pituitary were higher in Group A and B, whichdecreased during the sexual development (P<0.05), while the expression levelof MT2mRNA didn’t show significant difference in each group (P>0.05).Conclusion：Melatonin and melatonin receptor (MT1) would play an inhibitoryeffect in the onset of puberty, involving in the regulation of GnRH. Part II: Expression of melatonin receptor in thehypothalamus and pituitary in the female rats withprecocious pubertyObjective: To explore the effect of melatonin and melatonin receptors on theonset of the puberty by observing the plasma concentration of melatonin, theexpression levels of melatonin receptors (MT1, MT2) mRNA of hypothalamusand pituitary in precocious puberty female rats.Method:26-day-old female rats(n=40) were divided into precocious pubertygroup(A，n=10), treated group (B，n=10), control groups including prepubertygroup (C1，n=10) and normal puberty group(C2，n=10). N-methyl-DL-asparticacid(NMA) were used to induced precocious puberty in the female rats. GnRHanalog(Triptorelin) were used to treat the precocious puberty. The levels ofnocturnal plasma melatonin and luteinizing hormone were assayed by Enzymelinked Immunosorbent Assay (ELISA) and the expression levels of melatoninreceptors (MT1、MT2) and GnRH mRNA in the hypothalamus and pituitarywere measured by Real time-PCR.Results: There was no significant difference in the plasma melatonin levelsamong these groups. The expression level of MT1mRNA in the hypothalamusand pituitary of group A was significantly lower than those of the group B andC1, similar with Group C2. Each group showed no significant differences in theexpression level of MT2in the hypothalamus and pituitary.Conclusion: The expression level of MT1mRNA was ncreased in theprecocious puberty female rats. GnRH analog would up-regulate the expressionof MT1mRNA in hypothalamus and pituitary. Melatonin and its receptorswould participate in the onset of normal puberty or precocious puberty.