The Mechanism Of Melatonin And Leptin In The Process Of Sexual Maturity Of Hens

Melatonin has a wide range of physiological and pharmacological effects, can affect multiplesystems (including the immune system, endocrine system, nervous system), regulate biologicalcircadian rhythm, sexual maturation and reproduction, immune response, metabolism, cancer andsenium. Sexual maturity is a complex process, substances or mechanisms involved in triggering pubertyis not very clear, the mechanism of melatonin on the hypothalamic--pituitary--gonadal axis was not veryclear. Leptin has a wide range of biological effects, plays an important role in the regulation of energymetabolism, fat metabolism, reproduction, and so on. Melatonin could inhibit leptin secretion, leptinreceptor played an important role between metabolism and reproductive endocrinology. In this paper, inorder to study the effects of photoperiod and energy on regulating the reproductive axis mechanism, fourinfluencing factors were designed in this research: photoperiod: exogenous melatonin, feeding level andexogenous leptin. Using the molecular biology methods, we studied the interactions between thelight-sensitive nerve endocrine regulator--pineal gland and hypothalamus--pituitary--gonadal axis, inorder to reveal the mechanism of photoperiod and energy regulating the reproductive rhythms.Photoperiod treatment group selected10-week-old240Hyline gray hens, randomly divided intothree groups, photoperiod was8L:16D in group Ⅰ, photoperiod was12L:12D in group Ⅱ, photoperiodwas16L:8D in group Ⅲ, each group had four replicates of20chickens in each replicate. Melatonintreatment group selected10-week-old320Hyline gray hens, randomly divided into four groupswith fourreplicates of20chickens, the hens were maintained on8-h photoperiods(group Ⅰ),16-hphotoperiods(groupⅡ), and16-h photoperiods supplemented with20mg/kg melatonin (groupⅢ),200mg/kg melatonin(groupⅣ).Effect of photoperiod and melatonin on hypothalamic--pituitary--gonadalaxis, the main results obtained showed as follows:(1) There was a significant positive correlationbetween body weight and plantar long, long keel, pubic spacing, comb length, comb height and combthickness;(2) In experiment1, age of first egg in16-hour photoperiod group was21.75d earlier than8-hour photoperiod group. Body weight in16-hour photoperiod group was87.1g lighter than8-hourphotoperiod group. In experiment2, exogenous melatonin could delay age of first egg, increase sexualmaturity weight, absolute and relative abdominal fat weight;(3)There was a significant increase inplasma triglyceride concentrations at sexual maturity. photoperiod and exogenous melatonin showed nosignificant differences on plasma insulin and glucose concentrations at sexual maturity;(4)The shortphotoperiod inhibited GnRH receptor mRNA expression, long photoperiod promote GnRH receptormRNA expression; hypothalamic GnRH receptor mRNA expression was negatively correlated with senium;(5) FSHR mRNA gene expression showed a volatility increasing trend; given exogenousmelatonin reduced the mRNA expression of pituitary FSHR before sexual maturity, FSHR mRNA geneexpression was not significant different at sexual maturity;(6) Photoperiod and exogenous melatoninhad no significant effect on the pituitary LHR mRNA expression, which suggestted that, in addition tothe hypothalamic--pituitary--ovary axis, there were also some other multi-factors regulating systemsexisted in the ovary;(7) Ovary ER mRNA expression level showed an increasing trend with theincreasing age;(8) Pituitary MELR1A mRNA gene expression decreased according with the increasingage; Photoperiod did not show significant difference on MELR1A gene expression of hypothalamus,pituitary and ovary; long photoperiod promoted abdominal fat MELR1A mRNA expression at sexualmaturity; melatonin promoted pituitary MELR1A mRNA expression, but did not inhibit abdominal fatMELR1A mRNA expression at sexual maturity. MELR1B mRNA expression was highest in thehypothalamus, followed by the pituitary, ovary and abdominal fat; MELR1B mRNA expression in thehypothalamus reduced at sexual maturity, but did not change significantly in the pituitary, ovary andabdominal fat;(9) LEPR gene expression in the hypothalamus and pituitary reduced with the senium,while LEPR and ER mRNA expression in ovary increased with the senium;(10) In conclusion,melatonin receptor affected the HPG ’s maturity, MELR1A could inhibit hypothalamus and pituitarymelatonin receptor expression, or integrated with LEPR gene in the pituitary, or played a role from othersignal ways.Effects of feeding level and photoperiod on LEPR, MELR1A and MELR1B gene expression ofovary and abdominal fat.10weeks480Hy-Line Gray commercial henswere randomly divided into6groups, using2(ad libitum, voluntary feed intake80%)*3(8L:16D,12L:12D,16L:8D) two-factor design,each group of4replicates of20chickens. The results showed as follows: there is no interaction effectbetween photoperiod and feeding level on abdominal fatLEPR, MELR1A, MELR1B gene expression at14w and18w, there is no interaction effect between photoperiod and feeding level on ovaryER, MELR1A,MELR1B gene expression at14w and18w, but ovary LEPR gene expression showed interaction effect atsexual maturity. Restricted feeding significantly inhibited ovary ER, LEPR and MELR1B geneexpression at14weeks,18weeks and sexual maturity. At14-week-old, abdominal fat LEPR geneexpression was significantly lower in long photoperiod group compared with the short photoperiodgroup; At sexual maturity, short photoperiod and feed restriction group reduced abdominal fat LEPRgene expression; photoperiod did not show significant differences on ovary LEPR gene expression,feeding level showed a sinificant difference on ovary LEPR gene expression, the resutls indicated thatLEPR gene regulating sexual maturity mainly affected by feeding levels and body fat level.25Hy-Line Gray commercial hens (1560g±114g) were randomly divided into five groups:(I) adlibitum control group (n=5);(II) fasted control group (FC; n=5);(III) fasted+low concentration ofleptin (F+L; n=5);(IV) fasted+middle concentration of leptin (F+M; n=5);(V) fasted+highconcentration of leptin (F+H; n=5). All four fasted groups were fasted for2days and then ad libitumand the hens were treated with leptin at dose of0(control and FC),50(F+L),200(F+M) and400(F+H)ug/kg/d. on day3-5.The results of exogenous leptin on reproductive function of fasting hens were showed as follows: ovary function was affected by two-day fasting, appropriate concentrations ofexogenous leptin could recover the damaged ovary; exogenous injection of recombinant mouse leptincould promote pituitary MELR1A expression, suppressed pituitary LEPR gene expression; fasting couldinhibit ovary LEPR gene expression, and exogenous leptin could promote ovary LEPR gene expression,inhibit ovary MELR1A and MELRA1B gene expression. The results suggested exogenous leptin couldregulate ovary function’s recovery by central nervous system or directly through ovary regulatingsystem.