Changes in reproductive neuroendocrine mRNAs with decreasing ovarian function in ageing hens.

Egg production declines with advancing age in the domestic chicken and this is particularly pronounced in breeding stocks of meat type hens (broiler breeders). The objective of this study was to establish whether declining egg production with reproductive ageing in broiler breeders is correlated with plasma LH and FSH, and with mRNAs encoding hypothalamic gonadotrophin-releasing hormone-I (GnRH-I), gonadotrophin inhibitory hormone (GnIH), and gonadotrophin subunits. Comparisons were made between hens at the peak of egg laying (young: 30 weeks) and at the end of a laying year (old: 60 weeks). Old hens were subdivided into laying and out-of-lay groups. Plasma LH and FSH were lower in old than in young laying hens. Compared with old laying hens, old out-of-lay hens had significantly increased plasma FSH but not plasma LH. There were no differences in total hypothalamic GnRH-I and GnIH mRNAs between young and old hens. In old laying hens, the decrease in plasma LH was correlated with decreased gonadotrophin alpha-subunit but not LHbeta mRNAs. The decrease in plasma FSH was not associated with a change in FSHbeta mRNA. In old out-of-lay hens, the increase in plasma FSH was correlated with increased FSHbeta mRNA, while unchanged plasma LH was associated with increased LHbeta mRNA. A regression analysis of all plasma gonadotrophin and gonadotrophin subunit mRNA data collected from the study demonstrated that plasma LH is correlated with alpha-subunit but not LHbeta mRNAs, while plasma FSH is correlated with FSHbeta but not alpha-subunit mRNAs. It is concluded that the decrease in the rate of lay in ageing broiler breeders is not correlated with decreased GnRH-I mRNA nor with increased GnIH mRNA, but it is related to a decrease in alpha-subunit mRNA which may account for the associated reduction in plasma LH but not FSH.