Synthesis and biological activity of [D-Trp6] chicken luteinizing hormone-releasing hormone

Clock gene is associated with individual variation in the activation of reproductive endocrine and behavior of Asian short toed lark

Within year individual variation in the timing of seasonal reproduction within wild bird populations in highly seasonal environments can be pronounced, but the molecular and physiological mechanisms responsible for this variation are unclear. We investigated the relationship between Clock gene poly-Q length, activation of the HPG endocrine axis, and the timing of breeding behavior, in a wild population of the Asian short-toed lark (Calandrella cheleensis) in Inner Mongolia, China. Six variants of Clock gene poly-Q alleles were identified in this population. Clock poly-Q mean allele length was positively correlated with the mean peak date deviation of individual birds. The shorter an individual's Clock poly-Q mean allele length, the earlier its plasma LH, T and E2 values peaked. Mean Clock poly-Q allele length of nestlings in the same nest were positively correlated with the standardized laying date of the first egg in that nest. These results suggest that the Clock gene influences the reproductive timing of birds through its effect on the HPG endocrine axis, and that individual variation in the timing of reproduction may have a genetic basis.

Annual variation in the reproductive hormone and behavior rhythm in a population of the Asian short-toed lark: Can spring temperature influence activation of the HPG axis of wild birds?

Although a number of studies have demonstrated a correlation between ambient temperature and the timing of reproductive behavior in many bird species, the relationships between temperature, activation of the hypothalamus-pituitary-gonad (HPG) axis, laying, and hatching in free-living birds, remain unclear. We investigated the relationships between spring temperature, reproductive hormones, and behavior, in a population of the Asian short-toed lark (Calandrella cheleensis) on the Inner Mongolian Grasslands in 2014, 2015 and 2016. LH and T levels peaked earliest in the year with the highest April temperature (2014) and latest in the year with the lowest April temperature (2016), and rose faster in 2014 than in 2015 or 2016. Laying and hatching occurred earliest in 2014 and latest in 2016. E2 also peaked earlier in 2014 than in the other two years but there was no significant difference in peak E2 levels among the three years. The peak of hatching only coincided with the peak in grasshopper nymph (the main food of nestlings) abundance in 2015 and the nestling fledging rate in 2015 was significantly higher than that in 2014 and 2016. We also conducted a controlled experiment on the effect of temperature on plasma LH, T and E2 levels in wild-caught larks, which shows that mean plasma LH, T and E2 levels in the 16°C group all peaked 4days earlier than in the 13°C group. All these results suggest that activation of HPG endocrine axis in the Asia short-toed lark population is closely related to ambient temperature, and that this subsequently influences the timing of laying and hatching. The HPG axis' endocrine function is a physiological factor that mediates effects of ambient temperature on the timing of breeding. Temperature induced annual variation in the timing of hatching was, however, insufficient to synchronize the nestling period with the annual peak in grasshopper nymph abundance. Unusually warm, or cold, spring temperatures advanced, or delayed, grasshopper nymph development to an extent that exceeded the Asian short-toed lark's capacity to advance or delay the onset of breeding and reproductive success was lower in the two years of the study in which the nestling period was asynchronous with the peak of grasshopper nymph abundance. Long-term research on the relationship between the timing of reproduction and population demographics of the Asian short-toed lark is required to understand the effects of climate change on this species and on temperate birds in general.

Neuroendocrine regulation of gonadotropin secretion in seasonally breeding birds

Seasonally breeding birds detect environmental signals, such as light, temperature, food availability, and presence of mates to time reproduction. Hypothalamic neurons integrate external and internal signals, and regulate reproduction by releasing neurohormones to the pituitary gland. The pituitary gland synthesizes and releases gonadotropins which in turn act on the gonads to stimulate gametogenesis and sex steroid secretion. Accordingly, how gonadotropin secretion is controlled by the hypothalamus is key to our understanding of the mechanisms of seasonal reproduction. A hypothalamic neuropeptide, gonadotropin-releasing hormone (GnRH), activates reproduction by stimulating gonadotropin synthesis and release. Another hypothalamic neuropeptide, gonadotropin-inhibitory hormone (GnIH), inhibits gonadotropin synthesis and release directly by acting on the pituitary gland or indirectly by decreasing the activity of GnRH neurons. Therefore, the next step to understand seasonal reproduction is to investigate how the activities of GnRH and GnIH neurons in the hypothalamus and their receptors in the pituitary gland are regulated by external and internal signals. It is possible that locally-produced triiodothyronine resulting from the action of type 2 iodothyronine deiodinase on thyroxine stimulates the release of gonadotropins, perhaps by action on GnRH neurons. The function of GnRH neurons is also regulated by transcription of the GnRH gene. Melatonin, a nocturnal hormone, stimulates the synthesis and release of GnIH and GnIH may therefore regulate a daily rhythm of gonadotropin secretion. GnIH may also temporally suppress gonadotropin secretion when environmental conditions are unfavorable. Environmental and social milieus fluctuate seasonally in the wild. Accordingly, complex interactions of various neuronal and hormonal systems need to be considered if we are to understand the mechanisms underlying seasonal reproduction.

Stimulation of gonadotropin release by a non-GnRH peptide sequence of the GnRH precursor

The human gonadotropin-releasing hormone (GnRH) precursor comprises the GnRH sequence followed by an extension of 59 amino acids. Basic amino acid residues in the carboxyl terminal extension may represent sites of processing to biologically active peptides. A synthetic peptide comprising the first 13 amino acids (H X Asp-Ala-Glu-Asn-Leu-Ile-Asp-Ser-Phe-Gln-Glu-Ile-Val X OH) of the 59-amino acid peptide was found to stimulate the release of gonadotropic hormones from human and baboon anterior pituitary cells in culture. The peptide did not affect thyrotropin or prolactin secretion. A GnRH antagonist did not inhibit gonadotropin stimulation by the peptide, and the peptide did not compete with GnRH for GnRH pituitary receptors, indicating that the action of the peptide is independent of the GnRH receptor. The GnRH precursor contains two distinct peptide sequences capable of stimulating gonadotropin release from human and baboon pituitary cells.

Structure-activity relations of LHRH in birds

Structural requirements in LHRH for gonadotropin-releasing activity were investigated by comparing the activity of the natural vertebrate LHRH structural variants and synthetic analogues. Substitution of Arg8 results in a loss of activity of LHRH in mammals, whilst a number of amino acid substitutions for Arg8 retain high gonadotropin-releasing activity in the chicken. Thus Arg8 of LHRH comprises an integral part of the binding site of LHRH and/or contributes towards the conformation of the binding site for the mammalian receptor while this does not pertain in the bird. The possibility that relative conformational stabilization of LHRH is important for biological activity in mammals but not birds, was supported by the demonstration that a gamma-lactam conformationally constrained analogue of LHRH was more active than LHRH in the mammalian system but equipotent in the bird. The chicken LHRH receptor is also relatively undiscriminating with regard to amino acid substitutions in positions 5 and 7. A series of LHRH analogues with pure antagonist activity in rats exhibited a spectrum of activities, from pure agonist to mixed activity and pure antagonist, in the chicken. These differences in LHRH structural requirements of the mammalian and avian receptor are reflected by a difference in molecular size of the chicken receptor (67,000) and mammalian receptor (60,000). Nevertheless, like the mammalian pituitary, the chicken pituitary does exhibit "desensitization" on prolonged exposure to LHRH.