Gonadotropin releasing hormone is central to the control of reproduction in birds, as it is in other vertebrates. Not only does it play a key role in the aspects of reproduction that contribute to the success of the poultry industry but it is also a crucial part of the many fascinating strategies that control the timing of reproduction in other avian species. Two forms of GnRH have been identified in birds: chicken gonadotropin releasing hormone -I and -II (cGnRH-I and cGnRH-II). Both have been identified at the peptide level and the sequence of the cGnRH-I gene has been elucidated. The perikarya of cGnRH-l neurones are primarily located in the hypothalamus, whilst cGnRH-II perikarya have a mesencephalic distribution. The presence of a third form of GnRH in the lateral thalamus has been implied from immunological studies. Release of gonadotropin is mediated by receptors on the gonadotrope through protein kinase C and is calcium dependent. In chickens the release of gonadotropin in response to GnRH, both quantity and pattern, is sexually differentiated. There is also evidence that the relative potency of GnRH analogs is different in cockerels and hens. Expression of cGnRH-I, measured as the peptide or mRNA, changes in situations where there is long-term alteration of reproductive function, such as during incubation or after photostimulation. Expression is also negatively regulated by steroids. During the preovulatory surge of gonadotropin, when there is likely to be rapid changes in GnRH release, there is evidence of control at the level of the median eminence but there is little evidence for changes in gene expression. In contrast control of GnRH expression is an important component of the control of reproductive activity during incubation, photostimulation, photorefractoriness and negative steroid feedback where changes are over a longer time span.
|Number of pages||25|
|Journal||Avian and Poultry Biology Reviews|
|Publication status||Published - 1 Jan 1998|
- Gonadotropin releasing hormone