Endocrinology and genetics of the hypothalamic-pituitary-gonadal axis

Hypothalamic control of the pituitary -gonadal axis in broad terms operates at two levels. One mediates longer-term responses to maturational signalling, to environmental and behavioural factors such as changes in photoperiod and availability of food, and to the expression of incubation behaviour. This level of control is principally at gonadotrophin-releasing-hormone (GnRH-I) cell bodies in the hypothalamus, where GnRH-I synthesis occurs. The second level of control mediating shorter-term reproductive responses, for example the preovulatory releases of luteinizing hormone (LH) or the first-day hormone response to photostimulation, may be principally at the median eminence (ME), where GnRH-I release occurs. Dopamine is emerging as a potential key regulator at both levels of control, being stimulatory at the GnRH-I cell body and inhibitory at the ME. Glial cell plasticity may play a key role at both levels of control. At the level of the pituitary much has been achieved in understanding the control of follicle-stimulating hormone (FSH) secretion and synthesis. While it is clear that FSH secretion responds to feedback signals from the ovary, the mechanism responsible for the dramatic increase in circulating levels of FSH around puberty is not fully understood and may involve changing pulsatile patterns of GnRH-I release. GnRH-I signals through two receptors in the chicken, but further work is required to understand their respective functions in the regulation of gonadotrophin synthesis and release. Particularly exciting is the emerging role of the recently discovered gonadotrophin inhibitory hormone (GnIH), which is stimulating further research to understand how FSH and LH synthesis and release are controlled. A particular focus is on the role of GnIH in the regulation of common gondadotrophin a-subunit expression and con- sequent gonadotrophin secretion. This may lead to an understanding of how the complex functions of the ovary are regulated using a limited repertoire of primary hypothalamic signalling neuropeptides. Compared with egg-laying-type hens the reproductive neuroendocrine system in meat-type hens is abnormal, resulting in the requirement for feed restriction for satisfactory reproductive performance. In meat-type birds, feed restriction suppresses the activity of GnRH-I neurones. It is now possible to identify genes critical to the control of the hypothalamic -pituitary -gonadal axis, and the gene variation critical to the differences in activity of the axis. This, combined with further dissection of the genetics of reproduction, should result in new approaches to improving broiler breeder reproduction and ameliorating the negative effects of selection for muscle growth on ovarian function.