Fertility in male sheep: modulators of the acute effects of nutrition on the reproductive axis of male sheep.
Animals adjust the time of year that they reproduce through their ability to perceive and respond to critical aspects of their environment, such as photoperiod, nutrition or the socio-sexual milieu, and their genotype determines the degree of response to each stimulus. Ultimately, information from environmental cues filters through to the GnRH neurones in the brain which are the primary regulator of fertility. Each of these cues has been studied in isolation and the mechanisms by which they affect GnRH secretion are now better, if not fully, understood. In the field, the brain centres that control GnRH must integrate information from all cues at any given time before 'formulating a reproductive decision'. In this review, the effect of this integration is illustrated by showing how the acute GnRH response to a nutritional signal can be modulated by genotype, photoperiod and social cues, to the point of being completely blocked under some circumstances. Candidate pathways that may mediate these modulatory effects at both the whole body and brain have been proposed, although none of these pathways are confirmed and some have not yet been studied. As a guide for further research, we propose a working model that integrates the inputs and explains the interactions between them.
Blache
D
D
School of Animal Biology, Faculty of Natural and Agricultural Sciences, University of Western Australia, Crawley 6009, Australia. dblache@agric.uwa.edu.au
Zhang
S
S
Martin
G B
GB
D000824:Animal Nutritional Physiological Phenomena; D000818:Animals; D001921:Brain / Q000378:metabolism; D025461:Feedback, Physiological; D005298:Fertility / Q000502:physiology*; D005838:Genotype; D007987:Gonadotropin-Releasing Hormone / Q000502:physiology; D007031:Hypothalamus / Q000378:metabolism; D031297:Lupinus; D008297:Male; D008954:Models, Biological; D009474:Neurons / Q000502:physiology; D012621:Seasons; D012756:Sheep / Q000502:physiology*; D015398:Signal Transduction / Q000502:physiology; D012931:Social Environment; D013737:Testis / Q000378:metabolism
D007987:Gonadotropin-Releasing Hormone