Metabolic factors affecting the reproductive axis in male sheep

Abstract

Changes in food intake affect the reproductive axis in both sexes, and the nutritional signals involved and the sites that receive those signals are now beginning to be unravelled. Our studies have focussed on the mature male sheep, a model in which high food intake stimulates GnRH-LH pulse frequency for only 10-20 days but continues to promote testicular growth over several months. Different signals and different target organs seem to be responsible for these short- and long-term responses. Short-term dietary treatments lead to changes in blood concentrations of glucose, fatty acids, insulin and leptin, and concentrations of glucose, insulin, leptin and some amino acids in cerebrospinal fluid. It seems unlikely that amino acids affect GnRH-LH secretion directly in sheep. Intracerebroventricular infusions of insulin specifically increase LH pulse frequency, but intravenous, intra-abomasal or intracerebroventricular infusions of glucose have no effect, despite their effects on cerebrospinal fluid insulin concentrations. The addition of fatty acids to the diet also increases LH pulse frequency, but does not affect the concentrations of insulin or leptin in the cerebrospinal fluid. It appears that acute responses to changes in nutrition involve a range of alternative pathways, possibly including interactions among insulin, leptin and energy substrates. Effects of long-term dietary treatments on testicular size are only partly dependent on the GnRH-LH system (that is, on brain control) and so must also depend on other, as yet unknown, pathways. Concepts of 'metabolic sensing and integration' are being developed from the basis of existing knowledge of the central control of appetite and reproduction.

Identification of insulin-like growth factor-I in boar seminal plasma and its influence on sperm quality

“…Hence, several factors affecting gonadotropin secretion and testicular growth may influence IGF-I levels in the seminal plasma, such as genetics and nutrition. Nutrition is the most powerful factor that acts on the neuroendocrine system and controls testicular activity in ruminants (Blache et al, 2000;Brito et al, 2007). In the present study, no difference was observed between the genetic backgrounds (data not shown).…”

Effect of milk replacer and rumen inert fat on growth and reproduction of Malpura ram lambs

“…Earlier reports have shown greater testicular development in rams fed a high-energy diet compared with those fed a low-energy diet (Fernandez et al, 2004;Fourie et al, 2004). High food intake stimulates GnRH-LH pulse frequency for only 10 to 20 days but continues to promote testicular growth over several months in mature male sheep (Blache et al, 2000). However, the increased testicular length and volume of ram lambs fed milk replacer and rumen inert fat in the present study is contrary to the results of Bielli et al (1999) and Tufarelli et al (2011).…”

Interactions between nutrition and reproduction in the management of the mature male ruminant

“…Energy balance certainly affects plasma concentrations of GH under a wide variety of conditions but, in male sheep at least, an increase in nutrition induces a decrease in plasma GH concentrations (Miller et al, 1998). Circulating concentrations of IGF-I are also affected by diet, but concentrations of IGF-I in CSF are not (Miller et al, 1998) and we have not been able to demonstrate that IGF-I infusion into the third ventricle affects LH pulse frequency (Blache et al, 2000). It therefore seems likely that, in male sheep at least, IGF-I is not heavily involved in responses of the critical reproductive centres to changes in nutritional status.…”

“…There is a threshold frequency of GnRH pulses above which males will produce sperm and females will ovulate so, effectively, the 'decision' of an animal to reproduce or not is implemented through pathways, as yet not described, that control the production of GnRH. The GnRH neuroendocrine system is also the final common pathway via which gonadal activity is usually influenced by external factors, including socio-sexual signals and photoperiodic cues, as well as metabolic status (Figure 2; reviews: Blache et al, 2000Blache et al, , 2002Blache et al, , 2003Blache et al, and 2006 Figure 2 A schema describing the proposed relationships between photoperiodic, nutritional and social cues and the ways that they interact with genotype and steroid feedback in the control of hypothalamo-pituitary-testicular axis in the male sheep. Nutritional input is via 'metabolic status', a reflection of the difference between energy expenditure and the sum of energy available from food intake and from energy reserves, as measured by a hypothetical 'metabolic sensor'.…”