In mature Merino rams, changes in diet to below or above the requirements for maintenance of body weight lead to changes in gonadotrophin secretion and testicular growth. However, the effects on testicular growth persist for much longer than those on LH and FSH secretion so that the gonadal and gonadotrophin responses are poorly correlated over time. This suggests that the gonadal effects may be partly independent of changes in the hypothalamic secretion of GnRH, an hypothesis tested in this study. In a short-term experiment (November, late spring, non-breeding season), we tested whether a high frequency of exogenous GnRH pulses could override the endogenous system and mimic the change in gonadotrophins seen in rams fed a high plane of nutrition. Mature Merino rams (scrotal circumference (mean ± s.e.m.) 33·6 ±0·5 cm, body weight (mean ± s.e.m.) 59·0 ± 0·9 kg) were fed 900 g chaff+1·6 kg lupin grain (High diet) or 360 g chaff+60 g lupin grain (Low diet) and infused with 8 pulses of GnRH or saline daily for 5 weeks (n=5/group). Blood was sampled every 20 min for 12 h on days – 1 and 14 relative to the start of treatments. Relative to pre-treatment levels, LH pulse frequency and FSH concentrations were decreased on day 14 in saline-infused rams fed the Low diet and increased in saline-infused rams fed the High diet (P<0·001). In GnRH-infused rams, gonadotrophin secretion was not affected by diet and the patterns of secretion of LH and FSH were similar to those in saline-infused rams fed the High diet.
This model was used for a more complete endocrine analysis in a longer experiment designed to test the hypothesis that the effect of nutrition on testicular growth is partly independent of changes in the secretion of GnRH. The same treatments were imposed for 35 days on a different group of similar rams in March (autumn, mid-breeding season). Body weight and scrotal circumference were measured weekly and blood was sampled on days – 1 and 14. On days – 1 and 35, testosterone secretion in response to LH was tested by injecting exogenous ovine LH (NIADDK-oLH-25; 200 ng/kg body weight) to all rams. Body weight increased in rams fed the High diet and decreased in those fed the Low diet (P<0·001) and was not affected by infusion. The secretion of LH and FSH was affected by treatments as in experiment 1. There was an interaction between the effects of diet and infusion on change in scrotal circumference (P<0·02). In GnRH-infused rams fed the Low diet, scrotal circumference was not changed, so that from week 2 after the change in diet it was higher (P<0·05) than in saline-infused rams fed the Low diet and lower (P<0·05) than in both groups of rams fed the High diet. Changes in diet, GnRH pulse frequency or in testicular size did not affect mean plasma concentrations of inhibin or the testosterone response to LH.
In conclusion, we have shown that in mature rams pulsatile exogenous GnRH cannot fully reproduce the effect that feeding a high diet has on testicular growth, suggesting that the effect of nutrition on testicular growth is partly independent of changes in the secretion of GnRH. Our results also show that (i) testicular growth induced by nutrition is not associated with changes in plasma concentrations of inhibin, or peripheral concentrations of testosterone after a pulse of LH, suggesting a dissociation of the endocrine and spermatogenic functions of the testis; (ii) changes in diet alter the secretion of gonadotrophins primarily by changing GnRH pulse frequency; and (iii) an exogenous GnRH pulsatile regimen can override endogenous secretion of a similar pulse frequency.
Journal of Endocrinology (1995) 147, 75–85
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