THE EFFECT OF NUTRITION ON TESTICULAR GROWTH IN MATURE MERINO RAMS INVOLVES MECHANISMS THAT ARE INDEPENDENT OF CHANGES IN GNRH PULSE FREQUENCY

In mature Merino rams, changes in diet to below or above the requireme nts for maintenance of body weight lead to changes in gonadotrophin se cretion and testicular growth. However, the effects on testicular grow th persist for much longer than those on LH and FSH secretion so that the gonadal and gonadotrophin responses are poorly correlated over tim e. This suggests that the gonadal effects may be partly independent of changes in the hypothalamic secretion of GnRH, an hypothesis tested i n this study. In a short-term experiment (November, late spring, nonbr eeding season), we tested whether a high frequency of exogenous GnRH p ulses could override the endogenous system and mimic the change in gon adotrophins seen in rams fed a high plane of nutrition. Mature Merino rams (scrotal circumference (mean +/- S.E.M.) 33.6 +/- 0.5 cm, body we ight (mean +/- S.E.M.) 59.0 +/- 0.9 kg) were fed 900 g chaff+1.6 kg lu pin grain (High diet) or 360 g chaff+60 g lupin grain (Low diet) and i nfused 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 fr equency and FSH concentrations were decreased on day 14 in saline-infu sed rams fed the Low diet and increased in saline-infused rams fed the High diet (P<0.001). In GnRH-infused rams, gonadotrophin secretion wa s not affected by diet and the patterns of secretion of LH and FSH wer e similar to those in saline-infused rams fed the High diet. This mode l was used for a more complete endocrine analysis in a longer experime nt designed to test the hypothesis that the effect of nutrition on tes ticular growth is partly independent of changes in the secretion of Gn RH. The same treatments were imposed for 35 days on a different group of similar rams in March (autumn, mid-breeding season). Body weight an d 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 n g/kg body weight) to all rams. Body weight increased in rams fed the H igh 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 tre atments as in experiment 1. There was an interaction between the effec ts of diet and infusion on change in scrotal circumference (P<0.02). I n GnRH-infused rams fed the Low diet, scrotal circumference was not ch anged, 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) t han in both groups of rams fed the High diet. Changes in diet, GnRH pu lse frequency or in testicular size did not affect mean plasma concent rations of inhibin or the testosterone response to LH. In conclusion, we have shown that in mature rams pulsatile exogenous GnRH cannot full y reproduce the effect that feeding a high diet has on testicular grow th, suggesting that the effect of nutrition on testicular growth is pa rtly independent of changes in the secretion of GnRH. Our results also show that (i) testicular growth induced by nutrition is not associate d with changes in plasma concentrations of inhibin, or peripheral conc entrations of testosterone after a pulse of LH, suggesting a dissociat ion of the endocrine and spermatogenic functions of the testis; (ii) c hanges in diet alter the secretion of gonadotrophins primarily by chan ging GnRH pulse frequency; and (iii) an exogenous GnRH pulsatile regim en can override endogenous secretion of a similar pulse frequency.