Interactions between nutrition, testosterone and inhibin in the control of gonadotrophin secretion in mature rams

The role of negative feedback by two testicular hormones, testosterone and inhibin, in the gonadotrophin responses of mature male sheep to changes in nutrition was tested. Six days after castration, 24 Merino rams were assigned to groups that were fed either a diet that maintained their initial liveweight (Intermediate diet), or about half of the Intermediate diet (Low diet), or the Intermediate diet with a supplement of lupin grain (High diet). One week after the change of diet, all animals were given subcutaneous testosterone implants, providing plasma testosterone concentrations of 3.06 +/- 0.14 ng mL-1 (mean +/- s.e.m.; n = 24). The implants were left in place for 7 days, during which time all the rams were also injected subcutaneously with 2 mL steroid-free bovine follicular fluid (bFF) every 8 h, to provide inhibin at mean plasma concentrations of 0.40 +/- 0.04 ng mL-1 (n = 24; compared with 1.50 +/- 0.12 ng mL-1 when the animals were intact). Five days after castration, there were significant increases in the frequency of luteinizing hormone (LH) pulses (from 1.83 +/- 0.23 to 17.3 +/- 0.96 pulses per 12 h; mean +/- s.e.m.; n = 24) and concentrations of FSH (from 0.45 +/- 0.07 to 14.19 +/- 2.7 ng mL-1; n = 24). Dietary treatment did not significantly affect these responses. Treatment with bFF and testosterone for 7 days reduced LH-pulse frequencies and follicle-stimulating hormone (FSH) concentrations in all groups. The degree of reduction was least in the group on the High diet, for which the FSH concentration (6.49 +/- 1.96 ng mL-1) and frequency of LH pulses (7.00 +/- 2.31 pulses per 24 h) were significantly higher than those observed in the other groups. The mean frequency of LH pulses did not differ significantly between the Intermediate group (0.88 +/- 0.61 pulses per 24 h) and the Low group (0.25 +/- 0.25 pulses per 24 h). Similarly, FSH concentrations did not differ significantly between these two groups (1.78 +/- 0.46 for the Intermediate group v. 1.33 +/- 0.26 ng mL-1 for the Low group). It is concluded that there is no response to diet in the absence of testicular hormones and the effects of nutrition on LH secretion in castrated rams given exogenous testosterone and inhibin are similar to those observed in intact rams.