PARENT SESSION
NUTRITION AND REPRODUCTION

Monday, August 2, 2004
10:30 AM–12:30 PM
Buchanan Courtyard

(64) INHIBITION OF LH SECRETION BY ESTRADIOL IS ENHANCED IN YOUNG CASTRATED MALE SHEEP SUBJECTED TO SHORT-TERM FOOD WITHDRAWAL.

Hileman, Stanley¹, McManus, Christina¹, Kang, Lori¹, Llanza, Nancy¹, Dobbins, Adam¹, Valent, Miro¹, ¹ West Virginia University, Morgantown, WV

ABSTRACT- Previous work showed that inhibition of LH secretion by estradiol (E) is enhanced in wethers during long-term food restriction (i.e. >6 wk). The aims of this experiment are to test the hypothesis that E negative feedback on LH secretion is enhanced during short-term food withdrawal and to develop a model for future investigations of the underlying neural mechanisms. Chronically castrated male sheep that were approximately 5 months old were assigned to one of 4 groups: controls not treated with E (n=6) or implanted subcutaneously with a 1cm Silastic implant containing either 100% E (n=6), a mix of 50% E and 50% cholesterol (n=6), or a mix of 25%E and 75% cholesterol (n=5). Two weeks after receiving implants, all groups had food withdrawn for 4 days. To assess pulsatile patterns of LH secretion, blood samples were collected at 12-min intervals for 4 hours just prior to food removal (D0) and on D2 and D4. Mean concentrations of LH in control males did not differ significantly (P>0.10) on D0 (12.5 ± 1.8 ng/ml), D2 (15.6 ± 3.2 ng/ml), or D4 (12.4 ± 2.2 ng/ml). Compared to controls, mean LH levels were suppressed significantly (P<0.05) on D0 by the 100% E implant (2.4 ± 1.0 ng/ml), the 50% E implant (2.7 ± 0.8 ng/ml), and the 25% E implant (6.7 ± 1.4 ng/ml). Food withdrawal significantly (P<0.05) and maximally reduced mean concentrations of LH by D2 in all E-implanted groups. LH pulse frequency for controls on D0 (1.2 ± 0.1 pulses/h) was significantly greater (P<0.05) than that on D4 (0.8 ± 0.2 pulses/h). Compared to controls, LH pulse frequency was markedly reduced (P<0.05) on D0 in the 100% E (0.3 ± 0.2 pulses/h) and the 50% E (0.7 ± 0.2 pulses/h) groups, but was not (P>0.10) in 25% E animals (1.0 ± 0.1 pulses/h). Pulses of LH were virtually absent in all E-implanted groups by D2 of food withdrawal. In summary, while 4 days of food removal only marginally affected LH pulse frequency in control animals, it almost totally abolished LH pulses in E-implanted males. Thus, our findings support the hypothesis that short-term food withdrawal enhances the ability of E to inhibit LH secretion. This model will be useful for future investigation of neural mechanisms underlying the effect of undernutrition on reproduction. This work supported by USDA 2001-35203-10835 and USDA 2002-35203-11259.

KEY WORDS: estradiol, LH, sheep, nutrition