Gonadotropin-Releasing Hormone Overrides the Negative Effect of Reduced Dietary Energy on Gonadotropin Synthesis and Secretion in Ewes*

Abstract

During prolonged periods of reduced dietary energy, there is a reduction in secretion of LH in females. To test the hypothesis that decreased secretion of LH is due to reduced secretion of GnRH, 18 ovariectomized ewes were fed either a low-energy diet (LOW, 60% of the National Research Council recommendations, n = 12) or a normal diet, (control, n = 6), for 127 days. Each ewe received basal levels (approximately 5 pg/ml) of estradiol via sc Silastic implants. After 127 days, serum concentrations of FSH and LH were reduced (P less than 0.05) by 63% and 77%, respectively in LOW ewes compared to control ewes. Pituitary concentrations of FSH and LH in LOW ewes also were reduced by 56% and 80%, respectively. Compared to levels in control ewes, concentrations of messenger RNAs for alpha-, FSH beta-, and LH beta-subunits were reduced by 75%, 76%, and 91%, respectively. Pulsatile administration of GnRH (250 ng/2 h) for three weeks to LOW ewes restored each of the parameters to levels not different from those in controls. By the end of the study, serum concentrations of GH in all LOW animals had risen dramatically, but not in control ewes. Therefore, it appears that exogenous GnRH is capable of restoring synthesis and secretion of gonadotropins in ewes receiving low-energy diets. These results provide support for the hypothesis that reduced dietary energy results in decreased secretion of GnRH.

Parturition to resumption of ovarian cyclicity: comparative aspects of beef and dairy cows

“…More recent evidence suggests that the pituitary sensitivity to GnRH pulses is decreased during undernutrition. Tatman et al (1990) found that the pituitary content of LH was lower in thin ewes; Kile et al (1991) found that undernutrition suppressed pituitary synthesis of LH, as the concentration of mRNA for both α and β subunits of LH were less in nutritionally restricted ovariectomized ewes, although pulsatile administration of GnRH was capable of restoring LH synthesis and secretion. Similarly, cows fed restricted diets released more LH in response to exogenous GnRH than cows fed moderate or high diets (Whisnant et al, 1985;Rasby et al, 1991) and had increased concentrations of GnRH in the median eminence stalk of the hypothalamus (Rasby et al, 1992), suggesting that the greater sequestration of LH in the anterior pituitary gland and decreased LH secretion in nutritionally restricted animals is because of reduced GnRH release.…”

Increased Galanin and Neuropeptide-Y Immunoreactivity within the Hypothalamus of Ovariectomised Ewes following a Prolonged Period of Reduced Body Weight Is Associated with Changes in Plasma Growth Hormone but Not Gonadotropin Levels

“…Energy restric tion or high energy expenditure results in a failure of the hypothalamo-pituitary-gonadal axis in animals [1][2][3] and in humans, being particularly prevalent in eating disor ders [4,5], Plasma levels of luteinising hormone (LH) and follicle-stimulating hormone (FSH) tend to be lower when animals such as sheep [6. 7] or humans [8] arc underfed, although such changes may depend upon whether gonads are present or absent [9,10], the macronutrient makeup of the diet [11][12][13] and the length of caloric restriction [ 14], As there is a tight coupling between pulses of gonado tropin-releasing hormone (GnRH) and LH [ 15], and since the pituitary release adequate LH in response to exoge nous GnRH in most species, including sheep, during undernutrition [2,9,[16][17][18], it is most likely that pertur bations of reproductive function are related to changes in the secretion of GnRH.…”

“…food deprivation reduces GH secretion [19] due, in part, to a reduction of GH-releasing hormone (GHRH) mRNA lev els in the hypothalamus [20]. In contrast, plasma GH levels are elevated during undernutrition in man [21], swine [22], cows [23], rabbits [24] and sheep [3,6,7], These effects on GH secretion are thought to involve altered hypothalamic function and subsequent changes in GHRH and somatos tatin (SR1H) secretion which act in concert to influence pulsatile GH patterns [25]. For instance, a reduction in SRIH secretion into the hypophysial portal vessels has been detected in ewes on a restricted diet [26].…”

Nutritionally Induced Body Weight Loss and Ovarian Quiescence in Shiba Goats.

“…Anestrus or amenorrhea h a s b e e n d e m o n st r a t e d t o b e c a u s e d b y nutritionally induced body weight loss in cows [4], rats [10], hamsters [11] and women [12]. In small ruminants, several studies have examined the effects of long-term restricted feeding on metabolic and hormonal environments in sheep [13][14][15]. Recently, we h a v e s u c c e s sf u l l y a d o p t e d tr a n sr e c t a l ultrasonography for monitoring the follicular and luteal dynamics during the estrous cycle in Shiba goats [16].…”