Gonadotropin response by postpartum mares to gonadotropin-releasing hormone

LH and IGF-1 release during oestrus and early luteal phase in lactating and non-lactating horse mares

The aim of the present study was to determine effects of lactation on basal LH and IGF-1 concentrations and on the LH response to a GnRH-analogue at different stages of the oestrous cycle in mares. A total of 17 cyclic Haflinger mares were included in the study. Experiments were performed on lactating mares in first postpartum oestrus, the subsequent early luteal phase, and second postpartum oestrus. Non-lactating mares were used in oestrus and early luteal phase. Blood samples were taken for 1 h at 15 min intervals. Mares were then injected with the GnRH-analogue buserelin (GnRHa; 5 microg i.v.) and blood samples were drawn every 15 min for further 2 h. LH in all samples and basal IGF-1-concentrations were determined by RIA. In lactating mares, basal LH concentrations during the early luteal phase tended to be lower (p = 0.07) and the LH response to GnRHa, calculated as area under the curve, was significantly less pronounced compared to non-lactating mares (p < 0.01). As well in lactating mares, the basal LH concentration between first early luteal phase and second oestrus differed significantly (p < 0.05) and the net response to GnRHa was significantly lower between first oestrus as well as second oestrus and first early luteal phase (p < 0.05) but not between first and second oestrous postpartum. Within the group of non-lactating mares, the LH response to GnRHa was as well significantly lower during oestrus than during early luteal phase (p < 0.01). IGF-1 concentrations differed neither between groups nor stages of the cycle within groups. In conclusion, basal and GnRHa-stimulated LH release in lactating mares is lower than in non-lactating mares. This difference, however, occurs only in the early luteal phase. In lactating mares, concentrations of LH appear adequate to allow ovulation to occur.

Follicular and FSH responses to parturition during the anovulatory season in mares

The ovaries of periparturient pony mares (n=9 to 16 parturitions per month for January to April) were scanned ultrasonically on the day of parturition, while those of postpartum and control mares (n=12) were examined at least twice weekly. Four mares had apparent lactational anovulation (incidence, 7%) that corrected spontaneously (1 mare) or within 14 d after the weaning of foals on August 10 (3 mares). All but 2 of the postpartum ovulations occurred after April 29; that is, parturition did not effectively stimulate ovulation in ponies foaling during the anovulatory season. Mean diameter of the largest follicle per month increased (P<0.001) progressively in the controls (means: 11.4, 14.4, 19.0 and 24.5 mm for January to April, respectively). In the parturient mares, the diameter of the largest follicle on day of parturition did not increase over months (range of means: 13.6 to 16.9 mm), indicating that a suppressive effect of pregnancy counteracted the stimulatory effect of season. Within each month of parturition, diameter of the largest follicle increased (P<0.05) between Day 0 (day of parturition) and Day 3 or 7. Blood samples for FSH assay were taken daily for 14 d from 6 mares with parturition in the middle of each month and from 6 controls on the corresponding calendar days. In periparturient mares, a significant increase in mean FSH concentrations occurred for all months of parturition between Day-2 and Day 0, followed by a significant decrease between Day 3 and Day 7. Maximum means for the periparturient FSH profile were temporally related to the beginning of follicular growth. In the controls, FSH concentrations were not affected by month or day, or by their interaction. Within each month, mean FSH concentrations were lower (P<0.05) in the periparturient mares than in the controls (averaged over all months: 3.9+/-0.1 versus 7.9+/-0.3 ng/ml) even though follicular growth was greater following parturition than during the corresponding calendar days in controls.

Determinants of attenuated LH-release associated with the first ovulation of the equine breeding season

An attenuated ovulatory rise in circulating concentrations of LH is characteristically associated with the first seasonal reproductive cycle of horse mares. Unlike ovulations (OV) of subsequent estrous cycles, the first OV of the breeding season (OV1) is not preceded by elevated concentrations of progesterone (PROG). Hence, the ability of pretreatment with PROG to abolish attenuation of LH-secretion associated with OV1 was investigated. Ten nonpregnant anestrous mares were randomly divided into 2 groups; control (C) and treated (T). Per individual, when diameter of the largest follicle was consistently greater than or equal to 35 mm, C mares received 3 times daily injections of cottonseed oil (IM), for 15 d or until OV, while T mares received exogenous PROG (IM) for 15 d, in a manner designed to mimic a diestrous pattern of release. Jugular blood samples were collected daily from onset of treatment through 10 d following the third OV (OV3). Repeated measures analyses of area under the ovulatory LH-rise (AUC) and the maximum concentration of LH associated with OV (MAX) revealed a significant main effect of OV (P less than .005) but no main effect of group or OV by group interaction (P greater than or equal to .5). When groups were combined, a significant increase in mean AUC and MAX from OV1 to OV3 was observed (P less than .01). To evaluate the influence of hypothalamic-hypophyseal recrudescence on ovulatory LH-release at OV1, 16 mares were bred to foal, and subsequently initiate reproductive activity, early (E; mid April; n = 8) or late (L; mid July; n = 8) in the year. A significant OV (1, 2 or 3) by group (E or L) interaction was observed for AUC (P less than .06) and MAX (P less than .04). Mean AUC and MAX increased progressively from OV1 to OV3 in E mares (P less than .05). In L mares, neither AUC or MAX changed from OV1 to OV3 (P greater than .4). Based on these data, we suggest that attenuation of the LH-rise at OV1 in the mare, is a consequence of incomplete recrudescence of the hypothalamic-pituitary axis.