Endocrine Mechanisms of Puberty in Heifers. Role of Hypothalamo-Pituitary Estradiol Receptors in the Negative Feedback of Estradiol on Luteinizing Hormone Secretion1

Abstract

The hypothesis tested was that the decline in negative feedback of estradiol on secretion of luteinizing hormone (LH) that occurs as puberty approaches in heifers results from a decline in the number of receptors for estradiol in the hypothalamus and/or pituitary. In addition, associated changes in receptors for luteinizing hormone-releasing hormone (LHRH) in the pituitary, ovarian follicle development, and uterine growth were characterized. Fifty prepubertal heifers, 234 to 264 days of age, were used. Six heifers of median body weight were designated controls, and sequential blood samples were collected at 20-min intervals for 24 h every 2 wk from 249 days of age through puberty and analyzed for concentrations of LH. Frequency of LH pulses/24 h was regressed on number of days prepuberty to develop a prediction equation for puberty. Thirty of the remaining 44 heifers were killed at 253, 302, and 351 days of age (n = 10/group), and tissues for described analyses were collected. Three to 5 days before tissue collection, sequential blood samples were obtained from these heifers, as described for control heifers to determine frequency of release of LH. With this information, number of days prepuberty at the time of tissue collection was estimated from the prediction equation developed with data from control heifers. The average age at puberty in control heifers was 366 days. The average age at puberty of heifers that were not killed or included in the control group (n = 14) was 360 days. Receptor and morphological data were related to the estimated onset of puberty. Cytosolic concentration of receptors for estradiol (fmoles receptor/mg cytosolic protein) in the anterior hypothalamus, medial basal hypothalamus, and anterior pituitary declined (p less than 0.05) as puberty approached. No change in concentration of receptors for estradiol was observed in the stalk median eminence or preoptic area. The concentration of receptors for LHRH in the anterior pituitary did not change as puberty approached. Uterine weight increased rapidly during the 50 days preceding puberty. The number of small, medium, or large follicles and the wet, pressed, or dry weight of the ovaries did not change as puberty approached. Follicles with a diameter greater than 12 mm were found only in the 3 heifers estimated to be closest to puberty at the time of tissue collection. The hypothesis that the decline in estradiol feedback on secretion of LH during the prepubertal period in heifers may result from a decline in the concentration of binding sites for estradiol at the hypothalamus and/or pituitary is supported by this study.

The value of reproductive tract scoring as a predictor of fertility and production outcomes in beef heifers1

“…This is in contrast with an older theory that a heifer needs to reach a specific level of somatic development (BW) for the onset of puberty to be induced (Day et al, 1984(Day et al, , 1987Foster, 1994) and may indicate that there is Reproductive tract scoring in beef heifers an age-related induction of puberty that is not related to the BW of the heifer, in agreement with Yelich et al (1995) and Pence et al (2007). On the other hand, it may also indicate some variation in the critical BW of individual heifers that needed to be achieved to induce puberty, meaning that there was some scope for selection for AP in this population.…”

“…Previous studies have clarified the basic principles of the onset of puberty (Day et al, 1984(Day et al, , 1987Foster, 1994). Puberty in cattle occurs when a certain level of somatic development (critical BW) is reached, causing the prepubertal negative feedback of estradiol on the pituitary gland, hypothalamus, or both, to be terminated, which leads to the first ovulation.…”

Heifer fertility and carry over consequences for life time production in dairy and beef cattle

“…The diameter, duration and oestradiol secretion of the dominant follicle all increase over this final pre-pubertal period, driven by higher secretion of FSH, an increase in LH pulsatility in the last 50 days and a decline in the negative feedback effects of oestrogen on the hypothalamic-pituitary axis (Rawlings et al, 2003;Gasser, 2013). This finally triggers the switch to a positive feedback of oestradiol driving an LH surge (Day et al, 1987). A heifer reaches puberty when she ovulates a potentially fertile oocyte and some definitions also stipulate that this should be accompanied by visual signs of oestrus (Perry, 2012).…”

Evaluation of pre-breeding reproductive tract scoring as a predictor of long term reproductive performance in beef heifers

“…The onset of puberty in heifers is initiated by a decrease in oestradiol receptors in the hypothalamus and pituitary, ending the prepubertal negative feed-back and resulting in the first LH surge and ovulation (Day et al, 1984, Day et al, 1987. This shift occurs at a specific critical BW (as a proportion of adult BW) and critical age which varies amongst animals (Pence et al, 2007).…”