Regulation of reproductive seasonality in the red deer hind: oestradiol-dependent and -independent influences on the patterns of LH concentrations

Abstract

The control of reproductive seasonality was studied in farmed adult red deer hinds that had been either ovariectomized or ovariectomized and oestradiol-treated (s.c. implants). The breeding season, delineated by progesterone secretion in intact hind herdmates, was characterized by high (mean 0.6, range 0.1-2.5 ng ml-1 plasma) LH concentrations in ovariectomized oestradiol-treated hinds. In contrast, during the non-breeding season plasma LH concentrations in these animals were significantly lower (mean 0.1, range 0-0.9 ng ml-1 plasma). LH secretion in ovariectomized untreated hinds also displayed a marked seasonal pattern, approximately the inverse of daily photoperiod (that is, a winter peak and summer trough). The pituitary LH response to 10 micrograms exogenous GnRH was also maximal during the breeding season in ovariectomized (mean 7.4, range 1.2-14.6 ng ml-1) and ovariectomized, oestradiol-treated (mean 16.4, range 1.4-32.3 ng ml-1) hinds. These results indicate that LH secretion in the hind is regulated by both steroid-dependent and -independent mechanisms.

Pulsatile Luteinizing Hormone Secretion in the Ovariectomized, Thyroidectomized Red Deer Hind Following Treatment with Dopaminergic and Opioidergic Agonists and Antagonists1

“…Firstly, in the deer, both LH pulse frequency and pulse amplitude were suppressed in the presence of estradiol during the nonbreeding season, whereas in the ewe, estradiol primarily suppresses pulse frequency at this time [69]. Since estradiol greatly magnifies the seasonal fluctuations in pituitary responsiveness to GnRH that occur in red deer hinds [3], the decrease in LH pulse amplitude during the nonbreeding season is likely to reflect a reduction in the amount of LH released from the anterior pituitary gland in response to GnRH pulses, rather than a decrease in GnRH pulse size. Secondly, thyroidectomy during the breeding season was unable to overcome these effects on LH secretion, since LH pulse frequency and amplitude were as much suppressed by estradiol in THX hinds as in euthyroid hinds during the nonbreeding season.…”

“…This is in agreement with results from experiment 2, in which bromocriptine and sulpiride evoked prolactin but not pulsatile LH responses in ovariectomized hinds that had not received estradiol. Neural pathways that are not activated by gonadal steroids are of particular significance to red deer reproduction [3] since they appear to make a much larger contribution to seasonal breeding in this species than in the ewe. Therefore, in experiment 2 we examined the role of endogenous opioid peptides, as these appear to have an inhibitory role that is independent of estradiol in some [23][24][25] but not all [26][27][28] studies of seasonal LH secretion in ovariectomized sheep.…”

“…The red deer (Cervus elaphus) hind exhibits a circannual cycle of reproductive activity, with periods of estrous cyclicity occurring during winter under shortened ambient photoperiod [1][2][3], similar to that of the ewe [4,5]. However, recent studies indicate that differences in seasonal gonadotropin secretion patterns exist between the two species.…”

“…However, recent studies indicate that differences in seasonal gonadotropin secretion patterns exist between the two species. Red deer exhibit dramatic seasonal fluctuations of LH secretion even in the absence of gonadal steroids [3], as well as in exogenous GnRH-induced LH secretion [3,6], whereas the seasonal oscillations in these parameters in the domestic ewe are considerably smaller [7][8][9][10] or even undetectable [11,12], and seasonal reproductive quiescence is usually attributed primarily to the dramatic increase in negative feedback of estradiol on the hypothalamic GnRH pulse generator (e.g., [12]). …”

Cloning of the Elk TSH .BETA.-Subunit cDNA and Seasonal Expression of the Pituitary Glycoprotein Hormone Genes

“…Activity of their reproductive organs, including testis, ovaries and accessory sex glands, exhibit substantial annual changes along with the circulatory levels of sex and pituitary hormones [4,5]. In the absence of pregnancy, deer hinds are polyestrous and are capable of exhibiting between four to nine continuous 17-19 day oestrous cycles over a 3-6 month period between autumn and spring [6][7][8]. Thus, the pituitary gland responding to photoperiod stimulus would change expression of the pituitary gonadotropin genes according to the reproductive season.…”