Controlled, reversible suppression of estrous cycles in beef heifers and cows using agonists of gonadotropin-releasing hormone.

Abstract

Agonists of GnRH were examined for their potential to achieve controlled, reversible suppression of estrous cycles in beef cattle. In Exp. 1, cyclic heifers received two (Group B2) or four (Group B4) buserelin (D-Ser[Bu(t)]6-Pro9-LHRH[1-9] nonapeptide ethylamide) implants and degree of cessation of estrous cycles was monitored. Treatment with buserelin caused estrous cycles to cease, as indicated by basal (.2 ng/mL) concentrations of progesterone, for 48.4 +/- 3.8 d (mean +/- SEM) in Group B2, which was less (P = .6) than the 87.4 +/- 17.4 d in Group B4. In Exp. 2, heifers treated with one (Group D1) or two (Group D2) implants of deslorelin (D-Trp6-Pro9-des-Gly10-LHRH ethylamide) had basal progesterone concentrations for 203 +/- 26 d (Group D1) and 170 +/- 28 d (Group D2; P > .05). In Exp. 3, stage of the estrous cycle was synchronized in cows, and, on d 7 of the ensuing cycle, cows received four deslorelin implants for 28 (Group D28) or 56 d (Group D56). Treatment with deslorelin induced an acute increase in plasma concentrations of immunoactive and bioactive LH, which remained increased over 7 d of observation. Based on profiles of progesterone, cows did not develop a functional corpus luteum during deslorelin treatment. Days to first and second estrus after implant removal were similar for cows in Group D28 (23.6 +/- 2.1 and 40.2 +/- 4.2 d, respectively) and Group D56 (21.5 +/- 3.3 and 44.3 +/- 2.9 d). The findings indicated that GnRH agonists block estrous cycles in cattle. Also, estrous cycles returned after discontinuation of treatment. Furthermore, the consistent and predictable responses detected in cows after implant removal indicated that agonists should be suitable for achieving a controlled, reversible suppression of estrous cycles in cattle.

Evaluation of Remotely Delivered Leuprolide Acetate as a Contraceptive Agent in Female Elk (Cervus Elaphus Nelsoni)

“…The process of down-regulation has been described previously in other species and is initiated by long-term exposure to GnRH agonist. This effect causes a reduction in GnRH receptors on gonadotropes (Clayton, 1989), depletion of pituitary LH and follicle-stimulating hormone content (Aspden et al, 1996), and elimination of the preovulatory LH surge (Gong et al, 1995;D'Occhio et al, 1996). These responses have been shown to result in ovulation failure and infertility, which persists as long as the agonist is present in circulation at therapeutic levels (Melson et al, 1986;D'Occhio et al, 2000).…”

“…This effect causes a reduction in GnRH receptors on gonadotropes (Clayton, 1989), depletion of pituitary LH and follicle-stimulating hormone content (Aspden et al, 1996), and elimination of the preovulatory LH surge (Gong et al, 1995;D'Occhio et al, 1996). These responses have been shown to result in ovulation failure and infertility, which persists as long as the agonist is present in circulation at therapeutic levels (Melson et al, 1986;D'Occhio et al, 2000). Our findings are consistent with previous observations of acute and chronic responses reported in sheep (Dobson, 1985), cattle (D 'Occhio et al, 1989;Gong et al, 1996), horses (Montovan et al, 1990), deer (Becker andKatz, 1995), and elk (Baker et al, 2002) treated with GnRH agonist.…”

“…Therefore, the timing of treatment application is an important consideration in successful contraception. Because of the acute rise in LH concentrations that occurs following GnRH agonist treatments, ovulation of growing follicles can be induced (Macmillan and Thatcher, 1991;D'Occhio and Aspden, 1999). Therefore, to ensure effective contraception in female elk, leuprolide treatments should be applied before the initiation of seasonal estrus.…”

“…The use of gonadotropin-releasing hormone (GnRH) agonist implants to suppress short-term ovarian follicular growth and ovulation are well documented for a number of species including cattle (McLeod et al, 1991;D'Occhio et al, 1996), sheep (McNeilly and Fraser, 1987), monkeys , and humans (Broekmans et al, 1996). However, few studies have established the efficacy of these agents for long-term suppression of ovarian activity and contraception (Trigg et al, 2001;Baker et al, 2002Baker et al, , 2004D'Occhio et al, 2002) and, to our knowledge, none have previously demonstrated effective contraception by dart delivery of the implant.…”

Excretion rate of progesterone in milk and faeces in lactating dairy cows with two levels of milk yield

“…Our previous observations and other studies [7,30] showed that ovarian P4 production could be prevented by strategic use of a GnRH-agonist to create progesterone-free animals similar to ovarietomised cows. In order to block endogenous production of P4, each cow initially received a 6 mg Deslorelin (GnRH-agonist; D-Trp 6 -D-Trp 6 -Pro 9 -des-Gly 10 -GnRH ethylamide) ear implant (Peptech Animal Health, Sydney, Australia) initially, followed by two injections of prostaglandin F2α (2 mL Prosolvin, Intervet, Melbourne) at 0800 and 1600 h, 10 days later (Fig.…”