B.J. McLeod

Effects of a GnRH agonist (Buserelin) on LH secretion in post-partum beef cows

Plasma LH concentrations were monitored in post-partum beef cows treated with the GnRH agonist Buserelin, administered via biodegradable implants. Animals were given either untreated Buserelin implants (Group 1, n = 6) or implants that had been preincubated for 7 (Group 2, n = 6) or 14 (Group 3,n = 6) days. Control cows were sham-implanted (Group 4, n = 3). The implants were inserted subcutaneously at a mean time of 25 ± 2.8 days after caiving, and LH concentrations were monitored over 8 h periods of blood sampling (samples taken at 10 min intervals) undertaken on Days — 2, 1, 2, 3, 7, 14, 21 and 28 (Day 1 was the day of implantation). In all animals treated with Buserelin, plasma LH concentrations increased within 1 h of implantation to reach a maximum concentration (mean 35.4 ± 2.86 ng ml−1) within 4 h. In all three Buserelin-treated groups, mean LH concentrations were higher on Day 1 (P < 0.001) and on Days 2 and 3 (P < 0.05) than on Day — 2, before the start of treatment. On subsequent sampling days (Days 7, 14, 21 and 28), mean LH concentrations did not differ significantly from pretreatment values, but on Days 14, 21 and 28, mean LH concentrations were lower (P < 0.05) in Buserelin-treated than in control cows. There were no significant differences in either the magnitude or the duration of the LH response between Buserelin treatment groups. Before treatment, LH episodes occurred with a mean frequency of 1.9 ± 0.4 episodes per 8 h, and in control cows occurred at a frequency of 2.3 ± 0.2 episodes per 8 h throughout the 28-day sampling period. In contrast, LH episodes were not recorded in any of the Buserelin-treated cows on the first 3 days after implantation, and were recorded in only 1 of 18, 2 of 18, 1 of 18 and 4 of 18 animals on Days 7, 14, 21 and 28, respectively. Following luteal regression, plasma progesterone concentrations remained low for the remainder of the sampling period (mean 18.2 ± 1.2 days) in 17 of 18 Buserelin treated cows. These results demonstrate that Buserelin treatment induces a short period of increased LH secretion, after which there is a prolonged period of suppression of LH release which blocks ovulation.

The development of subcutaneous-delivery preparations of GnRH for the induction of ovulation in acyclic sheep and cattle

Abstract A series of oil-based preparations and biodegradable implants were tested for their efficacy to release low levels of GnRH for a prolonged period when administered subcutaneously, and thus to stimulate tonic LH secretion, promote preovulatory follicle growth and induce ovulation in acyclic sheep and cattle. A total of 15 oil-based formulations incorporating nine different carrier solutions were assessed. Wide between-animal variation in both the magnitude and the duration of the LH response to these preparations overshadowed differences between the doses of GnRH given, or the formulations used. Only six out of 55 seasonally anoestrous ewes, and none of the 57 heifers treated with these preparations produced an endogenous preovulatory LH surge. The first generation of biodegradable GnRH implants tested (ChB2) increased plasma LH concentrations from mean pretreatment values of 0.36 ± 0.08 ng/ml (mean ± s.e.m.) to a mean maximum concentration of 6.11 ± 0.89 ng/ml, and a preovulatory-surge and ovulation occurred in 61% of treated seasonally anoestrous ewes. The second GnRH implant-type (ChB3) resulted in mean maximum LH concentrations of 13.12 ± 1.72 ng/ml, and preovulatory surges and ovulation in 89% of treated ewes. The administration of two implants per animal induced ovulation in 88% and 94% of treated anoestrous ewes, for implant types ChB2 and ChB3, respectively. These results demonstrate the potential for using biodegradable implants, but not oil-based preparations, for inducing ovulation with GnRH in acyclic animals.

Ovarian control of gonadotrophin secretion in the prepubertal heifer

Abstract In order to investigate the development of ovarian feedback control of gonadotrophin secretion in the immature animal, three groups of eight Hereford X Friesian heifers were bilaterally ovariectomized at either 2, 6 or 12 weeks of age. Blood samples were collected for gonadotrophin analysis (15-min intervals for 6 h) on days −1, 4, 7, 14, 28, 55 and 110 relative to ovariectomy. The ovaries from these animals showed that follicular development was established soon after birth and the follicles were producing oestradiol. In all animals, ovariectomy was followed by a rise in plasma gonadotrophin concentrations although the response was delayed in heifers ovariectomized at 2 weeks compared to those ovariectomized at 6 or 12 weeks of age. Mean LH concentrations were significantly elevated 14, 7 or 4 days after ovariectomy and mean FSH concentrations were significantly higher than pre-treatment values 28, 7, or 4 days post-ovariectomy in heifers ovariectomized at 2, 6 or 12 weeks of age, respectively. The results suggest that an inhibitory mechanism, of ovarian origin, that controls gonadotrophin secretion develops between 2 and 12 weeks of age in the heifer.

Ovulatory responses to continuous administration of GnRH in nine-month-old prepubertal beef heifers.

Abstract Continuous infusion of gonadotrophin-releasing hormone (GnRH) will consistently induce ovulation in several species, but has had limited success in post-partum acyclic cattle. We suggest that this treatment is only effective when anovulation is due solely to a lack of gonadotrophic stimulation of the ovary, rather than to a hypothalamic-pituitary block. Therefore, we have attempted to induce ovulation in 9-month-old prepubertal heifers, an experimental model in which lack of ovarian stimulation is likely to be the only limitation to oestrous cycles. GnRH was administered either at a controlled rate (5 μg/h) via osmotic minipumps (inserted s.c.), or at a less predictable rate by s.c. implantation of a biodegradable implant (40 μg GnRH/implant) previously shown to be effective in sheep. The mean magnitude of the maximum luteinizing hormone (LH) concentration in response to GnRH treatment was significantly higher in heifers treated with 24 GnRH implants than in those given 8 implants or osmotic minipumps (18.02 ± 1.23 ng/ml, 11.55 ± 2.37 ng/ml and 8.6 ± 2.29 ng/ml, respectively). In addition, plasma LH concentrations reached preovulatory surge magnitude within 4 h of implantation in 5 5 , 1 6 and 0 5 animals in these treatments groups. Conversely, an endogenous preovulatory LH surge was recorded in 0 5 , 3 6 and 5 5 heifers treated with 24 implants, 8 implants or osmotic minipumps. The consistent ovulatory response recorded in those heifers treated with osmotic minipumps demonstrates the importance of administering GnRH at a constant and controlled rate.

Pituitary and ovarian responses to prolonged administration of low doses of GnRH in prepubertal heifers

Abstract The responses of prepubertal heifers to continuous infusion of GnRH were examined in 4 experiments. Eighteen 4 month old beef heifers received 0.25, 0.5 or 2.5 μg GnRH/h for 48 h by continuous intravenous infusion. No significant changes in plasma LH concentrations were observed during infusion of the two lower GnRH doses. However, in all six heifers receiving 2.5 μg GnRH/h, jugular plasma LH concentrations were significantly increased within 2 h of the start of infusion. Preovulatory-type gonadotrophin surges occurred in three of the six animals receiving 2.5 μg/h GnRH during the period of infusion and elevated plasma progesterone concentrations were detected in these animals 4 and 6 days after the end of treatment. Heifers that exhibited preovulatory gonadotrophin surges had higher plasma oestradiol-17β concentrations before treatment than those that did not. In a second experiment, five heifers received 1.25 μg/h GnRH for 120 h. Mean LH and oestradiol-17β concentrations increased in all animals over the first 60 h and then returned to baseline. Two heifers exhibited preovulatory-type LH surges and some luteal function. In Experiment 3, osmotic infusion pumps implanted s.c. were used to deliver 5.0, 7.5 or 10.0 μG GnRH for 8 h and the LH responses compared with the responses to i.v. infusion of 2.5 μg GnRH for 8 h. The latin square design used four heifers and 4 treatment days. Significant effects of both GnRH-dose and day of GnRH treatment were detected, with subcutaneous infusion of 10.0 μg GnRH/h producing a significantly greater LH response than i.v. infusion of 2.5 μg GnRH/h, and the mean LH response to all treatments being greater during the first treatment period than on subsequent treatment days. In Experiment 4, 12 heifers received progesterone pretreatment for 14 days followed by 5.0 μg GnRH/h or saline, infusion s.c. for 120 h. A third group of five heifers received the GnRH infusion only. Mean LH concentrations were significantly elevated in response to GnRH over the first 8 h of the infusion period in progesterone pretreated heifers and over the first 16 h of infusion in animals receiving GnRH alone. Preovulatory LH surges were detected in four GnRH-treated heifers, two animals that were progesterone pretreated and two that received GnRH alone. Laparoscopy revealed corpora lutea in three of these four heifers, and plasma progesterone concentrations were elevated in two of these. It is concluded that GnRH infusion does not consistently induce ovulation in prepubertal heifers.