Abstract
Using ovariectomised and intact Ile-de-France ewes, we tested the hypothesis than an increase in the secretion of GnRH is involved in the release of the preovulatory luteinizing hormone (LH) surge during the breeding season. In Experiment 1, hypophysial portal blood and peripheral blood were sampled simultaneously every 5 min for 25 h from 5 conscious, unrestrained ewes, 7 days after ovariectomy. Oestradiol (25 µg intramuscularly + 25 µg intravenously) was injected 6 h 25 min after the beginning of blood collection. In Experiment 2, cycling, intact ewes were treated with an intravaginal progesterone implant to control the time of the LH surge. The implants were inserted on day -12 and removed on day 0, and cannulae for sampling portal blood were inserted on day -6. Samples of peripheral and portal blood were collected every 6-10 min for 25-36 h after implant removal.
All 5 ewes exhibited a clear pulsatile pattern of LH secretion before oestradiol injection in the 1st experiment, followed by a typical biphasic decrease (negative feedback) and increase (positive feedback) in the mean concentration of LH. For analysis, the sampling period was divided into phases corresponding to pretreatment, negative feedback and positive feedback. During the pretreatment phase, GnRH pulses were clearly defined in portal blood and were always synchronized with LH pulses in the peripheral circulation. GnRH pulse frequency was 5.9 ± 0.6 pulses per 6 h and pulse amplitude was 31.6 ± 7.6 pg/ml. During negative feedback, the frequency (4.2 ± 0.5 pulses per 6 h, P < 0.01) and the amplitude (15.2 ± 4.6 pg/ml, P < 0.05) of the GnRH pulses decreased compared with the pretreatment phase. During positive feedback, there was a large surge in the concentration of GnRH, due primarily to an increase in pulse frequency (11.0 ± 1.3 pulses per 6 h, P < 0.01); pulse amplitude did not change significantly, but there was a large increase in the basal concentration of GnRH (P < 0.05). Consequently, the mean level of GnRH before injection with oestradiol (5.3 ± 1.0 pg/ml) differed (P < 0.05) from that during the negative (3.8 ± 0.5 pg/ml) and positive feedback phases (18.9 ± 4.7 pg/ml).
Preliminary data from the experiment 2 suggest that a similar large increase in GnRH secretion occurred during the preovulatory LH surge of intact ewes. In each of the 3 animals which had an LH surge during the period of portal blood collection, a sudden and unam-biguous increase in GnRH secretion was associated with initiation and maintenance of the LH surge. This was evident when the total amount of GnRH collected during 8 h just prior to the LH surge was compared with that collected during the 8 h after the onset of the surge: 12 vs. 96 pg (8-fold increase); 573 vs. 2618 pg (4.5-fold increase); 19 vs. 1008 pg (54-fold increase).
These studies indicate that an abrupt and large surge of GnRH secretion occurred at the time of the LH surge, in both castrated ewes treated with estrogen and intact, cycling ewes.
All 5 ewes exhibited a clear pulsatile pattern of LH secretion before oestradiol injection in the 1st experiment, followed by a typical biphasic decrease (negative feedback) and increase (positive feedback) in the mean concentration of LH. For analysis, the sampling period was divided into phases corresponding to pretreatment, negative feedback and positive feedback. During the pretreatment phase, GnRH pulses were clearly defined in portal blood and were always synchronized with LH pulses in the peripheral circulation. GnRH pulse frequency was 5.9 ± 0.6 pulses per 6 h and pulse amplitude was 31.6 ± 7.6 pg/ml. During negative feedback, the frequency (4.2 ± 0.5 pulses per 6 h, P < 0.01) and the amplitude (15.2 ± 4.6 pg/ml, P < 0.05) of the GnRH pulses decreased compared with the pretreatment phase. During positive feedback, there was a large surge in the concentration of GnRH, due primarily to an increase in pulse frequency (11.0 ± 1.3 pulses per 6 h, P < 0.01); pulse amplitude did not change significantly, but there was a large increase in the basal concentration of GnRH (P < 0.05). Consequently, the mean level of GnRH before injection with oestradiol (5.3 ± 1.0 pg/ml) differed (P < 0.05) from that during the negative (3.8 ± 0.5 pg/ml) and positive feedback phases (18.9 ± 4.7 pg/ml).
Preliminary data from the experiment 2 suggest that a similar large increase in GnRH secretion occurred during the preovulatory LH surge of intact ewes. In each of the 3 animals which had an LH surge during the period of portal blood collection, a sudden and unam-biguous increase in GnRH secretion was associated with initiation and maintenance of the LH surge. This was evident when the total amount of GnRH collected during 8 h just prior to the LH surge was compared with that collected during the 8 h after the onset of the surge: 12 vs. 96 pg (8-fold increase); 573 vs. 2618 pg (4.5-fold increase); 19 vs. 1008 pg (54-fold increase).
These studies indicate that an abrupt and large surge of GnRH secretion occurred at the time of the LH surge, in both castrated ewes treated with estrogen and intact, cycling ewes.
| Original language | English |
|---|---|
| Title of host publication | Recent progress on GnRH and gonadal peptides |
| Subtitle of host publication | Proceedings of an International Symposium, Paris, 1989 |
| Editors | P. Bouchard, F. Haour, P. Franchimont, B. Schatz |
| Place of Publication | Paris |
| Publisher | Elsevier |
| Pages | 59-70 |
| ISBN (Print) | 2906077143 |
| Publication status | Published - 1990 |