Release of LH in anoestrous and cyclic ewes

Summary. To characterize the changes in LH pulse frequency during the transition to breeding season, LH pulse patterns and serum progesterone profiles were determined in 8 intact ewes from mid-anoestrus to the early breeding season. Overall, 8 increases in LH pulse frequency were observed and these were restricted to 5 ewes. Of the 8 increases, 7 occurred during the 4 weeks before the first cycle, 5 of them within 1 week after a pulse frequency typical of anoestrus (0\p=n-\2per 8 h). Six of them occurred less than 1 week before either a full-length luteal phase (n = 2) or a 1\p=n-\3-day increment in progesterone (n = 4). Seven of these brief progesterone increases were observed in 6 ewes, 5 of them immediately preceding the first full-length luteal phase. These results are consistent with the hypothesis that the seasonal decrease in response to oestradiol negative feedback at the beginning of the breeding season causes an increase in GnRH, and thereby LH pulse frequency. In addition, they demonstrate that the first increase in tonic LH secretion occurs in < 1 week and, in most ewes, initiates either the first fulllength cycle or a transient increase in progesterone, the latter occurring more often.

Section: Methodssupporting

confidence: 80%

Changes in LH pulse frequency and serum progesterone concentrations during the transition to breeding season in ewes

I’Anson

Legan²

1988

“…Because episodic LH and oestrogen secretion is infrequent and irregular in anoestrus ewes (Yuthasastrakosol et al 1977) this could account for the inconsistency of NOS inhibitors in this study. One test of this hypothesis would be to determine if there is a negative correlation between peak oestrogen concentrations and LH secretion.…”

Section: Discussionmentioning

confidence: 88%

Does nitric oxide act in the ventromedial preoptic area to mediate oestrogen negative feedback in the seasonally anoestrous ewe?

McManus¹,

Valent²,

Hardy³

et al. 2007

Seasonal anoestrus in the ewe results from enhanced oestrogen negative feedback. Recent data have implicated the ventromedial preoptic area (vmPOA) as an important site of oestrogen action. This study addressed whether NO acts within the vmPOA to inhibit LH during seasonal anoestrus. In Experiment 1, microimplants containing Nu-nitro-L-arginine methyl ester (L-NAME, NOS inhibitor), S-methyl thiocitrulline (SMTC, neural NOS (nNOS) inhibitor) or empty implants (control) were administered during mid-anoestrus to the vmPOA. L-NAME, but not SMTC, significantly increased LH pulse frequency. For Experiment 2, ewes in late anoestrus were administered 7-nitroindazole (7NI; nNOS inhibitor), L-NAME, SMTC, or empty implants. 7NI, but not L-NAME or SMTC, increased LH pulse frequency. In Experiment 3, the effects of microimplants and microinjections of L-NAME were compared in mid-anoestrus. Microinjections of L-NAME (300 nl at 10 mg/ml) increased LH pulse frequency, but microimplants did not. In late anoestrus, similar microinjections were ineffective. Taken together, the results of Experiments 1-3 suggested that NO inhibition may be stronger during the middle than at the end of seasonal anoestrus. To test this hypothesis, ewes in Experiment 4 received microinjection of L-NAME or vehicle thrice during the non-breeding season; none of the treatments increased LH pulse frequency. These results indicate that NO plays a role in the vmPOA in suppressing LH secretion during seasonal anoestrus because NOS inhibitors were consistently stimulatory when LH pulse frequency was low. However, the inconsistent and modest effects of these inhibitors suggest that NO actions in this area cannot completely account for the effects of inhibitory photoperiod. Reproduction (2007) 134 137-145

“…The secretion of gonadotrophins, particularly the frequency of LH pulses, increases before and during the annual breeding season (Scaramuzzi & Martensz, 1975;Yuthasastrakosol et al, 1977). Studies with ovariectomized ewes bearing oestradiol implants suggest that this is mainly due to changes in the responsiveness of the hypothalamus to negative feedback by ovarian oestrogen (Legan et ai, 1977).…”

Section: Introductionmentioning

confidence: 99%

Seasonal and hormonal control of pulsatile LH secretion in the dairy goat (Capra hircus)

Chemineau¹,

Martin²,

Saumande³

et al. 1988

and 1 month later 6 of them were injected daily (10 h after 'dawn') with 2 mg melatonin. The other 5 animals served as controls. Blood samples (every 10 min for 6h) were taken just before and 38 and 72 days after the start of melatonin treatment. As the experiment progressed, LH pulse frequency increased by 20% in melatonin-treated goats but decreased by 43% in controls. Mean LH values were maintained in melatonin\x=req-\ treated females but decreased in the control group. Melatonin did not affect pulse amplitude.The results of the experiments with ovariectomized does suggest that seasonal reproductive cycles in the goat are caused by changes in the duration of melatonin secretion at night, which induce alterations in the intensity with which oestradiol inhibits LH secretion. In turn, this may be responsibile for the gradual changes in the frequency of LH pulses observed in entire does. The frequency of LH pulses is one of the most important endocrine signals controlling ovarian activity so would form the final link in the chain connecting environmental and reproductive cycles.