help button home button Endocrine Society Endocrinology
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Purchase Article
Right arrow View Shopping Cart
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow Request Copyright Permission
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Billings, H. J.
Right arrow Articles by Anderson, G. M.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Billings, H. J.
Right arrow Articles by Anderson, G. M.
Endocrinology Vol. 143, No. 7 2618-2625
Copyright © 2002 by The Endocrine Society

NEUROENDOCRINOLOGY

Temporal Requirements of Thyroid Hormones for Seasonal Changes in LH Secretion

Heather J. Billings1, Catherine Viguié2, Fred J. Karsch, Robert L. Goodman, John M. Connors and Greg M. Anderson3

Reproductive Sciences Program (H.J.B., C.V., F.J.K.) and Department of Physiology (H.J.B., F.J.K.), University of Michigan, Ann Arbor, Michigan 48109; and Department of Physiology, West Virginia University (G.M.A., R.L.G., J.M.C.), Morgantown, West Virginia 26506

Address all correspondence and requests for reprints to: Dr. Fred J. Karsch, Reproductive Sciences Program and Department of Physiology, 300 North Ingalls Building, 11th Floor, Ann Arbor, Michigan 48109-0404. E-mail: . fjkarsch{at}umich.edu


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 Summary
 References
 
The transition between breeding and anestrous seasons in ewes is driven by an endogenous rhythm in responsiveness to estradiol negative feedback. One stage of this rhythm, the transition to anestrus, requires the presence of thyroid hormone during a window of responsiveness that opens in the late breeding season. The primary goal of this study was to assess when ewes lose responsiveness to thyroid hormone (i.e. when the window closes). In addition, we investigated whether thyroid hormone influences aspects of seasonality other than the transition to anestrus. Ovariectomized ewes maintained in a simulated natural photoperiod were implanted with estradiol, thyroidectomized, and treated with T4 for 100 d beginning at progressively later dates during the anestrous season. Onset of neuroendocrine anestrus (decrease in LH), latency to anestrus, and time of onset of the subsequent neuroendocrine breeding season (rise in LH) were determined. Ewes gradually lost responsiveness to T4 during the latter half of the anestrous season, as judged by increasing latency to the decrease in LH and, eventually, failure to exhibit a decrease in LH. Progressively later T4 replacements also caused progressive delays in the subsequent breeding season. In contrast, the annual PRL cycle was not significantly affected by thyroidectomy or T4 replacement. These findings indicate that 1) responsiveness to T4 is lost gradually during the mid to late anestrous season; 2) thyroid hormones can influence the timing of the breeding season and thus may be required for the maintenance or entrainment of the endogenous reproductive rhythm; 3) thyroid hormones are not required for all seasonal neuroendocrine cycles.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 Summary
 References
 
IN THE EWE, seasonal breeding is generated by an endogenous circannual rhythm entrained to the seasons by changes in ambient day length, such that the breeding season begins in early autumn (September) and ends in winter (January-February) (1). At a neuroendocrine level, this rhythm is observed as cyclical changes in responsiveness to the negative feedback effects of estradiol on pulsatile GnRH and LH secretion (2, 3). During the transition to anestrus, GnRH and LH pulse frequencies decrease markedly due to enhanced responsiveness to the negative feedback action of estradiol. There is extensive evidence that the timing of events within this endogenous rhythm are synchronized to the seasons by the pineal hormone, melatonin (1, 4, 5, 6, 7, 8).

Thyroid hormones are critical for at least one aspect of this endogenous rhythm. Since the 1930s, when a relationship between thyroid and reproductive function was first recognized (see Ref. 9 for review), the requirement for thyroid hormones to terminate the breeding season has been established in a number of birds and mammals (10, 11, 12, 13, 14, 15, 16). In sheep, the transition to anestrus requires that thyroid hormones be present for a minimum of 60–90 d (17).

This experiment was designed to identify the stage(s) of the annual rhythm when thyroid hormones act to allow the development of anestrus. Prior work demonstrates that thyroidectomized (THX) ewes can respond to thyroid hormones and enter anestrus if T4 is replaced during late winter (i.e. late breeding season) (17, 18). However, if T4 replacement is delayed until late in the summer (i.e. late anestrus), it is no longer effective (17). This finding suggests that there is a critical period or window of responsiveness for thyroid hormone action in regulating seasonal reproduction. The window appears to open in the late breeding season (17), but it is not known how long into the typical nonbreeding season ewes remain responsive to T4. Thus, our main objective was to assess when ewes lose their responsiveness to thyroid hormone.

Subsequently, the experiment was extended to determine whether the onset of the breeding season was also delayed in ewes that had a delayed onset of anestrus due to timing of T4 treatment. This second part of the experiment was conducted to gain insight into whether thyroid hormones may influence aspects of the seasonal rhythm other than the transition to anestrus. Finally, to gain further insight into how thyroid hormones may be interacting with seasonal cycles, we determined whether another seasonal neuroendocrine change, the annual PRL cycle (19), is dependent upon thyroid hormones.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 Summary
 References
 
Animals
Thirty-four sexually mature Suffolk ewes were maintained at the Sheep Research Facility (Ann Arbor, MI; 42° 18'N), fed a diet of hay and alfalfa pellets, and provided with ad libitum access to water and mineral licks. All ewes were ovariectomized (OVX) in late October 1998, and each was treated sc with a 3-cm estradiol-containing SILASTIC brand implant (Dow Corning Corp., Midland, MI) (20) at the time of ovariectomy. These implants are known to maintain a luteal phase level (~3 pg/ml) of circulating estradiol (21). Twenty-eight of the ewes were THX in late November 1998. As two of the four parathyroid glands were also removed during thyroidectomy, plasma calcium levels were monitored, and the ewes were supplemented with calcium gluconate (23% solution; Vedco, Inc., St. Joseph, MO) if calcium deficiency was detected. Surgeries were performed using sterile procedures while ewes were maintained under appropriate anesthesia. All procedures were performed as approved by the University of Michigan Committee for the Use and Care of Animals.

Experimental design
From October 1998 through December 1999, thyroid-intact (n = 6) and THX ewes (n = 28) were housed indoors. Ewes were maintained in simulated natural photoperiod (42° 18'N), and the daily times when lights were on (350 lux at the level of the sheep’s heads) included the twilight transitions at that latitude. Temperature was not regulated, although temperature extremes indoors were attenuated compared with outdoor temperatures. The THX ewes were blocked by body weight and randomly assigned to one of five groups, as illustrated in Fig. 1Go: no T4 treatment (THX control; n = 5) or T4 replacement beginning on January 15 (late breeding season; n = 5), April 16 (early anestrus; n = 7), June 18 (mid-anestrus; n = 5), or August 13 (late anestrus; n = 6). These times were chosen to include the period when ewes are known to be either sensitive to T4 (January onset of treatment) or insensitive to T4 (August onset) as well as intermediate times when responsiveness to T4 was unknown. For injection, crystalline L-T4 (Sigma, St. Louis, MO) was dissolved in 0.3 N sodium hydroxide and 50% ethanol, then diluted in a sodium bicarbonate-buffered saline solution for injection, as described previously (22). T4 replacements lasted for 100 d, a duration sufficient for the transition to anestrus (17). Initially, T4 was injected daily (3 µg/kg, sc) for 6 d. Thereafter, 5 µg/kg T4 were injected sc, three times per week.



View larger version (32K):
[in this window]
[in a new window]
  		Figure 1. Experimental design depicting thyroidectomy status and time of T4 replacement. The time of exposure to T4 (either endogenous or exogenous) is indicated by hatched bars.

 
Monitoring hormonal status
To assess seasonal changes in reproductive neuroendocrine status, serum LH was measured in samples collected twice weekly via jugular venipuncture from October 1998 through December 1999. Serum was harvested and stored at -20 C for hormone assays. In estradiol-treated OVX ewes, high LH concentrations are indicative of the neuroendocrine breeding season (frequent GnRH pulses), and low LH concentrations are indicative of anestrus (infrequent GnRH pulses) (2, 3). To monitor another seasonal cycle of neuroendocrine activity, serum PRL concentrations were determined in selected samples (once every 2 wk). To confirm completeness of thyroidectomy and efficacy of T4 replacement, serum T4 concentrations were monitored twice weekly from 7 d before treatment until T4 concentrations returned to undetectable levels following cessation of treatment. In March 1999, blood was sampled every 8 h for 72 h from the ewes provided with T4 replacement to confirm that circulating concentrations of T4 remained elevated to euthyroid levels between injections. Serum T4 concentrations were also measured periodically at times when T4 was not replaced. Ewes with detectable T4 concentrations were judged to be incompletely THX and excluded from the experiment.

Assays
Serum LH concentrations were measured in duplicate aliquots of 10–200 µl using a previously described RIA (23, 24, 25). Intra- and interassay coefficients of variation were 5.7% and 6.4%, respectively. Values are expressed in terms of NIH-LH-S12. The mean (±SEM) assay sensitivity was 0.69 ± 0.03 ng/ml. Total T4 concentrations were determined in duplicate 50-µl aliquots by RIA using a Coat-A-Count kit (Diagnostic Products, Los Angeles, CA), previously validated for use in sheep (26). Intra- and interassay coefficients of variation were 7.5% and 7.0%, respectively. The mean (±SEM) assay sensitivity was 1.95 ± 0.23 ng/ml. Serum PRL concentrations were measured in duplicate aliquots of 0.5–200 µl using a previously described RIA (27, 28). Intra- and interassay coefficients of variation were 8.8% and 21.4%, respectively. Values are expressed in terms of NIH-P-S8. The mean assay sensitivity was 0.88 ± 0.12 ng/ml.

Data analysis and interpretation
Neuroendocrine anestrous and breeding seasons were defined by changes in serum LH concentrations. The onset of anestrus was taken as the day of the first of three consecutive samples in which LH concentrations fell below 1 ng/ml. Likewise, the onset of the subsequent breeding season was defined as the first day that serum LH rose above 1 ng/ml for at least three consecutive samples. The seasonal swings in LH in this animal model coincide with the transitions between breeding and anestrous seasons in ovary-intact ewes (2, 19).

To assess when ewes were no longer responsive to T4, serum LH concentrations were used to determine which groups of T4-replaced ewes either entered or failed to enter anestrus (1 ng/ml criterion). The proportions of ewes entering anestrus in each group were compared using Fisher’s exact test. As a further measure of responsiveness to T4, the latency from the start of T4 treatment to the onset of anestrus was compared between the late breeding season replaced ewes (positive control, January onset of T4) and the other groups of ewes entering anestrus using Mann-Whitney U tests. Nonparametric analysis was required due to nonhomogeneity of variance.

The duration of anestrus and the time of onset of the subsequent breeding season (autumn 1999) were compared across groups using ANOVA. As significant (P < 0.05) main effects were detected, Fisher’s least significant difference test was used for post hoc analysis. The effects of thyroidectomy and T4 replacement on the existence and timing of the seasonal PRL cycle were determined by repeated measures ANOVA.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 Summary
 References
 
T4 replacement
The T4 injections restored euthyroid (~30–60 ng/ml) serum T4 concentrations in the THX ewes within 3 d (Fig. 2AGo). T4 concentrations then remained at these levels throughout the 100-d treatment. The saw-tooth pattern of T4 concentrations observed in Fig. 2AGo is due to the variable timing of blood sampling (twice weekly) relative to the time of T4 injection (three times weekly). Between injections, T4 concentrations remained within the euthyroid range, although values tended to decline gradually (Fig. 2BGo).



View larger version (15K):
[in this window]
[in a new window]
  		Figure 2. Mean (±SEM) serum T4 concentrations in THX ewes provided T4 replacement. A, Concentrations combined for all groups (n = 23 ewes) for samples collected twice weekly beginning 7 d before T4 replacement through 7 d after the end of T4 replacement. B, Concentrations over the 72-h period between injections (n = 5).

 
Onset of anestrus
The time course of mean serum LH concentrations is illustrated for all groups in Fig. 3Go. The proportion of ewes in which LH fell below 1 ng/ml (i.e. entered neuroendocrine anestrus), the mean (±SEM) dates of onset of anestrus, and the latency from T4 treatment to onset of anestrus are summarized in Table 1Go. In thyroid-intact controls, LH fell below 1 ng/ml in mid-January, indicating the onset of anestrus (Fig. 3AGo and Table 1Go). In all five THX controls not receiving T4, LH remained elevated throughout the experiment (Fig. 3BGo). In four of the five ewes replaced with T4 beginning in January, LH fell to anestrus levels in late February, approximately 1 month later than in thyroid-intact controls (P < 0.05; Fig. 3CGo and Table 1Go). LH fell below 1 ng/ml by mid-June in all seven ewes replaced with T4 beginning in April (Fig. 3DGo and Table 1Go). Four of the five ewes treated with T4 beginning in June met the criteria for entering anestrus, with LH dropping below 1 ng/ml in late August (Fig. 3EGo and Table 1Go). One ewe in this group (no. 8059) failed to meet the criteria for anestrus, but did have a decrease in LH concentrations at this time. Further, the individual LH patterns for other ewes in this group were atypical (Fig. 4Go). Rather than having a prolonged period of uniformly undetectable serum LH concentrations, most of these ewes had several rises and falls in LH throughout a rather short period of anestrus as defined by the 1 ng/ml criteria. Finally, the six ewes replaced with T4 beginning in late anestrus failed to express a fall in LH (Fig. 3FGo).



View larger version (50K):
[in this window]
[in a new window]
  		Figure 3. Mean (±SEM) serum LH concentrations in thyroid-intact controls (A; n = 6), THX controls (B; n = 5), or THX ewes replaced with T4 beginning in the late breeding season (C; n = 5), early anestrus (D; n = 7), mid-anestrus (E; n = 5), or late anestrus (F; n = 6). Dashed lines indicate LH concentrations in individual ewes considered not to have entered anestrus in response to T4 replacement (i.e. LH did not fall to <1 ng/ml). Shaded bars depict the time of T4 replacement. LH concentrations are plotted on a logarithmic scale.

 

View this table:
[in this window]
[in a new window]
  		Table 1. Incidence of and latency to anestrus

 


View larger version (38K):
[in this window]
[in a new window]
  		Figure 4. Serum LH concentrations for individual ewes replaced with T4 beginning in mid-anestrus. Shaded bars indicate the time of T4 replacement.

 
Latency to onset of anestrus for ewes replaced with T4 beginning in April tended to be longer than for the January group (P < 0.10; Table 1Go). Latency to anestrus was significantly longer (P < 0.05) for the ewes replaced beginning in June compared with the January replaced ewes.

Duration of anestrus
The duration of anestrus (interval from decrease to subsequent increase in LH) was determined for all ewes that entered neuroendocrine anestrus. The duration of anestrus differed significantly among all groups entering anestrus (P < 0.05; Fig. 5AGo), with thyroid-intact ewes having the longest duration, and THX ewes treated with T4 beginning in June having the shortest duration. Overall, the later the onset of T4 replacement the shorter the anestrous season.



View larger version (20K):
[in this window]
[in a new window]
  		Figure 5. A, Mean (±SEM) duration of neuroendocrine anestrus in thyroid-intact ewes and THX ewes replaced with T4 at progressively later times of the year. B, Mean (±SEM) time of onset of the neuroendocrine breeding season for the same ewes as in A. a–d, Different letters indicate groups that are significantly different in either duration of anestrus or onset of the breeding season (P < 0.05).

 
Onset of the breeding season
The onset of the neuroendocrine breeding season, as defined by increased serum LH concentrations, was determined for all ewes that previously had entered neuroendocrine anestrus. In thyroid-intact controls, LH increased above 1 ng/ml in early September (Figs. 3AGo and 5BGo). This is typical for the seasonal onset of reproductive activity in our flock (7, 19, 29). LH increased at the same time in THX ewes replaced with T4 beginning in the late breeding season (Figs. 3CGo and 5BGo). In ewes treated with T4 beginning in early anestrus (April), however, LH did not increase until early October (Fig. 3DGo). This was a significant delay compared with either of the first two groups (P < 0.05; Fig. 5BGo). The increase in LH occurred even later (mid-November) in the four THX ewes in the mid anestrus (June) group that were judged to have entered anestrus (Figs. 3EGo and 5BGo). This was a significant delay compared with the other three groups in which LH increased (P < 0.05).

PRL
The time course of mean serum PRL concentrations for all groups of ewes is illustrated in Fig. 6Go. All groups exhibited a high amplitude annual PRL cycle with a nadir averaging 10–50 ng/ml in winter and peak values of 600–1200 ng/ml in spring/summer. Repeated measures ANOVA failed to identify a significant interaction between group and serum PRL concentrations over time (P > 0.05). Further, the mean PRL concentrations did not differ among any of the treatment groups. Thus, in marked contrast to LH, the overall annual cycle of PRL was not significantly altered by thyroidectomy or T4 replacement.



View larger version (43K):
[in this window]
[in a new window]
  		Figure 6. Mean (±SEM) serum PRL concentrations in thyroid-intact controls (A), THX controls (B; no T4), and THX ewes treated with T4 beginning late in the breeding season (C), in early anestrus (D), mid-anestrus (E), or late anestrus (F). Shaded bars indicate the time of T4 replacement in THX ewes. Due to the large range in PRL concentrations during the year, values are plotted on a logarithmic scale.

 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
Summary
 References
 
Prior observations suggest that there is a window of responsiveness to thyroid hormones in regulating seasonal reproduction in the ewe. This window appears to open in the mid to late breeding season (approximately December), but the time when it closes had not been ascertained. The primary goal of this study was to determine when the window closes.

As reported previously (13, 17, 18, 26), the transition to anestrus in the present study was blocked in THX ewes and reinstated by T4 replacement beginning in the late breeding season (January). The 1-month delay in the LH fall in this group relative to that in thyroid-intact controls was anticipated, as T4 replacement was not begun until after LH had begun to decline in the controls. When the onset of T4 treatment was delayed until April, LH still fell to an undetectable level. Thus, the window of responsiveness to thyroid hormones clearly remains open through early anestrus. By June to July, however, it appears that ewes begin to lose their responsiveness to T4. Although most ewes replaced with T4 beginning in June met our criteria for entering neuroendocrine anestrus, closer examination shows that serum LH concentrations were not fully suppressed. Of interest, the latency from the start of T4 treatment to the LH fall in these ewes was greater than that in ewes treated with T4 beginning in the late breeding season, adding further support to the idea that the ewes were losing responsiveness to T4 replacement by midsummer. Finally, as reported previously (17), THX ewes replaced with T4 beginning in late summer (August) did not have a fall in LH, suggesting that the window of responsiveness to T4 has ended by this time. It should be pointed out, however, that ewes require a minimum of 60–90 d of exposure to thyroid hormones for anestrus to occur (17). Thus, the window of responsiveness may not be completely closed until somewhat later than mid-August. Overall, our findings lead to the conclusion that the window of responsiveness to T4, which opens late in the breeding season (17), closes gradually beginning around mid-anestrus and is completely closed no later than the early breeding season.

There may, however, be a caveat to these interpretations. If the lack of thyroid hormones disrupts seasonal processes in general, then timing of the period of responsiveness to T4 may shift after thyroidectomy. Although current information does not allow us to resolve this issue, it is pertinent to consider whether thyroid hormones have a widespread influence on seasonal processes and influence aspects of the reproductive rhythm other than the transition to anestrus.

To investigate whether thyroid hormones influence seasonal processes in general, we considered whether they are required for all seasonal neuroendocrine cycles. In this regard, neither thyroidectomy nor time of year of T4 replacement affected the occurrence, timing, or amplitude of the annual PRL cycle. This is new information that is important for two reasons. First, it indicates that the role of thyroid hormones in the generation of the seasonal reproductive rhythm is not generalized to all seasonal mechanisms; rather, there is some degree of specificity for the influence of thyroid hormones on seasonal reproductive processes. Second, because photoperiod affects the timing of the annual PRL cycle (30, 31), our finding reinforces the prior conclusion of Dahl et al. (32) that THX does not block the relay of photoperiodic information to all neuroendocrine functions. In this regard, it is noteworthy that seasonal cycles of LH and PRL can be dissociated (33) and that melatonin, which mediates both LH and PRL responses to photoperiod (5, 34), acts at different sites in regulating seasonal cycles of these two hormones (35, 36, 37, 38, 39, 40). This neuroanatomical separation of the sites of melatonin action on PRL and GnRH/LH secretion may help to explain how thyroid hormones affect one, but not the other, of these seasonal processes.

The second way that we investigated the interaction of thyroid hormones with the mechanisms for seasonality was to determine the time of onset of the next breeding season in THX ewes that had entered anestrus in response to T4 treatment. This provided insight into whether thyroid hormones influence aspects of the endogenous rhythm other than the transition to anestrus. Our findings revealed a significant and progressive delay in breeding season onset with the successively later T4 treatments. This finding is consistent with the hypothesis that thyroid hormones impact the generation, maintenance, or timing of the endogenous reproductive rhythm, although we emphasize that this hypothesis was not directly tested by the present experiments. If thyroid hormones merely enabled the transition to anestrus, then other aspects of the underlying rhythm would not have been altered, and the breeding season would have begun at its usual time.

We can envision a number of ways that an interaction of thyroid hormones and the circannual rhythm mechanism could lead to the results of the present study. One arises from our finding that the anestrous period was shortened after the progressively later T4 replacements; the delay in onset of anestrus was considerably longer than the delay in onset of the subsequent breeding season. This is consistent with the possibility that there may be two separate rhythms, rather than a single rhythm for the entire circannual reproductive cycle. One would be a T4-dependent rhythm that drives the onset of anestrus; this would be disrupted by thyroidectomy. The other would be a T4-independent rhythm that drives the onset of the breeding season. In this scenario, the end of the window of responsiveness to thyroid hormones for onset of anestrus may actually be due to the driving force of the T4-independent rhythm to begin the next breeding season (i.e. an interaction of the two opposing rhythms). This may have masked the effects of the T4-dependent rhythm to drive the transition to anestrus in the presence of late T4 exposure and also may have caused the shortening of the anestrous season in the present study. Although this possibility may have merit, it cannot account for the progressive delay in onset of the breeding season resulting from the progressively later T4 treatments.

A second possibility related to the interaction between thyroid hormones and the rhythm is that thyroidectomy delayed the subsequent breeding season by lengthening the period of the underlying endogenous rhythm. That the duration of anestrus was not uniform among the groups could have been due to an interaction between an effect on lengthening the rhythm period and the entraining effect of the simulated natural photoperiod to which the ewes were exposed.

A third possibility is that thyroid hormones interact with the endogenous rhythm by influencing its photoperiodic entrainment. In ewes, the circadian patterns of melatonin secreted during the short nights of late spring and summer are particularly important for entraining the circannual reproductive rhythm (1, 6, 7, 8, 41). Although thyroid hormones are not required for generation or photoperiodic regulation of the circadian rhythm of melatonin secretion (26, 32), thyroid hormones are necessary for an appropriate reproductive response to melatonin (32). Nevertheless, our current observations, in conjunction with earlier findings, suggest that thyroid hormones are not needed during the time that melatonin actually entrains the rhythm. Specifically, the breeding season began at the usual time in THX ewes only when T4 was replaced during the late winter and early spring (January to April), i.e. before the time when melatonin is most effective in entraining the rhythm (7, 8, 41). Further, ewes treated with T4 during the spring and summer (April to July), when melatonin is most effective, had a delayed onset of the breeding season, suggesting that the rhythm was not entrained by the spring/summer melatonin signal. Thus, if thyroid hormones are needed for entrainment, they need not be present during the time melatonin is acting to entrain the rhythm. Rather, T4 may act to prime the endogenous rhythm mechanism for later entrainment by melatonin. This interpretation could accommodate the finding that ewes THX in March (just before the period when melatonin entrains the rhythm) enter the subsequent breeding season at the appropriate time despite the absence of thyroid hormones during the late spring and summer (42).


   Summary
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Summary
References
 
Based on the foregoing observations and considerations, we suggest that thyroid hormones have two distinct roles in the seasonal reproductive rhythm of ewes. First, thyroid hormones are required for the transition to anestrus. A window of responsiveness for this action of T4 opens in the late breeding season and then gradually closes between mid to late anestrus. The second role of thyroid hormones is for maintenance and/or entrainment of the circannual reproductive rhythm so the breeding season begins at the proper time. Finally, the persistence of the seasonal cycle of PRL secretion in THX ewes indicates that not all neuroendocrine processes are dependent upon thyroid hormones.


   Acknowledgments
 
We thank Mr. Douglas Doop and Mr. Gary McCalla for their excellent care and handling of the animals; Ms. Martha Brown for conducting RIAs; and Dr. Thomas Harris, Dr. Deborah Battaglia, Mr. Steve Hardy, Dr. Miro Valent, Dr. Nathalie Briard-Debus, Dr. Michael Lehman, Ms. Kellie Breen, Mr. Drew Beaver, Ms. Candace Williams, and Ms. Aphrodite Nikolovski for their help in designing and conducting this experiment, and in interpreting the results.


   Footnotes
 
This work was supported by USDA Grant 97-35203-4908, NIH-HD-03773 and HD-17864, and the Center for the Study of Reproduction (NIH P30-HD-18258) Standards and Reagents and Sheep Research Cores. A preliminary report of this work has appeared in Biol Reprod 62(Suppl):54, 2000.

1 Present address: Department of Cell Biology, Neurobiology, and Anatomy, University of Cincinnati College of Medicine, Cincinnati, Ohio 45267. Back

2 Present address: Institut National de la Recherche Agronomique, Unité Mixte de Recherche 181: Physiopathologie de Toxicologie Experimentales, Ecole Nationale Vétérinaire de Toulouse, 31076 Toulouse, France. Back

3 Present address: Department of Anatomy and Structural Biology, Otago University, Dunedin 9015, New Zealand. Back

Abbreviations: OVX, Ovariectomized; THX, thyroidectomized.

Received January 9, 2002.

Accepted for publication April 2, 2002.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Summary
 References
 

  1. Woodfill CJI, Robinson JE, Malpaux B, Karsch FJ 1991 Synchronization of the circannual reproductive rhythm of the ewe by discrete photoperiodic signals. Biol Reprod 45:110–121[Abstract]
  2. Legan SJ, Karsch FJ, Foster DL 1977 The endocrine control of seasonal reproductive function in the ewe: a marked change in response to the negative feedback action of estradiol on luteinizing hormone secretion. Endocrinology 101:818–824[Medline]
  3. Karsch FJ, Dahl GE, Evans NP, Manning JM, Mayfield KP, Moenter SM, Foster DL 1993 Seasonal changes in gonadotropin-releasing hormone secretion in the ewe: alteration in response to the negative feedback action of estradiol. Biol Reprod 49:1377–1383[Abstract]
  4. Bittman EL, Karsch FJ, Hopkins JW 1983 Role of the pineal gland in ovine photoperiodism: regulation of seasonal breeding and negative feedback effects of estradiol upon luteinizing hormone secretion. Endocrinology 113:329–336[Abstract]
  5. Bittman EL, Kaynard AH, Olster DH, Robinson JE, Yellon SM, Karsch FJ 1985 Pineal melatonin mediates photoperiodic control of pulsatile luteinizing hormone secretion in the ewe. Neuroendocrinology 40:409–418[Medline]
  6. Malpaux B, Robinson JE, Brown MB, Karsch FJ 1988 Importance of changing photoperiod and melatonin secretory pattern in determining the length of the breeding season in the Suffolk ewe. J Reprod Fert 83:461–470[Abstract/Free Full Text]
  7. Woodfill CJI, Wayne NL, Moenter SM, Karsch FJ 1994 Photoperiodic synchronization of a circannual reproductive rhythm in sheep: identification of season-specific time cues. Biol Reprod 50:965–976[Abstract]
  8. Barrell GK, Thrun LA, Brown ME, Viguié C, Karsch FJ 2000 Importance of photoperiodic signal quality to entrainment of the circannual reproductive rhythm of the ewe. Biol Reprod 63:769–774[Abstract/Free Full Text]
  9. Venzke WG 1938 Endocrines and their role in reproduction. J Am Vet Med Assoc 92:663–680
  10. Woitkevitsch AA 1940 Dependence of seasonal periodicity in gonadal changes in the thyroid gland in Sturnus vulgaris L. C R (Doklady) Acad Sci URSS 27:741–745
  11. Péczely P, Pethes G, Rudas P 1980 Interrelationship between thyroid and gonadal function in female Japanese quail kept under short and long photoperiods. J Endocrinol 87:55–63[Abstract/Free Full Text]
  12. Thapliyal JP, Lal P 1984 Light, thyroid, gonad and photorefractory state in the migratory redheaded bunting, Emberiza bruniceps. Gen Comp Endocrinol 56:41–52[CrossRef][Medline]
  13. Nicholls TJ, Follet BK, Goldsmith AR, Pearson H 1988 Possible homologies between photorefractoriness in sheep and birds: the effect of thyroidectomy on the length of the ewe’s breeding season. Reprod Nutr Dev 28:375–385
  14. Nicholls TJ, Goldsmith AR, Dawson A 1988 Photorefractoriness in birds and comparison with mammals. Physiol Rev 68:133–176[Free Full Text]
  15. Shi ZD, Barrell GK 1994 Thyroid hormones are required for the expression of seasonal changes in red deer (Cervus elaphus) stags. Reprod Fertil Dev 6:187–192[CrossRef][Medline]
  16. Anderson GM, Barrell GK 1998 Effects of thyroidectomy and thyroxine replacement in the red deer hind. J Reprod Fertil 113:239–250[Abstract/Free Full Text]
  17. Thrun LA, Dahl GE, Evans NP, Karsch FJ 1997 A critical period for thyroid hormone action on seasonal changes in reproductive neuroendocrine function in the ewe. Endocrinology 138:3402–3409[Abstract/Free Full Text]
  18. Thrun LA, Dahl GE, Evans NP, Karsch FJ 1996 Time-course of thyroid hormone involvement in the development of anestrus in the ewe. Biol Reprod 55:833–837[Abstract]
  19. Karsch FJ, Robinson JE, Woodfill CJI, Brown MB 1989 Circannual cycles of luteinizing hormone and prolactin secretion in ewes during prolonged exposure to a fixed photoperiod: evidence for an endogenous reproductive rhythm. Biol Reprod 41:1034–1046[Abstract]
  20. Karsch FJ, Dierschke DJ, Weick RF, Yamaji T, Hotchkiss J, Knobil E 1973 Positive and negative feedback control by estrogen of luteinizing hormone secretion in the rhesus monkey. Endocrinology 92:799–804[Medline]
  21. Karsch FJ, Goodman RL, Legan SJ 1980 Feedback basis of seasonal breeding: test of an hypothesis. J Reprod Fertil 58:521–535[Abstract/Free Full Text]
  22. Webster JR, Moenter SM, Woodfill CJI, Karsch FJ 1991 Role of the thyroid gland in seasonal reproduction. II. Thyroxine allows a season-specific suppression of gonadotropin secretion in sheep. Endocrinology 129:176–183[Abstract]
  23. Niswender GD, Midgley Jr AR, Reichert Jr LE 1968 Radioimmunologic studies with murine, ovine and porcine luteinizing hormone. In: Rosenberg E, ed. Gonadotropins. Los Altos: GERON-X; 299–306
  24. Niswender GD, Reichert Jr LE, Midgely Jr AR, Nalbandov AV 1969 Radioimmunoassay for bovine and ovine luteinizing hormone. Endocrinology 84:1166–1173[Medline]
  25. Hauger RL, Karsch FJ, Foster DL 1977 A new concept for control of the estrous cycle of the ewe based on the temporal relationships between luteinizing hormone, estradiol and progesterone in peripheral serum and evidence that progesterone inhibits tonic LH secretion. Endocrinology 101:807–817[Abstract]
  26. Moenter SM, Woodfill CJI, Karsch FJ 1991 Role of the thyroid gland in seasonal reproduction: thyroidectomy blocks seasonal suppression of reproductive neuroendocrine activity in ewes. Endocrinology 128:1337–1344[Abstract]
  27. Davis SL, Reichert Jr LE, Niswender GD 1971 Serum levels of prolactin in sheep as measured by radioimmunoassay. Biol Reprod 4:145–153[Abstract]
  28. Foster DL, Ebling FJP, Claypool LE, Woodfill CJI 1988 Cessation of long day melatonin rhythms time puberty in a short day breeder. Endocrinology 123:1636–1641[Abstract]
  29. Robinson JE, Wayne NL, Karsch FJ 1985 Refractoriness to inhibitory day lengths initiates the breeding season of the Suffolk ewe. Biol Reprod 32:1024–1030[Abstract]
  30. Pelletier J 1973 Evidence for photoperiodic control of prolactin release in rams. J Reprod Fertil 35:143–147[Abstract/Free Full Text]
  31. Lincoln GA, McNeilly AS, Cameron CL 1978 The effects of a sudden decrease or increase in daylength on prolactin secretion in the ram. J Reprod Fertil 52:305–311[Abstract/Free Full Text]
  32. Dahl GE, Evans NP, Moenter SM, Karsch FJ 1994 The thyroid gland is required for reproductive neuroendocrine responses to photoperiod in the ewe. Endocrinology 135:10–15[Abstract]
  33. Lincoln GA, Tortonese DJ 1999 Prolactin replacement fails to inhibit reactivation of gonadotropin secretion in rams treated with melatonin under long days. Biol Reprod 60:602–610[Abstract/Free Full Text]
  34. Kennaway DJ, Dunstan EA, Gilmore TA, Seamark RF 1982 /1983 Effects of shortened daylength and melatonin treatment on plasma prolactin and melatonin levels in pinealectomised and sham-operated ewes. Anim Reprod Sci 5:287–294
  35. Lincoln GA, Clarke IJ 1994 Photoperiodically-induced cycles in the secretion of prolactin in hypothalamo-pituitary disconnected rams: evidence for translation of the melatonin signal in the pituitary gland. J Neuroendocrinol 6:251–260[Medline]
  36. Malpaux B, Skinner DC, Maurice F 1995 The ovine pars tuberalis does not appear to be targeted by melatonin to modulate luteinizing hormone secretion, but may be important for prolactin release. J Neuroendocrinol 7:199–206[CrossRef][Medline]
  37. Malpaux B, Daveau A, Maurice-Mandon F, Duarte G, Chemineau P 1998 Evidence that melatonin acts in the premammillary hypothalamic area to control reproduction in the ewe: presence of binding sites and stimulation of luteinizing hormone secretion by in situ microimplant delivery. Endocrinology 139:1508–1516[Abstract/Free Full Text]
  38. Maywood ES, Hastings MH 1995 Lesions of the iodomelatonin-binding sites of the mediobasal hypothalamus spare the lactotropic, but block the gonadotropic response of male Syrian hamsters to short photoperiod and to melatonin. Endocrinology 136:144–153[Abstract]
  39. Tortonese DJ, Lincoln GA 1995 Effects of melatonin in the mediobasal hypothalamus on the secretion of gonadotrophins in sheep: role of dopaminergic pathways. J Endocrinol 146:543–552[Abstract/Free Full Text]
  40. Williams LM, Lincoln GA, Mercer JG, Barrett P, Morgan PJ, Clarke IJ 1997 Melatonin receptors in the brain and pituitary gland of hypothalamo-pituitary disconnected Soay rams. J Neuroendocrinol 9:639–643[CrossRef][Medline]
  41. Wayne NL, Malpaux B, Karsch FJ 1990 Photoperiodic requirements for timing onset and duration of the breeding season of the ewe: synchronization of an endogenous rhythm of reproduction. J Comp Physiol A 166:835–842[Medline]
  42. Thrun LA, Dahl GE, Evans NP, Karsch FJ 1997 Effect of thyroidectomy on maintenance of seasonal reproductive suppression in the ewe. Biol Reprod 56:1035–1040[Abstract]



This article has been cited by other articles:


Home page
 ReproductionHome page
N. Nakao, H. Ono, and T. Yoshimura
Thyroid hormones and seasonal reproductive neuroendocrine interactions
Reproduction, July 1, 2008; 136(1): 1 - 8.
[Abstract] [Full Text] [PDF]

Home page
 EndocrinologyHome page
S. Yasuo, N. Nakao, S. Ohkura, M. Iigo, S. Hagiwara, A. Goto, H. Ando, T. Yamamura, M. Watanabe, T. Watanabe, et al.
Long-Day Suppressed Expression of Type 2 Deiodinase Gene in the Mediobasal Hypothalamus of the Saanen Goat, a Short-Day Breeder: Implication for Seasonal Window of Thyroid Hormone Action on Reproductive Neuroendocrine Axis
Endocrinology, January 1, 2006; 147(1): 432 - 440.
[Abstract] [Full Text] [PDF]

Home page
 Biol. Reprod.Home page
J. H. Sliwowska, H. J. Billings, R. L. Goodman, L. M. Coolen, and M. N. Lehman
The Premammillary Hypothalamic Area of the Ewe: Anatomical Characterization of a Melatonin Target Area Mediating Seasonal Reproduction
Biol Reprod, June 1, 2004; 70(6): 1768 - 1775.
[Abstract] [Full Text] [PDF]

Home page
 EndocrinologyHome page
G. M. Anderson, S. L. Hardy, M. Valent, H. J. Billings, J. M. Connors, and R. L. Goodman
Evidence that Thyroid Hormones Act in the Ventromedial Preoptic Area and the Premammillary Region of the Brain to Allow the Termination of the Breeding Season in the Ewe
Endocrinology, July 1, 2003; 144(7): 2892 - 2901.
[Abstract] [Full Text] [PDF]

Home page
 Proc. Natl. Acad. Sci. USAHome page
B. J. Prendergast, B. Mosinger Jr., P. E. Kolattukudy, and R. J. Nelson
Hypothalamic gene expression in reproductively photoresponsive and photorefractory Siberian hamsters
PNAS, December 10, 2002; 99(25): 16291 - 16296.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Purchase Article
Right arrow View Shopping Cart
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow Request Copyright Permission
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Billings, H. J.
Right arrow Articles by Anderson, G. M.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Billings, H. J.
Right arrow Articles by Anderson, G. M.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Endocrinology Endocrine Reviews J. Clin. End. & Metab.
Molecular Endocrinology Recent Prog. Horm. Res. All Endocrine Journals