Reproductive Seasonality and the Effects of Thyroid Hormone on Neurotrophin Expression in the Ewe.
Bibliographic record
Abstract
Many animals exhibit an annual cycle of fertility that synchronizes the timing of reproductive activity and the birth of offspring. In the sheep, the transition from the breeding season to anestrus is associated with morphological and molecular changes resulting in an altered responsiveness of the gonadotropin-releasing hormone neurosecretory system to the negative feedback effects of estradiol. This transition to anestrus is known to be dependent on the thyroid hormone, thyroxine (T4), as removal of the thyroid gland (THX) during the late breeding season prevents the onset of anestrus. T4 can act in multiple sites to regulate this seasonal transition, including the ventral medial preoptic area (vmPOA) and premammillary region (PMR) of the hypothalamus, where T4 microimplants prevent the effects of THX. Moreover, the anatomy of dopamine neurons in the A15 region of the hypothalamus, which are involved in seasonal changes in estrogen negative feedback, is influenced by T4. Glutamatergic and synapsin-positive inputs onto A15 dopamine neurons are increased in anestrus, an effect correlated with longer dendrites of these A15 neurons. This latter observation is dependent on T4. Since neurotrophins and their receptors play an important role in mediating T4-dependent neuroplasticity during development, we examined neurotrophins as mediators of T4's influence during the transition to anestrus. In this study, we used Real Time-PCR to examine expression of brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) in hypothalamic regions of THX ewes compared to thyroid-intact controls during the transition to anestrus. Results indicate that thyroid-intact ewes have decreased BDNF mRNA expression in the A15 and PMR but increased expression in the vmPOA compared to THX ewes. In contrast, NGF mRNA expression is increased in the A15 and decreased in the vmPOA of thyroid-intact ewes. Thus both neurotrophins may be involved in mediating the T4 dependent changes by acting in different regions of the hypothalamus. In a second experiment, immunohistological staining for the receptor for BDNF, tyrosine kinase B (TrkB), was conducted in late breeding season thyroid-intact and THX animals. Preliminary analysis suggests that thyroid-intact animals may have decreased TrkB fiber density staining in the A15 compared to THX animals. This decreased receptor expression correlated with decreased BDNF mRNA expression in the A15 observed in the first study. The functional importance of this change in expression of BDNF and its receptor is currently unknown; however, it is clear that these factors are regulated by T4 and correlated with synaptic and morphological changes in the A15. Thus, our findings provide initial evidence pointing to alterations in NGF, BDNF and TrkB expression as possible mediators of the actions of T4 necessary for seasonal alterations in the neural circuitry mediating estrogen negative feedback in the sheep. (poster)
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.002 | 0.003 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".