Inter-relationships between the secretory dynamics of thyrotrophin-releasing hormone, thyrotrophin and prolactin in periovulatory mares: effect of hypothyroidism.

This research study investigated the interactions and secretory dynamics of three hormones – thyrotrophin-releasing hormone (TRH), thyrotrophin, and prolactin – in mares around ovulation. It was found that hypothyroidism, induced by propyl-thiouracil treatment, increased the secretion rates of TRH and thyrotrophin but not prolactin, and that all the hormones were secreted irregularly rather than rhythmically.

Investigation Methodology

• The study used a nonsurgical technique to collect pituitary venous blood from mares, either euthyroid (normal thyroid function) or hypothyroid (diminished thyroid function induced by propyl-thiouracil treatment).
• This blood collected continuously was divided into 1-minute samples for a 4-hour duration to investigate the dynamics of TRH, thyrotrophin, and prolactin secretion.
• To examine the effect of dopamine on the relationship between secretion patterns, a selective D2 receptor antagonist, sulpiride, was administered intramuscularly after 2 hours of sampling.
• The testing of the model and blood collection method established no TRH loss sites, confirming the reliability of these methods in investigating hormonal secretory dynamics.

Findings and Implications

• Hypothyroidism induced an increase in mean secretion rates of TRH and thyrotrophin, but not prolactin, indicating a specific hormonal response to decreased thyroid function.
• Sulpiride administration increased prolactin secretion rates in both hypothyroid and control mares. However, it did not affect TRH or thyrotrophin secretion, suggesting dopamine’s specific role in regulating prolactin but not TRH or thyrotrophin secretion.
• All three hormones were found to be secreted episodically, not rhythmically, showing irregular, rather than periodic, hormone release.
• The secretion pattern of TRH was found to be significantly related to that of thyrotrophin in both hypothyroid and euthyroid mares, albeit weakly, showing that TRH influences thyrotrophin release to a small degree.
• In hypothyroid mares, sulpiride enhanced the synchrony between TRH and prolactin patterns, implying that decreased inhibitory inputs might heighten TRH’s stimulating effect on prolactin secretion.
• The study concluded that TRH is not the major factor controlling minute-to-minute fluctuations in either thyrotrophin or prolactin.