Effects of physiologic and pharmacologic agents on serum prolactin concentrations in the nonpregnant mare.

The research article explores the effects of certain physiological and medical substances on serum prolactin levels in mares that are not pregnant. The study found similarities between how prolactin secretion is controlled in horses and other mammals, and stated that a seasonal decline in serum prolactin cannot be attributed to increased sensitivity to dopamine, a presumed inhibiting factor for prolactin.

Overall Design and Structure of the Study

The research consisted of four studies aimed at investigating the impact of different physiological and pharmacological agents on serum prolactin levels in nonpregnant mares. These agents included thyrotropin-releasing hormone (TRH), metoclopramide, sulpiride, and bromocriptine.

• TRH is a hormone that stimulates the pituitary gland to release thyroid-stimulating hormone and prolactin.
• Metoclopramide is a medication used to treat nausea and gastric reflux, which also has the effect of increasing prolactin levels.
• Sulpiride is typically used as an antipsychotic medication, and it too influences prolactin levels.
• Bromocriptine is a dopamine agonist, used to suppress prolactin production.

Key Findings of the Research

The investigation revealed several key findings about how these agents affect serum prolactin concentrations.

• An increase in serum prolactin due to the administration of TRH didn’t show any notable variation in mares in different stages of their reproductive cycle.
• Both metoclopramide and sulpiride rapidly increased serum prolactin levels. Importantly, the magnitude of the increase appeared to be dependent on the dose administered, rather than the mechanism of delivery.
• Contrastingly, bromocriptine was found to decrease serum prolactin levels. However, the inhibitory effect was only observed when the basic levels of serum prolactin were highest (in May).

Conclusion and Implication of Findings

The data gathered from these experiments suggested that the control mechanisms for prolactin secretion in horses are similar to those described in other mammals. An important conclusion was that the seasonal drop in serum prolactin levels could not be attributed to heightened sensitivity to dopamine, a proposed inhibitory factor for prolactin. This finding contradicts previously held beliefs and potentially opens new areas of study in understanding prolactin regulation.