Effects of various doses of ovine corticotrophin-releasing hormone on plasma and saliva cortisol concentrations in horses.

This study investigates the effects of different doses of ovine corticotrophin-releasing hormone (oCRH) on cortisol levels in both plasma and saliva of healthy horses. The results provide a valid test protocol for using oCRH to measure levels of activation in the horses’ major stress-response system.

Study Principles and Methodology

• The investigation aimed to compare the effects of intravenous (IV) administration of various doses of oCRH on plasma and saliva cortisol concentrations in horses.
• It also measured the validity of an oCRH challenge test protocol for use in adult horses.
• Twenty-four healthy Warmblood horses were evenly distributed into four groups – each receiving different doses of oCRH dissolved in saline – 0 (control group), 0.01, 0.1, or 1.0 Mg/kg.
• Both jugular blood and saliva samples were collected concurrently fifteen minutes before and immediately before the hormone injection. These measurements formed the baseline for comparing the effects of various oCRH doses.
• Post-injection blood and saliva samples were collected within a period of 210 minutes. Cortisol concentration in these samples was measured using a radioimmunoassay.

Key Findings

• All administered doses of oCRH significantly heightened cortisol concentrations in the plasma and saliva from baseline levels.
• The increase in plasma cortisol concentration was notably higher with the 1.0 Mg/kg dose of oCRH compared to lower doses.
• The study revealed a correlation (r = 0.518) between baseline cortisol concentrations in plasma and saliva.

Conclusion and Clinical Implications

• The study concluded that a valid CRH challenge test protocol for adult horses would involve the intravenous administration of at least 0.01 Mg of oCRH/kg.
• The ideal time frames recommended for post-injection sample collection were – blood samples from 10 to 180 minutes and saliva samples from 20 to 50 minutes.
• Implementing such a protocol could help identify chronic states of activation of the hypothalamic-pituitary-adrenocortical axis at the hypothalamic level in horses.