Preliminary investigation of ghrelin in horses and ponies: receptor expression and associations with prandial state, morphometry and signalment.

Overview

• This study explores the presence and role of the hormone ghrelin in horses and ponies, particularly its receptors, blood levels, and relationships with feeding state, age, breed, and body measurements.

Background on Ghrelin

• Ghrelin is a hormone known to regulate appetite, energy use, glucose metabolism, and thermoregulation in many species.
• It exists in two main forms circulating in the blood: acylated (active) and des-acylated; each form has distinct metabolic effects.
• Prior to this study, the role and presence of ghrelin and its receptors in horses and ponies were not well understood or extensively studied.

Study Objectives

• Confirm the presence of ghrelin receptor isoforms (1a and 1b) in various equine tissues.
• Validate a commercial assay kit to measure total ghrelin (both acylated and des-acylated forms) in horse blood samples.
• Examine associations between ghrelin concentrations and factors such as feeding state (prandial state), breed (signalment), and physical characteristics (morphometry) in horses and ponies without metabolic disease.

Methods

• Gene expression analysis was performed using PCR to detect ghrelin receptor isoforms in different tissues.
• Validation testing of a commercial assay kit for total ghrelin involved assessing parallelism and recovery during sample addition to confirm accuracy and reliability.
• Blood samples from 35 healthy horses and ponies were analyzed to measure active (acylated) ghrelin levels before and after feeding.
• Statistical analyses were conducted to investigate relationships between active ghrelin levels, breed, age, bodyweight, height, and body condition.

Key Findings: Ghrelin Receptor Expression

• Ghrelin receptor type 1a was confirmed in the pituitary gland and adrenal medulla and was also detected at lower levels in six other peripheral tissues.
• Receptor type 1b was detected in the pituitary, adrenal medulla, adrenal cortex, and ileum by PCR, but low product yields prevented sequencing confirmation, indicating less robust expression or lower abundance.
• The widespread receptor presence across diverse tissues suggests ghrelin plays multiple physiological roles in horses.

Key Findings: Assay Validation and Ghrelin Measurement

• The commercial assay kit for measuring total ghrelin (both forms combined) did not meet validation criteria due to poor parallelism and recovery results, meaning measurements of total ghrelin concentrations were unreliable and could not be reported.
• As a result, the analysis focused on active (acylated) ghrelin concentrations only.

Key Findings: Associations with Prandial State, Breed, Age, and Morphometry

• Active ghrelin levels decreased by approximately 19% after feeding (post-prandial state), which aligns with its role in hunger signaling.
• Active ghrelin concentrations were positively associated with age, indicating older horses and ponies had higher active ghrelin levels.
• No significant association was found between active ghrelin levels and bodyweight, height, or body condition score, suggesting ghrelin levels are not strongly related to these body measurements in healthy animals.
• Before feeding (pre-prandial state), Welsh ponies showed significantly higher active ghrelin concentrations compared to other horse and pony breeds, suggesting possible breed-specific metabolic differences.

Implications and Future Directions

• Confirmation of ghrelin receptor expression in multiple tissues emphasizes its likely complex and diverse roles in equine physiology beyond appetite control.
• Age and breed associations with active ghrelin suggest physiological diversity in metabolic regulation within equine species that warrants further investigation.
• The failure to validate total ghrelin assays highlights the need to develop or optimize reliable measurement methods for equine ghrelin to better understand its biology and potential clinical relevance.
• Future studies could explore the functional significance of the ghrelin receptor isoforms and the distinct roles of acylated and des-acylated ghrelin in horses and ponies, including potential links to metabolic health and disease.