β-endorphin immunoreactivity during anaesthesia in equidae.

This study investigates the effects of specific conditions such as surgery, hypoxia, hypercapnia and the administration of a drug called flunixin, on a hormone called β-endorphin in anaesthetized horses and ponies. It was found that β-endorphin is a sensitive marker of stress response in the body, but its exact role during equine anesthesia remains unclear.

Research Methods

The research comprised of a prospective crossover study involving six healthy adult Welsh Mountain ponies and seven healthy adult Thoroughbreds. The methods included:

• Anesthetizing the ponies with drugs like thiopentone and halothane or pentobarbitone, and anesthetizing the horses with guaiphenesin, thiopentone, and halothane.
• Subjecting these animals to 2-hour anaesthesia sessions where separate trials made them undergo hypoxia, hypercapnia, pure anaesthesia, or administration of flunixin at the onset.
• For the horses, some sessions involved a surgical procedure to subcutaneously relocate one carotid artery.
• Collecting plasma samples before, after, and at 20-minute intervals during anesthesia. These samples were later assessed for β-endorphin immunoreactivity (BEI) using a radioimmunoassay method.
• Using analysis of variance of the concentration-time curve for statistical analysis of collected data.

Research Findings

The study observed variations in BEI levels across different conditions and methods:

• The pre-anaesthetic BEI values ranged between 5.7 and 20.4 pmol L. Induction of anaesthesia resulted in a five to 10 fold increase in mean plasma BEI levels in all subjects except those in the hypercapnia group.
• Halothane anaesthesia increased BEI in both ponies and horses, but pentobarbitone anaesthesia did not bring about significant changes.
• Hypoxic conditions led to higher increase in BEI that was sustained for a longer period compared to subjects given halothane alone or those subjected to hypercapnia.
• While there was significant individual variation in the flunixin group, changes were not statistically significant.
• Surgery in horses led to the highest peak BEI values in the study but the overall integrated concentration over time (Area Under the Curve, AUC) was not significantly higher than in horses without surgery.

Overall Conclusion

The study found β-endorphin to be a sensitive indicator of an endocrine stress response in equidae under anaesthesia. Notably, factors such as the type of anaesthesia, the addition of surgery, and induced conditions like hypoxia and hypercapnia caused varying changes in β-endorphin levels. However, the physiological role of β-endorphin during equine anaesthesia is yet to be determined.