Determination of the source of increased serotonin (5-HT) concentrations in blood and peritoneal fluid of colic horses with compromised bowel.

The research explored the increase in serotonin (5-HT) in horses suffering from colic and compromised bowel. The aim was to identify the source of this serotonin overload and understand its implications in the development of ileus, a type of bowel obstruction.

Objective and Methods

The primary objective of the researchers was to examine the heightened systemic release of serotonin (5-HT) in colic stricken horses with compromised bowel and to find the source of the 5-HT overload.

• They measured 5-HT concentrations in both the plasma and peritoneal fluid (PF) of different categories of horses: healthy horses, horses suffering from strangulating small intestinal colic, non-surgical colic horses, and cryptorchid stallions.
• Further, the research tried to identify the source of the 5-HT overload by comparing the blood and PF 5-HT concentrations within the horses, and by determining the beta-thromboglobulin (beta-TG)/platelet factor 4 (PF4) ratio to assess the in vivo platelet activation.

Results

• All horses from the strangulating small intestinal colic group had higher levels of 5-HT in both their plasma (P = 0.006) and PF (P = 0.01) than those in a healthy condition.
• The plasma beta-TG/PF4 ratio in these horses always exceeded 2, a clear indication of in vivo platelet activation. This suggests that the elevated 5-HT level was due to a systemic release from activated platelets.
• The concentration of 5-HT found in the PF of colic horses with compromised bowel was consistently lower than the corresponding plasma concentrations (P = 0.005), suggesting a systemic origin rather than local production in the bowel.

Conclusions

The study concluded that significant amounts of 5-HT can be released into the systemic circulation of horses with compromised bowel. This happens through a massive release of platelet-stored 5-HT. Since 5-HT acts as a potent agent for inflammation, constriction of blood vessels, and modification of immune responses, this surge of systemic release could contribute to the pathophysiology of ileus in horses. The researchers suggested that, given the rapid and prolonged desensitization (tachyphylaxia) exhibited by the jejunal 5-HT1A-like receptors, this increased systemic release of 5-HT – particularly noticeable in colic horses – could represent a significant pathogenic factor in cases of ileus.