Clinical Research Abstracts of the British Equine Veterinary Association Congress 2015.

The research conducted investigates whether a specific type of receptor for the molecule serotonin, known as 5-HT4, is present in the small intestine of horses. The study found no evidence of the receptors, questioning the effectiveness of certain drugs used for digestion in horses.

About the Study

The primary intention of the study was to find out if functional 5-HT4 receptors, associated with the release of acetylcholine, exist in the equine small intestine.

• The 5-HT4 receptor is a type of serotonin receptor. In humans, activating these receptors can stimulate acetylcholine release, enhancing intestinal movement and aiding digestion.
• The researchers examined multiple muscle layers of the equine small intestine using an in vitro organ bath setup. To this end, they applied electric field stimulation (EFS) to longitudinal and circular smooth muscle strips.

Results

The outcomes of the experiments were intriguing for both muscle layers.

• Firstly, it was noted that EFS led to the creation of neurogenic , voltage-dependent on-contractions, which were eliminated by tetrodotoxin (a potent neurotoxin).
• The researchers confirmed these contractions as cholinergic. They achieved this by observing that atropine, a known inhibitor of acetylcholine, abolished these contractions produced at a specified voltage that induced 50% of the maximal amplitude.
• Significantly, the researchers found that prucalopride, a selective 5-HT4 receptor agonist, did not increase the amplitude of these contractions. They noted this even when using IBMX, a substance known to augment the effect of neuronal 5-HT4 receptors by stopping the breakdown of cAMP, their secondary messenger.
• Even serotonin, a full agonist at 5-HT4 receptors, did not amplify submaximal contractions when tested in the presence of other serotonin receptor subtype inhibitors, further asserting the absence of 5-HT4 receptors in horse intestinal neurons.

Conclusion and Impact

Ultimately, the research found no evidence of 5-HT4 receptors present in the cholinergic neurons of the horse’s small intestine.

• This knowledge challenges traditional use of 5-HT4 prokinetic drugs for aiding digestion in horses, and as such, practices may need to be revised.
• The research adhered to ethical standards by using material collected from an abattoir, rather than from live animals.
• No external funding or competing interests were associated with the study.