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5-Hydroxytryptamine-4 receptor messenger ribonucleic acid levels and densities in gastrointestinal muscle layers from healthy dairy cows¹

E. C. Ontsouka^*,,2,3, J. W. Blum^,3, A. Steiner^* and M. Meylan^*

^* Clinic for Ruminants, and and Division of Nutrition and Physiology, Institute of Animal Genetics, Nutrition and Housing, Vetsuisse Faculty, University of Berne, CH-3012 Berne, Switzerland

² Corresponding author: edgar.ontsouka{at}physio.unibe.ch

Serotonin (5-hydroxytryptamine, 5-HT) is involved in gastrointestinal tract (GIT) motor functions through binding to specific receptors located in the GIT walls. The objectives of the current study were to compare mRNA levels and binding sites of 5-HT₄ receptors (5-HTR₄) in smooth muscle layers from the fundus abomasi, pylorus, ileum, cecum, proximal loop of the ascending colon (PLAC), and external loop of the spiral colon (ELSC) of healthy dairy cows, and to verify whether mRNA and protein expression were correlated. Smooth muscle samples were prepared by scraping the mucosa and submucosa from full-thickness intestinal wall samples. The mRNA levels of 5-HTR₄ were measured by real-time PCR and expressed relative to those of the housekeeping gene glyceraldehyde phosphate dehydrogenase. Binding studies were performed using the 5-HTR₄ antagonist [³H]GR113808. The mRNA levels of 5-HTR₄ were affected (P < 0.05) by location along the GIT. The mRNA levels of 5-HTR₄ in the ELSC and the ileum were greater than in the PLAC (P = 0.05 and P = 0.07, respectively) but similar to those of all other locations. The competitive binding of [³H]GR113808 to suspended membranes from the fundus abomasi, pylorus, cecum, and ELSC was best fit by a 2-site receptor model, whereas it was best fit by a 1-site receptor model in the ileum and PLAC. The mRNA levels and numbers of 5-HTR₄ were not correlated (r = 0.14; P = 0.71). In conclusion, mRNA and binding sites for 5-HTR₄ are present in the smooth muscle layer of the entire GIT of dairy cows and may play a role with respect to motility. The effects of activation of this receptor subtype may be different among GIT locations due to differences in the amount of high- relative to low-affinity binding sites.

Key Words: dairy cow • gastrointestinal tract • 5-hydroxytryptamine (serotonin) receptor