Short communication: Differences in distribution of serotonin receptor subtypes in the mammary gland of sheep, goats, and cows during lactation and involution

Abstract

Serotonin receptors (5-HTR) are present in the mammary tissue of mouse, humans, cows, and rats. In these species, serotonin is important for the mammary gland function and lactation performance. The mammary gland expression of 5-HTR in small dairy ruminants has yet to be described. In the present study, primer sequences were developed to amplify 5-HTR (1A, 1D, 1E,1B, 1F, 2A, 2B, 2C, 3a, 4, 5a, 6, and 7) using real-time quantitative PCR for the detection of mRNA expression in mammary tissue of dairy sheep, goats, and cows. The distribution of commonly expressed 5-HTR between the 3 species (1B, 1E, 2A, 2B, 4, and 7) was analyzed in the mammary tissue of late-lactation and dried-off sheep, goats, and cows using immunohistochemical staining. Real-time quantitative PCR analysis showed that the 3 studied species expressed receptors 5-HTR1B, 1E, 2A, 2B, 4, and 7. Goats and sheep expressed 5-HTR1D and 5a; 5-HTR1A and 1F were expressed only in sheep. The mammary epithelial cells were positively stained for all the studied receptors by immunohistochemistry (5-HTR1B, 1E, 2A, 2B, 4, and 7). The endothelial cells of blood vessels were positively stained for 5-HTR1B, 2A, 2B, and 7 in all the species. Additionally, 5-HTR1E was present in cow endothelium. The myoepithelial cells stained positively for 5-HTR1E in all the species, and 5-HTR4 myoepithelial staining was present only in cows and sheep. Between the lactating and dried-off mammary glands, the location of 5-HTR in the epithelial cells changed from a cytoplasmic reaction in lactating udders to a reaction in the apical region in dry udders. These results showed that the distribution of 5-HTR subtypes in the mammary gland of dairy ruminants vary among species, tissue type, and stage of gland development. These findings warrant future studies aimed at understanding whether the differences in 5-HTR subtype expression and location accounts for the differences in milk secretion and lactocyte activity among cows, goats, and sheep.