Morphological identification of the leptin and its receptor in the abomasum of the sheep

The growing summer drought stress is affecting the nutritional value of pasture, no longer sufficient to support the nutritional status of the animals. This study aimed to describe the localization and distribution of both leptin (Ob) and its receptor (Ob-R) in sheep abomasum to identify biological markers of nutritional status and therefore sheep welfare. The Ob, an adipokine mainly produced by adipose tissue, has been detected in the human and rat gastrointestinal tract, where it regulates the rate of gastric emptying. Furthermore, Ob regulates food intake by an anorexigenic action. Abomasum samples of 15 adult female sheep reared in a semi-natural pasture were used to identify Ob and Ob-R by immunohistochemistry. Formalin-fixed paraffin-embedded sections, microwaved in the citrate buffer (pH 6) for antigen retrieval, were incubated with mouse monoclonal anti-Ob and rabbit polyclonal anti-Ob-R primary antibodies. Horse anti-mouse and goat anti-rabbit biotin-conjugated secondary antibodies were used. Immunofluorescent double-label localization of the Ob system with different neuroendocrine hormones (synaptophysin, chromogranin and serotonin) was conducted to distinguish the gland cell types. Both Ob and Ob-R have been detected in the mucous layer of the abomasum. The positive cells were localized in the lower half of fundic glands, and they were labelled as chief cells based on their morphological characteristics. Double-label immunohistochemistry showed that the cells positive to serotonin did not stain with Ob and Ob-R, while the positive ones to synaptophysin and chromogranin partially colocalize with chief cells secreting Ob and Ob-R highlighting a different behaviour of the neuroendocrine cell populations. The abundant presence of Ob and Ob-R in the gastric glands suggests a role of the leptin system in the regulation of abomasum functions in sheep.