Immunohistochemical analysis of the hypothalamic-pituitary-adrenal axis in dogs: Sex-linked and seasonal variation

•Corticotroph cells show sexual dimorphism in the dog.•ERα is expressed in the adrenal zona fasciculata and corticotroph cells of the dog.•MC2R in the dog's zona fasciculata shows seasonal variations and sexual dimorphism.

This study evaluated sexual dimorphism and seasonal variations in corticotrophs and adrenal zona fasciculata in dogs, as well as the expression of oestrogen receptor alpha (ERα). An immunohistochemical analysis was conducted in pituitaries for ACTH and in adrenal glands for ERα and for the melanocortin-2-receptor (MC2R) in winter and summer. Double immunofluorescence was performed to identify ERα in corticotrophs. Females had a greater proportion of corticotrophs per field (p < 0.01), with a greater cellular area and optical density (p < 0.001) than males. Optical density of corticotrophs was greater in winter for both sexes (p < 0.001). In zona fasciculata, ERα and MC2R expression was greater in females (p < 0.001) and was greater in winter (p < 0.001). ERα was identified in corticotrophs. This study is the first to demonstrate ERα expression in corticotrophs and the adrenal cortex in dogs, providing evidence for sexual dimorphism and seasonal variations.