Immunohistochemical localization of nitric oxide synthase (NOS) in mouse mammary gland during reproductive cycle

Nitric oxide (NO) has been proposed as a mediator of postnatal mammary gland development. We investigated the immunohistochemical localization of three isoforms of nitric oxide synthase (NOS) enzymes, neuronal NOS (nNOS), inducible NOS (iNOS) and endothelial NOS (eNOS), in the mouse mammary gland during reproductive cycle. The NOS isoforms detected in normal mouse mammary glands were the constitutive forms of NOS (nNOS and eNOS). nNOS was localized to the alveoli, myoepithelia, lactiferous ducts and blood vessel endothelia, while eNOS was localized in the alveoli, lactiferous ducts and blood vessel endothelia. The strongest immunoreactivity for both constitutive NOS isoforms was observed in pregnant mice. The differential staining intensity of NOS enzymes in the mammary gland led us to conclude that nitric oxide in the mouse mammary gland is mainly synthesized by constitutive NOS isoforms, and suggest that NO has functional roles in post-pubertal growth and differentiation of the mammary gland.