ABSTRACT. 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.KEY WORDS: immunohistochemistry, mammary gland, mouse, NOS, reproductive cycle.J. Vet. Med. Sci. 71 (7): [945][946][947][948][949] 2009 Nitric oxide (NO) is an intra-and inter-cellular mediator with diverse physiological and pathophysiological functions in numerous cell types [2,9,20,23]. In vivo, this inorganic free radical gas is generated by the conversion of L-arginine to L-citrulline catalyzed by a family of enzymes known as NO synthase (NOS), which exists in three isoforms. These isoforms were named for the tissues in which they were first identified; neuronal NOS (nNOS, NOS-I), endothelial NOS (eNOS, NOS-III), and the macrophage-inducible form (iNOS, NOS-II). In mammals, constitutive forms of NOS (nNOS and eNOS) are strictly Ca-dependent and generate small amounts of NO, which are responsible for modulating the physiological functions of cells. In contrast, under normal physiological conditions, expression of Ca-independent iNOS is very low, even undetectable, but it can be strongly upregulated as a result of cytokine and/or bacterial endotoxin stimulation [7]. However, the distinction between constitutive and inducible NOS has recently become less clearcut. Indeed, it has been shown that each of the three isoforms can be constitutively expressed in a variety of cells and tissues other than those in which they were initially characterized and cloned [1].NO is known to have a potent influence on the reproductive system and reproduction through numerous physiological processes, such as by altering vascular tone, platelet aggregation, cell growth, or by stimulating angiogenesis, remodeling or apoptosis [5,24,30,33,36]. As the mammary gland is regularly subjected to such processes throughout the reproductive life of mammalian females, NO and NOS have been proposed to play an important role in the growth and proliferation of the mammary gland.NO and NOS activity in the adult rat mammary gland was first reported by Onoda and Inano [22], and a role in the growth and differentiation of the mammary gland was sug...