الفهرس 
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Abstract
Bone is a dynamic tissue that is constantly remodeled to ensure correct mineral homeostasis and to maintain the structural integrity and thereby bone strength (Kular et al., 2012). A full remodeling cycle comprises bone resorption, followed by bone formation in tightly coordinated sequences. In humans, any defects in remodeling result in a range of metabolic bone diseases including osteoporosis, Paget’s disease of bone and osteopetrosis (Seeman, 2009; Kular et al., 2012).
Osteoporosis is the most prevalent metabolic bone disease characterized by low bone mass, micro-architectural deterioration of bone tissue, increase in bone fragility and susceptibility to fracture (Sànchez-Riera et al., 2010; Honig and Chang, 2012).
There are many mechanical, systemic and local factors, more or less dependent on each other, are essential for the dynamic balance between formation and resorption, maintenance of bone mass and skeletal integrity (Crockett et al., 2011).
Among these factors affecting bone remodeling is prolactin (PRL), the polypeptide hormone secreted by the lactotroph cells of the pituitary gland. PRL has long been known as a pituitary hormone that is important for mammary gland development and differentiation of mammary tissue during pregnancy and crucial for lactation and maternal behavior immediately after labor (Ben-Jonathan et al., 2008; Larsen and Grattan, 2012). However, nowadays, it is clear that PRL has several biological actions, which differ among species. In humans, the possible additional physiological roles of PRL are under investigation for the greater part; however, recent evidence suggests a modulatory role of PRL in the metabolism and immune response (Ben-Jonathan et al., 2008). In addition, PRL plays an important role in pathophysiological conditions: tumor growth in both breast cancer and possibly prostate cancer is promoted by PRL (Jacobson et al., 2011).
Hyperprolactinemia, the most common disorder of the hypothalamic- hypophyseal system is associated with a number of physiological conditions such as pregnancy, lactation and pathological conditions such as hypothyroidism, prolactinoma and chronic uses of dopamine-related antipsychotic drugs (Jung et al., 2006; Prabhakar and Davis, 2008; Holt and Peveler, 2011).
Although the effect of hyperprolactinemia on bone is, so far, believed to be mediated by hypogonadism, the presence of short and long isoforms of prolactin receptors (PRLRs) in tibiae, femora and vertebrae of adult female rats suggesting direct action of PRL on bone (Charoenphandhu et al., 2007).
Therefore, it is important to study the possible role of PRL in the regulation of bone remodeling.