Prolactin Evolution of

Prolactin (PRL) was so named as the essential factor for development and maintenance of lactation in mammals. Subsequently, it was shown to be vital for survival of euryhaline fish in fresh water and is now known to have over 300 physiological functions among the vertebrates. Originally, PRL was isolated from pituitaries, the source of the circulating hormone, but hypophysectomized female rat plasma has lactogenic activity and indicated that there were other sites of PRL production (such as, brain, thymus, lymph nodes). This local production implies a paracrine/autocrine function and is consistent with the relationship of prolactin with the family of cytokines that constitute the hematopoietic growth factors.