The high concentration of gonadotropinreleasing hormone (GnRH) In milk of several species implies that the mammary and is either a site of synthesis for this neuropeptide or that it is efficiently concentrated from plasma by this organ. By PCR amplification ofmammary gland cDNA, we have demonstrated expression of the mRNA for GnRH. The GnRH mRNA was present In the mammargland of pregnant and lactating rats but not of virgin rats, implying that expression ofthe GnRH gene is activated during pregnancy probably by prolactin. In contrast, actin mRNA was evident in all the preparations of mammary glands. Since GnRH is also known to be synthesized by the placenta, it is likely that the placenta and the mammary gland are complementary units by which the mother exercises control over the development and the metabolism of the infant during pregnancy as well as after parturition. In addition, GnRH synthesized by the mammary gland may also affect the mother by a paracrine and/or an endocrine mechanism.

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