Selective expression of neuropeptides in the rat mammary gland: somatostatin gene is expressed during lactation.

The existence of numerous neuropeptides in milk, in concentrations that exceed those in maternal plasma, is well established. It is still unclear whether these neuropeptides are produced by the mammary gland or that the gland concentrates them from the general circulation. In this study, we have examined the possibility that the genes of these neuropeptides are expressed in the rat mammary gland. RNA was extracted from the mammary glands of female rats during different stages of reproduction as well as from other tissues such as hypothalami, pancreas, pineal glands, small intestine, and ovaries. Following RT reaction, the resulting cDNA were amplified by radioactive PCR using specific oligonucleotide primers. We have used specific primers for the following neuropeptides: galanin, somatostatin, vasoactive intestinal peptide, TRH, GH-releasing hormone, cholecystokinin, neurotensin, oxytocin, and relaxin. We have also used primers for serotonin N-acetyl-transferase, the enzyme that is involved in melatonin biosynthesis. The ribosomal protein S-16 served as an internal control. Among all the neuropeptides that have been examined, somatostatin was the only one that was found to be expressed in the mammary gland. Somatostatin was expressed in the mammary gland of lactating rats, but not of virgin rats. Expression of the somatostatin gene was confirmed by Southern blot analysis and by sequencing of the PCR products. Immunohistochemical studies demonstrated somatostatin immunoreactivity in the epithelial cells that compose the secretory alveoli and in the secretory material. In addition, we have found that the mammary glands of the lactating rat express the PC-1 proteinase gene that process prosomatostatin to generate somatostatin-14, but do not express furin, the enzyme that is responsible for somatostatin-28 production. This finding substantiates previous studies that demonstrated that only somatostatin-14 is present in milk. The finding that most of the neuropeptides, examined by RT-PCR, are not expressed by the mammary gland suggest that these neuropeptides are actively concentrated by the mammary glands from the general circulation. The GnRH gene has been previously demonstrated to be expressed in the mammary gland, and in this study somatostatin was the only neuropeptide that was found to be produced by the mammary gland. The observation that only a small portion of the neuropeptides that are present in milk are being produced by the lactating mammary gland suggest that these neuropeptides have important functions in the biology of the suckling neonate and probably also in the development and function of the breast.