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 exc eed those in maternal plasma, is well established. It is still unclear whet her these neuropeptides are produced by the mammary gland or that the gland concentrates them from the general circulation. In this study, we have exa mined the possibility that the genes of these neuropeptides are expressed i n the rat mammary gland. RNA was extracted from the mammary glands of femal e rats during different stages of reproduction as well as from other tissue s such as hypothalami, pancreas, pineal glands, small intestine, and ovarie s. Following RT reaction, the resulting cDNA were amplified by radioactive PCR using specific oligonucleotide primers. We have used specific primers f or the following neuropeptides: galanin, somatostatin, vasoactive intestina l peptide, TRH, GH-releasing hormone, cholecystokinin, neurotensin, oxytoci n, and relaxin. We have also used primers for serotonin N-acetyl-transferas e, the enzyme that is involved in melatonin biosynthesis. The ribosomal pro tein S-16 served as an internal control. Among all the neuropeptides that h ave been examined, somatostatin was the only one that was found to be expre ssed in the mammary gland. Somatostatin was expressed in the mammary gland of lactating rats, but not of virgin rats. Expression of the somatostatin g ene was confirmed by Southern blot analysis and by sequencing of the PCR pr oducts. Immunohistochemical studies demonstrated somatostatin immunoreactiv ity in the epithelial cells that compose the secretory alveoli and in the s ecretory material. In addition, we have found that the mammary glands of th e lactating rat express the PC-1 proteinase gene that process prosomatostat in to generate somatostatin-14, but do not express furin, the enzyme that i s responsible for somatostatin-28 production. This finding substantiates pr evious studies that demonstrated that only somatostatin-14 is present in mi lk The finding that most of the neuropeptides, examined by RT-PCR, are not expressed by the mammary gland suggest that these neuropeptides are activel y 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 t o be produced by the mammary gland. The observation that only a small porti on of the neuropeptides that are present in milk are being produced by the lactating mammary gland suggest that these neuropepetides have important fu nctions in the biology of the suckling neonate and probably also in the dev elopment and function of the breast.