Changes in Vasopressin and Testosterone in the Senescent Brown-Norway (BN/BiRij) Rat

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Abstract
Although earlier reports in the literature suggested degenerative changes in the senescent hypothalamo-neurohypophyseal system (HNS), recent investigation showed hyperactivity of this system in the old Wistar (WAG/Rij) rat. In the brain, changes were found in those sites of extrahypothalamic vasopressin (VP) fiber termination that are testosterone-dependent. In the present study, further evaluation of these changes was performed in male Brown-Norway (BN/BiRij) rats. Immunocytochemical staining of VP binding sites in renal tubuli of the senescent, 33-month-old rats was occasional, and could only slightly be enhanced up to weak staining by in vitro preincubation with the peptide, in contrast to the intense staining observed in young, 3-month-old rats. Although VP plasma levels of young and old rats did not differ significantly and no change in either urine or plasma osmolality was observed, urinary VP concentration and 24-hour urinary VP excretion were significantly increased in senescent rats. The activation of the hypothalamo-neurohypophyseal system (HNS) in the senescent rat seems thus to compensate for a decreased responsiveness to VP in the aged kidney. Testicular weight in the senescent animals declined by 40% and a highly significant decrease of 80-90% was observed in total and free testosterone plasma levels. Consequently, changes in peripheral organs in senescent rats may well underly the changes observed in the brain of these animals.