CHANGING PRO-OESTROUS SURGES OF LUTEINIZING HORMONE IN AGEING 5-DAY CYCLIC RATS

Abstract

Department of Endocrinology, Growth and Reproduction, Erasmus University, Medical Faculty, P.O. Box 1738, Rotterdam, The Netherlands (Received 23 October 1975) Normal cyclic ovarian activity stops in old female rats: cyclicity may be replaced by absence of ovulation and persistent vaginal cornification (Everett, 1939; Aschheim, 1964/5). In the present study, factors were examined which could contribute to the change from cyclicity to absence of ovulation. Vaginal cyclicity, ovarian microscopy, and the pro-oestrous surge of luteinizing hormone (LH) were compared between old and young rats. Locally bred (R × U)F1 hybrid females were kept under standard lighting conditions (lights on at 05.00 h; off at 19.00 h). Three- to 5-month-old rats were all cyclic and showed predominantly 5-day cycles. From 7 months onwards a rapid increase occurred in the number of animals with persistent vaginal cornification; cyclic animals kept 5-day cycles (Table 1). In the first experiment sodium pentobarbitone (Nembutal) was injected