Attenuated proestrous luteinizing hormone surges in middle-aged rats are associated with decreased pituitary luteinizing hormone-B messenger ribonucleic acid expression Barbara B. Hogg, BA, Dennis W. Matt, PhD, and Timothy E. Sayles Richmond, Virginia OBJECTIVE: This study determined whether attenuated preovulatory luteinizing hormone surges in aging rats are associated with a decrease in pituitary luteinizing hormone content or luteinizing hormone l3-messenger ribonucleic acid expression on proestrus. STUDY DESIGN: Blood samples were taken every 90 minutes from 1:30 to 10:30 PM on proestrus in young (n = 8) and middle-aged (n = 12), regularly cyclic rats for plasma luteinizing hormone determination. On the next proestrus at 12 noon, rats were killed and the pituitaries were removed for luteinizing hormone content determination by radioimmunoassay and luteinizing hormone l3-messenger ribonucleic acid expression by dot blot analysis. Results were analyzed by one-way analysis of variance. RESULTS: Seven of the middle-aged rats had attenuated luteinizing hormone surges while the remaining five females had surges similar to those of young rats. On the next proestrus, all rats had similar quantities of pituitary luteinizing hormone. However, luteinizing hormone l3-messenger ribonucleic acid expression in middle-aged rats with attenuated luteinizing hormone surges was lower (p < 0.05) than that of middle-aged and young rats with normal surges. CONCLUSION: Decreased luteinizing hormone l3-messenger ribonucleic acid expression, but not pituitary luteinizing hormone content at 12 noon on proestrus is correlated with attenuated luteinizing hormone surges in middle-aged rats. (AMJ OSSTET GYNECOL 1992;167:303-7.)

Key words: Luteinizing hormone, luteinizing hormone f3-messenger ribonucleic acid expression, female reproductive aging, pituitary, ovulation Aging in the female rat is characterized by a progressive loss of regular estrous cyclicity and reproductive function.':' During middle age (10 to 12 months), the estrous cycle patterns of female rats change from regular 4-day cycles to irregular, extended cycles (2:6 days).' These irregular cycles are characterized by extended periods of vaginal cornification reflecting sustained estrogen release." After a transitional period of irregular cyclicity, female rats display sustained cornified vaginal epithelium and enter the anovulatory condition described as persistent estrus. Although the neuroendocrine mechanisms associated with the age-related loss in regular ovulatory cyclicity are currently unknown, previous studies have From the Departments of Obstetrics and Gynecology and Anatomy, Medical College of Virginia, Virginia Commonwealth University. Supported by The South Atlantic Association of Obstetricians and Gynecologists John R. McCain Fellowship, a Medical College of Virginia grant (A.D. Williams Summer Fellowship), National Institutes of Health grant No. AG-07452 (D.W.M.), and the Thomas F. and Kate Miller Jefferess Memorial Trust. John McCain Fellowship Lecture, presented at the Fifty-fourth Annual Meeting of the South Atlantic Association of Obstetricians and Gynecologists, Palm Beach, Florida, January 26-29, 1992. Reprint requests: Dennis W. Matt, PhD, Department of Obstetrics and Gynecology, Medical College of Virginia, Box 34, Richmond, VA 23298. 6/6/38063

revealed that the proestrous luteinizing hormone (LH) surges are attenuated in some middle-aged rats.?" Furthermore, female rats with attenuated proestrous LH surges cease to display regular estrous cyclicity within 2 months, whereas rats of similar age with normal LH surges continue to cycle regularly beyond 2 months." Thus changes in neuroendocrine function can be identified in aging rats before the loss of regular estrous cyclicity. The following experiments were conducted to investigate further the possible alterations in the hypothalamic-pituitary axis of the aging female rat, alterations that are associated with the loss of regular estrous cyclicity and ovulatory function. To determine if attenuated LH surges are related to decreases in pituitary LH synthesis, these studies examined the relation between the magnitude of the proestrous LH surge and pituitary LH content and LH -13 gene expression in young and middle-aged rats.

Material and methods Animals. Young (4 months) virgin and middle-aged (10 to 12 months) retired breeder female Long-Evan rats (Charles River Lab, Wilmington, Mass.) were maintained under standard laboratory conditions at room temperature (23° to 25° C) with a lighting schedule of

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Proestrus (h) Fig. 1. Patterns of LH secretion in young (open circles) and middle-aged female rats from 1:30 PM (1330 hours) to 10:30 PM (2230 hours) on proestrus. Middle-aged females demonstrating peak LH values of >7 ng/ml were considered to have normal LH surges (solid circles), whereas those middleaged rats with peak LH values of