Endocrine Status Versus Chronologic Age as Predictors of Altered Luteinizing Hormone Secretion in the “Aging” Rat*

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Abstract
LH [luteinizing hormone] release in response to GnRH [gonadotropin-releasing hormone] injected at 1400 h was measured in young (3-4 mo.), middle-aged (10-11 mo.) and old (19-20 mo.) female rats. Data were compared when animals were grouped by chronological age or according to their vaginal smear pattern 3-4 wk before treatment, i.e., regular 4-day cycles, constant vaginal cornification (CVC) or persistently leukocytic smears (PL). Compared to young, regularly cycling females (injected on vaginal estrus or diestrus II), the amount of LH released by all age groups with CVC or PL smears was significantly diminished. However, regardless of age, the amount of LH released by CVC females was similar for each dose of GnRH. Similarly, there was no difference in the amount of LH released by the young, middle-aged and old PL females at each dose of GnRH. Finally, the amount of LH released by the CVC females was uniformly higher than the amount of LH released by age-matched PL females. In contrast, comparison of LH release after GnRH injection in young and middle-aged rats released significantly less LH to protestrus. Apparently, changes in the ability of the pituitary to release LH in the older noncycling female rat occurs as a consequence of the altered endocrine status in the animal and not as a result of the chronological age per se. Changes in the ability of the pituitary of the middle-aged, regularly cycling female rat to release LH in response to GnRH may contribute to the age-related disruption of regular ovarian cycling.