Plasma and pituitary concentrations of luteinizing hormone, follicle-stimulating hormone and prolactin in aged, ovariectomized CD-1 and C57BL/6 mice

Plasma and pituitary concentrations of luteinizing hormone (LH), follicle-stimulating hormone (FSH), and prolactin (Prl) were determined by radioimmunoassay in young (2-4 months-old) and aged CD-1 (14-18 months-old) and C57BL/6 (16-22 months-old) mice one month after ovariectomy. In young, ovariectomized mice, plasma and pituitary concentrations of LH and FSH were significantly higher, whereas concentrations of Prl were significantly lower than those in control mice (sham-operated). In contrast, plasma concentrations of LH, FSH and Prl were not statistically different in aged, ovariectomized mice and aged, control mice. There were also no differences in pituitary concentrations of the three hormones when comparing the same aged C57BL/6 mice, although the aged, ovariectomized CD-1 mice exhibited higher pituitary levels of each hormone than those of their controls. The pituitary of the aged mouse responds differently to ovariectomy than that of the young mouse because of age-related changes in the ovary and/or hypothalamic-hypophyseal complex.

Plasma and pituitary concentrations of LH, FSH and prolactin after injection of GnRH in aged female C57BL/6 mice

Plasma and pituitary concentrations of LH, FSH and prolactin (Prl) were compared by radioimmunoassay (RIA) in young mature (2-4 month-old) and aged (16-22 month-old) female C57BL/6 mice, following various experiments with IP injections of GnRH. Unanesthetized mice were bled by cardiac puncture either (1) at various time intervals following a single injection of 25 ng GnRH, (2) after several injections of 25 ng GnRH, or (3) after receiving excessively large doses of GnRH. From the RIA of plasma and pituitary concentrations of LH, it appeared that the pituitaries of aged mice were, in general, capable of responding to GnRH treatment, only to a lesser degree than that of the younger females. The exception was that the pituitaries of aged mice that were subjected to unusually high doses of GnRH continued to secrete higher levels of LH, whereas the response of the pituitaries in younger mice plateaued after receiving 25 ng GnRH. Plasma and pituitary concentrations of FSH and Prl remained fairly constant when aged GnRH-treated females were compared with aged controls. This was also true when FSH and Prl concentrations in the younger mice were compared with their controls, except for significantly higher plasma FSH and significantly lower pituitary FSH following several injections of GnRH. When the hormonal concentrations of the young and aged groups were compared in the various experiments, the older mice consistently exhibited higher levels of gonadotropin and lower levels of prolactin than those of the younger mice. In a separate experiment, in which a radioreceptor assay (RRA) was used to quantitate plasma LH in GnRH-treated mice, both age groups were found to have lower concentrations of LH (p less than 0.05) than those obtained by RIA from the same animals. The RRA to RIA ratio, however, was 0.84 for 2-4 month-old and 0.57 for 16-22 month-old mice. These studies suggest that the anterior pituitary of the aged mouse is less sensitive to GnRH stimulation.

Plasma and pituitary concentrations of LH, FSH and prolactin in aged female C57BL/6 mice

Plasma and pituitary concentrations of LH, FSH and prolactin were determined by radioimmunoassay in 2-month-old (young) and 16-20-month-old (old) C56BL/6 mice. There were no statistical differences in hormonal levels between aged females in oestrus (those exhibiting a copulatory plug) and those in constant dioestrus. In the old females plasma levels of LH (P < 0.002) and FSH (P < 0.001) were significantly elevated, while levels of prolactin (P < 0.001) were significantly depressed when compared with those from young animals. Pituitary homogenates from old females also contained more gonadotrophins (P < 0.001) and less prolactin (P < 0.001) than those of the young females. A radioreceptor assay utilizing a plasma membrane of luteinized rat or mouse ovaries indicated that LH from 2-month-old animals bound better to ovarian receptors (P < 0.05) than did LH from old mice, although radioimmunoassay of the same samples gave higher (P < 0.01) plasma LH levels for the old mice. Since the radioreceptor assay is considered to be a more sensitive test for biologically active LH, the results from these two types of assays suggest that there may be an alteration in the mouse LH molecule with age.