RESUMO
The estrogen-induced luteinizing hormone (LH) surge is markedly attenuated in old (24 months) female mice compared with young (6 months) females. To test whether or not this attenuated LH response is due to a diminished capacity of the pituitary to respond to hypothalamic gonadotropin hormone-releasing hormone (GnRH), the pituitary response to exogenous GnRH was measured in young (5-6 months), normally cycling (n = 6) and old (24 months), acyclic constant diestrus (n = 6) C57BL/6J female mice. Mice were ovariectomized and estrogen-treated for 7 days. After intrajugular catheterization on Day 6, serial blood samples were taken at 15-min intervals for 165 min on Day 7. Serum LH was measured in samples obtained before and after infusion of either saline or GnRH (5 micrograms/5 microliters saline/kg body wt) and 15 micrograms/15 microliters saline/kg body wt 1 hr apart). Saline-treated animals demonstrated no LH response in either young (0.09 +/- 0.02 ng/ml baseline) or old (0.11 +/- 0.01 ng/ml baseline) females. However, a significant release of LH was obtained in response to each challenge of GnRH in both young (0.3 +/- 0.04 ng/ml first challenge, 0.69 +/- 0.1 ng/ml second challenge) and old (0.78 +/- 0.1 ng/ml first challenge, 1.76 +/- 0.2 ng/ml second challenge) mice. The LH response in the aged group was significantly greater (P < 0.05, analysis of variance) than in the young group. These results show that pituitaries of old female mice were at least as capable of responding to exogenous stimulation by GnRH as those of young. We conclude that alteration in the capacity of the pituitary to respond to GnRH is not likely to be a factor contributing to altered LH secretion with age in C57BL/6J mice.
