Luteinizing Hormone Response to N-Methyl-D,L-Aspartic Acid in the Presence of Physiological Estradiol Concentrations: Influence of Age and the Ovary

Abstract

We have previously reported that the pituitary of Intra-atrially cannulated old female C57BL/6J mice is as capable of responding to a GnRH challenge as is that of young females (10). We have observed elevated luteinizing hormone (LH) levels in ovariectomized (OVX) intra-atrially cannulated mice. Sustained physiologic levels of estradiol (E₂) for 6 days suppressed circulating LH to intact levels. However, in that model, a bolus of E₂ following E₂ priming was unable to elicit an LH surge (Joshi et al., unpublished findings). The present studies were designed to examine: first, whether GnRH neurons are competent to release GnRH in the presence of tonic physiologic levels of E₂ and, second, whether either age or the ovary can influence GnRH neuronal responsiveness. The N-methyl-D, L-aspartic acid (NMA)-evoked GnRH response was assessed indirectly by measuring LH in two groups of OVX C57BL/6J mice: short-term OVX (S-OVX) (1 week) mice were either prepubertal (5 weeks), postpuberal (10 weeks), young (5 months), middle aged (12 months), or old (24 months). Long-term OVX (L-OVX) mice were either young (5 months), or old (24 months) and OVX at puberty; middle-aged L-OVX mice were OVX at 8 months and examined at 12 months of age. Animals were administered physiologic levels of E₂ by subcutaneous silastic capsule for 1 week before testing. LH secretion was Inhibited by E₂ in S-OVX mice of all ages. In no case did NMA overcome this inhibition in E₂ primed S-OVX females. E₂ also inhibited LH secretion in L-OVX mice of all ages, but NMA was able to overcome the E₂ inhibition of LH secretion in L-OVX mice (young: 0.5 ± 0.1, 0.84 ± 0.19 ng/ml, first and second challenge, respectively; middle-aged: 0.46 ± 0.1, 1.08 ± 0.16 ng/ml; and old: 1.44 ± 0.19, 0.99 ± 0.27 ng/ml). This last effect was independent of animal maturity at the time of OVX or animal age at the time of experiment. These findings suggest that although the ovaries in the 24-month-old S-OVX mice had not produced enough E₂ to alter the vaginal cytology for 2 ± 0.5 months before the experiment, the ovarian modulation of the inhibitory effect of E₂ on NMA-induced LH secretion was still present. The nature of the ovarian factor(s) modulating this effect is unknown. These results demonstrate that in the intra-atrially cannulated female C57BL/6J mouse, the negative feedback effect of E₂ on hypothalamic GnRH release predominates and prevents the induction of an LH surge by a bolus of E_2. The ability of E₂ to inhibit the NMA response is mediated by the length of time between removal of the ovary and initiation of estrogen replacement, and this effect is independent of age.