The Increase of Anterior Pituitary Dopamine in Aging C57BL/6J Female Mice Is Caused by Ovarian Steroids, Not Intrinsic Pituitary Aging

Abstract

We describe how the increase of anterior pituitary dopamine (DA) during aging in female mice is related to altered secretion of ovarian steroids during reproductive senescence. A number of age-correlated neuroendocrine changes in female rodents result from cumulative exposure to ovarian steroids over a lifetime of estrous cycles, or from the altered pattern of ovarian steroid secretion concomitant with reproductive senescence. Pituitary DA has been shown to increase with age in female rats. To examine how the age-correlated increase of pituitary DA may depend on estradiol (E₂), we measured pituitary DA and serum prolactin (PRL) in the following groups of female mice: young (7 months) cycling, middle-aged (14 months) cycling and non-cycling, old (17 months) non-cycling, old (17 months) ovariectomized (OVX) at 4 months, and young mice given 0.2 mg E₂ valerate or E₂ implants. Mice from some of these groups were OVX 1, 4 or 8 weeks before sacrifice. Compared with young controls, 14-month-old cycling or non-cycling mice had 3-fold higher pituitary DA, and 17-month-old non-cycling mice had 5-fold higher pituitary DA. OVX for 2 or 13 months before sacrifice abolished the effect of age; OVX of young mice had no effect on pituitary DA. Three weeks after implantation of E₂ into OVX young mice or 7 weeks after injection of E₂ valerate in intact young mice, pituitary DA was elevated. The E₂-sensitive fraction of pituitary DA does not appear to decrease PRL secretion. We conclude that the age-correlated increase in pituitary DA is primarily dependent upon the effectively enhanced estrogenic stimulation concomitant with reproductive senescence, rather than upon intrinsic pituitary aging or irreversible effects from exposure to E₂ over a lifetime of estrous cycles.

N-Methyl-Aspartic Acid Lesions of the Arcuate Nucleus in Adult C57BL/6J Mice: A New Model for Age-Related Lengthening of the Estrous Cycle

“…Estrogen in creased the dopamine content of the anterior pituitaiy of OVX mice, but NMA treatment increased anterior pituitary dopa mine content above that seen with estrogen alone. Aging also increases dopamine content in the anterior pituitary of intact female mice but again, this change primarily is dependent upon the presence of estrogen; OVX of aged mice reduces anterior pituitary dopamine content toward levels seen in young intact females (34]. The present data suggest that NMA influenced do pamine clearance from an estrogen-sensitive site in the anterior pituitary and that such a paradigm may be useful in elucidating a function for anterior pituitary dopamine.…”

“…Dopamine and norepinephrine were measured in homogenates prepared from medial basal hypothalamus and anterior pituitary as described previously [27,34]. Briefly, frozen tissue was homogenized in 0.1 N HCI containing 1 m M EDTA and 0.57 m M ascorbic acid.…”

DOPA Accumulates in the Hypothalamic-Hypophysial Portal Vessels and Is Taken into the Anterior Pituitary of NSD-1015-Treated Rodents

“…Drinking water was adjusted to pH 2.2 to suppress Pseudomonas infections, as a general precaution [II]. In view of the sensitivity of pituitary' DA to estradiol and progesterone [4,12,13], mice were ovariectomized 3 weeks before experimentation. Mice were anesthetized with 2,2,2-tribromoethanol (Matheson, Coleman, Bell, Norwood, Ohio, USA) and bilaterally ovariectomized.…”

“…Mouse APs were homogenized [12] in 100 pi 0.1 N HCI, 1 mM EDTA, containing the appropriate internal standards: for DA, 4 ng/ml 3,4-dihydroxybenzylamine (Sigma Chemical Company, St. Louis, Mo., USA); for DOPA. 3,4-dihydroxyphenylacetic acid (DOPAC; Calbiochem-Behring, La Jolla, Calif., USA).…”

“…Aliquots of the homogenate (5-ul duplicates) were saved for protein assay [15]; the remainder was centrifuged at 1,800 g for 5 min. Supernatants were extracted for catechols by binding to alu mina [12,13,16,17], and analyzed by HPLC using a 5-gm C-18 reversed-phase column and precolumn (Brownlee Labs Inc., Santa Clara, Calif., USA) and an electrochemical detector with a glassy carbon electrode (Bioanalytical Systems, West Lafayette, Ind., USA). The mobile phase and chromatography conditions were as described [12,17] except that the mobile phase was acidified to pH 2.1-2.5 to resolve DOPA.…”