Lewis C. Krey

Prolonged glucocorticoid exposure reduces hippocampal neuron number: implications for aging

The hippocampus of the rat loses neurons with age, a loss which may eventuate in some of the functional impairments typical of senescence. Cumulative exposure to corticosterone (CORT) over the lifespan may be a cause of this neuronal loss, as it is prevented by adrenalectomy at mid- age. In this study, we demonstrate that prolonged exposure to CORT accelerates the process of cell loss. Rats were injected daily with sufficient CORT to produce prolonged elevations of circulating titers within the high physiological range. Animals treated for 3 months (chronic subjects) resembled aged rats in a number of ways. First, both groups had extensive and persistent depletions of CORT receptors in the hippocampus; in the case of chronic rats, no recovery of receptor concentrations occurred 4 months after the end of steroid treatment. Second, autoradiographic analysis revealed that the receptor depletion was due, in part, to a loss of CORT-concentrating cells, especially in the CA3 cell field. Remaining cells bound significantly less [3H]corticosterone than did those of control rats. Finally, analysis of size distributions of hippocampal cell bodies indicated that chronic subjects lost neurons of the same size as those lost in the aged hippocampus. Furthermore, chronic subjects also had increased numbers of small, darkly staining cells of CA3; these corresponded in size to the dark glia whose numbers increase in the aged hippocampus, and which are thought to infiltrate in response to neuronal damage or destruction. Thus, this study supports the hypothesis that cumulative exposure to CORT over the lifespan may contribute to age-related loss of neurons in the hippocampus, and that prolonged stress or exposure to CORT accelerates this process.

Aromatization: Important for sexual differentiation of the neonatal rat brain

This paper examines the hypothesis that testosterone (T) produces its differ-entiative effect on the neonatal rat brain after undergoing conversion in situ to estradiol-17β (E2). We examined the abilities of an aromatizing enzyme inhibitor, (1,4,6-androstatriene-3,17-dione) (ATD), and an anti-estrogen, CI628, to inhibit sexual differentiation. Male and female rats were treated during the first few days of postnatal life with ATD or CI628, and females were treated on the following day with T in Silastic capsules or its propionate (TP) in oil. ATD and ATD+T females were normal with respect to time of vaginal opening, ovarian weight, ability to demonstrate an LH surge, and lordosis behavior. T and TP females were masculinized with respect to all these measures. CI628 and CI628+TP females had impaired ovarian function and intermediate lordosis quotients (LQs) compared to controls, though they were higher in both measures than T females. ATD males demonstrated high LQs in response to estradiol benzoate (EB) + progesterone, comparable to those of control females. CI628 males had intermediate LQs, which were significantly higher than those of control males. These results indicate that ATD can substantially protect the neonatal rat brain from the differentiative effects of exogenous or endogenous T, probably by blocking aromatization. CI628 affords only partial protection against T and produces a weak differentiative effect due to its own weak estrogenicity. Thus, aromatization of T in newborn rat brains appears to be essential if sexual differentiation is to occur.

The adrenocorticol stress-response in the aged male rat: Impairment of recovery from stress

To test if the adrenocortical axis of the rat loses sensitivity to negative feedback control during aging, we examined corticosterone secretion under basal, stressed and post-stress conditions in young and in aged Fisher male rats. We find no age-related change in the speed or magnitude of the stress response or in the capacity to manifest a corticosterone response following chronic stress. However, we do observe in aging rats an elevation of basal corticosterone and an impaired capacity to adapt to and recover from stress. This latter finding is illustrated by longer latencies relative to young rats, in the return of serum corticosterone concentrations to basal values during sustained exposure to mild cold or following the end of immobilization stress. All of these deficits reflect an increased rate of corticosterone secretion during physiologically inappropriate circumstances. Such observations support the concept that there is an age-related loss of sensitivity of the brain and pituitary to the inhibitory effects of high circulating levels of corticosterone on ACTH release.

Subcutaneous implantation method for chronic glucocorticoid replacement therapy

Abstract A procedure is described for replacing glucocorticoids by subcutaneous implantation of pellets formed from molten corticosterone and cholesterol. Hormone dose is controlled by varying the ratio of these two substances in the pellet. The method produces stable blood and brain corticosterone levels in the physiological range within 3 days and is shown to reinstate normal brain glycerolphosphate dehydrogenase activity in previously adrenalectomized rats. The procedure appears to be well-suited to long-term psycho- and neuro-endocrine studies.

Steroid hormones: humoral signals which alter brain cell properties and functions.

Publisher Summary The steroid hormones influence the brain to affect behavior. This chapter discusses the intracellular steroid receptors in neural tissue; their anatomical distribution and properties, their role in carrying hormone to the cell nucleus, and their effects upon gene expression. It also describes the direct effects of steroids upon neural tissue which do not appear to be mediated by intracellular steroid receptors. It provides a more integrated picture of how steroid hormones influence patterns of behavior during the development and in adult life, using as an example the effects of estradiol in the rat brain. The brain contains receptor sites for all five classes of steroid hormones. Brain regions that contain such cytosol steroid receptors also display translocation of labeled hormone to the cell nuclear compartment, and this phenomenon underlies autoradiographic localization of steroid hormone concentrating cells, because the presence of label over the cell nucleus provides a visually striking and quantifiable endpoint. Cell nuclear translocation of steroid hormones in neural tissues is not necessarily accompanied by the extensive cytosol receptor depletion even when nuclear sites are loaded to capacity. There are other, indirect demonstrations of genomic involvement in steroid action in the brain.

Steroid receptor levels in intact and ovariectomized estrogen-treated rats: an examination of quantitative, temporal and endocrine factors influencing the efficacy of an estradiol stimulus.

Cell nuclear estrogen receptors and cytosol progestin receptors were measured in the pituitary gland, preoptic area and hypothalamus throughout the estrous cycle of the rat. Cell nuclear estrogen receptor levels paralleled changes in serum estradiol concentrations with highest values on proestrus and lowest on diestrus. Proestrous values were 50-60% of capacity for each tissue. Cytosol progestin receptor number in these tissues was also highest on proestrus and lowest on diestrus. With these data as a guide, Silastic capsules filled with estradiol-cholesterol mixtures were used to generate physiologic levels of estrogen receptor occupation within the brain-pituitary complex of ovariectomized rats and to examine the kinetics of estradiol stimulation of lordosis behavior and cyclic gonadotropin release. Our results indicate that the effectiveness of an estradiol stimulus to elicit lordosis or luteinizing hormone release depends on at least three factors: the magnitude of the increment in serum estradiol and brain and pituitary cell nuclear estradiol receptor levels; the duration over which these increments area maintained; and the interval from previous exposure to estrogen.

Corticosterone receptors decline in a site-specific manner in the aged rat brain.

Putative glucocorticoid receptors were measured in the brain and pituitary glands of young and aged rats in vivo and in vitro. Adrenalectomized rats were injected with a set dose of radiolabeled corticosterone plus unlabeled corticosterone; 2 h later [3H]corticosterone levels were measured in purified nuclear pellets from pituitary and 5 brain regions. Substantial decreases were seen in aged subjects in the maximal number of corticosterone binding sites in hippocampus and amygdala; all other regions showed no age-related changes. In contrast, there were no declines in the nuclear uptake of [3H]dexamethasone (DEX) in the aged rat brains. Since DEX interacts selectively with non-neuronal receptors in vivo, the deficit in glucocorticoid binding is selective for neurons. Subsequent studies assessing glucocorticoid receptor levels in cytosol preparations in vitro revealed significant declines in hippocampus and amygdala quantitatively comparable to the decline observed in nuclear binding in these loci. This suggests that the primary deficit leading to nuclear depletion may be the reduction of cytosolic receptor number, rather than other possible factors such as the reduction in receptor affinity or translocation of steroid-receptor complex. This decline may play a role in a number of limbic functions which are influenced by glucocorticoids and show deficits with age.

The adrenocortical axis in the aged rat: Impaired sensitivity to both fast and delayed feedback inhibition ☆

Aged rats secrete excessive amounts of the species-typical glucocorticoid, corticosterone, under basal conditions, following the end of stress and during habituation to mild stressors. Furthermore, the aged rat is resistant to the inhibitory effects of the synthetic glucocorticoid dexamethasone upon subsequent corticosterone secretion. These observations have led to the hypothesis that the aged adrenocortical axis is desensitized to the inhibitory effects of glucocorticoids. In the present study, we have defined this negative-feedback deficit more precisely. The aged adrenocortical axis is subject to both rate-sensitive fast feedback regulation by corticosterone and to level-sensitive delayed feedback. Moreover, there is no age difference in the maximal extent of feedback inhibition which can be attained. However, the sensitivity to both forms of feedback regulation is diminished in aged rats, in that the aged adrenocortical axes are responsive under feedback conditions which completely inhibit corticosterone secretion in young animals. Such insensitivity is likely to underlie the incidences of hyperadrenocorticism apparent in the aged rat; we speculate that progressive degeneration in the aged hippocampus might be the cause of this dampened sensitivity to feedback inhibition.

Elevated day 3 serum follicle stimulating hormone and/or estradiol may predict fetal aneuploidy

OBJECTIVE To determine whether baseline serum FSH and/or E2 concentrations can predict the risk for fetal chromosomal abnormalities. DESIGN Case control study. SETTING Reproductive technology program at a university hospital. PATIENT(S) Patients who underwent dilation and curettage (D + C), and whose products of conception were karyotyped. INTERVENTION(S) Patients underwent natural conception or controlled ovarian hyperstimulation followed by intrauterine insemination, in vitro fertilization and embryo transfer, gamete intrafallopian transfer, or zygote intrafallopian transfer. MAIN OUTCOME MEASURE(S) Baseline serum FSH and E2 concentrations and fetal karyotype. RESULT(S) Genetic evaluation of 78 D + C specimens revealed 34 normal and 44 abnormal fetal karyotypes. A significantly greater proportion of women with abnormal fetal karyotype had elevated baseline serum FSH (> or =15 mIU/mL [RIA] or 10 mIU/mL [Immulite]) and/or E2 > or = 50 pg/mL [Immulite]) compared with women of normal fetal karyotype. Among karyotypically abnormal abortuses, autosomal trisomy was the most common abnormality noted (79.5%), followed by mosaicism (6.8%), triploidy (6.8%), monosomy XO (4.5%), and balanced translocation (2.3%). CONCLUSION(S) Baseline serum FSH and/or E2 concentrations may be valuable as predictors of fetal aneuploidy.

Temporal relationship between cell nuclear progestin receptor levels and sexual receptivity following intravenous progesterone administration

This study was designed to assess the temporal relationship between the appearance and retention of cell nuclear progestin receptors in hypothalamus and the facilitation and decline of feminine sexual behavior following an i.v. injection of progesterone (P). Nuclear translocation of progestin receptors preceded the earliest appearance of behavior. The behavioral effects of P outlasted the nuclear progestin receptor elevation by several hours. Our results are consistent with the idea that P-induced effects on feminine sexual behavior involve genomic activation.