Estradiol enhances neurogenesis following ischemic stroke through estrogen receptors alpha and beta.

Neurogenesis persists throughout life under normal and degenerative conditions. The adult subventricular zone (SVZ) generates neural stem cells capable of differentiating to neuroblasts and migrating to the site of injury in response to brain insults. In the present study, we investigated whether estradiol increases neurogenesis in the SVZ in an animal model of stroke to potentially promote the ability of the brain to undergo repair. Ovariectomized C57BL/6J mice were implanted with capsules containing either vehicle or 17beta-estradiol, and 1 week later they underwent experimental ischemia. We utilized double-label immunocytochemistry to identify the phenotype of newborn cells (5-bromo-2'-deoxyuridine-labeled) with various cellular markers; doublecortin and PSA-NCAM as the early neuronal marker, NeuN to identify mature neurons, and glial fibrillary acidic protein to identify astrocytes. We report that low physiological levels of estradiol treatment, which exert no effect in the uninjured state, significantly increase the number of newborn neurons in the SVZ following stroke injury. This effect of estradiol is limited to the dorsal region of the SVZ and is absent from the ventral SVZ. The proliferative actions of estradiol are confined to neuronal precursors and do not influence gliosis. Furthermore, we show that both estrogen receptors alpha and beta play pivotal functional roles, insofar as knocking out either of these receptors blocks the ability of estradiol to increase neurogenesis. These findings clearly demonstrate that estradiol stimulates neurogenesis in the adult SVZ, thus potentially facilitating the brain to remodel and repair after injury.

Differential modulation of estrogen receptors (ERs) in ischemic brain injury: a role for ERalpha in estradiol-mediated protection against delayed cell death.

Estradiol enhances plasticity and survival of the injured brain. Our previous work demonstrates that physiological levels of estradiol protect against cerebral ischemia in the young and aging brain through actions involving estrogen receptors (ERs) and alterations in gene expression. The major goal of this study was to establish mechanisms of neuroprotective actions induced by low levels of estradiol. We first examined effects of estradiol on the time-dependent evolution of ischemic brain injury. Because estradiol is known to influence apoptosis, we hypothesized that it acts to decrease the delayed phase of cell death observed after middle cerebral artery occlusion (MCAO). Furthermore, because ERs are pivotal to neuroprotection, we examined the temporal expression profiles of both ER subtypes, ERalpha and ERbeta, after MCAO and delineated potential roles for each receptor in estradiol-mediated neuroprotection. We quantified cell death in brains at various times after MCAO and analyzed ER expression by RT-PCR, in situ hybridization, and immunohistochemistry. We found that during the first 24 h, the mechanisms of estradiol-induced neuroprotection after MCAO are limited to attenuation of delayed cell death and do not influence immediate cell death. Furthermore, we discovered that ERs exhibit distinctly divergent profiles of expression over the evolution of injury, with ERalpha induction occurring early and ERbeta modulation occurring later. Finally, we provide evidence for a new and functional role for ERalpha in estradiol-mediated protection of the injured brain. These findings indicate that physiological levels of estradiol protect against delayed cell death after stroke-like injury through mechanisms requiring ERalpha.

The morphometry of astrocytes in the rostral preoptic area exhibits a diurnal rhythm on proestrus: relationship to the luteinizing hormone surge and effects of age.

The morphometry of astrocytes in the arcuate nucleus exhibits cyclic changes during the estrous cycle leading to dynamic changes in the communication between neurotransmitters and neuropeptides that regulate pituitary hormone secretion. Data suggest that remodeling of direct and/or indirect inputs into GnRH neurons may influence the timing and/or amplitude of the preovulatory LH surge in young rats. We have previously found that aging alters the timing and amplitude of the LH surge. Therefore, the purpose of this study was to focus on the rostral preoptic area where GnRH cell bodies reside. We assessed the possibility that the morphometry of astrocytes in the rostral preoptic area displays time-related and age-dependent changes on proestrus. Our results demonstrate that, in young rats, astrocyte cell surface area decreases between 0800 h and 1200 h, before the initiation of the LH surge. Changes in surface area over the cycle were specific to astrocytes in close apposition to GnRH neurons. In contrast, in middle-aged rats astrocyte surface area was significantly less than in young rats and did not change during the day. These findings suggest that a loss of astrocyte plasticity could lead to the delayed and attenuated LH surge that has been previously observed in middle-aged rats.

Growth hormone-deficient dwarf animals are resistant to dimethylbenzanthracine (DMBA)-induced mammary carcinogenesis.

Increased plasma IGF-1 has consistently been associated with a variety of human cancers, whereas reduced levels of IGF-1 are associated with increased lifespan in other species. However, the aforementioned relationships are correlational or are derived from animal models that are not specific for growth hormone/IGF-1 excess or deficiency. This study was designed to assess the effects of physiological changes in growth hormone and IGF-1 expression on dimethylbenzanthracine (DMBA)-induced mammary carcinogenesis. At 50 days of age, female heterozygous (dw/+) and growth hormone deficient dwarf (dw/dw) rats of the Lewis strain received a single dose of DMBA (80 micro g/g of body weight) via oral gavage. Animals were assigned to one of four experimental groups: a) heterozygous animals (normal size), b) dwarf animals administered vehicle, c) dwarf animals administered low levels of porcine growth hormone (50 micro g twice daily), and d) dwarf animals administered high levels of porcine growth hormone (200 micro g twice daily). At study termination, heterozygous animals exhibited a 70% incidence of mammary tumors, whereas no tumors were observed in saline-treated dwarf animals. Administration of either 100 micro g or 400 micro g growth hormone/day resulted in a dose dependent increase in incidence of mammary tumors (83 and 100%, respectively). Furthermore, heterozygous animals exhibited 1.5 +/- 0.25 tumors per tumor-bearing animal, whereas dwarf animals administered 100 micro g and 400 micro g growth hormone per day had 1.9 +/- 0.63 and 3.4 +/- 0.83 tumors per animal, respectively. The present study demonstrates that DMBA-induced carcinogenesis is dependent on critical plasma levels of growth hormone and IGF-1, and that growth hormone/IGF-1 deficient animals are resistant to DMBA-induced carcinogenesis.

Models of growth hormone and IGF-1 deficiency: applications to studies of aging processes and life-span determination.

The remarkable progress in understanding the genetic basis of life-span determination in invertebrates indicates that impairments in the insulin-insulin-like growth factor 1 (IGF-1) signaling cascade increase longevity. Similarities among insulin and IGF-1-like signaling pathways in invertebrates and mammals raise the possibility that modifications of these pathways may extend life span in mammals. Investigators using Ames, Snell, and growth hormone receptor knockout models have concluded that decreased growth hormone and IGF-1 are responsible for increased life span. In this review, we critique the dwarf models and, based on multiple endocrine deficiencies and developmental anomalies, conclude that these models may not be sufficient to assess the consequences of growth hormone or IGF-1 deficiency on either biological aging or life span. We attempt to resolve some of these issues by presenting an alternative animal model of growth hormone-IGF-1 deficiency. Finally, we propose an integrated explanation of growth hormone and IGF-1's contribution to the aging phenotype and life-span determination.

Neuroendocrine modulation and repercussions of female reproductive aging.

The menopause marks the end of a woman's reproductive life. During the postmenopausal period, plasma estrogen concentrations decrease dramatically and remain low for the rest of her life, unless she chooses to take hormone replacement therapy. During the past 20 years, we have learned that changes in the central nervous system are associated with and may influence the timing of the menopause in women. Recently, it has become clear that estrogens act on more than just the hypothalamus, pituitary, ovary, and other reproductive organs. In fact, they play roles in a wide variety of nonreproductive functions. With the increasing life span of humans from approximately 50 to 80 years and the relatively fixed age of the menopause, a larger number of women will spend over one third of their lives in the postmenopausal state. It is not surprising that interest has increased in factors that govern the timing of the menopause and the repercussions of the lack of estrogen on multiple aspects of women's health. We have used animal models to better understand the complex interactions between the ovary and the brain that lead to the menopause and the repercussions of the hypoestrogenic state. Our results show that when rats reach middle age, the patterns and synchrony of multiple neurochemical events that are critical to the preovulatory gonadotropin-releasing hormone (GnRH) surge undergo subtle changes. The precision of rhythmic pattern of neurotransmitter dynamics depends on the presence of estradiol. Responsiveness to this hormone decreases in middle-aged rats. The lack of precision in the coordination in the output of neural signals leads to a delay and attenuation of the luteinizing hormone surge, which lead to irregular estrous cyclicity and, ultimately, to the cessation of reproductive cycles. We also have examined the impact of the lack of estrogen on the vulnerability of the brain to injury. Our work establishes that the absence of estradiol increases the extent of cell death after stroke-like injury and that treatment with low physiological levels of estradiol are profoundly neuroprotective. We have begun to explore the cellular and molecular mechanisms that underlie this novel nonreproductive action of estrogens. In summary, our studies show that age-related changes in the ability of estradiol to coordinate the neuroendocrine events that lead to regular preovulatory GnRH surges contribute to the onset of irregular estrous cycles and eventually to acyclicity. Furthermore, we have shown that the lack of estradiol increases the vulnerability of the brain to injury and neurodegeneration.