Estrogens: neuroprotective or neurotoxic?

Dose-related neuroprotective versus neurodamaging effects of estrogens in rat cerebral ischemia: a systematic analysis

Numerous studies of the effects of estrogens for stroke prevention have yielded conflicting results in human and animal studies alike. We present a systematical analysis of study design and methodological differences between 66 studies where estrogens' impact on ischemic brain damage in rat models has been investigated, providing evidence that the differences in results may be explained by high estrogen doses produced by slow-release pellets. These pellets have been used in all studies showing increased neurologic damage because of estrogens. Our data indicate that the increased neurologic damage is related to the pellets' plasma concentration profile with an early, prolonged, supraphysiological peak. Neither the method of inducing the ischemic brain lesions, the choice of variables for measuring outcome, the measured plasma concentrations of estrogens at the time of ischemia nor rat population attributes (sex, strain, age, and diseases) are factors contributing to the discrepancies in results. This suggests that the effects of estrogens for stroke prevention are concentration related with a complex dose-response curve, and underscores the importance of carefully validating the experimental methods used. Future studies of hormone-replacement therapy in women may have to take dosage and administration regimens into account.

Effects of sex, gonadal hormones, and augmented acoustic environments on sensorineural hearing loss and the central auditory system: insights from research on C57BL/6J mice

Mice of the C57BL/6J (B6) inbred strain exhibit genetic progressive sensorineural hearing loss and have been widely used as a model of adult-onset hearing loss and presbycusis. Males and females exhibit similar degrees of hearing loss until about 3 months of age, after which, the loss accelerates in females. This paper reviews research on how the B6 auditory system is affected by sex, gonadectomy (i.e., a reduction of gonadal hormone levels), and nightly exposure to moderately intense augmented acoustic environments (AAEs) - a low-frequency noise band (LAAE) or high-frequency band (HAAE). Several findings indicate a negative effect of ovarian hormones on the female B6 auditory system. Whereas the sex difference in high-frequency hearing loss was not significantly affected by gondadectomies, the female disadvantage in ABR thresholds at lower frequencies was erased by ovariectomy. Moreover, exposure to the LAAE or HAAE caused losses of hair cells that were more severe in intact females than in ovariectomized females or in males. Finally, intact females had more severe loss of neurons in the low-frequency region of the anterior ventral cochlear nucleus (AVCN) than other groups. In contrast, the presence of androgens had beneficial effects. Loss of hair cells and AVCN neurons after AAE exposure were more severe in orchidectomized males than in intact males. Ideas, hypotheses, and potential mechanisms concerning the findings are discussed.

Hormone replacement therapy, brain volumes and white matter in postmenopausal women aged 60-64 years

Research on the structural and functional effects of hormone replacement therapy on the brain has produced inconsistent results. This paper reports on cross-sectional associations between hormone replacement therapy use and volumes of brain structures measured using magnetic resonance imaging in 213 postmenopausal women aged 60-64 years recruited from a large population study. Of these, 64 were current hormone replacement therapy users, 69 previous users and 80 had never used hormone replacement therapy. No differences were observed between groups in total grey matter, white matter, hippocampal or amygdalar volumes, severity or volume of white matter hyperintensities, or in different measures of brain atrophy. While acknowledging the limitations of a cross-sectional study, the results argue against hormone replacement therapy being protective against brain changes associated with ageing in women in their early 60s.

Effect of estrogen upon methamphetamine-induced neurotoxicity within the impaired nigrostriatal dopaminergic system

In the present study, we investigated whether estrogen remains effective as a neuroprotectant within an impaired nigrostriatal dopaminergic (NSDA) system of gonadectomized female and male mice. In Experiment 1, mice were treated with four different regimens of methamphetamine (MA) to establish a protocol for an impaired NSDA system to be used in subsequent experiments. Based upon the results of Experiment 1, in Experiment 2 gonadectomized female mice received a treatment with either control (saline), low- or high-dose of MA to produce an initial NSDA impairment. At one week post-MA, mice received either estradiol benzoate (10 microg) or vehicle followed 24 h later with low-MA or saline. Estrogen altered the toxic effects of the second invasion of MA as indicated by a significant decrease in striatal dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) concentrations. In addition, DA and DOPAC depletion was greater in high- vs. low-dose MA. In gonadectomized male mice (Experiment 3), striatal DA and DOPAC concentrations showed greater decreases following high-, vs. low-doses of MA; however, estrogen did not alter these responses. These results demonstrate that the capacity for estrogen to protect or worsen MA-induced neurotoxicity of dopaminergic neurons is limited to female mice and depends on the condition of the NSDA system.

Sex dimorphisms in the neuroprotective effects of estrogen in an animal model of Parkinson's disease

The incidence of certain neurological disorders, including Parkinson's disease, appears to be more prevalent in men. Studies involving estrogen treatment of ovariectomised rodents attribute this largely to the neuroprotective effects of estrogen. However, a neuroprotective role for physiological levels of circulating hormones in males and females is less clear. Using the 6-hydroxydopamine (6-OHDA) model of Parkinson's disease to lesion the nigrostriatal dopaminergic (NSDA) pathway, we have shown that in females, endogenously produced estrogen is neuroprotective, whereas in males, gonadal factors increase striatal 6-OHDA toxicity. Intriguingly, estrogen, but not dihydrotestosterone, a nonaromatizable androgen, reversed the effects of orchidectomy on lesion size, raising the novel the hypothesis that enhanced male susceptibility may be attributable to the effects of endogenous testosterone only after its aromatization to estrogen. Thus, estrogen appears to exert opposite effects in the NSDA in males and females, being neuroprotective in females, but not in males, where it may even exacerbate neurodegenerative responses, with important implications for the clinical potential of estrogen-related compounds as neuroprotective agents. Preliminary experiments support the hypothesis that sex differences in the adult NSDA may result from the organisational actions of gonadal steroids during the critical neonatal period for the masculinization of the brain. Further studies are needed to determine whether this early organisation of a sexually differentiated neural circuitry may contribute to the emergence of neurodegenerative conditions such as Parkinson's disease.

Gender and age effects on outcome after pediatric traumatic brain injury

OBJECTIVE

To evaluate whether girls have better outcomes after traumatic brain injury than boys.

DESIGN

Retrospective cohort study.

SETTING

University hospital.

PATIENTS

A 16,586 patient subset of the National Pediatric Trauma Registry with nonpenetrating traumatic brain injury.

INTERVENTIONS

Retrospective review.

MEASUREMENTS AND MAIN RESULTS

The patients were subdivided by age into prepubertal (0-7 yrs), indeterminate pubertal (8-12 yrs), and probable pubertal (13-19 yrs). All analyses were adjusted for injury severity using the Injury Severity Score. Outcome variables were in-hospital death rate, intensive care unit length of stay, total length of stay, discharge to home vs. rehabilitation, and functional status at discharge. Overall, 6.1% of girls and 5.3% of boys died. A higher proportion of girls were injured in motor vehicle crashes. Gender did not have a significant effect on in-hospital mortality rate after adjustment for age, Injury Severity Score, and motor vehicle crashes. Boys had a shorter intensive care unit length of stay (p =.027). There were no statistically significant differences between boys and girls in total hospital length of stay, functional outcome, and discharge location, although for every outcome there was a trend toward girls doing worse.

CONCLUSIONS

There is evidence from this large study that girls do not have a better outcome after pediatric traumatic brain injury than boys, with a suggestion that girls may do worse.

Dose- and sex-dependent effects of the neurotoxin 6-hydroxydopamine on the nigrostriatal dopaminergic pathway of adult rats: differential actions of estrogen in males and females

Epidemiological and clinical studies provide growing evidence for marked sex differences in the incidence of certain neurological disorders that are largely attributed to the neuroprotective effects of estrogen. Thus there is a keen interest in the clinical potential of estrogen-related compounds to act as novel therapeutic agents in conditions of neuronal injury and neurodegeneration such as Parkinson's disease. Studies employing animal models of neurodegeneration in ovariectomised female rats treated with estrogen support this hypothesis, yet experimental evidence for sex differences in the CNS response to direct neurotoxic insult is limited and, as yet, few studies have addressed the role played by endogenously produced hormones in neuroprotection. Therefore, in this study we aimed to determine (1) whether the prevailing levels of sex steroid hormones in the intact rat provide a degree of protection against neuronal assault in females compared with males and (2) whether sex differences depend solely on male/female differences in circulating estrogen levels or whether androgens could also play a role. Using the selective, centrally administered neurotoxin 6-hydroxydopamine, which induces a lesion in the nigrostriatal dopaminergic pathway similar to that seen in Parkinson's disease, we have demonstrated a sexually dimorphic (male-dominant), dose-dependent susceptibility in rats. Furthermore, following gonadectomy, dopamine depletion resulting from a submaximal dose of 6-hydroxydopamine (1 microg) was reduced in male rats, whereas in females, ovariectomy enhanced dopamine depletion. Administration of the nonaromatizable androgen dihydrotestosterone to gonadectomized animals had no significant effect on 6-hydroxydopamine toxicity in either males or females, whereas treatment of gonadectomized males and females with physiological levels of estrogen restored the extent of striatal dopamine loss to that seen in intact rats, viz, estrogen therapy reduced lesion size in females but increased it in males. Taken together, our findings strongly suggest that there are sex differences in the mechanisms whereby nigrostriatal dopaminergic neurones respond to injury. They also reveal that the reported clinically beneficial effects of estrogen in females may not be universally adopted for males. While the reasons for this gender-determined difference in response to the activational action of estrogen are unknown, we hypothesize that they may well be related to the early organizational events mediated by sex steroid hormones, which ultimately result in the sexual differentiation of the brain.

Estrogen modulates the expression of L-arginine:glycine amidinotransferase in chick liver

Identification of estrogen-responsive genes is important to understand the molecular mechanisms of estrogen action. Suppression subtractive hybridization was employed to screen estrogen-responsive genes in chick liver. A single injection of estrogen into 6-week-old chick induced up-regulation of several known genes encoded for yolk proteins, such as Vitellogenin I and II and very low density lipoprotein II (apo-VLDL II). One novel sequence displayed a dramatic change (3-fold increase) in response to estrogen treatment. This cDNA fragment was extended and the resultant sequence was analyzed. Translated amino acid sequence was 90, 88, 83 and 87% identical to the L-arginine:glycine amidinotransferase of pig, rat, frog and human, respectively. The sequence has a conservative catalytic site of L-arginine: glycine amidinotransferase. The expression pattern of this gene in organs is consistent with previous reports of L-arginine:glycine amidinotransferase in chick. Thus, this clone represented the chicken L-arginine:glycine amidinotransferase. It appeared that estrogen-induced alteration of arginine:glycine amidinotransferase was not dependent on protein synthesis, because concurrent administration of cycloheximide did not affect the estrogen-mediated expression pattern. This is the first study demonstrating that L-arginine:glycine amidinotransferase is a target of the estrogen receptor.