Hypothalamic aromatase activity in young and old male rats

Brain-derived estrogen: a critical player in maintaining cognitive health of aged female rats, possibly involving GPR30

Brain-derived estrogen is an endogenous neuroprotective agent, whether and how might this protective function with aging, especially postmenopausal drops in circulating estrogen, remain unclear. We herein subjected 6, 14, and 18 Mon female rats to mimic natural aging, and found that estrogen synthesis is more active in the healthy aged brain, as evidenced by the highest levels of mRNA and protein expression of aromatase, the key enzyme of E2 biosynthesis, among the three groups. Aromatase knockout in forebrain neurons (FBN-Aro-/-) impaired hippocampal and cortical neurons, and cognitive function in 18 Mon rats, compared to wild-type controls. Furthermore, estrogen nuclear receptors (ERα/β) displayed opposite changes, with a significant ERα decrease and ERβ increase, while membrane receptor GPR30 expressed stably in hippocampus during aging. Intriguingly, GPR30, but not ERα and ERβ, was decreased by FBN-Aro-/-. The results indicate that GPR30 is more sensitive to brain local E2 synthesis. Our findings provide evidence of a critical role for brain-derived estrogen in maintaining healthy brain function in older individuals, possibly involving GPR30.

Brain-derived estrogen and neural function

Although classically known as an endocrine signal produced by the ovary, 17β-estradiol (E₂) is also a neurosteroid produced in neurons and astrocytes in the brain of many different species. In this review, we provide a comprehensive overview of the localization, regulation, sex differences, and physiological/pathological roles of brain-derived E₂ (BDE₂). Much of what we know regarding the functional roles of BDE₂ has come from studies using specific inhibitors of the E₂ synthesis enzyme, aromatase, as well as the recent development of conditional forebrain neuron-specific and astrocyte-specific aromatase knockout mouse models. The evidence from these studies support a critical role for neuron-derived E₂ (NDE₂) in the regulation of synaptic plasticity, memory, socio-sexual behavior, sexual differentiation, reproduction, injury-induced reactive gliosis, and neuroprotection. Furthermore, we review evidence that astrocyte-derived E₂ (ADE₂) is induced following brain injury/ischemia, and plays a key role in reactive gliosis, neuroprotection, and cognitive preservation. Finally, we conclude by discussing the key controversies and challenges in this area, as well as potential future directions for the field.

Effects of aging on testosterone and androgen receptors in the mesocorticolimbic system of male rats

As males age, systemic testosterone (T) levels decline. T regulates executive function, a collection of cognitive processes that are mediated by the mesocorticolimbic system. Here, we examined young adult (5 months) and aged (22 months) male Fischer 344 × Brown Norway rats, and measured systemic T levels in serum and local T levels in microdissected nodes of the mesocorticolimbic system (ventral tegmental area (VTA), nucleus accumbens (NAc), medial prefrontal cortex (mPFC), and orbitofrontal cortex (OFC)). We also measured androgen receptor (AR) immunoreactivity (-ir) in the mesocorticolimbic system. As expected, systemic T levels decreased with age. Local T levels in mesocorticolimbic regions - except the VTA - also decreased with age. Mesocorticolimbic T levels were higher than serum T levels at both ages. AR-ir was present in the VTA, NAc, mPFC, and OFC and decreased with age in the mPFC. Taken together with previous results, the data suggest that changes in androgen signaling may contribute to changes in executive function during aging.

Testosterone restores respiratory long term facilitation in old male rats by an aromatase-dependent mechanism

Steroidal sex hormones play an important role in the neural control of breathing. Previous studies in our laboratory have shown that gonadectomy in young male rats (3 months) eliminates a form of respiratory plasticity induced by intermittent hypoxia, known as long term facilitation (LTF). Testosterone replenishment restores LTF in gonadectomized male rats, and this is dependent on the conversion of testosterone to oestradiol by aromatase. By middle age (12 months), male rats no longer exhibit LTF of hypoglossal motor output; phrenic LTF is significantly reduced, and this persists into old age. We tested the hypothesis that LTF can be restored in old male rats by administration of testosterone. Intact Fischer 344 rats (>20 months) were implanted with Silastic tubing containing testosterone (T), T plus an aromatase inhibitor (T+ADT), or 5α-dihydrotestosterone (DHT), a form of testosterone not converted to oestradiol. One week post-surgery, LTF of hypoglossal and phrenic motor output was measured. By comparison with control rats, hypoglossal LTF was increased in testosterone-treated rats, with levels approaching that of normal young rats. LTF was not restored in T+ADT or DHT-treated rats. Aromatase levels in hypoglossal and phrenic nuclei did not change with age. As serum testosterone levels did not decline with age, local bioavailability of testosterone in old rats may be a limiting factor in the expression of this form of respiratory plasticity. Our findings suggest that testosterone supplementation could potentially be used to enhance upper airway control in the elderly.

Changes in androgen receptor, estrogen receptor alpha, and sexual behavior with aging and testosterone in male rats

Reproductive aging in males is characterized by a diminution in sexual behavior beginning in middle age. We investigated the relationships among testosterone, androgen receptor (AR) and estrogen receptor alpha (ERalpha) cell numbers in the hypothalamus, and their relationship to sexual performance in male rats. Young (3months) and middle-aged (12months) rats were given sexual behavior tests, then castrated and implanted with vehicle or testosterone capsules. Rats were tested again for sexual behavior. Numbers of AR and ERalpha immunoreactive cells were counted in the anteroventral periventricular nucleus and the medial preoptic nucleus, and serum hormones were measured. Middle-aged intact rats had significant impairments of all sexual behavior measures compared to young males. After castration and testosterone implantation, sexual behaviors in middle-aged males were largely comparable to those in the young males. In the hypothalamus, AR cell density was significantly (5-fold) higher, and ERalpha cell density significantly (6-fold) lower, in testosterone- than vehicle-treated males, with no age differences. Thus, restoration of serum testosterone to comparable levels in young and middle-aged rats resulted in similar preoptic AR and ERalpha cell density concomitant with a reinstatement of most behaviors. These data suggest that age-related differences in sexual behavior cannot be due to absolute levels of testosterone, and further, the middle-aged brain retains the capacity to respond to exogenous testosterone with changes in hypothalamic AR and ERalpha expression. Our finding that testosterone replacement in aging males has profound effects on hypothalamic receptors and behavior has potential medical implications for the treatment of age-related hypogonadism in men.

Age-related changes in serum and brain levels of androgens in male Brown Norway rats

Age-related depletion of androgens in men results in functional impairments in androgen-responsive tissues, such as the brain, resulting in increased risk for Alzheimer's disease. To investigate the relationship between normal age-related hormone loss and Alzheimer's disease risk, we evaluated the brain and serum levels of androgens and estrogen in aging male rats. We observed that increasing age was associated with a significant reduction in brain levels of the potent androgen dihydrotestosterone and a trend toward decreased testosterone. Brain levels of soluble beta-amyloid were observed to increase with age. Collectively, these findings highlight differences in brain and circulating levels of androgens during aging, and identify an inverse correlation with beta-amyloid levels that may be relevant to Alzheimer's disease risk.

Androgens induce dopaminergic neurotoxicity via caspase-3-dependent activation of protein kinase Cdelta

Aged men have a greater incidence of Parkinson's disease (PD) than women. PD is a neurodegenerative condition associated with the loss of dopamine neurons in the nigrostriatal pathway. This study examined the neurotoxic effects of androgens in a dopaminergic cell line (N27 cells) and the downstream signaling pathways activated by androgens. Treatment of N27 cells with testosterone- and dihydrotestosterone-induced mitochondrial dysfunction, protein kinase C (PKC)-delta cleavage, and apoptosis in dopaminergic neuronal cells. Inhibition of caspase-3 prevented the cleavage of PKCdelta from the full-length element to the catalytic fragment and apoptosis in N27 cells, suggesting that androgen-induced apoptosis is mediated by caspase-3-dependent activation of PKCdelta. Androgen-induced apoptosis may be specific to dopamine neurons as evidenced by a lack of testosterone-induced apoptosis in GnRH neurons. These results support a neurotoxic consequence of testosterone on dopaminergic neurons and may provide insight into the gender bias found in PD.

Sexual experience changes sex hormones but not hypothalamic steroid hormone receptor expression in young and middle-aged male rats

Testosterone is well known to regulate sexual behavior in males, but this is dependent upon prior sexual experience. Aging is associated with decreased libido and changes in testosterone, but the role of experience in these age-related processes has not been systematically studied. We examined effects of age and sexual experience on serum hormones (total testosterone, free testosterone, estradiol, LH) and on numbers of androgen receptor (AR) and estrogen receptor alpha (ERalpha) immunoreactive cells in the hypothalamus. Extensive sexual experience was given to male rats at 4 months of age. Rats were euthanized at either 4 months (young) or 12 months (middle-aged (MA)). Comparable sexually naïve male rats were handled and placed into the testing arena but did not receive any sexual experience. Thus, we had four groups: young-naïve, young-experienced, MA-naïve and MA-experienced. Serum hormone levels were assayed, and numbers of AR and ERalpha cells were quantified stereologically in the medial preoptic nucleus (MPN) and the anteroventral periventricular nucleus (AVPV). Sexually experienced males had significantly elevated serum testosterone and free testosterone in both age groups. Both total and free testosterone were higher, and estradiol lower, in middle-aged than young rats. Experience did not alter either AR or ERalpha expression in the preoptic brain regions studied. Aging was associated with increased expression of AR, but no change in ERalpha. These results show that sexual experience can induce short-term and long-term alterations in serum hormones but these effects are not manifested upon their receptors in the hypothalamus.

Porcine hypothalamic aromatase cytochrome P450: isoform characterization, sex-dependent activity, regional expression, and regulation by enzyme inhibition in neonatal boars

Domestic pigs have three CYP19 genes encoding functional paralogues of the enzyme aromatase cytochrome P450 (P450arom) that are expressed in the gonads, placenta, and preimplantation blastocyst. All catalyze estrogen synthesis, but the gonadal-type enzyme is unique in also synthesizing a nonaromatizable biopotent testosterone metabolite, 1OH-testosterone (1OH-T). P450arom is expressed in the vertebrate brain, is higher in males than females, but has not been investigated in pigs, to our knowledge. Therefore, these studies defined which of the porcine CYP19 genes was expressed, and at what level, in adult male and female hypothalamus. Regional expression was examined in mature boars, and regulation of P450arom expression in neonatal boars was investigated by inhibition of P450arom with letrozole, which is known to reprogram testicular expression. Pig hypothalami expressed the gonadal form of P450arom (redesignated the "gonadal/hypothalamic" porcine CYP19 gene and paralogue) based on functional analysis confirmed by cloning and sequencing transcripts. Hypothalamic tissue synthesized 1OH-T and was sensitive to the selective P450arom inhibitor etomidate. Levels were 4-fold higher in male than female hypothalami, with expression in the medial preoptic area and lateral borders of the ventromedial hypothalamus of boars. In vivo, letrozole-treated neonates had increased aromatase activity in hypothalami but decreased activity in testes. Therefore, although the same CYP19 gene is expressed in both tissues, expression is regulated differently in the hypothalamus than testis. These investigations, the first such studies in pig brain to our knowledge, demonstrate unusual aspects of P450arom expression and regulation in the hypothalamus, offering promise of gaining better insight into roles of P450arom in reproductive function.

Estrogen, testosterone, and sequential movement in men

Behavioral and physiological data suggest that the striatal dopaminergic system is important in the production and execution of sequential movements. Striatal function is also modulated by sex hormones, and previous studies show that estradiol is related to sequential movement in women. The authors examined whether sex hormones are involved in the production of sequential movement in healthy older and younger men. Testosterone was modified for a 6-week period such that levels in older men matched those of younger men, the conversion of testosterone to estradiol was blocked, the production of testosterone was blocked, or the men received no treatment (placebo). Sequential movement was measured before and after hormone treatment. Older men were slower and more accurate than younger men on the sequential movement task pre- and posttreatment. Hormone manipulation had no effect on movement speed. Hormone levels were not correlated with sequential movement performance in either older or younger men, suggesting that sex hormones do not modulate sequential movement in men, and hormone replacement may not restore a loss of sequential movement ability in elderly men or men with Parkinson's disease.

Age-related changes in hypothalamic androgen receptor and estrogen receptor alpha in male rats

The control of reproductive function involves actions of sex steroids upon their nuclear receptors in the hypothalamus and preoptic area (POA). Whether hypothalamic hormone receptors change their expression in aging male mammals has not been extensively pursued, although such changes may underlie functional losses in reproductive physiology occurring with aging. We performed a stereologic analysis of immunoreactive androgen receptor (AR) and estrogen receptor alpha (ERalpha) cells in three POA nuclei of male Sprague-Dawley rats (anteroventral periventricular nucleus [AVPV], median preoptic area [MePO], and medial preoptic nucleus [MPN]), at young (3 months), middle-aged (12 months), and old (20 months) ages. Serum testosterone and estradiol levels were assayed. Testosterone concentrations decreased significantly and progressively with aging. Estradiol concentrations were significantly higher in middle-aged than either young or old rats. Stereologic analyses of the POA demonstrated that AR-immunoreactive cell numbers and density in the AVPV, MePO, and MPN were significantly higher in old compared with young or middle-aged rats. No change in the total number or density of ERalpha-immunoreactive cells was detected with age, although when cells were subdivided by intensity of immunolabeling, the most heavily labeled ERalpha cells increased in number with aging in the AVPV and MePO, and in density in the AVPV. There are several interpretations to our finding of substantially increased AR cell numbers during aging, including a potential compensatory upregulation of the AR under diminished testosterone concentrations. These results provide further information about how the neural targets of steroid hormones change with advancing age.

Testosterone, but not nonaromatizable dihydrotestosterone, improves working memory and alters nerve growth factor levels in aged male rats

Recent studies have suggested that testosterone levels are lower in men with Alzheimer's disease and that testosterone treatment improves cognition in older men. Since testosterone can be aromatized to estrogen, testosterone's effects could be due to conversion into estrogen. We treated aged male rats with either testosterone or dihydrotestosterone (DHT), the latter of which is not aromatized to estrogen, in order to determine whether these treatments improve spatial working and reference memory as assessed in the water radial arm maze. We also tested whether such effects are related to beta-amyloid levels in the hippocampus or neurotrophin levels in the hippocampus, entorhinal cortex, frontal cortex, or striatum. Aged rats made more errors than young rats on all memory measures. Testosterone, but not DHT, improved working memory and decreased hippocampal NGF protein in aged rats, while having no effect on beta-amyloid. However, higher beta-amyloid levels were correlated with poorer working memory performance in young rats. Neurotrophin levels in entorhinal cortex were positively correlated with errors for all memory measures in androgen-treated rats. Similar to findings in human studies, in our study androgen treatment lowered circulating estradiol levels in aged rats, suggesting that androgen treatment exerts feedback to the hypothalamic pituitary axis and that conversion to estrogen may not be the underlying biological mechanism of testosterone's effects on memory and growth factor levels. The ratio of estradiol to testosterone, or the actions of the aromatase enzyme itself, may be responsible for the observed effects. These data support the hypothesis that testosterone therapy in aging men may provide positive effects on cognition and that neural regions that are linked to cognition, such as the hippocampus and/or entorhinal cortex, may be involved in such effects.

Age-related deficits in brain androgen binding and metabolism, testosterone, and sexual behavior of male rats

Brain androgen binding and metabolism, serum testosterone (T), and sexual behavior were measured in old and young male Fischer 344 rats. After completion of sexual behavior tests, blood was collected for T assay and brains were removed for simultaneous measurements of cytosolic (ARc) and nuclear (ARn) androgen receptors and aromatase activity (AA) in the preoptic area (POA), hypothalamus (HYP) and amygdala (AMG). In Experiment 1, old and young intact males were examined. None of the old males ejaculated in any of the tests of sexual behavior whereas all of the young males ejaculated. The old males had lower levels of serum T, lower levels of ARn in the POA and HYP and lower levels of AA in the POA. The ARc levels of the old and young males did not differ. Experiment 2 was designed to determine if the deficits in brain androgen binding and metabolism were due to low levels of T. Old and young T-treated gonadectomized (GX-T) males and young intact (I) males were examined. T levels were comparable in the young and old GX-T males and were higher in each of these groups than in the young I males. In sexual behavior tests, all of the young but only 25% of the old GX-T males ejaculated. Although ARn levels in the old GX-T males were lower than in the young GX-T males, they were comparable to the young I male levels. No age-related differences in T induction of AA were observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Aging in male rats modifies castration and testosterone-induced neuropeptide Y response in various microdissected brain nuclei

Neuropeptide Y (NPY) is localized in several hypothalamic sites which are implicated in the control of hypothalamic luteinizing hormone-releasing hormone (LHRH) and pituitary luteinizing hormone (LH) release. We have observed previously that in young rats castration decreases and testosterone (T) replacement restores NPY levels in selected hypothalamic sites. However, in aged male rats, NPY levels were decreased in all hypothalamic sites studied. Since testicular function is diminished in aged rats, we reasoned that decreased T feedback may be responsible for the reduction of NPY in the hypothalamus. Therefore, we compared the effects of castration and T-replacement on NPY levels in microdissected hypothalamic sites of 2.5-month- (young) and 15-month-old (aged) male rats. Serum LH and T levels were markedly reduced in aged as compared to those observed in young rats. In association with the decreased hormone levels, NPY levels were significantly reduced in each of the 7 hypothalamic sites of aged as compared to young rats. Further, in young rats, castration reduced and T-replacement prevented the castration-induced depletion in only 3 sites, viz. the ventromedial hypothalamic nucleus (VMN), arcuate nucleus (ARC) and median eminence (ME). In contrast, castration in aged rats reduced NPY levels not only in the VMN as in young rats, but also in the medial preoptic area (MPOA) and dorsomedial nucleus (DMN). However, the marked reduction in the ME and ARC NPY levels of young rats following castration was not observed in the ME and ARC of aged rats.(ABSTRACT TRUNCATED AT 250 WORDS)

The distribution and regulation of aromatase activity in the central nervous system

Our data demonstrate that androgen-dependent AA is found in areas of the brain that are essential for the neuroendocrine control of gonadotropin secretion and sexual behavior. However, until we know more about the neurons that contain AA, e.g., whether they are peptidergic or catecholaminergic, we can not speculate about the neuronal functions that depend on local estrogen formation. In fact, the association of AA with neurons and not glia has only recently been demonstrated. That estrogens and androgens synergize in the regulation of various neuroendocrine functions has been known for many years, but an explanation of the synergism at the cellular level was not available. One explanation for this synergism may lie in our recent observation that the administration of exogenous estradiol to castrated rats increases androgen-receptor concentrations in specific brain nuclei. Perhaps locally formed estrogens work in a similar fashion to regulate androgen receptors in the brain of the intact male.