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

Advances in Knowledge of Androgens: How Intentional and Accidental Neurosteroid Changes Inform Us of Their Action and Role

Purpose of Review

Here, we summarize current knowledge of androgens’ action gained over the recent years.

Recent Findings

Neurosteroids are produced in the brain and peripheral nerves, independent of endocrine glands have been investigated for how they are regulated, and have actions via non-steroid receptor targets to mediate social, affective, and cognitive behavior and to protect the brain. Androgens’ organizing actions in the peri-natal period have effects throughout the lifetime that may be recapitulated later in life during critical periods and at times of challenge. Developmental changes in androgens occur during mid-childhood, adrenarche, puberty, adolescence, young adulthood, middle age, and andropause. Changes in androgens with a 5α-reductase inhibitor, such as finasteride, result in disruptions in organizational and activational functions of androgens that can be unremitting.

Summary

Normal developmental or perturbation in androgens through other means can cause changes in androgen-sensitive phenotypes throughout the lifespan, in part through actions of neurosteroids.

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.

Age-related changes in sexual function and steroid-hormone receptors in the medial preoptic area of male rats

Testosterone is the main circulating steroid hormone in males, and acts to facilitate sexual behavior via both reduction to dihydrotestosterone (DHT) and aromatization to estradiol. The mPOA is a key site involved in mediating actions of androgens and estrogens in the control of masculine sexual behavior, but the respective roles of these hormones is not fully understood. As males age they show impairments in sexual function, and a decreased facilitation of behavior by steroid hormones compared to younger animals. We hypothesized that an anatomical substrate for these behavioral changes is a decline in expression and/or activation of hormone receptor-sensitive cells in the mPOA. We tested this by quantifying and comparing numbers of AR- and ERα-containing cells, and Fos as a marker of activated neurons, in the mPOA of mature (4-5months) and aged (12-13months) male rats, assessed one hour after copulation to one ejaculation. Numbers of AR- and ERα cells did not change with age or after sex, but the percentage of AR- and ERα-cells that co-expressed Fos were significantly up-regulated by sex, independent of age. Age effects were found for the percentage of Fos cells that co-expressed ERα (up-regulated in the central mPOA) and the percentage of Fos cells co-expressing AR in the posterior mPOA. Interestingly, serum estradiol concentrations positively correlated with intromission latency in aged but not mature animals. These data show that the aging male brain continues to have high expression and activation of both AR and ERα in the mPOA with copulation, raising the possibility that differences in relationships between hormones, behavior, and neural activation may underlie some age-related impairments.

Tyramide Signal Amplification Permits Immunohistochemical Analyses of Androgen Receptors in the Rat Prefrontal Cortex

Research on neural androgen receptors (ARs) has traditionally focused on brain regions that regulate reproductive and aggressive behaviors, such as the hypothalamus and amygdala. Although many cells in the prefrontal cortex (PFC) also express ARs, the number of ARs per cell appears to be much lower, and thus, AR immunostaining is often hard to detect and quantify in the PFC. Here, we demonstrate that biotin tyramide signal amplification (TSA) dramatically increases AR immunoreactivity in the rat brain, including critical regions of the PFC such as the medial PFC (mPFC) and orbitofrontal cortex (OFC). We show that TSA is useful for AR detection with both chromogenic and immunofluorescent immunohistochemistry. Double-labeling studies reveal that AR+ cells in the PFC and hippocampus are NeuN+ but not GFAP+ and thus primarily neuronal. Finally, in gonadally intact rats, more AR+ cells are present in the mPFC and OFC of males than of females. Future studies can use TSA to further examine AR immunoreactivity across ages, sexes, strains, and different procedures (e.g., fixation methods). In light of emerging evidence for the androgen regulation of executive function and working memory, these results may help understand the distribution and roles of ARs in the PFC.

Exogenous Testosterone, Aging, and Changes in Behavioral Response of Gonadally Intact Male Mice

This study tested the hypothesis that aging significantly affects the influence of exogenous testosterone on neurobehavior in gonadally intact male mice. Groups of prepubertal and aged male mice received daily vehicle or testosterone propionate (TP; 2.5 or 5.0 mg/kg intraperitoneal [i.p.]) for 21 days. Behaviors were assessed on days 1 and 21. Weight gain was significant in prepubertal mice. Locomotion and rearing increased in prepubertal mice after first dose and decreased after last dose of TP. Rearing was suppressed in aged mice throughout. Suppression of grooming occurred in both age groups at day 21. Significant increase in working memory in both age groups was seen in the radial-arm maze (at specific doses) and in prepubertal mice in the Y-maze. Elevated plus maze test showed mixed anxiolytic/anxiogenic effects. Aged mice had higher serum testosterone. In conclusion, age is an important determinant for the influence of exogenous testosterone on behavior in gonadally intact male mice.

Sexual Differentiation of Circadian Clock Function in the Adrenal Gland

Sex differences in glucocorticoid production are associated with increased responsiveness of the adrenal gland in females. However, the adrenal-intrinsic mechanisms that establish sexual dimorphic function remain ill defined. Glucocorticoid production is gated at the molecular level by the circadian clock, which may contribute to sexual dimorphic adrenal function. Here we examine sex differences in the adrenal gland using an optical reporter of circadian clock function. Adrenal glands were cultured from male and female Period2::Luciferase (PER2::LUC) mice to assess clock function in vitro in real time. We confirm that there is a pronounced sex difference in the intrinsic capacity to sustain PER2::LUC rhythms in vitro, with higher amplitude rhythms in adrenal glands collected from males than from females. Changes in adrenal PER2::LUC rhythms over the reproductive life span implicate T as an important factor in driving sex differences in adrenal clock function. By directly manipulating hormone levels in adult mice in vivo, we demonstrate that T increases the amplitude of PER2::LUC rhythms in adrenal glands of both male and female mice. In contrast, we find little evidence that ovarian hormones modify adrenal clock function. Lastly, we find that T in vitro can increase the amplitude of PER2::LUC rhythms in male adrenals but not female adrenals, which suggests the existence of sex differences in the mechanisms of T action in vivo. Collectively these results reveal that activational effects of T alter circadian timekeeping in the adrenal gland, which may have implications for sex differences in stress reactivity and stress-related disorders.

Steroid Receptors and Aromatase Gene Expression in Different Brain Areas of Copulating and Sexually Sluggish Male Rats

INTRODUCTION

Sexually sluggish (SS) males have been identified in several species of mammals including rats. These animals take more than 30 minutes to ejaculate; they do not ejaculate or do so inconsistently despite being tested repeatedly with sexually receptive females. Different brain areas and hormones play an important role in the control of male sexual behavior.

AIMS

Determine gene expression for the androgen receptor (AR), the estrogen receptor alpha (ERα), the progesterone receptor (PR), and the aromatase enzyme (ARO), in brain regions important in the control of male sexual behavior including the medial preoptic area (MPOA), the amygdala (AMG), the olfactory bulb (OB), and, as a control, the cortex (CTX) of copulating (C) and SS male rats.

METHODS

Males that ejaculated within 30 minutes in three tests with receptive females were included in the C group, while those males that ejaculated in one or none of the four tests were included in the SS group. RNA was isolated 1 week after the last test of sexual behavior, and cDNA was synthesized from the brain areas listed above.

MAIN OUTCOMES MEASURES

Expression of the AR, ERα, PR, and ARO genes was determined by quantitative polymerase chain reaction (qPCR). Cyclophilin A (CycA) and tyrosine 3-monooxygenase-tryptophan activation protein zeta (Ywhaz) were housekeeping genes used to determine relative gene expression with the 2(-ΔΔCt) method.

RESULTS

The expression of mRNA for AR and ARO increased in the MPOA of SS males. ARO mRNA was increased in the AMG of SS males. In the OB, ERα mRNA was increased and AR mRNA reduced in SS males.

CONCLUSION

These results indicate SS and C males show differences in gene expression within brain regions controlling sexual behavior.

Aging impairs VEGF-mediated, androgen-dependent regulation of angiogenesis

There is a progressive impairment of vascular repair mechanisms with advancing age concomitant with a steady decline in circulating androgen levels in men. Emerging evidence indicates androgens regulate angiogenesis; however, little research has focused on the impact of age upon androgen-mediated regulation of angiogenic mechanisms. Human dermal fibroblasts from young (<30 years) and older (>65 years) men were incubated with DHT, with or without androgen receptor antagonist hydroxyflutamide, or phosphoinositide 3-kinase inhibitor. Fibroblast-conditioned medium was used to stimulate angiogenic functions in human umbilical vein endothelial cells. Nuclear fractionation and fluorescence microscopy were used to study androgen receptor (AR) distribution. Conditioned medium from fibroblasts of young men, but not old men, treated with DHT produced a 3-fold increase in human umbilical vein endothelial cell tubulogenesis and 2-fold increase in migration via increased vascular endothelial growth factor (VEGF) expression and secretion, predominantly of VEGF145. DHT-induced VEGF secretion from fibroblasts of young men was AR-dependent and increased AKT phosphorylation, which was abrogated by phosphoinositide 3-kinase inhibition. By contrast, fibroblasts from older men were unresponsive to DHT and lacked androgen-mediated enhancement in VEGF production. These findings were associated with reduced AR nuclear translocation in old fibroblasts. The failure of DHT-induced paracrine stimulation of angiogenesis in fibroblasts from older men is likely due to defective nuclear translocation of AR. This first demonstration of androgen resistance (or insensitivity) acquired by human fibroblasts with aging suggests that pharmacological testosterone therapy for old men may be less effective in enhancing angiogenesis and facilitating tissue regeneration mechanisms reliant on paracrine release of VEGF.

Testosterone or oestradiol implants in the medial preoptic area induce mating in noncopulating male rats

Noncopulating (NC) male rats are those males that do not mount, intromit or ejaculate when repeatedly tested with receptive females. The lack of sexual behaviour in these males is not associated with alterations in testosterone or oestradiol (E2) plasma concentrations. Instead, it has been shown that androgen receptors are higher and oestrogen receptors are lower in the medial preoptic area (MPOA) of NC male rats than those observed in copulating (C) male rats. We have also observed reduced aromatase activity in the MPOA (but not in other brain regions) of NC male rats. The aim of the present study was to determine whether testosterone or E2 implants in the MPOA of NC male rats could induce sexual behaviour. Accordingly, in Experiment 1, we evaluated the long-term effects of testosterone or E2 implants in the MPOA, the ventromedial nucleus of the hypothalamus or the medial amygdala with respect to inducing sexual behaviour in castrated C male rats. Male rats were bilaterally implanted with a guide cannula, either empty or containing testosterone or E2. Starting 1 week later, all male rats were mated once weekly for 5 months. As described previously, the site where hormone implants most consistently induced sexual behaviour in castrated C male rats was the MPOA. Experiment 1 extended these findings showing that the males continued mating even 5 months after the implant. In the second experiment, NC males were implanted in the MPOA with a guide cannula empty or filled with testosterone or E2. One week after the testosterone or E2 implant, the percentage of males that mounted and intromitted started to increase and, 5 weeks after the implant, 50% of the subjects displayed mounts and intromissions. All NC males implanted with testosterone ejaculated consistently from week 11 after the implant until the end of testing (5 months), whereas all subjects implanted with E2 ejaculated from week 16 after the implant until the end of testing. These results support the hypothesis that, in the MPOA of NC male rats, there is a hormonal alteration associated with the lack of sexual behaviour.

Males, but not females, lose tyrosine hydroxylase fibers in the medial prefrontal cortex and are impaired on a delayed alternation task during aging

The structure of the prefrontal cortex (PFC) is particularly vulnerable to the effects of aging, and behaviors mediated by the PFC are impaired during aging in both humans and animals. In male rats, behavioral deficits have been correlated with a decrease in dopaminergic functioning. However, studies have found that anatomical changes associated with aging are sexually dimorphic, with males experiencing greater age-related loss than females. The present study investigated the effects of sex and aging on performance of a delayed alternation t-maze, a task mediated by the medial prefrontal cortex (mPFC), and on tyrosine hydroxylase (TH) immunoreactivity in this brain region using adult (7 months) and aged (21 months) male and female F344 rats. There was a sex by age interaction in performance of the delayed alternation task such that adult males performed better than aged males, but aged females were not different than adult females. Adult males performed better than adult females across all delays; however, this sex difference was reversed during aging and aged males performed worse than aged females. In addition, TH immunoreactivity decreased during aging in layers 2/3 in the male, but not female mPFC. Thus females were less sensitive to the effects of aging on the prefrontal dopaminergic system and on performance of a delayed alternation task. These effects may be due to decreases in testosterone in aging males, as well as the protective effects of ovarian hormones, which continue to be secreted after cessation of the estrous cycle in aging females.

Testosterone impairs the acquisition of an operant delayed alternation task in male rats

The current study examined the effects of gonadectomy (GDX) and subsequent testosterone treatment of male Long-Evans rats on an operant variable delay spatial alternation task (DSA). Gonadally-intact rats (intact-B), GDX rats receiving implants that delivered a physiological level of testosterone (GDX-T), and GDX rats receiving blank implants (GDX-B) were tested for 25 sessions on a DSA task with variable inter-trial delays ranging from 0 to 18 s. Acquisition of the DSA task was found to be enhanced following GDX in a time and delay dependent manner. Both the GDX-T and the intact-B rats had lower performance accuracies across delays initially, relative to GDX-B rats, and this deficit persisted into subsequent testing sessions at longer delays. The GDX-T and intact-B rats also had a tendency to commit more perseverative errors during the early testing sessions, with both groups persisting in pressing a lever which had not been associated with reinforcement for at least two consecutive trials. However, both the GDX-T and intact-B groups were able to achieve performance accuracy similar to that of the GDX-B rats by the final sessions of testing. Overall, these results suggest that castration of adult male rats enhances their acquisition of an operant DSA task.

Sex hormones, aging, and Alzheimer's disease

A promising strategy to delay and perhaps prevent Alzheimer's disease (AD) is to identify the age-related changes that put the brain at risk for the disease. A significant normal age change known to result in tissue-specific dysfunction is the depletion of sex hormones. In women, menopause results in a relatively rapid loss of estradiol and progesterone. In men, aging is associated with a comparatively gradual yet significant decrease in testosterone. We review a broad literature that indicates age-related losses of estrogens in women and testosterone in men are risk factors for AD. Both estrogens and androgens exert a wide range of protective actions that improve multiple aspects of neural health, suggesting that hormone therapies have the potential to combat AD pathogenesis. However, translation of experimental findings into effective therapies has proven challenging. One emerging treatment option is the development of novel hormone mimetics termed selective estrogen and androgen receptor modulators. Continued research of sex hormones and their roles in the aging brain is expected to yield valuable approaches to reducing the risk of AD.

Sex hormones, aging, and Alzheimer's disease

A promising strategy to delay and perhaps prevent Alzheimer's disease (AD) is to identify the age-related changes that put the brain at risk for the disease. A significant normal age change known to result in tissue-specific dysfunction is the depletion of sex hormones. In women, menopause results in a relatively rapid loss of estradiol and progesterone. In men, aging is associated with a comparatively gradual yet significant decrease in testosterone. We review a broad literature that indicates age-related losses of estrogens in women and testosterone in men are risk factors for AD. Both estrogens and androgens exert a wide range of protective actions that improve multiple aspects of neural health, suggesting that hormone therapies have the potential to combat AD pathogenesis. However, translation of experimental findings into effective therapies has proven challenging. One emerging treatment option is the development of novel hormone mimetics termed selective estrogen and androgen receptor modulators. Continued research of sex hormones and their roles in the aging brain is expected to yield valuable approaches to reducing the risk of AD.

Effect of gonadal hormones and sexual experience on vocalizations and behavior of male bank voles (Myodes glareolus)

This study investigated the influence of gonadal hormones and sexual experience on behavior, especially vocalizations, of male bank voles ( Myodes glareolus (Schreber, 1780)) during same-sex encounters in a neutral arena. Interactions within pairs of castrated males, castrated but testosterone-treated males, and sham-operated intact males were studied in the first experiment and within pairs of sexually experienced males and sexually inexperienced males in the second experiment. Castration reduced the number of ultrasonic vocalizations emitted and androgen treatment restored it to levels seen in intact males. Ultrasounds were emitted more frequently during amicable encounters than during aggressive ones, but ultrasonic calling was reduced during interactions between sexually experienced males, possibly because of the high level of aggression seen in such encounters. In contrast, audible sounds were associated with aggressive behavior and were positively affected by social experience, but they were not testosterone dependent. Neither testosterone nor sexual experience appeared to have any effect on the spectral and temporal characteristics of either audible or ultrasonic calls. The results indicate that emission of ultrasounds during same-sex encounters of male bank voles is regulated by hormonal and social factors and seems to be correlated with type of behavior shown.

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.

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.

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.

Sexual behavior in male rodents

The hormonal factors and neural circuitry that control copulation are similar across rodent species, although there are differences in specific behavior patterns. Both estradiol (E) and dihydrotestosterone (DHT) contribute to the activation of mating, although E is more important for copulation and DHT for genital reflexes. Hormonal activation of the medial preoptic area (MPOA) is most effective, although implants in the medial amygdala (MeA) can also stimulate mounting in castrates. Chemosensory inputs from the main and accessory olfactory systems are the most important stimuli for mating in rodents, especially in hamsters, although genitosensory input also contributes. Dopamine agonists facilitate sexual behavior, and serotonin (5-HT) is generally inhibitory, though certain 5-HT receptor subtypes facilitate erection or ejaculation. Norepinephrine agonists and opiates have dose-dependent effects, with low doses facilitating and high doses inhibiting behavior.

Relationship between sexual satiety and brain androgen receptors

Recently we showed that 24 h after copulation to satiety, there is a reduction in androgen receptor density (ARd) in the medial preoptic area (MPOA) and in the ventromedial hypothalamic nucleus (VMH), but not in the bed nucleus of the stria terminalis (BST). The present study was designed to analyze whether the ARd changes in these and other brain areas, such as the medial amygdala (MeA) and lateral septum, ventral part (LSV), were associated with changes in sexual behavior following sexual satiety. Males rats were sacrificed 48 h, 72 h or 7 days after sexual satiety (4 h ad libitum copulation) to determine ARd by immunocytochemistry; additionally, testosterone serum levels were measured in independent groups sacrificed at the same intervals. In another experiment, males were tested for recovery of sexual behavior 48 h, 72 h or 7 days after sexual satiety. The results showed that 48 h after sexual satiety 30% of the males displayed a single ejaculation and the remaining 70% showed a complete inhibition of sexual behavior. This reduction in sexual behavior was accompanied by an ARd decrease exclusively in the MPOA-medial part (MPOM). Seventy-two hours after sexual satiety there was a recovery of sexual activity accompanied by an increase in ARd to control levels in the MPOM and an overexpression of ARd in the LSV, BST, VMH and MeA. Serum testosterone levels were unmodified during the post-satiety period. The results are discussed on the basis of the similarities and discrepancies between ARd in specific brain areas and male sexual behavior.

Exogenous testosterone reverses age-related atrophy in a spinal neuromuscular system

Aging is associated with a variety of pathologies, including motor dysfunctions and reductions in sexual behavior. In male rats, declines in sexual behavior during the aging process may be caused in part by the loss of the lumbar spinal cord motoneurons that innervate the penile musculature. Alternatively, declining sexual behavior may be caused by the precipitous reductions in circulating testosterone that occur during aging. In this paper, we report two experiments examining these issues. In Experiment 1, we counted motoneurons in the lumbar motor nuclei and measured several androgen-sensitive morphological properties of the penile muscles and their innervating motoneurons at several time points during the aging process. Motoneuron number in the lumbar nuclei did not change over time, even with very advanced age. In contrast, the penile muscles and their innervating motoneurons underwent profound atrophy, with muscle weight and motoneuron dendritic length declining to less than 50% of young adult levels. In Experiment 2, we treated aged animals with exogenous testosterone, and then examined their penile neuromuscular systems for morphological changes. Testosterone treatment, both acute and chronic, completely reversed age-related declines in the weight of the penile muscles and in the soma size and dendritic length of their innervating motoneurons. Together, these data suggest that reductions in male sexual behavior during the aging process are caused primarily by declines in testosterone levels rather than motoneuron loss. Furthermore, they raise the possibility that testosterone treatment could play an important role in maintaining neuronal connectivity in the aging body.

Comparative analysis of immunoreactive cells for androgen receptors and oestrogen receptor alpha in copulating and non-copulating male rats

In some species, including gerbils, guinea pigs, mice, rams and rats, some apparently normal males fail to mate. These kinds of animals have been named 'noncopulating (NC)'. The cause of this behavioural deficit is unknown. The present study aimed to determine whether NC male rats have alterations in the amount of androgen (AR) and oestrogen receptor alpha (ERalpha) in a neuronal circuit important for the control of male sexual behaviour; the vomeronasal projection pathway. We evaluated the number of AR and ERalpha immunoreactive (AR-IR and ERalpha-IR) cells in the accessory olfactory bulb (AOB), the bed nucleus of the stria terminalis (BNST), the anterior-dorsal medial amygdala (MeAD), the posterior dorsal amygdala (MePD) and the medial preoptic area (MPOA). The results demonstrate that the number of AR-IR cells in NC males was significantly higher compared to copulating (C) males in the MePD, but no significant differences were found in any of the other structures analysed. ERalpha-IR cells were more abundant in NC than in C males in the MeAD and the MePD. However, in the MPOA the number of ERalpha-IR cells was significantly reduced in NC males. No significant differences were found in the AOB or in the BNST. A similar pattern of results was observed when regions within these structures that are activated by Fos expression, on mating or exposure to sexually relevant cues were analysed. The differences in the number of AR and ER in particular brain areas could be associated with alterations in sexual behaviour as well as partner and olfactory preference for receptive females seen in NC male rats.

Middle-Age Male Mice Have Increased Severity of Experimental Autoimmune Encephalomyelitis and Are Unresponsive to Testosterone Therapy

Treatment with sex hormones is known to protect against experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis. However, little is known about how age affects the course of EAE or response to hormone treatment. This study demonstrates striking differences between middle-age vs young C57BL/6 male mice in the clinical course of EAE and response to both testosterone (T4) and estrogen (E2) hormone therapy. Unlike young males that developed an acute phase of EAE followed by a partial remission, middle-age males suffered severe chronic and unremitting EAE that was likely influenced by alterations in the distribution and function of splenic immunocytes and a significant reduction in suppressive activity of CD4+CD25+ regulatory T cells in the spleen and spinal cord. Middle-age males had reduced numbers of splenic CD4+ T cells that were generally hypoproliferative, but enhanced numbers of splenic macrophages and MHC class II-expressing cells, and increased secretion of the proinflammatory factors IFN-gamma and MCP-1. Surprisingly, middle-age males were unresponsive to the EAE-protective effects of T4 and had only a transient benefit from E2 treatment; young males were almost completely protected by both hormone treatments. T4 treatment of young males inhibited proliferation of myelin oligodendrocyte glycoprotein 35-55-specific T cells and secretion of TNF-alpha and IFN-gamma. The effects of T4 in vivo and in vitro were reversed by the androgen receptor antagonist, flutamide, indicating that the regulatory effects of T4 were mediated through the androgen receptor. These data are the first to define age-dependent differences in EAE expression and response to hormone therapy.

The role of ovarian hormones in preserving cognition in aging

Hormone replacement therapy is now the standard of care for amelioration of the symptoms of menopause and prevention of osteoporosis. More recently, it has been appreciated that hormone replacement may also have beneficial effects on the brain. This review will contrast the biologic data showing that estrogen has significant neuroprotectant effects on the brain, with data from cross-sectional and epidemiologic studies of women showing that hormone replacement may have beneficial effects for cognition in aging. Together, studies suggest that the temporal pattern of hormone replacement may be critical for maintaining cognitive health in aging.

Estradiol accelerates extinction of a conditioned taste aversion in female and male rats

Exogenous testosterone treatment prolongs extinction of conditioned taste aversions and estradiol treatment prevents testosterone from prolonging extinction in both gonadectomized males and females. Estradiol could require the presence of testosterone for its effect or its action alone could accelerate extinction. The first series of experiments were designed to test the hypothesis that estradiol accelerates extinction when it is given in the absence of testosterone. The results showed that estradiol accelerates extinction of conditioned taste aversions in the absence of testosterone in gonadectomized Sprague-Dawley females and Fischer 344 females and males. The second series of experiments were designed to determine whether estradiol and testosterone differ in the temporal requirements for their opposite effects on extinction. The results showed that estradiol can accelerate extinction when it is present before and during acquisition (from 8 days before until 3 days after acquisition) or when it is present before and during extinction (from 2 days after acquisition, which was 23 days before extinction, until extinction trials were terminated). This is in contrast to a previous finding that testosterone prolongs extinction only when it is present before and during extinction. The following two hypotheses were suggested to account for the temporal effects of estradiol on extinction of conditioned taste aversions: (1) the presence of estradiol during acquisition reduces the effectiveness of LiCl through its action on the opioid system, and the presence of estradiol during extinction activates a neural pathway, such as that associated with activity levels, that accelerates extinction of passive avoidance tasks in general or (2) the presence of estradiol before, not during, acquisition or extinction accelerates extinction because of its illness-inducing properties. Most of the evidence supports the second hypothesis.

Sex differences in male-typical copulatory behaviors in response to androgen and estrogen treatment in rats

The present experiment was performed to test the hypothesis that gender differences in the capacity for brain estrogen synthesis could constitute a sexually dimorphic mechanism that limits the activational effects of testosterone (T) in females, and enhances them in males. We determined the effects of treatments with equivalent levels of either T or estradiol (E2) on olfactory behavior and mounting in age-matched heterosexually naive gonadectomized male and female rats that were genitally ansthetized with lidocaine paste in order to minimize the contribution of sexually dimorphic somatosensory inputs to the expression of copulatory behavior. We found that T stimulated mounting to a greater extent in males than in females, but had equivalent effects on mount latency and genital investigation in the two sexes. On the other hand, E2 stimulated equivalent levels of mounting in males and females and reduced mount latency to a similar extent in males and females. However, E2 had a pronounced effect on the levels of genital investigation in males but not in females. Serum steroid levels and the levels of nuclear steroid receptor occupation in the brain were not different between males and females, suggesting that the behavioral differences between males and females cannot be attributed to differences in peripheral steroid metabolism or brain uptake. The results obtained corroborate previous studies suggesting that female rats normally undergo considerable male-typical behavioral masculinization during fetal development. However, such male-typical features of normal development in female rats do not extend to the regulation of preoptic aromatase activity or to the capacity of females to display olfactory behaviors in response to adult E2 exposure, functions which are sexually dimorphic even in the rat. The present results support the view that gender differences in the capacity for brain estrogen synthesis contribute to the sexually dimorphic display of T-stimulated male-typical sexual motivation and copulatory behavior in rats.

Sex differences in estrogenic regulation of preproenkephalin mRNA levels in the medial preoptic area of prepubertal rats

Opioids have been implicated in sexual differentiation of the brain and in the regulation of reproductive behavior and endocrinology of mammals. Previous studies have indicated that estrogen administration in adults regulates preproenkephalin MRNA levels in several hypothalamic brain nuclei. We have determined preproenkephalin mRNA levels in estrogen-treated juvenile male and female rats to investigate the developmental pattern of estrogenic regulation of enkephalinergic neurons in the medial preoptic area. Rats were treated with estradiol benzoate (20 microgram/kg/day) or oil from day 21 to 23. Sections of the medial preoptic area (mPOA) were studied by in situ hybridization histochemistry at the single cell level and quantified with the assistance of an image analysis system. Our data indicate that males contain higher levels of preproenkephalin mRNA per neuron than females. In addition, our results indicate that estrogen causes an upward shift in the amount of mRNA expressed per cell, females demonstrating a greater response to estrogen than males. An increase in soma cell area following estrogen treatment was observed only in female mPOA enkephalinergic neurons. Taken together, these results indicate a sex difference in total preproenkephalin levels and in estrogenic regulation of preproenkephalin mRNA in the POA of juvenile rats. These results are discussed in relation to the differential role opioids may play in male and female reproductive physiology.

Pregnenolone reverses the age-dependent accumulation of glial fibrillary acidic protein within astrocytes of specific regions of the rat brain

Although aged-related modifications of astrocytes have been frequently described, little is known so far about the signals responsible for these modifications. Since it is well demonstrated that astrocytes are highly responsive to a variety of steroids, we hypothesized that modifications of cerebral astrocytes may result from the age-related decrease of circulating steroids. In the present study, we investigated the effects of the chronic administration of pregnenolone (PREG), the precursor of all steroid hormones, on the age-related extension of astrocytic processes in various brain regions. In adult (2-3 month-old) and aged (22-24 month-old) rats, quantitative image analysis was used to estimate, within each region, the number of astrocyte cell bodies immunostained (IS) for S100, and the surface occupied by astrocytic cell bodies and processes IS for glial fibrillary acidic protein (GFAP). In all regions, the surface occupied by GFAP-IS structures was increased in the aged vs. the adult rats, whereas no significant modifications were observed in the number of S100-IS cell bodies. Chronic administration of PREG to aged rats induced a marked decrease in the surface occupied by GFAP-IS structures in the cortex, amygdala and thalamus, without any significant effect on the number of S100-IS cell bodies present in these regions. By contrast, PREG had no significant effect when administered to adult animals. These data suggest that decreased levels of circulating steroid hormones may be responsible for the age-dependent modifications of the astrocytes present in various brain regions, and that these modifications can be at least partly corrected by the administration of PREG.

Age-dependent changes in androgen receptor immunoreactivity in motoneurons of the spinal nucleus of the bulbocavernosus of male rats

Androgen receptor (AR) immunoreactivity was examined in androgen-sensitive motoneurons of the spinal nucleus of the bulbocavernosus (SNB) in young and old male rats by immunohistochemistry using the polyclonal antibody, PG21. In young animals, intense AR immunoreactivity was confined to the cell nucleus, but not in the nucleolus of SNB motoneurons. In old animals, both the intensity of AR immunoreactivity in the nuclei and number of AR immunoreactive nuclei of the SNB motoneurons were significantly reduced. Plasma levels of testosterone in old animals were significantly smaller than those in young ones. Age-dependent changes both in AR expression of SNB motoneurons and plasma levels of androgen seem to correlate with the aging of the SNB system.

Hypothalamic aromatase cytochrome P450 and 5alpha-reductase enzyme activities in pregnant and female rats

The metabolism of steroid hormones in the medial basal hypothalamus (MBH) is known to play a critical role in neural development, the modulation of neuroendocrine function and regulating sexual behavior. While the important biological functions of the aromatase enzyme are well established, the importance of brain 5alpha-reductase has been revealed and elucidated only in the last few years. The distribution and regulation of brain aromatase and 5alpha-reductase enzyme activities have been investigated for the most part in male rats. Therefore, in the present study, MBH aromatase cytochrome P450 and 5alpha-reductase activities were characterized in pregnant and female rats during postnatal development under various hormonal conditions. MBH aromatase activity was determined in each tissue sample using the 'tritiated water release' assay, whereas, the 5alpha-reductase rates were determined by thin layer chromatography and scintillation counting of the isolated 5alpha-metabolites. Both activities were highest in infantile animals, then declined with increasing postnatal age; whereas, in aged non-cycling or ovariectomized/adrenalectomized (Ovx/Adx) rats high rates of androgen metabolism were seen in MBH tissue. No significant alterations in MBH aromatase were observed when the 5alpha-reductase pathway was blocked in pregnant animals during late gestation with a known 5alpha-reductase inhibitor (Proscar). However, plasma estradiol levels were significantly increased in the Proscar-treated animals. These results indicate that: 1) the decreasing MBH aromatase and 5alpha-reductase profile (in infantile to adult cycling animals) is developmentally regulated, 2) evidently, there is a divergent regulatory mechanism controlling MBH aromatase versus 5alpha-reductase in aged animals where the aromatase activity increased in aged non-cycling and Ovx/Adx rats while 5alpha-reductase rates remained at moderate levels and, 3) apparently, the 5alpha-reductase pathway is not involved in regulating MBH aromatase activity during late pregnancy.

Sexual function in altered physiological states: comparison of effects of hypertension, diabetes, hyperprolactinemia, and others to "normal" aging in male rats

In this review, we examine the changes in sexual function that accompany deviations from "normal" physiological states. We propose that the changes one observes in many altered physiological states should not be viewed in isolation. We describe our paradigms for assessing sexual function, and proceed to evaluate how sexual function changes with hormonal deprivation and aging, in rat models for hypertension, in severe hyperprolactinemia, in streptozotocin-induced diabetes, after chronic alcohol intake, after chronic morphine administration, and after exposure to the heavy metal, cadmium. We will provide evidence for the involvement of adrenergic transmitters and two neuropeptides, neuropeptide Y and somatostatin, in the neuroendocrine regulation of sexual behavior. Finally, we compare and contrast the changes observed relative to the changes seen in "normal" aging in rats. The sequence of age-related changes in sexual function is distinct. The first change observed is a decrement in ex copula erectile reflexes. Next are decreases in ejaculatory threshold, followed shortly by increases in initiation and reinitiation of copulation after ejaculation. This is followed by a decrement in the number of males copulating to ejaculation. Finally, there is a failure to initiate the copulatory process. This sequelae is relatively common, being evident after castration, with hyperprolactinemia, and after exposure to cadmium. The data available for sexual function in hypertension is incomplete and modified by the etiology, but a suggestion for this sequelae is seen in SHR. In contrast, sexual dysfunction associated with chronic morphine administration appears to be due to an initial deficit in motivational aspects. Testosterone reverses sexual dysfunction associated with castration, but not with idiopathic sexual inactivity, nor with sexual dysfunction associated with aging, diabetes, or chronic morphine administration. Comparing sexual function in rat models for hypertension, diabetes and chronic ethanol leads to the conclusion that increases in blood pressure, like decreases in testosterone, cannot be the primary causal factor for sexual dysfunction. Age, hormonal history of the subject, and the age at castration influence changes in sexual function. Age-related sexual dysfunction appears to be contributed to by changes in adrenergic-neuropeptidergic, to include sympathetic, systems. Site-specific administration of NPY induces alterations in parameters of copulatory behavior which mimic those seen in aging and the retention of ejaculatory behavior with aging is associated with site-selective attenuation (or reversal) of age-associated changes in NPY content. Yohimbine enhances copulatory activity in castrated and aging rats, and attenuates or reverses the antisexual effects of clonidine, epinephrine and somatostatin.(ABSTRACT TRUNCATED AT 400 WORDS)

Neurobiological correlates of masculine sexual behavior

The experimental analysis of the neuroendocrine interactions regulating sexual behavior has traditionally relied on studying the effects of CNS lesions and pharmacological treatments with hormones or drugs purportedly acting through specific neurotransmitter systems. New methodological developments have allowed the assessment of several indices of neural function in experimental animals, particularly the rat, as they relate to behavioral changes. In the field of sexual behavior, ex vivo analyses have been used to measure markers of energy metabolism, such as 2-deoxyglucose uptake and Na,K-ATPase activity, the tissue content of neurotransmitters and metabolites, the levels of steroid receptors and neurosteroids, and immediate-early gene expression products in different areas of the CNS. In vivo studies have monitored brain electrical activity and temperature, as well as the extracellular levels of neurotransmitters and metabolites by cerebrospinal fluid sampling, push-pull perfusion and, especially, electrochemical recordings and microdialysis, in the course of mating and exposure to various relevant stimuli. The findings with the different methodologies are generally consistent and agree with those of previous surgical and pharmacological manipulations. They provide data on temporal relationships between neurobiological and behavioral events and suggest new interpretations for different aspects of the male copulatory pattern.

Age-related changes in androgen receptor levels in cranial nerve nuclei of male rats

Sensory and motor events are important for mating, and several cranial nerve nuclei mediating these events contain androgen receptors (AR). Since mating behavior declines with age, we tested whether AR binding is decreased in cranial nerve nuclei of old male rats. Cytosol and cell nuclear androgen receptor (AR) binding was assessed in the cochlear, hypoglossal and facial nuclei in young (4 months) and old (20 months) male Fischer 344 rats. For comparison with other neural and non-neural tissue, AR binding in combined hypothalamus, preoptic area and amygdala (HPA), and muscle tissue from tongue and masseter were examined. Cytosol AR binding was significantly decreased in all three cranial nerve nuclei of old males. No age-related changes were observed in HPA or muscle. Cell nuclear AR binding was unaffected by age in all of the tissues analyzed. Neuron numbers in motor nuclei of the hypoglossal, facial and trigeminal nerves were compared between young and old rats. A significant decrease in neuron number was found only in the hypoglossal nucleus of old rats, indicating that neuronal loss is not a factor in the reduction of AR's in cranial nerve nuclei. It is suggested that the loss of AR's in cranial nerve nuclei of old rats contributes to the decline in male copulatory behavior by reducing responsiveness to sensory and motor cues.

GFAP mRNA increases with age in rat and human brain

Glial fibrillary acidic protein (GFAP) mRNA was examined by RNA blot hybridization in three age groups of two cohorts of male F-344 rats and in 47 human postmortem brain samples. GFAP mRNA increased in the hippocampus and striatum of 24 versus 6- to 7-month-old rats. Another astrocytic molecular marker, glutamine synthetase mRNA, did not change with age in rat brain. Rat GFAP mRNA prevalence was inversely correlated with serum testosterone but not correlated with serum corticosterone. In human hippocampus, frontal and temporal cortex, GFAP mRNA also increased in older (60-79 years) compared with middle-aged (25-59 years) individuals. In contrast, mitochondrial cytochrome oxidase subunit 1 mRNA did not change between age groups in any region. By combining the three regions for further analysis, GFAP mRNA increased with age irregardless of gender, alcoholism in the middle-aged group, or whether brains were classified as normal or neuropathologic (excluding Alzheimer's disease pathology). These data indicate that increased GFAP protein or GFAP-immunoreactive astrocytes in rats and humans may result from transcriptional or post-transcriptional regulation and extend the number to three species (including mouse) showing an increase in GFAP mRNA with age. Factors that are known to regulate GFAP mRNA expression in young brains are considered as possible causes of age-related increases.

Reduced sensitivity of penile mechanoreceptors in aging rats with sexual dysfunction

Male sexual activity gradually declines with age in most mammalian species and may be associated with a decrease in vibrotactile sensitivity of the penis. This study was designed to determine if the penile mechanoreceptors in aging rats exhibit reduced sensitivity to vibrotactile stimuli. Electrophysiological and behavioral data was obtained from mature adult (9 months) and old (23 months) rats. Single penile mechanoreceptor afferent fibers from aging animals exhibited a decrease in the ability to transduce high-frequency vibratory stimuli when compared to afferents from mature adult animals. In addition, there was a reduction in single-fiber and sensory nerve conduction velocity. Copulatory performance was reduced in the aging animals as determined by the inability to ejaculate with a receptive female. The results of this study demonstrate that the male rat exhibits an age-related decrease in penile vibrotactile sensitivity mediated by mechanoreceptor afferents which may be partly responsible for the decrease in parameters of sexual activity.