Brain-derived neurotrophic factor regulates expression of androgen receptors in perineal motoneurons

Muscle, a conduit to brain for hormonal control of behavior

SBN Elsevier Lecture Investigation into mechanisms whereby hormones control behavior often starts with actions on central nervous system (CNS) motivation and motor systems and is followed by assessment of CNS drive of coordinated striated muscle contractions. Here we turn this perspective on its head by discussing ways in which hormones might first act on muscle that then secondarily drive upstream the evolution and function of the CNS. While there is a lengthy history for consideration of this perspective, newly discovered properties of muscle signaling reveal novel mechanisms that may well be captured by endocrine systems and thus of interest to behavioral endocrinologists.

Transsynaptic trophic effects of steroid hormones in an avian model of adult brain plasticity

The avian song control system provides an excellent model for studying transsynaptic trophic effects of steroid sex hormones. Seasonal changes in systemic testosterone (T) and its metabolites regulate plasticity of this system. Steroids interact with the neurotrophin brain-derived neurotrophic factor (BDNF) to influence cellular processes of plasticity in nucleus HVC of adult birds, including the addition of newborn neurons. This interaction may also occur transsynpatically; T increases the synthesis of BDNF in HVC, and BDNF protein is then released by HVC neurons on to postsynaptic cells in nucleus RA where it has trophic effects on activity and morphology. Androgen action on RA neurons increases their activity and this has a retrograde trophic effect on the addition of new neurons to HVC. The functional linkage of sex steroids to BDNF may be of adaptive value in regulating the trophic effects of the neurotrophin and coordinating circuit function in reproductively relevant contexts.

Voluntary exercise does not ameliorate spatial learning and memory deficits induced by chronic administration of nandrolone decanoate in rats

Chronic exposure to the anabolic androgenic steroids (AAS) nandrolone decanoate (ND) in supra-physiological doses is associated with learning and memory impairments. Given the well-known beneficial effects of voluntary exercise on cognitive functions, we examined whether voluntary exercise would improve the cognitive deficits induced by chronic administration of ND. We also investigated the effects of ND and voluntary exercise on hippocampal BDNF levels. The rats were randomly distributed into 4 experimental groups: the vehicle-sedentary group, the ND-sedentary group, the vehicle-exercise group, and the ND-exercise group. The vehicle-exercise and the ND-exercise groups were allowed to freely exercise in a running wheel for 15 days. The vehicle-sedentary and the ND-sedentary groups were kept sedentary for the same period. Vehicle or ND injections were started 14 days prior to the voluntary exercise and continued throughout the 15 days of voluntary exercise. After the 15-day period, the rats were trained and tested on a water maze spatial task using four trials per day for 5 consecutive days followed by a probe trial two days later. Exercise significantly improved performance during both the training and retention of the water maze task, and enhanced hippocampal BDNF. ND impaired spatial learning and memory, and this effect was not rescued by exercise. ND also potentiated the exercise-induced increase in hippocampal BDNF levels. These results seem to indicate that voluntary exercise is unable to improve the disruption of cognitive functions by chronic ND. Moreover, increased levels of BDNF may play a role in ND-induced impairments in learning and memory. The harmful effects of ND and other AAS on learning and memory should be taken into account when athletes decide to use AAS for performance or body image improvement.

With a little help from my friends: androgens tap BDNF signaling pathways to alter neural circuits

Gonadal androgens are critical for the development and maintenance of sexually dimorphic regions of the male nervous system, which is critical for male-specific behavior and physiological functioning. In rodents, the motoneurons of the spinal nucleus of the bulbocavernosus (SNB) provide a useful example of a neural system dependent on androgen. Unless rescued by perinatal androgens, the SNB motoneurons will undergo apoptotic cell death. In adulthood, SNB motoneurons remain dependent on androgen, as castration leads to somal atrophy and dendritic retraction. In a second vertebrate model, the zebra finch, androgens are critical for the development of several brain nuclei involved in song production in males. Androgen deprivation during a critical period during postnatal development disrupts song acquisition and dimorphic size-associated nuclei. Mechanisms by which androgens exert masculinizing effects in each model system remain elusive. Recent studies suggest that brain-derived neurotrophic factor (BDNF) may play a role in androgen-dependent masculinization and maintenance of both SNB motoneurons and song nuclei of birds. This review aims to summarize studies demonstrating that BDNF signaling via its tyrosine receptor kinase (TrkB) receptor may work cooperatively with androgens to maintain somal and dendritic morphology of SNB motoneurons. We further describe studies that suggest the cellular origin of BDNF is of particular importance in androgen-dependent regulation of SNB motoneurons. We review evidence that androgens and BDNF may synergistically influence song development and plasticity in bird species. Finally, we provide hypothetical models of mechanisms that may underlie androgen- and BDNF-dependent signaling pathways.

Turning sex inside-out: Peripheral contributions to sexual differentiation of the central nervous system

Sexual differentiation of the nervous system occurs via the interplay of genetics, endocrinology and social experience through development. Much of the research into mechanisms of sexual differentiation has been driven by an implicit theoretical framework in which these causal factors act primarily and directly on sexually dimorphic neural populations within the central nervous system. This review will examine an alternative explanation by describing what is known about the role of peripheral structures and mechanisms (both neural and non-neural) in producing sex differences in the central nervous system. The focus of the review will be on experimental evidence obtained from studies of androgenic masculinization of the spinal nucleus of the bulbocavernosus, but other systems will also be considered.

Androgen and estrogen receptor expression in the spinal segments of the genitofemoral nerve during testicular descent

AIM

During testicular descent (TD), the genitofemoral nerve (GFN) is masculinized by androgen. This study aimed to test whether androgen receptor (AR), estrogen receptor α (ERA), or estrogen receptor β (ERB) are expressed during TD in the GFN spinal segments and dorsal root ganglia (DRG) in normal and flutamide-treated rats.

METHODS

Time-mated Sprague-Dawley dams were injected with flutamide (75 mg/kg, subcutaneously (S/C) in sunflower oil) on embryonic (E) days 16 to 19. Embryonic and postnatal (P) male L1-2 spinal cord segments were collected (E16, E17, E19, P0, P2, and P4) in control and flutamide-treated groups (n = 5-10). Samples were fixed in 4% paraformaldehyde. Five-micrometer-thick sections were prepared immunohistochemically for AR, ERA, and ERB.

RESULTS

During TD, ERB was expressed in L1-2 DRG. Surprisingly, AR was not expressed in prenatal DRG, only after P2. There was no ERA expression. Flutamide had no effect on AR, ERB, or ERA expression in the L1-2 DRG during TD.

CONCLUSION

During the E window of androgen sensitivity, the GFN is not directly masculinized, with little AR expression and no change with flutamide over this period. Estrogen receptor β is expressed in the DRG during TD. However, its relevance is yet to be determined.

Time course of adult castration-induced changes in soma size of motoneurons in the rat spinal nucleus of the bulbocavernosus

The spinal nucleus of the bulbocavernosus (SNB) innervates striated muscles, the bulbocavernosus and levator ani (BC/LA), which control penile reflexes. Castration results in shrinkage in the size of SNB somata and dendrites, as well as BC/LA muscle mass. However, there is no information about how quickly these regressive changes occur compared to the rapid effects of castration upon penile reflexes, which are greatly diminished a few days after surgery. Therefore we examined the time course of change in the size of SNB somata after castration of adult male rats. Males were sacrificed 2, 14, or 28 days after either castration or sham surgery and somata were measured in the SNB and in a control population of motoneurons, the retrodorsolateral nucleus (RDLN). BC/LA weight was reduced in castrates compared to intact males 14 and 28 days post surgery, but SNB somata were significantly smaller in castrates only at 28 days after surgery. As has been previously observed, castration did not affect soma size in the RDLN. These data indicate that SNB somata respond more slowly after castration than BC/LA mass or penile reflexes, suggesting that the size of SNB somata cannot account for the loss of penile reflexes. Androgenic effects on SNB somata may contribute to aspects of reproductive behavior that are not apparent in penile reflexes tested ex copula.

Androgen regulation of axon growth and neurite extension in motoneurons

Androgens act on the CNS to affect motor function through interaction with a widespread distribution of intracellular androgen receptors (AR). This review highlights our work on androgens and process outgrowth in motoneurons, both in vitro and in vivo. The actions of androgens on motoneurons involve the generation of novel neuronal interactions that are mediated by the induction of androgen-dependent neurite or axonal outgrowth. Here, we summarize the experimental evidence for the androgenic regulation of the extension and regeneration of motoneuron neurites in vitro using cultured immortalized motoneurons, and axons in vivo using the hamster facial nerve crush paradigm. We place particular emphasis on the relevance of these effects to SBMA and peripheral nerve injuries.

Testosterone prevents synaptic loss in the perineal motoneuron pool in the spinal cord in male rats exposed to chronic stress

Chronic stress is known to induce disorders of reproductive neuroendocrine functions. Motoneurons of the spinal nucleus of the bulbocavernosus (SNB) in male rats play an important role in copulatory behavior. In the present study, it was examined whether chronic stress would alter synaptic organization of the SNB motoneurons and whether androgen would modify the changes under chronic stress. Five male rats were under restraint stress for 5 days per week for 3 weeks, and five males implanted subcutaneously with Silastic capsules containing testosterone were also exposed to stress. Five males served as unstressed controls. After 3 weeks of restraint stress, cholera toxin-horseradish peroxidase (CT-HRP) was injected into the bulbocavernosus muscles and animals were killed 2 days later. The spinal cords containing the SNB were dissected, processed with a modified tetramethylbenzidine (TMB) method for visualization of retrogradely transported CT-HRP, and examined ultrastructurally. Neuronal structures apposing the membranes of 150 SNB motoneurons (total for three groups) were analyzed by measuring the percentage of somatic membranes covered by synaptic contacts. The mean percentage of somatic membranes covered by synapses in males exposed to chronic stress was significantly less than that in controls or stressed males treated with testosterone. Size and number of synaptic contacts per unit length of somatic membranes in males exposed to stress were also significantly less than those in controls or stressed males treated with testosterone. There was no significant difference in any of the parameters between controls and stressed males treated with testosterone. Changes in plasma levels of testosterone showed the same profile as changes in the synaptic contacts. These results suggest that the SNB motoneurons of male rats exposed to chronic stress retain a considerable synaptic plasticity in response to androgen, and that androgen treatment can rescue the SNB system in male rats when under chronic restraint stress.

Sexually dimorphic expression of trkB, a Z-linked gene, in early posthatch zebra finch brain

Sexual differentiation of the zebra finch (Taeniopygia guttata) neural song circuit is thought to be initiated by sex differences in sex chromosome gene expression in brain cells. One theory is that Z-linked genes, present in the male's ZZ genome at double the dose of females' (ZW), are expressed at higher levels and trigger masculine patterns of development. We report here that trkB (tyrosine kinase receptor B) is Z-linked in zebra finches. trkB is the receptor for neurotrophic factors BDNF and neurotrophin 4, and mediates their influence on neuronal survival, migration, and specification. trkB mRNA is expressed at a higher level in the male telencephalon or whole brain than in corresponding regions of the female in adulthood, and at posthatch day (P) 6, when the song circuit is undergoing sexual differentiation. Moreover, this expression is higher in the song nucleus high vocal center (HVC) than in the surrounding telencephalon at P6, and in males relative to females. In addition, trkB protein is expressed more highly in male than female whole brain at P6. These results establish trkB as a candidate factor that contributes to masculine differentiation of HVC because of its Z-linkage, which leads to sex differences in expression. BDNF is known to be stimulated by estrogen and to be expressed at higher levels in males than females at later ages in HVC. Thus, the trkB-BDNF system may be a focal point for convergent masculinizing influences of Z-linked factors and hormones.

Is Melanoma a stem cell tumor? Identification of neurogenic proteins in trans-differentiated cells

Background

Although several genes and proteins have been implicated in the development of melanomas, the molecular mechanisms involved in the development of these tumors are not well understood. To gain a better understanding of the relationship between the cell growth, tumorigenesis and differentiation, we have studied a highly malignant cat melanoma cell line that trans-differentiates into neuronal cells after exposure to a feline endogenous retrovirus RD114.

Methods

To define the repertoire of proteins responsible for the phenotypic differences between melanoma and its counterpart trans-differentiated neuronal cells we have applied proteomics technology and compared protein profiles of the two cell types and identified differentially expressed proteins by 2D-gel electrophoresis, image analyses and mass spectrometry.

Results

The melanoma and trans-differentiated neuronal cells could be distinguished by the presence of distinct sets of proteins in each. Although approximately 60–70% of the expressed proteins were shared between the two cell types, twelve proteins were induced de novo after infection of melanoma cells with RD114 virus in vitro. Expression of these proteins in trans-differentiated cells was significantly associated with concomitant down regulation of growth promoting proteins and up-regulation of neurogenic proteins (p = < 0.001). Based on their physiologic properties, >95% proteins expressed in trans-differentiated cells could be associated with the development, differentiation and regulation of nervous system cells.

Conclusion

Our results indicate that the cat melanoma cells have the ability to differentiate into distinct neuronal cell types and they express proteins that are essential for self-renewal. Since melanocytes arise from the neural crest of the embryo, we conclude that this melanoma arose from embryonic precursor stem cells. This model system provides a unique opportunity to identify domains of interactions between the expressed proteins that halt the tumorigenic potential of melanoma cells and drive them toward neurogenerative pathways involved in early neurogenesis. A better understanding of these proteins in a well-coordinated signaling network would also help in developing novel approaches for suppression of highly malignant tumors that arise from stem-like embryonic cells.

Minireview: Neuronal steroid hormone receptors: they're not just for hormones anymore

The ovarian steroid hormones have numerous effects on the brain, many of which are mediated, at least in part, by interaction with intracellular steroid hormone receptors acting as regulators of transcription. These intracellular steroid hormone receptors have often been considered to be activated solely by cognate hormone. However, during the past decade, numerous studies have shown that the receptors can be activated by neurotransmitters and intracellular signaling systems, through a process that does not require hormone. Although most of these have been in vitro experiments, others have been in vivo. Evidence from a wide variety of tissues and cells suggests that steroid hormone receptors are transcription factors that can be activated by a wide variety of factors, only one of which is cognate hormone. Furthermore, ligand-independent activation of neural steroid hormone receptors, rather than being a pharmacological or in vitro curiosity, seems to be a process that occurs in the normal physiology of animals. Thinking of steroid hormone receptors only as ligand-activated proteins may constrain our thinking about the many factors that may activate the receptors and cause receptor-dependent changes in neural gene expression and neuroendocrine function.

Expression of androgen receptor mRNA in the late embryonic and early posthatch zebra finch brain

Zebra finch males sing and females do not, and the underlying neural circuitry in males is more developed than that in females. Sex steroid hormones influence the development of sex differences in this circuitry, including differences in androgen receptor (AR) expression, although the role of androgens has been controversial. We isolated a cDNA encoding a portion of the zebra finch AR and used in situ hybridization to examine the spatiotemporal pattern of AR mRNA expression in the brain during late embryonic development and at hatching. We detected AR mRNA in all the major subdivisions of the brain as early as embryonic day 10. No qualitative sex differences in AR mRNA expression patterns were observed. Cells lining the ventral arm of the lateral telencephalic ventricles expressed AR mRNA on embryonic day 11 and posthatching day 1, as did cells lining the third ventricle at all three developmental stages examined, suggesting that androgens may play a role in early stages of cellular proliferation, migration, or differentiation. AR mRNA was also detected in the hippocampus, neostriatum, septum, ventromedial archistriatum, hypothalamic regions, dorsal mesencephalon, and in and around the brainstem nucleus tracheosyringealis. Our results suggested that androgens act early in neural development and therefore may contribute to the process of sexual differentiation.

Development of androgen receptor and p75(NTR) mRNAs and peptides in the lumbar spinal cord of the gerbil

Development of sex differences in the spinal cord appears to be largely under the control of androgen and although neurotrophins may also have a role. Spinal cords of male and female neonatal gerbils (postnatal days 1, 5, 7, 10, 23) and adult gerbils (postnatal day 150) were examined to determine the relative temporal expression of androgen receptor (AR) and the low-affinity neurotrophin receptor (p75) mRNAs within the spinal nucleus of the bulbocavernosus (SNB) and dorsolateral nucleus (DLN). Furthermore, prepubertal male gerbils were placed into one of six gonadal hormone treatment groups at weaning: Either sham castrate, castrated with gonadal hormone replacement, or castrated without gonadal hormone replacement. Ten weeks later gerbils were aldehyde-perfused, spinal cords removed and processed for presence of AR and p75 immunoreactivity (ir) in motoneurons of the SNB and DLN. During neonatal development, there were significant increases in androgen receptor mRNA within the SNB and DLN. In the SNB, the increase in androgen receptor mRNA preceded the increase in p75 mRNA. Peripubertally, significantly more SNB than DLN motoneurons contained AR- and p75-ir. These data demonstrate that AR expression occurs along the same developmental time frame as the development of the SNB and DLN and the organizational effects of androgens on their development continues through puberty in the male gerbil.

Distributions of estrogen receptors alpha and beta in sympathetic neurons of female rats: enriched expression by uterine innervation

Estrogen modulates many features of the sympathetic nervous system, including cell numbers and ganglion synapses, and can induce uterine sympathetic nerve degeneration. However, distributions of estrogen receptors alpha and beta within sympathetic neurons have not been described, and their regulation by target tissue or estrogen levels has not been explored. We used immunofluorescence and retrograde tracing to define estrogen receptor expression in sympathetic neurons at large in pre- and paravertebral ganglia and in those projecting to the uterine horns. Estrogen receptor alpha immunoreactivity was present in 29 +/- 1%, while estrogen receptor beta was expressed by 92 +/- 1% of sympathetic neurons at large. The proportions of neurons expressing these receptors were comparable in the superior cervical and thoraco-lumbar paravertebral ganglia from T11 through L5, and in the suprarenal, celiac, and superior mesenteric prevertebral ganglia. Injections of FluoroGold into the uterine horns resulted in labeled neurons, with peak occurrences in T13, L1, and the suprarenal ganglion. Uterine-projecting neurons showed small but significantly greater incidence of estrogen receptor beta expression relative to the neuronal population at large, whereas the proportion of uterine-projecting neurons with estrogen receptor alpha-immunoreactivity was nearly threefold greater. Numbers of estrogen receptor-expressing neurons were not altered by acute estrogen administration. We conclude that the vast majority of sympathetic neurons express estrogen receptor beta immunoreactive protein, whereas a smaller, presumably overlapping subset expresses the estrogen receptor alpha. Expression of the latter apparently can be enhanced by target-mediated mechanisms.

Development of sex differences in the rabbit masseter muscle is not restricted to a critical period

The proportions of muscle fibers of different phenotype in the adult rabbit masseter differ greatly in different sexes. These sex differences are not apparent in young adults, but arise under the influence of testosterone in the males. We examined whether this switch occurred during a critical period of postnatal development. Testosterone was administered to young adults 1, 2, or 4 mo after castration, and also to adult females. Samples of masseter muscle were taken at four monthly intervals after the onset of treatment and examined for the expression of different myosin heavy chain (MyHC) isoforms by using a panel of monoclonal antibodies. Despite the length of androgen deprivation, treatment with testosterone produced a marked MyHC isoform switch from alpha-slow/beta to IIa. This male proportion of fibers of different phenotypes persisted well beyond the return of serum testosterone levels to pretreatment levels. Thus brief exposure to testosterone produces a permanent change in the proportions of masseter muscle fibers of different phenotypes, and the capacity for this change is not restricted to a critical period.

Blockade of endogenous neurotrophic factors prevents the androgenic rescue of rat spinal motoneurons

Target-derived neurotrophic factors are assumed to regulate motoneuron cell death during development but remain unspecified. Motoneuron cell death in the spinal nucleus of the bulbocavernosus (SNB) of rats extends postnatally and is controlled by androgens. We exploited these features of the SNB system to identify endogenously produced trophic factors regulating motoneuron survival. Newborn female rat pups were treated with the androgen, testosterone propionate, or the oil vehicle alone. In addition, females received trophic factor antagonists delivered either into the perineum (the site of SNB target muscles) or systemically. Fusion molecules that bind and sequester the neurotrophins (trkA-IgG, trkB-IgG, and trkC-IgG) were used to block activation of neurotrophin receptors, and AADH-CNTF was used to antagonize signaling through the ciliary neurotrophic factor receptor-alpha (CNTFRalpha). An acute blockade of trkB, trkC, or CNTFRalpha prevented the androgenic sparing of SNB motoneurons when antagonists were delivered to the perineum. Trophic factor antagonists did not significantly reduce SNB motoneuron number when higher doses were injected systemically. These findings demonstrate a requirement for specific, endogenously produced trophic factors in the androgenic rescue of SNB motoneurons and further suggest that trophic factor interactions at the perineum play a crucial role in masculinization of this neural system.

Blockade of endogenous neurotrophic factors prevents the androgenic rescue of rat spinal motoneurons

Target-derived neurotrophic factors are assumed to regulate motoneuron cell death during development but remain unspecified. Motoneuron cell death in the spinal nucleus of the bulbocavernosus (SNB) of rats extends postnatally and is controlled by androgens. We exploited these features of the SNB system to identify endogenously produced trophic factors regulating motoneuron survival. Newborn female rat pups were treated with the androgen, testosterone propionate, or the oil vehicle alone. In addition, females received trophic factor antagonists delivered either into the perineum (the site of SNB target muscles) or systemically. Fusion molecules that bind and sequester the neurotrophins (trkA-IgG, trkB-IgG, and trkC-IgG) were used to block activation of neurotrophin receptors, and AADH-CNTF was used to antagonize signaling through the ciliary neurotrophic factor receptor-alpha (CNTFRalpha). An acute blockade of trkB, trkC, or CNTFRalpha prevented the androgenic sparing of SNB motoneurons when antagonists were delivered to the perineum. Trophic factor antagonists did not significantly reduce SNB motoneuron number when higher doses were injected systemically. These findings demonstrate a requirement for specific, endogenously produced trophic factors in the androgenic rescue of SNB motoneurons and further suggest that trophic factor interactions at the perineum play a crucial role in masculinization of this neural system.

Neuronal size in the spinal nucleus of the bulbocavernosus: direct modulation by androgen in rats with mosaic androgen insensitivity

The motoneurons of the spinal nucleus of the bulbocavernosus (SNB) and its target muscles, the bulbocavernosus and levator ani, form a sexually dimorphic circuit that is developmentally dependent on androgen exposure and exhibits numerous structural and functional changes in response to androgen exposure in adulthood. Castration of male adult rats causes shrinkage of SNB somata, and testosterone replacement reverses this effect, but the site at which androgen is acting to cause this change is undetermined. We exploited the X-chromosome residency of the androgen receptor (AR) gene to generate androgenized female rats that were heterozygous for the testicular feminization mutant (tfm) AR mutation and that, as a consequence of ontogenetic random X-inactivation, expressed a blend of androgen-sensitive wild-type cells and tfm-affected androgen-insensitive cells in the SNB. Chronic testosterone treatment of adult mosaics increased soma sizes only in androgen-competent wild-type SNB cells. The size of tfm-affected SNB somata in the same animals did not differ from the size of either the wild-type or tfm-affected SNB neurons in control mosaics that did not receive androgen treatment in adulthood. Because the muscle targets of the SNB are known to be uniformly androgen-sensitive in tfm mosaics, this mosaic analysis provides unambiguous evidence that androgenic effects on motoneuron soma size are mediated locally in the SNB. It is possible that the neuronal AR plays a permissive role in coordinating the actions of androgen.

Androgen stimulates neuronal plasticity in the perineal motoneurons of aged male rats

Ten aged male rats (24 months of age) were castrated and implanted subcutaneously with Silastic capsules containing testosterone (T)(5 males) or nothing (5 males). Five sham-castrated males (25 months of age) served as controls. Four weeks after castration, cholera toxin-horseradish peroxidase (CT-HRP) was injected into the bulbocavernosus muscles and animals were killed 2 days later. The spinal cords containing the spinal nucleus of the bulbocavernosus (SNB) were dissected, processed with a modified tetramethylbenzidine method for visualization of retrogradely transported CT-HRP, and examined ultrastructurally. Neuronal structures apposing the membranes of 150 CT-HRP-labeled SNB motoneurons were analyzed by measuring the percentage of somatic membranes covered by synaptic contacts, synaptoid contacts, and neuron-neuron contacts. Most of the neuronal structures in the control and experimental SNB motoneurons consisted of synaptic contacts. The mean percentage of somatic membranes covered by synapses in castrated, aged males treated with T was significantly greater than that in control or castrated animals. Size and number of synaptic contacts per unit length of somatic membranes in castrated, aged males treated with T were also significantly greater than those in control or castrated animals. Plasma levels of T in castrated, aged males treated with T were significantly greater than that in controls. These results suggest that the SNB motoneurons of aged male rats retain a considerable synaptic plasticity in response to androgen, and that androgen may be, at least in part, involved in the process of aging of the SNB system in male rats.

Neuronal size in the spinal nucleus of the bulbocavernosus: direct modulation by androgen in rats with mosaic androgen insensitivity

The motoneurons of the spinal nucleus of the bulbocavernosus (SNB) and its target muscles, the bulbocavernosus and levator ani, form a sexually dimorphic circuit that is developmentally dependent on androgen exposure and exhibits numerous structural and functional changes in response to androgen exposure in adulthood. Castration of male adult rats causes shrinkage of SNB somata, and testosterone replacement reverses this effect, but the site at which androgen is acting to cause this change is undetermined. We exploited the X-chromosome residency of the androgen receptor (AR) gene to generate androgenized female rats that were heterozygous for the testicular feminization mutant (tfm) AR mutation and that, as a consequence of ontogenetic random X-inactivation, expressed a blend of androgen-sensitive wild-type cells and tfm-affected androgen-insensitive cells in the SNB. Chronic testosterone treatment of adult mosaics increased soma sizes only in androgen-competent wild-type SNB cells. The size of tfm-affected SNB somata in the same animals did not differ from the size of either the wild-type or tfm-affected SNB neurons in control mosaics that did not receive androgen treatment in adulthood. Because the muscle targets of the SNB are known to be uniformly androgen-sensitive in tfm mosaics, this mosaic analysis provides unambiguous evidence that androgenic effects on motoneuron soma size are mediated locally in the SNB. It is possible that the neuronal AR plays a permissive role in coordinating the actions of androgen.

Interaction of BDNF and testosterone in the regulation of adult perineal motoneurons

In androgen-sensitive motoneurons of the spinal nucleus of the bulbocavernosus (SNB), we investigated the interaction of BDNF (brain-derived neurotrophic factor) and testosterone to understand whether each factor gates the ability of the other to regulate androgen receptor expression and soma size, and whether each factor requires the presence of the other for its action. We axotomized SNB motoneurons and applied BDNF or PBS (phosphate-buffered saline) to the cut ends of the axons in rats that were castrated and treated with either testosterone or placebo. Control groups were either not castrated or not axotomized, or had intact SNB axons and were castrated and treated with testosterone or placebo. We found that testosterone determined the expression of nuclear androgen receptor, and this effect was enhanced by both BDNF and contact with the target muscles. The effect of BDNF on androgen receptor expression was seen only when testosterone was present. In the regulation of soma size, BDNF dominated. The application of BDNF completely compensated for the loss of testosterone in castrated males so that the testosterone effect on soma size was seen only in intact SNB motoneurons and in axotomized motoneurons treated with PBS. Moreover, testosterone increased androgen receptor and soma size in axotomized SNB motoneurons, indicating that testosterone can act on sites other than the target muscles of the SNB to regulate each of these. These results indicate that the regulation of androgen receptor by testosterone does not require BDNF, but the regulation of androgen receptor by BDNF does require testosterone. The regulation of soma size by BDNF does not require high expression of nuclear androgen receptor.

Muscle-dependent and hormone-dependent differentiation of the vocal control premotor nucleus robustus archistriatalis and the motornucleus hypoglossus pars tracheosyringealis of the zebra finch

Sex differences in the vertebrate brain (brain sex) are thought to develop owing to the tissue specific action of gonadal hormones similar to the development of secondary sex characteristics of the body. Small sex differences in body anatomy could, however, retrogradely control the sexual differentiation of the central nervous system. This possibility has so far been verified only for motorneuron pools, since the connectivity of sex-specific higher brain areas to the sexual dimorphic periphery is frequently not well known. Here, we tested whether somatic sex differences feed back on higher brain areas by bilateral denervation of the syringeal musculature of zebra finches before, during, and after onset of estrogen-sensitive sexual differentiation of forebrain vocal nuclei such as RA (nucleus robustus archistriatalis). In the zebra finch, the sound-producing musculature (the syrinx), the syrinx motornucleus hypoglossus pars tracheosyringealis (nXIIts), and the RA are much larger in males compared to females. Tract tracing studies revealed that the volume and neuron size distribution of the nXIIts was sexually dimorphic in intact but not in animals denervated as juveniles. In contrast, the volume of RA and size of RA neurons of denervated animals were highly sexually dimorphic. Furthermore, estrogen masculinized the RA of denervated females. Thus, sexual differentiation of the RA but not of the nXIIts appears independent of somatic sex differences. The syrinx muscles are, however, important for the soma size of those RA neurons that project to the nXIIts.

Age-dependent changes in phosphorylated cAMP response element-binding protein immunoreactivity in motoneurons of the spinal nucleus of the bulbocavernosus of male rats

Phosphorylated cAMP response element-binding protein (CREB) immunoreactivity was examined in motoneurons of the spinal nucleus of the bulbocavernosus (SNB) in young and old male rats by immunohistochemistry. In young animals, intense CREB immunoreactivity was confined to the cell nucleus, but not in the nucleolus of SNB motoneurons. In old animals, both the intensity of CREB immunoreactivity in the nuclei and number of CREB immunoreactive nuclei of the SNB motoneurons were significantly reduced. A marked decline in expression of CREB in the aged SNB motoneurons suggests alternation of cAMP-mediated regulation of gene expression in the SNB system with advancing age.

BDNF regulation of androgen receptor expression in axotomized SNB motoneurons of adult male rats

Brain-derived neurotrophic factor (BDNF) prevents the axotomy-induced loss of androgen receptor-like immunoreactivity (AR-LI) in the spinal nucleus of the bulbocavernosus (SNB) motoneurons of adult male rats. In this report, we investigated the dose-response effect of BDNF on androgen receptor expression in axotomized SNB motoneurons, and examined whether delayed application of BDNF to the cut SNB axons can completely reverse the axotomy-induced loss of androgen receptor expression. We also used autoradiography to test whether axotomy decreases the ability of SNB motoneurons to accumulate androgens. SNB motoneurons were axotomized bilaterally and BDNF or PBS was applied to the proximal ends of the axons. The percentage of SNB motoneurons expressing medium or high AR-LI was the major measure of androgen receptor expression. AR-LI was significantly higher on the BDNF-treated side than on the contralateral side treated with phosphate-buffered saline (PBS) for all three doses of BDNF (1.45, 2.9, and 5.8 mg/ml) and was higher than in rats treated bilaterally with PBS. Moreover, AR-LI at the highest dose of BDNF was not different from that in intact SNB motoneurons. Delayed application of BDNF to the axotomized SNB motoneurons restored the AR-LI to the intact level. The AR-LI decreased by axotomy started to increase significantly 4 days after BDNF application and returned to the intact level by 10 days. Furthermore, axotomy significantly decreased the percentage of SNB motoneurons to accumulate tritiated testosterone or its metabolites. In conclusion, our data demonstrate that BDNF completely prevents and reverses the axotomy-induced loss of AR-LI. Moreover, decrease of AR-LI by axotomy reflects the decrease in the ability of SNB motoneurons to accumulate androgens.

Synthesis and phosphorylation of androgen receptor of the mouse brain cortex and their regulation by sex steroids during aging

To examine the synthesis and phosphorylation of androgen receptor (AR) and their regulation by sex steroids, adult (24 weeks) and old (65 weeks) male and female mice were gonadectomized and administered with testosterone and estradiol. AR amount, synthesis and phosphorylation were measured in the brain cortex by immunoblotting and immunoprecipitation using antibody raised against rat AR transactivation domain (TAD) which was expressed in E. coli as a fusion protein. We found that the amount of AR was high in adult and declined in old mice of both sexes. Administration of testosterone and estradiol significantly down-regulated the level of AR in old male and adult female. Similarly, the rate of AR synthesis also declined with age. Exogenous treatment of gonadectomized mice with testosterone and estradiol reduced the extent of synthesis significantly in all groups except in old female. No sex-dependent variation was noticed either in the level or synthesis of AR. In contrast, the extent of phosphorylation was higher in old mice of both sexes as compared to their adult counterparts. Testosterone and estradiol supplementation resulted in remarkable increase in AR phosphorylation in all groups. Thus it is evident from our findings that the amount and synthesis of AR decrease but phosphorylation of AR increases in the brain cortex with advancing age of mice and they are regulated by testosterone and estradiol in age- and sex-specific manner.

Androgens rescue avian embryonic lumbar spinal motoneurons from injury-induced but not naturally occurring cell death

The regulation of survival of spinal motoneurons (MNs) has been shown to depend during development and after injury on a variety of neurotrophic molecules produced by skeletal muscle target tissue. Increasing evidence also suggests that other sources of trophic support prevent MNs from undergoing naturally occurring or injury-induced death. We have examined the role of endogenous and exogenous androgens on the survival of developing avian lumbar spinal MNs during their period of programmed cell death (PCD) between embryonic day (E)6 and E11 or after axotomy on E12. We found that although treatment with testosterone, dihydrotestosterone (DHT), or the androgen receptor antagonist flutamide (FL) failed to affect the number of these MNs during PCD, administration of DHT from E12 to E15 following axotomy on E12 significantly attenuated injury-induced MN death. This effect was inhibited by cotreatment with FL, whereas treatment with FL alone did not affect MN survival. Finally, we examined the spinal cord at various times during development and following axotomy on E12 for the expression of androgen receptor using the polyclonal PG-21 antibody. Our results suggest that exogenously applied androgens are capable of rescuing MNs from injury-induced cell death and that they act directly on these cells via an androgen receptor-mediated mechanism. By contrast, endogenous androgens do not appear to be involved in the regulation of normal PCD of developing avian MNs.

Neurotrophic factors and neuro-muscular disease: II. GDNF, other neurotrophic factors, and future directions

This is the second of two reviews in which we discuss the essential aspects of neurotrophic factor neurobiology, the characteristics of each neurotrophic factor, and their clinical relevance to neuromuscular diseases. The previous paper reviewed the neurotrophin family and neuropoietic cytokines. In the present article, we focus on the GDNF family and other neurotrophic factors and then consider future approaches that may be utilized in neurotrophic factor treatment.

Effects of spinal cord injury on spermatogenesis and the expression of messenger ribonucleic acid for Sertoli cell proteins in rat Sertoli cell-enriched testes

The study was an examination of the effects of spinal cord injury (SCI) on spermatogenesis and Sertoli cell functions in adult rats with Sertoli cell-enriched (SCE) testes. The effects of SCI on the seminiferous epithelium were characterized by abnormalities in the remaining spermatogenic cells during the first month after SCI. Three days after SCI, serum testosterone levels were 80% lower, while serum FSH and LH levels were 25% and 50% higher, respectively, than those of sham control SCE rats. At this time, the levels of mRNA for androgen receptor (AR), FSH receptor (FSH-R), and androgen-binding protein (ABP) were normal whereas those for transferrin (Trf) had decreased by 40%. Thereafter, serum testosterone levels increased, but they remained lower than those of the sham control rats 28 days after SCI; and serum FSH and LH levels returned to normal. The levels of mRNA for AR, ABP, and Trf exhibited a biphasic increase 7 days after SCI and remained elevated 28 days after SCI. FSH-R mRNA levels were also elevated 90 days after SCI. Unexpectedly, active spermatogenesis, including qualitatively complete spermatogenesis, persisted in > 40% of the tubules 90 days after SCI. These results suggest that the stem cells and/or undifferentiated spermatogonia in SCE testes are less susceptible to the deleterious effects of SCI than the normal testes and that they were able to proliferate and differentiate after SCI. The presence of elevated levels of mRNA for Sertoli cell FSH-R and AR, as well as of that for the Sertoli cell proteins, in the SCE testes during the chronic stage of SCI suggests a modification of Sertoli cell physiology. Such changes in Sertoli cell functions may provide a beneficial environment for the proliferation of the stem cells and differentiation of postmeiotic cells, thus resulting in the persistence of spermatogenesis in these testes.

BDNF mediates the effects of testosterone on the survival of new neurons in an adult brain

New neurons are incorporated into the high vocal center (HVC), a nucleus of the adult canary (Serinus canaria) brain that plays a critical role in the acquisition and production of learned song. Recruitment of new neurons in the HVC is seasonally regulated and depends upon testosterone levels. We show here that brain-derived neurotrophic factor (BDNF) is present in the HVC of adult males but is not detectable in that of females, though the HVC of both sexes has BDNF receptors (TrkB). Testosterone treatment increases the levels of BDNF protein in the female HVC, and BDNF infused into the HVC of adult females triples the number of new neurons. Infusion of a neutralizing antibody to BDNF blocks the testosterone-induced increase in new neurons. Our results demonstrate that BDNF is involved in the regulation of neuronal replacement in the adult canary brain and suggest that the effects of testosterone are mediated through BDNF.

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.