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.

Androgen receptor expression in the levator ani muscle of male mice

The spinal nucleus of the bulbocavernosus (SNB) is a sexually dimorphic group of motoneurones that innervates the bulbocavernosus (BC) and levator ani (LA), skeletal muscles that attach to the base of the penis. In many species, including mice, rats and hamsters, the LA and BC have been found to be highly responsive to androgen and, in rats, these muscles mediate several effects of androgen on the SNB system. However, characterising the SNB system in mice is important because of the availability of genetic models in this species. In the present study, we examined AR expression in skeletal muscles of C57/BlJ6 adult male mice using immunoblotting and immunocytochemistry, comparing the BC/LA to the androgen-unresponsive extensor digitorum longus (EDL). We found similar differences in AR expression for these muscles in the mouse as previously reported for rats. In mice, the BC/LA contains more AR protein than does the EDL. At the cellular level, the LA contains a higher percentage of AR positive myonuclei and fibroblasts than does the EDL. Finally, AR expression is enriched at the neuromuscular junction of mouse LA fibres. The increased expression of AR in the LA compared to the EDL in both muscle fibres and fibroblasts indicates that each cell type may critically mediate androgen action on the SNB system in mice.

Photoperiod and androgens act independently to induce spinal nucleus of the bulbocavernosus neuromuscular plasticity in the Siberian hamster, Phodopus sungorus

In the Siberian hamster, Phodopus sungorus, short-day photoperiods induce the winter phenotype, which in males includes a decrease in the production of androgens and changes in physiology to inhibit reproduction. Motoneurones of the spinal nucleus of the bulbocavernosus (SNB) and their target muscles, the bulbocavernosus and the levator ani, a neuromuscular system involved in male copulation, also display seasonal plasticity in P. sungorus. It is not known whether the plasticity seen in the SNB system of gonadally intact hamsters is due to the effects of photoperiod per se, or to the photoperiod-induced changes in androgen production. To answer this question, we castrated adult male hamsters from long days and then implanted them with capsules containing either testosterone or blanks. Half of the hamsters from each hormone condition were moved into short photoperiod (8 : 16 h light/dark cycle) while the rest were maintained under long-day conditions (15 : 9 h light/dark cycle). After 15 weeks, many measures of the SNB system, such as somata size and weight of target muscles, responded only to androgen, not to photoperiod. However, there were effects of photoperiod on the neuromuscular junctions (NMJs) that were independent of androgen status. For example, the number of synaptic zones per NMJ and the area of the NMJs were significantly increased by short days and/or testosterone treatment. The two factors exerted an additive, rather than an interactive, effect on these measures. Another striated muscle, the extensor digitorum longus, which is present in both sexes and plays no specialized role in reproduction, displayed neither an effect of androgen nor of photoperiod on fibre size or NMJ structure. These results suggest that, in addition to androgenic effects on SNB plasticity, there is also an androgen-independent effect of photoperiod on the SNB neuromuscular system.

Photoperiod and social cues influence the medial amygdala but not the bed nucleus of the stria terminalis in the Siberian hamster

We investigated whether the posterodorsal nucleus of the medial amygdala (MePD) and the posteromedial nucleus of the bed nucleus of the stria terminalis (BSTpm) undergo structural changes in response to photoperiod or social environment in the Siberian hamster, a seasonally breeding rodent. Adult male hamsters were either kept in long days (LD; 15:9 h light:dark) from birth or were transferred at 12-16 weeks of age to short days (SD; 8:16) and housed with a male conspecific for 11 weeks. Other males were transferred to SD but were housed with an unrelated female conspecific from LD. Males transferred to SD without a female cagemate displayed testicular regression, but males transferred to SD with a female cagemate did not. The regional volume and average soma size of the BSTpm and the MePD were estimated using Nissl-stained brain sections. Neither photoperiod nor social condition modified either of the BSTpm measures. Among males housed in same-sex groups, the average soma size in the MePD was significantly smaller in SD males than in LD males. Cohabitation with a female resulted in MePD volumes indistinguishable from LD males. These results indicate that the MePD, a nucleus implicated in socio-sexual behavior, can respond to photoperiodic as well as to social cues.

Androgens regulate the mammalian homologues of invertebrate sex determination genes tra-2 and fox-1

Androgens, like other steroid hormones, exert profound effects on cell growth and survival by modulating the expression of target genes. In vertebrates, androgens play a critical role downstream of the testis determination pathway, influencing the expression of sexually dimorphic traits. Among cells of the nervous system, motor neurons respond to trophic effects of androgen stimulation, with a subpopulation of spinal motor neurons exhibiting sexually dimorphic survival. To study the mechanisms of androgen action in these cells, we performed a subtractive screen for genes upregulated by androgen in a motor neuron cell line. We show androgen-inducible expression of two RNA-binding proteins that are the mammalian homologues of invertebrate sex determination genes. Androgens upregulate the expression of tra-2alpha, an enhancer of RNA splicing homologous to Drosophila tra-2, and promote redistribution of the protein from a diffuse to a speckled pattern within the nucleus. Similarly, androgens upregulate the expression of a novel gene homologous to Caenorhabditis elegans fox-1. These data indicate that androgens exert their effects, in part, by modulating the expression and function of genes involved in RNA processing, and identify homologues of invertebrate sex determination genes as androgen-responsive genes in mammals.

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.

Post-weaning social isolation of male rats reduces the volume of the medial amygdala and leads to deficits in adult sexual behavior

At 21 days of age, gonadally intact male Long Evans rats were weaned and placed into standard laboratory conditions (three per cage) or housed singly. They were tested for noncontact erections and sexual performance at 90 and 220 days of age. Rats raised in isolation displayed significantly fewer noncontact erections in response to sensory cues from an estrous female and fewer intromissions when allowed to mate with a female than did males raised in groups. The volume of the posterodorsal component of the medial amygdala (MePD) and the size of neurons within the MePD were significantly smaller in the isolated males than in socially housed males. Similarly, neurons in the sexually dimorphic nucleus of the preoptic area (SDN-POA) were smaller in isolate animals than in controls. As both MePD volume and SDN-POA soma size are responsive to sex steroids, these differences could result if the isolates experienced lower testosterone levels. Finally, the volume of the overall medial amygdala (MeA) correlated significantly with the number of noncontact erections, a relationship that was not explained by housing condition. These findings highlight the role of social experience as a factor in the sexual differentiation of the brain and suggest a positive relationship between the volume of a brain structure and the display of sexual behaviors.

The orthodox view of brain sexual differentiation

The standard view of sexual differentiation of the brain, derived primarily from work with mammals, is that the same steroidal signal which permanently masculinizes the body early in life, androgen, also permanently masculinizes the nervous system. This oversimplified view overlooks the rich diversity of mechanisms produced by natural selection. We review the mechanisms underlying sexual differentiation of what may be the simplest mammalian model, the spinal nucleus of the bulbocavernosus (SNB), a system that is intimately associated with sexual differentiation of the periphery. Indeed, in many instances, early androgen can permanently masculinize the SNB system but, surprisingly, these early influences may depend to some extent on social mediating factors. Furthermore, in adulthood, androgen continues to affect the SNB system in diverse ways, acting on several different loci, indicating a life-long plasticity in even this simple system. Finally, there is evidence that adult androgens interact with social experience in order to affect the SNB system. Thus the SNB system displays a far richer array of interactions than the standard view of sexual differentiation would predict. Examination of other systems and other species, as depicted in the following reports, reveals a far more complicated, and far more interesting perspective on how the brains and behaviors of males and females diverge.

Short day lengths affect perinatal development of the male reproductive system in the Siberian hamster, Phodopus sungorus

The Siberian hamster, Phodopus sungorus, breeds seasonally. In the laboratory, seasonal breeding can be controlled by photoperiod, which affects the duration of nightly melatonin secretion. Winterlike, short day lengths induce gonadal regression in adult animals, and pups born and maintained in short days undergo pubertal gonadal development later than animals born into long days. However, to date there have been no reports of gestational photoperiod affecting fetal development of reproductive systems. The spinal nucleus of the bulbocavernosus (SNB) and its target muscles, the bulbocavernosus (BC) and levator ani (LA), compose a sexually dimorphic, androgen-sensitive neuromuscular system involved in male reproduction. The SNB neuromuscular system was studied in male Siberian hamsters maintained from conception in short-day (8 h light, 16 h dark; 8L:16D) versus long-day (16L:8D) conditions. On the day of birth, and at postnatal (PN) days 2 and 18, the BC/LA muscles of hamsters gestated and raised in the short photoperiod were significantly reduced relative to those of their long-day counterparts. Testes weights were not significantly different between groups until day 18. Thus, photoperiod exposure during gestation and after birth affects perinatal development of the SNB system in this species, and these effects can be seen as early as the day of birth. Because photoperiod did not significantly affect testes weights until PN18, these results suggest that either perinatal photoperiod affects fetal androgen production without affecting testes weight or it influences BC/LA development independently from androgen.

A brain sexual dimorphism controlled by adult circulating androgens

Reports of structural differences between the brains of men and women, heterosexual and homosexual men, and male-to-female transsexuals and other men have been offered as evidence that the behavioral differences between these groups are likely caused by differences in the early development of the brain. However, a possible confounding variable is the concentration of circulating hormones seen in these groups in adulthood. Evaluation of this possibility hinges on the extent to which circulating hormones can alter the size of mammalian brain regions as revealed by Nissl stains. We now report a sexual dimorphism in the volume of a brain nucleus in rats that can be completely accounted for by adult sex differences in circulating androgen. The posterodorsal nucleus of the medial amygdala (MePD) has a greater volume in male rats than in females, but adult castration of males causes the volume to shrink to female values within four weeks, whereas androgen treatment of adult females for that period enlarges the MePD to levels equivalent to normal males. This report demonstrates that adult hormone manipulations can completely reverse a sexual dimorphism in brain regional volume in a mammalian species. The sex difference and androgen responsiveness of MePD volume is reflected in the soma size of neurons there.

Sex difference and laterality in the volume of mouse dentate gyrus granule cell layer

Sex differences in spatial learning have been reported in both humans and rodents. Correspondingly, there have been reports of sexual dimorphism in the morphology of the hippocampal formation (HF), a brain structure implicated in spatial cognition. In Experiment 1, we confirmed earlier reports that the overall volume of the granule cell layer (GCL) of the dentate gyrus (DG) of A/J mice is larger in males than in females. We also found that male A/J mice have a larger GCL volume in the right hemisphere than the left. Female A/J mice displayed no such laterality. A similar pattern of laterality, favoring the right HF, had been reported previously in male, but not female, rats. In Experiment 2, we examined mice with a defective structural gene for androgen receptors (testicular feminization mutant, or tfm mice) on a C57/BL6J background. The C57/J strain had not previously been examined for hippocampal sexual dimorphism. We found no sexual dimorphism in the left, right, or total volume of the GCL in C57/BL6J mice whether they were wildtype or tfm. However, the right GCL volume was greater than the left in wildtype C57/BL6J mice of either sex. No lateralization of GCL volume was found in the androgen-insensitive tfm-affected males or the partially androgen-insensitive tfm-carrier females. These findings confirm earlier reports that sexual dimorphism in mouse HF is found in some inbred strains but not others, and indicate for the first time that mouse HF structures are lateralized. The absence of lateralization in partially or wholly androgen-insensitive mice suggests that androgen receptors may play a role in development of laterality in the GCL independently of any sexual dimorphism in this structure.

Seasonal plasticity of neuromuscular junctions in adult male Siberian hamsters (Phodopus sungorus)

Transfer of adult Siberian hamsters, Phodopus sungorus, from long day (16 h light and 8 h dark; 16L:8D) to short day (8L:16D) photoperiods induces an involution of the gonads and a cessation of reproductive behavior 8-10 weeks later. The motoneurons of the spinal nucleus of the bulbocavernosus and their target muscles, the bulbocavernosus and the levator ani, are sexually dimorphic and are necessary for successful reproduction by male mammals. We demonstrate that after transfer of adult male Siberian hamsters to short photoperiods, the bulbocavernosus motoneurons, their target muscles and neuromuscular junctions are all significantly smaller than those of males that remain under long day conditions. Photoperiod also affected the number of active zones within each neuromuscular junction, an apparent remodeling of these synapses. Thus, this neuromuscular system of adult Siberian hamsters demonstrates considerable seasonal plasticity in response to changes in photoperiod.

Social cues attenuate photoresponsiveness of the male reproductive system in Siberian hamsters (Phodopus sungorus)

Transfer of adult Siberian hamsters (Phodopus sungorus) from long (16 h light and 8 h dark, 16L:8D) to short (8L:16D) daily photoperiods induces an involution of the gonads and a cessation of reproductive behavior 8 to 10 weeks later. However, when male and female long-day hamsters were paired on transfer to short photoperiods, the males' gonads did not undergo the typical short-day response. Similarly, when male long-day hamsters were paired with refractory females (i.e., females housed in short photoperiods for at least 28 weeks so that they became unresponsive to short photoperiods), the response of the males' reproductive system to short photoperiods also was attenuated. Thus, social cues can override or delay the effects of photoperiod on the testes of this species. These results suggest that the inhibitory effects of long durations of melatonin secretion (in response to short photoperiods) on the male hypothalamic-pituitary-gonadal axis may be attenuated by social cues such as contact with the opposite sex.

Sexual differentiation of the vertebrate brain: principles and mechanisms

A wide variety of sexual dimorphisms, structural differences between the sexes, have been described in the brains of many vertebrate species, including humans. In animal models of neural sexual dimorphism, gonadal steroid hormones, specifically androgens, play a crucial role in engendering these differences by masculinizing the nervous system of males. Usually, the androgen must act early in life, often during the fetal period to masculinize the nervous system and behavior. However, there are a few examples of androgen, in adulthood, masculinizing both the structure of the nervous system and behavior. In the modal pattern, androgens are required both during development and adulthood to fully masculinize brain structure and behavior. In rodent models of neural sexual dimorphism, it is often the aromatized metabolites of androgen, i.e., estrogens, which interact with estrogen receptors to masculinize the brain, but there is little evidence that aromatized metabolites of androgen play this role in primates, including humans. There are other animal models where androgens themselves masculinize the nervous system through interaction with androgen receptors. In the course of masculinizing the nervous system, steroids can affect a wide variety of cellular mechanisms, including neurogenesis, cell death, cell migration, synapse formation, synapse elimination, and cell differentiation. In animal models, there are no known examples where only a single neural center displays sexual dimorphism. Rather, each case of sexual dimorphism seems to be part of a distributed network of sexually dimorphic neuronal populations which normally interact with each other. Finally, there is ample evidence of sexual dimorphism in the human brain, as sex differences in behavior would require, but there has not yet been any definitive proof that steroids acting early in development directly masculinize the human brain.

Short day lengths delay development of the SNB neuromuscular system in the Siberian hamster, Phodopus sungorus

The Siberian hamster, Phodopus sungorus, breeds seasonally. In the laboratory, the seasonal breeding can be controlled by photoperiod, which affects the durations of nightly melatonin secretions. Winterlike short day lengths induce gonadal regression in adult animals, and pups born and maintained in short days undergo gonadal development much later than animals born into long days. The spinal nucleus of the bulbocavernosus (SNB) and its target muscles, the bulbocavernosus (BC) and levator ani (LA), comprise a sexually dimorphic, androgensensitive neuromuscular system involved in male reproduction. The SNB neuromuscular system was studied in male Siberian hamsters maintained from conception in short-day (8:16 h light/dark cycle) versus long-day (16:8 h light/dark cycle) conditions. At 40-47 days of age, development of three components of the SNB neuromuscular system were all significantly delayed in hamsters raised in the short photoperiod: BC/LA muscle weight, the size of SNB motoneuronal somata, and the area of the neuromuscular junctions at the BC/LA muscles of short-day hamsters were each significantly reduced relative to those of longday counterparts. Thus, development of the SNB reproductive system is delayed under short day lengths in this species.

Neonatal androgen and estrogen treatments masculinize the size of motoneurons in the rat spinal nucleus of the bulbocavernosus

1. Newborn female rats were injected with either a nonaromatizable androgen, dihydrotestosterone propionate (DHTP; 1 mg), or estrogen benzoate (EB; 100 micrograms), or both, or sesame oil vehicle only as a control. In the first experiment, females were injected only on the day of birth (day 1). In the second experiment, females were given daily injections on either days 1, 3, and 5 of life or on days 6, 8, and 10. At 60 days of age the animals were sacrificed and the size of the somata and nuclei of motoneurons in the spinal nucleus of the bulbocavernous (SNB) was determined. 2. As in earlier studies, neonatal EB had no effect on the adult numbers of SNB cells, and the present study demonstrated estrogen's ineffectiveness in this regard in either the absence or the presence of DHTP. Nor did neonatal estrogen influence the survival of the SNB target musculature. 3. In agreement with previous studies, early DHTP treatment resulted in more SNB cells in adulthood and both late and early neonatal treatment with DHTP also resulted in larger SNB cells in adulthood. 4. We report for the first time that neonatal EB treatment also resulted in larger adult SNB cells. EB exerted this effect after a single injection on the day of birth or after multiple injections during the early neonatal period (days 1-5) but not after late neonatal injections. 5. These data suggest that both androgens and estrogens normally act to masculinize the size of SNB motoneurons, while only androgens affect the number of SNB cells.

Androgen spares androgen-insensitive motoneurons from apoptosis in the spinal nucleus of the bulbocavernosus in rats

The spinal nucleus of the bulbocavernosus (SNB) is a sexually dimorphic motor nucleus in the rat lumbar spinal cord. The sex difference arises through the androgenic sparing of the motoneurons and their target muscles from ontogenetic cell death. Indirect evidence suggests that androgen acts on the target muscles rather than directly on SNB motoneurons to spare them from death. The testicular feminization mutation (Tfm), a defect in the androgen receptor (AR), blocks androgenic sparing of SNB motoneurons and their targets. The pattern of AR immunocytochemistry was previously found to be different in adult Tfm and wild-type rats: immunostaining was nuclear in most SNB cells of wild-type rats, but very few SNB cells display nuclear AR immunostaining in affected Tfm rats. Because the Tfm mutation is carried on the X chromosome, random X inactivation during development makes female carriers of Tfm (+/Tfm) genetic mosaics for androgen sensitivity. Tfm carriers, their wild-type sisters, and affected Tfm males were treated with perinatal testosterone and immunocytochemistry was used to detect androgen receptor in the SNB when the rats reached adulthood. Mosaic females could be distinguished from their wild-type sisters by external morphology. In such perinatally androgenized mosaics, adult SNB cells were equally divided between wild-type and Tfm genotype, as indicated by AR immunocytochemistry. In contrast, the pattern of AR immunocytochemistry in target muscles of mosaics appeared similar to that of wild-type females. These results indicate that early androgen spared both androgen-sensitive and -insensitive motoneurons from cell death, confirming a site of androgen action other than the motoneurons themselves.

Sexual dimorphism of perineal muscles and motoneurons in spotted hyenas

Female spotted hyenas are known for their male-like genitalia, high levels of aggression, and dominance over males, characteristics which are attributed to exposure to elevated levels of testosterone in utero. Although the nervous system of spotted hyenas has not previously been examined, one might predict that neural systems which are sexually dimorphic in other mammals would be monomorphic in this species. Spinal motoneurons which innervate muscles associated with the phallus are located in Onuf's nucleus and are more numerous in males than in females in a wide array of mammals. Onuf's nucleus was examined in adult and neonatal spotted hyenas and, contrary to expectation, was found to be sexually dimorphic in the typical mammalian pattern: Males have significantly more motoneurons in Onuf's nucleus than do females. This dimorphism was correlated with a previously undescribed dimorphism in the relevant target musculature. Specifically, the morphology of the bulbocavernosus muscle is distinctly different in male and female spotted hyenas. Pregnant hyenas were treated with anti-androgen in an attempt to interfere with the actions of androgen during fetal development. Motoneuron number in Onuf's nucleus and the morphology of the bulbocavernosus muscle were feminized in males exposed to anti-androgen in utero.

Ciliary neurotrophic factor arrests muscle and motoneuron degeneration in androgen-insensitive rats

Steroid hormones and neurotrophic factors exert profound and widespread effects on the developing nervous system, including regulation of the size, connectivity, and survival of neurons. Androgenic control of the survival of motoneurons in the spinal nucleus of the bulbocavernosus (SNB) of rats has been well documented. We previously found that ciliary neurotrophic factor (CNTF) mimics many effects of androgen on the developing SNB. Whether effects of CNTF depend on the presence of a functional androgen receptor was evaluated in the present study. Androgen-insensitive male rats bearing the testicular feminization mutation, Tfm, and female litter-mates were treated with CNTF or with vehicle alone from embryonic day 22 through postnatal day 3. On postnatal day 4 SNB cell number was elevated in both groups receiving CNTF. Volumes of the bulbocavernosus (BC) and levator ani (LA) muscles, targets of SNB motoneurons, were also markedly increased by CNTF. Since the BC appears to degenerate completely in untreated females, these results indicate that CNTF can delay or prevent muscle fiber death. The relative sparing of muscles and motoneurons did not differ for Tfm males and females, demonstrating that effects of CNTF on the SNB neuromuscular system do not require functional androgen receptors.

Motoneurons innervating guinea pig perineal muscles are sexually dimorphic in size but not number

Sexual differentiation occurs prenatally in guinea pigs but extends into the postnatal period in rats. Steroids affect the development of two motoneuron nuclei of the rat lumbar spinal cord that innervate sexually dimorphic perineal muscles. The spinal nucleus of the bulbocavernosus (SNB) innervates the bulbocavernosus (BC) and levator ani (LA) muscles while the dorsolateral nucleus (DLN) innervates the ischiocavernosus (IC). In male rats, perinatal testosterone prevents degeneration of these muscles and results in a sex difference in both motoneuron size and number in adulthood. For comparative purposes, we examined the guinea pig motoneurons innervating these muscles, as well as those innervating the retractor penis (RP) and retractor clitoris (RC), muscles that have no counterpart in rats. Injections of horseradish peroxidase localized the BC/LA and IC motoneurons of guinea pigs to discrete columns in spinal levels L6 and S1, with the BC/LA motoneurons occupying a more medial position. The RP/RC motoneurons were found in L5. Motoneuronal soma area was larger in males in all examined motor pools, as was nuclear area of BC/LA and IC motoneurons. Although raw counts suggested a sex difference in cell number in the motor columns containing BC/LA and IC motoneurons, either of two different correction procedures for split nuclei error eliminated the sex difference in cell number, emphasizing the importance of such corrections when comparing neurons of different size.

Androgen alters the dendritic arbors of SNB motoneurons by acting upon their target muscles

In adult male rats, motoneurons of the spinal nucleus of the bulbocavernosus (SNB) have been shown to retract and reextend their dendritic branches in response to systemic androgen deprivation and readministration. Furthermore, other studies have suggested that the dendritic complexity of neurons can be regulated by their targets. To assess whether androgens might act upon the target muscles to mediate changes in SNB dendrites, adult male rats were castrated and implanted with a small capsule filled with testosterone (T) next to the bulbocavernosus and levator ani muscle complex (BC/LA) on one side, while the muscles on the contralateral side were implanted with another capsule containing hydroxyflutamide (hFl), an anti-androgen. We have previously shown that after 30 d of this focused, lateralized androgen treatment the BC/LA complex is significantly larger on the T-treated side. We now report that the total dendritic lengths of SNB motoneurons innervating muscles given androgen blockade are reduced by 44% compared to SNB motoneurons innervating muscles given androgen stimulation. Dendrite lengths within three regions of the spinal cord were altered in a nonuniform manner: large changes occurred in the dorsal and contralateral dendritic fields while there was no difference in the ipsilateral dendritic field. These results suggest that BC/LA muscles, in response to androgen stimulation, produce a trophic substance which regulates the dendritic organization of SNB motoneurons in adulthood.

Distribution of androgen receptor immunoreactivity in the spinal cord of wild-type, androgen-insensitive and gonadectomized male rats

The polyclonal antiserum PG21 was used to detect androgen receptor (AR) protein in three motoneuronal pools of the male rat lumbar spinal cord. In gonadally intact, wild-type males, the spinal nucleus of the bulbocavernosus (SNB), dorsolateral nucleus (DLN), and retrodorsolateral nucleus (RDLN) all displayed immunolabeling of cell nuclei. The percentage of motoneurons displaying such labeling was highest in the SNB and lowest in the RDLN. This pattern of AR immunocytochemical labeling agrees well with previous steroid autoradiographic studies of androgen accumulation in the rat spinal cord. In contrast, virtually no motoneurons in any of the three pools displayed nuclear AR immunostaining in long-term gonadectomized males or in gonadally intact males carrying the Tfm mutation, which renders the AR incompetent. In gonadectomized males, labeling was restored in the SNB and DLN, but not the RDLN, 30 min after an injection of replacement testosterone. Eight hours of testosterone exposure restored immunolabeling in all three motor nuclei. Apparent cytoplasmic staining was seen in SNB motoneurons of untreated castrates and Tfm rats, but not intact rats, suggesting that AR residing in the cytoplasm translocates to the nucleus on binding to androgen in these motoneurons.

Ciliary neurotrophic factor maintains motoneurons and their target muscles in developing rats

Ciliary neutrophic factor (CNTF) can enhance motoneuron survival during naturally occurring cell death in the chick (Oppenheim et al, 1991). Because receptors for CNTF are expressed in both motoneurons and their target muscles (Davis et al., 1991; lp et al., 1993), both tissues are potential sites of CNTF action in development. We examined the ability of CNTF to prevent the degeneration of a neuromuscular system in developing female rats. The death of motoneurons in the spinal nucleus of the bulbocavernosus (SNB) extends postnatally and is sexually dimorphic, with many more motoneurons dying in females than in males. The bulbocavernosus (BC), a target muscle of SNB motoneurons, also degenerates postnatally in females. Female rats treated with daily injections of 1 microgram CNTF from embryonic day 22 through postnatal day 3 (P3) had 70% more SNB motoneurons on P4 than did control animals, and the number of pyknotic profiles in the SNB area was markedly reduced by CNTF. In addition, the degeneration of the BC was completely prevented by CNTF treatment of perinatal female rats. These results demonstrate that CNTF can preserve mammalian motoneurons from developmental death, but also suggest that the sparing effect of CNTF on motoneurons in vivo may be a secondary consequence of effects on target muscles.

Ontogeny of calcitonin gene-related peptide immunoreactivity in rat lumbar motoneurons: delayed appearance and sexual dimorphism in the spinal nucleus of the bulbocavernosus

Immunoreactivity for calcitonin gene-related peptide (CGRP) has been observed in both adult and embryonic rat motoneurons. However, the developmental pattern of CGRP expression in motoneurons has not been systematically examined and the role of CGRP in neuromuscular development is poorly understood. We have mapped the ontogeny of CGRP-like immunoreactivity in three motoneuron pools of the rat lumbar spinal cord from birth through adulthood. Immunoreactivity was uniformly high in lateral horn motoneurons (the retrodorsolateral nucleus) of males and females at all ages examined. The majority of motoneurons of the dorsolateral nucleus also were positive throughout postnatal development although the percentage of positive motoneurons was slightly higher in males than in females. In contrast, virtually no motoneurons of the spinal nucleus of the bulbocavernosus, located in the medial ventral horn, were positive for CGRP in neonatal rats. CGRP-like immunoreactivity was delayed in this nucleus until approximately postnatal day 6 in males and day 27 in females. Because these three motoneuronal nuclei are differentially sensitive to early androgen and differ with respect to the timing of several developmental milestones, these observations have implications for the regulation and possible roles of CGRP in developing motor systems.

Brain lesions affect penile reflexes

Electrolytic lesions of several potential brain afferents to the spinal nucleus of the bulbocavernosus (SNB) affect the display of penile reflexes. Ablation of the median and pontine raphe areas significantly potentiates the expression of cups and flips. Animals with a bilateral lesion of the paraventricular nucleus of the hypothalamus have a shorter latency to the first erection but otherwise display normal reflex behavior. Although bilateral destruction of the lateral vestibular nucleus (LVN) completely eliminated penile reflex activity, it also caused significant motor impairment thus clouding conclusions concerning the normal role of the LVN in penile reflex behavior. These and other results support the hypothesis that these brain regions which project to the SNB region normally modulate spinal reflex behavior of the rat penis.

Hormone-sensitive periods for the control of motoneuron number and soma size in the dorsolateral nucleus of the rat spinal cord

The dorsolateral nucleus (DLN) of the rat lumbosacral spinal cord is sexually dimorphic, with males having more and larger DLN motoneurons than do females. The development of this dimorphism depends on the presence of perinatal androgens. The present study sought to determine the periods in development during which the DLN is sensitive to the masculinizing effects of the androgen testosterone propionate (TP). The size and number of DLN motoneurons in neonatally ovariectomized female rats that were exposed to TP during either the late prenatal, early postnatal, or late postnatal period were compared to control males and females. Both late prenatal and early postnatal TP injections significantly increased DLN number by 48% and 50%, respectively, but the sensitive period for TP masculinization of soma size seems to be primarily postnatal, because prenatal TP injections had little or no effect on that measure. The sensitive period for TP masculinization of DLN neuron number is similar to that of the sexually dimorphic spinal nucleus of the bulbocavernosus (SNB). However, the sensitive period for TP masculinization of DLN soma size appears to begin later than for the SNB.

Local perineal implants of anti-androgen block masculinization of the spinal nucleus of the bulbocavernosus

Female rats were injected with testosterone propionate on the 1st and 3rd days of life. In addition, some females received 200 micrograms of the anti-androgen, hydroxyflutamide. Females receiving anti-androgen directly to the perineum, including the muscles bulbocavernosus and levator ani, had fewer surviving spinal nucleus of the bulbocavernosus (SNB) motoneurons than did females receiving the anti-androgen systemically. These results indicate that androgen acts upon the target muscles to spare developing SNB motoneurons from death.

Regulation of motoneuron death in the spinal nucleus of the bulbocavernosus

A sexual dimorphism in the number of motoneurons in the spinal nucleus of the bulbocavernosus (SNB) of rats is engendered by a sex difference in ontogenetic cell death. Testicular secretions, specifically androgenic steroids, reduce SNB motoneuron death in males. The fate of the target muscles generally mirrors that of the motoneurons, and androgens appear to exert their effects upon the target muscles, sparing the motoneurons as a secondary consequence. Treatment with ciliary neurotrophic factor can also spare SNB motoneurons in newborn females, raising the possibility that this factor normally mediates androgen's effect upon motoneuron survival. The ontogeny of calcitonin gene-related peptide immunoreactivity is delayed in SNB cells compared with other motoneurons and is further delayed in the SNB cells of females. In both sexes, calcitonin gene-related peptide is detected after the period of SNB motoneuron death is complete. A sex difference in motoneuron number is also seen in the human homologue of the SNB and, because ontogenetic death of motoneurons in humans overlaps the period of androgen secretion, may arise in a manner similar to that in the rat SNB.

The role of the bulbocavernosus in penile reflex behavior in rats

The sexually dimorphic spinal nucleus of the bulbocavernosus (SNB) innervates the bulbocavernosus (BC) and levator ani (LA), striated perineal muscles. By transecting the pudendal nerve just proximal to the BC muscle, we removed spinal input to both muscles while leaving them mechanically intact. Such denervation of the BC/LA muscles virtually eliminated the display of reflexive penile cups, while having no significant effect on penile erections, or flips.

Differential effects of testosterone metabolites upon the size of sexually dimorphic motoneurons in adulthood

This study examined the effect of testosterone and two of its metabolites on the size of motoneurons in the sexually dimorphic spinal nucleus of the bulbocavernosus (SNB) in adult male rats. Treatment of castrates with either testosterone or dihydrotestosterone maintained SNB cell size, although testosterone was more effective in this regard. However, estradiol, either alone or in conjunction with dihydrotestosterone treatment, had no effect on the size of the somata or nuclei of SNB motoneurons. These results indicate that testosterone affects SNB cell size by interacting with androgen receptors and that aromatized metabolites of testosterone are not involved in this aspect of motoneuronal plasticity in adulthood. Because the penile reflexes mediated by the SNB neuromuscular system are also sensitive to androgen but not estrogen treatment, morphological changes in SNB cells may contribute to the androgenic modulation of these reflexes.

Androgen locally regulates rat bulbocavernosus and levator ani size

Adult male rats were gonadectomized, and small Silastic capsules filled with hormone were sutured to each bulbocavernosus and levator ani muscle complex (BC/LA). In the first experiment, one capsule contained testosterone (T), while the capsule on the contralateral muscles contained the antiandrogen hydroxyflutamide (hFl). The intent of this treatment was to provide a focus of androgenic stimulation to the muscles on one side. After 30 days, animals were sacrificed, and the BC/LA muscle pairs were removed, weighed, and compared. BC/LAs receiving T treatment were heavier than those receiving hFl treatment (p less than 0.0001), with an average weight difference of 12%. Muscle fibers from T-treated BCs were significantly larger in diameter than those from contralateral, hFl-treated BCs. These results indicate that androgen exerts its anabolic effect by acting locally upon a cell population within or near the BC/LA. When hFl and blank capsules were implanted in castrated males, the hFl-treated muscles were significantly heavier (by 9%), demonstrating an anabolic effect of hFl in the absence of androgen, and refuting the idea that hFl may have caused local toxic effects in the first experiment. Gonadectomized animals given T versus blank capsules had T-treated muscles that were 8% heavier than the blank-treated side. Muscle weights were also compared in animals receiving bilateral denervation of the BC/LA at the time of T and hFl capsule implantation and gonadectomy; local testosterone treatment failed to affect BC/LA weights in these denervated muscles.

Ontogeny of steroid accumulation in spinal lumbar motoneurons of the rat: implications for androgen's site of action during synapse elimination

Androgens influence the postnatal development of motoneurons in the spinal nucleus of the bulbocavernosus (SNB) by regulating neuromuscular synapse elimination, the process through which multiple axonal inputs are retracted from each muscle fiber until single innervation is established. In the rat levator ani (LA), one of the target muscles for SNB motoneurons, much of this loss of multiple innervation can be prevented by prepubertal androgen treatment. We used steroid autoradiography to measure the ontogeny of steroid accumulation in the SNB and the retrodorsolateral nucleus (RDLN), two motoneuronal groups thought to differ in their sensitivity to androgens. Spinal cord tissue was analyzed from castrated male rats at 7, 14, 21, and 60 days of age after injection of radiolabelled testosterone. SNB and RDLN motoneurons differ in the ontogeny of androgen accumulation. Over 80% of SNB motoneurons develop the capacity to accumulate androgen during the second week after birth, during the period when androgen regulates synapse elimination in the LA. In contrast, androgen accumulation in RDLN motoneurons develops much later (after 21 days). These data suggest that androgen may act directly on SNB motoneurons to influence synapse elimination.

Sexual dimorphism and androgen effects on spinal motoneurons innervating the rat flexor digitorum brevis

The foot muscle, flexor digitorum brevis, is innervated by motoneurons in the retrodorsolateral nucleus of the lumbar spinal cord in rats. We found this muscle to be sexually dimorphic, but insensitive to the anabolic or catabolic effects of androgen manipulation in adulthood: the flexor digitorum brevis was larger in adult male rats than in females, with no decrease in mass after castration in males nor any increase in size after androgen treatment of ovariectomized females. The cross-sectional area of motoneurons innervating this muscle was also sexually dimorphic, i.e., the motoneurons were larger in males. In contrast to the absence of an androgen effect on target muscle size, however, cross-sectional area of motoneurons decreased in adult males as a result of castration, and increased in adult females after androgen treatment. The dissociation of androgen effects on muscle mass and motoneuron size suggests the possibility of steroid effects upon motoneurons independent of effects upon target musculature.

Does androgen affect axonal transport of cholera toxin HRP in spinal motoneurons?

We examined the effect of systemic androgen levels upon the rate at which lumbosacral motoneurons are labeled with cholera toxin-conjugated horseradish peroxidase (CT-HRP) injected into target muscles. CT-HRP first reaches the spinal nucleus of the bulbocavernosus between 8 and 10 h after injection into the bulbocavernosus muscle of adult male rats, but the number of motoneurons filled with CT-HRP does not differ between androgen-treated and control castrates at any of the time points examined. Thus, contrary to current speculation, we found no evidence that androgen can affect retrograde transport of CT-HRP by rat motoneurons.

Brain sites projecting to the spinal nucleus of the bulbocavernosus

Motoneurons of the spinal nucleus of the bulbocavernosus (SNB) occupy a distinct dorsomedial position in the ventral horn of lumbar segments 5 and 6 and innervate sexually dimorphic striated muscles of the rat perineum, including the bulbocavernosus and levator ani. To begin the study of brain influences upon SNB function, we used retrograde tracers to identify brain regions that project to the area of SNB motoneurons. Our findings provide strong evidence that lateral vestibular and several reticular nuclei innervate the SNB. Additional possible afferents include the superior and medial vestibular nuclei, raphe nucleus, red nucleus, interstitial nucleus of the medial longitudinal fasciculus, and paraventricular nucleus.

Neuromuscular junctions shrink and expand as muscle fiber size is manipulated: in vivo observations in the androgen-sensitive bulbocavernosus muscle of mice

Neuromuscular synapses in an androgen-sensitive muscle of sexually mature male mice were repeatedly observed over several-month intervals in normal animals and in animals in which testosterone levels were manipulated. In normal bulbocavernosus muscles, pre- and postsynaptic regions of neuromuscular junctions enlarge as muscle fibers grow. After castration, junctional area decreased in parallel with muscle fiber atrophy. When testosterone was resupplied to castrated animals, junctions that previously decreased in size then enlarged in parallel with muscle fiber hypertrophy. Surprisingly, these size changes occurred without loss or addition of motor nerve terminal branches or acetylcholine (ACh) receptor regions. Rather, each nerve terminal branch and underlying receptor region became smaller following castration and reenlarged following testosterone treatment. Several lines of evidence argued that the size changes observed after castration and testosterone treatment were secondary to shrinkage and stretching of the postsynaptic muscle fiber membrane. Following castration, the spaces between synaptic regions decreased in size at the same time and to a similar extent as the regions themselves. Following testosterone replacement, the spaces between synaptic regions expanded and each existing ACh receptor region enlarged. Ultrastructural analysis showed that there was no loss or addition of postsynaptic secondary junctional folds in the muscle fiber membrane (where ACh receptors are located) as junctions shrank and expanded. Rather, folds became more densely packed as muscle fibers atrophied following castration and less densely packed as muscle fibers hypertrophied following testosterone replacement. From these studies of the bulbocavernosus muscle, as from our previous studies of the sternomastoid muscle, we conclude that neuromuscular junction size is directly coupled to muscle fiber size. Androgens modulate muscle fiber volume directly, leading to a change in the surface area of the muscle fiber membrane, which in turn causes the postsynaptic specializations to shrink or expand. The concomitant shrinkage and stretching of motor nerve terminals that we observed can only be accounted for by their adhesion to postsynaptic specializations that are also changing size. Thus adhesion, rather than an interchange of diffusible factors, trophic or otherwise, is likely to be the primary determinant of coordinated pre- and postsynaptic enlargement in growing mammalian skeletal muscles.

Lineage, arrangement, and death of clonally related motoneurons in chick spinal cord

We have used recombinant retroviruses as lineage markers to study the genealogy of motoneurons (MNs) in the chick spinal cord. We infected individual progenitors by injecting virions into the neural tube at stages 11-18, a few cell divisions before MNs are born. The descendants of infected cells were subsequently detected with a histochemical stain for beta-galactosidase (lacZ), the product of the retrovirally introduced gene. Clonally related, lacZ-positive cells formed clusters that were usually radial or planar in shape. The cells that comprised these clones were classified by morphology, size, and location. About 15% of the clones in the spinal cord contained MNs, and these were studied further. Multicellular clones that contained only MNs were infrequent. Instead, close relatives of MNs included a variety of other neurons, as well as glia and ependymal cells. Most non-MNs in these clones were found in the ventral and intermediate parts of the spinal cord. Neurons included interneurons and autonomic preganglionic neurons in the column of Terni. Labeled glia were found in both the gray and white matter and included astrocytes and cells tentatively identified as oligodendrocytes. Thus, even shortly before MNs are born, their progenitors are multipotential. Clonally related MNs were not restricted to a single motor pool. Some clones contained MNs in both the medial and lateral parts of the lateral motor column, which are known to innervate distinct groups of limb muscles. Furthermore, some clones contained MNs in the medial motor column (which innervate axial muscles) as well as in the lateral motor column. In contrast, the dispersal of clonally related MNs (and other neurons) was restricted in the rostrocaudal axis; most clones were less than one-quarter segment in length. Thus, MNs derived from a single progenitor are more likely to share rostrocaudal position than synaptic targets. To investigate the fate of clonally related MNs, we counted the number of MNs per clone at times before, during, and after the major period of MN death. The number of MNs per clone declined in precise parallel with the total number of MNs during this period, suggesting that neurons are eliminated without regard to their clone of origin. This result implies that the decision to live or die occurs on a cell-by-cell rather than a clone-by-clone basis.

Evidence for androgen receptors in sexually dimorphic perineal muscles of neonatal male rats. Absence of androgen accumulation by the perineal motoneurons

During development, survival of the sexually dimorphic spinal nucleus of the bulbocavernosus (SNB) and its target perineal muscles, the bulbocavernosus (BC) and the levator ani (LA) is androgen-dependent. To define androgen's site of action in masculinizing SNB system structures, we examined whether or not androgen receptors are present in SNB motoneurons and/or BC/LA muscles of neonatal male rats. Using a receptor binding assay, we have identified androgen-binding factors in the neonatal BC/LA (Bmax = 13.5 fmol/mg protein; Kd = 4.69 nM) for the first time. In contrast, androgen autoradiography provided no evidence that neonatal spinal motoneurons accumulate androgens. These results support the hypothesis that BC/LA muscles are a primary site of androgen action for masculinizing SNB system structures, and that androgen need not interact with SNB motoneurons directly to sexually differentiate them.

Neonatal androgen maintains sexually dimorphic muscles in the absence of innervation

We examined the site of androgen action in maintaining the sexually dimorphic spinal nucleus of the bulbocavernosus (SNB) and its target perineal muscles, the bulbocavernosus (BC) and levator ani (LA), in rats. To determine whether androgen action on SNB motoneurons is crucial for BC/LA survival, we removed SNB cells in newborn female rats by lumbosacral spinalectomy, administered testosterone propionate (TP) on days 1 and 3 of life, and examined for the presence of BC/LA muscles in adulthood. BC/LA muscles were present in all TP-treated spinalectomized females, and staining of these muscles with alpha-bungarotoxin or for acetylcholinesterase showed no evidence of cholinergic innervation. Thus, complete neonatal denervation of the BC/LA does not prevent TP from sparing these muscles, suggesting that androgen acts directly upon BC/LA muscles to maintain them during development. This androgenic maintenance of the BC/LA may be crucial for the survival of SNB motoneurons.

Motoneuronal death during human fetal development

The total number of ventral horn motoneurons throughout the spinal cord was determined for 19 human fetuses ranging in age from 11 to 32 menstrual weeks. There was a significant (approximately 35%) decline in motoneuron number between wks 11 and 25 of gestation, but no further decline from wks 25-32. Counts of pyknotic cells indicated a peak of motoneuronal degeneration between about wks 12 and 16 of age. The normal period of motoneuronal death observed here overlaps with the initiation of functional neuromuscular contact as well as the period of androgen production by the human fetal testes. As in rats, androgen may influence final motoneuron number in the human spinal cord by attenuating cell death in sexually dimorphic motor nuclei.

Ontogeny of functional innervation of bulbocavernosus muscles in male and female rats

Electrical stimulation of the sixth lumbar ventral root reliably elicited contractions of the bulbocavernosus muscle (BC) in untreated female rats on the day before birth. This functional contact indicates that testosterone probably does not act to save the spinal nucleus of the bulbocavernosus (SNB) and its BC target muscles through the establishment of an active neuromuscular synapse, since this is already present in animals with an SNB system which is fated to die.

Seasonal variation in mammalian striated muscle mass and motoneuron morphology

Feral white-footed mice are seasonal breeders that undergo predictable cycles of reproductive function. Photoperiod-induced fluctuations in gonadal function of white-footed mice were associated with morphological changes in perineal muscles and their motoneurons. Exposure to short daylengths resulted in testicular regression, decreased perineal muscle mass, and shrinkage of somata and nuclei of motoneurons of the spinal nucleus of the bulbocavernosus (SNB). These effects were reversed by reinstatement of long daylengths. Similar reductions in muscle mass and SNB soma size were seen following gonadectomy of white-footed mice. In addition, dendritic trees of SNB motoneurons were reduced in gonadectomized mice compared with dendritic arbors of intact mice or castrates provided with testosterone capsules. Androgen-mediated annual changes in muscle mass and motoneuron morphology appear to be a natural part of this species' physiology.

Sexual dimorphism in human and canine spinal cord: role of early androgen

Onuf's nucleus, located in the sacral spinal cord of dogs, cats, and primates, innervates perineal muscles involved in copulatory behavior. A sexual dimorphism in Onuf's nucleus was found in humans and dogs: males have significantly more motoneurons in this nucleus than do females. Prenatal androgen treatment of female dogs eliminated the dimorphism. In the homologous nucleus in rats, a similar effect of androgen has been shown to involve sparing of motoneurons from cell death. These results establish a morphological sex difference in a human central nervous system region of known function; well-studied animal models suggest explanations of the development of this dimorphism.

Cellular analyses of hormone influence on motoneuronal development and function

The striated bulbocavernosus (BC) muscles of the rodent perineum are innervated by motoneurons in the spinal nucleus of the bulbocavernosus (SNB). In adulthood, the BC muscles are present in males only. However, newborn female rats have BC muscles, and SNB cells have made both anatomical and functional contact with them. Nevertheless, both motoneurons and muscles will degenerate unless androgens are administered perinatally. Such androgen treatment appears to be acting primarily on the BC muscles themselves, since the muscles are spared by androgen even after the loss of supraspinal neural afferents or even the entire lumbosacral spinal cord. Furthermore, androgen can spare SNB motoneurons that are themselves androgen insensitive. Perinatal steroid treatments can also alter the final spinal location of SNB cells as determined by retrograde tracing studies. Androgen continues to modify the morphology of the SNB system in adulthood, altering the size of both motoneurons and targets, which may be important for the reproductive function of BC muscles. Finally, the sexually dimorphic character of motoneuronal groups innervating perineal muscles seems to be common in mammals, since the homologue of the SNB, Onuf's nucleus, has more cells in males than in females in both dogs and humans.