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

Cellular and molecular mechanisms of sexual differentiation in the mammalian nervous system

Neuroscientists are likely to discover new sex differences in the coming years, spurred by the National Institutes of Health initiative to include both sexes in preclinical studies. This review summarizes the current state of knowledge of the cellular and molecular mechanisms underlying sex differences in the mammalian nervous system, based primarily on work in rodents. Cellular mechanisms examined include neurogenesis, migration, the differentiation of neurochemical and morphological cell phenotype, and cell death. At the molecular level we discuss evolving roles for epigenetics, sex chromosome complement, the immune system, and newly identified cell signaling pathways. We review recent findings on the role of the environment, as well as genome-wide studies with some surprising results, causing us to re-think often-used models of sexual differentiation. We end by pointing to future directions, including an increased awareness of the important contributions of tissues outside of the nervous system to sexual differentiation of the brain.

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.

Neuroprotective actions of androgens on motoneurons

Androgens have a variety of protective and therapeutic effects in both the central and peripheral nervous systems. Here we review these effects as they related specifically to spinal and cranial motoneurons. Early in development, androgens are critical for the formation of important neuromuscular sex differences, decreasing the magnitude of normally occurring cell death in select motoneuron populations. Throughout the lifespan, androgens also protect against motoneuron death caused by axonal injury. Surviving motoneurons also display regressive changes to their neurites as a result of both direct axonal injury and loss of neighboring motoneurons. Androgen treatment enhances the ability of motoneurons to recover from these regressive changes and regenerate both axons and dendrites, restoring normal neuromuscular function. Androgens exert these protective effects by acting through a variety of molecular pathways. Recent work has begun to examine how androgen treatment can interact with other treatment strategies in promoting recovery from motoneuron injury.

The organizational hypothesis and final common pathways: Sexual differentiation of the spinal cord and peripheral nervous system

In honor of the 50th anniversary of the "organizational hypothesis," this paper reviews work on sexual differentiation of the spinal cord and peripheral nervous system. Topics considered include the spinal nucleus of the bulbocavernosus, the ejaculation center, the cremaster nucleus, sensory and autonomic neurons, and pain. These relatively simple neural systems offer ample confirmation that early exposure to testicular hormones masculinizes the nervous system, including final common pathways. However, I also discuss findings that challenge, or at least stretch, the organizational hypothesis, with important implications for understanding sex differences throughout the nervous system.

The spinal nucleus of the bulbocavernosus: firsts in androgen-dependent neural sex differences

Cell number in the spinal nucleus of the bulbocavernosus (SNB) of rats was the first neural sex difference shown to differentiate under the control of androgens, acting via classical intracellular androgen receptors. SNB motoneurons reside in the lumbar spinal cord and innervate striated muscles involved in copulation, including the bulbocavernosus (BC) and levator ani (LA). SNB cells are much larger and more numerous in males than in females, and the BC/LA target muscles are reduced or absent in females. The relative simplicity of this neuromuscular system has allowed for considerable progress in pinpointing sites of hormone action, and identifying the cellular bases for androgenic effects. It is now clear that androgens act at virtually every level of the SNB system, in development and throughout adult life. In this review we focus on effects of androgens on developmental cell death of SNB motoneurons and BC/LA muscles; the establishment and maintenance of SNB motoneuron soma size and dendritic length; BC/LA muscle morphology and physiology; and behaviors controlled by the SNB system. We also describe new data on neurotherapeutic effects of androgens on SNB motoneurons after injury in adulthood.

Androgen-dependent pathology demonstrates myopathic contribution to the Kennedy disease phenotype in a mouse knock-in model

Kennedy disease, a degenerative disorder characterized by androgen-dependent neuromuscular weakness, is caused by a CAG/glutamine tract expansion in the androgen receptor (Ar) gene. We developed a mouse model of Kennedy disease, using gene targeting to convert mouse androgen receptor (AR) to human sequence while introducing 113 glutamines. AR113Q mice developed hormone and glutamine length-dependent neuromuscular weakness characterized by the early occurrence of myopathic and neurogenic skeletal muscle pathology and by the late development of neuronal intranuclear inclusions in spinal neurons. AR113Q males unexpectedly died at 2-4 months. We show that this androgen-dependent death reflects decreased expression of skeletal muscle chloride channel 1 (CLCN1) and the skeletal muscle sodium channel alpha-subunit, resulting in myotonic discharges in skeletal muscle of the lower urinary tract. AR113Q limb muscles show similar myopathic features and express decreased levels of mRNAs encoding neurotrophin-4 and glial cell line-derived neurotrophic factor. These data define an important myopathic contribution to the Kennedy disease phenotype and suggest a role for muscle in non-cell autonomous toxicity of lower motor neurons.

Androgens regulate the sexually dimorphic production of co-contained galanin and cholecystokinin in lumbar laminae VII and X neurons

A population of rat lumbar laminae VII and X putative spinothalamic (STT) neurons that co-contain cholecystokinin-8 (CCK) and galanin (GAL) are sexually dimorphic. Males have a significantly greater number of these neurons, as well as having greater optical densities for both neuropeptides than females. Optical densities for GAL and CCK immunoreactivities in these lumbar neurons in rats that have the testicular feminization mutation (Tfm) are not significantly different from females; however, the number of these lumbar neurons in Tfm rats is significantly smaller than in females. These data suggest that androgens, as well as functional androgen receptors (that Tfm rats lack), are necessary for the establishment of these sexual dimorphisms. Functionally, these CCK- and GAL-containing neurons in the deep lumbar laminae may contribute to the establishment of known sex differences in the affective component of somatic and visceral nociception, as well as the sexually dimorphic nature of some pelvic diseases, e.g., irritable bowel syndrome or cystitis.

Cell death and sexual differentiation of the nervous system

Sex differences in nuclear volume or neuron number often are attributed to the hormonal control of cell death. In the spinal nucleus of the bulbocavernosus, the central portion of the medial preoptic nucleus, and the principal nucleus of the bed nucleus of the stria terminalis testicular hormones decrease cell death during perinatal life, resulting in a male advantage in neuron number in adulthood. Conversely, males have more dying cells during development and fewer neurons in adulthood than do females in the anteroventral periventricular nucleus of the hypothalamus. This review discusses several limitations and unresolved issues in the literature on sexually dimorphic cell death, and identifies molecular mechanisms by which gonadal steroids may control cell survival. In particular, evidence is presented for the hormonal regulation of neurotrophic factors and involvement of Bcl-2 family proteins in the determination of sex differences in neuron number.

Expression of nuclear receptor coactivators in androgen-responsive and -unresponsive motoneurons

Adult rat lumbar motoneurons in the spinal nucleus of the bulbocavernosus (SNB) respond to androgens with an increase in soma size. This response is mediated by the androgen receptor (AR) in these motoneurons. Interestingly, other lumbar motoneurons in the rat possess the AR, yet do not respond to androgens in this fashion. This paradox suggests the existence and participation of nuclear receptor coregulators in conferring direct androgen-responsiveness to select motoneurons in the adult rat spinal cord. Nuclear receptor coregulators have received much attention recently for their proposed role in enhancing or repressing the transcriptional activity of steroid hormone receptors. The present study used immunocytochemistry to identify a number of nuclear receptor coactivators that are expressed by adult lumbar motoneurons: SRC-1, SRC-2, CBP, p300, and cJUN. Results of this study indicate that all five of these coactivators are abundantly expressed in the androgen-responsive SNB, and in two adjacent motor pools, the androgen-responsive dorsolateral nucleus (DLN), and the androgen-unresponsive retrodorsolateral nucleus (RDLN). While we detected significant regional differences for only SRC-1 and cJUN, the SNB consistently contained the highest percentage of immunoreactive motoneurons for all five cofactors examined. Our results indicate five different putative cofactors have the potential to participate in motoneuronal responses to androgens, since their distribution overlaps well with the distribution of ARs in these motoneurons.

Reduction of the spinal nucleus of the bulbocavernosous volume by experimental diabetes

The sexually dimorphic nuclei, spinal nucleus of the bulbocavernosus (SNB) and dorsolateral nucleus, are located at the lumbar segment of the rat spinal cord. These nuclei innervate perineal muscles involved in penile erection and ejaculation. Testosterone levels modulate their size in adult male rats. Because diabetes is associated with low levels of testosterone, we have evaluated morphological changes on spinal cord of diabetic animals. Significant reduction in the SNB volume was observed 4 weeks after diabetes induction accomplished by a reduction on the motoneuronal size. Insulin prevents the morphological alterations. No significant changes were observed on other dimorphic nucleus. The altered sexual behavior of diabetic rats could be consequence of the detected reduction in the SNB volume.

Effects of testosterone on the development of neuromuscular systems and their target tissues involved in courtship and copulation in green anoles (Anolis carolinensis)

Male green anole lizards court females using a red throat fan (dewlap) and copulate by intromitting one of two penises (hemipenes). These structures begin sexually monomorphic, but by adulthood males have larger dewlaps, only males have hemipenes, and many of the neuromuscular components of both systems show male-biased dimorphisms. We hypothesized that testosterone (T), which increases in juvenile males but not females about a month after hatching, facilitates masculinization. To test this idea, on post-hatching day 30, gonadally intact females received either a blank or T implant, and males were either castrated or sham-castrated. At day 90, juveniles were euthanized and the length of the cartilage and cross-sectional areas of the muscle fibers and motoneurons required for dewlap extension were examined. We also measured the cross-sectional areas of the hemipenes and associated muscle fibers and motoneurons, and counted the motoneurons. T-treated females had longer cartilages and larger dewlap muscle fibers compared to those with blank implants. No effects on motoneurons were detected, and no females possessed hemipenes or associated musculature. In males, castration produced shorter dewlap cartilages and smaller hemipenes; other measures were not affected by treatment. These data indicate that components of the dewlap system differentiate relatively late in development, that T likely mediates the process, and that although components of the copulatory system are plastic in juvenile males, sexual differentiation of peripheral features is complete before day 30. The data also suggest that target structures (dewlap cartilage and hemipenes), compared to their neuromuscular effectors, are particularly sensitive to developmental T exposure.

Increase in motoneurons in the spinal nucleus of the bulbocavernosus of prepubertally castrated male Mongolian gerbils following delayed treatment with testosterone

Sexual dimorphism in the spinal nucleus of the bulbocavernosus (SNB) of the Mongolian gerbil is achieved by two periods of postnatal increase, one in the first month after birth and one at puberty. The pubertal increase in motoneuron number is of particular interest because it occurs in a nearly adult animal. The purpose of this research was threefold. The first was to determine the response of the SNB in prepubertally castrated male gerbils receiving delayed hormone replacement as adults. Testosterone propionate (TP) treatment resulted in numbers of SNB motoneurons comparable to those seen in intact males, whereas androgen metabolites were less effective. The second purpose was to determine the latency of motoneurons to appear in response to TP. New SNB motoneurons appeared within 2 days of delayed TP replacement in prepubertally castrated males, and 16 days of treatment did not further increase SNB motoneuron numbers. The response of the motoneurons to TP appeared more rapid than the response of the bulbocavernosus (BC) muscle, scent gland, and seminal vesicles. The third purpose was to determine whether the new cells were connected to a target muscle. After 16 days of TP treatment, more motoneurons were labeled in the SNB following injection of a retrograde tract tracer into the BC muscle compared with the number seen in control animals. Thus, new motoneurons appeared in the SNB of prepubertally castrated male Mongolian gerbils within 2 days of the start of delayed TP treatment and were connected to a target within 16 days of TP treatment.

Normal ontogeny of perineal muscles and testosterone levels in Mongolian gerbils; Response to testosterone in developing females

The spinal nucleus of the bulbocavernosus (SNB) of Mongolian gerbils (Meriones unguiculatus) becomes sexually dimorphic during postnatal life, rather than prenatally as in rats. We therefore examined the early postnatal ontogeny of Mongolian gerbils, focusing on growth, serum testosterone (T) levels, and the sexually dimorphic perineal musculature innervated by the SNB. Serum T levels were higher in males than in females from birth through adulthood, with several early postnatal peaks and a large increase in T occurring during puberty in males. The SNB target muscles-the bulbocavernosus (BC) and levator ani (LA)-were present in both sexes on postnatal day 1 (PND1). Cross-sectional areas of BC fibers in males increased with age, and concurrently the myofibers of the BC became more fully developed and organized. In PND10 female pups, the BC muscle was virtually absent, while the LA muscle remained (although it was reduced in size). Postnatal treatment of female gerbils with androgen caused the BC muscle to remain and the LA muscle to become larger by PND10. Sexual dimorphism of the SNB develops differently in gerbils compared to other species, although its target muscles appear to respond to androgen in a manner similar to that in rats.

Early postnatal response of the spinal nucleus of the bulbocavernosus and target muscles to testosterone in male gerbils

This study examined the response of the spinal nucleus of the bulbocavernosus (SNB) and the bulbocavernosus (BC) muscle, to testosterone in male Mongolian gerbils (Meriones unguiculatus) during the early postnatal period. Male gerbil pups were given testosterone propionate (TP) or vehicle for 2 days, then perfused on postnatal day (PND) 3, 5, 10 or 15. The BC and levator ani (LA) muscles were removed, weighed, and sectioned. Cross-sections of BC muscle fibers were measured and muscle fiber morphology examined. Spinal cords were removed and coronally sectioned in order to count and measure the SNB motoneurons. Following TP treatment, male pups of all ages had significantly heavier BC-LA muscles and larger fibers in the BC muscle compared to age-matched controls. The increase in muscle weight following TP treatment was greatest at PND10, while fiber size increased to a similar degree at all ages suggesting that hyperplasia as well as hypertrophy was responsible for the increase in muscle mass at this time. SNB motoneurons increased significantly in number and size with age and TP treatment. We hypothesize that the increase in SNB motoneuron number during normal ontogeny that can be augmented by TP treatment and represents an unusual means of establishing sexual dimorphism in the nervous system of a mammal through cell recruitment to the motor pool of a postnatal animal.

Lumbar motoneuron fate in a mouse model of amyotrophic lateral sclerosis

Onuf's nucleus, a collection of motoneurons within the spinal cord, is often spared in the neurodegenerative disorder amyotrophic lateral sclerosis. To assess whether these cells survive in a rodent model of this disease, motoneurons were counted in the spinal nucleus of the bulbocavernosus (an homologous structure to Onuf's), as well as in two other cell groups at the same level of the spinal cord, the dorsolateral nucleus and the retrodorsolateral nucleus. In mice displaying signs of neurodegeneration, both the dorsolateral and retrodorsolateral nuclei displayed significant motoneuron loss compared to controls; this cell loss was particularly exaggerated in the retrodorsolateral nucleus of animals displaying a rapid disease progression. However, no significant decline in motoneuron number was observed in the spinal nucleus of the bulbocavernosus, and the perineal muscle bulbocavernosus, which is innervated by this nucleus, appeared to be unaffected. This was in stark contrast to the thigh muscles, which displayed significant atrophy. Overall, these data indicate that the spinal nucleus of the bulbocavernosus is spared from degeneration in an animal model of amyotrophic lateral sclerosis, paralleling observations in patients suffering from this disease. Further study of this resistance to motoneuron loss may provide useful insights into the pathophysiology of the degenerative process.

Androgen receptor messenger RNA and protein in adult rat sciatic nerve: implications for site of androgen action

Gonadal androgens exert a wide variety of effects on several neuromuscular systems, including controlling the developmental fate of motoneurons and neuromuscular synapses and promoting the growth of adult dendrites and axons. Paramount in understanding the molecular mechanisms behind androgen action is determining where androgen acts; does androgen act directly or indirectly on cells to change their fate and function? One step toward answering this question has been to determine which cells express androgen receptors (ARs). Motoneurons and skeletal muscles both have ARs and are, therefore, potential sites of androgen action. Recent evidence indicates that the sciatic nerve in rats also contains AR mRNA (Magnaghi et al. [1999] Brain Res. Mol. Brain Res. 70:36-44), although which cell type expresses ARs remains unanswered. In this study, we explored the question of which cell populations in the rat sciatic nerve express ARs. Using immunocytochemistry and reverse transcriptase-PCR, we confirmed the presence of AR protein and mRNA in sciatic nerve from adult rats and found a sex difference, favoring males, in the number of cell nuclei immunopositive for AR. This difference was not due to a sex difference in the overall number of cell nuclei. We also found a difference favoring males in AR mRNA, evidence also suggesting that AR expression is higher in males than in females. Results from double-immmunolabeling experiments in sciatic nerve from adult males suggest that, within the endoneurial compartment, endoneurial fibroblasts stain prominently for AR, with some endothelial cells also AR(+). Although Schwann cells showed light AR immunostaining, this staining is apparently nonspecific. We conclude that cells within peripheral nerve have ARs and may, therefore, mediate some of the effects of androgens on neuromuscular systems.

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.

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.

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.

Testosterone mediates satellite cell activation in denervated rat levator ani muscle

Denervation stimulates quiescent satellite cells in skeletal muscle to reenter the cell cycle. In the androgen-sensitive rat levator ani muscle (LA), this mitotic response to loss of neural input fails to occur in castrated animals. To elucidate the role of androgens in denervation-induced satellite cell proliferation, the denervated LA of castrated rats (Group A) was compared with that of animals infixed with testosterone implants after castration (Group B). Mean myofiber cross-sectional areas (Group A: 362.95 microm(2) +/- 27.74; Group B: 403.13 microm(2) +/- 53.87) and linear nuclear densities (Group A: 74.07 mm(-1) +/- 17.58; Group B: 104.13 mm(-1) +/- 4.06) were similar (P > 0.05) in both groups. The androgen-deprived myofibers of Group A, however, had a significantly lower nuclear content (271.0 +/- 74.91 vs. 1,285.80 +/- 81.74 in Group B; P < 0.05) on account of their considerably shorter mean length (3.44 mm +/- 0.29 vs. 12.31 mm +/- 0.92 in Group B; P < 0.05). The proportional representation of satellite cells in hormone-replaced, denervated muscle was more than twice that in the untreated group (Group B: 5.15 +/- 0.83% vs. Group A: 2.28 +/- 0.23%; P < 0.05). In absolute terms, the satellite cell number in Group B was approximately an order of magnitude greater than in Group A (408.4 x 10(3) vs. 38.08 x 10(3)). The results confirm the absence of testosterone as the factor responsible for the inability of satellite cells in the LA of castrated rats to respond mitotically to the withdrawal of neural input after denervation.

N-cadherin expression in motoneurons is directly regulated by androgens: a genetic mosaic analysis in rats

We have recently reported that systemic androgens regulate adult N-cadherin (N-cad) expression in spinal motoneurons. However, the mechanism through which androgen mediates this effect remains undetermined. Androgen may act directly on motoneurons to regulate N-cad expression, or indirectly, via effects on androgen-sensitive afferent or efferent structures. Here, we describe a genetic mosaic investigation of this site-of-action indeterminacy. Following developmental random X chromosome inactivation, androgenized female rats heterozygous for the tfm androgen receptor mutation (X(WT)X(tfm)) are phenotypic mosaics of androgen-sensitive wild-type (WT) and androgen-insensitive (tfm) motoneurons. We compared steroid effects on WT and tfm cells in two sexually-dimorphic motoneuron pools, the spinal nucleus of the bulbocavernosus (SNB) and the dorsolateral nucleus (DLN), as well as a less steroid responsive motoneuron pool, the sexually monomorphic retrodorsolateral nucleus (RDLN). Independent of steroid treatment, a greater proportion of wild-type cells were N-cad immunoreactive (IR) in the DLN and RDLN. Following testosterone treatment, increased N-cad expression was observed in both cell types in the DLN, but in the SNB only the androgen-competent WT cells increased N-cad expression. Testosterone treatment did not significantly alter N-cad expression in the mosaic RDLN. The results indicate both cell autonomous and cell non-autonomous androgenic regulation of N-cad expression in spinal motoneurons.

N-cadherin is regulated by gonadal steroids in adult sexually dimorphic spinal motoneurons

Gonadal steroids influence the morphology and function of neurons in the adult spinal cord through cellular and molecular mechanisms that are largely unknown. The cadherins are cell adhesion molecules that participate in the formation and organization of the CNS during embryonic development, and recent evidence suggests that the cadherins continue to regulate neural structure and function in adulthood. Using degenerate oligonucleotides coding conserved regions of the catenin-binding domain of classical cadherins in a RT-PCR cloning strategy, we identified several cadherin subtypes, the most frequently cloned being N-, E-, and R-cadherin, suggesting that these are the major classical cadherin subtypes present in the adult male rat lumbosacral spinal cord. We then examined cadherin expression levels of these cadherin subtypes under steroid conditions known to induce plastic changes in spinal motoneurons. Semiquantitative PCR revealed that mRNA levels of N-cadherin, but not E-cadherin or R-cadherin, are elevated in castrated rats treated with testosterone, 17 beta-estradiol, or dihydrotestosterone relative to castrate rats not treated with steroids. Immunolocalization of N-cadherin revealed that steroid treatment increased N-cadherin expression levels in functionally related neural populations whose morphology and function are regulated by steroids. These results suggest a role for N-cadherin in steroid-induced neuroplastic change in the adult lumbar spinal cord.

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.

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.

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 receptor (AR) immunoreactivity in rat pudendal motoneurons: implications for accessory proteins

Pudendal motoneurons in male rats are located in two sexually dimorphic motoneuronal pools: the spinal nucleus of the bulbocavernosus (SNB) and the dorsolateral nucleus (DLN). SNB motoneurons innervate sexually dimorphic muscles bulbocavernosus (BC) and levator ani (LA) and the sexually monomorphic external anal sphincter (EAS) muscle. DLN motoneurons innervate either the sexually dimorphic ischiocavernosus (IC) muscle or the sexually monomorphic external urethral sphincter (EUS) muscle. Previous observations indicate that the size of BC, LA, and IC motoneurons in males is sensitive to adult androgen manipulations, whereas the size of EAS and EUS motoneurons is not, raising the question of whether this difference in androgen sensitivity among pudendal motoneurons reflects a difference in androgen receptor (AR) expression. AR immunocytochemistry using the PG-21 antiserum was carried out on spinal cord tissue from normal adult male rats in which specific pudendal motoneuronal subpopulations were identified with retrograde markers. Over 90% of BC, LA, and IC motoneurons displayed AR immunoreactivity in their nuclei. Among motoneurons in the SNB, significantly fewer EAS motoneurons had AR-positive nuclei, which may contribute to the reported failure of EAS motoneurons to morphologically respond to changes in androgen levels. However, within the DLN, despite the fact that IC but not EUS motoneurons are reported to respond to androgen with an increase in soma size, IC and EUS motoneurons had the same proportion of AR-positive nuclei. These results indicate that androgen receptors, while necessary, are not sufficient to confer androgen sensitivity to cells.

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.

Number, size, and regional distribution of motor neurons in the dorsolateral and retrodorsolateral nuclei as a function of sex and neonatal stimulation

Motor neurons were measured in the retrodorsolateral nucleus (RDLN) and the dorsolateral nucleus (DLN) of adult male and female rats that were reared with normal or reduced levels of maternal anogenital stimulation. In contrast with findings for the spinal nucleus of the bulbocavernosus, which is located in the same spinal segments, reduced stimulation had no effect on neuron number in either nucleus. However, several regional and sex differences were observed. Rostrally located neurons were larger in both the RDLN and the DLN; these location effects were greater in females. There was no sex difference in RDLN neuron size, but DLN neurons were larger in females, particularly in the rostral region. Females had significantly more cells in the RDLN, a nucleus previously considered nondimorphic, whereas males had more DLN neurons. Both regional and sex differences may reflect local differences in trophic factors from targets or afferents.

Roles of steroid hormones and their receptors in structural organization in the nervous system

Due to their chemical properties, steroid hormones cross the blood-brain barrier where they have profound effects on neuronal development and reorganization both in invertebrates and vertebrates, including humans mediated through their receptors. Steroids play a crucial role in the organizational actions of cellular differentiation representing sexual dimorphism and apoptosis, and in the activational effects of phenotypic changes in association with structural plasticity. Their sites of action are primarily the genes themselves but some are coupled with membrane-bound receptor/ion channels. The effects of steroid hormones on gene transcription are not direct, and other cellular components interfere with their receptors through cross-talk and convergence of the signaling pathways in neurons. These genomic and non-genomic actions account for the divergent effects of steroid hormones on brain function as well as on their structure. This review looks again at and updates the tremendous advances made in recent decades on the study of the role of steroid (gonadal and adrenal) hormones and their receptors on developmental processes and plastic changes in the nervous system.

Importance of target innervation in recovery from axotomy-induced loss of androgen receptor in rat perineal motoneurons

In adult male rats, axotomy of the spinal nucleus of the bulbocavernosus (SNB) motoneurons transiently down-regulates androgen receptor (AR) immunoreactivity. The present study investigates the importance of target reinnervation in the recovery of AR expression in axotomized SNB motoneurons after short (up to 5 days) and long (1 to 6 weeks) periods of recovery. In the long-term recovery experiment, animals were divided into two groups. In one, the two stumps of the cut pudendal nerve, which carries the axons of the SNB motoneurons, were sutured together immediately after axotomy. In the second group, the proximal stump was ligated immediately after axotomy to prevent target reinnervation. Axotomy of the SNB motoneurons caused a significant down-regulation in AR immunoreactivity within 3 days. At 6 weeks, AR immunoreactivity was still depressed in ligated animals but had recovered to control levels in resutured animals. The recovery in the resutured group was coincident with the first signs of reinnervation of the target perineal muscles, although reinnervation seemed to lag behind AR immunoreactivity. SNB soma size was significantly reduced 2 weeks after axotomy and returned to control levels after 6 weeks of recovery only in the resutured animals. These findings suggest that the target perineal muscles play a role in the regulation of AR expression and androgen sensitivity in the SNB motoneurons, perhaps mediated by muscle-derived trophic factors.

Axotomy transiently down-regulates androgen receptors in motoneurons of the spinal nucleus of the bulbocavernosus

Testosterone is an important trophic factor for motoneurons in the spinal nucleus of the bulbocavernosus (SNB), and SNB motoneurons are more responsive to testosterone than are other motoneurons. Axonal injury during early postnatal life prevents the normal development of steroid-sensitivity by adult SNB motoneurons. Axonal injury also causes changes in the expression by motoneurons of a wide range of proteins, including the up-regulation of trophic factor receptors. We have used a polyclonal antibody (PG-21; G.S. Prins) to study the expression of androgen receptors in SNB motoneurons after axonal injury. PG-21 labeled motoneuronal nuclei in the lower lumbar spinal cord of rats in a pattern that matched autoradiographic reports of androgen accumulation in this region of the nervous system. A population of numerous, small cells located dorsal to the central canal also showed evidence of androgen receptor expression. Cutting the axons of SNB motoneurons in adulthood or in development caused a decrease in androgen receptor immunoreactivity in SNB motoneurons. This is the first report that a trophic factor receptor in motoneurons is down-regulated after axonal injury, and is interesting in light of reports that testosterone treatment can facilitate motoneuronal regeneration after nerve cut. Androgen receptor levels subsequently returned to normal, regardless of the age at axotomy, providing no evidence for a lasting effect of developmental axotomy on androgen receptor levels in SNB motoneurons. Thus, axotomy-induced down-regulation of androgen receptors does not underlie the inability of SNB motoneurons to respond to androgen treatment several months after pudendal nerve cut in development.

Androgenic, not estrogenic, steroids alter neuromuscular synapse elimination in the rat levator ani

Developmental synapse elimination in the rat levator ani (LA) muscle is sensitive to gonadal androgen. This process occurs faster in castrated male rats that lack gonadal testosterone and is largely prevented by testosterone treatment. Because testosterone can be irreversibly converted into either androgenic metabolites such as dihydrotestosterone or estrogenic metabolites such as estradiol, the present experiment sought to determine which of these metabolites account for testosterone's effect. Male rat pups at postnatal day 7 (P7) were castrated and given daily subcutaneous injections of one of 5 possible treatments for 3 weeks (P7-P28): (1) testosterone propionate (TP), (2) dihydrotestosterone propionate (DHTP), (3) estradiol benzoate (EB), (4) a combination of DHTP and EB or (5) sesame oil vehicle. At the end of treatment, the LA and extensor digitorum longus (EDL) muscles were dissected and their motor nerve terminals were stained with tetranitroblue tetrazolium. Hormone effects on synapse elimination were evaluated by counting the number of motor axons that contacted individual muscle fibers. The lumbosacral spinal cord was also dissected and processed histologically to examine the motoneurons in the spinal nucleus of the bulbocavernosus (SNB), which innervates the LA. Hormone effects on SNB motoneuron size were assessed by measuring the cross-sectional area of SNB motoneuronal somata and nuclei. We report that DHTP mimics the effects of TP on synapse elimination in the LA muscle, but that EB, acting either alone or together with DHTP, has little or no effect on this process. Synapse elimination in the EDL was unaffected by any hormone treatment. TP or DHTP, but not EB, increase the size of SNB motoneurons. We conclude that testosterone or its androgenic metabolites influence synapse elimination in the LA and probably exert these effects via androgen receptors.

Axotomy of developing rat spinal motoneurons: cell survival, soma size, muscle recovery, and the influence of testosterone

During the period of synapse elimination, motoneurons are impaired in their ability to generate or regenerate axonal branches: following partial denervation of their target muscle, young motoneurons do not sprout to nearby denervated fibers and after axonal injury, they fail to reinnervate the muscle. In the rat levator ani (LA) muscle, which is innervated by motoneurons in the spinal nucleus of the bulbocavernosus (SNB), synapse elimination ends relatively late in development and can be regulated by testosterone. We took advantage of this system to determine if the end of synapse elimination and the development of regenerative capabilities by motoneurons share a common mechanism, or, alternatively, if these two events can be dissociated in time. Axotomy on or before postnatal day 14 (P14) caused the death of SNB motoneurons. By P21, toward the end of synapse elimination in the LA muscle, SNB motoneurons had developed the ability to survive axonal injury. Altering testosterone levels by castration on P7 followed by 4 weeks of either testosterone propionate or control injections did not change the ability of SNB motoneurons to survive axonal injury during development, although these same treatments alter the time course of synapse elimination in the LA muscle. Thus, we dissociated the inability of SNB motoneurons to recover from axonal injury from their developmental elimination of synaptic terminals. We also measured the effect of early axotomy on motoneuronal soma size and on target muscle weight. Axotomy on P14 caused a long-lasting decrease in the soma size of surviving SNB motoneurons, whereas motoneurons axotomized on P28 recovered their normal soma size. Axotomy on or before P7 caused severe atrophy of the target muscles, matching the extensive loss of motoneurons. However, target muscle recovery after axotomy on P14 was as good as recovery after axotomy at later ages, despite greater motoneuronal death after axotomy on P14. This result may reflect an increase in motor unit size, a decrease in polyneuronal innervation by SNB motoneurons that survive axotomy on P14, or a combination of the two.

Expression of androgen receptor protein within the lumbar spinal cord during ontologic development and following antiandrogen induced cryptorchidism

One of the leading hypotheses regarding androgenic regulation of testicular descent is that the lumbar genitofemoral nucleus is morphologically altered by testosterone during a specific prenatal period. This hypothesis is based on the unproved assumption that androgen receptor is present in the fetal spinal cord. Using immunohistochemistry we identified androgen receptor in the rat lumbar spinal cord on gestational day 15, a day before the onset of maximal androgenic action for testicular descent. Experiments were performed to determine whether alterations in the morphology of the genitofemoral nucleus are associated with flutamide induced cryptorchidism. Studies revealed a reduction of motoneuron number in rats with flutamide induced cryptorchidism (124.2 +/- 18.9) compared to rats exposed to flutamide without cryptorchidism (269.3 +/- 20.2) and/or male controls (291.0 +/- 14.5, p < 0.01). These findings support the hypothesis that androgens can directly regulate development and morphology of the genitofemoral nucleus.

Sexually dimorphic neuron number in lumbosacral dorsal root ganglia of the rat: development and steroid regulation

Rats possess a sexually dimorphic neuromuscular system that controls penile reflexes critical for copulation. This system includes two motor nuclei in the lumbar cord and their target musculature in the perineum. The spinal nucleus of the bulbocavernosus (SNB) and the dorsolateral nucleus (DLN) motoneuron populations and their target perineal muscles are much larger in males than in females. The sex difference in motoneuron number develops via androgen-regulated differential cell death during the perinatal period; androgen also regulates retention of the target muscles. The developmental pattern and steroid sensitivity of peripheral afferents to the SNB/DLN motor nuclei were previously unknown. In order to characterize the peripheral sensory component of the dimorphic SNB/DLN system, the neurons of the relevant dorsal root ganglia (DRGs) were quantified in terms of number, size, and androgen sensitivity at various perinatal ages. DRG neuron number is greatest prenatally, then decreases in both sexes after birth; the timing and pattern of neuron number development are similar to those seen in the SNB and DLN. Postnatally, males have more DRG neurons than females, as a result of greater neuron death in the DRGs of females. Females treated with testosterone propionate during the perinatal period exhibit masculine development of DRG neuron number. Thus, the normal development of DRG neuron number parallels that of the SNB/DLN motor nuclei and target muscles in pattern and timing, is sexually dimorphic, and is regulated by androgen.

Androgenic regulation of expression of beta-tubulin messenger ribonucleic acid in motoneurons of the spinal nucleus of the bulbocavernosus

Expression of beta-tubulin mRNA was examined in androgen-sensitive motoneurons of the spinal nucleus of the bulbocavernosus (SNB) in adult male rats by in situ hybridization histochemistry using cDNA encoding mouse beta-tubulin. Hybridizable beta-tubulin mRNA was localized in the somata and proximal dendrites of SNB motoneurons. Removal of androgen by castration significantly reduced the expression level of beta-tubulin mRNA in the SNB motoneurons, whereas the change was prevented by testosterone treatment. On the contrary, castration or testosterone treatment did not induce any changes in the expression level of beta-tubulin mRNA in the androgen-insensitive motoneurons of the retrodorsolateral nucleus. These results suggest that androgen regulates the expression of beta-tubulin gene in the SNB motoneurons and may provide evidence for the molecular mechanisms of hormonally-induced neuronal plasticity in the SNB motoneurons.

Estrogen receptor immunoreactive neurons in the fetal ferret forebrain

The development of estrogen receptors was studied in the preoptic area/anterior hypothalamus (POA/AH) of fetal male and female ferrets. In males this region includes a nucleus (MN-POA/AH), delineated by Nissl stains, which is not discernible in females. The results reveal the distribution of estrogen receptor containing cells during the period when estrogen is known to induce the differentiation of the male ferret's MN-POA/AH. Brains were taken from ferret kits on days 30, 34, 37 and 40 of a 41-42 day gestation, and were processed utilizing the H222 monoclonal antibody to reveal estrogen receptors. At E30 there were numerous H222 immunoreactive (ir) cells in central regions of the POA/AH. From E30 to E40 there was a striking increase in the number of H222ir cells in the POA/AH. A broad sweep of H222ir cells extended from the ventral POA dorsally and laterally into the caudal POA and AH of both males and females. H222ir cells were not restricted to the region of the MN-POA/AH at any fetal age. H222 immunoreaction product at E30 was restricted to nuclear compartments. By E40, H222ir processes extended from some cells with H222ir nuclei in the medial and lateral POA/AH in both males and females. At the older fetal ages immunopositive cell numbers increased in lateral positions. At E34 and E37 (but not E30) selective ventricular zones, and regions between the hypothalamus and amygdala contained H222ir cells, suggesting the presence of estrogen receptors in cells during migration. Although the amygdala contained a few H222ir cells as early as E34, the cortex lacked H222ir cells even as late as E40. The appearance of H222ir cells in positions suggestive of migration is consistent with the hypothesis that estrogen receptors play some role in determining cell positions in certain regions of the developing nervous system.

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.

Testosterone fails to save androgen-sensitive rat motoneurons following early target removal

Axotomy during development can result in the death of up to 100% of the affected motoneurons. However, axotomy-induced death can be significantly reduced by administration of androgens in young rats. Motoneuron death in the spinal nucleus of the bulbocavernosus (SNB) has previously been shown to be regulated by androgens during development. the present experiment examined the effects of androgen treatment on the survival of SNB motoneurons after target removal and concomitant axotomy early in development. On the day of birth, two target muscles of SNB motoneurons of male and female rats were bilaterally extirpated. Target removal resulted in a dramatic loss of SNB motoneurons within 48 h of surgery, with an ultimate loss of virtually all motoneurons projecting to the extirpated muscles by postnatal day 10. Treatment with testosterone failed to save SNB motoneurons from target removal-induced death. Pups treated with testosterone after target removal did not differ in the pattern or timing of motoneuron loss from untreated pups at any age examined. Counts of degenerating cells in the SNB reflected the extensive motoneuron loss and also did not differ with testosterone treatment. These results indicate that testosterone cannot save the androgen-sensitive SNB motoneurons from death after target removal and concomitant axotomy early in development. The failure of testosterone treatment to rescue SNB motoneurons in the absence of the SNB target musculature further suggests that during normal development, both androgens and target muscles are necessary for SNB motoneuron survival.

Timing and duration of dihydrotestosterone treatment affect the development of motoneuron number and morphology in a sexually dimorphic rat spinal nucleus

The spinal nucleus of the bulbocavernosus (SNB) is a sexually dimorphic motor nucleus in the rat lumbar spinal cord. SNB motoneurons and their perineal target muscles are present in adult males, but reduced or absent in adult females. This dimorphism is due to the presence of androgens during development. Perinatal treatment of females with testosterone (T), or a combination of dihydrotestosterone (DHT) and estrogen (E+D females) from embryonic (E) day 16 through postnatal (P) day 5, results in a masculine number of SNB motoneurons and the retention of the target muscles. Perinatal treatment with estrogen alone does not masculinize the SNB; prenatal treatment with DHT alone from E17-E22 results in a feminine number of SNB motoneurons and a significantly altered motoneuron morphology and connectivity. To determine if masculinization of the SNB involves the interaction of estrogen and DHT or results from a longer exposure to DHT alone, the number, morphology, and connectivity of SNB motoneurons in females treated with DHT both pre- and post-natally (from E16-P5) were examined. At E22, DHTP (E16-P5) females have SNB motoneuron numbers identical to E+D and normal females, but far fewer than normal males, thus indicating that T is essential for prenatal masculinization. After E22, SNB motoneuron number declines precipitously in normal females but remains stable in DHTP (E16-P5) females and E+D females, which do not differ from normal males at P10. These results demonstrate that DHT can completely masculinize SNB motoneuron number without any synergistic actions with estrogen, and suggest that the development of SNB motoneuron number is strictly an androgen-mediated event. In adulthood, horseradish peroxidase histochemistry reveals that the connectivity, dendritic length, and soma size of SNB motoneurons in DHTP (E16-P5) females are identical to those of normal males but differ significantly from those of DHTP (E17-E22) females. These data suggest that the altered connectivity in DHTP (E17-E22) females is not simply a hormone-specific effect, but the result of a truncated hormone exposure. Thus, DHT can fully masculinize SNB morphology and connectivity if given during the appropriate period of development. It is suggested that while T may be required to masculinize the SNB prenatally, DHT may be involved in masculinizing postnatal aspects of SNB development.