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. © 1995 John Wiley & Sons, Inc.     

Androgenic, but not estrogenic, protection of motoneurons from somal and dendritic atrophy induced by the death of neighboring motoneurons

Motoneuron loss is a significant medical problem, capable of causing severe movement disorders or even death. We have been investigating the effects of motoneuron loss on surviving motoneurons in a lumbar motor nucleus, the spinal nucleus of the bulbocavernosus (SNB). SNB motoneurons undergo marked dendritic and somal atrophy following the experimentally induced death of other nearby SNB motoneurons. However, treatment with testosterone at the time of lesioning attenuates this atrophy. Because testosterone can be metabolized into the estrogen estradiol (as well as other physiologically active steroid hormones), it was unknown whether the protective effect of testosterone was an androgen effect, an estrogen effect, or both. In the present experiment, we used a retrogradely transported neurotoxin to kill the majority of SNB motoneurons on one side of the spinal cord only in adult male rats. Some animals were also treated with either testosterone, the androgen dihydrotestosterone (which cannot be converted into estradiol), or the estrogen estradiol. As seen previously, partial motoneuron loss led to reductions in soma area and in dendritic length and extent in surviving motoneurons. Testosterone and dihydrotestosterone attenuated these reductions, but estradiol had no protective effect. These results indicate that the neuroprotective effect of testosterone on the morphology of SNB motoneurons following partial motoneuron depletion is an androgen effect rather than an estrogen effect.     

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. © 1993 John Wiley & Sons, Inc.     

Androgen-sensitivity of somata and dendrites of spinal nucleus of the bulbocavernosus (SNB) motoneurons in male C57BL6J mice

In rats, androgens in adulthood regulate the morphology of motoneurons in the spinal nucleus of the bulbocavernosus (SNB), including the size of their somata and the length of their dendrites. There are conflicting reports about whether androgens exert similar influences on SNB motoneurons in mice. We castrated or sham-operated C57BL6J mice at 90 days of age and, thirty days later, injected cholera toxin conjugated horseradish peroxidase into the bulbocavernosus muscle (to label SNB motoneurons) on one side, and into intrinsic foot muscles contralaterally (to label motoneurons of the retrodorsolateral nucleus (RDLN)). Castrated mice had significantly smaller SNB somas compared to sham-operated mice while there were no differences in soma size of RDLN motoneurons. Dendritic length in C57BL6J mice, estimated in 3-dimensions, also decreased significantly after adult castration. In rats, androgens act directly through androgen receptors (AR) in SNB motoneurons to control soma size and nearly all SNB motoneurons contain AR. Since SNB somata in C57BL6J mice shrank after adult castration, we used immunocytochemistry to characterize AR expression in SNB cells as well as motoneurons in the RDLN and dorsolateral nucleus (DLN). A pattern of labeling matched that seen previously in rats: the highest percentage of AR-immunoreactive motoneurons are in the SNB (98%), the lowest in the RDLN (25%) and an intermediate number in the DLN (78%). This pattern of AR labeling is consistent with the possibility that androgens also act directly on SNB motoneurons in mice to regulate soma size in mice.     

Hormonally mediated plasticity of motoneuron morphology in the adult rat spinal cord: A cholera toxin-HRP study

The dorsolateral nucleus (DLN) and the spinal nucleus of the bulbocavernosus (SNB) of the rat lumbar spinal cord are sexually dimorphic groups of motoneurons that innervate striated perineal muscles involved in male copulatory behavior. Androgens control the development of these motoneurons and their target muscles, and continue to influence the system in adulthood. Given that several features of SNB motoneuron morphology have been shown to be androgen sensitive in adult male rats, we examined the effects of androgen manipulations on the morphology of motoneurons in the DLN in adult rats. Adult male rats were castrated and implanted with testosterone-filled or blank implants, or were subjected to a sham-castration procedure. Six weeks after treatment, motoneurons in the DLN were retrogradely labeled with cholera toxin-horseradish peroxidase (HRP) after injection into the ischiocavernosus (IC) muscle and their morphology assessed. Measures of the radial extent and coverage of the dendritic arbor of DLN motoneurons projecting to the IC (DLN-IC motoneurons) were similar across the groups, indicating comparable degrees of HRP transport. However, DLN-IC motoneurons in castrates with blank implants possessed both shorter dendritic lengths and smaller somas than those of castrates treated with testosterone. Castrates with testosterone implants had DLN-IC motoneurons that were significantly larger than those of sham castrates in dendritic length and soma area. These results suggest that motoneurons in the DLN, like those in the SNB, possess a significant degree of structural plasticity in adulthood which is influenced by androgens.     

Hormonally mediated plasticity of motoneuron morphology in the adult rat spinal cord: a cholera toxin-HRP study.

The dorsolateral nucleus (DLN) and the spinal nucleus of the bulbocavernosus (SNB) of the rat lumbar spinal cord are sexually dimorphic groups of motoneurons that innervate striated perineal muscles involved in male copulatory behavior. Androgens control the development of these motoneurons and their target muscles, and continue to influence the system in adulthood. Given that several features of SNB motoneuron morphology have been shown to be androgen sensitive in adult male rats, we examined the effects of androgen manipulations on the morphology of motoneurons in the DLN in adult rats. Adult male rats were castrated and implanted with testosterone-filled or blank implants, or were subjected to a sham-castration procedure. Six weeks after treatment, motoneurons in the DLN were retrogradely labeled with cholera toxin-horseradish peroxidase (HRP) after injection into the ischiocavernosus (IC) muscle and their morphology assessed. Measures of the radial extent and coverage of the dendritic arbor of DLN motoneurons projecting to the IC (DLN-IC motoneurons) were similar across the groups, indicating comparable degrees of HRP transport. However, DLN-IC motoneurons in castrates with blank implants possessed both shorter dendritic lengths and smaller somas than those of castrates treated with testosterone. Castrates with testosterone implants had DLN-IC motoneurons that were significantly larger than those of sham castrates in dendritic length and soma area. These results suggest that motoneurons in the DLN, like those in the SNB, possess a significant degree of structural plasticity in adulthood which is influenced by androgens.     

Hormonal control of neuron number in sexually dimorphic spinal nuclei of the rat: IV. Masculinization of the spinal nucleus of the bulbocavernosus with testosterone metabolites

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 females. This sexual dimorphism is due to the presence of androgen during development; females treated with testosterone (T) perinatally have a masculine SNB system. To assess whether masculinization of the SNB could involve the conversion of testosterone into its active metabolites, dihydrotestosterone (DHT) and estrogen, we examined the development of the SNB in females treated perinatally with estrogen alone or in combination with dihydrotestosterone. Counts of motoneurons in the developing SNB in all groups showed the typical prenatal increase followed by a differential postnatal decline; the incidence of degenerating cells reflected this decline. Motoneuron numbers and the frequency of degenerating cells in females treated with estrogen (E) alone did not differ from those of normal females, with both groups losing large numbers of motoneurons and having a high incidence of degenerating cells. In contrast, females treated with both estrogen and dihydrotestosterone did not show the female-typical decline in motoneuron number and had a low, masculine incidence of degenerating cells. By postnatal day 10, females treated with estrogen and dihydrotestosterone had a fully masculine SNB motoneuron number, suggesting that dihydrotestosterone alone or in conjunction with estrogen may be involved in the development of the sexually dimorphic SNB system.     

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.     

Regulation of motoneuron death in the spinal nucleus of the bulbocavernsus

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. © 1992 John Wiley & Sons, Inc.     

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 littermates 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. © 1995 John Wiley & Sons, Inc.     

Neurotrophic agents prevent motoneuron death following sciatic nerve section in the neonatal mouse

We have examined the ability of different neurotrophic and growth factors to prevent axotomy-induced motoneuron cell death in the developing mouse spinal cord. After postnatal unilateral section of the mouse sciatic nerve, most motoneuron (MN) loss occurs in the lateral motor column of the fourth lumbar segment (L4). Significant axotomy-induced cell death occurred after surgery performed on or before postnatal day (PN) 5. In contrast, no significant cell loss was found when axotomy was performed after PN10. Axotomy on PN2 or PN5 resulted in a 44% loss of L4 motoneurons by 7 days, and a 66% loss of motoneurons by 10 days postsurgery. Implantation of gelfoam presoaked in various neurotrophic factors at the lesion site rescued axotomized motoneurons. Nerve growth factor (NGF), nedurotrophin-4/5 (NT-4/5) and ciliary neurotrophic factor (CNTF) rescued 20%–30% of motoneurons, whereas brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and insulin-like growth factor 1 (IGF-1) rescued virtually all motoneurons from axotomy-induced death. By contrast, platelet-derived growth factor (PDGF)-AA, PDGF-AB, basic fibroblast growth factor (bFGF), and interleukin (IL-6) were ineffective on motoneuron survival following axotomy. NGF, BDNF, NT-3, IGF-1, and CNTF also prevented axotomy-induced atrophy of surviving motoneurons. These data show that mouse lumbar motoneurons continue to be vulnerable to axotomy up to about 1 week after birth and that a number of trophic agents, including the neurotrophins, CNTF, and IGF-1, can prevent the death of these neurons following axotomy. Our studies confirm and extend previous reports on the time course of axotomy-induced mouse motoneuron death and the survival promoting effects of neurotrophic factors. 1994 John Wiley & Sons, Inc.     

Critical Periods of Sensitivity of Sexually Dimorphic Spinal Nuclei to Prenatal Testosterone Exposure in Female Rats

Male rats normally have more neurons than do females in two nuclei of the lumbar spinal cord, the spinal nucleus of the bulbocavernosus (SNB) and the dorsolateral nucleus (DLN). Female rats exposed to testosterone propionate (TP) on the 2 days of gestation (Days 18 and 19) when males normally experience a surge in plasma testosterone showed a maximal increase in both SNB and DLN neuronal number. TP exposure just prior to, or following, Days 18 and 19 led to smaller increments. Administration of a small (5 μg) dose of TP after birth, while having no effect by itself, synergized with prenatal TP to enhance the number of SNB neurons. DLN neurons were less responsive to postnatal TP. The somal and nuclear size of SNB, but not DLN, neurons was increased by perinatal TP. Paradoxically, the number of DLN neurons with large somas (1358 μm²or larger) was reduced by perinatal TP, a finding congruent with a previous report that females and feminized males have more of these large DLN neurons than control males. Our data suggest an exquisite sensitivity of the developing spinal nuclei to the timing of hormonal surges normally found in fetal males. Exposure to androgens during a brief prenatal period is needed to assure responsiveness to the low amounts of androgen circulating during neonatal ontogeny, when the process of sexual differentiation is completed.     

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 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 adultTfmand wild-type rats: immunostaining was nuclear in most SNB cells of wild-type rats, but very few SNB cells display nuclear AR immunostaining in affectedTfmrats. Because theTfmmutation is carried on the X chromosome, random X inactivation during development makes female carriers ofTfm(+/Tfm) genetic mosaics for androgen sensitivity.Tfmcarriers, their wild-type sisters, and affectedTfmmales 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 andTfmgenotype, 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.     

Neuroprotective effect of testosterone treatment on motoneuron recruitment following the death of nearby motoneurons

Motoneuron loss is a significant medical problem, capable of causing severe movement disorders or even death. We have previously shown that motoneuron death induces marked dendritic atrophy in surviving nearby motoneurons. Additionally, in quadriceps motoneurons, this atrophy is accompanied by decreases in motor nerve activity. However, treatment with testosterone partially attenuates changes in both the morphology and activation of quadriceps motoneurons. Testosterone has an even larger neuroprotective effect on the morphology of motoneurons of the spinal nucleus of the bulbocavernosus (SNB), in which testosterone treatment can completely prevent dendritic atrophy. The present experiment was performed to determine whether the greater neuroprotective effect of testosterone on SNB motoneuron morphology was accompanied by a greater neuroprotective effect on motor activation. Right side SNB motoneurons were killed by intramuscular injection of cholera toxin‐conjugated saporin in adult male Sprague‐Dawley rats. Animals were either given Silastic testosterone implants or left untreated. Four weeks later, left side SNB motor activation was assessed with peripheral nerve recording. The death of right side SNB motoneurons resulted in several changes in the electrophysiological response properties of surviving left side SNB motoneurons, including decreased background activity, increased response latency, increased activity duration, and decreased motoneuron recruitment. Treatment with exogenous testosterone attenuated the increase in activity duration and completely prevented the decrease in motoneuron recruitment. These data provide a functional correlate to the known protective effects of testosterone treatment on the morphology of these motoneurons, and further support a role for testosterone as a therapeutic agent in the injured nervous system. © 2009 Wiley Periodicals, Inc. Develop Neurobiol, 2009     

Developmental motoneuron cell death and neurotrophic factors

During the development of higher vertebrates, motoneurons are generated in excess. In the lumbar spinal cord of the developing rat, about 6000 motoneurons are present at embryonic day 14. These neurons grow out axons which make contact with their target tissue, the skeletal muscle, and about 50% of the motoneurons are lost during a critical period from embryonic day 14 until postnatal day 3. This process, which is called physiological motoneuron cell death, has been the focus of research aiming to identify neurotrophic factors which regulate motoneuron survival during this developmental period. Motoneuron cell death can also be observed in vitro when the motoneurons are isolated from the embryonic avian or rodent spinal cord. These isolated motoneurons and other types of primary neurons have been a useful tool for studying basic mechanisms underlying neuronal degeneration during development and under pathophysiological conditions in neurodegenerative disorders. Accumulating evidence from such studies suggests that some specific requirements of motoneurons for survival and proper function may change during development. The focus of this review is a synopsis of recent data on such specific mechanisms.     

Differential effects of dihydrotestosterone and estrogen on the development of motoneuron morphology in a sexually dimorphic rat spinal nucleus

The rat lumbar spinal cord contains a sexually dimorphic motor nucleus, the spinal nucleus of the bulbocavernosus (SNB), whose motoneurons innnervate perineal muscles involved in copulatory reflexes. Dendritic development of SNB motoneurons is biphasic and androgen dependent. During the first 4 postnatal weeks, SNB dendrites grow exuberantly, and subsequently retract to mature lengths by 7 weeks of age. After early postnatal castration, SNB dendrites fail to grow, and testosterone replacement restores this growth. In other systems, testosterone and its metabolites, dihydrotestosterone and estrogen, are important for somatic and neural sexual differentiation. The purpose of the present study was to examine the effects of castration and dihydrotestosterone or estrogen replacement on the growth of SNB motoneuron somata and dendritic arbors. Male rat pups were castrated on postnatal (P) day 7 and treated daily with either dihydrotestosterone propionate (DHTP; 2 mg) or estradiol benzoate (EB; 100 μg) until P28 or P49. By using cholera toxin horseradish peroxidase (BHRP) histochemistry, the soma size, dendritic length, dendritic extent, and arbor area of BHRP-labeled SNB motoneurons were measured and analyzed. Both DHTP and EB treatment supported the initial exuberant growth of SNB dendrites through P28, but EB treatment was ineffective in maintaining mature, adult lengths at P49. The possible sites of hormone action and functional implications of these hormonal treatments are discussed. 1994 John Wiley & Sons, Inc.     

Exogenous testosterone prevents motoneuron atrophy induced by contralateral motoneuron depletion

Gonadal steroids exhibit neuroprotective and neurotherapeutic effects. The lumbar spinal cord of male rats contains a highly androgen-sensitive population of motoneurons, the spinal nucleus of the bulbocavernosus (SNB), whose morphology and function are dependent on testosterone in adulthood. Unilateral SNB motoneuron depletion induces dendritic atrophy in contralateral SNB motoneurons, but this atrophy is reversed in previously castrated males treated with testosterone. In the present experiment we test the hypothesis that the morphology of SNB motoneurons is protected from atrophy after contralateral motoneuron depletion by exogenous testosterone alone (i.e., with no delay between castration and testosterone replacement). We unilaterally depleted SNB motoneurons by intramuscular injection of cholera toxin conjugated saporin. Simultaneously, some saporin-injected rats were castrated and immediately given replacement testosterone. Four weeks later, contralateral SNB motoneurons were labeled with cholera toxin conjugated HRP, soma sizes were measured, and dendritic arbors were reconstructed. Contralateral SNB motoneuron depletion induced somal atrophy and dendritic retraction, but testosterone treatment prevented both of these effects. Thus, the presence of high-normal levels of testosterone prevents motoneuron atrophy induced by contralateral motoneuron depletion. These data support a therapeutic role for testosterone in preventing atrophy induced by motoneuron injury.     

Aromatase inhibition reduces dendritic growth in a sexually dimorphic rat spinal nucleus

The rat lumbar spinal cord contains the steroid-sensitive spinal nucleus of the bulbocavernosus (SNB), whose motoneurons innervate perineal muscles involved in copulatory reflexes. In normal males, SNB motoneuron dendrites grow exuberantly through postnatal (P) day 28. This growth is steroid dependent: Dendrites fail to grow in males castrated at P7, but grow normally in castrates treated with testosterone or its metabolites, dihydrotestosterone combined with estrogen. Treatment with either metabolite alone supports dendritic growth, but not to the level of testosterone-treated or intact males. In this study, we tested the hypothesis that aromatization of androgens to estrogens was involved in the masculine development of SNB dendrites. Motoneuron morphology was assessed in normal males and males treated daily (P7-28) with fadrozole, a potent aromatase inhibitor (0.25 mg/kg, subcutaneously) or saline vehicle (n = 4-6/group). SNB motoneurons were retrogradely labeled with cholera toxin-horseradish peroxidase at P28 (when dendritic length is normally maximal) and reconstructed in three dimensions. Comparable labeling was seen across groups; it was equivalent in both the rostrocaudal and radial extents. However, dendritic lengths in fadrozole-treated males were significantly below those of intact or saline-treated males. Neither SNB somata size nor target muscle weight differed across groups. These results suggest that aromatization of androgens to estrogens is necessary for development of masculine SNB dendritic morphology.