Castration reduces motoneuron soma size but not dendritic length in the spinal nucleus of the bulbocavernosus of wild-type and BCL-2 overexpressing mice

Motoneurons in the spinal nucleus of the bulbocavernosus (SNB) and their target muscles, bulbocavernosus and levator ani (BC/LA), constitute an androgen-sensitive neuromuscular system. Testosterone regulates SNB soma size, SNB dendritic length, and BC/LA muscle mass in adult male rats. Recent evidence indicates that the cell death-regulatory protein, Bcl-2, may also play a role in adult neural plasticity. The present study examined whether gonadal hormones and/or the Bcl-2 protein influence the morphology of the SNB neuromuscular system in adult B6D2F1 mice. In Experiment 1, adult wild-type and Bcl-2 overexpressing males were castrated and implanted with silastic capsules containing testosterone or left blank. Six weeks after castration, cholera toxin-horseradish peroxidase was injected into the BC muscle to label SNB dendrites. Animals were killed 48 h later, and BC/LA muscle mass, SNB soma size, and SNB dendritic arbors were examined. In Experiment 2, wild-type and Bcl-2 overexpressing males were castrated or sham castrated, implanted with testosterone-filled or blank capsules, and examined 12 weeks later. In both experiments, BC/LA muscle mass and SNB soma size were significantly reduced in castrates receiving blank capsules. Surprisingly, however, there was no effect of hormone manipulation on any of several measures of dendritic length. Thus, the dendritic morphology of SNB motoneurons appears to be relatively insensitive to circulating androgen levels in B6D2F1 mice. Bcl-2 overexpression did not influence BC/LA muscle mass, SNB soma size, or SNB dendritic length, indicating that the morphology of this neuromuscular system and the response to castration are not altered by forced expression of the Bcl-2 protein.     

Castration reduces motoneuron soma size but not dendritic length in the spinal nucleus of the bulbocavernosus of wild‐type and BCL‐2 overexpressing mice

Motoneurons in the spinal nucleus of the bulbocavernosus (SNB) and their target muscles, bulbocavernosus and levator ani (BC/LA), constitute an androgen‐sensitive neuromuscular system. Testosterone regulates SNB soma size, SNB dendritic length, and BC/LA muscle mass in adult male rats. Recent evidence indicates that the cell death‐regulatory protein, Bcl‐2, may also play a role in adult neural plasticity. The present study examined whether gonadal hormones and/or the Bcl‐2 protein influence the morphology of the SNB neuromuscular system in adult B6D2F1 mice. In Experiment 1, adult wild‐type and Bcl‐2 overexpressing males were castrated and implanted with silastic capsules containing testosterone or left blank. Six weeks after castration, cholera toxin‐horseradish peroxidase was injected into the BC muscle to label SNB dendrites. Animals were killed 48 h later, and BC/LA muscle mass, SNB soma size, and SNB dendritic arbors were examined. In Experiment 2, wild‐type and Bcl‐2 overexpressing males were castrated or sham castrated, implanted with testosterone‐filled or blank capsules, and examined 12 weeks later. In both experiments, BC/LA muscle mass and SNB soma size were significantly reduced in castrates receiving blank capsules. Surprisingly, however, there was no effect of hormone manipulation on any of several measures of dendritic length. Thus, the dendritic morphology of SNB motoneurons appears to be relatively insensitive to circulating androgen levels in B6D2F1 mice. Bcl‐2 overexpression did not influence BC/LA muscle mass, SNB soma size, or SNB dendritic length, indicating that the morphology of this neuromuscular system and the response to castration are not altered by forced expression of the Bcl‐2 protein. © 2002 Wiley Periodicals, Inc. J Neurobiol 53: 403–412, 2002     

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.     

Effects of testosterone on the development of a sexually dimorphic neuromuscular system in ciliary neurotrophic factor receptor knockout mice.

Motoneurons in the spinal nucleus of the bulbocavernosus (SNB) innervate the perineal muscles, bulbocavernosus (BC), and levator ani (LA). Testosterone regulates the survival of SNB motoneurons and BC/LA muscles during perinatal life. Previous findings suggest that effects of testosterone on this system may be mediated by trophic factors-in particular, by a factor acting through the ciliary neurotrophic factor alpha-receptor (CNTFRalpha). To test the role of CNTFRalpha in the response of the developing SNB system to testosterone, CNTFRalpha +/+ and -/- mice were treated with testosterone propionate (TP) or oil during late embryonic development. BC/LA muscle size and SNB motoneuron number were evaluated on the day of birth. Large sex differences in BC and LA muscle size were present in newborn mice of both genotypes, but muscle volumes were reduced in CNTFRalpha -/- animals relative to same-sex, wild-type controls. Prenatal testosterone treatment completely eliminated the sex difference in BC/LA muscle size in wild-type animals, and eliminated the effect of the CNTFRalpha gene deletion on muscle size in males. However, the effect of TP treatment on BC and LA muscle sizes was blunted in CNTFRalpha -/- females. SNB motoneuron number was sexually dimorphic in oil-treated, wild-type mice. In contrast, there was no sex difference in SNB motoneuron number in oil-treated, CNTFRalpha knockout mice. Prenatal treatment with testosterone did not increase SNB motoneuron number in CNTFRalpha -/- mice, but also did not significantly increase SNB motoneuron number in newborn wild-type animals. These findings confirm the absence of a sex difference in SNB motoneuron number in CNTFRalpha -/- mice. Moreover, the CNTFRalpha gene deletion influences perineal muscle development and the response of the perineal muscles to testosterone. Prenatal TP treatment of CNTFRalpha -/- males overcomes the effects of the gene deletion on the BC and LA muscles without a concomitant effect on SNB motoneuron number.     

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.     

Effects of testosterone on the development of a sexually dimorphic neuromuscular system in ciliary neurotrophic factor receptor knockout mice

Motoneurons in the spinal nucleus of the bulbocavernosus (SNB) innervate the perineal muscles, bulbocavernosus (BC), and levator ani (LA). Testosterone regulates the survival of SNB motoneurons and BC/LA muscles during perinatal life. Previous findings suggest that effects of testosterone on this system may be mediated by trophic factors—in particular, by a factor acting through the ciliary neurotrophic factor α‐receptor (CNTFRα). To test the role of CNTFRα in the response of the developing SNB system to testosterone, CNTFRα +/+ and −/− mice were treated with testosterone propionate (TP) or oil during late embryonic development. BC/LA muscle size and SNB motoneuron number were evaluated on the day of birth. Large sex differences in BC and LA muscle size were present in newborn mice of both genotypes, but muscle volumes were reduced in CNTFRα −/− animals relative to same‐sex, wild‐type controls. Prenatal testosterone treatment completely eliminated the sex difference in BC/LA muscle size in wild‐type animals, and eliminated the effect of the CNTFRα gene deletion on muscle size in males. However, the effect of TP treatment on BC and LA muscle sizes was blunted in CNTFRα −/− females. SNB motoneuron number was sexually dimorphic in oil‐treated, wild‐type mice. In contrast, there was no sex difference in SNB motoneuron number in oil‐treated, CNTFRα knockout mice. Prenatal treatment with testosterone did not increase SNB motoneuron number in CNTFRα −/− mice, but also did not significantly increase SNB motoneuron number in newborn wild‐type animals. These findings confirm the absence of a sex difference in SNB motoneuron number in CNTFRα −/− mice. Moreover, the CNTFRα gene deletion influences perineal muscle development and the response of the perineal muscles to testosterone. Prenatal TP treatment of CNTFRα −/− males overcomes the effects of the gene deletion on the BC and LA muscles without a concomitant effect on SNB motoneuron number. © 1999 John Wiley & Sons, Inc. J Neurobiol 41: 317–325, 1999     

Testosterone metabolites differentially maintain adult morphology in a sexually dimorphic neuromuscular system

The lumbar spinal cord of rats contains the sexually dimorphic, steroid‐sensitive spinal nucleus of the bulbocavernosus (SNB). Androgens are necessary for the development of the SNB neuromuscular system, and in adulthood, continue to influence the morphology and function of the motoneurons and their target musculature. However, estrogens are also involved in the development of the SNB system, and are capable of maintaining function in adulthood. In this experiment, we assessed the ability of testosterone metabolites, estrogens and nonaromatizable androgens, to maintain neuromuscular morphology in adulthood. Motoneuron and muscle morphology was assessed in adult normal males, sham‐castrated males, castrated males treated with testosterone, dihydrotestosterone, estradiol, or left untreated, and gonadally intact males treated with the 5α‐reductase inhibitor finasteride or the aromatase inhibitor fadrozole. After 6 weeks of treatment, SNB motoneurons were retrogradely labeled with cholera toxin‐HRP and reconstructed in three dimensions. Castration resulted in reductions in SNB target muscle size, soma size, and dendritic morphology. Testosterone treatment after castration maintained SNB soma size, dendritic morphology, and elevated target muscle size; dihydrotestosterone treatment also maintained SNB dendritic length, but was less effective than testosterone in maintaining both SNB soma size and target muscle weight. Treatment of intact males with finasteride or fadrozole did not alter the morphology of SNB motoneurons or their target muscles. In contrast, estradiol treatment was completely ineffective in preventing castration‐induced atrophy of the SNB neuromuscular system. Together, these results suggest that the maintenance of adult motoneuron or muscle morphology is strictly mediated by androgens. © 2009 Wiley Periodicals, Inc. Develop Neurobiol 70: 206–221, 2010.     

Estrogenic support of motoneuron dendritic growth via the neuromuscular periphery in a sexually dimorphic motor system

The lumbar spinal cord of rats contains the sexually dimorphic, steroid-sensitive spinal nucleus of the bulbocavernosus (SNB). In males, the growth of SNB dendrites is steroid-dependent: dendrites fail to grow after castration, but grow in castrates treated with androgens or estrogens. Blocking estradiol synthesis or estrogen receptors in gonadally intact males attenuates SNB dendritic growth, suggesting that estrogens are required and must be able to act at their receptors to support normal masculine dendritic growth. However, SNB motoneurons do not accumulate estrogens, suggesting that estrogens act indirectly to support SNB dendritic growth. In this experiment, we examined whether local estrogen action in the neuromuscular periphery was involved in the postnatal development of SNB motoneurons. Motoneuron morphology was assessed in gonadally intact and castrated males. Gonadally intact males were left untreated or given either blank or tamoxifen implants sutured to the target musculature, or tamoxifen interscapular implants. Castrated males were left untreated or were given estradiol by muscle or interscapular implants or systemic injection during the period of SNB dendritic growth. At postnatal day 28, when SNB dendritic length is normally maximal, SNB motoneurons were retrogradely labeled with cholera toxin-HRP and reconstructed in three dimensions. While interscapular tamoxifen implants were ineffective, blocking estrogen receptors at the target musculature resulted in attenuation of SNB dendritic growth. In contrast, while interscapular implants of estradiol were ineffective, local treatment with estradiol at the target musculature in castrated males resulted in masculinization of dendritic growth. Thus, estrogens may act by an indirect action in the neuromuscular periphery to support SNB dendritic growth.     

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.     

Castration‐induced upregulation of muscle ERα supports estrogen sensitivity of motoneuron dendrites in a sexually dimorphic neuromuscular system

The spinal cord of rats contains the sexually dimorphic motoneurons of the spinal nucleus of the bulbocavernosus (SNB). In males, SNB dendrites fail to grow after castration, but androgen or estrogen treatment supports dendritic growth in castrated males. Estrogenic support of SNB dendrite growth is mediated by estrogen receptors (ER) in the target muscle. ERα expression in cells lacking a basal lamina (referred to as “extra‐muscle fiber cells”) of the SNB target musculature coincides with the period of estrogen‐dependent SNB dendrite growth. In the SNB target muscle, extra‐muscle fiber ERα expression declines with age and is typically absent after postnatal (P) day 21 (P21). Given that estradiol downregulates ERα in skeletal muscle, we tested the hypothesis that depleting gonadal hormones would prevent the postnatal decline in ERα expression in the SNB target musculature. We castrated male rats at P7 and assessed ERα immunolabeling at P21; ERα expression was significantly greater in castrated males compared with normal animals. Because ERα expression in SNB target muscles mediates estrogen‐dependent SNB dendrogenesis, we further hypothesized that the castration‐induced increase in muscle ERα would heighten the estrogen sensitivity of SNB dendrites. Male rats were castrated at P7 and treated with estradiol from P21 to P28; estradiol treatment in castrates resulted in dendritic hypertrophy in SNB motoneurons compared with normal males. We conclude that early castration results in an increase in ERα expression in the SNB target muscle, and this upregulation of ERα supports estrogen sensitivity of SNB dendrites, allowing for hypermasculinization of SNB dendritic arbors. © 2013 Wiley Periodicals, Inc. Develop Neurobiol 73: 921–935, 2013     

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.     

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.     

Androgen action at the target musculature regulates brain‐derived neurotrophic factor protein in the spinal nucleus of the bulbocavernosus

We have previously demonstrated that brain‐derived neurotrophic factor (BDNF) interacts with testosterone to regulate dendritic morphology of motoneurons in the highly androgen‐sensitive spinal nucleus of the bulbocavernosus (SNB). Additionally, in adult male rats testosterone regulates BDNF in SNB motoneurons and its target muscle, the bulbocavernosus (BC). Because BDNF is retrogradely transported from skeletal muscles to spinal motoneurons, we hypothesized that testosterone could regulate BDNF in SNB motoneurons by acting locally at the BC muscle. To test this hypothesis, we restricted androgen manipulation to the SNB target musculature. After castration, BDNF immunolabeling in SNB motoneurons was maintained at levels similar to those of gonadally intact males by delivering testosterone treatment directly to the BC muscle. When the same implant was placed interscapularly in castrated males it was ineffective in supporting BDNF immunolabeling in SNB motoneurons. Furthermore, BDNF immunolabeling in gonadally intact adult males given the androgen receptor blocker hydroxyflutamide delivered directly to the BC muscle was decreased compared with that of gonadally intact animals that had the same hydroxyflutamide implant placed interscapularly, or when compared with castrated animals that had testosterone implants at the muscle. These results demonstrate that the BC musculature is a critical site of action for the androgenic regulation of BDNF in SNB motoneurons and that it is both necessary and sufficient for this action. Furthermore, the local action of androgens at the BC muscle in regulating BDNF provides a possible mechanism underlying the interactive effects of testosterone and BDNF on motoneuron morphology. © 2013 Wiley Periodicals, Inc. Develop Neurobiol 73: 587–598, 2013     

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     

Maternal care effects on SNB motoneuron development: The mediating role of sensory afferent distribution and activity

Maternal licking in rats affects the development of the spinal nucleus of the bulbocavernosus (SNB), a sexually dimorphic motor nucleus that controls penile reflexes involved with copulation. Reduced maternal licking produces decreased motoneuron number, size, and dendritic length in the rostral portion of the adult SNB as well as deficits in adult male copulatory behavior. Previous research suggests that decreases in perineal tactile stimulation may be responsible for these effects. To determine whether the regional effects of maternal licking on SNB morphology are driven by sensory afferent innervation of the lumbosacral spinal cord, we used WGA‐HRP to reconstruct the location of sensory afferent fibers from the perineal skin. We found that these fibers are caudally concentrated relative to the area of the SNB dendritic field, with the rostral dendritic arbor receiving little perineal afferent innervation. We also assessed Fos expression following perineal tactile stimulation to determine whether it increased local spinal cord activity in the SNB dendritic field. Sixty seconds of licking‐like perineal stimulation produced a transient 115% increase in Fos expression in the area of the SNB dendritic field. This effect was driven by a significant increase in Fos in the caudal portion of the SNB dendritic field, matching the pattern of perineal afferent fiber labeling. Perineal tactile stimulation also produced significantly greater Fos expression in male pups than in female pups. Together, these results suggest that perineal sensory afferent activity mediates the effects of early maternal care on the masculinization of the SNB and resultant male copulatory behavior. © 2009 Wiley Periodicals, Inc. Develop Neurobiol 2009     

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. © 1995 John Wiley & Sons, Inc.     

Cellular analyses of hormone influence on motoneuronal development and function

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

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.     

Tactile stimulation during artificial rearing influences adult function and morphology in a sexually dimorphic neuromuscular system

Maternal licking of rat pups affects the development of the spinal nucleus of the bulbocavernosus (SNB), a sexually dimorphic motor nucleus that controls penile reflexes involved with copulation. Maternal licking influences SNB motoneurons, with reductions in licking producing decreased SNB number, size, and dendritic length in adulthood. Reduced maternal licking also produces deficits in adult male copulatory behavior. In this experiment, we used an artificial rearing paradigm to assess the potential role of tactile stimulation in mediating the effects of maternal licking on the SNB neuromuscular system. During artificial rearing, pups were stroked with a paintbrush to mimic maternal licking, receiving low, medium, or high levels of daily stimulation. In adulthood, ex copula penile reflex behavior was tested and the morphology of SNB motoneurons assessed. SNB motoneurons were retrogradely labeled with cholera toxin-conjugated HRP and dendritic arbor was reconstructed in three dimensions. Animals that received low levels of stimulation showed deficits in penile reflexes relative to maternally reared controls, including a longer latency to erection, fewer cup erections, and fewer erection clusters. SNB dendritic morphology was also shaped by stimulation condition, with animals that received low or medium levels of stimulation showing an average 27% reduction in dendritic length. In addition, several reflex behaviors were significantly correlated with dendritic length, including latency to first erection, percent of cup erections, and number of erection clusters. These results suggest that tactile stimulation provided by maternal licking mediates some of the effects of maternal care on the development of male copulatory behavior.