Motoneuron development after deafferentation. I. dorsal rhizotomy does not alter growth in the spinal nucleus of the bulbocavernosus (SNB)

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.

Critical period for estrogen-dependent motoneuron dendrite growth is coincident with ERα expression in target musculature

The spinal cord of rats contains the sexually dimorphic, steroid-sensitive motoneurons of the spinal nucleus of the bulbocavernosus (SNB). In males, SNB dendrite growth is dependent on gonadal steroids: dendrite growth is inhibited after castration, but supported in androgen- or estrogen-treated castrated males. Furthermore, estrogenic support of SNB dendrite growth is mediated by estrogen action at the target musculature, inhibited by estrogen receptor (ER) blockade at the muscle and supported by local estradiol treatment. However, this estrogenic support is restricted to the early postnatal period, after which the morphology of SNB dendrites is insensitive to estrogens. To test if the developmentally restricted effects of estrogens on SNB dendrite growth coincide with the transient expression of ER in the target musculature, ERα expression was assessed during development and in adulthood. ERα expression in extra-Muscle fiber cells was greatest from postnatal day 7 (P7) to P14 and declined after P21. Because this pattern of ERα expression coincided with the period of estrogen-dependent dendrite growth, we tested if limiting hormone exposure to the period of maximal ERα expression in extra-muscle fiber cells could fully support estrogen-dependent SNB dendrite growth. We restricted estradiol treatment in castrated males from P7 to P21 and assessed SNB dendritic morphology at P28. Treating castrates with estradiol implants at the muscle from P7 to P21 supported dendrite growth to normal levels through P28. These data suggest that the transient ERα expression in target muscle could potentially define the critical period for estrogen-dependent dendrite growth in SNB motoneurons.

Trophic effects of brain-derived neurotrophic factor blockade in an androgen-sensitive neuromuscular system

In adult male rats, androgens are necessary for the maintenance of the motoneurons and their target muscles of the sexually dimorphic, steroid-sensitive spinal nucleus of the bulbocavernosus (SNB) neuromuscular system, regulating motoneuron and muscle morphology, function, and expression of trophic factors. Castration of males results in somal, dendritic, and muscle atrophy as well as increases in brain-derived neurotrophic factor (BDNF) in the target musculature. Because BDNF can have either facilitative or inhibitory effects in other systems, we examined SNB neuromuscular morphology after BDNF blockade using a fusion protein (tyrosine kinase receptor type B IgG). Blockade of BDNF in gonadally intact males resulted in hypertrophy of SNB motoneuron dendrites and target musculature, suggesting that normal levels of BDNF are inhibitory in SNB neuromuscular system. BDNF blockade in castrated males prevented SNB motoneuron atrophy and attenuated target muscle weight loss. This is the first demonstration that the highly androgen-sensitive SNB motoneuron dendrites and target muscles can be maintained in the absence of gonadal hormones and, furthermore, that blocking BDNF can have trophic effects on skeletal muscle. These results suggest that whereas BDNF is involved in the signaling cascade mediating the androgenic support of SNB neuromuscular morphology, its action can be inhibitory. Furthermore, the elevations in BDNF after castration may be responsible for the castration-induced atrophy in SNB motoneurons and target muscles, and the trophic effects of androgens may be mediated in part through a suppression of BDNF. These results may have relevance to therapeutic approaches to the treatment of neurodegenerative disease or myopathies.

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 5alpha-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.

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.

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.

Testosterone manipulation protects motoneurons from dendritic atrophy after contralateral motoneuron depletion

Dendritic morphology is reactive to many kinds of injuries, including axotomy and deafferentation. In this study, we examined the response of motoneurons in the spinal nucleus of the bulbocavernosus (SNB), an androgen-dependent population of motoneurons in the lumbar spinal cord of the rat, to partial motoneuron depletion. We depleted SNB motoneurons on one side only of the spinal cord by unilateral intramuscular injection of a retrogradely transported form of saporin, and examined the morphology of contralateral SNB motoneurons. Motoneuron morphology was assessed in normal control males, gonadally intact saporin-treated males, and saporin-treated males who had been castrated 6 weeks previously and given testosterone replacement beginning at the time of saporin injection. Untreated castrated males served as an additional control group. Four weeks after saporin treatment, SNB motoneurons contralateral to the saporin injection were retrogradely labeled with horseradish peroxidase conjugated to the cholera toxin B subunit and reconstructed in three dimensions. In gonadally intact males, unilateral motoneuron depletion caused regressive changes in contralateral SNB motoneurons: Soma size and dendritic length were both decreased. However, testosterone manipulation (i.e., castration followed by testosterone replacement) completely prevented the dendritic retraction. These data suggest a therapeutic role for testosterone in preventing, or accelerating recovery from, dendritic atrophy induced by motoneuron injury.

Development of a sexually dimorphic neuromuscular system in male rats after spinal transection: morphologic changes and implications for estrogen sites of action

The lumbar spinal cord of rats contains the sexually dimorphic, steroid-sensitive spinal nucleus of the bulbocavernosus (SNB). In male rats, SNB motoneurons exhibit a biphasic pattern of dendritic growth, having an initial period of exuberant growth followed by a period of retraction to mature lengths by 7 weeks of age. This growth is steroid dependent: dendrites fail to grow after castration, but growth is supported in castrates treated with estradiol. In this experiment, we examined whether supraspinal afferent input by means of descending spinal tracts to the SNB was involved in the normal postnatal development of SNB motoneurons, and whether the effect of estradiol on SNB dendritic growth could be explained by an indirect action of estradiol on supraspinal afferents. Motoneuron morphology was assessed in normal males, early- or late-postnatally transected males, castrated males left untreated or treated with estradiol, and transected castrates treated with estradiol. SNB motoneurons were retrogradely labeled with cholera toxin-horseradish peroxidase during both the growth and retraction phases of dendritic development and reconstructed in three dimensions. The removal of supraspinal afferents resulted in extremely local effects within the developing SNB arbor, as well as transient alterations in somal growth. Furthermore, spinal transection did not block the trophic effect of estradiol on supporting SNB dendritic growth, indicating that estrogens do not act by means of supraspinal input to support SNB motoneuron development.

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.

N-methyl-D-aspartate receptor blockade inhibits estrogenic support of dendritic growth in a sexually dimorphic rat spinal nucleus

The lumbar spinal cord of rats contains the sexually dimorphic, steroid-sensitive spinal nucleus of the bulbocavernosus (SNB). Dendritic development of SNB motoneurons requires the action of both androgens and estrogens. Estrogenic effects are limited to the initial growth of SNB dendrites through 4 weeks of age. During this postnatal period, dendritic growth in other spinal motoneurons is regulated by N-methyl-D-aspartate (NMDA) receptor activation. In this study, we tested whether NMDA receptor activation was involved in SNB dendritic growth and whether the estrogenic support of SNB dendritic growth was dependent on the activation of NMDA receptors. Motoneuron morphology was assessed in normal males, intact males treated daily with the NMDA receptor antagonist MK-801, castrated males treated with estradiol benzoate (EB), and castrated males treated with both EB and MK-801. SNB motoneurons were retrogradely labeled with cholera toxin-horseradish peroxidase at 4 weeks of age (when dendritic length is normally maximal) and reconstructed in three dimensions. Somal area and dendritic length of SNB motoneurons in MK-801-treated, intact males were below those of normal males. Dendritic growth was partially supported in EB-treated castrates, but this growth was blocked by MK-801 treatment. These results suggest that, as in other motoneurons, dendritic development in the SNB involves NMDA receptors and, furthermore, that the estrogen-sensitive component of SNB dendritic development requires their activation.

Synergistic effects of testosterone metabolites on the development of motoneuron morphology in a sexually dimorphic rat spinal nucleus

The rat lumbar spinal cord contains the testosterone-dependent spinal nucleus of the bulbocavernosus (SNB), whose motoneurons innervate perineal muscles involved in copulatory reflexes. In normal males, SNB dendrites grow exuberantly through the first 4 weeks postnatally. This growth is steroid-dependent: dendrites fail to grow in males castrated at P7, but grow normally in castrates treated with testosterone (T). Treatment with either of the T metabolites, dihydrotestosterone or estrogen, supports dendritic growth in castrates, but not to the lengths characteristic of intact males or T-treated castrates. The present study tested the hypothesis that dihydrotestosterone and estrogen act together to support development of SNB dendrites. Male rat pups were castrated on P7 and treated daily with dihydrotestosterone propionate (DHT) (2 mg), estradiol benzoate (E) (100 microg), DHT (2 mg) combined with estradiol benzoate in either 5 microg (E5) or 100 microg (E100) doses, or vehicle alone. On P28, when SNB dendritic length is normally maximal, motoneurons were retrogradely labeled with cholera toxin-HRP (BHRP). Soma size and dendritic lengths of labeled motoneurons were assessed and compared to those of age-matched, intact male rats. Soma areas of DHT + E5-treated and DHT + E100-treated castrates did not differ from those of castrates treated with DHT alone, although somata of all three groups were significantly larger than those of normal males and E- or oil-treated castrates. Dendritic lengths in DHT + E5-treated castrates were significantly shorter than those of normal males, and did not differ from those of castrates receiving DHT or E alone, although all hormone-treated groups had dendritic lengths that were significantly longer than untreated castrates. However, treatment of castrates with DHT + E100 fully supported dendritic growth to levels characteristic of normal males. These results suggest that somal and dendritic growth may occur through separate developmental mechanisms, and that E and DHT act synergistically to support normal masculine SNB dendritic development.

Motoneuron development after deafferentation: II. dorsal rhizotomy does not block estrogen-supported growth in the dorsolateral nucleus (DLN)

The lumbar spinal cord of the rat contains two sexually dimorphic motor nuclei, the spinal nucleus of the bulbocavernosus (SNB) and the dorsolateral nucleus (DLN). Postnatally, SNB and DLN motoneurons grow substantially and reach their adult morphology by 7 weeks of age. The masculinization of SNB and DLN motoneuron dendrites depends upon steroid hormones. After early castration, the growth of SNB and DLN dendrites is markedly attenuated, but testosterone replacement restores this growth. In the SNB, initial dendritic growth is also supported in castrates treated with estrogen. By using castration and hormone replacement techniques, we examined the development of DLN motoneuron morphology in estrogen-treated castrated rats to determine if estrogen also supports the growth of DLN motoneurons. In addition, given that dorsal root ganglia may be a site of estrogen action, we tested the hypothesis that estrogen acts at primary afferents to support DLN dendritic growth. Thus, we attempted to block the potential trophic effect of estrogen by performing unilateral dorsal rhizotomies in estrogen-treated castrates. DLN motoneuron morphology was analyzed at 4 and 7 weeks of age by using cholera toxin horseradish peroxidase (BHRP) histochemistry. As found for SNB motoneurons, estrogen treatment transiently supported development. DLN motoneurons in estrogen-treated castrates developed normally through 4 weeks of age, but by 7 weeks, DLN motoneuron morphology in estrogen-treated castrates was no longer different from that in oil-treated castrates. Moreover, deafferentation via unilateral dorsal rhizotomy did not inhibit estrogen's ability to masculinize the early development of DLN motoneurons. Thus, the trophic effect of estrogen did not appear to act via the dorsal root ganglia to support the early postnatal development of DLN motoneurons.