Motoneuron development after deafferentation: II. Dorsal rhizotomy does doe 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.     

Androgen regulation of dendritic growth and retraction in the development of a sexually dimorphic spinal nucleus

The spinal nucleus of the bulbocavernosus (SNB) is a sexually dimorphic group of motoneurons whose development and maintenance are under androgenic control. Exposure to androgens early in development permanently alters SNB motoneuron number and soma size; in adulthood, androgens regulate dendritic and synaptic architecture. The present set of experiments investigates the influence of androgens on the development of SNB dendritic morphology. In normal males, SNB dendritic growth is biphasic, reaching exuberant lengths by the fourth postnatal week and then retracting to adult lengths by 7 weeks of age. This dendritic growth is androgen dependent--males castrated on postnatal day (P) 7 and given daily injections of testosterone propionate (TP) had exuberant dendritic lengths similar to those of normal males; dendritic length in oil-treated males remained at P7 levels. The early exuberant dendritic length was retained in TP-treated males through P49. The retraction of SNB dendrites after P28 is also influenced by androgens. Males castrated at P28 and given testosterone implants retained exuberant dendritic length at P49; blank-implanted males had significantly shorter dendritic lengths by P70. These results suggest that androgens are necessary for the early exuberant growth of SNB dendrites. Furthermore, the subsequent retraction of SNB dendrites may be halted when testosterone titers reach a critical level during puberty, stabilizing their adult length.     

Motoneuron morphology in the dorsolateral nucleus of the rat spinal cord: Normal development and androgenic regulation

The rat lumbar spinal cord contains two sexually dimorphic motor nuclei, the spinal nucleus of the bulbocavernosus (SNB), and the dorsolateral nucleus (DLN). These motor nuclei innervate anatomically distinct perineal muscles that are involved in functionally distinct copulatory reflexes. The motoneurons in the SNB and DLN have different dendritic morphologies. The dendrites of motoneurons in the medially positioned SNB have a radial, overlapping arrangement, whereas the dendrites of the laterally positioned DLN have a bipolar and strictly unilateral organization. During development, SNB motoneuron dendrites grow exuberantly and then retract to their mature lengths. In this experiment we determined whether the adult difference in SNB and DLN motoneuron morphology was reflected in different patterns of dendritic growth during normal development. Furthermore, the development of both these nuclei is under androgenic control. In the absence of androgens, SNB dendrites fail to grow; testosterone replacement supports normal dendritic growth. Thus, we also examined the development of DLN dendrites for similar evidence of androgenic regulation. By using cholera toxin-horseradish peroxidase (BHRP) to label motoneurons retrogradely, we measured the morphology of DLN motoneurons in normal males, and in castrates treated with testosterone or oil/blank implants at postnatal day (P) 7, P28, P49, and P70. Our results demonstrate that in contrast to the biphasic pattern of dendritic development in the SNB, dendritic growth in the DLN was monotonic; the dendritic length of motoneurons increased more than 500% between P7 and P70. However, as in the SNB, development of DLN motoneuron morphology is androgen-dependent. In castrates treated with oil/blank implants, DLN somal and dendritic growth were greatly attenuated compared to those of normal or testosterone-treated males. Thus, while androgens are clearly necessary for the growth of motoneurons in both the SNB and DLN, their different developmental patterns suggest that other factors must be involved in regulating this growth. © 1993 Wiley-Liss, Inc.     

Morphology of rat spinal motoneurons with normal and hormonally altered specificity

Potential determinants of motoneuronal morphology were examined by using a sexually dimorphic, steroid-sensitive neuromuscular system in the rat spinal cord. In males, the spinal nucleus of the bulbocavernosus (SNB) innervates the perineal muscles bulbocavernosus (BC) and levator ani (LA), and the dorsolateral nucleus (DLN) innervates the ischiocavernosus muscle (IC). Adult females normally lack these motoneurons and the peripheral targets. Prenatal exposure of females to the androgen dihydrotestosterone propionate (DHTP) partially masculinizes this neuromuscular system and alters moto-neuron-to-muscle specificity, resulting in retained SNB target muscles anomalously innervated by motoneurons in the DLN. Because the morphology of SNB and DLN motoneurons normally differs significantly, the influence of spinal cord location and peripheral target on motoneuron morphology can be directly compared. Injection of cholera toxin conjugated to horseradish peroxidase (CTHRP) into the LA of DHTP-treated females labeled motoneurons predominantly in the SNB. These (SNB-LA) motoneurons in DHTP females were identical in all morphological measures to those of normal males. CTHRP injection into the BC of DHTP females labeled motoneurons in both the SNB and the DLN. SNB-BC motoneurons in DHTP females resembled those of normal males in process number and orientation, but were significantly smaller in dendritic length per motoneuron and in soma size. The DLN motoneurons anomalously projecting to the BC in DHTP females differed significantly from SNB-BC motoneurons in soma size and number and orientation of primary processes. However, these motoneurons were identical in all respects to DLN-IC motoneurons in DHTP females; DLN-IC motoneurons were similar to those of normal males in the orientation of their dendritic arbor, but were significantly smaller in dendritic length, soma size, and number of primary processes. These comparisons make it clear that DHTP selectively affects motoneuronal specificity and morphology in specific motoneuron classes. Further, motoneuronal morphology in the SNB/DLN system appears to be influenced more by spinal cord location than by peripheral target.     

Hormonal control of neuron number in sexually dimorphic spinal nuclei of the rat: I. Testosterone-regulated death in the dorsolateral nucleus

Adult male rats have substantially more motoneurons than do females in two motor nuclei in the lumbar spinal cord: the spinal nucleus of the bulbocavernosus (SNB) and the dorsolateral nucleus (DLN). Previous studies of the development of the SNB revealed that the sex difference in SNB motoneuron number is established through a differential motoneuron death which is under the control of androgens. In this study the development of the sexually dimorphic DLN was examined to test the hypothesis that early androgen action also determines the sex difference in DLN motoneuron number by regulating normally occurring motoneuron death. Because SNB motoneurons may migrate from the DLN, quantitative examination of DLN development was necessary in order to understand more completely the cellular mechanisms contributing to the establishment of dimorphic motoneuron number. At 5 days before birth, the number of motoneurons in the DLN is significantly higher than in adulthood in both sexes, and no sex difference is present. There is a decrease in motoneuron numbers prenatally in both sexes, which is consistent with the emigration of presumptive SNB motoneurons. Motoneuron number declines differentially through the first week of postnatal life and by postnatal day 10 motoneuron numbers are in the adult range and the sex difference is fully expressed. Females lose significantly more DLN motoneurons than males through a differential death as revealed by the higher incidence of degenerating cell profiles. Females treated with testosterone propionate have a male-typical motoneuron loss and incidence of degenerating cells. These results indicate that steroid hormones establish the sex difference in DLN motoneuron number by regulating normally occurring cell death.     

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.     

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.     

Hormonal control of neuron number in sexually dimorphic spinal nuclei of the rat: III. Differential effects of the androgen dihydrotestosterone

The spinal cord of the rat contains two sexually dimorphic nuclei: the spinal nucleus of the bulbocavernosus (SNB) and the dorsolateral nucleus (DLN). These nuclei and the perineal muscles they innervate are present in males but reduced or absent in females. The sex difference in motoneuron number in these nuclei is due to an androgen-regulated motoneuron death. Developing females treated with the androgen testosterone propionate (TP) have a fully masculine number of SNB and DLN motoneurons and retain the perineal muscles they would normally have lost. Paradoxically, females treated prenatally with the androgen dihydrotestosterone propionate (DHTP) also retain the perineal musculature but as adults lack the SNB motoneurons which would normally innervate them. The SNB target muscles retained by DHTP females are anomalously innervated by motoneurons in the DLN. Counts of motoneurons and degenerating cells in the developing SNB of DHTP-treated females showed that their feminine number is the result of a failure of DHTP to prevent the death of SNB motoneurons. Furthermore, the peak number of SNB motoneurons was below that of normal females, suggesting that DHTP treatment may also have inhibited motoneuronal migration. However, DHTP treatment fully masculinized both motoneuron number and degenerating cell counts in the DLN of these females, and it is this masculinized DLN that gives rise to the anomalous projection. Taken together, these results suggest that the effects of different androgens during development are specific and complex, involving the regulation of motoneuron death, migration, and specification of peripheral projections.     

Development of androgen receptor and p75(NTR) mRNAs and peptides in the lumbar spinal cord of the gerbil

Development of sex differences in the spinal cord appears to be largely under the control of androgen and although neurotrophins may also have a role. Spinal cords of male and female neonatal gerbils (postnatal days 1, 5, 7, 10, 23) and adult gerbils (postnatal day 150) were examined to determine the relative temporal expression of androgen receptor (AR) and the low-affinity neurotrophin receptor (p75) mRNAs within the spinal nucleus of the bulbocavernosus (SNB) and dorsolateral nucleus (DLN). Furthermore, prepubertal male gerbils were placed into one of six gonadal hormone treatment groups at weaning: Either sham castrate, castrated with gonadal hormone replacement, or castrated without gonadal hormone replacement. Ten weeks later gerbils were aldehyde-perfused, spinal cords removed and processed for presence of AR and p75 immunoreactivity (ir) in motoneurons of the SNB and DLN. During neonatal development, there were significant increases in androgen receptor mRNA within the SNB and DLN. In the SNB, the increase in androgen receptor mRNA preceded the increase in p75 mRNA. Peripubertally, significantly more SNB than DLN motoneurons contained AR- and p75-ir. These data demonstrate that AR expression occurs along the same developmental time frame as the development of the SNB and DLN and the organizational effects of androgens on their development continues through puberty in the male gerbil.     

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.     

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.     

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.     

Maternal licking influences dendritic development of motoneurons in a sexually dimorphic neuromuscular system

Maternal licking of pups' perineal regions affects the development of the spinal nucleus of the bulbocavernosus (SNB), a sexually dimorphic motor nucleus in the lumbar spinal cord that controls penile reflexes involved with copulation. Maternal licking influences SNB motoneuron number, with reductions in licking resulting in fewer motoneurons. Reduced maternal licking also has functional consequences in adulthood, resulting in increased latency to ejaculation and postejaculatory intromission and longer interintromission intervals. In this experiment, we assessed the potential effect of maternal licking on the development of SNB dendritic morphology. To reduce maternal licking, dams were treated with intranasal application of zinc sulfate during the first two postnatal weeks, which interferes with their ability to detect the pup odors that drive the licking behavior. At either postnatal day (P) 28 (when SNB dendritic length is normally maximal) or P49 (when SNB dendritic morphology is normally mature), SNB motoneurons were retrogradely labeled with cholera toxin-conjugated HRP, and dendritic arbor was reconstructed in three dimensions. At P28, the dendritic arbor of reduced maternal licking pups was not different from controls; however at P49, reduced licking pups showed a 23% reduction in dendritic arbor in the SNB, an effect that was especially pronounced in the rostral end of the nucleus, where reductions reached 48%. These results suggest that reductions in perineal stimulation provided by maternal licking could affect adult male copulatory behavior via alterations in SNB motoneuron morphology, and thus support maternal licking as an important factor in normal neural and behavioral development.     

Hormonal control of neuron number in sexually dimorphic spinal nuclei of the rat: II. Development of the spinal nucleus of the bulbocavernosus in androgen-insensitive (Tfm) rats

The spinal nucleus of the bulbocavernosus (SNB) is a sexually dimorphic motor nucleus whose development is under the control of steroid hormones. The SNB contains many more motoneurons in adult male rats than in females, and this sex difference is produced by a sexually dimorphic motoneuron death which is regulated by androgens. To study further the role of androgens in the development of sex differences in SNB motoneuron number, we examined SNB development in males with the testicular feminization mutation (Tfm) which renders them insensitive to androgens. Counts of SNB motoneurons perinatally revealed that SNB development in normal male and female King-Holtzman rats was similar to that reported previously for Sprague-Dawley rats; SNB motoneuron number increased from initially low levels at embryonic day 18 through the day before birth, when motoneuron numbers in both sexes were substantially higher than adult levels. After this prenatal increase, motoneuron number declined in both sexes, until by postnatal day 10 motoneuron numbers were in their adult ranges and the sex difference was fully expressed. Females lost more motoneurons than did males during this period, and this loss was due to motoneuron death as revealed by counts of degenerating cells. SNB development in King-Holtzman Tfm males was similar to that of normal males through embryonic day 20, suggesting that androgens may not be necessary for the initial increase in motoneuron numbers in the SNB. Thereafter, SNB motoneuron numbers in Tfm males declined in a female-typical fashion; Tfm males and normal females did not differ at any postnatal age.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Distribution of dendrites from biventer cervicis and complexus motoneurons stained intracellularly with horseradish peroxidase in the adult cat

The three-dimensional distribution of dendrites from the dorsal neck muscles biventer cervicis (BC) and complexus (CM) was examined in the adult cat using intracellular staining techniques. Motoneurons were electrophysiologically identified, stained with injection of horseradish peroxidase, and reconstructurcted from serial histological sections. The dendritic distributions of all motoneurons examined followed an orderly pattern. Many dendrites extended rostrally and caudally to form a complex parallel collection of dendrites in the ventromedial nucleus. Other dendrites projected dorsolaterally into the spinal accessory nucleus and lateral parts of lamina VII and VIII. Dorsomedial dendrites followed a path parallel to the medial border of the ventral horn and frequently terminated near the central canal. A few scattered dendrites were usually found directly dorsal to the soma in lamina VIII. This pattern of dendritic distribution differed distinctly from the dendritic distribution of motoneurons in other spinal regions. However, in all spinal regions, including the upper cervical spinal cord where BC and CM motoneurons were found, the pattern of dendritic distribution from different motoneurons was similar if their somata were located in the same region. For 15 motoneurons with well-stained dendrites, the mean rostral-caudal extent of the dendritic tree was 2,860 μm. The mean total dendritic length of three of these motoneurons measured 73,100 μm, almost four times larger than hindlimb motoneurons involved in planter reflexes. Despite the large size of the dendritic trees of BC and CM motoneurons, the surface areas of BC and CM cell bodies were smaller than most large hindlimb motoneurons. These quantitative differences in motoneuron dimensions may in turn be reflected by differences in the electrotonic properties of motoneurons in different motoneuron nuclei.     

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.     

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^WTX^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.     

Morphometric analysis of phrenic motoneurons in the cat during postnatal development.

The dendritic geometry of 20 phrenic motoneurons from four postnatal ages (2 weeks, 1 and 2 months, and adult) was examined by using intracellular injection of horseradish peroxidase. The number of primary dendrites (approximately 11-12) remained constant throughout postnatal development. In general, postnatal growth of the dendrites resulted from an increase in the branching and in the length and diameter of segments at all orders of the dendritic tree. There was one exception. Between 2 weeks and 1 month, the maximum extent of the dendrites increased in parallel with the growth of the spinal cord; however, there was no increase in either combined dendritic length or total membrane surface area. In addition, there was a significant decrease in the number of dendritic terminals per cell (59.8 +/- 9.3 vs. 46.4 +/- 7.4 for 2 weeks and 1 month, respectively). The distance from the soma, where the peak number of dendritic terminals per cell occurred, ranged from 700-900 microns at 2 weeks and 2 months to 1,300-1,700 microns in the adult. The diameter of dendrites as a function of distance from the soma along the dendritic path increased with age. The process of maturation tended to increase the distance from the soma over which the surface area and dendritic trunk parameter (sigma d1.5/D1.5) remained constant. The three-dimensional distribution of dendrites was analyzed by dividing space into six equal volumes or hexants. This analysis revealed that the postnatal growth in surface area in the rostral and caudal hexants was proportionately larger than that in either the medial, lateral, dorsal, or ventral hexants. Strong linear correlations were found between the diameter of the primary dendrite and the combined length, surface area, volume, and number of terminals of the dendrite at all ages studied.     

Dendritic distribution of motoneurons innervating the three heads of the trapezius muscle in the cat

The organization of the nuclei and dendritic architecture of motoneurons innervating the three heads of the trapezius muscle, clavotrapezius (CT), acromiotrapezius (AT), and spinotrapezius (ST), have been examined by using intracellular staining techniques. CT, AT, and ST motoneurons were found in the spinal accessory nucleus and were arranged in three overlapping subnuclei. CT motoneurons were primarily found in C2 and C3. In contrast, most AT motoneurons were found in C3, C4, and C5 and ST motoneurons were found in C4, C5, and the rostral parts of C6. Most dendrites of CT motoneurons, located in rostral C2, extended dorsally and many of these dendrites spread medially and laterally to encompass all of lamina VIII and the dorsolateral part of lamina VII. When viewed in the horizontal plane these motoneurons had a stellate appearance. The dendritic tree structure of CT motoneurons changed abruptly between rostral C2 and mid-C2. The majority of dendrites of CT motoneurons located in the central and caudal parts of C2 projected rostrally and caudally to form a complex bundle of dendrites in the motoneuron nucleus. Small numbers of dendrites were also found ventromedial and dorsal to the soma. The dendritic trees of CT motoneurons in C3 and C4 and AT and ST motoneurons located in C4 and the rostral parts of C5 also followed this fusiform distribution pattern. The dendritic trees of AT and ST motoneurons in caudal C5 were not fusiform but instead had a complex distribution pattern which consisted of dendrites projecting in several directions. Many dendrites projected rostrally and caudally, and in addition, there were major dendritic projections ventrolateral and dorsolateral to the soma. These results indicate that each head of the trapezius muscle is innervated by two structurally dissimilar groups of motoneurons which occupy different spinal segments. Trapezius motoneurons at the same segmental level, regardless of which head of the trapezius muscle they innervated, have similar dendritic trees whose structure differs from those of neighbouring dorsal neck muscle motoneurons in C2, C3, and C4. Thus, the organization of motoneuron dendritic trees appears to be governed by several factors including the muscle innervated by the motoneuron and the transverse and segmental position of the motoneuron's soma.