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D B Kelley

Publications and source records attributed to D B Kelley.

At least 37 records · Page 2Linked to original sources
PubMed1995

Sexual differentiation and hormonal regulation of the laryngeal synapse in Xenopus laevis.

In Xenopus laevis frogs, sex differences in adult laryngeal synapses contribute to sex differences in vocal behavior. This study explores the development of sex differences in types of neuromuscular synapses and the development and hormone regulation of sex differences in transmitter release. Synapses in the juvenile larynx have characteristics not found in adults: juvenile muscle fibers can produce subthreshold or suprathreshold potentials in response to the same strength of nerve stimulation and can also produce multiple spikes to a single nerve stimulus. Juvenile laryngeal muscle also contains the same synapse types (I, II, and III) as are found in adult laryngeal muscle. The distribution of laryngeal synapse types in juveniles is less sexually dimorphic than the distribution in adults. Analysis of quantal content indicates that laryngeal synapses characteristically release low amounts of transmitter prior to sexual differentiation. Quantal content values from male and female juveniles are similar to values for adult males and are lower than values for adult females. When juveniles are gonadectomized and treated with exogenous estrogen, quantal content values increase significantly, suggesting that this hormone may increase transmitter release at laryngeal synapses during development. Gonadectomy alone does not affect quantal content of laryngeal synapses in either sex. Androgen treatment decreases quantal content in juvenile females but not males; the effect is opposite to and smaller than that of estrogen. Thus, muscle fiber responses to nerve stimulation and transmitter release are not sexually dimorphic in juvenile larynges. Transmitter release is strengthened, or feminized, by the administration of estradiol, an ovarian steroid hormone.

Animals↗
PubMed1995

Androgen-directed development of the Xenopus laevis larynx: control of androgen receptor expression and tissue differentiation.

The larynx of adult male Xenopus laevis differs markedly from that of the female; masculinization of the larynx requires androgen secretion during postmetamorphic development. Early in postmetamorphic development, androgen stimulates laryngeal cell proliferation and androgen binding activity is high. Later, androgen induces laryngeal cell differentiation and binding levels decrease. Here we explore the relation between laryngeal differentiation and androgen receptor expression. In untreated females, the larynx expresses high levels of androgen receptor mRNA early in postmetamorphic life; levels decline as females mature. The highest level of androgen receptor message is found in the undifferentiated laryngeal elastic precartilage of both sexes. When juveniles are exposed to androgen, laryngeal cell proliferation is stimulated within 48 hr in both sexes. Short exposures to androgen result in a biphasic response of AR mRNA levels; a marked down-regulation (4 hr) is followed by recovery at 8 hr (males) or 48 hr (females). Following longer periods of androgen treatment (3 weeks), AR mRNA expression is down-regulated and male-typical differentiation of elastic cartilage is induced in both sexes. Thus laryngeal growth responses to androgen are closely related to expression levels of androgen receptor mRNA.

Animals↗
PubMed1995

Androgen regulation of a laryngeal-specific myosin heavy chain mRNA isoform whose expression is sexually differentiated.

The larynx, the vocal organ of Xenopus laevis, is sexually differentiated; male laryngeal muscle fibers are entirely fast twitch while female fibers are predominantly slow twitch. In adults, all male laryngeal muscle fibers express the mRNA for a laryngeal-specific myosin heavy chain (MHC), LM; female laryngeal muscle expresses LM in a subset of fast-twitch fibers. In juvenile females, LM expression is increased by exposure to exogenous androgen, suggesting that sexually differentiated expression of this laryngeal-specific MHC is regulated by exposure to male sex hormones. Here we examine hormonal regulation of LM expression in juvenile male and female frogs. Exposure to exogenous androgen increases LM expression in both sexes. In situ hybridization analyses of larynges reveal upregulation of LM expression within 8 hr of androgen treatment in males; upregulation is not evident until after 48 hr in females. The upregulation in juvenile females includes both an increase in the number of muscle fibers expressing LM and an increase in expression in fibers already expressing LM. In juvenile males, all fibers express LM from the end of metamorphosis on; levels of LM expression are increased in all fibers by androgen treatment. Androgen-induced increases in LM expression are prevented by treatment with cycloheximide and are thus dependent upon protein synthesis. Castration of juvenile males results in diminished LM expression relative to intact animals. We conclude that expression of LM is regulated by exposure to androgen and that this regulation accounts for the sexually differentiated phenotype.

Androgens↗
PubMed1995

A sex difference in synaptic efficacy at the laryngeal neuromuscular junction of Xenopus laevis.

Under physiological conditions, the response of Xenopus laevis laryngeal muscle fibers to nerve stimulation is sexually differentiated; subthreshold potentials are common in males and rare in females. This sex difference in muscle fiber response is correlated with sex differences in vocal behavior. Quantal analyses at male and female laryngeal synapses were performed to determine if there is a sex difference in synaptic strength. Quantal content at laryngeal synapses is significantly higher in females than in males. Values for quantal content in males can be increased by raising extracellular calcium concentration. There is no sex difference in miniature endplate potential amplitude suggesting that ACh receptor number or properties are not different in the sexes. Sex difference in synaptic strength thus appear presynaptic in origin; transmitter release is less in males. Ultrastructural analyses of the laryngeal motor terminal indicate that there is no sex difference in the length of active zones or in the number of channels per length of active zone. Thus, ultrastructural characteristics of the laryngeal motor terminal do not account for the pronounced sex difference in quantal content.

Animals↗
PubMed1994

Androgen directs sexual differentiation of laryngeal innervation in developing Xenopus laevis.

In adult Xenopus laevis, innervation of the vocal organ is more robust in males than in females. This sex difference originates during tadpole development; at stage 56, when the gonads first differentiate, the number of axons entering the larynx is the same in the sexes, but by stage 62, innervation is greater in males. To determine if androgen secretion establishes sex differences in axon number, we treated tadpoles with antiandrogen or androgen beginning at stage 48 or 54 and counted laryngeal nerve axons at stage 62 using electron microscopy. When male tadpoles were treated with the antiandrogen hydroxyflutamide, axon numbers were reduced to female-typical values; axon numbers in females were unaffected by antiandrogen treatment. When female tadpoles were treated with the androgen DHT (dihydrotestosterone), axon numbers were increased to male-like values. These findings suggest that endogenous androgen secretion during late tadpole stages in males is required for the sexual differentiation of laryngeal innervation observed from stage 62 on. Because androgen treatment and laryngeal innervation affect myogenesis in postmetamorphic frogs, numbers of laryngeal dilator muscle fibers were determined for hormonally manipulated tadpoles. At stage 62, vehicle-treated males had more laryngeal axons than females; laryngeal muscle fiber numbers did not, however, differ in the sexes. Both male and female tadpoles, treated from stage 54 with DHT, had more muscle fibers at stage 62 than vehicle-treated controls. Thus, while endogenous androgen secretion during late tadpole stages is subthreshold for the establishment of masculinized muscle fiber numbers, laryngeal myogenesis is androgen sensitive at this time and can be increased by suprathreshold provision of exogenous DHT. A subgroup of tadpoles, DHT treated from stage 54 to 62, was allowed to survive, untreated, until postmetamorphic stage 2 (PM2: 5 months after metamorphosis is complete). Androgen treatment between tadpole stages 54 and 62 does not prevent the ontogenetic decrease in axon numbers characteristic of laryngeal development. In addition, the elevation in stage 62 axon numbers produced by DHT-treatment at late tadpole stages was not associated with elevated numbers of laryngeal muscle fibers at PM2. Juvenile males normally maintain elevated axon numbers (relative to final adult values) through PM2 and the presence of these additional axons may result from--rather than contribute directly to--laryngeal muscle fiber addition.

Animals↗
PubMed1993

Laryngeal muscle and motor neuron plasticity in Xenopus laevis: testicular masculinization of a developing neuromuscular system.

In Xenopus laevis, the sexual differentiation of the neuromuscular system responsible for courtship song is controlled by testicular androgen secretion. To explore the sensitivity of this system to androgenic stimulation, male and female frogs were gonadectomized and given testis transplants at seven different developmental stages between the end of metamorphosis and adulthood, grown to sexual maturity, and the laryngeal muscle fibers and motor axons were counted. Muscle fiber and axon numbers in males were not affected by the testicular transplant at any stage. In females, testicular transplants at all developmental stages increased muscle fiber numbers in adulthood. Values attained were, however, significantly less than those of adult intact or testis-transplanted males. Testis transplantation increased laryngeal axon numbers in females to levels equivalent to those of intact males; this effect was obtained at every stage of postmetamorphic development including adulthood. To further explore androgen regulation in adults, males and females were gonadectomized and implanted with silicone tubes containing testosterone propionate for 1.5-3 years and laryngeal muscle fibers and axon numbers compared to those of gonadectomized or sham-operated adult controls. Neither treatment with exogenous androgen nor gonadectomy had any effect on laryngeal muscle fiber or axon number in either males or females; values did not differ from those of sham-operated controls. We conclude that testicular secretions can induce laryngeal muscle fiber and axon addition in females throughout postmetamorphic life. This degree of plasticity, exhibited after the period when adult values are normally attained, stands in contrast to the effects of administration of synthetic androgen and suggests that the degree of plasticity in adult females may be underestimated if exogenous hormones rather than testicular transplants are provided.

Animals↗
PubMed1993

The roles of sex, innervation, and androgen in laryngeal muscle of Xenopus laevis.

The relative contributions of innervation and androgen to three muscle fiber properties--twitch type, size, and number--were examined in the sexually dimorphic, androgen-sensitive laryngeal muscle of Xenopus laevis. In adults, the muscle contains all fast-twitch fibers in males and fast- and slow-twitch fibers in females; laryngeal muscle fibers are larger and more numerous in males than in females. Juvenile larynges are female-like in both sexes; male laryngeal muscle is subsequently masculinized by androgen secretion during postmetamorphic development. Because both laryngeal motor neurons and muscle fibers are androgen sensitive during masculinization, we examined the role of the nerve in androgen-regulated muscle fiber development. Laryngeal muscle of male and female juvenile frogs was unilaterally denervated, and effects on muscle fiber type, size, and number were examined 4 weeks later. Half of the frogs received a dihydrotestosterone pellet at the time of denervation. Androgen treatment converts laryngeal muscle from mixed slow and fast to all fast twitch in both innervated and denervated muscle. Thus, the nerve is not required for androgen-regulated fiber type expression in either sex. Denervation produces muscle fiber atrophy and androgen treatment induces muscle fiber hypertrophy in male and female larynx. Nerve and hormone effects are independent and additive; fiber size in androgen-treated denervated muscle is greater than in untreated innervated muscle, and fiber size in androgen-treated denervated muscle is smaller than in androgen-treated innervated muscle. There is no sex difference in the effects of innervation or androgen on fiber size. Denervation causes laryngeal muscle fiber loss in males but not in females. Androgen treatment protects male laryngeal muscle from denervation-induced fiber loss and causes fiber addition in innervated female laryngeal muscle. We conclude that there is a sexually dimorphic interaction between innervation and androgen in control of laryngeal muscle fiber number.

Androgens↗
PubMed1992

Testicular masculinization of vocal behavior in juvenile female Xenopus laevis reveals sensitive periods for song duration, rate, and frequency spectra.

In Xenopus laevis, adult males but not females produce courtship songs comprised of rapid trills. Two experiments were conducted to determine whether male-typical singing could be induced in females. At 6 different juvenile stages, male and female frogs were gonadectomized and implanted with testes, grown to sexual maturity, and tested for vocal behavior. All frogs with functional testicular implants sang; females sang as much as males. The frequency spectra of the clicks within trills were fully masculinized in females implanted at PM0, PM1, and PM2. There were deficiencies in song quality in females implanted late in juvenile life. Females receiving testis implants at PM3, PM4, and PM5 did not produce clicks with masculine spectral qualities. In a concurrent experiment, adult males and females were gonadectomized and implanted with testes or silicone tubes containing testosterone propionate. When tested for vocal behavior 10 to 15 months after implantation, 8/10 androgen-treated males, 3/12 androgen-treated females, 5/5 testes-implanted males, and 2/4 testes-implanted females sang. The females that did sing spent much less time singing than males. The click rates of females were uniformly slower than males and no female produced clicks with a masculine frequency spectrum. Thus, testicular secretions can induce male-typical singing in females until late in juvenile development. However, females exhibit a progressive decline in vocal potential with increasing age, culminating in an almost complete loss of singing ability by adulthood.

Acoustic Stimulation↗
PubMed1992

Sexually dimorphic expression of a laryngeal-specific, androgen-regulated myosin heavy chain gene during Xenopus laevis development.

Masculinization of the larynx in Xenopus laevis frogs is essential for the performance of male courtship song. During postmetamorphic (PM) development, the initially female-like phenotype of laryngeal muscle (slow and fast twitch fibers) is converted to the masculine form (entirely fast twitch) under the influence of androgenic steroids. To explore the molecular basis of androgen-directed masculinization, we have isolated cDNA clones encoding portions of a new Xenopus myosin heavy chain (MHC) gene. We have detected expression of this gene only in laryngeal muscle and specifically in males. All adult male laryngeal muscle fibers express the laryngeal myosin (LM). Adult female laryngeal muscle expresses LM only in some fibers. Expression of LM during PM development was examined using Northern blots and in situ hybridization. Males express higher levels of LM than females throughout PM development and attain adult levels by PM3. In females, LM expression peaks transiently at PM2. Treatment of juvenile female frogs with the androgen dihydrotestosterone masculinizes LM expression. Thus, LM appears to be a male-specific, testosterone-regulated MHC isoform in Xenopus laevis. The LM gene will permit analysis of androgen-directed sexual differentiation in this highly sexually dimorphic tissue.

Amino Acid Sequence↗
PubMed1991

Development of functional sex differences in the larynx of Xenopus laevis.

Three laryngeal properties associated with the production of masculine song--laryngeal muscle tension, fiber twitch type, and fiber recruitment--are markedly sexually dimorphic in adult Xenopus laevis frogs. To elucidate the pattern of sexual differentiation, tension and fiber recruitment in male and female larynges and fiber twitch type in male larynges were examined throughout postmetamorphic development. Masculinization of male laryngeal properties begins early in postmetamorphic development and continues until adulthood. In contrast, tension and fiber recruitment in females do not change after the end of metamorphosis. Laryngeal muscle tension and fiber type are gradually and progressively masculinized; the temporal pattern of masculinization is very similar for these properties. Fiber recruitment, on the other hand, appears to masculinize in a stepwise manner. Masculinization of all three properties is highly correlated with larynx weight in males. We have used this relation to divide postmetamorphic development into seven stages associated with key events in sexual differentiation. This staging scheme provides an important experimental tool for studying the hormonal regulation of sexual differentiation, the subject of the accompanying paper.

Animals↗
PubMed1991

Temporal constraints on androgen directed laryngeal masculinization in Xenopus laevis.

Temporal constraints on androgen regulated masculinization of three sexually dimorphic laryngeal properties--tension, fiber type, and fiber recruitment--were examined in Xenopus laevis frogs. Endocrine state was manipulated at PM0 when the larynx is similar in males and females, at PM2 when the larynx begins sexual differentiation, and at PM6 when sexual differentiation is complete. Removing the testes in developing males (PM0 or PM2) completely arrests laryngeal masculinization. Masculinization resumes when testosterone is replaced later in development (PM2 or PM6, respectively). Thus, testicular secretions, in particular androgens, are required for laryngeal masculinization. The ability of androgens to masculinize tension, fiber type, and fiber recruitment in developing and adult larynges was also determined. Five weeks of testosterone treatment in PM0 or PM2 males and females completely masculinizes laryngeal tension and fiber type, but only partially masculinizes fiber recruitment. However, fiber recruitment can be fully masculinized in PM6 males castrated at PM2. We conclude that androgen induced masculinization of tension and fiber type are not temporally constrained but that androgen induced masculinization of fiber recruitment is. Prolonged androgen treatment can override the temporal constraints on masculinization of the larynx. Testosterone treatment for more than 6 months fully masculinizes fiber recruitment in developing (PM0 or PM2) females. In addition, prolonged treatment (greater than 9 months) completely masculinizes tension, fiber type, and fiber recruitment in adult females; these properties were not fully masculinized by shorter (1-3 months) treatments in adult females. Testosterone induced masculinization in females is maintained for up to 8 months following testosterone removal; thus androgen effects are long lasting and possibly permanent.

Aging↗
PubMed1990

Molecular cloning of androgen receptors from divergent species with a polymerase chain reaction technique: complete cDNA sequence of the mouse androgen receptor and isolation of androgen receptor cDNA probes from dog, guinea pig and clawed frog.

We have cloned and sequenced 2.8 kilobases of cDNA encoding the mouse androgen receptor by RNA amplification with transcript sequencing. Sequence analysis predicts that this cDNA contains an open reading frame of 2697 nucleotides encoding a polypeptide of 899 amino acids. Androgen receptor cDNA probes of dog, guinea pig, and frog were also isolated and sequenced using consensus primers derived from human and rat androgen receptor cDNAs. Northern blot analysis with the species-specific probes revealed similarities in size between amphibian and mammalian mRNAs. These results demonstrate the utility of this technique in obtaining nucleic acid probes and sequence information of steroid receptors from different species. The sequence data and the Northern blot analysis of the receptors in different species demonstrate that the androgen receptor has been well-conserved during evolution.

Amino Acid Sequence↗
PubMed1990

The vocal motor neurons of Xenopus laevis: development of sex differences in axon number.

Sex differences in the number of muscle fibers in the larynx of clawed frogs (Xenopus laevis) develop after metamorphosis. In order to examine possible contributions of neural innervation to this process, we prepared sections of the laryngeal nerve from tadpole stage 56, when the sexes can first be distinguished, through adulthood, and counted axons on electron micrographs. The adult number of axons is achieved by a sexually differentiated pattern of axonal addition and loss. Axon numbers are high at tadpole stage 56 and equal for males and females; thereafter, males have more axons. Sex differences are most pronounced at tadpole stage 62 because between stages 59 and 62 the number of laryngeal axons in males increases by an average of 119 per nerve. Ultrastructural evidence is congruent with the hypothesis that new axons are added to the laryngeal nerve between tadpole stages 56 and 62. The loss of axons from the laryngeal nerve is greater for females than for males. Between tadpole stages 56 and adulthood, overall axon number decreases by 47% in males and by 64% in females. Signs of axonal degeneration are present in both sexes before metamorphosis but are rare at juvenile or adult stages. The numbers of axons in juvenile frogs do not differ from those in adults and continue to be greater in males than in females. In contrast to the amount of axon addition and loss, the timing of axon loss and the percentage of myelinated axons is the same for males and females throughout development. Thus sex differences in the innervation of laryngeal muscle originate before metamorphosis and could contribute to the marked sex differences in muscle fiber addition that occur thereafter.

Animals↗
PubMed1990

An improved method for mounting frozen-section specimens.

We describe an efficient method for embedding frozen-section specimens. Transparent tape is used to produce a well on the surface of the tissue holder ("chuck"). This well contains a quantity of nonmotile mounting media in which a tissue specimen can be easily oriented. After freezing, the tape is removed, leaving a relatively smooth surface.

Specimen Handling↗
PubMed1990

Hormone-sensitive stages in the sexual differentiation of laryngeal muscle fiber number in Xenopus laevis.

The number of muscle fibers in the vocal organ of the adult male African clawed frog, Xenopus laevis, exceeds that of adult females. This sex difference is the result of rapid fiber addition in males between the end of metamorphosis, post-metamorphic stage 0 (PM0) and PM2. At PM0, male and female frogs have similar numbers of laryngeal muscle fibers. Males then add more muscle fibers than females and achieve an adult value that is 1.7 times the female number. Males castrated at PM0 have the same fiber number as females. Ovariectomy at PM0 does not alter muscle fiber addition in females. Gonadectomy at PM2 has no effect on fiber addition in either sex. Females attain masculine muscle fiber number if their ovaries are replaced with a testis at metamorphosis. Exogenous testosterone treatment at PM0 significantly increases fiber number in females but not in males. Exogenous testosterone given at PM2 has no effect on fiber number in females but decreases fiber number in males. We conclude that the testes are necessary for the marked addition of laryngeal muscle fibers seen in male X. laevis between PM0 and PM2. The masculine pattern of muscle fiber addition can be induced in females provided with a testis. Androgen secretion from the testes most probably accounts for masculinization of laryngeal muscle fiber number. After PM2, androgens are no longer necessary for muscle fiber addition and cannot increase fiber number in females.

Androgens↗
PubMed1989

Connective tissue disease in southeastern Georgia: a case-control study of etiologic factors.

The authors conducted a population-based case-control study of connective tissue disease in a four-county area in southeastern Georgia. Interviews with 44 cases and 88 controls were conducted in September and October of 1986. The association with a family history of these disorders was reconfirmed, and a positive association with the use of hair dyes was found. A negative association was found with a family history of diabetes mellitus.

Amines↗