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

A novel degree of sex difference in laryngeal physiology of Xenopus muelleri: behavioral and evolutionary implications

Characterizing sex and species differences in muscle physiology can contribute to a better understanding of proximate mechanisms underlying behavioral evolution. In Xenopus, the laryngeal muscle's ability to contract rapidly and its electromyogram potentiation allows males to produce calls that are more rapid and intensity-modulated than female calls. Prior comparative studies have shown that some species lacking typical male features of vocalizations sometimes show reduced sex differences in underlying laryngeal physiology. To further understand the evolution of sexually differentiated laryngeal muscle physiology and its role in generating behavior, we investigated sex differences in the laryngeal physiology of X. muelleri, a species in which male and female calls are similar in rapidity but different with respect to intensity modulation. We delivered ethologically relevant stimulus patterns to ex vivo X. muelleri larynges to investigate their ability to produce various call patterns, and we also delivered stimuli over a broader range of intervals to assess sex differences in muscle tension and electromyogram potentiation. We found a small but statistically significant sex difference in laryngeal electromyogram potentiation that varied depending on the number of stimuli. We also found a small interaction between sex and stimulus interval on muscle tension over an ethologically relevant range of stimulus intervals; male larynges were able to produce similar tensions to female larynges at slightly smaller (11-12 ms) inter-stimulus intervals. These findings are consistent with behavioral observations and present a previously undescribed intermediate sex difference in Xenopus laryngeal muscle physiology.

Differences in neuromuscular junctions of laryngeal and limb muscles in rats

OBJECTIVES/HYPOTHESIS

Laryngeal muscles are specialized for fine control of voice, speech, and swallowing, and may differ from limb muscles in many aspects. Because muscles and their controlling motor neurons communicate via neuromuscular junctions (NMJs), we hypothesized that NMJs in laryngeal muscles have specialized characteristics different from limb muscles.

STUDY DESIGN

In vivo study.

METHODS

Single muscle fibers from 12 Sprague-Dawley rats (six male, six female) were used to analyze the postsynaptic side of NMJs from laryngeal thyroarytenoid (TA), cricothyroid (CT), posterior cricoarytenoid (PCA), limb soleus (SOL), and extensor digitorum longus (EDL) muscles. NMJs were labeled with rhodamine-conjugated α-bungarotoxin. With confocal microscopy, we counted cluster fragments and measured the NMJ area, both absolute and normalized (corrected by muscle fiber diameter), for at least 10 single fibers from each muscle of each animal. Differences between genders were also compared.

RESULTS

Cluster fragments of postsynaptic NMJs were more numerous in PCA and TA compared to CT, SOL, and EDL muscles (P < .01) in both male and female rats. NMJ cluster fragments were more numerous in female than in male rats only in the TA muscle (P < .01). The absolute area covered by the NMJs showed SOL > EDL > PCA > CT > TA (P < .01); however, with normalization the SOL = EDL = PCA > CT = TA.

CONCLUSIONS

Differences found in NMJ surface and organization between laryngeal and limb muscle fibers may relate to specialized laryngeal muscle functions. Differences in NMJs between male and female rats were found only in the TA muscle, suggesting an underlying mechanism for some gender-specific laryngeal disorders related to abnormal TA muscle activity.

A neuroendocrine basis for the hierarchical control of frog courtship vocalizations

Seasonal courtship signals, such as mating calls, are orchestrated by steroid hormones. Sex differences are also sculpted by hormones, typically during brief sensitive periods. The influential organizational-activational hypothesis [50] established the notion of a strong distinction between long-lasting (developmental) and cyclical (adult) effects. While the dichotomy is not always strict [1], experimental paradigms based on this hypothesis have indeed revealed long-lasting hormone actions during development and more transient anatomical, physiological and behavioral effects of hormonal variation in adulthood. Sites of action during both time periods include forebrain and midbrain sensorimotor integration centers, hindbrain and spinal cord motor centers, and muscles. African clawed frog (Xenopus laevis) courtship vocalizations follow the basic organization-activation pattern of hormone-dependence with some exceptions, including expanded steroid-sensitive periods. Two highly-tractable preparations-the isolated larynx and the fictively calling brain-make this model system powerful for dissecting the hierarchical action of hormones. We discuss steroid effects from larynx to forebrain, and introduce new directions of inquiry for which Xenopus vocalizations are especially well-suited.

Historical perspective: Hormonal regulation of behaviors in amphibians

This review focuses on research into the hormonal control of behaviors in amphibians that was conducted prior to the 21st century. Most advances in this field come from studies of a limited number of species and investigations into the hormonal mechanisms that regulate reproductive behaviors in male frogs and salamanders. From this earlier research, we highlight five main generalizations or conclusions. (1) Based on studies of vocalization behaviors in anurans, testicular androgens induce developmental changes in cartilage and muscles fibers in the larynx and thereby masculinize peripheral structures that influence the properties of advertisement calls by males. (2) Gonadal steroid hormones act to enhance reproductive behaviors in adult amphibians, but causal relationships are not as well established in amphibians as in birds and mammals. Research into the relationships between testicular androgens and male behaviors, mainly using castration/steroid treatment studies, generally supports the conclusion that androgens are necessary but not sufficient to enhance male behaviors. (3) Prolactin acts synergistically with androgens and induces reproductive development, sexual behaviors, and pheromone production. This interaction between prolactin and gonadal steroids helps to explain why androgens alone sometimes fail to stimulate amphibian behaviors. (4) Vasotocin also plays an important role and enhances specific types of behaviors in amphibians (frog calling, receptivity in female frogs, amplectic clasping in newts, and non-clasping courtship behaviors). Gonadal steroids typically act to maintain behavioral responses to vasotocin. Vasotocin modulates behavioral responses, at least in part, by acting within the brain on sensory pathways that detect sexual stimuli and on motor pathways that control behavioral responses. (5) Corticosterone acts as a potent and rapid suppressor of reproductive behaviors during periods of acute stress. These rapid stress-induced changes in behaviors use non-genomic mechanisms and membrane-associated corticosterone receptors.

Sexual differentiation of the copulatory neuromuscular system in green anoles (Anolis carolinensis): Normal ontogeny and manipulation of steroid hormones

The copulatory neuromuscular system of green anoles is sexually dimorphic and differentiates during embryonic development, although details of the process were unknown. In Experiment 1, we determined the time course of normal ontogeny. Both male and female embryos possessed bilateral copulatory organs (hemipenes) and associated muscles until incubation day 13; the structures completely regressed in female embryos by incubation day 19 (total incubation 34 days). In Experiment 2, we treated eggs with testosterone, dihydrotestosterone, estradiol, or vehicle on both incubation days 10 and 13 to determine whether these steroid hormones mediate sexual differentiation. These time points fall between gonadal differentiation, which was determined in Experiment 1 to complete before day 10, and regression of the peripheral copulatory system in females. Tissue was collected on the day of hatching. Gonads were classified as testes or ovaries; presence versus absence of hemipenes and muscles, and the number and size of copulatory motoneurons were determined. Copulatory system morphology of vehicle-treated animals matched their gonadal sex. Hemipenes and muscles were absent in estradiol-treated animals, and androgens rescued the hemipenes and muscles in most females. Both testosterone and dihydrotestosterone treatment also caused hypertrophy of the hemipenes, which were everted in animals treated with these steroids. Copulatory motoneurons, assessed on the day of hatching in both experiments, were not dimorphic in size or number. Steroid treatment significantly increased motoneuron size and number overall, but no significant differences were detected in pairwise comparisons. These data demonstrate that differentiation of peripheral copulatory neuromuscular structures occurs during embryonic development and is influenced by gonadal steroids (regression by estradiol and enhancement by androgens), but associated motoneurons do not differentiate until later in life.

Seasonal plasticity in the copulatory neuromuscular system of green anole lizards: a role for testosterone in muscle but not motoneuron morphology

The copulatory system of green anoles is highly sexually dimorphic. Males possess bilateral copulatory organs called hemipenes, each independently controlled by two muscles: the transversus penis (TPN) and retractor penis magnus (RPM). The TPN everts the hemipene through the cloaca and the RPM retracts it. Adult females do not possess hemipenes or either of these two muscles. The spinal nucleus projecting to the TPN and RPM contains more and larger motoneurons in males than females. Because anoles breed seasonally, two experiments were designed to test whether adult copulatory morphology varies with environmental condition, and if so, whether the effect is mediated by testicular androgens. Three groups of adult males were used in each experiment: males from breeding environmental conditions with reproductive testes (BS); males in breeding conditions with regressed testes (BS-X); and males in nonbreeding conditions with regressed testes (NBS). Experiment 1 compared gonadally intact males and Experiment 2 compared castrated males treated with either testosterone (T) or an empty implant. In both experiments, copulatory and control motoneurons appeared smaller in NBS males, but T did not affect their size. In contrast, while hemipene and RPM muscle fiber size were not plastic across season in gonadally intact males, T in castrated males significantly increased both measures under BS and BS-X, but not NBS, conditions. These results demonstrate that neuron soma size might change on a general level and environmental cues can mediate T-induced changes in peripheral structures, suggesting that plasticity across copulatory system components is regulated by different mechanisms.

Estrogen alters excitability but not morphology of a sexually dimorphic neuromuscular system in adult rats

In rats, motoneurons of the spinal nucleus of the bulbocavernosus (SNB) innervate the bulbocavernosus (BC) muscle, which surrounds the base of the penis. The SNB/BC is a sexually dimorphic, steroid-sensitive neuromuscular system, which is critically important in male reproductive behavior. Androgens are necessary for the development, morphology, and function of the SNB/BC system. However, estradiol (E) is also necessary for the development of the SNB/BC system, and E is capable of maintaining BC EMG activity in adulthood. In this study, we used electrophysiological and anatomical methods to examine estrogenic effects on BC EMG activity. We used a modified H-reflex testing method to investigate polysynaptic reflex characteristics in intact males, castrates, and castrates treated short term with estradiol benzoate (EB). Measures of EMG activity, response latency, and spike count were altered in castrates, but maintained in EB-treated castrates to the levels of intact males. Furthermore, estrogenic effects were found in EMG activity that could be isolated to the periphery of the SNB/BC system. BC NMJ size and muscle fiber area have been demonstrated to be hormone sensitive, and we examined these for possible correlates of E's effects on BC EMG activity. BC muscles of intact males, castrates, and short-term EB-treated castrates were fixed and stained with zinc iodide and osmium tetroxide. NMJ size and muscle fiber area did not differ between groups. Together, these data suggest that E treatment results in changes in the neuromuscular periphery that maintain BC EMG activity, but this effect cannot be accounted for by changes in NMJ size or muscle fiber area.

Androgens modulate NMDA receptor-mediated EPSCs in the zebra finch song system

Androgens potently regulate the development of learned vocalizations of songbirds. We sought to determine whether one action of androgens is to functionally modulate the development of synaptic transmission in two brain nuclei, the lateral part of the magnocellular nucleus of the anterior neostriatum (LMAN) and the robust nucleus of the archistriatum (RA), that are critical for song learning and production. We focused on N-methyl-D-aspartate-excitatory postsynaptic currents (NMDA-EPSCs), because NMDA receptor activity in LMAN is crucial to song learning, and because the LMAN synapses onto RA neurons are almost entirely mediated by NMDA receptors. Whole cell recordings from in vitro brain slice preparations revealed that the time course of NMDA-EPSCs was developmentally regulated in RA, as had been shown previously for LMAN. Specifically, in both nuclei, NMDA-EPSCs become faster over development. We found that this developmental transition can be modulated by androgens, because testosterone treatment of young animals caused NMDA-EPSCs in LMAN and RA to become prematurely fast. These androgen-induced effects were limited to fledgling and juvenile periods and were spatially restricted, in that androgens did not accelerate developmental changes in NMDA-EPSCs recorded in a nonsong area, the Wulst. To determine whether androgens had additional effects on LMAN or RA neurons, we examined several other physiological and morphological parameters. In LMAN, testosterone affected alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprianate-EPSC (AMPA-EPSC) decay times and the ratio of peak synaptic glutamate to AMPA currents, as well as dendritic length and spine density but did not alter soma size or dendritic complexity. In contrast, testosterone did not affect any of these parameters in RA, which demonstrates that exogenous androgens can have selective actions on different song system neurons. These data are the first evidence for any effect of sex steroids on synaptic transmission within the song system. Our results support the idea that endogenous androgens limit sensitive periods for song learning by functionally altering synaptic transmission in song nuclei.

Generating sexually differentiated songs

The past year has witnessed increased confusion as to the role of gonadal hormones in the development of neuroeffectors for sexually differentiated vocalizations in several species. Are sex differences in levels of circulating gonadal hormones robust enough to account for the full spectrum of male/female differences? Understanding how vocal behaviors are generated has improved, permitting greater insights into how differences in cell number and type contribute to male- and female-specific songs in frogs and birds.

Steroid hormone effects on neurons subserving behavior

Recent advances in understanding effects of steroid hormones at the level of individual neurons have been achieved using model systems. Steroid hormone effects on dendritic morphology, synaptic function and ionic conductances have been implicated in the regulation of behavior in both vertebrates and invertebrates. Particularly exciting are studies demonstrating steroid hormone effects on specific synaptic connections and ionic currents. There also has been important progress in understanding the diversity of sites and mechanisms of hormone action, encompassing both genomic and non-genomic effects of steroids on neuronal properties.