Breathing and calling: neuronal networks in the Xenopus laevis hindbrain

Evolution of vertebrate respiratory central rhythm generators

Tracing the evolution of the central rhythm generators associated with ventilation in vertebrates is hindered by a lack of information surrounding key transitions. To begin with, central rhythm generation has been studied in detail in only a few species from four vertebrate groups, lamprey, anuran amphibians, turtles, and mammals (primarily rodents). Secondly, there is a lack of information regarding the transition from water breathing fish to air breathing amniotes (reptiles, birds, and mammals). Specifically, the respiratory rhythm generators of fish appear to be single oscillators capable of generating both phases of the respiratory cycle (expansion and compression) and projecting to motoneurons in cranial nerves innervating bucco-pharyngeal muscles. In the amniotes we find oscillators capable of independently generating separate phases of the respiratory cycle (expiration and inspiration) and projecting to pre-motoneurons in the ventrolateral medulla that in turn project to spinal motoneurons innervating thoracic and abdominal muscles (reptiles, birds, and mammals). Studies of the one group of amphibians that lie at this transition (the anurans), raise intriguing possibilities but, for a variety of reasons that we explore, also raise unanswered questions. In this review we summarize what is known about the rhythm generating circuits associated with breathing that arise from the different rhombomeric segments in each of the different vertebrate classes. Assuming oscillating circuits form in every pair of rhombomeres in every vertebrate during development, we trace what appears to be the evolutionary fate of each and highlight the questions that remain to be answered to properly understand the evolutionary transitions in vertebrate central respiratory rhythm generation.

Inspiring song: The role of respiratory circuitry in the evolution of vertebrate vocal behavior

Vocalization is a common means of communication across vertebrates, but the evolutionary origins of the neural circuits controlling these behaviors are not clear. Peripheral mechanisms of sound production vary widely: fish produce sounds with a swimbladder or pectoral fins; amphibians, reptiles, and mammalians vocalize using a larynx; birds vocalize with a syrinx. Despite the diversity of vocal effectors across taxa, there are many similarities in the neural circuits underlying the control of these organs. Do similarities in vocal circuit structure and function indicate that vocal behaviors first arose in a single common ancestor, or have similar neural circuits arisen independently multiple times during evolution? In this review, we describe the hindbrain circuits that are involved in vocal production across vertebrates. Given that vocalization depends on respiration in most tetrapods, it is not surprising that vocal and respiratory hindbrain circuits across distantly related species are anatomically intermingled and functionally linked. Such vocal-respiratory circuit integration supports the hypothesis that vocal evolution involved the expansion and functional diversification of breathing circuits. Recent phylogenetic analyses, however, suggest vocal behaviors arose independently in all major tetrapod clades, indicating that similarities in vocal control circuits are the result of repeated co-options of respiratory circuits in each lineage. It is currently unknown whether vocal circuits across taxa are made up of homologous neurons, or whether vocal neurons in each lineage arose from developmentally and evolutionarily distinct progenitors. Integrative comparative studies of vocal neurons across brain regions and taxa will be required to distinguish between these two scenarios.

Generation, Coordination, and Evolution of Neural Circuits for Vocal Communication

In many species, vocal communication is essential for coordinating social behaviors including courtship, mating, parenting, rivalry, and alarm signaling. Effective communication requires accurate production, detection, and classification of signals, as well as selection of socially appropriate responses. Understanding how signals are generated and how acoustic signals are perceived is key to understanding the neurobiology of social behaviors. Here we review our long-standing research program focused on Xenopus, a frog genus which has provided valuable insights into the mechanisms and evolution of vertebrate social behaviors. In Xenopus laevis, vocal signals differ between the sexes, through development, and across the genus, reflecting evolutionary divergence in sensory and motor circuits that can be interrogated mechanistically. Using two ex vivo preparations, the isolated brain and vocal organ, we have identified essential components of the vocal production system: the sexually differentiated larynx at the periphery, and the hindbrain vocal central pattern generator (CPG) centrally, that produce sex- and species-characteristic sound pulse frequencies and temporal patterns, respectively. Within the hindbrain, we have described how intrinsic membrane properties of neurons in the vocal CPG generate species-specific vocal patterns, how vocal nuclei are connected to generate vocal patterns, as well as the roles of neurotransmitters and neuromodulators in activating the circuit. For sensorimotor integration, we identified a key forebrain node that links auditory and vocal production circuits to match socially appropriate vocal responses to acoustic features of male and female calls. The availability of a well supported phylogeny as well as reference genomes from several species now support analysis of the genetic architecture and the evolutionary divergence of neural circuits for vocal communication. Xenopus thus provides a vertebrate model in which to study vocal communication at many levels, from physiology, to behavior, and from development to evolution. As one of the most comprehensively studied phylogenetic groups within vertebrate vocal communication systems, Xenopus provides insights that can inform social communication across phyla.

Development of central respiratory control in anurans: The role of neurochemicals in the emergence of air-breathing and the hypoxic response

Physiological and environmental factors impacting respiratory homeostasis vary throughout the course of an animal's lifespan from embryo to adult and can shape respiratory development. The developmental emergence of complex neural networks for aerial breathing dates back to ancestral vertebrates, and represents the most important process for respiratory development in extant taxa ranging from fish to mammals. While substantial progress has been made towards elucidating the anatomical and physiological underpinnings of functional respiratory control networks for air-breathing, much less is known about the mechanisms establishing these networks during early neurodevelopment. This is especially true of the complex neurochemical ensembles key to the development of air-breathing. One approach to this issue has been to utilize comparative models such as anuran amphibians, which offer a unique perspective into early neurodevelopment. Here, we review the developmental emergence of respiratory behaviours in anuran amphibians with emphasis on contributions of neurochemicals to this process and highlight opportunities for future research.

Feedback to the future: motor neuron contributions to central pattern generator function

Motor behaviors depend on neural signals in the brain. Regardless of where in the brain behavior patterns arise, the central nervous system sends projections to motor neurons, which in turn project to and control temporally appropriate muscle contractions; thus, motor neurons are traditionally considered the last relay from the central nervous system to muscles. However, in an array of species and motor systems, an accumulating body of evidence supports a more complex role of motor neurons in pattern generation. These studies suggest that motor neurons not only relay motor patterns to the periphery, but directly contribute to pattern generation by providing feedback to upstream circuitry. In spinal and hindbrain circuits in a variety of animals - including flies, worms, leeches, crustaceans, rodents, birds, fish, amphibians and mammals - studies have indicated a crucial role for motor neuron feedback in maintaining normal behavior patterns dictated by the activity of a central pattern generator. Hence, in this Review, we discuss literature examining the role of motor neuron feedback across many taxa and behaviors, and set out to determine the prevalence of motor neuron participation in motor circuits.

The return to water in ancestral Xenopus was accompanied by a novel mechanism for producing and shaping vocal signals

Listeners locate potential mates using species-specific vocal signals. As tetrapods transitioned from water to land, lungs replaced gills, allowing expiration to drive sound production. Some frogs then returned to water. Here we explore how air-driven sound production changed upon re-entry to preserve essential acoustic information on species identity in the secondarily aquatic frog genus Xenopus. We filmed movements of cartilage and muscles during evoked sound production in isolated larynges. Results refute the current theory for Xenopus vocalization, cavitation, and favor instead sound production by mechanical excitation of laryngeal resonance modes following rapid separation of laryngeal arytenoid discs. Resulting frequency resonance modes (dyads) are intrinsic to the larynx rather than due to neuromuscular control. Dyads are a distinctive acoustic signature. While their component frequencies overlap across species, their ratio is shared within each Xenopus clade providing information on species identity that could facilitate both conspecific localization and ancient species divergence.

Editorial note

This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Development of an Acute Method to Deliver Transgenes Into the Brains of Adult Xenopus laevis

The central vocal pathway of the African clawed frog, Xenopus laevis, is a powerful vertebrate model to understand mechanisms underlying central pattern generation. However, fast and efficient methods of introducing exogenous genes into the neurons of adult X. laevis are currently not available. Here, we systematically tested methods of transgene delivery into adult X. laevis neurons. Although successfully used for tadpole neurons for over a decade, electroporation was not efficient in transfecting adult neurons. Similarly, adeno-associated virus (AAV) was not reliable, and lentivirus (LV) failed to function as viral vector in adult Xenopus neurons. In contrast, vesicular stomatitis virus (VSV) was a fast and robust vector for adult X. laevis neurons. Although toxic to the host cells, VSV appears to be less virulent to frog neurons than they are to mice neurons. At a single cell level, infected neurons showed normal physiological properties up to 7 days post infection and vocal circuits that included infected neurons generated normal fictive vocalizations up to 9 days post infection. The relatively long time window during which the physiology of VSV-infected neurons can be studied presents an ideal condition for the use of optogenetic tools. We showed that VSV does not gain entry into myelinated axons, but is taken up by both the soma and axon terminal; this is an attractive feature that drives transgene expression in projection neurons. Previous studies showed that VSVs can spread across synapses in anterograde or retrograde directions depending on the types of glycoprotein that are encoded. However, rVSV did not spread across synapses in the Xenopus central nervous system. The successful use of VSV as a transgene vector in amphibian brains not only allows us to exploit the full potential of the genetic tools to answer questions central to understanding central pattern generation, but also opens the door to other research programs that focus on non-genetic model organisms to address unique questions.

Premotor Neuron Divergence Reflects Vocal Evolution

To identify mechanisms of behavioral evolution, we investigated the hindbrain circuit that generates distinct vocal patterns in two closely related frog species. Male Xenopus laevis and Xenopus petersii produce courtship calls that include a fast trill: trains of ∼60 Hz sound pulses. Although fast trill rates are similar, X. laevis fast trills have a longer duration and period than those of X. petersii To pinpoint the neural basis of these differences, we used whole-cell patch-clamp recordings in a key premotor hindbrain nucleus (the Xenopus parabrachial area, PB_X) in ex vivo brains that produce fictive vocalizations, vocal nerve activity corresponding to advertisement call patterns. We found two populations of PB_X neurons with distinct properties: fast trill neurons (FTNs) and early vocal neurons (EVNs). FTNs, but not EVNs, appear to be intrinsically tuned to produce each species' call patterns because: (1) X. laevis FTNs generate longer and slower depolarizations than X. petersii FTNs during their respective fictive vocalizations, (2) current steps in FTNs induce burst durations that are significantly longer in X. laevis than X. petersii, and (3) synaptically isolated FTNs oscillate in response to NMDA in a species-specific manner: longer and slower in X. laevis than in X. petersii Therefore, divergence of premotor neuron membrane properties is a strong candidate for generating vocal differences between species.SIGNIFICANCE STATEMENT The vertebrate hindbrain includes multiple neural circuits that generate rhythmic behaviors including vocalizations. Male African clawed frogs produce courtship calls that are unique to each species and differ in temporal patterns. Here, we identified two functional subtypes of neurons located in the parabrachial nucleus: a hindbrain region implicated in vocal and respiratory control across vertebrates. One of these neuronal subtypes exhibits distinct properties across species that can account for the evolutionary divergence of song patterns. Our results suggest that changes to this group of neurons during evolution may have had a major role in establishing novel behaviors in closely related species.

Synaptic up-scaling preserves motor circuit output after chronic, natural inactivity

Neural systems use homeostatic plasticity to maintain normal brain functions and to prevent abnormal activity. Surprisingly, homeostatic mechanisms that regulate circuit output have mainly been demonstrated during artificial and/or pathological perturbations. Natural, physiological scenarios that activate these stabilizing mechanisms in neural networks of mature animals remain elusive. To establish the extent to which a naturally inactive circuit engages mechanisms of homeostatic plasticity, we utilized the respiratory motor circuit in bullfrogs that normally remains inactive for several months during the winter. We found that inactive respiratory motoneurons exhibit a classic form of homeostatic plasticity, up-scaling of AMPA-glutamate receptors. Up-scaling increased the synaptic strength of respiratory motoneurons and acted to boost motor amplitude from the respiratory network following months of inactivity. Our results show that synaptic scaling sustains strength of the respiratory motor output following months of inactivity, thereby supporting a major neuroscience hypothesis in a normal context for an adult animal.

Neurons in the brain communicate using chemical signals that they send and receive across junctions called synapses. To maintain normal behavior over time, circuits of neurons must reliably process these signals. A variety of nervous system disorders may result if they are unable to do so, as may occur when neural activity changes as a result of disease or injury.

The processes underlying the stability of a neuron’s synapses is referred to as “homeostatic” synaptic plasticity because the changes made by the neuron directly oppose the altered level of activity. In one form of homeostatic plasticity, known as synaptic scaling, neurons modify the strength of all of their synapses in response to changes in neural activity.

There is substantial experimental evidence to show that in young animals, neurons that communicate using a chemical called glutamate undergo synaptic scaling in response to artificial changes in activity. It had not been directly shown that synaptic scaling protects the neural activity of adult animals in their natural environments, in part, because neural activity in most healthy animals generally only goes through small changes. However, the neurons in the brain that cause the breathing muscles of bullfrogs to contract are ideal for studying homeostatic plasticity because they are naturally inactive for several months when frogs hibernate in ponds during the winter. During this time, the bullfrogs do not need to use their lungs to breathe because enough oxygen passes through their skin to keep them alive.

Santin et al. have now observed synaptic scaling of glutamate synapses in individual bullfrog neurons that had been inactive for two months. Further experiments that examined the activity of the breathing control circuit in the brainstem provided evidence that synaptic scaling leads to sufficient amounts of neural activity that would activate the breathing muscles when frogs emerge from hibernation. Therefore neural activity after prolonged, natural inactivity relies on synaptic scaling to preserve life-sustaining behavior in frogs.

These results open up new questions: mainly, how do synaptic scaling and other forms of homeostatic plasticity operate in animals as they experience normal variations in neural activity? Determining how homeostatic plasticity works normally in an animal will help us to understand what happens when plasticity mechanisms go wrong, as is thought to occur in several human nervous system diseases including nervous system injury, Alzheimer’s disease, and epilepsy.

Motor Neurons Tune Premotor Activity in a Vertebrate Central Pattern Generator

Central patterns generators (CPGs) are neural circuits that drive rhythmic motor output without sensory feedback. Vertebrate CPGs are generally believed to operate in a top-down manner in which premotor interneurons activate motor neurons that in turn drive muscles. In contrast, the frog (Xenopus laevis) vocal CPG contains a functionally unexplored neuronal projection from the motor nucleus to the premotor nucleus, indicating a recurrent pathway that may contribute to rhythm generation. In this study, we characterized the function of this bottom-up connection. The X. laevis vocal CPG produces a 50-60 Hz "fast trill" song used by males during courtship. We recorded "fictive vocalizations" in the in vitro CPG from the laryngeal nerve while simultaneously recording premotor activity at the population and single-cell level. We show that transecting the motor-to-premotor projection eliminated the characteristic firing rate of premotor neurons. Silencing motor neurons with the intracellular sodium channel blocker QX-314 also disrupted premotor rhythms, as did blockade of nicotinic synapses in the motor nucleus (the putative location of motor neuron-to-interneuron connections). Electrically stimulating the laryngeal nerve elicited primarily IPSPs in premotor neurons that could be blocked by a nicotinic receptor antagonist. Our results indicate that an inhibitory signal, activated by motor neurons, is required for proper CPG function. To our knowledge, these findings represent the first example of a CPG in which precise premotor rhythms are tuned by motor neuron activity.SIGNIFICANCE STATEMENT Central pattern generators (CPGs) are neural circuits that produce rhythmic behaviors. In vertebrates, motor neurons are not commonly known to contribute to CPG function, with the exception of a few spinal circuits where the functional significance of motor neuron feedback is still poorly understood. The frog hindbrain vocal circuit contains a previously unexplored connection from the motor to premotor region. Our results indicate that motor neurons activate this bottom-up connection, and blocking this signal eliminates normal premotor activity. These findings may promote increased awareness of potential involvement of motor neurons in a wider range of CPGs, perhaps clarifying our understanding of network principles underlying motor behaviors in numerous organisms, including humans.

Rhythm generation, coordination, and initiation in the vocal pathways of male African clawed frogs

Central pattern generators (CPGs) in the brain stem are considered to underlie vocalizations in many vertebrate species, but the detailed mechanisms underlying how motor rhythms are generated, coordinated, and initiated remain unclear. We addressed these issues using isolated brain preparations of Xenopus laevis from which fictive vocalizations can be elicited. Advertisement calls of male X. laevis that consist of fast and slow trills are generated by vocal CPGs contained in the brain stem. Brain stem central vocal pathways consist of a premotor nucleus [dorsal tegmental area of medulla (DTAM)] and a laryngeal motor nucleus [a homologue of nucleus ambiguus (n.IX-X)] with extensive reciprocal connections between the nuclei. In addition, DTAM receives descending inputs from the extended amygdala. We found that unilateral transection of the projections between DTAM and n.IX-X eliminated premotor fictive fast trill patterns but did not affect fictive slow trills, suggesting that the fast and slow trill CPGs are distinct; the slow trill CPG is contained in n.IX-X, and the fast trill CPG spans DTAM and n.IX-X. Midline transections that eliminated the anterior, posterior, or both commissures caused no change in the temporal structure of fictive calls, but bilateral synchrony was lost, indicating that the vocal CPGs are contained in the lateral halves of the brain stem and that the commissures synchronize the two oscillators. Furthermore, the elimination of the inputs from extended amygdala to DTAM, in addition to the anterior commissure, resulted in autonomous initiation of fictive fast but not slow trills by each hemibrain stem, indicating that the extended amygdala provides a bilateral signal to initiate fast trills.

NEW & NOTEWORTHY

Central pattern generators (CPGs) are considered to underlie vocalizations in many vertebrate species, but the detailed mechanisms underlying their functions remain unclear. We addressed this question using an isolated brain preparation of African clawed frogs. We discovered that two vocal phases are mediated by anatomically distinct CPGs, that there are a pair of CPGs contained in the left and right half of the brain stem, and that mechanisms underlying initiation of the two vocal phases are distinct.

Evolution of vocal patterns: tuning hindbrain circuits during species divergence

The neural circuits underlying divergent courtship behaviors of closely related species provide a framework for insight into the evolution of motor patterns. In frogs, male advertisement calls serve as unique species identifiers and females prefer conspecific to heterospecific calls. Advertisement calls of three relatively recently (∼8.5 Mya) diverged species - Xenopus laevis, X. petersii and X. victorianus - include rapid trains of sound pulses (fast trills). We show that while fast trills are similar in pulse rate (∼60 pulses s^-1) across the three species, they differ in call duration and period (time from the onset of one call to the onset of the following call). Previous studies of call production in X. laevis used an isolated brain preparation in which the laryngeal nerve produces compound action potentials that correspond to the advertisement call pattern (fictive calling). Here, we show that serotonin evokes fictive calling in X. petersii and X. victorianus as it does in X. laevis As in X. laevis, fictive fast trill in X. petersii and X. victorianus is accompanied by an N-methyl-d-aspartate receptor-dependent local field potential wave in a rostral hindbrain nucleus, DTAM. Across the three species, wave duration and period are strongly correlated with species-specific fast trill duration and period, respectively. When DTAM is isolated from the more rostral forebrain and midbrain and/or more caudal laryngeal motor nucleus, the wave persists at species-typical durations and periods. Thus, intrinsic differences within DTAM could be responsible for the evolutionary divergence of call patterns across these related species.

Evolution of Courtship Songs in Xenopus: Vocal Pattern Generation and Sound Production

The extant species of African clawed frogs (Xenopus and Silurana) provide an opportunity to link the evolution of vocal characters to changes in the responsible cellular and molecular mechanisms. In this review, we integrate several robust lines of research: evolutionary trajectories of Xenopus vocalizations, cellular and circuit-level mechanisms of vocalization in selected Xenopus model species, and Xenopus evolutionary history and speciation mechanisms. Integrating recent findings allows us to generate and test specific hypotheses about the evolution of Xenopus vocal circuits. We propose that reduced vocal sex differences in some Xenopus species result from species-specific losses of sexually differentiated neural and neuromuscular features. Modification of sex-hormone-regulated developmental mechanisms is a strong candidate mechanism for reduced vocal sex differences.

Harnessing vocal patterns for social communication

Work on vocal communication, influenced by a drive to understand the evolution of language, has focused on auditory processing and forebrain control of learned vocalizations. The actual hindbrain neural mechanisms used to create communication signals are understudied, in part because of the difficulty of experimental studies in species that rely on respiration for vocalization. In these experimental systems-including those that embody vocal learning-vocal behaviors have rhythmic qualities. Recent studies using molecular markers and 'fictive' patterns produced by isolated brains are beginning to reveal how hindbrain circuits generate vocal patterns. Insights from central pattern generators for respiration and locomotion are illuminating common neural and developmental mechanisms. Choice of vocal patterns is responsive to socially salient input. Studies of the vertebrate social brain network suggest mechanisms used to integrate socially salient information and produce an appropriate vocal response.

The Xenopus amygdala mediates socially appropriate vocal communication signals

Social interaction requires that relevant sensory information is collected, classified, and distributed to the motor areas that initiate an appropriate behavioral response. Vocal exchanges, in particular, depend on linking auditory processing to an appropriate motor expression. Because of its role in integrating sensory information for the purpose of action selection, the amygdala has been implicated in social behavior in many mammalian species. Here, we show that two nuclei of the extended amygdala play essential roles in vocal communication in the African clawed frog, Xenopus laevis. Transport of fluorescent dextran amines identifies the X. laevis central amygdala (CeA) as a target for ascending auditory information from the central thalamic nucleus and as a major afferent to the vocal pattern generator of the hindbrain. In the isolated (ex vivo) brain, electrical stimulation of the CeA, or the neighboring bed nucleus of the stria terminalis (BNST), initiates bouts of fictive calling. In vivo, lesioning the CeA of males disrupts the production of appropriate vocal responses to females and to broadcasts of female calls. Lesioning the BNST in males produces an overall decrease in calling behavior. Together, these results suggest that the anuran CeA evaluates the valence of acoustic cues and initiates socially appropriate vocal responses to communication signals, whereas the BNST plays a role in the initiation of vocalizations.

Coding rate and duration of vocalizations of the frog, Xenopus laevis

Vocalizations involve complex rhythmic motor patterns, but the underlying temporal coding mechanisms in the nervous system are poorly understood. Using a recently developed whole-brain preparation from which "fictive" vocalizations are readily elicited in vitro, we investigated the cellular basis of temporal complexity of African clawed frogs (Xenopus laevis). Male advertisement calls contain two alternating components--fast trills (∼300 ms) and slow trills (∼700 ms) that contain clicks repeated at ∼60 and ∼30 Hz, respectively. We found that males can alter the duration of fast trills without changing click rates. This finding led us to hypothesize that call rate and duration are regulated by independent mechanisms. We tested this by obtaining whole-cell patch-clamp recordings in the "fictively" calling isolated brain. We discovered a single type of premotor neuron with activity patterns correlated with both the rate and duration of fast trills. These "fast-trill neurons" (FTNs) exhibited long-lasting depolarizations (LLDs) correlated with each fast trill and action potentials that were phase-locked with motor output-neural correlates of call duration and rate, respectively. When depolarized without central pattern generator activation, FTNs produced subthreshold oscillations and action potentials at fast-trill rates, indicating FTN resonance properties are tuned to, and may dictate, the fast-trill rhythm. NMDA receptor (NMDAR) blockade eliminated LLDs in FTNs, and NMDAR activation in synaptically isolated FTNs induced repetitive LLDs. These results suggest FTNs contain an NMDAR-dependent mechanism that may regulate fast-trill duration. We conclude that a single premotor neuron population employs distinct mechanisms to regulate call rate and duration.

Shared developmental and evolutionary origins for neural basis of vocal-acoustic and pectoral-gestural signaling

Acoustic signaling behaviors are widespread among bony vertebrates, which include the majority of living fishes and tetrapods. Developmental studies in sound-producing fishes and tetrapods indicate that central pattern generating networks dedicated to vocalization originate from the same caudal hindbrain rhombomere (rh) 8-spinal compartment. Together, the evidence suggests that vocalization and its morphophysiological basis, including mechanisms of vocal-respiratory coupling that are widespread among tetrapods, are ancestral characters for bony vertebrates. Premotor-motor circuitry for pectoral appendages that function in locomotion and acoustic signaling develops in the same rh8-spinal compartment. Hence, vocal and pectoral phenotypes in fishes share both developmental origins and roles in acoustic communication. These findings lead to the proposal that the coupling of more highly derived vocal and pectoral mechanisms among tetrapods, including those adapted for nonvocal acoustic and gestural signaling, originated in fishes. Comparative studies further show that rh8 premotor populations have distinct neurophysiological properties coding for equally distinct behavioral attributes such as call duration. We conclude that neural network innovations in the spatiotemporal patterning of vocal and pectoral mechanisms of social communication, including forelimb gestural signaling, have their evolutionary origins in the caudal hindbrain of fishes.

Effects of environmentally relevant concentrations of the xeno-androgen, methyldihydrotestosterone, on male and female mating behavior in Xenopus laevis

Endocrine disrupting compounds (EDCs) are well known to interfere with the hormone system of aquatic vertebrates and to affect their reproductive biology. 17α-Methyldihydrotestosterone (MDHT) is a widely used model compound for the assessment of androgenic EDCs, because it binds with high affinity to nuclear androgen receptors. It was previously shown to affect various aspects of reproductive biology in aquatic vertebrates, however, evidence for MDHT affecting mating behavior of aquatic vertebrate species is lacking. In order to test the assumption that MDHT affects reproductive behavior of aquatic vertebrates, we exposed male and female Xenopuslaevis to three environmentally relevant concentrations of MDHT (30.45 ng L(-1), 3.05 μg L(-1) and 30.45 μg L(-1)). In males, MDHT at all concentrations led to enhanced levels of advertisement calling and decreased the relative proportions of rasping, a call type characterizing a sexually unaroused state of the male, indicating an increase in sexual arousal of MDHT exposed males. Temporal and spectral parameters of the advertisement call itself, however, were not affected by MDHT exposure. In females, MDHT (30.45 ng L(-1)) did not have any effects, while MDHT at 3.05 μg L(-1) increased female receptivity, increased the duration of time females spent close to the speaker playing male advertisement calls and reduced their latency to respond. MDHT at 30.45 μg L(-1), on the other hand, decreased female receptivity and increased their latency to respond. In summary, this study illustrates that exposure to environmentally relevant concentrations of the androgenic EDC MDHT affects male and female mating behavior of X. laevis. Hence, we suggest that nonaromatizable androgens might play a direct and predominant role in the physiology and regulation of reproduction not only in male but also in female frogs.

Corticosterone promotes emergence of fictive air breathing in Xenopus laevis Daudin tadpole brainstems

The emergence of air breathing during amphibian metamorphosis requires significant changes to the brainstem circuits that generate and regulate breathing. However, the mechanisms controlling this developmental process are unknown. Because corticosterone plays an important role in the neuroendocrine regulation of amphibian metamorphosis, we tested the hypothesis that corticosterone augments fictive air breathing frequency in Xenopus laevis. To do so, we compared the fictive air breathing frequency produced by in vitro brainstem preparations from pre-metamorphic tadpoles and adult frogs before and after 1 h application of corticosterone (100 nmol l–1). Fictive breathing measurements related to gill and lung ventilation were recorded extracellularly from cranial nerve rootlets V and X. Corticosterone application had no immediate effect on respiratory-related motor output produced by brainstems from either developmental stage. One hour after corticosterone wash out, fictive lung ventilation frequency was increased whereas gill burst frequency was decreased. This effect was stage specific as it was significant only in preparations from tadpoles. GABA application (0.001–0.5 mmol l–1) augmented fictive air breathing in tadpole preparations. However, this effect of GABA was no longer observed following corticosterone treatment. An increase in circulating corticosterone is one of the endocrine processes that orchestrate central nervous system remodelling during metamorphosis. The age-specific effects of corticosterone application indicate that this hormone can act as an important regulator of respiratory control development in Xenopus tadpoles. Concurrent changes in GABAergic neurotransmission probably contribute to this maturation process, leading to the emergence of air breathing in this species.

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.

Cannabinoids and Reproduction: A Lasting and Intriguing History

Starting from an historical overview of lasting Cannabis use over the centuries, we will focus on a description of the cannabinergic system, with a comprehensive analysis of chemical and pharmacological properties of endogenous and synthetic cannabimimetic analogues. The metabolic pathways and the signal transduction mechanisms, activated by cannabinoid receptors stimulation, will also be discussed. In particular, we will point out the action of cannabinoids and endocannabinoids on the different neuronal networks involved in reproductive axis, and locally, on male and female reproductive tracts, by emphasizing the pivotal role played by this system in the control of fertility.

An environmentally relevant endocrine-disrupting antiandrogen, vinclozolin, affects calling behavior of male Xenopus laevis

Vinclozolin (VIN) is an antiandrogenic model substance as well as a common fungicide that can affect the endocrine system of vertebrates. The objective of this study was to investigate how VIN affects mate calling behavior of South African clawed frogs (Xenopus laevis) and whether it is effective at environmentally relevant concentrations. Male X. laevis were injected with human chorionic gonadotropin (hCG) to stimulate their androgen-controlled mate calling behavior and were treated with VIN at concentrations of 10(-6), 10(-8) and 10(-10)M. VIN at 10(-6)M reduced calling activity. Furthermore, the vocalization composition of VIN-treated X. laevis was altered. The call types advertisement calls and chirping are uttered by reproductively active males, whereas the call types growling, ticking, and rasping indicate a sexually unaroused state of a male. VIN at any of the tested concentrations led to a decrease in utterance of calls, which indicate a sexually aroused state of the males, and an increase in relative proportions of calls, indicating a sexually unaroused state of the males. Additionally, the mean duration of clicks and the number of accentuated clicks during the advertisement calls decreased at all concentrations of VIN. No significant differences were observed in any other temporal or spectral calling parameters between the treatments. This study illustrates that exposure to the antiandrogen VIN might result in a reduced reproductive success by altering mate calling behavior of X. laevis. Moreover, it suggests that the behavioral parameters examined in this study can be used as sensitive biomarkers for detecting antiandrogenic endocrine disrupting compounds in amphibians.

NMDAR-dependent control of call duration in Xenopus laevis

Many rhythmic behaviors, such as locomotion and vocalization, involve temporally dynamic patterns. How does the brain generate temporal complexity? Here, we use the vocal central pattern generator (CPG) of Xenopus laevis to address this question. Isolated brains can elicit fictive vocalizations, allowing us to study the CPG in vitro. The X. laevis advertisement call is temporally modulated; calls consist of rhythmic click trills that alternate between fast (approximately 60 Hz) and slow (approximately 30 Hz) rates. We investigated the role of two CPG nuclei--the laryngeal motor nucleus (n.IX-X) and the dorsal tegmental area of the medulla (DTAM)--in setting rhythm frequency and call durations. We discovered a local field potential wave in DTAM that coincides with fictive fast trills and phasic activity that coincides with fictive clicks. After disrupting n.IX-X connections, the wave persists, whereas phasic activity disappears. Wave duration was temperature dependent and correlated with fictive fast trills. This correlation persisted when wave duration was modified by temperature manipulations. Selectively cooling DTAM, but not n.IX-X, lengthened fictive call and fast trill durations, whereas cooling either nucleus decelerated the fictive click rate. The N-methyl-d-aspartate receptor (NMDAR) antagonist dAPV blocked waves and fictive fast trills, suggesting that the wave controls fast trill activation and, consequently, call duration. We conclude that two functionally distinct CPG circuits exist: 1) a pattern generator in DTAM that determines call duration and 2) a rhythm generator (spanning DTAM and n.IX-X) that determines click rates. The newly identified DTAM pattern generator provides an excellent model for understanding NDMAR-dependent rhythmic circuits.

Reproductive and diurnal rhythms regulate vocal motor plasticity in a teleost fish

Seasonal and circadian rhythms control fundamental physiological processes including neural excitability and synaptic plasticity that can lead to the periodic modulation of motor behaviors like social vocalizations. Parental male midshipman fish produce three call types during the breeding season: long duration (min to >1 h) advertisement 'hums', frequency and amplitude modulated agonistic 'growls' (s), and very brief (ms) agonistic 'grunts' produced either singly or repetitively as ;grunt trains' for up to several minutes. Fictive grunts that establish the temporal properties of natural grunts are readily evoked and recorded in vivo from vocal occipital nerve roots at any time of day or year by electrical microstimulation in either the midbrain periaqueductal gray or a hindbrain vocal pre-pacemaker nucleus. Now, as shown here, the longer duration fictive growls and hums can also be elicited, but are restricted to the nocturnal reproductive season. A significant drop in call threshold accompanies the fictive growls and hums that are distinguished by their much longer duration and lower and more regular firing frequency. Lastly, the long duration fictive calls are dependent upon increased stimulation time and intensity and hence may result from activity-dependent changes in the vocal motor circuit that are themselves modulated by seasonal and circadian rhythms.

Endogenous serotonin acts on 5-HT2C-like receptors in key vocal areas of the brain stem to initiate vocalizations in Xenopus laevis

Serotonin initiates various rhythmic behaviors in vertebrates. Previously we have shown that serotonergic neurons innervate the central vocal pathway in the African clawed frog (Xenopus laevis). We also discovered that exogenous serotonin applied to isolated brains in vitro activates fictive vocalizations by activating 5-HT(2C)-like receptors. In this study, we examined the location of 5-HT(2C)-like receptors and determined whether endogenously released serotonin also initiates vocalizations by activating 5-HT(2C)-like receptors in male Xenopus brains. To this end, we first identified the specific location of 5-HT(2C)-like receptors using immunohistochemistry. We next examined which of the populations of neurons that express 5-HT(2C)-like receptors are functionally relevant for initiating fictive vocalizations by applying a 5-HT(2C) receptor agonist to brains transected at various levels. Of four populations of immunopositive neurons, we showed that 5-HT(2C)-like receptors located in two areas of the brain stem vocal circuit, the raphe nucleus and motor nucleus IX-X, initiate fictive vocalizations. We next showed that endogenous serotonin can also activate fictive vocalizations by increasing the extracellular concentration of endogenous serotonin using a selective serotonin reuptake inhibitor (SSRI). The SSRI-induced vocal initiation is also mediated by activation of 5-HT(2C)-like receptors because blockade of these receptors prevents fictive vocalization. The results suggest that in vivo release of serotonin initiates male vocalizations by activating 5-HT(2C)-like receptors in the brain stem vocal nuclei.

5-HT2C-like receptors in the brain of Xenopus laevis initiate sex-typical fictive vocalizations

Vocalizations of male and female African clawed frogs (Xenopus laevis) are generated by brain stem central pattern generators. Serotonin (5-HT) is likely important for vocal initiation because, when applied in vitro, sex-typical fictive vocalizations are evoked from isolated brains. To explore the mechanisms underlying vocal initiation, we identified the types of serotonin receptors mediating vocal activation pharmacologically using a whole brain, fictive preparation. The results showed that 5-HT(2C)-like receptors are important for activation of fictive vocalizations in the sexes. 5-HT(2C) receptor agonists elicited fictive vocalizations, and 5-HT(2C) receptor antagonists blocked 5-HT-induced fictive vocalizations, whereas agonists and antagonists of 5-HT(2A) and 5-HT(2B) receptors failed to initiate or block 5-HT-induced fictive vocalizations in the sexes. The results indicate that serotonin initiates fictive vocalizations by binding to 5-HT(2C)-like receptors located either within or upstream of the vocal central pattern generator in both sexes. We conclude that the basic mechanism of vocal initiation is shared by the sexes despite the differences in the actual vocalizations between males and females. Sex-typical vocalizations, therefore, most likely arise from activation of different populations of 5-HT(2C) receptor expressing cells or from differential activation of downstream pattern generating neurons.

Hormones and acoustic communication in anuran amphibians

Circulating hormone levels can mediate changes in the quality of courtship signals by males and/or mate choice by females and may thus play an important role in the evolution of courtship signals. Costs associated with shifts in hormone levels of males, for example, could effectively stabilize directional selection by females on male signals. Alternatively, if hormone levels affect the selection of mates by females, then variation in hormone levels among females could contribute to the maintenance of variability in the quality of males' signals. Here, I review what is known regarding the effects of hormone levels on the quality of acoustic signals produced by males and on the choice of mates by females in anuran amphibians. Surprisingly, despite the long history of anuran amphibians as model organisms for studying acoustic communication and physiology, we know very little about how variation in circulating hormone levels contributes to variation in the vocal quality of males. Proposed relationships between androgen levels and vocal quality depicted in recent models, for example, are subject to the same criticisms raised for similar models proposed in relation to birds, namely that the evidence for graded effects of androgens on vocal performance is often weak or not rigorously tested and responses seen in one species are often not observed in other species. Although several studies offer intriguing support for graded effects of hormones on calling behavior, additional comparative studies will be required to understand these relationships. Recent studies indicate that hormones may also mediate changes in anuran females' choice of mates, suggesting that the hormone levels of females can influence the evolution of males' mating signals. No studies to date have concurrently addressed the potential complexity of hormone-behavior relationships from the perspective of sender as well as receiver, nor have any studies addressed the costs that are potentially associated with changes in circulating hormone levels in anurans (i.e., life-history tradeoffs associated with elevations in circulating androgens in males). The mechanisms involved in hormonally induced changes in signal production and selectivity also require further investigation. Anuran amphibians are, in many ways, conducive to investigating such questions.

Temperature-dependent regulation of vocal pattern generator

Vocalizations of Xenopus laevis are generated by central pattern generators (CPGs). The advertisement call of male X. laevis is a complex biphasic motor rhythm consisting of fast and slow trills (a train of clicks). We found that the trill rate of these advertisement calls is sensitive to temperature and that this rate modification of the vocal rhythms originates in the central pattern generators. In vivo the rates of fast and slow trills increased linearly with an increase in temperature. In vitro a similar linear relation between temperature and compound action potential frequency in the laryngeal nerve was found when fictive advertisement calls were evoked in the isolated brain. Temperature did not limit the contractile properties of laryngeal muscles within the frequency range of vocalizations. We next took advantage of the temperature sensitivity of the vocal CPG in vitro to localize the source of the vocal rhythms. We focused on the dorsal tegmental area of the medulla (DTAM), a brain stem nucleus that is essential for vocal production. We found that bilateral cooling of DTAM reduced both fast and slow trill rates. Thus we conclude that DTAM is a source of biphasic vocal rhythms.

Evolutionary origins for social vocalization in a vertebrate hindbrain-spinal compartment

The macroevolutionary events leading to neural innovations for social communication, such as vocalization, are essentially unexplored. Many fish vocalize during female courtship and territorial defense, as do amphibians, birds, and mammals. Here, we map the neural circuitry for vocalization in larval fish and show that the vocal network develops in a segment-like region across the most caudal hindbrain and rostral spinal cord. Taxonomic analysis demonstrates a highly conserved pattern between fish and all major lineages of vocal tetrapods. We propose that the vocal basis for acoustic communication among vertebrates evolved from an ancestrally shared developmental compartment already present in the early fishes.

Sexually differentiated central pattern generators in Xenopus laevis

Understanding the neural mechanisms that underlie the function of central pattern generators (CPGs) presents a formidable challenge requiring sophisticated tools and well-chosen model systems. In this article, we describe recent work on vocalizations of the African clawed frog Xenopus laevis. These behaviors are driven by sexually differentiated CPGs and are exceptionally well suited to this objective. In particular, a simplified mechanism of vocal production (independent of respiratory musculature) allows straightforward interpretations of nerve activity with respect to behavior. Furthermore, the development of a fictively vocalizing isolated brain, together with the finding of rapid androgen-induced masculinization of female vocalizations, provides an invaluable tool for determining how new behaviors arise from existing circuits.

Central pattern generators for social vocalization: androgen-dependent neurophysiological mechanisms

Historically, most studies of vertebrate central pattern generators (CPGs) have focused on mechanisms for locomotion and respiration. Here, we highlight new results for ectothermic vertebrates, namely teleost fish and amphibians, showing how androgenic steroids can influence the temporal patterning of CPGs for social vocalization. Investigations of vocalizing teleosts show how androgens can rapidly (within minutes) modulate the neurophysiological output of the vocal CPG (fictive vocalizations that mimic the temporal properties of natural vocalizations) inclusive of their divergent actions between species, as well as intraspecific differences between male reproductive morphs. Studies of anuran amphibians (frogs) demonstrate that long-term steroid treatments (wks) can masculinize the fictive vocalizations of females, inclusive of its sensitivity to rapid modulation by serotonin. Given the conserved organization of vocal control systems across vertebrate groups, the vocal CPGs of fish and amphibians provide tractable models for identifying androgen-dependent events that are fundamental to the mechanisms of vocal motor patterning. These basic mechanisms can also inform our understanding of the more complex CPGs for vocalization, and social behaviors in general, that have evolved among birds and mammals.

Regulation of respiratory and vocal motor pools in the isolated brain of Xenopus laevis

The aquatic frog Xenopus laevis uses a complex vocal repertoire during mating and male-male interactions. Calls are produced without breathing, allowing the frogs to vocalize for long periods underwater. The Xenopus vocal organ, the larynx, is innervated by neurons in cranial motor nucleus (n.) IX-X, which contains both vocal (laryngeal) and respiratory (glottal) motor neurons. The primary descending input to n.IX-X comes from the pretrigeminal nucleus of the dorsal tegmental area of the medulla (DTAM), located in the rostral hindbrain. We wanted to characterize premotor inputs to respiratory and vocal motor neurons and to determine what mechanisms might be involved in regulating two temporally distinct rhythmic behaviors: breathing and calling. Using isolated brain and larynx preparations, we recorded extracellular activity from the laryngeal nerve and muscles and intracellular activity in laryngeal and glottal motor neurons. Spontaneous nerve activities mimicking respiratory and vocal patterns were observed. DTAM projection neurons (DTAM(IX-X) neurons) provide direct input to glottal and laryngeal motor neurons. Electrical stimulation produced short-latency coordinated activity in the laryngeal nerve. DTAM(IX-X) neurons provide excitatory monosynaptic inputs to laryngeal motor neurons and mixed excitatory and inhibitory inputs to glottal motor neurons. DTAM stimulation also produced a delayed burst of glottal motor neuron activity. Together, our data suggest that neurons in DTAM produce vocal motor output by directly activating laryngeal motor neurons and that DTAM may coordinate vocal and respiratory motor activity.

Xenopus vocalizations are controlled by a sexually differentiated hindbrain central pattern generator

Male and female African clawed frogs (Xenopus laevis) produce rhythmic, sexually distinct vocalizations as part of courtship and mating. We found that Xenopus vocal behavior is governed by a sexually dimorphic central pattern generator (CPG) and that fictive vocalizations can be elicited from an in vitro brain preparation by application of serotonin or by electrical stimulation of a premotor nucleus. Male brains produced fictive vocal patterns representing two calls commonly produced by males in vivo (advertisement and amplectant call), as well as one call pattern (release call) that is common for juvenile males and females in vivo but rare for adult males. Female brains also produced fictive release call. The production of male calls is androgen dependent in Xenopus; to test the effects of androgens on the CPG, we examined fictive calling in the brains of testosterone-treated females. Both fictive male advertisement call and release call were produced. This suggests that all Xenopus possess a sexually undifferentiated pattern generator for release call. Androgen exposure leads to a gain-of-function, allowing the production of male-specific call types without prohibiting the production of the undifferentiated call pattern. We also demonstrate that the CPG is located in the brainstem and seems to rely on the same nuclei in both males and females. Finally, we identified endogenous serotonergic inputs to both the premotor and motor nuclei in the brainstem that may regulate vocal activity in vivo.

Direct action of gonadotropin in brain integrates behavioral and reproductive functions

Essential roles for gonadotropins in gonadal development and reproduction are well established. Over the past decade, however, the expression of luteinizing hormone receptor (LHR) has also been reported in the brain of various mammals and birds. Although suggestive, it has not yet been determined whether this expression pattern supports a novel function for gonadotropins. Here, we demonstrate a CNS-mediated role of gonadotropins in a reproductive behavior: the courtship songs of the South African clawed frog, Xenopus laevis. Male advertisement calling in this species depends on a nongonadal action of gonadotropin. To determine whether this effect is due to action on the CNS, we administered gonadotropin intracerebroventricularly (ICV) or systemically to intact or castrated males with or without concomitant androgen replacement. In intact and androgen-replaced gonadectomized males, gonadotropin significantly increased calling within 1 h after ICV injection. The effective dosage via ICV injections was less than one hundredth of the effective systemic dose. In situ hybridization with a cloned fragment of Xenopus LHR revealed strong expression in ventral forebrain areas important for vocal control. Further, gonadotropin treatment of brain in vitro up-regulates immunoreactivity for the LHR downstream target, egr-1, specifically in these vocal forebrain areas. Up-regulation occurs even when synaptic transmission is suppressed by incubation in Ca2+ free/high magnesium saline. These results demonstrate a neural role for gonadotropin in the control of calling behavior, potentially mediated via LHRs in forebrain vocal nuclei. Gonadotropin may play a novel integrative role in modulating both reproductive physiology and behavior.