Autoradiographic localization of hormone-concentrating cells in the brain of an amphibian, Xenopus laevis. II. Estradiol

Neural Control of Sexual Behavior in Fish

Many vertebrate species show breeding periods and exhibit series of characteristic species-specific sexual behaviors only during the breeding period. Here, secretion of gonadal sex hormones from the mature gonads has been considered to facilitate sexual behaviors. Thus, the sexual behavior has long been considered to be regulated by neural and hormonal mechanisms. In this review, we discuss recent progress in the study of neural control mechanisms of sexual behavior with a focus on studies using fish, which have often been the favorite animals used by many researchers who study instinctive animal behaviors. We first discuss control mechanisms of sexual behaviors by sex steroids in relation to the anatomical studies of sex steroid-concentrating neurons in various vertebrate brains, which are abundantly distributed in evolutionarily conserved areas such as preoptic area (POA) and anterior hypothalamus. We then focus on another brain area called the ventral telencephalic area, which has also been suggested to contain sex steroid-concentrating neurons and has been implicated in the control of sexual behaviors, especially in teleosts. We also discuss control of sex-specific behaviors and sexual preference influenced by estrogenic signals or by olfactory/pheromonal signals. Finally, we briefly summarize research on the modulatory control of motivation for sexual behaviors by a group of peptidergic neurons called terminal nerve gonadotropin-releasing hormone (TN-GnRH) neurons, which are known to be especially developed in fishes among various vertebrate species.

Petroleum-derived naphthenic acids disrupt hormone-dependent sexual behaviours in male Western clawed frogs

Naphthenic acids (NAs), the carboxylic acids found in petroleum, are of emerging concern as they contaminate coastlines after oil spills, leech into freshwater ecosystems of oil sands areas and have wide industrial applications. They are acutely toxic in fish and tadpoles and may be endocrine disruptors at sublethal levels. We characterized androgen-dependent courtship behaviours and their disruption by NAs in male Western clawed frogs, Silurana tropicalis. Courtship primarily consists of males producing low trills and achieving amplexus, a mating position where a male clasps a female. Adult males were exposed for 5 days to 20 mg/l NA and injected with human chorionic gonadotropin to induce calling. The duration of calling activity was significantly reduced by NA exposure. Other acoustic parameters such as dominant frequency, click rate and trill length were not affected. Vocalization and amplexus were both inhibited after NA exposure and restored after 2 weeks of recovery in clean water. To determine possible disruption at the level of the testes, the effects of NA exposure on gene expression of key players in steroidogenesis was determined. Exposure to NAs decreased srd5a on average by ~ 25%. The enzyme 5α-reductase, encoded by srd5a, converts testosterone to its more bioactive form 5α-dihydrotestosterone (DHT), so NAs may be affecting this steroidogenic step. However, the observed upregulation of lhr, star and cyp17a1 suggests that NA-exposed males may be attempting to counteract the reduced potential to produce DHT. Yet, these NA-exposed frogs have dramatically reduced calling duration, so the observed upregulation of star and cyp17a1 is decoupled from the vocalizations. Calling duration and the ability of males to amplex females is reversibly disrupted by NA exposure, implying that environmental reduction and removal of NAs may help improve habitability of contaminated ecosystems.

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.

Sex-specific distribution of Neuropeptide Y (NPY) in the brain of the frog, Microhyla ornata

Neuropeptide Y (NPY) is involved in sex-specific behavioural processes in vertebrates. NPY integrates energy balance and reproduction in mammals. However, the relevance of NPY in reproduction of lower vertebrates is understudied. In the present study, we have investigated neuroanatomical distribution and sex-specific differences of NPY in the brain of Microhyla ornata using immunohistochemistry and quantitative real time PCR. NPY is widely distributed throughout the brain of M. ornata. We observed NPY immunoreactivity in the cells of the nucleus accumbens, striatum pars dorsalis, dorsal pallium, medial pallium, ventral pallium, bed nucleus of stria terminalis, preoptic nucleus, infundibular region, median eminence and pituitary gland of adult M. ornata. A higher number of NPY- immunoreactive cells were observed in the preoptic nucleus (p < .01), nucleus infundibularis ventralis (p < .001) and anteroventral tegmental nucleus (p < .001) of the female as compared to that of the male frog. Real-Time PCR revealed higher mRNA levels of NPY in the female as compared to male frogs in the mid-brain region that largely contains the hypothalamus. Sexual dimorphism of NPY expression in M. ornata suggests that NPY may be involved in the reproductive physiology of anurans.

Diclofenac can exhibit estrogenic modes of action in male Xenopus laevis, and affects the hypothalamus-pituitary-gonad axis and mating vocalizations

Diclofenac (DCF) is a non-steroidal analgesic and antiphlogistic. Due to its tremendous use, DCF can be found in the environment, especially in sewage, but also surface waters, ground and drinking water. Previous studies indicated that DCF can modulate the reproductive physiology of fish by altering the expression of important key enzymes of the hypothalamus-pituitary-gonad-axis (HPG-axis) and might act as an estrogenic endocrine disrupting chemical (EDC). Other studies, however, demonstrated that DCF does not exhibit any estrogenicity. Thus, in the present study we investigated whether an exposure to DCF can affect reproductive behavior and physiology of adult male X. laevis by analyzing DCF effects on the mate calling behavior of the frogs and on gene expression patterns of key biomarkers of the HPG-axis. In addition, plasma sex steroid levels were determined to gain detailed insights into the mechanisms of DCF action. We could demonstrate that DCF can act as EDC by exhibiting slight estrogenic modes of action. In addition, pharmacological impacts on gonadal steroidogenesis could be revealed leading to imbalances in sex steroid levels and ratios. DCF furthermore altered the calling behavior of exposed males, potentially reducing the mating and reproductive success of the frogs, possibly leading to severe population effects.

What's in a name? Considerations of homologies and nomenclature for vertebrate social behavior networks

Behavioral neuroendocrinology is an integrative discipline that spans a wide range of taxa and neural systems, and thus the appropriate designation of homology (sameness) across taxa is critical for clear communication and extrapolation of findings from one taxon to another. In the present review we address issues of homology that relate to neural circuits of social behavior and associated systems that mediate reward and aversion. We first address a variety of issues related to the so-called "social behavior network" (SBN), including homologies that are only partial (e.g., whereas the preoptic area of fish and amphibians contains the major vasopressin-oxytocin cell groups, these populations lie in the hypothalamus of other vertebrates). We also discuss recent evidence that clarifies anterior hypothalamus and periaqueductal gray homologies in birds. Finally, we discuss an expanded network model, the "social decision-making network" (SDM) which includes the mesolimbic dopamine system and other structures that provide an interface between the mesolimbic system and the SBN. This expanded model is strongly supported in mammals, based on a wide variety of evidence. However, it is not yet clear how readily the SDM can be applied as a pan-vertebrate model, given insufficient data on numerous proposed homologies and a lack of social behavior data for SDM components (beyond the SBN nodes) for amphibians, reptiles or fish. Functions of SDM components are also poorly known for birds. Nonetheless, we contend that the SDM model provides a very sound and important framework for the testing of many hypotheses in nonmammalian vertebrates.

Characterization of the bed nucleus of the stria terminalis in the forebrain of anuran amphibians

Major common features have been reported for the organization of the basal telencephalon in amniotes, and most characteristics were thought to be acquired in the transition from anamniotes to amniotes. However, gene expression, neurochemical, and hodological data obtained for the basal ganglia and septal and amygdaloid complexes in amphibians (anamniotic tetrapods) have strengthened the idea of a conserved organization in tetrapods. A poorly characterized region in the forebrain of amniotes has been the bed nucleus of the stria terminalis (BST), but numerous recent investigations have characterized it as a member of the extended amygdala. Our study analyzes the main features of the BST in anuran amphibians to establish putative homologies with amniotes. Gene expression patterns during development identified the anuran BST as a subpallial, nonstriatal territory. The BST shows Nkx2.1 and Lhx7 expression and contains an Islet1-positive cell subpopulation derived from the lateral ganglionic eminence. Immunohistochemistry for diverse peptides and neurotransmitters revealed that the distinct chemoarchitecture of the BST is strongly conserved among tetrapods. In vitro tracing techniques with dextran amines revealed important connections between the BST and the central and medial amygdala, septal territories, medial pallium, preoptic area, lateral hypothalamus, thalamus, and prethalamus. The BST receives dopaminergic projections from the ventral tegmental area and is connected with the laterodorsal tegmental nucleus and the rostral raphe in the brainstem. All these data suggest that the anuran BST shares many features with its counterpart in amniotes and belongs to a basal continuum, likely controlling similar reflexes, reponses, and behaviors in tetrapods.

Estrogens can disrupt amphibian mating behavior

The main component of classical contraceptives, 17α-ethinylestradiol (EE2), has high estrogenic activity even at environmentally relevant concentrations. Although estrogenic endocrine disrupting compounds are assumed to contribute to the worldwide decline of amphibian populations by adverse effects on sexual differentiation, evidence for EE2 affecting amphibian mating behaviour is lacking. In this study, we demonstrate that EE2 exposure at five different concentrations (0.296 ng/L, 2.96 ng/L, 29.64 ng/L, 2.96 µg/L and 296.4 µg/L) can disrupt the mating behavior of adult male Xenopus laevis. EE2 exposure at all concentrations lowered male sexual arousal, indicated by decreased proportions of advertisement calls and increased proportions of the call type rasping, which characterizes a sexually unaroused state of a male. Additionally, EE2 at all tested concentrations affected temporal and spectral parameters of the advertisement calls, respectively. The classical and highly sensitive biomarker vitellogenin, on the other hand, was only induced at concentrations equal or higher than 2.96 µg/L. If kept under control conditions after a 96 h EE2 exposure (2.96 µg/L), alterations of male advertisement calls vanish gradually within 6 weeks and result in a lower sexual attractiveness of EE2 exposed males toward females as demonstrated by female choice experiments. These findings indicate that exposure to environmentally relevant EE2 concentrations can directly disrupt male mate calling behavior of X. laevis and can indirectly affect the mating behavior of females. The results suggest the possibility that EE2 exposure could reduce the reproductive success of EE2 exposed animals and these effects might contribute to the global problem of amphibian decline.

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.

Female sexual arousal in amphibians

Rather than being a static, species specific trait, reproductive behavior in female amphibians is variable within an individual during the breeding season when females are capable of reproductive activity. Changes in receptivity coincide with changes in circulating estrogen. Estrogen is highest at the point when females are ready to choose a male and lay eggs. At this time female receptivity (her probability of responding to a male vocal signal) is highest and her selectivity among conspecific calls (measured by her probability of responding to a degraded or otherwise usually unattractive male signal) is lowest. These changes occur even though females retain the ability to discriminate different acoustic characteristics of various conspecific calls. After releasing her eggs, female amphibians quickly become less receptive and more choosy in terms of their responses to male sexual advertisement signals. Male vocal signals stimulate both behavior and estrogen changes in amphibian females making mating more probable. The changes in female reproductive behavior are the same as those generally accepted as indicative of a change in female sexual arousal leading to copulation. They are situationally triggered, gated by interactions with males, and decline with the consummation of sexual reproduction with a chosen male. The changes can be triggered by either internal physiological state or by the presence of stimuli presented by males, and the same stimuli change both behavior and physiological (endocrine) state in such a way as to make acceptance of a male more likely. Thus amphibian females demonstrate many of the same general characteristics of changing female sexual state that in mammals indicate sexual arousal.

Vocal pathway degradation in gonadectomized Xenopus laevis adults

Reproductive behaviors of many vertebrate species are activated in adult males by elevated androgen levels and abolished by castration. Neural and muscular components controlling these behaviors contain numerous hormone-sensitive sites including motor initiation centers (such as the basal ganglia), central pattern generators (CPGs), and muscles; therefore it is difficult to confirm the role of each hormone-activated target using behavioral assays alone. Our goal was to address this issue by determining the site of androgen-induced vocal activation using male Xenopus laevis, a species in which androgen dependence of vocal activation has been previously determined. We compared in vivo calling patterns and functionality of two in vitro preparations-the isolated larynx and an isolated brain from which fictive courtship vocalizations can be evoked--in castrated and control males. The isolated larynx allowed us to test whether castrated males were capable of transducing male-typical nerve signals into vocalizations and the fictively vocalizing brain preparation allowed us to directly examine vocal CPG function separate from the issue of vocal initiation. The results indicate that all three components--vocal initiation, CPG, and larynx--require intact gonads. Vocal production decreased dramatically in castrates and laryngeal contractile properties of castrated males were demasculinized, whereas no changes were observed in control animals. In addition, fictive calls of castrates were degraded compared with those of controls. To our knowledge, this finding represents the first demonstration of gonad-dependent maintenance of a CPG for courtship behavior in adulthood. Because previous studies showed that androgen-replacement can prevent castration-induced vocal impairments, we conclude that degradation of vocal initiation centers, larynx, and CPG function are most likely due to steroid hormone deprivation.

Sexually dimorphic androgen and estrogen receptor mRNA expression in the brain of túngara frogs

Sex steroid hormones are potent regulators of behavior and they exert their effects through influences on sensory, motor, and motivational systems. To elucidate where androgens and estrogens can act to regulate sex-typical behaviors in the túngara frog (Physalaemus pustulosus), we quantified expression of the androgen receptor (AR), estrogen receptor alpha (ERalpha), and estrogen receptor beta (ERbeta) genes in the brains of male and females. To do so, we cloned túngara-specific sequences for AR, ERalpha, and ERbeta, determined their distribution in the brain, and then quantified their expression in areas that are important in sexual communication. We found that AR, ERalpha, and ERbeta were expressed in the pallium, limbic forebrain (preoptic area, hypothalamus, nucleus accumbens, amygdala, septum, striatum), parts of the thalamus, and the auditory midbrain (torus semicircularis). Males and females had a similar distribution of AR and ER expression, but expression levels differed in some brain regions. In the auditory midbrain, females had higher ERalpha and ERbeta expression than males, whereas males had higher AR expression than females. In the forebrain, females had higher AR expression than males in the ventral hypothalamus and medial pallium (homolog to hippocampus), whereas males had higher ERalpha expression in the medial pallium. In the preoptic area, striatum, and septum, males and females had similar levels of AR and ER expression. Our results suggest that sex steroid hormones have sexually dimorphic effects on auditory processing, sexual motivation, and possibly memory and, therefore, have important implications for sexual communication in this system.

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.

Sex-specific modulation of cell proliferation by socially relevant stimuli in the adult green treefrog brain (Hyla cinerea)

Social experience plays an important role in regulating the neural, physiological and hormonal changes that accompany the expression of reproductive behavior in vertebrates. This suite of functions is sexually dimorphic, with different neural control areas preeminent in males and females. In anuran amphibians, social experience comes in the form of acoustic communication, which is central to their reproductive behavior. We sought to determine whether acoustic cues regulate cell proliferation in the brain of adult green treefrogs (Hyla cinerea). Our results show that both male and female treefrogs that heard a conspecific chorus during the breeding season exhibited increased brain cell proliferation compared to animals that heard random tones. Increased cell proliferation, as assessed by the number of 5-bromo-2'-deoxyuridine-immunoreactive (BrdU+) cells, were found near the ventricles of acoustically sensitive brain regions such as the preoptic area (POA) and the infundibular hypothalamus (IF). Sex differences emerged in the location of this socially modulated cell proliferation: increases occurred primarily in the male POA and the female IF. In addition, gonadal steroid hormones might have played a role in the social modulation of cell proliferation: by statistically control- ling for hormone level, we revealed that androgens might influence socially induced increases in BrdU+ cells in the male POA, but estrogen did not contribute to socially induced increases in the female IF. These results indicate that the reception of social cues increases cell proliferation in brain regions mediating sexual behavior and endocrine regulation, and moreover that social modulation of cell proliferation occurs in a sexually differentiated fashion.

Sexual hearing: the influence of sex hormones on acoustic communication in frogs

The majority of anuran amphibians (frogs and toads) use acoustic communication to mediate sexual behavior and reproduction. Generally, females find and select their mates using acoustic cues provided by males in the form of conspicuous advertisement calls. In these species, vocal signal production and reception are intimately tied to successful reproduction. Research with anurans has demonstrated that acoustic communication is modulated by reproductive hormones, including gonadal steroids and peptide neuromodulators. Most of these studies have focused on the ways in which hormonal systems influence vocal signal production; however, here we will concentrate on a growing body of literature that examines hormonal modulation of call reception. This literature suggests that reproductive hormones contribute to the coordination of reproductive behaviors between signaler and receiver by modulating sensitivity and spectral filtering of the anuran auditory system. It has become evident that the hormonal systems that influence reproductive behaviors are highly conserved among vertebrate taxa. Thus, studying the endocrine and neuromodulatory bases of acoustic communication in frogs and toads can lead to insights of broader applicability to hormonal modulation of vertebrate sensory physiology and behavior.

Neurogenic development of the auditory areas of the midbrain and diencephalon in the Xenopus laevis and evolutionary implications

To study whether the core-versus-shell pattern of neurogenesis occurred in the mesencephalic and diencephalic auditory areas of amniotes also appears in the amphibian, [(3)H]-thymidine was injected into tadpoles at serial developmental stages of Xenopus laevis. Towards the end of metamorphism, [(3)H]-thymidine labeling was examined and led to two main observations: 1) neuron generation in the principal nucleus (Tp) started at stage 50, and peaked at stage 53, whereas it began at stage 48.5, and peaked around stage 49 in the other two mesencephalic auditory areas, the laminar nucleus (Tl) and the magnocellular nucleus (Tmc). 2) Neuron generation appeared at stage 40, and peaked around stage 52 in the posterior thalamic nucleus (P) and the central thalamic nucleus (C). Our study revealed that, like the cores of mesencephalic auditory nuclei in amniotes, Tp showed differences from Tl and Tmc in the onset and the peak of neurogenesis. However, such differences did not occur in the P and C. Our neurogenetic data were consistent with anatomical and physiological reports indicating a clear distinction between the mesencephalic, but not the diencephalic auditory areas of the amphibian. Our data are helpful to get insights into the organization of auditory nuclei and its evolution in vertebrates.

Reproductive hormones modify reception of species-typical communication signals in a female anuran

In many vertebrates, the production and reception of species-typical courtship signals occurs when gonadotropin and gonadal hormone levels are elevated. These hormones may modify sensory processing in the signal receiver in a way that enhances behavioral responses to the signal. We examined this possibility in female túngara frogs (Physalaemus pustulosus) by treating them with either gonadotropin (which elevated estradiol) or saline and exposing them to either mate choruses or silence. Expression of an activity-dependent gene, egr-1, was quantified within two sub-nuclei of the auditory midbrain to investigate whether gonadotropin plus chorus exposure induced greater egr-1 induction than either of these stimuli alone. The laminar nucleus (LN), a sub-nucleus of the torus semicircularis that contains steroid receptors, exhibited elevated egr-1 induction in response to chorus exposure and gonadotropin treatment. Further analysis revealed that neither chorus exposure nor gonadotropin treatment alone elevated egr-1 expression in comparison to baseline levels whereas gonadotropin + chorus exposure did. This suggests that mate signals and hormones together produce an additive effect so that together they induce more egr-1 expression than either alone. Our previously published studies of female túngara frogs reveal that (1) gonadotropin-induced estradiol elevations also increase behavioral responses to male signals, and (2) reception of male signals elevates estradiol levels in the female. Here, we report data that reveal a novel mechanism by which males exploit female sensory processing to increase behavioral responses to their courtship signals.

The septal complex of the fire-bellied toad Bombina orientalis: Chemoarchitecture

In order to investigate whether chemoarchitecture would support the subdivision of the anuran septum based on cytoarchitectonic and hodological studies, we performed enzyme-histochemical detection of NADPH-diaphorase and immunohistological demonstration of choline-acetyl transferase (ChAT), aspartate, calretinin, gamma-aminobutyric acid (GABA), 5-hydroxy-tryptamine, tyrosine hydroxylase, neuropeptide Y (NPY), somatostatin, Leu- and Leu + Met-enkephalin, and substance P in the fire-bellied toad Bombina orientalis. Labeling of cell bodies matched well the previously defined subnuclei: The dorsolateral septal nucleus contains enkephalin-immunoreactive (-ir) and weakly stained GABA-ir neurons; calretinin-ir and weakly labeled GABA-ir neurons are found in the ventrolateral septal nucleus. The medial septal nucleus is characterized by the presence of numerous ChAT-ir and some tyrosine hydroxylase-ir neurons, while the dorsal septal nucleus is outlined by its NPY-ir neurons. Many ChAT-ir and some aspartate-ir and somatostatin-ir neurons are found in the diagonal band of Broca, and the central septal nucleus contains some GABA-ir and ChAT-ir neurons. In contrast, labeled fibers form a pattern which does not match the boundaries of septal subnuclei. Comparing the anuran septal complex with that of other vertebrates reveals that the complexity of the lateral septum has increased during the evolution from anamniote to amniote vertebrates. In spite of this fact, many similarities in chemoarchitecture between anurans and other vertebrates are evident. Some basal septal functions such as involvement in learning and memory formation or inhibition of sexual behavior appear to have persisted during vertebrate evolution.

Functional mapping of the auditory midbrain during mate call reception

We examined patterns of neural activity as assayed by changes in gene expression to localize representation of acoustic mating signals in the auditory midbrain of frogs. We exposed wild-caught male Physalaemus pustulosus to conspecific mating calls that vary in their behavioral salience, nonsalient mating calls, or no sound. We measured expression of the immediate early gene egr-1 (also called ZENK, zif268, NGFI-A, and krox-24) throughout the torus semicircularis, the auditory midbrain homolog of the inferior colliculus. Differential egr-1 induction in response to the acoustic stimuli occurred in the laminar, midline, and principal nuclei of the torus semicircularis, whereas the ventral region did not show significant effects of stimulus. The laminar nucleus differentially responded to conspecific mating calls compared with nonsalient mating calls, whereas the midline and principal nuclei responded preferentially to one of two conspecific calls. These responses were not explained by simple acoustic properties of the stimuli, and they demonstrate a functional heterogeneity of auditory processing of complex biological signals within the frog midbrain. Moreover, using analyses that assess the ability of the torus semicircularis as a whole to discriminate among acoustic stimuli, we found that activity patterns in the four regions together provide more information about biologically relevant acoustic stimuli than activity in any single region.

Seasonal changes in frequency tuning and temporal processing in single neurons in the frog auditory midbrain

Frogs rely on acoustic signaling to detect, discriminate, and localize mates. In the temperate zone, reproduction occurs in the spring, when frogs emerge from hibernation and engage in acoustically guided behaviors. In response to the species mating call, males typically show evoked vocal responses or other territorial behaviors, and females show phonotactic responses. Because of their strong seasonal behavior, it is possible that the frog auditory system also displays seasonal variation, as evidenced in their vocal control system. This hypothesis was tested in male Northern leopard frogs by evaluating the response characteristics of single neurons in the torus semicircularis (TS; a homolog of the inferior colliculus) to a synthetic mating call at different times of the year. We found that TS neurons displayed a seasonal change in frequency tuning and temporal properties. Frequency tuning shifted from a predominance of TS units sensitive to intermediate frequencies (700-1200 Hz) in the winter, to low frequencies (100-600 Hz) in the summer. In winter and early spring, most TS neurons showed poor, or weak, time locking to the envelope of the amplitude-modulated synthetic call, whereas in late spring and early summer the majority of TS neurons showed robust time-locked responses. These seasonal differences indicate that neural coding by auditory midbrain neurons in the Northern leopard frog is subject to seasonal fluctuation.

Vocal circuitry in Xenopus laevis: telencephalon to laryngeal motor neurons

Sexually differentiated calling patterns of Xenopus laevis are conveyed to the vocal organ by a dedicated neuromuscular system. Here, we define afferents to vocal motor neurons and determine whether the connectivity of the vocal pathway is sexually differentiated. The use of fluorescent dextran amines and the isolated brain preparation readily permitted identification of anterograde and retrograde connectivity patterns. The whole-mount preparation allowed us to observe projections in their entirety, including cells of origin of a projection (for retrograde projections), terminal fields (for anterograde connections), and fiber tracts. Major findings are the confirmation of a robust and reciprocal connection between cranial nucleus (n.) IX-X and the pretrigeminal nucleus of the dorsal tegmental area of the medulla (DTAM) as well as between DTAM and the ventral striatum (VS). Newly revealed is the extensive connectivity between the rostral subdivision of the dorsal nucleus raphe (rRpd) and candidate vocal nuclei. In contrast to previous results using peroxidase, we did not observe dramatic sex differences in connectivity, although some connections were less robust in female than in male brains. Some retrograde connections previously observed (e.g., anterior preoptic area to DTAM) were not confirmed. Plausible hypotheses are that a set of rhombencephalic neurons located in DTAM, the inferior reticular formation and n.IX-X are responsible for generating patterned vocal activity, that activity is modulated by neurons in rRpd, and that activity in VS (particularly that evoked by conspecific calls), together with effects of steroid hormones at many sites in the vocal circuit, contribute to the initiation of calling.

Estrogen receptors in Xenopus: duplicate genes, splice variants, and tissue-specific expression

The estrogenic steroid hormones, acting primarily through the nuclear estrogen receptors ERalpha and ERbeta, regulate sexual differentiation in a wide variety of vertebrates. In the frog Xenopus laevis, estrogen regulates the strength of vocal neuromuscular synapses and contributes to the physiological basis of sexually differentiated songs. To understand the mechanisms by which estrogen produces these effects, we have characterized the ERs of X. laevis and their expression in laryngeal muscle and other tissues. We found a remarkable molecular diversity in the estrogen receptor population within individuals. First, we have identified two distinct ERalpha genes, xlERalpha1 and xlERalpha2, which represent, to our knowledge, the first discovery of retained duplicates of the ERalpha gene in any species. These two genes are highly conserved at the amino acid level but have distinct nucleotide sequences; moreover, ERalpha2 has no N-terminal domain. Cloning of ERalpha and ERbeta in the related species Xenopus tropicalis and phylogenetic analysis indicate that the two xlERalpha loci were generated by a duplication specific to the X. laevis lineage-most likely the genome duplication that led to a doubling of the X. laevis chromosome number about 30 million years ago. The primary ER expressed in X. laevis laryngeal muscle is the novel gene xlERalpha2; ERalpha1 is primarily expressed in liver, forebrain, and oviduct. Alternatively spliced transcripts of both xlERalpha1 and xlERalpha2 are also expressed in a tissue-specific manner. We propose that complementary spatial expression of these two genes and their alternatively spliced transcripts contributes to their conservation over such a long period of time, consistent with the subfunctionalization model for evolution after gene duplication.

Environmental estrogens and reproductive biology in amphibians

In this paper, the effects of an estrogenic compound, 4-nonyl-phenol (NP), on the amphibians Rana esculenta and Triturus carnifex are described together with those on sexual differentiation in Xenopus laevis. NP increased plasma vitellogenin in male frogs and newts in a dose-related manner; moreover, inhibitory effects on gonadotropin and prolactin (PRL) secretion by pituitary were found together with an elevation of plasma androgens. NP treatment also caused a remarkable increase in number of prolactin-immunolabeled cells, suggesting that xenoestrogen might induce, at least in the newt pituitary, a PRL accumulation possibly due to a reduction of the hormone release. In addition, both NP and bisphenol A caused feminization by increasing the percentage of female phenotypes in X. laevis, and the in vivo effects were more pronounced than those of estradiol-17beta.

Social context influences androgenic effects on calling in the green treefrog (Hyla cinerea)

Courtship behavior in frogs is an ideal model for investigating the relationships among social experience, gonadal steroids, and behavior. Reception of mating calls causes an increase in androgen levels in listening males, and calling, in turn, depends on the presence of androgens. However, previous studies found that androgen replacement does not always restore calling to intact levels, and the relationship between androgens and calling may be context dependent. We examined the influence of androgens on calling behavior in the presence and the absence of social signals in male green treefrogs (Hyla cinerea). We categorized calling during an acoustic stimulus (mating chorus or tones) as evoked and calling in the absence of a stimulus as spontaneous. Intact males received a cholesterol implant, castrated males were castrated and received a cholesterol implant, and T-implanted males were castrated and received a testosterone implant. The androgen levels (mean +/- SE ng/ml of plasma) achieved by the implants were as follows: castrated males, 1.2 +/- 0.2; intact males 21.9 +/- 7.0; T-implanted males, 254.6 +/- 39.5. As in other frogs, calling depends on the presence of androgens, as castration abolished and T replacement maintained calling. However, among intact and T-implanted males, the influence of androgens on calling differed between spontaneous and evoked calling. There was a positive effect of androgen treatment on spontaneous call rate and a positive correlation between spontaneous call rate and androgen levels. The influence of androgen levels on evoked call rate was more complex and interacted with acoustic treatment. Surprisingly, T implants suppressed the chorus-specific increase in calling that is evident in intact males. In addition, in response to the chorus, T-implanted males called less than did intact males, in spite of higher androgen levels. Furthermore, variation in androgens did not explain variation in evoked call rate. These data indicate that androgens influence the motivation to call, but that, when socially stimulated, androgens are necessary but insufficient for calling.

Auditory and lateral line inputs to the midbrain of an aquatic anuran: neuroanatomic studies in Xenopus laevis

Computation of rate in auditory signals is essential to call recognition in anurans. This task is ascribed to a group of central nervous system nuclei in the dorsal midbrain or torus semicircularis, homologous to the inferior colliculus of mammals. We have mapped the connections of the subnuclei of the torus semicircularis in Xenopus laevis to determine which receive auditory and which receive lateral line information. Relative to terrestrial anurans, the torus of X. laevis is hypertrophied and occupies the entire caudal, dorsal midbrain. Auditory input to the torus, that arising directly from the dorsal medullary nucleus, is present only in the laminar nucleus. The principal and magnocellular nuclei receive their input from the lateral line nucleus of the medulla. All three nuclei of the torus also have reciprocal connections with the superior olive and the nucleus of the lateral lemniscus. Ascending efferents from all three nuclei of the torus innervate central and lateral thalamic nuclei, and all have a weak reciprocal connection with the posterior thalamus. The laminar and magnocellular nuclei have reciprocal connections with the ventral thalamus, and all three nuclei of the torus receive descending input from the anterior entopeduncular nucleus. The laminar and magnocellular nuclei also receive descending input from the preoptic area. Based on our identification of toral nuclei and these results we assign a major function for the detection of water-borne sounds to the laminar nucleus and a major function for the detection of near field disturbances in water pressure to the principal and magnocellular nuclei.

Social signals influence hormones independently of calling behavior in the treefrog (Hyla cinerea)

Social signals play an important role in regulating hormone-behavior relationships. In anurans (frogs and toads), acoustic signals are an essential aspect of reproductive behavior; however, the physiological consequences of receiving social signals has remained largely undescribed. Each night for 5, 10, or 20 days, we presented acoustically isolated male treefrogs with a conspecific mating chorus, an array of tones, or no sound. We recorded calling rate of individuals throughout the experiment and collected blood before and after treatment. Days of stimulus exposure had no effect on any dependent measure. Acoustic treatment influenced steroid levels; testosterone, dihydrotestosterone, and corticosterone increased only in the group exposed to the chorus. Chorus-exposed males also showed an increase in stimulus-evoked calling. We found no correlation between androgens and calling within each treatment group. In addition, noncallers in the chorus group had higher levels of androgens than males in the tone or no sound groups. Further, chorus-exposed males with zero, low, or high rate of calling had similar levels of androgens. These data indicate that social signals increase circulating androgens independently of calling behavior. Elevated corticosterone associated with chorus reception did not inhibit calling behavior, and corticosterone showed no correlation with androgen levels.

Estrogen receptors and aromatase activity in the hypothalamus of the female frog, Rana esculenta. Fluctuations throughout the reproductive cycle

It is well known that certain actions of androgen are mediated through in situ aromatization to estrogen in neural target tissues. This study was undertaken to investigate androgen utilization in the hypothalamus of the female frog, Rana esculenta, through a quantification of estrogen receptors and aromatase activity during the reproductive cycle. 3H-estradiol-binding molecules were present in both the cytosol and the nuclear extract of the hypothalamus. These molecules bound specifically 3H-estradiol with high affinity (Kd 10(-10) M) and low capacity (cytosol: 1.2+/-0.4 fmol/mg protein; nuclear extract: 7.9+/-0.6 fmol/mg protein). Aromatase activity was detected in the microsomal fraction of the hypothalamus using a sensitive in vitro radiometric assay. Both aromatase activity and nuclear estrogen receptor binding fluctuated in synchrony throughout the reproductive cycle. Western blot analysis of aromatase protein revealed one immunoreactive band with a molecular weight of approximately 56 kDa. In contrast to aromatase enzyme activity, the relative levels of aromatase protein changed little during the reproductive cycle suggesting that post-translational mechanisms may be involved in regulating estrogen synthesis in the frog brain. A possible role for estrogens in the modulation of the reproductive behavior in this species is suggested.

Chemoarchitecture of the anuran auditory midbrain

The anuran torus semicircularis consists of several subnuclei that are part of the ascending auditory pathway as well as audiomotor interface structures. Additionally, recent anatomical studies suggest that the midbrain tegmentum is an integral part of the audiomotor network. To describe the chemoarchitecture of these nuclei, taking into account the toral subdivisions, we investigated the distribution of serotonin, leucine-enkephalin, substance P, tyrosine-hydroxylase, dopamine D2-receptor, parvalbumin, aspartate, GABA, and estrogen-binding protein-immunoreactivity in the midbrain of Bombina orientalis, Discoglossus pictus and Xenopus laevis. In the torus semicircularis, the highest density of immunoreactive fibers and terminals for all transmitters was found in the laminar nucleus. Parvalbumin-like immunoreactivity was highest in the principal nucleus, and D2-receptor-like immunoreactivity was uniformly distributed throughout the torus. In the tegmentum, axons and/or dendrites were stained with all antibodies except estrogen-binding protein. Additionally, heavily stained enkephalin and substance P-immunopositive fiber plexus were found in the lateral and dorsal tegmentum. The immunostainings revealed no qualitative differences between the three species. Immunopositive cell bodies were labeled in several brain areas, the connectivity of which with torus and tegmentum is discussed on the background of functional questions. The putative neuromodulatory innervation of both the laminar nucleus of the torus semicircularis and the tegmentum may be the anatomical basis for the influence of the animal's endogenous state on the behavioral reaction to sensory stimuli. These data corroborate earlier anatomical and physiological findings that the neurons of these nuclei are key elements in the audio-motor interface.

Estrogen regulates gonadotropin-releasing hormone in the nervus terminalis of Xenopus laevis

The nervus terminalis or terminal nerve (TN) is a neuronal plexus found in the nasal cavity and rostral forebrain of most vertebrates. The hormone gonadotropin-releasing hormone (GnRH) is found in a population of TN neurons as well as hypothalamic neurons which regulate pituitary secretion of the gonadotropins. The GnRH-containing neurons of the TN appear to represent a rostral continuation of the hypothalamic population since they both originate from the olfactory placode and are frequently anatomically continuous. Previous studies have shown that the hypothalamic GnRH neurons are regulated by circulating estrogen levels. Ovariectomy decreases while estrogen administration increases GnRH content in these neurons. It is not known whether the GnRH-containing TN neurons are also regulated in a similar manner. This study demonstrates that ovariectomy and estrogen readministration alters GnRH-immunoreactive (ir) levels in the TN of female Xenopus laevis in a manner similar to that seen in the hypothalamus. One week after ovariectomy, the density of TN GnRH-ir fibers in the olfactory bulb region (one site of TN termination) is significantly decreased. In contrast, a significant increase in GnRH-ir TN fiber density is observed following estrogen readministration to ovariectomized frogs. These findings demonstrate that estrogen regulates GnRH metabolism in neurons of the TN.

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.

Brain vasotocin pathways and the control of sexual behaviors in the bullfrog

The neurohypophysial peptide arginine vasotocin (AVT) alters the display of several sexually dimorphic behaviors in the bullfrog (Rana catesbeiana). These behaviors include mate calling, release calling, call phonotaxis, and locomotor activity. Populations of AVT-immunoreactive cells are present in six areas of bullfrog brain and fibers are widespread. Neural areas involved in vocalization, in particular, contain AVT cells and fibers. As well, AVT concentrations in a subset of brain areas are sexually dimorphic and steroid sensitive. Effects of gonadectomy and gonadal steroid treatment vary, depending on the brain area and sex of the frog. For example, some anterior areas are sensitive to changes in both dihydrotestosterone (DHT) and estradiol. In some posterior brain areas, on the other hand, AVT levels are affected only by DHT. A similar situation exists for putative AVT receptors in bullfrogs. Receptors are widespread, occurring in many areas that have been linked to behavior. Receptor concentrations are sexually dimorphic in the amygdala pars lateralis, hypothalamus, pretrigeminal nucleus, and dorsolateral nucleus. Estradiol alters AVT receptor level in the amygdala of both sexes of bullfrog and both estradiol and DHT alter the receptor number in the pretrigeminal nucleus, but only in males. The mechanisms responsible for steroid effects on vasotocin neurons and their targets are unknown. Specific AVT cells, fiber terminal fields, and receptor populations are likely influenced by gonadal steroids for effective timing of individual behaviors displayed by bullfrogs.

Neuroanatomical distribution of androgen and estrogen receptor-immunoreactive cells in the brain of the male roughskin newt

Immunohistochemistry was used to investigate the neuroanatomical distribution of androgen and estrogen receptors in brains of adult male roughskin newts, Taricha granulosa, collected during the breeding season. Immunoreactive cells were found to be widely distributed in specific brain areas of this urodele amphibian. Androgen receptor-immunoreactive (AR-ir) cells were observed in the olfactory bulbs, habenula, pineal body, preoptic area, hypothalamus, interpeduncular nucleus, area acusticolateralis, cerebellum, and motor nuclei of the medulla oblongata. Estrogen receptor-immunoreactive (ER-ir) cells were found in the lateral septum, amygdala pars lateralis, pallium, preoptic area, hypothalamus, and dorsal mesencephalic tegmentum. This immunocytochemical study of the newt brain reveals AR-ir and ER-ir cells in several regions that have not been previously reported to contain androgen and estrogen receptors in non-mammalian vertebrates. Additionally, the distribution of AR-ir and ER-ir cells in the newt brain, in general, is consistent with previous studies, suggesting that the distribution of sex steroid receptor-containing neurons in some brain regions is relatively conserved among vertebrates.

Autoradiographic localization of dihydrotestosterone and testosterone concentrating neurons in the brain of the oyster toadfish

Vertebrate species with male mating calls or songs tend to have sexually dimorphic sonic neurons that concentrate gonadal steroids. The distribution of [3H]dihydrotestosterone- and testosterone-concentrating neurons was examined in oyster toadfish (Opsanus tau), males of which produce a courtship boatwhistle call. Labeled cells in the forebrain were found in the posterior nucleus of the dorsal telencephalon (Dp), a pallial structure, the supracommissural nucleus of the ventral telencephalon (Vs), nucleus propticus parvocellularis anterior (PPa) and other preoptic nuclei, the ventral, dorsal and caudal hypothalamus. Positive brainstem areas included the optic tectum, torus semicircularis, nucleus lateralis valvula, a periventricular nucleus of the rostral medulla and the inferior reticular formation. Compared to estrogen, androgens labeled fewer sites in the forebrain and more in the brainstem. Two of the positive sites, Vs and PPa, have been implicated in boatwhistle production. Many sites that connect to these areas in teleosts likewise concentrate steroids. Unlike the situation in frogs, birds, and one other teleost, the toadfish sonic motor nucleus did not concentrate androgens. Androgen labeling in the posterior nucleus of the dorsal telencephalon represents the first autoradiographic demonstration of steroid concentration in the pallium of a teleost forebrain.

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.

Comparative distribution of neurotensin-like immunoreactivity in the brain of a teleost (Carassius auratus), an amphibian (Hyla meridionalis), and a reptile (Gallotia galloti)

The distribution of neurotensin (NT) was studied in the brain of three species belonging to the three major classes of cold-blooded vertebrates: teleost fishes (Carassius auratus), anuran amphibians (Hyla meridionalis), and reptiles (Gallotia galloti; Lacertidae). By using antibodies directed against synthetic bovine NT in the three species, immunoreactive cell bodies were discovered mostly in the telencephalon and diencephalon, in particular at the level of the preoptic region the mediobasal hypothalamus, and the thalamus. In the frog and the lizard, additional immunoreactive (ir) structures were observed in the optic tectum and the tegmentum of the mesencephalon. In the goldfish pituitary, an extensive innervation was consistently observed at the level of the rostral pars distalis, whereas in both frog and lizard, positive fibers were only detected in the external layer of the median eminence. In the three species there is a striking overlap between the distribution of the NT-ir cell bodies and that of the target cells for sexual steroids. The results are discussed in relation with those reported in birds and mammals, and with the possible interactions among NT, sexual steroids, and the neuroendocrine control of pituitary hormone release, in particular prolactin and gonadotrophin.

Sexual differences in hormonal control of release calls in bullfrogs

Release calls in anuran amphibians are given when animals are inappropriately clasped by others. Since other call types, such as mate calls, are sexually dimorphic in frogs and toads, sonogram analysis was used to determine whether release call characteristics might also be sexually dimorphic in bullfrogs (Rana catesbeiana). Only intercall intervals differed significantly between males and females. Call duration, dominant frequency characteristics, and the display of secondary or tertiary harmonic frequencies were similar in both sexes. In the spring, but not the fall, calling rates were significantly lower in female bullfrogs, compared to males. Females also had significantly lower plasma androgen concentrations and higher plasma estrogen in the spring, compared to males. In both sexes, plasma androgen and estrogen were significantly higher in the spring, compared to the fall. The neuropeptide arginine vasotocin significantly decreased release call rates in females in the spring while it significantly increased rates in males. Vasotocin had no significant effect in the fall. Prostaglandin E2 significantly inhibited release calling in both males and females. On the other hand, prolactin significantly inhibited calling in female bullfrogs, but had no affect in males. Thus, although acoustic characteristics of release calls were similar in male and female bullfrogs, hormonal control of call rates was sexually dimorphic and seasonally variable.

Sexual dimorphism in the vasotocin system of the bullfrog (Rana catesbeiana)

Arginine vasotocin (AVT) is widespread in amphibian brains, where its levels have been correlated with reproductive behaviors. To better understand which neural systems are involved in central actions of AVT, we used immunocytochemistry to compare the distribution of AVT in the brains of male and female bullfrogs (Rana catesbeiana). AVT-immunoreactive cells were observed in the septal nucleus, amygdala pars lateralis, magnocellular preoptic area, suprachiasmatic nucleus, and hypothalamus. AVT-immunoreactive cells were also found in the pretrigeminal nucleus, but only in animals killed in the fall. Immunoreactive fibers were broadly distributed in hypothalamic and extrahypothalamic areas. The most obvious sex differences were found in the amygdala pars lateralis, where the density of immunoreactive cells and fibers was significantly greater in male than in female bullfrogs. In addition, in the habenular nucleus, males had a denser distribution of AVT-immunoreactive fibers than females. In the suprachiasmatic nucleus, AVT-immunoreactive cells were larger in females than in males but did not differ in number. Since the areas that showed sex differences in AVT distribution have also been implicated in control of reproductive behaviors, they may form the neural substrates for the effects of AVT on sexually dimorphic behaviors in amphibians.

Acoustic modulation of neural activity in the preoptic area and ventral hypothalamus of the green treefrog (Hyla cinerea)

Responses of neurons in the preoptic area and ventral hypothalamus to conspecific mating calls or white noise bursts were examined in male green treefrogs (Hyla cinerea) during different seasons. In the winter, 34.3% of preoptic neurons and 46.7% of ventral hypothalamic cells demonstrated significant changes in activity level during presentation of a conspecific mating call. In contrast, only 13.3% of preoptic units and 16.7% of ventral hypothalamic cells responded to the white noise. The percentage of preoptic and hypothalamic units responding to the advertisement call did not differ significantly during the summer breeding season. Type I units exhibited a dramatic increase in activity during acoustic stimulation followed by a rapid return to baseline activity levels after stimulus offset. Type II cells showed a robust activity increase during stimulation, but maintained an intermediate activity level after stimulus offset. In the preoptic area, a third response type exhibited suppressed activity during acoustic stimulation. Although seasonal condition did not alter the percentage of acoustically responsive units within either nucleus, the proportion of Type I units in the ventral hypothalamus was greatest during the summer.

Immunocytochemical localization of a galanin-like peptidergic system in the brain of two urodele and two anuran species (Amphibia)

Galanin-like immunoreactivity was localized in the brain of Urodela (Ambystoma, Pleurodeles) and Anura (Bufo, Xenopus) by immunocytochemistry with anti-porcine galanin antiserum. In the four species, immunoreactive perikarya were observed in the telencephalon (striatum, amygdala), diencephalon preoptic area mainly along the anterodorsal wall of the preoptic recessus, suprachiasmatic nucleus, lateral hypothalamus, ventral and dorsal infundibular nuclei, paraventricular organ, and rhombencephalon (nucleus of the solitary tract). Galaninergic fibres extended in similar regions and in the medial septum, ventral telencephalon, ventral hypothalamus, median eminence, and various mesencephalic and rhombencephalic regions. Contacts with the cerebrospinal fluid cavity occurred along the preoptic recessus (Ambystoma) and the ventral infundibular wall (all species). Fibres were scarce in the neurohypophysis. The distal and intermediate lobes of the pituitary were virtually devoid of immunoreactivity. The galaninergic system appeared more developed in adult amphibia than in young animals, suggesting the stimulating influence of sex steroids on the expression of galanin as previously described in Anguilla. The extensive distribution of the galanin-like immunoreactive neurons in amphibian brains suggests that this peptide may act as a neuromodulatur and/or neurotransmitter.

Evolutionary precedents for behavioral actions of oxytocin and vasopressin

It is clear that the behavioral actions of oxytocin and vasopressin in mammals are not newly acquired, but have evolutionary antecedents. Injection studies with fish, amphibians, reptiles, and birds indicate that AVT can activate certain reproductive behaviors. The strongest evidence that AVT acts centrally to control reproductive behaviors comes from research on T. granulosa. In this amphibian, injections of AVT agonists activate courtship behaviors (amplectic clasping) in males and egg-laying behaviors in females, whereas injections of AVT antagonists inhibit the behaviors. Also, in Taricha males, AVT concentrations in specific brain areas are associated with the expression of courtship behaviors. Several conclusions about steroid-peptide interactions can be drawn, based on research with this amphibian. First, gonadal steroid hormones act to maintain the behavioral actions of AVT in both males and females. In Taricha, gonadectomy abolishes and steroid implants restore AVT-induced courtship in males and egg-laying in females. Second, gonadal steroids maintain the behavioral actions of AVT, in part, by modulating AVT receptor numbers on target neurons. In Taricha males and females, gonadectomy reduces AVT receptor concentrations (but not binding affinity) in certain brain areas (amygdala pars lateralis) and not others. Third, the type of gonadal steroid determines whether AVT elicits male-like or female-like reproductive behaviors. Ovariectomized Taricha females respond to AVT injections with egg-laying behaviors when implanted with estradiol and with male-like amplectic clasping when implanted with dihydrotestosterone. Fourth, the masculinization of AVT-induced behaviors in females most likely reflects site-specific actions of androgens on AVT-synthesizing neurons. In Taricha, AVTir concentrations in the optic tectum are sexually dimorphic (higher in males than females) and reach peak levels in males during the breeding season. Fifth, AVT content in specific brain areas increase as a function of performing the behaviors. In Taricha, AVTir concentrations in DPOA, CSF, and ventral infundibulum are higher in males that exhibit courtship behaviors than in males that do not. These conclusions illustrate how steroid-peptide interactions in the control of behaviors entail multiple neuroanatomical sites and neurochemical actions.

Autoradiographic localization of estrogen-concentrating cells in the brain and pituitary of the oyster toadfish

We examined the distribution of [3H] estradiol-concentrating neurons in gonadectomized male and female oyster toadfish (Opsanus tau) by thaw-mount autoradiography. Compared to other teleosts studied in this fashion, the toadfish is of interest because of its reliance on male sound production during courtship. Labeled cells were found in the pars distalis of the pituitary and in periventricular midline regions of the forebrain. These included the ventral, supracommissural and posterior nuclei of the ventral telencephalon, nucleus preopticus parvocellularis anterior, the ventral, lateral, dorsal and caudal hypothalamus, and the periventricular nucleus of the posterior tuberculum of the thalamus. For the first time in a teleost, estrogen-concentrating cells were recorded in the brainstem, in sites including the optic tectum and torus semicircularis. In contrast to a previous study with testosterone, two prominent nuclei in the medulla were negative. With some additions, the distribution of estrogen-concentrating neurons conformed to the general pattern in other teleosts.

The response of nucleus preopticus neurosecretory cells to ovarian pressure in the frog, Rana tigrina

Intraovarian pressure (IOP) of 5, 15, and 25 mm Hg was administered in the frog, Rana tigrina, and the response of the nucleus preopticus (NPO) pars magnocellularis was investigated with aldehyde fuchsin (AF) stain and immunocytochemical method using neurophysin (NP) antisera. The 5 mm Hg IOP treatment resulted in cell and cell nuclear hypertrophy (P less than 0.001); discrete signs of de novo synthesis of AF-positive and NP-immunoreactive material in the perikarya and remarkable increases in the number and size of "Herring bodies" in the processes were observed. Stimulatory response after 15 mm Hg IOP treatment was characterized by dramatic augmentation of the AF-positive and NP-immunoreactive material in the processes; the engorged and coalescing Herring bodies totally predominated the lateral preoptic area. IOP of 25 mm Hg resulted in extensive loss of secretory material; the processes revealed the presence of vacuoles indicative of the rapid anterograde transport of the neurosecretory material. Furthermore, the application of IOP seemed to promote the transport of NP-immunoreactive material toward the anterior preoptic area and amygdala pars medialis and the release of secretory material into the cerebrospinal fluid. No changes were observed in the NPO when treatment was preceded by the transection of ipsilateral dorsal spinal nerve roots or the spinal cord. The results suggest the existence of an afferent neural pathway from the ovary capable of conducting the stretch signals to the NPO and triggering the synthesis and release of neurohypophysial hormones.

Immunoreactive luteinizing hormone-releasing hormone in the frog (Rana esculenta) brain: distribution pattern in the adult, seasonal changes, castration effects, and developmental aspects

The present work describes the neuroanatomical distribution of the immunoreactive luteinizing hormone-releasing hormone (ir-LHRH) system in the brain of adult male and female, castrated male and developing Rana esculenta. No obvious sex differences in the distribution pattern of ir-LHRH were observed. Immunoreactive neuronal cell bodies are not contained within a single anatomically defined area of the brain. They are present as distinct groups in the olfactory bulbs, medial septal area, anterior preoptic area (APOA), retrochiasmatic area of the infundibulum, and interpeduncular nucleus-tegmentum area. Of the entire brain, the medial septal-APOA region exhibits the highest frequency of ir-LHRH cell bodies in both sexes. ir-LHRH fiber projections are present in the olfactory bulbs, medial septal area, APOA, floor of the diencephalon, subhabenular-periventricular area in the epithalamus, lateral suprachiasmatic area, ventrolateral infundibulum, median eminence, pars nervosa, optic tectum, interpeduncular nucleus-tegmentum area, and rhombencephalon grey. Castration seems to bear no effect on the pattern of ir-LHRH system in the frog brain. The influence of castration consisted in decreased intensity of the immunostaining and frequency of occurrence of the septal-APOA neuronal cell bodies. In median eminence, castration also induced a sensible decrease in the immunoreactivity, whereas in the pars nervosa of 50-day castrates ir-LHRH fibers totally disappeared. During ontogenesis, ir-LHRH elements first become evident in stage 31 tadpoles (beginning of metamorphic climax); LHRH immunoreaction is restricted to the cell bodies and fibers in the APOA and some fibers in the ventral hypothalamus and a few in median eminence. This condition remains unaltered until stage 33 when the tail is almost totally resorbed. The possible implications of the ir-LHRH-containing brain areas in the different aspects of reproduction in the frog are briefly discussed.

Localization of steroid-concentrating cells in the central nervous system of the frog Rana esculenta

Steroid-concentrating cells were localized using steroid hormone autoradiography in the brain of Rana esculenta after systemic administration of tritiated estradiol and testosterone. Highest numbers of labeled cells were found in the anterior preoptic area, in the ventral infundibular nucleus, and in the pituitary. Other sites of tritiated sex steroid uptake were in the striatum, septum, amygdala, thalamus, tectum, torus semicircularis, and dorsal tegmental area of the medulla. No differences were seen between males and females. These results complement the ones obtained in Rana pipiens and confirm that in the genus Rana the distribution of testosterone- and estradiol-concentrating neurons is invariant in many respects.

Neurogenesis in the vocalization pathway of Xenopus laevis

We examined possible contributions of neurogenesis to sex differences in the vocalization pathway of the South African clawed frog, Xenopus laevis. Birthdates of neurons were obtained from autoradiograms of animals receiving tritiated thymidine from gastrulation through 1 month after metamorphosis. Thymidine availability studies showed that 80% of the [3H]-thymidine injected into embryos and tadpoles was incorporated into the DNA of dividing cells within 3 hours. We observed 3 patterns of neurogenesis: late-short, a short burst of proliferation occurred late in development in the anterior preoptic area, the ventromedial nucleus of the thalamus, and the pretrigeminal nucleus of the dorsal tegmental area of the medulla; protracted-bimodal, a prolonged period of proliferation with an early and a late peak in the number of labeled cells occurred in the ventral striatum and in the ventrolateral and posterior nuclei of the thalamus; protracted-unimodal, a prolonged period of proliferation with a single early peak occurred in the inferior reticular formation and in the medial and lateral nucleus IX-X (containing laryngeal motor neurons). There were no differences between sexes in the number of tritiated thymidine labeled cells in any nucleus. The difference in nucleus IX-X neuron number in adults does not appear to result from sex differences in the proliferation of these cells during development. Since neurons in the vocalization pathway do not exhibit androgen receptors until after neurogenesis is complete, we also conclude that androgen probably does not regulate the genesis of these cells.