Differential effects of arginine vasotocin and gonadotropin-releasing hormone on sexual behaviors in an anuran amphibian

Arginine vasotocin affects vocal behavior but not selective responses to conspecific calls in male túngara frogs

Arginine vasotocin (AVT) and its homolog arginine vasopressin (AVP) modulate social behavior, including social communication. In anuran amphibians, male-male competition and female mate choice rely heavily on acoustic signaling. Behavioral experiments show that AVT influences motivation to call and vocal production. It may also influence how males process and respond to socially relevant auditory stimuli, but few studies have explored this possibility in this taxon. Túngara frogs produce a "whine" that is used for species recognition; in competition with other males they append one or more attractive "chucks" to the whine. Frequency modulation in the whine is an important cue for recognizing conspecifics, and gating of conspecific signals begins in the auditory midbrain. We used dynamic playback experiments to investigate the effects of exogenous AVT on males' responses to stimuli with species-typical and altered frequency modulation. We used avoidance of call overlap as evidence that a male recognizes a stimulus as salient and the production of attractive chucks as evidence of his competitive response to a proximate rival. We used call rate, whine duration, and whine frequency as measures of motivation and motor production. Males responded selectively to a stimulus with species-typical frequency modulation. Following treatment with AVT, they increased call rate and altered whines and chucks in a way that suggests increased air flow during the whine. We did not, however, find evidence that treatment with AVT alters the salience of frequency modulation in recognizing and responding to acoustic signals, at least for the stimuli used in this study.

Vasotocin receptor blockade disrupts maternal care of offspring in a viviparous snake, Sistrurus miliarius

Parental care is a complex social behavior that is widespread among vertebrates. The neuroendocrine regulation of parent-offspring social behavior has been well-described in mammals, and to a lesser extent, in birds and fish. However, little is known regarding the underlying mechanisms that mediate the expression of care behaviors in squamate reptiles. In mammalian model species and humans, posterior pituitary hormones of the oxytocin and vasopressin families mediate parental care behaviors. To test the hypothesis that the regulatory role of posterior pituitary neuropeptides is conserved in a viviparous squamate reptile, we pharmacologically blocked the vasotocin receptor in post-parturient pigmy rattlesnakes, Sistrurus miliarius, and monitored the spatial relationship between mothers and offspring relative to controls. Mothers in the control group demonstrated spatial aggregation with offspring, with mothers having greater post-parturient energy stores aggregating more closely with their offspring. Blockade of vasotocin receptors eliminated evidence of spatial aggregation between mothers and offspring and eliminated the relationship between maternal energetic status and spatial aggregation. Our results are the first to implicate posterior pituitary neuropeptides in the regulation of maternal behavior in a squamate reptile and are consistent with the hypothesis that the neuroendocrine mechanisms underlying social behaviors are broadly conserved among vertebrates.

Arginine Vasotocin, the Social Neuropeptide of Amphibians and Reptiles

Arginine vasotocin (AVT) is the non-mammalian homolog of arginine vasopressin (AVP) and, like vasopressin, serves as an important modulator of social behavior in addition to its peripheral functions related to osmoregulation, reproductive physiology, and stress hormone release. In amphibians and reptiles, the neuroanatomical organization of brain AVT cells and fibers broadly resembles that seen in mammals and other taxa. Both parvocellular and magnocellular AVT-containing neurons are present in multiple populations located mainly in the basal forebrain from the accumbens-amygdala area to the preoptic area and hypothalamus, from which originate widespread fiber connections spanning the brain with a particularly heavy innervation of areas associated with social behavior and decision-making. As for mammalian AVP, AVT is present in greater amounts in males in many brain areas, and its presence varies seasonally, with hormonal state, and in males with differing social status. AVT's social influence is also conserved across herpetological taxa, with significant effects on social signaling and aggression, and, based on the very small number of studies investigating more complex social behaviors in amphibians and reptiles, AVT may also modulate parental care and social bonding when it is present in these vertebrates. Within this conserved pattern, however, both AVT anatomy and social behavior effects vary significantly across species. Accounting for this diversity represents a challenge to understanding the mechanisms by which AVT exerts its behavioral effects, as well are a potential tool for discerning the structure-function relationships underlying AVT's many effects on behavior.

Arginine vasotocin V1a2 receptor and GnRH-I co-localize in preoptic neurons of the sex changing grouper, Epinephelus adscensionis

The arginine vasotocin/vasopressin (AVT/AVP) and gonadotropin releasing hormone (GnRH) systems are known to control sexual behaviors and reproduction, respectively, in different vertebrate groups. However, a direct functional connection between these two neuroendocrine systems has not been demonstrated for any vertebrate species. Therefore, the objective of this research was to test the hypothesis that AVT acts on the GnRH system via an AVT V1a receptor in a sex changing grouper species, the rock hind, Epinephelus adscensionis. AVT V1a2 receptors were co-localized with GnRH-I on neurons in the preoptic anterior hypothalamus identifying a structural linkage between the AVT system and GnRH-I. Transcripts for avt, gnrh-I, and two AVT receptor subtypes (v1a1 and v1a2) were isolated and characterized for E. adscensionis and their expression was measured in males and females by q-RT-PCR. Translation of V1a-type cDNA sequences revealed two distinct forms of the AVT V1a receptor in E. adscensionis brain similar to those reported for other species. The observation of significantly higher gnrh-I mRNA in the POA+H of rock hind males as compared to females suggests differential regulation of the gnrh-I transcripts in the two sexes of this protogynous species. In male E. adscensionis, but not in females, a negative relationship was seen between plasma 11-ketotestosterone (11-KT) and the v1a1 receptor mRNA levels in the POA+H, while a positive trend was observed between 11-KT and v1a2 receptor mRNA levels, indicating that these receptor forms may be differentially regulated.

Gonadotropin-releasing hormone stimulates the biosynthesis of pregnenolone sulfate and dehydroepiandrosterone sulfate in the hypothalamus

The sulfated neurosteroids pregnenolone sulfate (Δ(5)PS) and dehydroepiandrosterone sulfate (DHEAS) are known to play a role in the control of reproductive behavior. In the frog Pelophylax ridibundus, the enzyme hydroxysteroid sulfotransferase (HST), responsible for the biosynthesis of Δ(5)PS and DHEAS, is expressed in the magnocellular nucleus and the anterior preoptic area, two hypothalamic regions that are richly innervated by GnRH1-containing fibers. This observation suggests that GnRH1 may regulate the formation of sulfated neurosteroids to control sexual activity. Double labeling of frog brain slices with HST and GnRH1 antibodies revealed that GnRH1-immunoreactive fibers are located in close vicinity of HST-positive neurons. The cDNAs encoding 3 GnRH receptors (designated riGnRHR-1, -2, and -3) were cloned from the frog brain. RT-PCR analyses revealed that riGnRHR-1 is strongly expressed in the hypothalamus and the pituitary whereas riGnRHR-2 and -3 are primarily expressed in the brain. In situ hybridization histochemistry indicated that GnRHR-1 and GnRHR-3 mRNAs are particularly abundant in preoptic area and magnocellular nucleus whereas the concentration of GnRHR-2 mRNA in these 2 nuclei is much lower. Pulse-chase experiments using tritiated Δ(5)P and DHEA as steroid precursors, and 3'-phosphoadenosine 5'-phosphosulfate as a sulfonate moiety donor, showed that GnRH1 stimulates, in a dose-dependent manner, the biosynthesis of Δ(5)PS and DHEAS in frog diencephalic explants. Because Δ(5)PS and DHEAS, like GnRH, stimulate sexual activity, our data strongly suggest that some of the behavioral effects of GnRH could be mediated via the modulation of sulfated neurosteroid production.

Sexually dimorphic effects of melatonin on brain arginine vasotocin immunoreactivity in green treefrogs (Hyla cinerea)

Arginine vasotocin (AVT) and its mammalian homologue, arginine vasopressin (AVP), regulate a variety of social and reproductive behaviors, often with complex species-, sex- and context-dependent effects. Despite extensive evidence documenting seasonal variation in brain AVT/AVP, relatively few studies have investigated the environmental and/or hormonal factors mediating these seasonal changes. In the present study, we investigated whether the pineal hormone melatonin alters brain AVT immunoreactivity in green treefrogs (Hyla cinerea). Reproductively active male and female frogs were collected during the summer breeding season and a melatonin-filled or blank silastic capsule was surgically implanted subcutaneously. The duration of hormone treatment was 4 weeks, at which time frogs were eutha-nized and the brains and blood collected and processed for AVT immunohistochemistry and steroid hormone assay. We quantified AVT-immunoreactive (AVT-ir) cell bodies in the nucleus accumbens (NAcc), caudal striatum and amygda- la (AMG), anterior preoptic area, suprachiasmatic nucleus (SCN) and infundibular region of the ventral hypothalamus. Sex differences in AVT-ir cell number were observed in all brain regions except in the anterior preoptic area and ventral hypothalamus, with males having more AVT-ir cells than females in the NAcc, amygdala and SCN. Brain AVT was sensitive to melatonin signaling during the breeding season, and the effects of melatonin varied significantly with both region and sex. Treatment with melatonin decreased AVT immunoreactivity in both the NAcc and SCN in male H. cinerea. In contrast, brain AVT was relatively insensitive to melatonin signaling in females, indicating that the regulation of the AVT/AVP neuropeptide system by melatonin may be sexually dimorphic. Finally, melatonin did not significantly influence testosterone or estradiol concentrations of male or female frogs, respectively, suggesting that the effects of melatonin on AVT immunoreactivity are independent of changes in gonadal sex steroid hormones. Collectively, our results indicate that the AVT/AVP neuronal system may be an important target for melatonin in facilitating seasonal changes in reproductive physiology and social behavior.

The synthetic progestogen, Levonorgestrel, but not natural progesterone, affects male mate calling behavior of Xenopus laevis

Worldwide, more than 100 million women use hormonal contraceptives, which act through progestogenic modes of action. These man-made hormones can enter the aquatic environment as they are excreted via feces and urine. Xeno-progestins are able to interfere with the endocrine system of female aquatic vertebrates impairing oogenesis and reproduction. However, data on progestogenic effects on reproductive behavior of male aquatic vertebrates are lacking. To evaluate whether progestins affect the mating behavior of male Xenopus laevis, we exposed male frogs to three environmentally relevant concentrations (10(-7) M, 10(-8) M and 10(-10) M) of the synthetic progestin Levonorgestrel (LNG) and the corresponding natural steroid progesterone (PRG), respectively. LNG at all exposure concentrations increased the proportions of advertisement calling, indicating a sexually aroused state of the males. Furthermore LNG at 10(-7) M decreased the relative proportions of rasping, a call type indicating a sexually unaroused state of the male. PRG, on the other hand, did not affect any of those parameters. Temporal and spectral features of the advertisement call itself were not affected by any of the two exposure treatments. Since LNG exhibits slight androgenic activity, the results suggest that LNG effects on male mate calling behavior of X. laevis are due to its moderate androgenic but not to its progestogenic activities. However, although males' sexual arousal seems to be enhanced by LNG, the adverse effects of LNG on female reproduction presumably outweigh these enhancing effects and LNG exposure nonetheless might result in reduced reproductive success of these animals.

Treatment with arginine vasotocin alters mating calls and decreases call attractiveness in male túngara frogs

The peptide hormone arginine vasotocin (AVT) and its mammalian homolog arginine vasopressin modulate a variety of social behaviors in vertebrates. In anurans, AVT influences the production of advertisement calls, the acoustic signals that males use to attract females and repel rival males. In this study, we investigate the effects of AVT on call characteristics in the túngara frog (Physalaemus pustulosus). Túngara frogs produce a "whine" that is important for species recognition; they may also produce a second, attractive call component, the "chuck". We used a field playback experiment to determine changes in male calling behavior following treatment with AVT. A previous study showed that AVT alters call rate and the production of chucks; in the current analysis, we focus on changes in the whine. Males produce shorter whines with higher initial frequencies following treatment with AVT. Call changes do not vary with a social stimulus. We also used female phonotaxis experiments to investigate the effects of call changes on female mate choice. Females disfavor the calls produced by males treated with exogenous AVT. We suggest that AVT influences motivation to call and the motor control of call production, but that over-stimulation of the vocal system limited the production of attractive calls in this experimental context.

Neurosteroid biosynthesis: enzymatic pathways and neuroendocrine regulation by neurotransmitters and neuropeptides

Neuroactive steroids synthesized in neuronal tissue, referred to as neurosteroids, are implicated in proliferation, differentiation, activity and survival of nerve cells. Neurosteroids are also involved in the control of a number of behavioral, neuroendocrine and metabolic processes such as regulation of food intake, locomotor activity, sexual activity, aggressiveness, anxiety, depression, body temperature and blood pressure. In this article, we summarize the current knowledge regarding the existence, neuroanatomical distribution and biological activity of the enzymes responsible for the biosynthesis of neurosteroids in the brain of vertebrates, and we review the neuronal mechanisms that control the activity of these enzymes. The observation that the activity of key steroidogenic enzymes is finely tuned by various neurotransmitters and neuropeptides strongly suggests that some of the central effects of these neuromodulators may be mediated via the regulation of neurosteroid production.

Rhabdias pseudosphaerocephalainfection inBufo marinus: lung nematodes reduce viability of metamorph cane toads

Cane toads (Bufo marinus) were introduced to Australia in 1935 and have since spread widely over the continent, generating concern regarding ecological impacts on native predators. Most Australian cane toad populations are infected with lung nematodesRhabdias pseudosphaerocephala, a parasite endemic to New World (native-range) cane toad populations; presumably introduced to Australia with its toad host. Considering the high intensities and prevalence reached by this parasite in Australian toad populations, and public ardour for developing a control plan for the invasive host species, the lack of experimental studies on this host-parasite system is surprising. To investigate the extent to which this lungworm influences cane toad viability, we experimentally infected metamorph toads (the smallest and presumably most vulnerable terrestrial phase of the anuran life cycle) with the helminth. Infected toads exhibited reduced survival and growth rates, impaired locomotor performance (both speed and endurance), and reduced prey intake. In summary,R. pseudosphaerocephalacan substantially reduce the viability of metamorph cane toads.

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.

Changes in brain mRNA levels of gonadotropin-releasing hormone, pituitary adenylate cyclase activating polypeptide, and somatostatin during ovulatory luteinizing hormone and growth hormone surges in goldfish

In goldfish, circulating LH and growth hormone (GH) levels surge at the time of ovulation. In the present study, changes in gene expression of salmon gonadotropin-releasing hormone (sGnRH), chicken GnRH-II (cGnRH-II), somatostatin (SS) and pituitary adenylate cyclase activating polypeptide (PACAP) were analyzed during temperature- and spawning substrate-induced ovulation in goldfish. The results demonstrated that increases in PACAP gene expression during ovulation are best correlated with the GH secretion profile. These results suggest that PACAP, instead of GnRH, is involved in the control of GH secretion during ovulation. Increases of two of the SS transcripts during ovulation are interpreted as the activation of a negative feedback mechanism triggered by high GH levels. The results showed a differential regulation of sGnRH and cGnRH-II gene expression during ovulation, suggesting that sGnRH controls LH secretion, whereas cGnRH-II correlates best with spawning behavior. This conclusion is further supported by the finding that nonovulated fish induced to perform spawning behavior by prostaglandin F2α treatment increased cGnRH-II expression in both forebrain and midbrain, but decreased sGnRH expression in the forebrain.

New insights into neuropeptide modulation of aggression: field studies of arginine vasotocin in a territorial tropical damselfish

The neuropeptides arginine vasotocin (AVT) and arginine vasopressin are key modulators of affiliation and aggression among non-mammalian and mammalian vertebrates, respectively. Here, we explored AVT's effect on aggression in a wild population of beaugregory damselfish, Stegastes leucostictus, a highly territorial species. Aggression by territorial males towards 'intruders' (bottled fishes) was assessed before and after each male received intramuscular injections of either AVT, Manning compound (an AVT V1a receptor antagonist), isotocin (the teleost homologue of mammalian oxytocin differing from AVT by two amino acids) or saline (vehicle control). Compared to saline controls, AVT and Manning increased and decreased aggression, respectively, while isotocin had no effect. Response selectivity was further established in a dose-response study that revealed an inverted U-shaped function. Compared to saline controls, aggression levels for low and high AVT doses were similar, while medium dose treatments were significantly greater. This type of behavioural response, the first that we know of for a vertebrate neuropeptide, could depend on the binding of AVT to both V1-type and other AVT or non-AVT receptors. The pattern revealed here for damselfish may be symptomatic of species- and context-dependent specificity of AVT's modulation of aggression across teleosts, as is currently proposed for tetrapods.

Arginine Vasotocin Promotes Calling Behavior and Call Changes in Male Túngara Frogs

In the túngara frog, Physalaemus pustulosus, males alter calling behavior with changes in their social environment, adding 'chucks' to their advertisement calls in response to the calls of conspecific males. Other studies demonstrate that adding chucks increases the attractiveness of calls to females but also increases the risk of bat predation. In the current study, subcutaneous injections of the neuropeptide hormone arginine vasotocin (AVT) significantly increased chuck production in male túngara frogs. The effects of AVT on chuck production did not depend on the presence of playback stimuli, suggesting that AVT increased either the males' general motivation to produce chucks or their responsiveness to the calls of distant males. Injections of AVT also increased the probability that males would call and decreased the latency to call after injection, supporting the hypothesis that AVT influences motivation to call. Finally, AVT inhibited a drop in call rate after the termination of a playback stimulus and increased call rate at a lower dose of AVT. The effects of AVT on chucks and call rate appear to be independent of each other, as there was no correlation between change in chuck production and change in call rate in individual males. We conclude that AVT may play an important role in socially-mediated call changes that result from competition for mates. The behavioral changes induced by AVT might increase a male's attractiveness to females, and also may be consistent with an aggressive response to another túngara frog male.

Stress Hormone Is Implicated in Satellite‐Caller Associations and Sexual Selection in the Great Plains Toad

The effects of androgens on male-typical traits suggest that variation among males in circulating levels can play a major role in sexual selection. We examined whether variation in vocal attractiveness is attributable to differences in androgen levels among Great Plains toads (Bufo cognatus). We found that noncalling "satellite" males practicing an alternative mating tactic were more likely to associate with males producing long calls. However, callers with satellites did not have higher androgen levels than callers without satellites. Rather, callers with satellites had significantly lower corticosterone (CORT) levels than callers without satellites. A CORT manipulation experiment suggested that differences in calls for males with and without satellites were related to differences in CORT levels. Furthermore, there was a negative correlation between CORT level and call duration within most nights of chorus activity. However, the correlation was weak for the pooled data (across all nights), suggesting that local environmental and/or social factors also affect call duration. Last, we show that females preferred broadcast calls of longer duration, characteristic of males with satellites and low CORT. These results imply that satellites optimize their reproductive success by associating with males producing long calls. However, this association should negatively affect the fitness of attractive callers.

Vasotocin and mesotocin stimulate the biosynthesis of neurosteroids in the frog brain

The neurohypophysial nonapeptides vasopressin (VP) and oxytocin (OT) modulate a broad range of cognitive and social activities. Notably, in amphibians, vasotocin (VT), the ortholog of mammalian VP, plays a crucial role in the control of sexual behaviors. Because several neurosteroids also regulate reproduction-related behaviors, we investigated the possible effect of VT and the OT ortholog mesotocin (MT) in the control of neurosteroid production. Double immunohistochemical labeling of frog brain sections revealed the presence of VT/MT-positive fibers in close proximity of neurons expressing the steroidogenic enzymes 3beta-hydroxysteroid dehydrogenase/delta5-delta4 isomerase (3beta-HSD) and cytochrome P450 17alpha-hydroxylase/c17, 20-lyase (P450(C17)). High concentrations of VT and MT receptor mRNAs were observed in diencephalic nuclei containing the 3beta-HSD and P450(C17) neuronal populations. Exposure of frog hypothalamic explants to graded concentrations of VT or MT produced a dose-dependent increase in the formation of progesterone, 17-hydroxypregnenolone, 17-hydroxyprogesterone, and dehydroepiandrosterone. The stimulatory effect of VT and MT on neurosteroid biosynthesis was mimicked by VP and OT, as well as by a selective V1b receptor agonist, whereas V2 and OT receptor agonists had no effect. VT-induced neurosteroid production was completely suppressed by selective V1a receptor antagonists and was not affected by V2 and OT receptor antagonists. Concurrently, the effect of MT on neurosteroidogenesis was markedly attenuated by selective OT and V1a receptor antagonists but not by a V2 antagonist. The present study provides the first evidence for a regulatory effect of VT and MT on neurosteroid biosynthesis. These data suggest that neurosteroids may mediate some of the behavioral actions of VT and MT.

Elevated corticosterone levels elicit non-calling mating tactics in male toads independently of changes in circulating androgens

Competition among males for a limited number of females may result in the expression of condition-dependent alternative mating tactics. In such cases, decision rules mediating mating tactic expression are likely to be influenced by a male's external as well as internal environment. For example, experimental studies with anuran amphibians (frogs and toads) indicate that changes in the social-acoustic environment alter the probability that an individual adopts a calling versus non-calling "satellite" mating tactic. However, there is considerable variation in the behavioral responses of individuals in such studies, suggesting that physiological differences among individuals play an important role in tactic expression. For instance, recent models predict that natural elevation in adrenal glucocorticoids during vocal production alters androgen production and/or neural activity to mediate transitions between reproductive tactic expression in anuran amphibians. Using corticosteroid injections, we show that elevation in circulating corticosterone levels significantly increase the probability that free-ranging male Great Plains toads (Bufo cognatus) and Woodhouse's toads (B. woodhousii) adopt a non-calling satellite tactic. Corticosterone-induced behavioral transitions occurred rapidly (<1 h) and independently of fluctuations in circulating androgen levels, suggesting a direct effect of glucocorticoids on brain centers controlling vocal production. We discuss our findings in the context of behavioral studies that alter the social-acoustic environment to examine its influence on tactic expression.

Current research in amphibians: studies integrating endocrinology, behavior, and neurobiology

Amphibian behavioral endocrinology has focused on reproductive social behavior and communication in frogs and newts. Androgens and estrogens are critical for the expression of male and female behavior, respectively, and their effects are relatively clear. Corticosteroids have significant modulatory effects on the behavior of both sexes, as does the peptide neuromodulator arginine vasotocin in males, but their effects and interactions with gonadal steroids are often complex and difficult to understand. Recent work has shown that the gonadal hormones and social behavior are mutually reinforcing: engaging in social interactions increases hormone levels just as increasing hormone levels change behavior. The reciprocal interactions of hormones and behavior, as well as the complex interactions among gonadal steroids, adrenal steroids, and peptide hormones have implications for the maintenance and evolution of natural social behavior, and suggest that a deeper understanding of both endocrine mechanisms and social behavior would arise from field studies or other approaches that combine behavioral endocrinology with behavioral ecology.

Historical perspective: Hormonal regulation of behaviors in amphibians

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

Effects of arginine vasotocin (AVT) on the behavioral, cardiovascular, and corticosterone responses of starlings (Sturnus vulgaris) to crowding

Previous studies in European starlings have concluded that conspecific crowding can be a significant stressor that is capable of simultaneously altering behavior, heart rate, and corticosterone (CORT) concentrations. It was hypothesized that the peptide hormone arginine vasotocin (AVT) has a role in the regulation of these three types of responses to crowding. Four male and four female resident starlings were submitted to nine combinations of 3 crowding treatments (0, 1, or 5 intruder starlings) and 3 subcutaneous injections (1, 4 microg AVT, and saline control). Resident starlings were given a treatment injection, their heart rate and behavior were monitored for 30 min, 0, 1, or 5 intruder Starlings were allowed to enter the residents cage, and HR and behavior were monitored for another 30 min. Blood samples were taken before and after all treatments to assess CORT concentrations. Exogenous AVT decreased the frequency of maintenance behaviors (feeding, drinking, preening, and beak wiping), as well as activity in resident starlings. Although aggressive behaviors upright posture, head feather expansion, and pecking) increased during crowding, these increases were significantly attenuated by AVT. Heart rate was significantly lower during these behavioral effects, and the CORT data indicate that the cardiovascular and behavioral effects are not dependent on significant increases in CORT. These data support the hypothesis that AVT's attenuation of general behavior and crowding induced aggression are modulated by a cardiovascular mechanism.

Arginine vasotocin activates advertisement calling and movement in the territorial Puerto Rican frog, Eleutherodactylus coqui

Arginine vasotocin (AVT) is a neuropeptide that modulates social behavior in amphibians and activates calling in frogs. The Puerto Rican coqui frog, Eleutherodactylus coqui, is a terrestrial anuran that exhibits complex social behaviors, including territoriality and paternal care. Males have a distinctive social hierarchy and can be any of the following: satellite (non-calling), territorial (calling), or paternal (guards and broods embryos). Field experiments were conducted to determine the effects of AVT on satellite behavior and the male social hierarchy of E. coqui. Satellite males were captured within the territory held by a resident male, given injections (i.p.) of AVT or saline (control) and placed back in their original location. To determine if AVT affects all males, not merely satellite males, territorial (calling) males were injected with AVT. Significantly more satellite males commenced advertisement calling following AVT injections than did control males injected with saline. AVT-activated satellites did not challenge the resident territorial male for possession of the territory but instead moved into a new area before commencing to call. In fact, AVT-activated satellite males were significantly more likely to move into a new territory following AVT injections than AVT-injected territorial males. The effect of AVT was short lived, lasting only one night in all but two cases. It is concluded that AVT stimulates advertisement calling and AVT-activated males displayed territorial characteristics of E. coqui.

Evolutionary insights into the regulation of courtship behavior in male amphibians and reptiles

Comparative studies of species differences and similarities in the regulation of courtship behavior afford an understanding of evolutionary pressures and constraints shaping reproductive processes and the relative contributions of hormonal, genetic, and ecological factors. Here, we review species differences and similarities in the control of courtship and copulatory behaviors in male amphibians and reptiles, focusing on the role of sex steroid hormones, the neurohormone arginine vasotocin (AVT), and catecholamines. We discuss species differences in the sensory modalities used during courtship and in the neural correlates of these differences, as well as the value of particular model systems for neural evolution studies with regard to reproductive processes. For example, in some genera of amphibians (e.g., Ambystoma) and reptiles (e.g., Cnemidophorus), interspecific hybridizations occur, making it possible to compare the ancestral with the descendant species, and these systems provide a window into the process of behavioral and neural evolution as well as the effect of genome size. Though our understanding of the hormonal and neural correlates of mating behavior in a variety of amphibian and reptilian species has advanced substantially, more studies that manipulate hormone or neurotransmitter systems are required to assess the functions of these systems.

Gonadotropin-releasing hormones modulate electrical activity of vasotocin and isotocin neurons in the brain of rainbow trout

Gonadotropin-releasing hormone (GnRH) is widely distributed in the vertebrate brains; however, its significance in the brain function is poorly understood. Both GnRH and vasopressin-family hormones are involved in control of reproductive behavior. Anatomical evidence indicated the possible action of GnRH on classical neurosecretory neurons. In the present study, we examined whether GnRH modulates electrical activity of vasotocin (VT) and isotocin (IT) neurons in the brain of rainbow trout (Oncorhynchus mykiss). Two forms of GnRH, salmon GnRH and chicken GnRH II, are present in the rainbow trout brain, and their fibers are localized in the close vicinity of VT and IT neurons. Applications of both GnRH forms elevated the frequency of cell-type-specific synchronous Ca(2+) pulses in VT and IT neurons that are blocked by a GnRH-receptor antagonist. Our results showed facilitatory actions of GnRHs on VT and IT neurons, suggesting that GnRH neurons modulate classical neurosecretory neurons to control reproductive behavior.

Involvement of arginine vasotocin in reproductive events in the male newt Cynops pyrrhogaster

Effects of arginine vasotocin (AVT) on reproductive events such as courtship behavior, pheromone release, and spermatophore discharge were investigated in the male newt Cynops pyrrhogaster. AVT enhanced the incidence and frequency of androgen-induced courtship behavior. In this case, AVT was likely to act centrally because the behavior was evoked with a much smaller amount of AVT when the hormone was administered intracerebroventricularly than when given intraperitoneally. Involvement of endogenous AVT in spontaneously occurring courtship behavior was also evidenced by the fact that administration of a V1 (vasopressor) receptor antagonist, [d(CH2)5(1), Tyr(Me)2, Arg8-vasopressin] suppressed the expression of the courtship behavior. The water in which AVT-treated males had been kept showed considerable female-attracting activity as compared with the water in which saline-injected males had been kept. Moreover, the content of sodefrin, a female-attracting pheromone in the abdominal gland, was decreased by the intraperitoneal injection of AVT, suggesting that the neurohypophyseal hormone stimulated the release of sodefrin from the abdominal gland into the water. AVT induced contraction of the excised abdominal gland concentration-dependently, and, again, the V1 receptor antagonist suppressed the AVT-induced contraction. Thus, we concluded that AVT induces the pheromone discharge, acting peripherally on a contractile structure of the abdominal gland. AVT was also found to induce spermatophore deposition in the male kept in the absence of the female. Administration of the V1 receptor blocker to the sexually developed males suppressed the spermatophore deposition. All these results indicate the involvement of AVT in reproductive events acting centrally and peripherally.

Arginine vasotocin interacts with the social environment to regulate advertisement calling in the gray treefrog (Hyla versicolor)

Arginine vasotocin (AVT) has different effects on social behaviors across species with different social systems. However, less attention has been directed towards the way environmental stimuli interact with AVT to affect social behaviors. Previous work found that AVT affects the production of advertisement calls and the outcome of competitive interactions between male gray treefrogs (Hula versicolor). We conducted a field study to investigate how the presence of other males in a chorus influenced the production of advertisement calls in males treated with AVT or saline. After hormone treatment we placed individuals in call sites either within 50 cm of another calling male (proximate), or more than 2 m away from any calling male (distant). Males treated with AVT produced significantly longer calls with more pulses when placed at proximate call sites, but not distant call sites. Nonsignificant trends indicated that these increases in call length coincided with a decrease in call rate. There was no effect of call site distance on advertisement calls when males were treated with saline. These results indicate that the social environment can modulate the action of AVT on an important reproductive behavior, and that AVT does not influence call duration and pulse number unless another male is nearby. Understanding how social stimuli regulate the action of AVT on behavior could provide insight on the diversity of AVT function across species.

Neurochemical correlates of male polymorphism and alternative reproductive tactics in the Azorean rock-pool blenny, Parablennius parvicornis

In the common Azorean rock-pool blenny, Parablennius parvicornis, males exhibit alternative reproductive morphologies: (1) larger males defend nest sites, provide parental care, have anal glands (involved in pheromone release), testicular glands, and low gonad:body weight ratio (GSI) and (2) smaller, younger, males do not defend nests, have reduced glands and high GSI. These smaller non-nesting males behave as satellites (associated with nests) or sneakers (moving among nests), attempting to achieve parasitic fertilizations via sperm competition. In non-mammals, arginine vasotocin (AVT) is a key hypothalamic peptide involved in the control of reproductive behavior and physiology, and several fish species that exhibit alternative male reproductive morphs show polymorphism in AVT brain chemistry. We conducted an immunocytochemical study to generate comparative data on this intertidal blenny. Our analysis showed no difference in AVT-immunoreactive cell number or size between the male morphs, which is consistent with studies on other fish, including blennies. The number of AVT cells was positively correlated to fish body mass, while cell size showed no such relation. If corrected for body mass, the smaller non-nesting males have significantly more cells than the large nesting males. Our data suggest that the size and number of forebrain AVT cells develops initially to allow for reproduction in the young non-nesting males and this pattern does not appear to change when males take on the nesting morphotype later in life. This result appears to be consistent in many fishes with alternative male morphotypes.

Behavioral neuroendocrinology of vasotocin and vasopressin and the sensorimotor processing hypothesis

Vasotocin (AVT) and vasopressin (AVP) are potent modulators of social behaviors in diverse species of vertebrates. This review addresses questions about how and where AVT and AVP act to modulate social behaviors, focusing on research with an amphibian model (Taricha granulosa). In general, the behaviorally important AVT and AVP neurons occur in the forebrain and project to sites throughout the brain. Social behaviors are modulated by AVT and AVP acting at multiple sites in the brain and at multiple levels in the behavioral sequence. This review proposes that AVT and AVP can act on sensory pathways to modulate the responsiveness of neurons to behaviorally relevant sensory stimuli and also can act on motor pathways in the brainstem and spinal cord to modulate the neuronal output to behavior-specific pattern generators. This neurobehavioral model, in which AVT and AVP are thought to modulate social behaviors by affecting sensorimotor processing, warrants further research.

Arginine vasotocin effects on courtship behavior in male white perch (Morone americana)

Arginine vasotocin (AVT) and its mammalian homologue, arginine vasopressin (AVP), have been shown to have widespread behavioral effects in vertebrates. AVT was evaluated for its effectiveness in stimulating an important courtship behavior termed 'attending' in male white perch, Morone americana. Attending consists of close and continuous following of the female with occasional contact in the abdominal area. We tested the behavioral effectiveness of AVT in stimulating attending when administered either intraperitoneally (IP) or intracerebroventricularly (ICV). We also tested IP injections of AVT alone and in combination with an AVP V(1) receptor antagonist (Manning compound). None of the IP injections of either AVT or Manning compound produced consistent effects on attending behavior. In contrast, ICV injections of AVT did significantly increase attending behavior and at low dosages. Circulating levels of testosterone and 11-ketotestosterone were not affected approximately 80 min following injection by any of the treatments. The strong behavioral effects observed with ICV administration support a central site of action for AVT in stimulating attending behavior. This is a complex behavior that shows similarities to behaviors mediated by AVT and AVP in other vertebrates, providing further evidence of a conserved behavioral role for these peptides.

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.

Manipulations of the AVT system shift social status and related courtship and aggressive behavior in the bluehead wrasse

Arginine vasotocin (AVT) and its mammalian homologoue arginine vasopressin (AVP) influence male sexual and aggressive behaviors in many species. We tested the effects of AVT and an AVP-V(1a) receptor antagonist on the display of alternative male tactics in a tropical coral reef fish, the bluehead wrasse Thalassoma bifasciatum. We gave AVT injections to territorial and nonterritorial males of the large and colorful phenotype (terminal phase) and an AVP-V(1a) receptor antagonist, Manning compound, to territorial males in the field. AVT increased courtship independent of status, while its effects on territoriality and aggression were dependent upon male status. In territorial males, AVT increased courtship and tended to decrease the number of chases toward initial phase individuals. In nonterritorial males, AVT increased courtship, chases toward initial phase individuals, and territorial behavior while decreasing feeding. These are all behaviors rarely seen in nonterritorial males, so AVT made these males act like territorial TP males. The AVP-V(1a) receptor antagonist had opposite effects. It decreased courtship and territorial defense, making these males act more like nonterritorial males. Manipulations of the AVT system shifted males within a single phenotype from the nonterritorial social status to the territorial social status and vice versa. Since the entire suite of behaviors related to territoriality was affected by AVT system manipulations, our results suggest that the AVT system may play a key role in motivation of behaviors related to mating.

Social behavior functions and related anatomical characteristics of vasotocin/vasopressin systems in vertebrates

The neuropeptide arginine vasotocin (AVT; non-mammals) and its mammalian homologue, arginine vasopressin (AVP) influence a variety of sex-typical and species-specific behaviors, and provide an integrational neural substrate for the dynamic modulation of those behaviors by endocrine and sensory stimuli. Although AVT/AVP behavioral functions and related anatomical features are increasingly well-known for individual species, ubiquitous species-specificity presents ever increasing challenges for identifying consistent structure-function patterns that are broadly meaningful. Towards this end, we provide a comprehensive review of the available literature on social behavior functions of AVT/AVP and related anatomical characteristics, inclusive of seasonal plasticity, sexual dimorphism, and steroid sensitivity. Based on this foundation, we then advance three major questions which are fundamental to a broad conceptualization of AVT/AVP social behavior functions: (1) Are there sufficient data to suggest that certain peptide functions or anatomical characteristics (neuron, fiber, and receptor distributions) are conserved across the vertebrate classes? (2) Are independently-evolved but similar behavior patterns (e.g. similar social structures) supported by convergent modifications of neuropeptide mechanisms, and if so, what mechanisms? (3) How does AVT/AVP influence behavior - by modulation of sensorimotor processes, motivational processes, or both? Hypotheses based upon these questions, rather than those based on individual organisms, should generate comparative data that will foster cross-class comparisons which are at present underrepresented in the available literature.

Behavioral and hormonal effects of exogenous vasotocin and corticosterone in the green treefrog

Vasotocin (AVT) promotes courtship in a wide range of vertebrates. However, this effect is not independent of steroid hormones. For example, androgens may work in concert with AVT and corticosterone (CORT) may work to oppose AVT action. In frogs, AVT promotes calling, and in some species, CORT inhibits calling. In addition, androgens are known to modulate AVT in the brain, and CORT may depress androgen secretion. Previous work in amphibians has suggested that AVT promotes courtship by overcoming a CORT-mediated stress response. Possible behavioral and hormonal interactions among AVT, CORT, and androgens were investigated in wild, free-living green treefrogs (Hyla cinerea). Saline, AVT, CORT, or a combination of AVT and CORT were administered to calling males, and several measures of spontaneous calling were evaluated for 1.5 h following injection. Plasma testosterone, dihydrotestosterone, and CORT were also measured. Saline-injected males had low CORT levels, and AVT and CORT injection elevated plasma CORT levels. AVT increased the likelihood of calling, but, in males who did call, AVT did not influence latency to call or how often they were observed calling. Very few saline-injected males resumed calling after injection, and therefore a CORT effect was only detectable in AVT-injected males. CORT inhibited calling in AVT-injected males only at the highest dose of CORT (40 microg); lower levels of CORT were unsuccessful at inhibiting AVT-induced calling. AVT appeared to have a specific effect on calling motivation. Further, the data suggest that disinhibition of a CORT response is not the primary mechanism by which AVT increases calling. In addition, CORT injection reduced endogenous androgen levels. Finally, endogenous androgens were negatively correlated with latency to begin calling, suggesting that they may have a positive effect on calling. These data indicate that AVT has positive effects on calling but provide only weak evidence that CORT inhibits courtship in this species.

Vasotocin stimulates appetitive responses to the visual and pheromonal stimuli used by male roughskin newts during courtship

It is now well established that vasotocin (AVT) and its mammalian homologue vasopressin influence various social behaviors in vertebrates, but less is known about the mechanisms through which these peptides modulate behavior. In male roughskin newts, Taricha granulosa, AVT stimulates a courtship behavior, amplectic clasping. Three general explanations for how AVT affects male courtship behavior have been considered: by enhancing a central state of sexual motivation, by affecting sensorimotor integration mechanisms in individual sensory modalities, or by influencing a nonspecific state of attention, arousal, or anxiety. AVT administration enhanced appetitive responses to visual and olfactory sexual stimuli, as would be expected if AVT affects a state of sexual motivation that affects behavioral responses to sexual stimuli regardless of the sensory modality in which they are processed. However, AVT selectively enhanced responses to female olfactory stimuli (sex pheromones), but similarly enhanced responses to female and food-related visual stimuli (worms), thus questioning the utility of such a motivational mechanism, as responses to female stimuli were not selectively enhanced in all sensory modalities. We therefore propose that exogenous AVT independently influences olfactory processes associated with orientation/attraction toward a female sex pheromone and visual processes associated with orientation/attraction toward a visual feature common to females and worms. In further experiments AVT administration failed to stimulate feeding behavior but did decrease locomotor activity. Thus, AVT does not stimulate courtship behavior in this species by enhancing the animals' general state of attention or by decreasing general anxiety, as responses to nonsexual, attractive stimuli were not uniformly enhanced, nor by stimulating general arousal, as activity levels did not increase. Rather, the data support the conclusion that AVT affects courtship by influencing specific sensorimotor processes associated with behavioral responses to individual releasing stimuli, which suggests a mechanistic framework for understanding socially motivated behavior is this species.

Mating vocalizations of female frogs: control and evolutionary mechanisms

Vocalization behaviors of anuran amphibians are universally sexually dimorphic. Usually, only male frogs give an advertisement call, while female frog calls are limited to a soft and simple release call which is specifically suppressed at mating. In a very few species, however, female frogs also give mating vocalizations. We examined possible mechanisms for control of this rare heterotypical behavior. At the peripheral level, most differences in temporal and spectral characteristics between female mating calls and the calls of conspecific males related directly to sexual dimorphisms in laryngeal and oblique muscle morphology. At the neural and hormonal level, we first developed an integrated model for control of vocalizations, based primarily on male frog data. When this model is applied to females, female mating vocalizations were most similar to male advertisement calls, rather than being modified release calls. Females may have conscripted preexisting androgen-sensitive neural pathways typically used only by males but present in both sexes. Female mating calls have been heard only during courtship and amplexus. Androgen levels in females at this time are significantly higher than even those levels in males. Because this situation is common in frogs, female mating vocalizations likely evolved independently multiple times. Character optimization suggests that mate location is the most common biological role for female mate calling, but the particular aspects of reproductive biology vary widely across species.

Vasotocin innervation and modulation of vocal-acoustic circuitry in the teleost Porichthys notatus

Arginine vasotocin (AVT) and its mammalian homologue arginine vasopressin (AVP) modulate reproduction-related and other social behaviors in a broad range of vertebrate species. These functions of AVT/AVP may be in part achieved through the modulation of sensorimotor integration, although experimental evidence supporting this hypothesis remains limited. In the present experiments, we demonstrate (1) AVT innervation of candidate vocal-acoustic brain regions, and (2) AVT modulation of vocal-motor physiology in the plainfin midshipman fish (Porichthys notatus), which uses vocalizations in both mate attraction and agonistic contexts. AVT distribution was compared with known vocally active brain regions and to central auditory and vocal pathways. AVT-immunoreactive fibers and putative terminals descend almost exclusively from the preoptic area and are found in two primary candidate sites for vocal-acoustic integration - the anterior tuberal hypothalamus and paralemniscal midbrain tegmentum. AVT immunoreactivity is also located in several other vocally active regions, including the ventral tuberal nucleus, periaqueductal gray, and paraventricular regions of the isthmus and rostral hindbrain. The parvocellular preoptic area itself is also vocally active, although thresholds are substantially higher than for other regions. The functional significance of AVT input to vocal-acoustic regions was demonstrated in the paralemniscal midbrain where local delivery of AVT modulated electrically evoked, rhythmic vocal-motor output, which precisely mimicked natural vocalizations. AVT produced dose-dependent inhibitions of parameters associated with call initiation (burst latency and number of vocal-motor bursts elicited) but not of vocal-motor patterning (fundamental frequency and burst duration). Together, these findings provide support for the proposal that AVT modulates sensorimotor processes underlying social/acoustic communication.

Effects of gonadotropin-releasing hormones, corticotropin-releasing hormone, and vasopressin on female sexual behavior

The effects of intracerebroventricular (icv) infusion of four neuropeptides on female sexual behavior were examined in the female musk shrew (Suncus murinus). In the first experiment, (icv) infusion of 100 ng of the mammalian form of gonadotropin-releasing hormone (mGnRH) facilitated rapid display of receptivity. Gonadotropin-releasing hormone-infused females had shorter latencies to rump present and tail wag, compared with controls. In a second experiment, icv administration of the other form of GnRH present in musk shrew brain, the chicken GnRH-II form, produced no changes in female behavior relative to the control condition. In Experiment 3, icv delivery of corticotropin-releasing hormone (CRH) facilitated female sexual behavior, relative to vasopressin and controls. The females treated with CRH had shorter latencies to display rump present, tail wag, and for the receipt of the first missed intromission compared with females in the other treatment groups. Vasopressin increased female scent marking relative to that of CRH-treated females. These data indicate that neurohormones of the hypothalamic-pituitary-gonadal and the hypothalamic-pituitary-adrenal axes can facilitate reproductive behavior in S. murinus.

The neuropeptide arginine vasotocin alters male call characteristics involved in social interactions in the grey treefrog, Hyla versicolor

We investigated the effects of different doses (0, 2.5, 25 and 250 µg) of the neuropeptide arginine vasotocin (AVT) on the calling characteristics of the grey treefrog in a chorus in its natural habitat. AVT changed some call characteristics known to influence social behaviour in grey treefrogs. It increased call duration and number of pulses in a call, but not dominant frequency, call rate or pulse effort. Saline injections and handling did not produce significant changes in any of the call characteristics. In addition, individual animals injected with AVT only rarely produced call characteristics that were outside of the range found in the preinjection measurements, suggesting that AVT does not cause abnormal calling behaviour. Other researchers have demonstrated that longer calls with more pulses are produced by males when chorus densities increase, and females display a strong preference for longer calls with more pulses. This suggests that the changes induced by AVT injections may have functional consequences in social interactions. We previously demonstrated that AVT-injected males (25 µg AVT) displaced resident males from calling sites through changes in calling behaviour under natural field conditions. Our results indicate that changes in call duration and pulse number could contribute to the unmanipulated resident male's behaviour towards the AVT-injected intruder, perhaps because the calls are more attractive to females or because the calls are perceived as more aggressive. Copyright 2000 The Association for the Study of Animal Behaviour.

Sexual dimorphism in numbers of vasotocin-immunoreactive neurons in brain areas associated with reproductive behaviors in the roughskin newt

Vasotocin (VT) and vasopressin control many endocrine and neuroendocrine functions, including the regulation of reproductive behaviors. In the roughskin newt (Taricha granulosa), VT administration can enhance courtship behaviors in males and egg-laying behaviors in females. This study used immunohistochemistry to investigate whether there are sex differences in VT in specific brain areas, and whether these differences persist in nonbreeding animals. Numbers of VT immunoreactive (ir) cell bodies were counted in males and females collected in February, April, June, and August. Radioimmunoassay of plasma samples confirmed that testosterone and 5alpha-dihydrotestosterone concentrations were higher in males than females, and that 17beta-estradiol concentrations were higher in females than males. In 11 brain areas, no sexual or seasonal differences in the number of VTir cells were found. But in 3 brain regions-the bed nucleus of the stria terminalis (BNST), the nucleus amygdalae dorsolateralis (AMYG), and the anterior preoptic area (aPOA)-there were significantly greater numbers of VTir cells in males than in females, and these differences did not change seasonally. In the aPOA, an area important to male sex behaviors, the sexual dimorphism in VTir was particularly pronounced. In four brain regions, there were significantly greater numbers of VTir cells in females than males, but only in specific seasons. In April-collected (breeding) animals, more VTir cells were found in females than in males in the populations of VT cells within the pars dorsalis hypothalami and ventromedial hypothalamus, brain regions frequently associated with stress responses and female mating behaviors. In August-collected (nonbreeding) animals, more VTir cells were found in females than in males, in the region of the bed nucleus of the decussation of the fasciculus lateralis telencephali and in the nucleus visceralis superior, nucleus isthmi region. Significantly greater numbers of VTir cells were observed in the magnocellular preoptic area of males and females collected in February. These results indicate that the functional interactions between gonadal steroid hormones and VT are complex and appear to involve site-, sex-, and season-specific regulatory mechanisms. Furthermore, it seems likely that populations of VT neurons in the BNST, AMYG, and aPOA are involved in regulating male-specific behaviors, and that the VT neurons in the pars dorsalis hypothalami/ventromedial hypothalamus may be involved in female-specific behaviors.

Actions of two forms of gonadotropin releasing hormone and a GnRH antagonist on spawning behavior of the goldfish Carassius auratus

The central effects of two native forms of gonadotropin-releasing hormone (GnRH), salmon GnRH (sGnRH) and chicken GnRH-II (cGnRH-II), and a GnRH antagonist, ¿Ac-delta 3-Pro(1), 4FD-Phe(2), D-Trp(3, 6)mGnRH (analog E), on the spawning behavior of sexually recrudescent female goldfish were investigated. The effects of analog E were also observed in mature males. Female spawning behavior was induced by intramuscular injection of females with prostaglandin F(2alpha) and placing them in the presence of mature males. Behavioral responses were quantified by recording the numbers of spawning acts performed by each pair of fish for 2 h following brain intracerebroventricular (icv) injection of different dosages of peptide or saline as control. For males, the time spent courting the female was recorded. Each pair of fish was pretested to determine their level of spawning behavior, for comparison to spawning behavior following icv treatment. Icv injection of analog E caused a significant decrease in the number of spawning acts performed by females, suggesting a role of endogenous GnRH in modulating female spawning behavior. icv injection of 0.5 ng/g of sGnRH or cGnRH-II significantly stimulated female spawning behavior, whereas doses of 1 ng/g and higher resulted in an almost complete inhibition of spawning, reflecting a down-regulation as a result of the excessive dosages. Analog E suppressed the actions of exogenous sGnRH and cGnRH-II on spawning behavior, as both the sGnRH- and cGnRH-II-induced increases in the number of spawning acts were inhibited by concomitant treatment with analog E. Analog E-injected males showed no alteration in courtship behavior. These results indicate that GnRH peptides play a major role in the control of female reproductive behavior in goldfish, but have little or no role in the control of male behavior.

Vasotocin maintains multiple call types in the gray treefrog, Hyla versicolor

The neuropeptide arginine vasotocin (AVT) influences vocalizations in some anuran amphibians but it is unknown whether AVT alters all vocal behaviors of a species similarly. We first characterized the vocal repertoire of male gray treefrogs (Hyla versicolor). Three different call types were distinguished by unique sets of temporal and spectral features. Second, we examined the effects of AVT on each call type by injecting frogs with either AVT (100 microg; intraperitoneal) or saline and recording subsequent behavior. In the field, AVT maintained advertisement calling, whereas calling ceased in saline-injected animals. Advertisement call rate in AVT-injected males fell significantly and dominant frequency of the call was significantly higher. In the laboratory, AVT induced advertisement calling in males that were not initially vocalizing and dominant frequency was also significantly higher in these males. AVT maintained aggressive calling similarly but the characteristics of aggressive calls were not altered by AVT. There were no significant differences in release call behavior between AVT- and saline-injected groups; however, release call duration decreased significantly in AVT-injected animals, compared with preinjection values for the same animals. The effects of AVT on vocal behavior in this species are therefore not the same for each call type. AVT may act at more general motivational levels in the central nervous system and other neural or endocrine factors may control choice of call type and direct motor output.

Forebrain arginine vasotocin correlates of alternative mating strategies in cricket frogs

In cricket frogs, Acris crepitans, sexually active males can switch between calling and noncalling (satellite) mating strategies and injections of the neuropeptide arginine vasotocin (AVT) stimulate calling behavior. We report here that this behavioral variation of animals under field conditions is associated with variations in AVT-immunoreactive (AVT-ir) staining in distinct brain nuclei. In both calling and satellite males, one AVT-ir brain region was found in a continuous string of cells between the medial amygdala and the nucleus accumbens (ACC). Satellite males possessed significantly more AVT-ir staining in the brain (cells and fibers) than calling males at the level of the ACC, although not in the medial amygdala. This difference in AVT-ir staining in the ACC can, in part, be explained by differences in the density of staining within the cells and in cell size. In addition, satellite males had significantly higher AVT-ir staining in the fibers medial to the ACC than calling males. Because other studies have demonstrated that AVT stimulates calling behavior, a plausible hypothesis is that calling males are releasing more AVT from neurons in the ACC, depleting reserves within the cells, and that the released AVT elicits calling behavior. AVT immunoreactivity levels are also higher in the ACC in both calling and satellite males than in female cricket frogs, which do not call. Satellite males may therefore have AVT reserves that might allow them to call depending on the social conditions.

Effect of intraseptal vasotocin and vasoactive intestinal polypeptide infusions on courtship song and aggression in the male zebra finch (Taeniopygia guttata)

The present experiments were conducted to test the hypothesis that septal arginine vasotocin (AVT) and vasoactive intestinal polypeptide (VIP) modulate directed song (a courtship behaviour) and aggression in male zebra finches (Taeniopygia guttata). Subjects were surgically fitted with a guide cannula directed at the septum. Following recovery they were tested for aggression and directed song following infusions of AVT, its antagonist (anti-vasopressin, AVP), and saline volume control. Infusion of the AVT antagonist significantly reduced all three aggressive behaviours measured (pecks, beak fences and chases); and AVT infusion significantly facilitated beak fencing. Vasoactive intestinal polypeptide treatment significantly reduced pecking. No treatment produced a change in directed song. Comparison with findings in mammals suggests that modulation of aggression by septal AVT (or AVP) is evolutionarily conserved in vertebrates, but modulation of aggression by VIP has not previously been reported for any vertebrate.