Changes in serum sex steroid levels throughout the reproductive cycle of Bufo arenarum females

Using the Ratio of Urine Testosterone to Estrone-3-Glucuronide to Identify the Sex of Chinese Giant Salamanders (Andrias davidianus)

Minimally invasive sampling was used to determine the sex of Chinese giant salamanders (Andrias davidianus). Urine samples (n = 25) were collected from 6 adults in the breeding season and from 19 individuals (7 adults and 12 juveniles) in the non-breeding season. The hormone testosterone (T) and estrone-3-glucuronide (E1G) in urine were collected from Chinese giant salamanders (CGSs), and the hormone extracts were analyzed by enzyme immunoassays (EIA). The data demonstrated that the urine T concentration of the male CGSs was significantly higher than that of the females during the breeding season (p < 0.05) and even more pronounced during the non-breeding season (p < 0.01). The urine E1G concentration of the males was less pronounced than that of the females during the breeding season (p < 0.01) and significantly lower during the non-breeding season (p < 0.05). The urine T/E1G values of all the male salamanders were significantly higher than those of the females (p < 0.01) during both the breeding season and the non-breeding season. An interesting pattern was found in this study: the value of urine log10(T/E1G) of the male CGSs was higher than 1, whereas the value for the females was lower than 1, during both the breeding and non-breeding seasons, and in the adult and sub-adult age groups of CGSs. There were 25 salamanders in this study and the accuracy rate reached 100% by using a log10(T/E1G) value of 1. The results of the log10(T/E1G) value provide new insight into the future development of the sex identification of CGSs and also lay the foundation for accurate sex identification in the preparation for artificial release. This is the first study to show that the T/E1G ratio in urinary hormones is reliable for the sex identification of CGSs. Additionally, urinary hormone T/E1G measures are promising sex identification tools for amphibian or monomorphic species and for those whose secondary sex characteristics are visible only during the breeding season.

Effect of nervous stimulation on ovarian steroid secretion in amphibians

In the present study, the effect of nerve stimulation on the secretory activity of the ovary of adult females was analyzed for the first time in amphibians. Results revealed that in Rhinella arenarum the stimulation of nerves that supply the gonad induced an increase in estradiol and progesterone secretion, this response showing differences during the reproductive cycle of the species. During the postreproductive period, an increase in estradiol secretion was observed while, in the reproductive period, progesterone secretion increased. Our results suggest that the sympathetic division of the autonomic nervous system would be responsible for this increase, taking into account that, under our experimental conditions, acetylcholine did not affect the endocrine activity of the gonad, while adrenaline (epinephrine) was effective in inducing steroid secretion an effect that could be due to interaction with β receptors. On the other hand, our data show that the association of adrenaline with follicle-stimulating hormone increased estradiol secretion during the postreproductive period, while the association of catecholamine with LH or hCG increased progesterone secretion during the reproductive period. Our results would suggest that nerve stimulation, mediated by the release of adrenaline, would act synergistically with gonadotrophins to stimulate steroid secretion.

Dynamics of steroid-induced oocyte maturation in three amphibian species: Mathematical modeling and simulation

Oocyte maturation in vertebrates involves morphological, biochemical, and functional changes through which meiosis, previously arrested in Profase I, is resumed. Within these changes, germinal vesicle breakdown (GVBD) constitutes morphological evidence of the resumption of the cellular cycle. Sex steroids (progestins and androgens) play a key role in oocyte maturation and are considered inducers of this process. The aims of this study were to analyze the effect of sex steroids on the oocyte maturation of amphibians and to model and simulate the dynamics of this process through the experimental data obtained. The oocytes of sexually mature females of Scinax fuscovarius, Pleurodema borellii, and Physalaemus biligonigerus were treated with different concentrations (0.001-1 µM) of sex steroids progesterone (P ₄ ) and testosterone (T) for 24 hr. Dose- and time-response curves were performed with the results achieved. Sex steroids induced oocyte maturation in a dose-dependent manner in all three species. The dose at which the highest maturation percentage was found was 1 μM. No significant differences were observed between GVBD percentages of P ₄ and T. However, time-response curves show that oocytes responded earlier and achieved higher GVBD percentages when treated with P ₄ . Gompertz mathematical model was proposed in this study to simulate GVBD dynamics. This model properly fits the corresponding experimental data and allows the analysis of the behavior of oocyte maturation in amphibians and the quantification of parameters with biological meaning that are indicative of sex steroids efficiency in this process.

Oocyte maturation in the toad Rhinella arenarum (Amphibia, Anura): Evidence of cAMP involvement in steroid production and action

In this work, we describe the participation of the adenylate cyclase/3'-5'-cyclic adenonsine monophosphate (cAMP) pathway in the seasonal follicular secretion of progesterone (P₄ ) and testosterone (T), and its relationship with the maturation of Rhinella arenarum oocytes. Under gonadotropin stimulation, P₄ secretion was the dominant steroid produced during the reproductive period, resulting in 100% germinal vesicle breakdown (GVBD) in oocytes in vitro; in contrast, T and estradiol (E₂ ) secretion increased (∼16 nM/20 follicles and ∼80 pM/20 follicles, respectively) during the non-reproductive period, but only yielded 50% GVBD. Treatment of the follicles with dibutyryl-cAMP or forskolin induced a significant increase in T secretion during both periods, but P₄ secretion did not significantly change and GVBD did not occur. These results suggest that high cAMP levels in the oocyte maintain meiotic arrest and prevent the induction effect of follicular steroids. An increase in cAMP levels in denuded oocytes, however, negatively regulated T-induced maturation since treatment with increasing db-cAMP or forskolin inhibited their maturation. Therefore, we hypothesize that an elevation in T during the non-reproductive period favors its aromatization to E₂ , leading to follicle growth. During the reproductive period, P₄ production might promote oocyte maturation when environmental conditions are favorable for reproduction. Together, the results indicate that steroidogenesis is seasonal and depends on gonadotropic activity in R. arenarum.

Effects of nerve stimulation on amphibian oviductal activity

The present study describes, for the first time in an anuran amphibian, the nerve stimulation effects on the secretory and motor activity of the oviduct of adult females. The results reveal that in Rhinella arenarum oviducts, the epithelial and glandular secretory cells of the mucosa of the pars convoluta respond to nerve stimulation secreting the products synthetized and stored in their cytoplasm. The ultrastructural analysis showed that the cell content released is made up of granular, fibrillar and floccular material, exocytosis being the main secretory mechanism found in epithelial secretory cells, although apocrine and holocrine processes could also be observed. In contrast, in glandular cells only exocytosis processes were found. With respect to the participation of the nervous system in the motility of the duct, observations under our experimental conditions indicated that oviductal nerve stimulation promotes motor activity as manifested by a succession of coordinated contractions and relaxations that generate movements similar to peristaltic waves. These results were observed in oviducts from animals captured during the reproductive and post reproductive periods. However, it is important to note that both the secretory response and duct motility are markedly decreased during the post reproductive period of the species.

Ovarian control and monitoring in amphibians

Amphibian evolution spans over 350 million years, consequently this taxonomic group displays a wide, complex array of physiological adaptations and their diverse modes of reproduction are a prime example. Reproduction can be affected by taxonomy, geographic and altitudinal distribution, and environmental factors. With some exceptions, amphibians can be categorized into discontinuous (strictly seasonal) and continuous breeders. Temperature and its close association with other proximate and genetic factors control reproduction via a tight relationship with circadian rhythms which drive genetic and hormonal responses to the environment. In recent times, the relationship of proximate factors and reproduction has directly or indirectly lead to the decline of this taxonomic group. Conservationists are tackling the rapid loss of species through a wide range of approaches including captive rescue. However, there is still much to be learned about the mechanisms of reproductive control and its requirements in order to fabricate species-appropriate captive environments that address a variety of reproductive strategies. As with other taxonomic groups, assisted reproductive technologies and other reproductive monitoring tools such as ultrasound, hormone analysis and body condition indices can assist conservationists in optimizing captive husbandry and breeding. In this review we discuss some of the mechanisms of ovarian control and the different tools being used to monitor female reproduction.

Patterns of Apoptosis and Proliferation throughout the Biennial Reproductive Cycle of Viviparous Female Typhlonectes compressicauda (Amphibia, Gymnophiona)

Typhlonectes compressicauda is an aquatic gymnophionan amphibian living in South America. Its breeding cycle is linked to seasons, characterized by a regular alternation of rainy and dry seasons. During a complex biennial cycle, the female genital tract undergoes a series of alternations of increasing and decreasing, governed by equilibrium of proliferation and apoptotic phenomena. Immunohistochemical methods were used to visualize cell proliferation with the detection of Ki67 antibody, a protein present in proliferative cells; terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) and Apostain were performed to detect apoptotic cells on sections of ovaries and oviducts. In ovaries, both phenomena affect the germinal nests and follicles according to the cycle period. In the oviduct, the balance was in favor of proliferation during preparation for reproduction, and in favor of apoptosis when genital ducts regress. Apoptosis and proliferation are narrowly implicated in the remodeling of the genital tract and they are accompanied by the differentiation of tissues according to the phase of the breeding cycle. These variations permit the capture of oocytes at ovulation, always at the same period, and the parturition after 6-7 months of gestation, at a period in which the newborns live with their mother, protected in burrows in the mud. During the intervening year of sexual inactivity, the female reconstitutes body reserves.

Biological sex identification in the endangered dusky gopher frog (Lithobates sevosa): a comparison of body size measurements, secondary sex characteristics, ultrasound imaging, and urinary hormone analysis methods

BACKGROUND

Accurate sex identification techniques are important for wildlife demographic studies and for genetic management of captive breeding colonies. Various non-invasive methods for identification of biological sex in the weakly dimorphic endangered dusky gopher frog (DGF; Lithobates sevosa) were explored to support planned recovery efforts for this species including breeding and augmentation of wild populations.

METHODS

Body size (snout-vent length and body weight) measurements, observation of nuptial pads, ultrasound imaging, and urinary hormone analysis for testosterone and estrone were performed on 27 male and 19 female DGFs. For each method, the mean and range of measurement values were determined for male and female DGFs housed in a captive breeding population. The ability of these methods to accurately predict the true biological sex of the individuals was assessed retrospectively.

RESULTS

Body size measurements were of limited use for sex identification purposes, as males and females demonstrated overlapping body lengths and weights. Observation of the presence/absence of nuptial pads in males and females, respectively, proved to be accurate and easy to perform in most cases. Ultrasound imaging was useful for predicting the sex of female frogs, particularly when females were gravid. Commercial enzyme immunoassay kits were validated to measure urinary hormones in the DGF. Mean urinary testosterone (males: 2.22 ± 0.38 ng/ml; females: 0.92 ± 0.11 ng/ml) and estrone (males: 0.08 ± 0.01 ng/ml; females: 1.50 ± 0.39 ng/ml) concentrations were significantly (p < 0.05) different between the sexes. However, there was some overlap in hormone concentrations between the sexes. When a ratio of testosterone (T) to estrone (E) concentrations was calculated for each individual, males demonstrated significantly greater T/E ratios compared to females (p < 0.05). Use of this ratio showed greater accuracy in predicting the sex of the animal compared to using testosterone or estrone concentrations alone.

CONCLUSIONS

Monitoring for presence/absence of nuptial pads and using urinary testosterone to estrone hormone ratios were the most accurate methods for identifying the biological sex of adult DGFs. Urinary hormone measurements for sex identification may be useful in other weakly dimorphic and monomorphic amphibian species in both ex situ and in situ settings.

Progesterone and prostaglandin F2α induce species-typical female preferences for male sexual displays in Cope's gray treefrog (Hyla chrysoscelis)

Endocrine systems play critical roles in facilitating sexual behavior in seasonally breeding vertebrates. Much of the research exploring this topic has focused on the endocrine correlates of signaling behavior in males and sexual proceptivity in females. What is less understood is how hormones promote the expression of the often complex and highly selective set of stimulus-response behaviors that are observed in naturally breeding animals. In female frogs, phonotaxis is a robust and sensitive bioassay of mate choice and is exhibited by gravid females during the breeding season. In stark contrast, females exhibit low phonotactic responsiveness outside the breeding season, but the administration of hormones can induce sexual proceptivity. Here we test the hypothesis that manipulation of a minimal set of reproductive hormones-progesterone and prostaglandin F2α-are capable of evoking not only proceptive behavior in non-breeding females, but also the patterns of intraspecific selectivity for male sexual displays observed in gravid females tested during the breeding season. Specifically, we investigated whether preferences for faster call rates, longer call durations, and higher call efforts were similar between breeding and hormone-treated females of Cope's gray treefrog (Hyla chrysoscelis). Hormone injections induced patterns of selective phonotaxis in non-breeding females that were remarkably similar to those observed in breeding females. These results suggest that there may be an important contribution of hormonal pleiotropy in regulating this complex, acoustically-guided sexual behavior. Our findings also support the idea that hormonal induction could be used to evaluate hypotheses about selective mate choice, and its underlying mechanisms, using non-breeding females.

In vitro steroid-induced meiosis in Rhinella arenarum oocytes: role of pre-MPF activation

In this work we showed the relationship between seasonal periods and the response of R. arenarum follicles and oocytes to different steroids. Using in vitro germinal vesicle breakdown (GVBD) assays, we demonstrated that P4 is the main steroid capable of inducing maturation in R. arenarum oocytes and follicles. In the second part of this work we showed that androgens can activate pre-maturation promoting factors (pre-MPFs) such as P4, by cytoplasm microinjection experiments. The results indicated that the steroids assayed induced oocyte and follicle maturation in a dose- and time-dependent manner. In oocytes, P4 was the most efficient steroid as a maturation inducer (EC50 of the reproductive period, 6 nM, EC50 of the non-reproductive period ≅ 30 nM). Androgens (DHEA, dehydroepiandrosterone; T, testosterone; and AD, androstenedione) were less efficient maturation inducers than P4 (EC50 reproductive period ≅ 50, 120 and 600 nM respectively). Similar results were obtained with intact follicles in both seasonal periods. Although the response of follicles to the different androgens was variable, in no case was it above the above the response induced by P4. Independently of the season, oocytes and follicles incubated in P4, P5 and T underwent GVBD after 6–10 h while oocytes and follicles incubated in DHEA and AD matured more slowly. Furthermore, we demonstrated that microinjection of mature cytoplasm from androgen-treated oocytes is sufficient to promote GVBD in immature recipient oocytes (DHEA, 57 ± 12%; AD, 60 ± 8%; T, 56 ± 13%). Thus, androgens such as DHEA, T and AD are as competent as P4 to activate pre-MPF.

Role of arachidonic acid cascade in Rhinella arenarum oocyte maturation

There are no studies that document the production of prostaglandins (PGs) or their role in Rhinella arenarum oocyte maturation. In this study, we analysed the effect of arachidonic acid (AA) and prostaglandins (PGs) on maturation, activation and pronuclear formation in R. arenarum oocytes. Our results demonstrated that AA was capable of inducing maturation in time-dependent and dose-dependent manner. Arachidonic acid-induced maturation was inhibited by indomethacin. PGs from AA hydrolysis, such as prostaglandin F2α (PGF2α) and, to a lesser extent, PGE2, induced meiosis resumption. Oocyte maturation in response to PGF2α was similar to that produced by progesterone (P4). Oocyte response to PGE1 was scarce. Rhinella arenarum oocyte PGF2α-induced maturation showed seasonal variation. From February to June, oocytes presented low sensitivity to PGF2α. In following periods, this response increased until a maximum was reached during October to January, a close temporal correlation with oocyte response to P4 being observed. The effect of PGF2α on maturation was verified by analysing the capacity of oocytes to activate and form pronuclei after being injected with homologous sperm. The cytological analysis of activated oocytes demonstrated the absence of cortical granules in oocytes, suggesting that PGF2α induces germinal vesicle breakdown (GVBD) and meiosis resumption up to metaphase II. In turn, oocytes matured by the action of PGF2α were able to form pronuclei after fertilization in a similar way to oocyte maturated by P4. In microinjection of mature cytoplasm experiments, the transformation of pre-maturation promoting factor (pre-MPF) to MPF was observed when oocytes were treated with PGF2α. In summary, our results illustrated the participation of the AA cascade and its metabolites in maturation, activation and pronuclei formation in R. arenarum.

Homeostasis and secretion of calcium in the oviductal mucosa of toad Rhinella arenarum

The presence of a calcium pump, calbindin D-28KD, and calmodulin in the secretory cells (SC) of the oviductal pars convoluta (PC) of Rhinella arenarum was established for the first time in amphibians using immunohistochemical techniques. Marked variations were observed in the localization and degree of expression of these proteins according to the duct segment and the period of the sexual cycle analyzed. During the preovulatory and ovulatory periods the calcium pump colocalized with calbindin D-28KD can be seen mainly in the apical border of the SC, which are located in the first zones of PC and synthesize and secrete the components of the inner jelly coat layers. These envelopes, which surround the oocytes, contain the molecules indispensable for fertilization, probably inducing the sperm acrosome reaction (AR). Our results suggest that calmodulin, colocalized with the calcium pump at the SC cytoplasmic level, would be involved in the active transport of the cation inside the secretory granules, maintaining adequate levels of intracellular Ca(2+) . During the postreproductive period, a calcium pump colocalized with calbindin D-28KD appears for the first time in the cycle in the basal zones of the SC. This system may be related to the replenishing of intracellular Ca(2+) stores. In contrast, in R. arenarum the Ca(2+) present in the jelly coats that surround the oocytes participates in the AR during fertilization, suggesting that this secretion system of the cation provided by the oviductal mucosa is functionally more active during the reproductive period of this species.

Involvement of PLA2, COX and LOX in Rhinella arenarum oocyte maturation

In Rhinella arenarum, progesterone is the physiological nuclear maturation inducer that interacts with the oocyte surface and starts a cascade of events that leads to germinal vesicle breakdown (GVBD). Polyunsaturated fatty acids and their metabolites produced through cyclooxygenase (COX) and lipoxygenase (LOX) pathways play an important role in reproductive processes. In amphibians, to date, the role of arachidonic acid (AA) metabolites in progesterone (P4)-induced oocyte maturation has not been clarified. In this work we studied the participation of three enzymes involved in AA metabolism - phospholipase A2 (PLA2), COX and LOX in Rhinella arenarum oocyte maturation. PLA2 activation induced maturation in Rhinella arenarum oocytes in a dose-dependent manner. Oocytes when treated with 0.08 μM melittin showed the highest response (78 ± 6% GVBD). In follicles, PLA2 activation did not significantly induce maturation at the assayed doses (12 ± 3% GVBD). PLA2 inhibition with quinacrine prevented melittin-induced GVBD in a dose-dependent manner, however PLA2 inactivation did not affect P4-induced maturation. This finding suggests that PLA2 is not the only phospholipase involved in P4-induced maturation in this species. P4-induced oocyte maturation was inhibited by the COX inhibitors indomethacin and rofecoxib (65 ± 3% and 63 ± 3% GVBD, respectively), although COX activity was never blocked by their addition. Follicles showed a similar response following the addition of these inhibitors. Participation of LOX metabolites in maturation seems to be correlated with seasonal variation in ovarian response to P4. During the February to June period (low P4 response), LOX inhibition by nordihydroguaiaretic acid or lysine clonixinate increased maturation by up to 70%. In contrast, during the July to January period (high P4 response), LOX inhibition had no effect on hormone-induced maturation.

Structural analysis of toad oviductal mucosa in relation to jelly components secretion throughout the reproductive cycle

In amphibians, the components of the jelly coats that surround the oocytes at the time of fertilization and coordinate gamete interaction are secreted by the oviduct. We analysed the histological variations in the mucosa of the oviductal pars convoluta (PC) of Rhinella arenarum during the reproductive cycle and its relationship with secretion. During the preovulatory period, the mucosa reaches a high degree of morphological and functional development, with a large number of epithelial (ESC) and glandular secretory cells (GSC) loaded with contents that are secreted into the oviductal lumen. During the ovulatory period, the secretory cells (SC) of both layers present maximum secretory activity through apocrinia and merocrinia. While the ESC located at the tips of the folds release their content directly in contact with the oocytes, the GSC secrete material from the bottom of the epithelial folds that, by interaction with the secretion of the ESC in the lateral faces, form a product with a certain degree of organization. Secretion is a continuous process with formation of coats of increasing complexity from the intermediate proximal zone (IPZ) to the pars convoluta (pc) itself, and the passage of the oocyte is a requisite for the organization of the jelly coats around the gamete. During the early postovulatory period, although there is a marked decrease in the number and volume of the SC, the ESC still release material into the oviductal lumen. In the late postovulatory period the morphological characteristics of the PC begin to recovery although there is no evidence of secretion.

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

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

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.

Hormonal induction of gamete release, and in-vitro fertilisation, in the critically endangered southern corroboree frog, Pseudophryne corroboree

BACKGROUND

Conservation Breeding Programs (CBP's) are playing an important role in the protection of critically endangered anuran amphibians, but for many species recruitment is not successful enough to maintain captive populations, or provide individuals for release. In response, there has been an increasing focus on the use of Assisted Reproductive Technologies (ART), including the administration of reproductive hormones to induce gamete release followed by in vitro fertilisation. The objective of this study was to test the efficacy of two exogenous hormones to induce gamete release, for the purpose of conducting in vitro fertilisation (IVF), in one of Australia's most critically endangered frog species, Pseudophryne corroboree.

METHODS

Male frogs were administered a single dose of either human chorionic gonadotropin (hCG) or luteinizing hormone-releasing hormone (LHRHa), while female frogs received both a priming and ovulatory dose of LHRHa. Spermiation responses were evaluated at 3, 7, 12, 24, 36, 48, 60 and 72 h post hormone administration (PA), and sperm number and viability were quantified using fluorescent microscopy. Ovulation responses were evaluated by stripping females every 12 h PA for 5 days. Once gametes were obtained, IVF was attempted by combining spermic urine with oocytes in a dilute solution of simplified amphibian ringer (SAR).

RESULTS

Administration of both hCG and LHRHa induced approximately 80% of males to release sperm over 72 h. Peak sperm release occurred at 12 h PA for hCG treated males and 36 h PA for LHRHa treated males. On average, LHRHa treated males released a significantly higher total number of live sperm, and a higher concentration of sperm, over a longer period. In female frogs, administration of LHRHa induced approximately 30% of individuals to release eggs. On average, eggs were released between 24 and 48 h PA, with a peak in egg release at 36 h PA. IVF resulted in a moderate percentage (54.72%) of eggs being fertilised, however all resultant embryos failed prior to gastrulation.

CONCLUSIONS

Hormone treatment successfully induced spermiation and ovulation in P. corroboree, but refinement of gamete induction and IVF techniques will be required before ART protocols can be used to routinely propagate this species.

Analysis of Bufo arenarum oviductal secretion during the sexual cycle

SummaryBufo arenarum oocytes are oviposited surrounded by jelly coats, one component of the extracellular matrix required for fertilization. The secretion, released to the oviductal lumen, was analysed by SDS-PAGE. The coomassie blue staining evidenced an electrophoretic pattern with molecules ranging between 300 and 19 kDa that showed variations in their secretion profiles during the sexual cycle. In the preovulatory period the densitometric analysis showed the presence of nine peaks with marked predominance of the 74 kDa molecule. Once ovulation has occurred, the jelly coats become arranged around the oocytes during their transit throughout the oviductal pars convoluta (PC), revealing the addition of three proteins only observed during this period, which suggests a differential secretion. Some of these proteins could not diffuse under any extraction treatment, indicating for them a structural or in situ function. Proteins of low molecular mass diffused totally while others showed a partial diffusing capacity. After ovulation a marked decrease in the relative amount of all the proteins released to the lumen, especially the 74 kDa protein, could be detected. During this period, unlike the other stages of the sexual cycle, a differential secretion pattern was observed along the PC. The histochemical analysis performed during the ovulatory period showed the presence of glycoconjugates including both acidic and neutral groups. The present results are in agreement with previous ultrastructural and histochemical studies that describe the role of Bufo arenarum jelly coats in fertilization.

Subcellular localization of calcium and Ca-ATPase activity during nuclear maturation in Bufo arenarum oocytes

The localization of calcium and Ca-ATPase activity in Bufo arenarum oocytes was investigated by ultracytochemical techniques during progesterone-induced nuclear maturation, under in vitro conditions. No Ca2+ deposits were detected in either control oocytes or progesterone-treated ones for 1-2 h. At the time when nuclear migration started, electron dense deposits of Ca2+ were visible in vesicles, endoplasmic reticulum cisternae and in the space between the annulate lamellae membranes. Furthermore, Ca-ATPase activity was also detected in these membrane structures. As maturation progressed, the cation deposits were observed in the cytomembrane structures, which underwent an important reorganization and redistribution. Thus, they moved from the subcortex and became located predominantly in the oocyte cortex area when nuclear maturation ended. Ca2+ stores were observed in vesicles surrounding or between the cortical granules, which are aligned close to the plasma membrane. The positive Ca-ATPase reaction in these membrane structures could indicate that the calcium deposit is an ATP-dependent process. Our results suggest that during oocyte maturation calcium would be stored in membrane structures where it remains available for release at the time of fertilization. Data obtained under our experimental conditions indicate that calcium from the extracellular medium would be important for the oocyte maturation process.

Sex differences and androgen influences on midbrain auditory thresholds in the green treefrog, Hyla cinerea

Reproductive hormones can modulate communication-evoked behavior by acting on neural systems associated with motivation; however, recent evidence suggests that modulation occurs at the sensory processing level as well. The anuran auditory midbrain processes communication stimuli, and is sensitive to steroid hormones. Using multiunit electrophysiology, we tested whether sex and circulating testosterone influence auditory sensitivity to pure tones and to the natural vocalization in the green treefrog, Hyla cinerea. Sex did not influence audiogram best frequencies although sexes did differ in the sensitivities at those frequencies with males more sensitive in the lower frequency range. Females were more sensitive than males in response to the natural vocalization, despite showing no difference in response to pure tones at frequencies found within the advertisement call. Thresholds to frequencies outside the range of the male advertisement call were higher in females. Additionally, circulating testosterone increased neural thresholds in females in a frequency-specific manner. These results demonstrate that sex differences are limited to frequency ranges that relate to the processing of natural vocalizations, and depend on the type of stimulus. The frequency-dependent and stimulus-dependent nature of sex and testosterone influences suggests that reproductive hormones influence the filtering properties of the auditory system.

Hormonal modulation of phonotaxis and advertisement-call preferences in the gray treefrog (Hyla versicolor)

Hormonal levels fluctuate during the breeding season in many anurans, but the identity of the hormones that modulate breeding behavior and their effects remain unclear. We tested the influence of a combined treatment of progesterone and prostaglandin on phonotaxis, the key proceptive reproductive behavior of female anurans. First, we found that female gray treefrogs (Hyla versicolor) treated with progesterone and prostaglandin exhibited phonotaxis to synthetic male advertisement signals significantly more often than animals treated with ringers vehicle or uninjected controls. Responsive females had greater levels of plasma progesterone and estradiol compared to both control groups, suggesting that these steroids may be promoting phonotaxis. Second, we found that the selectivity of hormonally-induced phonotaxis in H.versicolor was similar to that observed in freshly captured breeding animals. Females made the same choices between acoustic signals after hormone treatments in tests of frequency, call rate and pulse rate, compared to their responses without treatment immediately after collection from the breeding chorus. The preference for a longer call was, however, significantly weaker after hormonal induction of phonotaxis. Hormonally primed females were also less likely to respond in any test and took longer to respond than did freshly collected females. Consequently, our study shows how progesterone-prostaglandin induced phonotaxis in female treefrogs influences both the quality and quantity of phonotaxis, relative to that exhibited by naturally breeding females.

Modulators of Bufo arenarum ovulation

In this study we investigated ovulation in vitro using ovary samples from Bufo arenarum with respect to their response to stimulation with homologous pituitary homogenate (HPH) or with progesterone and prostaglandins (PGF2α and PGE1) as intermediates of pituitary action. Ovary samples were obtained from animals captured during the breeding period. Our results demonstrate that the ovulatory response to all different inducers was dose dependent, the highest percentage of ovulated oocytes being obtained with HPH treatment. An important increase in the ovulatory response was obtained by the association of PGF2α with either HPH or progesterone at suboptimal doses, indicating that this prostaglandin induced a synergistic potentiating effect. Incubation with cyclooxygenase inhibitors (indomethacin or diclofenac sodium) produced a significant decrease in the ovulation induced by HPH, demonstrating that prostaglandins are involved in the action of the pituitary gland in this process. According to our results, PGE1 not only had no participation in the ovulatory process, but also produced an inhibitory effect on ovulation induced by HPH treatment.

Effect of steroid hormones onBufo arenarum oviduct. Ultrastructural study

The endocrine regulation of the mucosa of the oviductal pars convoluta was analyzed by ultrastructural studies demonstrating that ovariectomy, together with a decrease in ovarian steroids circulating levels, caused a marked regression in this portion of Bufo arenarum oviduct. Twenty-five days after ovariectomy, a decrease in the depth of the epithelial and glandular layers was observed due to the notable loss of secretory cells, whose number was clearly smaller than in nonovariectomized females. The remaining secretory cells showed involution signs, with few secretory granules in their cytoplasm, little endoplasmic reticulum near poorly developed Golgi complexes and a large amount of lipid droplets. Cells in an advanced autolysis state were found in the lumen. These characteristics evidence a nonfunctional state of the pars convoluta. Treatment with 5alpha-dihydrotestosterone (DHT) completely reversed the ovariectomy effect, inducing pars convoluta growths and restoring the characteristics of epithelial and glandular secretory cells in the whole pars convoluta, with micrographs similar to the control. These same effects were observed after treatment with estradiol-17beta (E2), progesterone (P) o E(2)+P in the glandular layer of the whole pars convoluta, but only in the epithelial layer of the most anterior region of this duct. In the secretory cells of other segments these treatments induced the formation of granules of high electron density and homogeneous aspect. Each steroid had a particular effect on the pars convoluta. Although E2 and DHT induced the development of the organoids involved in the proteins biosynthesis, P and DHT acted as secretagogues.

Hormonal state influences aspects of female mate choice in the Túngara Frog (Physalaemus pustulosus)

Females alter their mate choices as they transition through different reproductive stages; however, the proximal mechanisms for such behavioral fluctuation are unclear. In many taxa, as females transition through different reproductive stages, there is an associated change in hormone levels; therefore, we examined whether fluctuation in hormone levels serves as a proximal mechanism for within-individual variation in mate choice in female túngara frogs (Physalaemus pustulosus). We manipulated hormone levels of females by administering 0, 10, 100, 500 or 1,000 IU of human chorionic gonadotropin (HCG), which is a ligand for luteinizing hormone (LH) receptors and will therefore cause increased gonadal hormone production. Phonotaxis assays were conducted to measure three aspects of mate choice behavior before and after HCG administration; receptivity (response to a conspecific mate signal), permissiveness (response to a signal that is less attractive than conspecific signals) and discrimination (ability to discern signal differences). The probability of response to a conspecific and an artificial hybrid signal significantly increased at the highest HCG doses. The difference in mean response time between pre- and post-HCG tests was significantly different for both the receptivity and permissiveness tests among the five doses. Increased permissiveness, however, was not due to decreased discrimination because females could discriminate between calls even at the highest HCG doses. These hormonal manipulations caused the same behavioral pattern we reported in females as they transitioned through different reproductive stages (Lynch, K.S., Rand, A.S., Ryan, M.J., Wilczynski, W., 2005. Plasticity in female mate choice associated with changing reproductive states. Anim. Behav. 69, 689-699), suggesting that changes in hormone levels can influence the female's mate choice behavior.

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.

Gonadal steroids vary with reproductive stage in a tropically breeding female anuran

Tropically breeding anurans that require heavy rainfall in order to reproduce are subject to favorable breeding conditions that are sporadic. Although there is an increased probability of rain during the rainy season, the probability of local rainfall is unpredictable and this may influence female anuran reproductive strategies. The female túngara frog, a neotropical frog that requires standing water to breed, maintains readiness to breed at any time via asynchronous oogenesis. Although females constantly produce and maintain oocytes during the breeding season, this study shows that they have cyclic fluctuations in gonadal hormone levels. Plasma levels of estrogen significantly change during three reproductive stages within a single reproductive cycle (P=0.03), as do plasma levels of progesterone and androgen (P<0.001 and P=0.001, respectively). Furthermore, elevation in plasma estrogen and progesterone concentrations occurs during the same reproductive stage in which it has been reported that females display the maximum frequency of reproductive behaviors, the amplexed stage. Androgen levels, however, are elevated prior to the reproductive stage in which females display maximal reproductive behavior, that is, the unamplexed stage. Our study suggests that the pattern of gonadal hormone fluctuation in a tropically breeding female anuran is similar to the classic paradigm in which there is a temporal relationship between the appearance of reproductive hormones and reproductive behaviors.