Celebrating 50+ years of research on the reproductive biology and endocrinology of the green frog: An overview

The Aromatase-Estrogen System in the Testes of Non-Mammalian Vertebrates

Estrogens are important physiological regulators of testicular activity in vertebrates. Estrogen levels depend on the activity of P450 aromatase, the enzyme responsible for the irreversible conversion of testosterone into 17β-estradiol. Therefore, P450 aromatase is the key player in the aromatase-estrogen system. The present review offers a comparative overview of P450 aromatase activity in male gonads of amphibians, reptiles, and birds, with a particular emphasis on the functions of the aromatase-estrogen system in these organisms during their developmental and adult stages. The aromatase-estrogen system appears to be crucial for the sex differentiation of gonads in vertebrates. Administration of aromatase inhibitors prior to sexual differentiation of gonads results in the development of males rather than females. In adults, both aromatase and estrogen receptors are expressed in somatic cells, Leydig and Sertoli cells, as well as germ cells, with certain differences among different species. In seasonal breeding species, the aromatase-estrogen system serves as an "on/off" switch for spermatogenesis. In some amphibian and reptilian species, increased estrogen levels in post-reproductive testes are responsible for blocking spermatogenesis, whereas, in some species of birds, estrogens function synergistically with testosterone to promote spermatogenesis. Recent evidence indicates that the production of the aromatase enzyme in excessive amounts reduces the reproductive performance in avian species of commercial interest. The use of aromatase inhibitors to improve fertility has yielded suitable positive results. Therefore, it appears that the role of the aromatase-estrogen system in regulating the testicular activity differs not only among the different classes of vertebrates but also among different species within the same class.

JNK/Elk1 signaling and PCNA protein expression in the brain of hibernating frog Pelophylax esculentus

Mitogen activated protein kinase (MAPK) activation and neurogenesis are known to play a role in neuronal survival during hibernation. Herein, we investigate the activity of c-Jun N-terminal kinases (JNK) and Ets like-1 protein (Elk1) kinase involved in cell survival, as well as the expression of proliferating cell nuclear antigen (PCNA), a cell proliferation marker, in the brain of the frog Pelophylax esculentus. The study was conducted on female and male frogs collected during the annual cycle. Our results demonstrated that JNK activity increased during the hibernating phase in relation to the active phase. Interestingly, P-Elk1 levels were positively correlated with P-JNK levels, suggesting that the JNK/Elk1 pathway is pivotal in mediating neuroprotective adaptations that are essential to successful hibernation. On the contrary, we detected higher PCNA expression levels during the active period compared with the hibernating period. A sex dimorphism was observed in the expression levels of P-JNK/P-Elk1 that were specifically higher in males, and in the expression of PCNA reporting higher levels in female brains. Much remains to be learned regarding the regulation of hibernation, however, our findings provide new insights into the role of MAPK and proliferative pathways in hibernation, adding new knowledge concerning the mechanisms activated in the brain of ectothermic species to counteract the damage resulting from extreme temperatures.

Amphibian reproductive technologies: approaches and welfare considerations

Captive breeding and reintroduction programs have been established for several threatened amphibian species globally, but with varied success. This reflects our relatively poor understanding of the hormonal control of amphibian reproduction and the stimuli required to initiate and complete reproductive events. While the amphibian hypothalamo-pituitary-gonadal (HPG) axis shares fundamental similarities with both teleosts and tetrapods, there are more species differences than previously assumed. As a result, many amphibian captive breeding programs fail to reliably initiate breeding behaviour, achieve high rates of fertilization or generate large numbers of healthy, genetically diverse offspring. Reproductive technologies have the potential to overcome these challenges but should be used in concert with traditional methods that manipulate environmental conditions (including temperature, nutrition and social environment). Species-dependent methods for handling, restraint and hormone administration (including route and frequency) are discussed to ensure optimal welfare of captive breeding stock. We summarize advances in hormone therapies and discuss two case studies that illustrate some of the challenges and successes with amphibian reproductive technologies: the mountain yellow-legged frog (Rana muscosa; USA) and the northern corroboree frog (Pseudophryne pengilleyi; Australia). Further research is required to develop hormone therapies for a greater number of species to boost global conservation efforts.