<p class="first" id="d4332901e101">Rhinella arenarum is a South American toad with
wide geographic distribution. Testes
of this toad produce high amount of androgens during the non reproductive season and
shift steroid synthesis from androgens to 5α-pregnanedione during the breeding. In
addition, plasma estradiol (E2) in males of this species shows seasonal variations
but, since testes of R. arenarum do not express aromatase, the source of plasma E2
remained unknown for several years. However, the Bidder's organ (BO), a structure
located at one pole of each testis, is proposed to be the main source of E2 in male's
toads since it expresses several steroidogenic enzymes and is able to produce E2 from
endogenous substrates throughout the year. In addition, there were significant correlations
between plasma E2 and total activity of BO aromatase, and between plasma E2 and the
amount of hormone produced by the BO in vitro. In the toad, apoptosis induced by in
vitro treatment with E2 was mostly detected in spermatocytes during the breeding and
in spermatids during the post-reproductive season, suggesting that this steroid has
an important role in controlling spermatogenesis. However, in vitro treatment with
E2 had no effect on proliferation. This evidence suggests that the mechanism of action
of E2 on amphibian spermatogenesis is complex and more studies are necessary to fully
understand the role of estrogens regulating the balance between cellular proliferation
and apoptosis. In addition, in R. arenarum in vitro studies suggested that E2 has
no effect on CypP450c17 protein levels or enzymatic activity, while it reduces 3β-hydroxysteroid
dehydrogenase/isomerase (3β-HSD/I) activity during the post reproductive season. As
well, E2 regulates FSHβ mRNA expression all over the year suggesting a down regulation
process carried out by this steroid. The effect on LHβ mRNA is dual, since during
the reproductive season estradiol increases the expression of LHβ mRNA while in the
non-reproductive season it has no effect. In conclusion, the effect of E2 on gonadotropins
and testicular function is complex, not clearly understood and probably varies depending
on the species. The aim of the current article is to review evidence on reproductive
endocrinology and on the role of estradiol regulating reproduction in amphibians,
with emphasis on the South American species Rhinella arenarum.
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