Estrogens in the plasma of females of Rana esculenta during the annual cycle and following ovariectomy

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

This issue is dedicated to the late Professor Giovanni Chieffi, and this article is an overview of the research on Comparative Endocrinology of reproduction using Rana esculenta (alias Pelophylax esculentus) as a model system. Starting from the early 1970s till today, a large quantity of work have been conducted both in the fields of experimental endocrinology and in the definition of the diffuse neuroendocrine system, with a major focus on the increasing role of regulatory peptides. The various aspects investigated concerned the histological descriptions of principal endocrine glands of the hypothalamic-pituitary-gonadal (HPG) axis, the localization and distribution in the HPG of several different substances (i.e. neurosteroids, hypothalamic peptide hormones, pituitary gonadotropins, gonadal sex steroids, and other molecules), the determination of sex hormone concentrations in both serum and tissues, the hormone manipulations, as well as the gene and protein expression of steroidogenic enzymes and their respective receptors. All together these researches, often conducted considering different periods of the annual reproductive cycle of the green frog, allowed to understand the mechanism of cascade control/regulation of the HPG axis of R. esculenta, characterizing the role of different hormones in the two sexes, and testing the hypotheses about the function of single hormones in different target organs. It becomes evident from the review that, in their simplest form, several features of this species are specular as compared to those of other vertebrate species and that reproduction in this frog species is either under endogenous multi-hormonal control or by a wide array of different factors. Our excursus of this research, spanning almost five decades, shows that R. esculenta has been intensively and successfully used as an animal model in reproductive endocrinology as well as several field studies such as those involving environmental concerns that focus on the effects of endocrine disruptors and other environmental contaminants.

[Plasma androgens and 17 beta-estradiol of the Pyrenean urodele Euproctus asper during the period of reproduction]

The concentration of plasma androgens (A) and 17 beta-estradiol (E) in both males and females of Euproctus asper from June to November was determined by radioimmunoassay. The levels of androgens in males varied from a maximum of 980 ng/100 ml in June to a minimum of 172.5 ng/100 ml in July. A small increase was observed in September and a decrease in October-November. The concentration of androgens in females was comparable to that of males at the end of the cycle: 320 ng/100 ml; the values were high (470 ng/100 ml) in July and decreased to a minimum (80 ng) in September. The values of 17 beta-estradiol in females showed a different pattern during the reproductive cycle. The concentration was at a maximum in September, at the same time of the low concentration of androgens. In the males, the concentration of estradiol was very low (less than 17 ng/100 ml). Measurements of adenohypophysial volume and gonadotrophic cell diameter are discussed with respect to levels of circulating steroids.

Seasonal plasma sex steroid levels in the female Rana esculenta

Seasonal plasma progesterone, androstenedione, estrone, and 17 beta-estradiol concentrations in the female Rana esculenta were determined by radioimmunoassay during the 1979 and 1981 seasons. Plasma levels of these steroids were highest just before the first ovulatory wave in spring and lowest after the breeding season. In the 1979 season (during the 1981 season hormones were not assayed in January, November, and December) progesterone, androstenedione, and estradiol levels showed another peak in November-December. During the breeding months, i.e., late March to late June, progesterone, androstenedione, and estradiol levels showed intermittent ups and downs corresponding roughly to the ovulatory waves. In addition, during the breeding season progesterone and androstenedione levels had a higher average in frogs with "ripe" ovaries than in those with "spent" ovaries. Relationships between seasonal steroid levels and ovarian activity are briefly discussed.

Vitellogenic cycles in laboratory-maintained females of the leopard frog, Rana pipiens

As a part of studies on the reproduction of laboratory maintained frogs, wild-caught Rana pipiens were ovulated and maintained at 22-27 degrees C for up to 18 months. Vitellogenic oocytes were periodically staged and counted, and a "maturity index" was calculated to assess the progress of the vitellogenic cycle. The initial cycle was similar to that of wild frogs except that the first oocytes to reach stage 5 (mature eggs) usually began to degenerate before later starting oocytes became mature. In addition, a second cycle began before the first was completed. After more than 1 year at room temperature, abnormal cycles were common. Ovaries of such animals contained very few mature eggs. Many of their oocytes were in early stages of vitellogenesis or, if pigmented, had begun to degenerate. These deficiencies were partially corrected in females placed in 4 degrees C for 4-6 weeks. The average number of mature eggs increased 15-fold and ovary weights more than doubled. Oviduct weights almost doubled. Although the rates of cooling, photoperiod, and nutritional status could be important influences, the results imply that cold treatment alone increases estrogen secretion. We suggest that low estrogen secretion may account for the reproductive deficiencies seen in R. pipiens cultured at room temperature.

Initiation and inhibition of the release croak of Rana pipiens

A release croak is emitted by unreceptive female frogs when they are clasped by males; receptive females are silence. This report investigates the sources of stimuli which initiate and inhibit the release croak of Rana pipiens. Experiment 1 demonstrates that manual clasping of the trunk rather than the legs elicits the croak. Experiment 2 shows that denervation of the skin of the trunk prevents the release call in response to manual stimulation. Experiment 3 confirms that artificial distension of the body with fluid inhibits the release croak in response to manual stimulation. Experiment 4 shows that artifically-distended females, who are not in the normal endocrinological state that accompanies mating, are silent and receptive in response to clasping by sexually-active male frogs. The first two experiments imply that stimulation of the skin of the trunk initiates the release croak; Experiments 3 and 4 suggest that an internal afferent source inhibits the release croak and might mediate an important aspect of receptivity in female frogs.