Thyroid in reproduction of reptiles

Thyroid and reproduction in amphibians and reptiles

The relationship between the thyroid gland and reproduction in amphibians and reptiles has been studied for more than 100 years. Most studies suggest a positive involvement of thyroid hormones with some aspects of reproduction, but some studies support a negative role for thyroid hormones at certain life stages. Comprehensive studies of gene activation/suppression by thyroid hormones and their absence at various levels of the hypothalamo-pituitary-gonadal axis coupled with observations of adrenocorticoid activity, reproductive performance, and metabolic involvement are needed to understand this complex relationship.

Sex steroids are correlated with environmental factors and body condition during the reproductive cycle in females of the lizard Sceloporus torquatus

Reproduction is regulated by multiple factors that influence physiology and behavior to ensure the continuity of species. However, more work is needed to examine the complex relationships between environmental factors and endocrine transducers that modulate reproductive cycles, particularly in lizards. Here, we aimed to characterize the variation in plasma sex steroid levels in different stages of the reproductive cycle in the lizard Sceloporus torquatus and assess whether sex steroid levels were related to environmental factors (temperature, photoperiod, precipitation, and relative humidity) and body condition. Plasma concentrations of estradiol (E₂) and progesterone (P₄) from blood samples were quantified by enzyme-linked immunosorbent assay (ELISA) and radioimmunoanalysis (RIA), respectively. Our results indicate that sex steroid concentrations were positively related to follicular development but negatively related to temperature and precipitation. E₂ increased as the follicles grew, and its concentrations were highest in the preovulatory phase. P₄ showed a similar pattern and persisted during pregnancy. Changes in body condition were non-significant and mainly unrelated to the reproductive stage and plasma sex steroids. Our findings indicate that sex steroids change depending on the season and reproductive stage. We observed high concentrations of E₂ and P₄ in the late vitellogenic and preovulatory stages, probably because of their role in promoting vitellogenesis and ovulation. Additionally, we observed that follicular development is correlated with temperature and photoperiod. To better understand the mechanisms underlying reproduction, future studies of captive populations where environmental factors can be manipulated are needed.

Thyroid hormone actions on male reproductive system of teleost fish

Thyroid hormones (THs) play important roles in the regulation of many biological processes of vertebrates, such as growth, metabolism, morphogenesis and reproduction. An increasing number of studies have been focused on the involvement of THs in the male reproductive system of vertebrates, in particular of fish. Therefore, this mini-review aims to summarize the main findings on THs role in male reproductive system of fish, focusing on sex differentiation, testicular development and spermatogenesis. The existing data in the literature have demonstrated that THs exert their roles at the different levels of the hypothalamic-pituitary-gonadal (HPG) axis. In general a positive correlation has been shown between THs and fish reproductive status; where THs are associated with testicular development, growth and maturation. Recently, the molecular mechanisms underlying the role of THs in spermatogenesis have been unraveled in zebrafish testis. THs promote germ cell proliferation and differentiation by increasing a stimulatory growth factor of spermatogenesis produced by Sertoli cells. In addition, THs enhanced the gonadotropin-induced androgen release in zebrafish testis. Next to their functions in the adult testis, THs are involved in the gonadal sex differentiation through modulating sex-related gene expression, and testicular development via regulation of Sertoli cell proliferation. In conclusion, this mini-review showed that THs modulate the male reproductive system during the different life stages of fish. The physiological and molecular mechanisms showed a link between the thyroid and reproduction, suggesting a possibly co-evolution and interdependence of these two systems.

Thyroid hormones in male reproductive development: evidence for direct crosstalk between the androgen and thyroid hormone axes

Thyroid hormones (THs) exert a broad range of effects on development in vertebrate species, demonstrating connections in nearly every biological endocrine system. In particular, studies have shown that THs play a role in sexual differentiation and gonadal development in mammalian and non-mammalian species. There is considerable evidence that the effects of THs on reproductive development are mediated through the female hormonal axis; however, recent findings suggest a more direct crosstalk between THs and the androgen axis. These findings demonstrate that THs have considerable influence in the sexual ontogeny of male vertebrates, through direct interactions with select sex-determining-genes and regulation of gonadotropin production in the hypothalamus-pituitary-gonad axis. THs also regulate androgen biosynthesis and signaling through direct and indirect regulation of steroidogenic enzyme expression and activity. Novel promoter analysis presented in this work demonstrates the potential for direct and vertebrate wide crosstalk at the transcriptional level in mice (Mus musculus), Western clawed frogs (Silurana tropicalis) and medaka (Oryzias latipes). Cumulative evidence from previous studies; coupled with novel promoter analysis suggests mechanisms for a more direct crosstalk between the TH and male reproductive axes across vertebrate species.

Thyroid disruption in the lizard Podarcis bocagei exposed to a mixture of herbicides: a field study

Pesticide exposure has been related with thyroid disrupting effects in different vertebrate species. However, very little is known about the effects of these compounds in reptiles. In the Mediterranean area, lacertid lizards are the most abundant vertebrate group in agroecosystems, and have been identified as potential model species for reptile ecotoxicology. The aim of this study was to understand if the herbicides applied in corn fields have thyroid disruptive effects in the lizard Podarcis bocagei. Adult male lizards were captured in north-western Portugal in corn fields treated with herbicides (exposed sites), and in organic agricultural fields (reference sites). Thyroid and male gonad morphology and functionality, and testosterone levels were investigated through histological, immunohistochemical and biochemical techniques. Lizards from exposed locations displayed thyroid follicular lumens with more reabsorption vacuoles and significantly larger follicular area than those from reference fields. Furthermore, testes of lizards from exposed locations had significantly larger seminiferous tubule diameters, significantly higher number of spermatogenic layers and displayed an up-regulation of thyroid hormone receptors when compared with lizards from reference areas. These findings strongly suggest that the complex mixture of herbicides that lizards are exposed to in agricultural areas have thyroid disrupting effects which ultimately affect the male reproductive system. Alachlor, which has demonstrated thyroid effects in mammals, may be largely responsible for the observed effects.

The reptilian thyroid and parathyroid glands

The field of reptilian clinical endocrinology is still in its infancy. The thyroid and parathyroid glands are intimately involved with many basic metabolic functions. These glands have been the subject of extensive research studies in reptilian species; however, the effects of abnormal gland function have been poorly documented in clinical cases. These glands play a major role in maintaining physiologic homeostasis in all vertebrates. With the advent of more sensitive assays, it should be possible to measure the small amounts of hormones found in reptilian species. The purpose of this article is to review the literature regarding clinical endocrinology of the thyroid and parathyroid glands in reptiles.

The expression of androgen receptor messenger RNA is regulated by tri-iodothyronine in lizard testis

The network of hormonal and non-hormonal signals required for testicular activity during the reproductive cycle of the seasonal breeding lizard, Podarcis sicula, are not yet well understood. Androgens are significantly involved in meiosis and spermiogenesis, and such an effect is mediated through their receptor (AR). Estrogens also affect the testicular activity down-regulating the expression of AR mRNA. Since over the last few years, extensive works have reported, in mammals, a clear influence of tri-iodothyronine (T(3)), the biologically active thyroid hormone, on Sertoli cell activities, we carried out a study to shead light on the effect/s exerted by T(3) in lizard testis. A thyroid hormone receptor mRNA (TR mRNA) has been found in the testis indicating that T(3) might be involved in the regulation of gonadal activity. In in vivo experiments, injection of T(3) to male lizards, captured during the recrudescence period (March) and maintained under experimental photothermal conditions (24 degrees C and 15 h daylight), increased the expression of AR mRNA. The in vitro results confirmed the stimulatory effect of T(3) on AR mRNA levels. Thus, in testosterone (T) exposed cells, the highest values of AR mRNA were observed in T(3)-primed animals, indicating that T and T(3) increase AR gene transcription independently. The present data suggest that, in lizards, the combined action of androgens, estrogen and T(3) might regulate testicular activity, modulating AR mRNA levels.

Hepatic outer-ring deiodinase in a Mexican endemic lizard (Sceloporus grammicus)

The kinetic characterization of the outer-ring deiodination pathway using rT(3) (rT(3)-ORD) in male, female, and pregnant female livers of an endemic lizard, Sceloporus grammicus, is reported. The ORD pathway does not have the characteristics of deiodinase type II; it is exclusively carried out by deiodinase type I (DI). DI enzymatic activity in lizard liver contains one of the highest activities reported in vertebrates. This activity is sexually dimorphic, with males presenting the highest activity during the reproductive season. The properties of this enzyme correspond to those described in mammals, such as specificity for rT(3), susceptibility to inhibition by 6-n-propyl-2-thiouracil and gold-thioglucose, cofactor requirement, and kinetic pattern. Unlike other vertebrates, the lizard DI exhibits conspicuous stability in the thermal range of 15 to 42 degrees C and in the pH range of 5.0 to 9.0. Male true kinetic constants exhibit a direct correlation with temperature. This is in agreement with short-term adaptation to microenvironmental changes and the feasible expression of enzymatic forms/variants which, together, endow this lizard species with a greater adaptation to natural daily ambient thermal fluctuations.

Photoperiodic response of pineal-thyroid axis of the female Indian palm squirrel, Funambulus pennanti

Effect of short (SP: 10 L: 14 D: Light: Dark) and long (LP: 14 L: 10 D) photoperiod was tested on pineal-thyroid axis in a tropical seasonally breeding rodent F. pennanti during sexually regressive phase of reproduction. SP reduced thyroid weight and plasma thyroxine (T4) as well as increased melatonin (aMT) while pinealectomy (Px) prevented such an effect of SP. LP and Px plus LP had no significant effect on thyroid activity when compared with their respective controls. These results suggest that SP stimulated pineal activity which increased the concentration of aMT, hence inhibited thyroid gland activity. However, the plasma aMT concentration under LP regime had no significant difference from controls, as the ambient photoperiodic length (13.6 hr) was almost similar to the experimental one (14.0 hr). It is suggested that being innervated from the same ganglion (SCG) which conveys photic stimuli to the pineal gland, the thyroid gland like the pineal is also under the influence of photoperiodic responses. Further pineal/melatonin also influences thyroid function indirectly.

Circulating thyroid hormone concentrations in relation to seasonal events in the male Indian garden lizard, Calotes versicolor

Circulating thyroxine (T4) and triiodothyronine (T3) were estimated by RIA in plasma samples of Calotes versicolor collected over a period of 28 months. Monthly observations on seasonal events, viz., molting, whole body O2 consumption, abdominal fat deposition, and testes weight were also made. Plasma T4 ranged from 0.16 to 4.63 ng/ml and T3 from 0 to 1.76 ng/ml during the course of the year. Peaks of T4 and T3 could be related with prehibernating (October/November) and posthibernating (March/April) periods, when scale shedding and O2 consumption were also enhanced. Maximum abdominal fat deposition was found in prehibernatory months (October/November). Thyroid hormone levels declined sharply with entry into hibernation. Abdominal fat deposits gradually decreased, being almost depleted by the end of hibernation. The gradual increase in thyroid hormone concentration in the later half of hibernation may be associated with increased lipolysis and preparation of the oncoming reproductive period. Although T4 and T3 profiles were apparently parallel, discrepancies were observed. Posthibernatory peak of T4 was maintained much longer than that of T3 which declined earlier. Also the rise in the prehibernatory T3 concentration was delayed as compared to that in T4.

Role of thyroid and testicular hormones in the oxidative metabolism of the Indian garden lizard, Calotes versicolor

L-Thyroxine (L-T4) and testosterone were administered separately to intact, thyroidectomized, and castrated male Indian garden lizards, Calotes versicolor (maintained under natural climatic conditions) throughout the year to study their effects on the whole-body oxygen consumption and on tissue (liver, muscle, kidney, and brain) respiration. The findings indicate that the calorigenic actions of L-T4 and testosterone are independent of ambient temperature, and different tissues have different, temperature-independent, annual rhythms of sensitivity to L-T4 and testosterone. Internal factors seem to play a major role in the determination of these annual tissue sensitivity rhythms. Effects of thyroidectomy and castration are brought about by decreased levels of both thyroidal and testicular hormones. It is suggested that gonadal hormones are directly involved in the oxidative metabolism of Calotes which is regulated jointly by thyroid and gonadal steroids, and the degree of involvement of the glands depends upon the energy demand and sensitivity of tissues during different months of the year.