Effects of thyroidectomy and thyroxine replacement on seasonal reproduction in the red deer hind

The Role of the Thyroid Axis in Fish

In all vertebrates, the thyroid axis is an endocrine feedback system that affects growth, differentiation, and reproduction, by sensing and translating central and peripheral signals to maintain homeostasis and a proper thyroidal set-point. Fish, the most diverse group of vertebrates, rely on this system for somatic growth, metamorphosis, reproductive events, and the ability to tolerate changing environments. The vast majority of the research on the thyroid axis pertains to mammals, in particular rodents, and although some progress has been made to understand the role of this endocrine axis in non-mammalian vertebrates, including amphibians and teleost fish, major gaps in our knowledge remain regarding other groups, such as elasmobranchs and cyclostomes. In this review, we discuss the roles of the thyroid axis in fish and its contributions to growth and development, metamorphosis, reproduction, osmoregulation, as well as feeding and nutrient metabolism. We also discuss how thyroid hormones have been/can be used in aquaculture, and potential threats to the thyroid system in this regard.

Estrogen is neuroprotective against hypoglycemic injury in murine N38 hypothalamic cells

Estrogen (E2) has been demonstrated to possess protective effects from hypoglycemic toxicity, particularly in the pancreas. In the central nervous system, several brain regions, such as the hypothalamus, are highly vulnerable to hypoglycemic injuries that may lead to seizures, coma, and mortality. The present study performed a novel in vitro assay of hypoglycemic injury to hypothalamic cells, and is the first study, to the best of our knowledge, to demonstrate that E2 protects hypothalamic cells from hypoglycemic toxicity. The toxic effects of hypoglycemia on hypothalamic cells in vitro was determined by performing cell counts, together with MTT and lactate dehydrogenase assays, using the N38 murine hypothalamic cell line. Following 24 and 48 h in hypoglycemic conditions, a 60 and 75% reduction in cell number and mitochondrial function was observed, which reached 80 and ~100% by 72 and 96 h, respectively. E2 treatment prevented the hypoglycemia‑induced loss in cell number and mitochondrial toxicity at 24 and 48 h. However at 72 and 96 h of hypoglycemic conditions, the neuroprotective effects of E2 on cell number or mitochondrial function was not significant or not present at all. In order to determine whether E2 exerted its effects through the AKT signaling pathway, the expression of proline‑rich AKT substrate of 40 kDa (PRAS40) was analyzed. No alterations in PRAS40 expression were observed when N38 cells were exposed to hypoglycemic shock. From the biochemical and molecular data obtained, the authors speculated that E2 exhibits neuroprotective effects against hypoglycemic shock in hypothalamic cells, which dissipates with time. Despite demonstrating no significant effect on total AKT/PRS40 activity, it is possible that E2 may mediate these neuroprotective effects by upregulating the phosphorylated‑AKT/pPRAS40 signaling pathway. The present study presented, to the best of our knowledge, the first in vitro model for hypoglycemic toxicity to hypothalamic cells, and provided evidence to suggest that E2 may protect hypothalamic cells from the damaging effects of hypoglycemia.

Caloric restriction, but not caloric loading, affects circulating fetal and maternal C-type natriuretic peptide concentrations in late ovine gestation

The factors regulating the greatly elevated concentrations of maternal plasma C-type natriuretic peptide (CNP) forms in ruminant pregnancy are largely unknown, but nutrient status is likely to be important. Previous work has shown that increases in maternal plasma CNP, sourced from the placenta, occur in response to caloric restriction in late gestation. Whether oversupply of nutrients also regulates CNP secretion in pregnancy has not been studied. Hypothesising that CNP in fetal and maternal tissues will be responsive to both deficiency and excess, we studied changes in CNP and a cosecreted fragment, namely N-terminal pro-CNP (NTproCNP), during short-term periods of caloric restriction (CR) and loading (CL). Twin-bearing ewes received CR (fasted Days 121–124), CL (Days 110–124) or control maintenance diets. During CR, fetal plasma CNP forms, insulin-like growth factor (IGF)-1 and liveweight all fell, and maternal plasma NTproCNP increased. During CL, fetal IGF-1 increased, whereas CNP forms and liveweight were unchanged, as were maternal concentrations of CNP forms. The high abundance of CNP peptides in placental tissues was unaffected by these short-term changes in nutrient supply. We conclude that CNP in the fetal–maternal unit is acutely responsive to undernutrition, but is unaffected by oversupply in late gestation.

C-type natriuretic peptide forms in pregnancy: maternal plasma profiles during ovine gestation correlate with placental and fetal maturation

Circulating concentrations of C-type natriuretic peptide (CNP) and a related amino terminal fragment (NTproCNP) were measured at weekly intervals from preconception to 3 wk postpartum in ewes with twins (n = 8) and nonpregnant ewes (n = 8). In contrast to low and stable values in nonpregnant ewes (CNP, 0.75 +/- 0.08; NTproCNP, 22 +/- 2 pmol/liter), CNP forms increased abruptly at 40-50 d of gestation and rose to peak values (CNP, 31 +/- 5, NTproCNP, 270 +/- 16 pmol/liter) at about d 120. Approximately 7 d prepartum, the concentration of both CNP forms fell precipitously to preconception values immediately postpartum. In separate studies, circulating maternal CNP forms were positively related to fetal number at d 120. Consistent with a major contribution from the placenta to circulating levels, the concentrations of CNP forms were elevated in the placentome (cotyledon: CNP, 18 +/- 4, NTproCNP, 52 +/- 10 pmol/g; caruncle: CNP, 13 +/- 3, NTproCNP, 31 +/- 6 pmol/g) and much higher than those of intercaruncular uterine tissue (CNP, 0.19 +/- 0.05, NTproCNP, 0.98 +/- 0.2 pmol/g) in late-gestation ewes (P < 0.001, n = 4). These distinctive patterns of maternal plasma CNP forms, positive relation with fetal number, and greatly elevated protein concentrations in the placentome demonstrate the hormone's strong relation to placental and fetal maturation. The findings provide a firm basis for future studies of the functional role of CNP in fetal-maternal welfare.

Morphological plasticity in the neural circuitry responsible for seasonal breeding in the ewe

An increase in the response of GnRH neurons to estrogen negative feedback is responsible for seasonal anestrus in the ewe, but the underlying neural mechanisms remain largely unknown. Neural plasticity may play an important role because the density of synaptic input to GnRH neurons changes with seasons. Moreover, the transition from breeding to anestrous season requires thyroid hormones, which are also required for neuronal development. In the first experiment, we examined whether the decrease in synapses on GnRH neurons is critical for the transition to anestrus by comparing synaptic input in thyroidectomized and thyroid-intact controls, using electron microscopic analysis. Thyroidectomized ewes remained in the breeding season, but the number of synaptic contacts on their GnRH cells was not different from those in thyroid-intact ewes that were anestrus. The next experiment tested whether there was a seasonal change in morphology of the A15 dopaminergic neurons that mediate estrogen negative feedback during anestrus by analyzing synapsin-positive close contacts onto A15 neurons with confocal microscopy. There was a 2-fold increase in these close contacts onto dendrites of A15 neurons in anestrus and a corresponding increase in the length of A15 dendrites at this time of year. The increase in dendritic length was blocked by thyroidectomy, but this procedure did not significantly affect synaptic input to A15 neurons. These results provide initial evidence that the seasonal change in synapses on GnRH neurons is not sufficient for the transition into anestrus but that plasticity of the A15 dopaminergic neurons mediating estrogen negative feedback may contribute to this seasonal alteration.

Laparoscopic ovum pick-up and in vitro production of sika deer embryos: effect of season and culture conditions

Amongst the 200 deer subspecies worldwide, more than 40 are considered as endangered. In vitro embryo production may represent an efficient way to produce and disseminate offspring from sparse remaining individuals in these species. With a view to establishing a method of in vitro embryo production, we assessed the ovarian response after hormonal stimulation (oFSH), oocyte yield following laporoscopic ovum pick-up (LOPU) and oocyte developmental competence according to seasonal reproductive status in sika deer (Cervus nippon nippon). Twelve adult sika deer hinds were allocated between two groups and submitted weekly to oFSH follicular growth stimulation followed by LOPU. Hinds in Group A (n=6) were treated first during the breeding season (5 weeks), and then during the non-breeding season (3 weeks). Hinds in Group B (n=6) were submitted to similar procedures but in the reverse order (treated first during the non-breeding season). Cumulus-oocytes complexes (COC) recovered from Group B were allowed to mature in vitro for 24 h in TCM-199 medium supplemented with oFSH, goat follicular fluid and 100 microM cysteamine. In vitro fertilization was performed with frozen/thawed semen in SOFaa medium supplemented with 20% estrous sheep serum and presumptive zygotes were cultured in the presence or absence of ovine oviductal epithelial cell monolayer (oOEC) in SOFaa-BSA medium. Mean number of follicles aspirated per hind per session decreased significantly between breeding and non-breeding season in Group A (9.8+/-0.7 versus 3.2+/-0.7, mean+/-S.E.M., respectively, P<0.001) but did not change between the non-breeding and the subsequent breeding season in Group B (5.3+/-0.7 and 5.7+/-0.7, respectively, P>0.05). Irrespective of the season, good quality COC with complete and compact cumulus investments were recovered allowing a high cleavage rate after in vitro maturation and fertilization. Whereas development to the blastocyst stage did not occur in SOF medium alone, high development rates to the blastocyst stage were observed in oOEC co-culture regardless of season (22% and 34% of total oocytes in co-culture during non-breeding and breeding season, respectively).

Reciprocal interaction between seasonal testis and thyroid activity in Zembra Island wild rabbits (Oryctolagus cuniculus): effects of castration, thyroidectomy, temperature, and photoperiod

The reproductive cycle of wild rabbits (Oryctolagus cuniculus) living in Zembra Island (North Tunisia) is dependent on an external factor, the photoperiod: the gonads are inhibited by long days and stimulated by short days or melatonin implants. Here we studied the role of an internal factor, thyroid hormones and the possible thyroid-gonadal interrelationships, in animals captured on Zembra Island and maintained in natural conditions of photoperiod and temperature. We determined the seasonal profile of the thyroid and testis cycles and investigated the effects of castration and thyroidectomy on the seasonal testosterone and thyroxine cycles. Plasma thyroxine and testosterone levels followed similar, parallel seasonal patterns, with a peak in autumn (October) and low values from January to August. In thyroidectomized animals, plasma testosterone levels, although significantly higher than those in controls (P < 0.001), remained low throughout the 13 mo of the experiment, and no testicular reactivation was observed in the fall. In castrated animals, despite the increase in thyroxine concentration in the 3 mo following castration (P < 0.01), plasma thyroxine levels remained low during the 2 yr of the study. We then investigated the combined effects of long days (16L:8D) and moderately high temperature (25 degrees C) on these two endocrine axes. In constant gonado-inhibiting conditions (16L:8D), whether the temperature was kept constantly high or allowed to fluctuate naturally, no reactivation of the thyroid and testicular axes was observed in the fall. In control animals, the peaks of testosterone and thyroxine concentrations observed in September were larger (P < 0.001) than those in animals subjected to the same natural photoperiod conditions but with constantly high temperature. The lower level of autumnal testis stimulation (P < 0.001) in animals maintained in conditions of constant high temperature (25 degrees C) may be attributed to the low thyroxine levels induced by high temperature. These results clearly confirm that the thyroid and testicular cycles display similar seasonal variations and show that the thyroid and gonadal axes are strictly interdependent. This study provides the first demonstration, for a given species, that the seasonal reactivation of gonad activity is controlled by the thyroid, and thyroid activity is controlled by the gonads.

Temporal requirements of thyroid hormones for seasonal changes in LH secretion

The transition between breeding and anestrous seasons in ewes is driven by an endogenous rhythm in responsiveness to estradiol negative feedback. One stage of this rhythm, the transition to anestrus, requires the presence of thyroid hormone during a window of responsiveness that opens in the late breeding season. The primary goal of this study was to assess when ewes lose responsiveness to thyroid hormone (i.e. when the window closes). In addition, we investigated whether thyroid hormone influences aspects of seasonality other than the transition to anestrus. Ovariectomized ewes maintained in a simulated natural photoperiod were implanted with estradiol, thyroidectomized, and treated with T(4) for 100 d beginning at progressively later dates during the anestrous season. Onset of neuroendocrine anestrus (decrease in LH), latency to anestrus, and time of onset of the subsequent neuroendocrine breeding season (rise in LH) were determined. Ewes gradually lost responsiveness to T(4) during the latter half of the anestrous season, as judged by increasing latency to the decrease in LH and, eventually, failure to exhibit a decrease in LH. Progressively later T(4) replacements also caused progressive delays in the subsequent breeding season. In contrast, the annual PRL cycle was not significantly affected by thyroidectomy or T(4) replacement. These findings indicate that 1) responsiveness to T(4) is lost gradually during the mid to late anestrous season; 2) thyroid hormones can influence the timing of the breeding season and thus may be required for the maintenance or entrainment of the endogenous reproductive rhythm; 3) thyroid hormones are not required for all seasonal neuroendocrine cycles.

Thyroid hormones mediate steroid-independent seasonal changes in luteinizing hormone pulsatility in the ewe

Thyroid hormones permit the increase in response to estradiol negative feedback in ewes at the transition to anestrus. In this study, we tested whether the thyroid hormones are also required for steroid-independent seasonal changes in pulsatile LH secretion. In experiment 1, Suffolk ewes were ovariectomized and thyroidectomized (THX) or ovariectomized only (controls) in late November. LH pulse frequency and amplitude were measured for 4 h in December, April, May, June, and August. Pulse frequency was also measured in the presence of estradiol-containing implants during the breeding (December) and early anestrus (March) seasons. As expected, in the presence of estradiol, pulse frequency declined between December and March in control but not THX ewes. In the absence of estradiol, a seasonal decline in frequency and an increase in amplitude occurred in control ewes, concurrent with lengthening photoperiod. A similar trend was seen in THX ewes, but the seasonal changes were lower in magnitude and not significant. In experiment 2, the same protocol was used (pulse measurements in December, May, and June) with a larger THX group size (n = 7). Results were similar to those of experiment 1 for controls. In THX ewes, pulse frequency did not change over time and was significantly elevated relative to that of controls during the summer. Pulse amplitude in THX ewes tended to increase during summer and did not differ from pulse amplitudes in control ewes. These results demonstrate that thyroid hormones are required for steroid-independent cycles in LH pulse frequency; however, some seasonal changes in amplitude still occur in the absence of thyroid hormones. This finding contrasts with the changes in estradiol negative feedback at the transition to anestrus, which are entirely thyroid hormone dependent.

The relationship between thyroxine, oestradiol, and postnatal alopecia, with relevance to women's health in general

Post-partum hair loss is possibly due to a reduction in the levels of oestradiol and thyroxine postnatally. Alopecia and/or a persistent loss of hair condition postnatally is associated with a group of symptoms (a syndrome), wherein postnatal depression is significant, as a result of physiologically inadequate levels of thyroxine (T4) and oestradiol (E2), secondary to physiological postnatal anterior pituitary dysfunction. Using this hypothesis, the author began to apply the same hypothesis to other female patients, who were not postpartum, but with similar symptomatology. The author became aware of the necessity for an adequate level of T4 to be present for correct oestrogenization to occur. He then goes on to hypothesize on the synergistic relationship that T4 and oestradiol may have in premenstrual syndrome (PMS), infertility, dysfunctional uterine bleeding, poor placental function, osteoporosis, and anorexia nervosa. He also discusses the role lowering T4 could play in the treatment of terminal cancer breast in premenopausal women.