Seasonal changes in thyroid-gonadal interactions in the edible dormouse, Glis glis

Abstract

This study was conducted to determine changes in thyroid-gonadal interaction in the edible dormouse during the phase of the annual cycle that corresponds to the end of the breeding season (from June to September). We evaluated intra-hypothalamic luteinizing hormone-releasing hormone (LHRH) content, and plasma concentrations of luteinizing hormone (LH), testosterone, thyroid-stimulating hormone (TSH) and thyroxine (T4) in three groups of dormice: (1) controls; (2) dormice receiving sufficient T4 supplementation to maintain June levels in control animals until September, thus counteracting the seasonal reduction of T4 that normally begins in July; and (3) thyroidectomized dormice. The effect of thyroidectomy was only detectable in June, when plasma T4 concentration in the control group was maximal, and consisted of a significant decrease in plasma testosterone levels. This provides strong support for the hypothesis that thyroid function positively influences gondal function during the breeding season. The T4 supplementation resulted in a decrease in hypothalamic LHRH concentration, suggesting that an increased LHRH release led to the observed stimulated hypophyseal secretion of LH in June and September and the increased circulating testosterone levels in September. There was no detectable effect in July and August. These results show that thyroid axis activation of the hypothalamic-pituitary-gondal system is only possible during certain phases of the annual cycle, particularly evidenced here during the breeding season. They also reinforce our conclusions drawn from the thyroidectomy results. Conversely, the summer testicular regression which normally occurs after the breeding season is no longer controlled by plasma T4 levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Clinical Biochemistry and Hematology

“…The annual cycle of plasma testosterone levels in the dormouse is parallel and correlated to thyroxine concentrations (Jallageas and Assenmacher, 1983). Testosterone levels are decreased compared to reference levels in the dormouse when the thyroid gland is removed in June, at the beginning of the breeding season (Jallageas et al, 1992). Constant pattern of photoperiod and warm temperatures were found to interfere with the normal annual cycle of testosterone levels in this species (Jallageas and Assenmacher, 1986).…”

Effects of Superior Cervical Ganglionectomy and Melatonin Replacement on Intra-hypothalamic LHRH Content and Pulsatile Luteinizing Hormone Release in the Mink

“…Intra-hypothalamic content of LHRH was estimated by radioimmunoassay, from triplicate aliquots of acidic tissue extracts (Jallageas et al, 1992). The anti-LHRH antibody numbered 6, and directed against the C-terminal part of synthetic LHRH, was used (Pique et al, 1978;Gautron et al, 1991).…”

Physiological, Behavioral, and Life-History Adaptations to Environmental Fluctuations in the Edible Dormouse

“…Other, equally important facets of this cycle include seasonal growth and shrinkage of alimentary organs such as the liver, which, along with the up-and downregulation of foraging activity contribute to the seasonal cycle of body energy reserves (Bieber et al, 2011(Bieber et al, , 2017. Also, dormice do not only reproduce seasonally but undergo gonadal involution during winter (Jallageas et al, 1992), which will further add to energy savings. Since all of these responses in mammals are interconnected, in fact they have been called an adaptive syndrome (Heldmaier and Lynch, 1986), it appears that any variation in seasonality is not only due to environmental factors but also affected by the state of animals within their life cycles.…”