![brigitte siberian mouse brigitte siberian mouse](https://i.ytimg.com/vi/uANVznko0eY/hqdefault.jpg)
001), and this divergence was amplified by PND32 (t 17 = 8.3, P <. 20), but by PND25, testes of SD hamsters weighed significantly less than those of LD hamsters (t 17 = 6.4, P <. Mean ± SEM (A) paired testes mass, (B) hypothalamic dio2 mRNA expression (relative to 18S rRNA), and (C) hypothalamic dio3 mRNA expression of adult male Siberian hamsters that were housed in either LD (15 h light/d) or SD (9 h light/d) photoperiods for 6 weeks. 70 Figure 1B), but dio3 expression was 25-fold greater in hypothalami of SD relative to LD hamsters (t 10 = −6.37, P <.
![brigitte siberian mouse brigitte siberian mouse](https://absolute-siberia.com/userfiles/image/sib_3.jpg)
SD treatments caused the expected gonadal regression (t 12 = 11.1, P. dio2 and dio3 mRNA responses to photoperiod in adult male hamsters Differences were only considered significant if P ≤. Where warranted by a significant F statistic, pairwise comparisons were performed using t tests. Indeed, a 4-hour extension of the photophase at weaning has rapid (. An augmented sensitivity to the effects of photoperiod and MEL on reproductive physiology evidently exists around the time of weaning ( 30). However, transfer of juveniles to SD at or before PND18 inhibits gonadal development, with effects evident in less than 7 days ( 25, 28, 29). Hamsters raised from birth in LD initiate puberty within a few days after weaning (typically on postnatal day 18 see Refs. Juvenile Siberian hamsters, unlike adults, exhibit rapid reproductive responses to changes in photoperiod. It is not known whether MEL is sufficient in this regard, nor has any study to date evaluated iodothyronine deiodinase responses to photoperiod in females. In Siberian hamsters, pineal MEL is necessary for photoperiodic regulation of dio3 ( 15). Although the dio2 response to exogenous MEL in adult Syrian hamsters is rapid, occurring in less than 24 hours, photoperiod-driven changes in dio3 in adult Siberian hamsters have been suggested to occur relatively slowly, paralleling the concomitant changes in the reproductive system ( 15, 24). Considered together, the data suggest that in male Syrian and Siberian hamsters, dio2 and dio3, respectively, are under strong photoperiodic control in the MBH. In long-day (LD) breeding Syrian ( Mesocricetus auratus) and Siberian ( Phodopus sungorus) hamsters, canonical model species for the dissection of neuroendocrine mechanisms of photoperiodic timing, short-day (SD) photoperiods (14 h light/d) rapidly up-regulate dio2 mRNA, and SD-like MEL treatments inhibit dio2 ( 22, 23). In mammals, changes in photoperiod are internalized via entrainment of the nocturnal circadian rhythm of melatonin (MEL) synthesis and release ( 4– 7). Many temperate zone species use changes in day length as a predictive cue to initiate and terminate seasonal breeding, such that offspring develop under environmentally favorable conditions ( 1– 3). SD photoperiods and MEL exert rapid, sustained, and additive effects on hypothalamic dio3 mRNA, which may play a central role in inhibiting maturation of the peripubertal hypothalamo-pituitary-gonadal axis. In LD, a single afternoon melatonin (MEL) injection on PND18 or PND20 was sufficient to increase hypothalamic dio3 mRNA, and dio3 increased in proportion to the number of successive days of MEL treatment. In contrast, dio2 mRNA rose conspicuously on PND21, independent of photoperiod, returning to basal levels thereafter. In LD males, dio3 remained low and invariant from PND18–PND32. Females exhibited comparable dio3 responses to SD. In SD males, hypothalamic dio3 mRNA was elevated 2.5-fold within 3 days (PND21) and continued to increase (>20-fold) through PND32 changes in dio3 mRNA preceded inhibition of gonadotropin (FSH) secretion and gonadal regression in SD. Hypothalamic dio2 and dio3 expression was measured via quantitative PCR in hamsters born and reared in a long-day photoperiod (15L:9D) and weaned on PND18 into short-day photoperiods (9L:15D). These experiments tested the hypothesis that juvenile Siberian hamsters, which are highly responsive to photoperiod at weaning (postnatal day 18), exhibit rapid and sustained changes in hypothalamic dio3 mRNA expression during photoperiod-induced and photoperiod-inhibited puberty. In several seasonally-breeding vertebrates, DIO2 and DIO3 expression is implicated in photoperiod signal transduction in adulthood. Type 2 iodothyronine deiodinase (DIO2) and DIO3 enzymes catalyze the prohormone T 4 into biologically-active T 3 and biologically-inactive rT 3, respectively.
![brigitte siberian mouse brigitte siberian mouse](http://www.kukik.de/wp-content/uploads/2011/09/BSneu.jpg)
Production of T 3 in the mediobasal hypothalamus is critical for regulation of seasonal reproductive physiology.