Circadian clock mechanism driving mammalian photoperiodism.

The annual photoperiod cycle provides the critical environmental cue synchronizing rhythms of life in seasonal habitats. In 1936, Bünning proposed a circadian-basis for photoperiodic synchronization. Here, light-dark cycles entrain a circadian rhythm of photosensitivity, and the expression of summer or winter biology depends on whether light coincides with the phase of high photosensitivity. Formal studies support the universality of this so-called coincidence timer, but we lack understanding of the mechanisms involved. Here we show in mammals that coincidence timing takes place in the pars tuberalis of the pituitary, through a melatonin-dependent flip-flop switch between circadian transcriptional activation and repression. Long photoperiods produce short night-time melatonin signals, leading to induction of the circadian transcription factor BMAL2, in turn triggering summer biology through the eyes absent / thyrotrophin (EYA3 / TSH) pathway. Conversely, short photoperiods produce long melatonin signals, inducing circadian repressors including DEC1, in turn suppressing BMAL2 and the EYA3/TSH pathway, triggering winter biology. These actions are associated with progressive genome-wide changes in chromatin state, elaborating the effect of the circadian coincidence timer. Hence, circadian clock interactions with pituitary epigenetic pathways form the basis of the mammalian coincidence timer mechanism. Our results constitute a blueprint for circadian-based seasonal timekeeping in vertebrates

Radiations and male fertility

During recent years, an increasing percentage of male infertility has to be attributed to an array of environmental, health and lifestyle factors. Male infertility is likely to be affected by the intense exposure to heat and extreme exposure to pesticides, radiations, radioactivity and other hazardous substances. We are surrounded by several types of ionizing and non-ionizing radiations and both have recognized causative effects on spermatogenesis. Since it is impossible to cover all types of radiation sources and their biological effects under a single title, this review is focusing on radiation deriving from cell phones, laptops, Wi-Fi and microwave ovens, as these are the most common sources of non-ionizing radiations, which may contribute to the cause of infertility by exploring the effect of exposure to radiofrequency radiations on the male fertility pattern. From currently available studies it is clear that radiofrequency electromagnetic fields (RF-EMF) have deleterious effects on sperm parameters (like sperm count, morphology, motility), affects the role of kinases in cellular metabolism and the endocrine system, and produces genotoxicity, genomic instability and oxidative stress. This is followed with protective measures for these radiations and future recommendations. The study concludes that the RF-EMF may induce oxidative stress with an increased level of reactive oxygen species, which may lead to infertility. This has been concluded based on available evidences from in vitro and in vivo studies suggesting that RF-EMF exposure negatively affects sperm quality

Circadian organization of tau mutant hamsters: Aftereffects and splitting

Homozygous tau mutant (τss) hamsters show an extremely short (20 h) circadian period (τ) that is attributable to altered enzymatic activity of casein kinase 1ε. It has been proposed that coupling of constituent circadian oscillators is strengthened in τss hamsters, explaining their tendency to show strong resetting after prolonged exposure to constant darkness. To evaluate further the circadian organization of τss hamsters, the authors assessed the extent of shortening of period as an aftereffect of exposure to light:dark cycles whose period (T) is 91% of τ and the ability of constant light to induce splitting. They find that τss hamsters show aftereffects comparable to wild types, indicating that normal CK1ε activity is not required for T cycles to shorten τ. This finding also contradicts the proposal that circadian period is homeostatically conserved. However, the authors find that τss hamsters rarely show splitting in constant light. Furthermore, LL does not induce lengthening of τ or reduction of activity duration (α) in these mutants. The authors\u27 findings support the conclusion that the τ mutation alters the coupling between constituent circadian oscillators

Period gene expression in mouse endocrine tissues

Circadian rhythms are generated by the oscillating expression of the Per1 and Per2 genes, which are expressed not only in the central brain pacemaker but also in peripheral tissues. Hormones are likely to coordinate physiological function in time. We performed in situ hybridization to localize mPer1 and mPer2 mRNA to particular cell types and tissue compartments in adrenal, thyroid, and testis. BALB/c mice maintained in a 12:12-h light-dark cycle expressed mPer1 in adrenal medulla, particularly in late afternoon and early night. mPer2 mRNA was more intensely expressed in adrenal cortex, especially in afternoon and evening. mPer1 mRNA was detected in thyroid. mPer1 was found in some but not all seminiferous tubules of each mouse at all times of day. Quantitation in C57BL/6 mice revealed a significant increase in the number of heavily labeled seminiferous tubules early in the night. Consistent with in situ hybridization, immunocytochemistry showed PER1 protein in spermatocytes and spermatids (spermatogenic stages VII-XII). Staining in spermatogonia and interstitial cells was inconsistent. Double labeling with 5′-bromodeoxyuridine showed PER1 expression first occurring 5 days after DNA replication. We conclude that mPeriod genes are expressed in peripheral endocrine glands. Central regulation, adenohypophyseal control, and functional importance of expression and phase remain to be elucidated. </jats:p

Suprachiasmatic regulation of circadian rhythms of gene expression in hamster peripheral organs: Effects of transplanting the pacemaker

Neurotransplantation of the suprachiasmatic nucleus (SCN) was used to assess communication between the central circadian pacemaker and peripheral oscillators in Syrian hamsters. Free-running rhythms of haPer1, haPer2, and Bmal1 expression were documented in liver, kidney, spleen, heart, skeletal muscle, and adrenal medulla after 3 d or 11 weeks of exposure to constant darkness. Ablation of the SCN of heterozygote tau mutants eliminated not only rhythms of locomotor activity but also rhythmic expression of these genes in all peripheral organs studied. The Per:Bmal ratio suggests that this effect was attributable not to asynchronous rhythmicity between SCN-lesioned individuals but to arrhythmicity within individuals. Grafts of wild-type SCN to heterozygous, SCN-lesioned tau mutant hamsters not only restored locomotor rhythms with the period of the donor but also led to recovery of rhythmic expression of haPer1, haPer2, and haBmal1 in liver and kidney. The phase of these rhythms most closely resembled that of intact wild-type hamsters. Rhythmic gene expression was also restored in skeletal muscle, but the phase was altered. Behaviorally effective SCN transplants failed to reinstate rhythms of clock gene expression in heart, spleen, or adrenal medulla. These findings confirm that peripheral organs differ in their response to SCN-dependent cues. Furthermore, the results indicate that conventional models of internal entrainment may need to be revised to explain control of the periphery by the pacemaker