Disruption of the Pituitary Circadian Clock Induced by Hypothyroidism and Hyperthyroidism: Consequences on Daily Pituitary Hormone Expression Profiles.

BACKGROUND: The secretion of pituitary hormones oscillates throughout the 24-hour period, indicating that circadian clock-mediated mechanisms regulate this process in the gland. Additionally, pituitary hormone synthesis has been shown to be altered in hypo- and hyperthyroidism. Although thyroid hormones can modulate the other peripheral clocks, the interaction between thyroid hormone levels and circadian clock gene expression in the anterior pituitary has yet to be elucidated. METHODS: Male Wistar rats were divided into three groups: control, hypothyroid, and hyperthyroid. Following the experimental procedures, animals were euthanized every three hours over the course of a 24-hour period. The anterior pituitary glands were excised and processed for mRNA expression analysis by quantitative reverse transcriptase polymerase chain reaction. One- and two-way analysis of variance as well as cosinor analysis were used to evaluate the time-of-day-dependent differential expression for each gene in each experimental group and their interactions. RESULTS: Hyperthyroidism increased the mRNA expression of core clock genes and thyrotrophic embryonic factor (Tef), as well as the mesor and amplitude of brain and muscle Arnt-like protein-1 (Bmal1) and the mesor of nuclear receptor subfamily 1 (Nr1d1) group D member 1, when compared to euthyroid animals. Hypothyroidism disrupted the circadian expression pattern of Bmal1 and period circadian regulator 2 (Per2) and decreased the mesor of Nr1d1 and Tef. Furthermore, it was observed that the pituitary content of Dio2 mRNA was unaltered in hyperthyroidism but substantially elevated in hypothyroidism during the light phase. The upregulated expression was associated with an increased mesor and amplitude, along with an advanced acrophase. The gene expression of all the pituitary hormones was found to be altered in hypo- and hyperthyroidism. Moreover, prolactin (Prl) and luteinizing hormone beta subunit (Lhb) displayed circadian expression patterns in the control group, which were disrupted in both the hypo- and hyperthyroid states. CONCLUSION: Taken together, the data demonstrate that hypo- and hyperthyroidism alter circadian clock gene expression in the anterior pituitary. This suggests that triiodothyronine plays an important role in the regulation of pituitary gland homeostasis, which could ultimately influence the rhythmic synthesis and/or secretion of all the anterior pituitary hormones.

Early appearance of nonvisual and circadian markers in the developing inner retinal cells of chicken.

The retina is a key component of the vertebrate circadian system; it is responsible for detecting and transmitting the environmental illumination conditions (day/night cycles) to the brain that synchronize the circadian clock located in the suprachiasmatic nucleus (SCN). For this, retinal ganglion cells (RGCs) project to the SCN and other nonvisual areas. In the chicken, intrinsically photosensitive RGCs (ipRGCs) expressing the photopigment melanopsin (Opn4) transmit photic information and regulate diverse nonvisual tasks. In nonmammalian vertebrates, two genes encode Opn4: the Xenopus (Opn4x) and the mammalian (Opn4m) orthologs. RGCs express both Opn4 genes but are not the only inner retinal cells expressing Opn4x: horizontal cells (HCs) also do so. Here, we further characterize primary cultures of both populations of inner retinal cells (RGCs and HCs) expressing Opn4x. The expression of this nonvisual photopigment, as well as that for different circadian markers such as the clock genes Bmal1, Clock, Per2, and Cry1, and the key melatonin synthesizing enzyme, arylalkylamine N-acetyltransferase (AA-NAT), appears very early in development in both cell populations. The results clearly suggest that nonvisual Opn4 photoreceptors and endogenous clocks converge all together in these inner retinal cells at early developmental stages.

Daily patterns of clock and cognition-related factors are modified in the hippocampus of vitamin A-deficient rats.

The circadian expression of clock and clock-controlled cognition-related genes in the hippocampus would be essential to achieve an optimal daily cognitive performance. There is some evidence that retinoid nuclear receptors (RARs and RXRs) can regulate circadian gene expression in different tissues. In this study, Holtzman male rats from control and vitamin A-deficient groups were sacrificed throughout a 24-h period and hippocampus samples were isolated every 4 or 5 h. RARα and RXRß expression level was quantified and daily expression patterns of clock BMAL1, PER1, RORα, and REVERB genes, RORα and REVERB proteins, as well as temporal expression of cognition-related RC3 and BDNF genes were determined in the hippocampus of the two groups of rats. Our results show significant daily variations of BMAL1, PER1, RORα, and REVERB genes, RORα and REVERB proteins and, consequently, daily oscillating expression of RC3 and BDNF genes in the rat hippocampus. Vitamin A deficiency reduced RXRß mRNA level as well as the amplitude of PER1, REVERB gene, and REVERB protein rhythms, and phase-shifted the daily peaks of BMAL1 and RORα mRNA, RORα protein, and RC3 and BDNF mRNA levels. Thus, nutritional factors, such as vitamin A and its derivatives the retinoids, might modulate daily patterns of BDNF and RC3 expression in the hippocampus, and they could be essential to maintain an optimal daily performance at molecular level in this learning-and-memory-related brain area.