Adrenal peripheral oscillator in generating the circadian glucocorticoid rhythm
ABSTRACT The mammalian circadian timing system is organized into hierarchical structures with a central clock in the suprachiasmatic nucleus (SCN) and subsidiary peripheral oscillators. After the discovery of the local clockwork in peripheral organs and tissues, which have a molecular makeup similar to the central pacemaker SCN, uncovering the roles of the peripheral clock in the rhythmic physiology has been an emerging goal in chronobiology. Glucocorticoid (GC) is a multifunctional adrenal steroid hormone that shows a robust circadian rhythm. The daily GC rhythm has long been thought to be governed by the SCN via the hypothalamus-pituitary-adrenal neuroendocrine axis. Recent findings, however, indicate that multiple regulatory mechanisms, including the adrenal intrinsic mechanism by the peripheral clock, are also involved. GC regulates diverse physiological processes and acts as a signal for resetting peripheral clocks, which suggests the importance of the GC rhythm in harmonizing overall circadian physiology and behavior. Therefore, in this review, we will discuss the important role of the adrenal peripheral clockwork in the circadian regulation of GC and its physiological relevance in the circadian timing system.
SourceAvailable from: Patricia J Sollars[Show abstract] [Hide abstract]
ABSTRACT: The suprachiasmatic nucleus (SCN) is a circadian oscillator entrained to the day/night cycle via input from the retina. Serotonin (5-HT) afferents to the SCN modulate retinal signals via activation of 5-HT1B receptors, decreasing responsiveness to light. Consequently, 5-HT1B receptor knockout (KO) mice entrain to the day/night cycle with delayed activity onsets. Since circulating corticosterone levels exhibit a robust daily rhythm peaking around activity onset, we asked whether delayed entrainment of activity onsets affects rhythmic corticosterone secretion. Wheel-running activity and plasma corticosterone were monitored in mice housed under several different lighting regimens. Both duration of the light∶dark cycle (T cycle) and the duration of light within that cycle was altered. 5-HT1B KO mice that entrained to a 9.5L:13.5D (short day in a T = 23 h) cycle with activity onsets delayed more than 4 h after light offset exhibited a corticosterone rhythm in phase with activity rhythms but reduced 50% in amplitude compared to animals that initiated daily activity <4 h after light offset. Wild type mice in 8L:14D (short day in a T = 22 h) conditions with highly delayed activity onsets also exhibited a 50% reduction in peak plasma corticosterone levels. Exogenous adrenocorticotropin (ACTH) stimulation in animals exhibiting highly delayed entrainment suggested that the endogenous rhythm of adrenal responsiveness to ACTH remained aligned with SCN-driven behavioral activity. Circadian clock gene expression in the adrenal cortex of these same animals suggested that the adrenal circadian clock was also aligned with SCN-driven behavior. Under T cycles <24 h, altered circadian entrainment to short day (winter-like) conditions, manifest as long delays in activity onset after light offset, severely reduces the amplitude of the diurnal rhythm of plasma corticosterone. Such a pronounced reduction in the glucocorticoid rhythm may alter rhythmic gene expression in the central nervous system and in peripheral organs contributing to an array of potential pathophysiologies.PLoS ONE 11/2014; 9(11):e111944. DOI:10.1371/journal.pone.0111944 · 3.53 Impact Factor
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ABSTRACT: Hormone receptors are a necessary (although not sufficient) part of the process through which hormones like corticosterone create physiological responses. However, it is currently unknown to what extent receptor concentrations across different target tissues may be correlated within individual animals. In this study, we examined this question using a large dataset of radioligand binding data for glucocorticoid receptors (GR) and mineralocorticoid receptors (MR) in 13 different tissues in the house sparrow (Passer domesticus, n = 72). Our data revealed that individual house sparrows tended to exhibit higher or lower receptor binding across all tissues, which could be part of what creates the physiological and behavioral syndromes associated with different hormonal profiles. However, although statistically significant, the correlations between tissues were very weak. Thus, when each tissue was independently regressed on receptor concentrations in the other tissues, multivariate analysis revealed significant relationships only for subcutaneous fat (for GR) and whole brain, hippocampus, kidney, omental fat and subcutaneous fat (for MR). We also found significant pairwise correlations only between receptor concentrations in brain and hippocampus, and brain and kidney (both for MR). This research reveals that although there are generalized individual consistencies in GR and MR concentrations, possibly due to such factors as hormonal regulation and genetic effects, the ability of two different tissues to respond to the same hormonal signal appears to be affected by additional factors that remain to be identified.Endocrinology 02/2015; DOI:10.1210/en.2014-1811 · 4.64 Impact Factor
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ABSTRACT: Glutamate, the major excitatory amino acid, activates a wide variety of signal transduction cascades. This neurotransmitter is involved in photic entrainment of circadian rhythms, which regulate physiological and behavioral functions. The circadian clock in vertebrates is based on a transcription-translation feedback loop in which Brain and muscle aryl hydrocarbon receptor nuclear translocator (ARNT)-like protein 1 (BMAL1) acts as transcriptional activator of others clock genes. This protein is expressed in nearly all suprachiasmatic nucleus neurons, as well as in the granular layer of the cerebellum. In this context, we decided to investigate the role of glutamate in the molecular mechanisms involved in the processes of transcription/translation of BMAL1 protein. To this end, primary cultures of chick cerebellar Bergmann glial cells were stimulated with glutamatergic ligands and we found that BMAL1 levels increased in a dose- and time dependent manner. Additionally, we studied the phosphorylation of serine residues in BMAL1 under glutamate stimulation and we were able to detect an increase in the phosphorylation of this protein. The increased expression of BMAL1 is most probably the result of a stabilization of the protein after it has been phosphorylated by the cyclic AMP-dependent protein kinase and/or the Ca(2+)/diacylglycerol dependent protein kinase. The present results strongly suggest that glutamate participates in regulating BMAL1 in glial cells and that these cells might prove to be important in the control of circadian rhythms in the cerebellum.Neurochemical Research 03/2015; DOI:10.1007/s11064-015-1551-z · 2.55 Impact Factor