Cyclic AMP and protein kinase A rhythmicity in the mammalian suprachiasmatic nuclei

Neural function of Bmal1: an overview

Bmal1 (Brain and muscle arnt-like, or Arntl) is a bHLH/PAS domain transcription factor central to the transcription/translation feedback loop of the biologic clock. Although Bmal1 is well-established as a major regulator of circadian rhythm, a growing number of studies in recent years have shown that dysfunction of Bmal1 underlies a variety of psychiatric, neurodegenerative-like, and endocrine metabolism-related disorders, as well as potential oncogenic roles. In this review, we systematically summarized Bmal1 expression in different brain regions, its neurological functions related or not to circadian rhythm and biological clock, and pathological phenotypes arising from Bmal1 knockout. This review also discusses oscillation and rhythmicity, especially in the suprachiasmatic nucleus, and provides perspective on future progress in Bmal1 research.

Melanopsins: Localization and Phototransduction in Xenopus laevis Melanophores

Xenopus laevis melanophores express two melanopsins, Opn4x and Opn4m. We identified Opn4x immunoreactivity throughout the melanophore cytoplasm and in the cell membrane. The strongest immunopositivity for Opn4m was observed in the nuclear region, and no labeling was seen in the cell membrane. This immunodistribution suggests Opn4x as the functional photopigment. In X. laevis melanophores, light triggers pigment dispersion and clock gene induction at blue wavelength, which maximally activates melanopsins. Although light stimulation activates phospholipase C and increases intracellular calcium and cGMP, this nucleotide does not participate in photo-induced melanin dispersion. Nevertheless, the guanylyl cyclase activator YC-1 stimulates Per1 expression, similar to blue light pulse, and the use of pharmacological inhibitors indicates the participation of the phosphoinositide cascade. Since cAMP levels does not change after blue light stimulation, the cAMP/PKA pathway most probably is not involved in blue light induction of Per in X. laevis melanophores. Given the localization of melanopsins and our pharmacological data, the light-induced clock gene expression seems to be mediated by Opn4x through phosphoinositide cascade and rise in cGMP, thus leading to the reset of the biological clock in our model.

From blue light to clock genes in zebrafish ZEM-2S cells

Melanopsin has been implicated in the mammalian photoentrainment by blue light. This photopigment, which maximally absorbs light at wavelengths between 470 and 480 nm depending on the species, is found in the retina of all classes of vertebrates so far studied. In mammals, melanopsin activation triggers a signaling pathway which resets the circadian clock in the suprachiasmatic nucleus (SCN). Unlike mammals, Drosophila melanogaster and Danio rerio do not rely only on their eyes to perceive light, in fact their whole body may be capable of detecting light and entraining their circadian clock. Melanopsin, teleost multiple tissue (tmt) opsin and others such as neuropsin and va-opsin, are found in the peripheral tissues of Danio rerio, however, there are limited data concerning the photopigment/s or the signaling pathway/s directly involved in light detection. Here, we demonstrate that melanopsin is a strong candidate to mediate synchronization of zebrafish cells. The deduced amino acid sequence of melanopsin, although being a vertebrate opsin, is more similar to invertebrate than vertebrate photopigments, and melanopsin photostimulation triggers the phosphoinositide pathway through activation of a G(q/11)-type G protein. We stimulated cultured ZEM-2S cells with blue light at wavelengths consistent with melanopsin maximal absorption, and evaluated the time course expression of per1b, cry1b, per2 and cry1a. Using quantitative PCR, we showed that blue light is capable of slightly modulating per1b and cry1b genes, and drastically increasing per2 and cry1a expression. Pharmacological assays indicated that per2 and cry1a responses to blue light are evoked through the activation of the phosphoinositide pathway, which crosstalks with nitric oxide (NO) and mitogen activated protein MAP kinase (MAPK) to activate the clock genes. Our results suggest that melanopsin may be important in mediating the photoresponse in Danio rerio ZEM-2S cells, and provide new insights about the modulation of clock genes in peripheral clocks.

Linking neural activity and molecular oscillations in the SCN

Neurons in the suprachiasmatic nucleus (SCN) function as part of a central timing circuit that drives daily changes in our behaviour and underlying physiology. A hallmark feature of SCN neuronal populations is that they are mostly electrically silent during the night, start to fire action potentials near dawn and then continue to generate action potentials with a slow and steady pace all day long. Sets of currents are responsible for this daily rhythm, with the strongest evidence for persistent Na(+) currents, L-type Ca(2+) currents, hyperpolarization-activated currents (I(H)), large-conductance Ca(2+) activated K(+) (BK) currents and fast delayed rectifier (FDR) K(+) currents. These rhythms in electrical activity are crucial for the function of the circadian timing system, including the expression of clock genes, and decline with ageing and disease. This article reviews our current understanding of the ionic and molecular mechanisms that drive the rhythmic firing patterns in the SCN.

Physiology of circadian entrainment

Mammalian circadian rhythms are controlled by endogenous biological oscillators, including a master clock located in the hypothalamic suprachiasmatic nuclei (SCN). Since the period of this oscillation is of approximately 24 h, to keep synchrony with the environment, circadian rhythms need to be entrained daily by means of Zeitgeber ("time giver") signals, such as the light-dark cycle. Recent advances in the neurophysiology and molecular biology of circadian rhythmicity allow a better understanding of synchronization. In this review we cover several aspects of the mechanisms for photic entrainment of mammalian circadian rhythms, including retinal sensitivity to light by means of novel photopigments as well as circadian variations in the retina that contribute to the regulation of retinal physiology. Downstream from the retina, we examine retinohypothalamic communication through neurotransmitter (glutamate, aspartate, pituitary adenylate cyclase-activating polypeptide) interaction with SCN receptors and the resulting signal transduction pathways in suprachiasmatic neurons, as well as putative neuron-glia interactions. Finally, we describe and analyze clock gene expression and its importance in entrainment mechanisms, as well as circadian disorders or retinal diseases related to entrainment deficits, including experimental and clinical treatments.

Tóth Z, House SB, Gainer H. Depolarization and neurotransmitter regulation of vasopressin gene expression in the rat suprachiasmatic nucleus in vitro

Vasopressin (VP) transcription in the rat suprachiasmatic nucleus (SCN) in organotypic culture was studied by in situ hybridization histochemistry using an intron-specific VP heteronuclear RNA probe. The circadian peak of VP gene transcription in the SCN in vitro is completely blocked by a 2 h exposure to tetrodotoxin (TTX) in the culture medium, and this TTX inhibition of VP gene transcription is reversed by exposure of the SCN to either forskolin or potassium depolarization. This suggests that an intrinsic, spontaneously active neuronal mechanism in the SCN is responsible for the cAMP- and depolarization-dependent pathways involved in maintaining peak VP gene transcription. In this paper, we evaluate a variety of neurotransmitter candidates, membrane receptors, and signal-transduction cascades that might constitute the mechanisms responsible for the peak of VP gene transcription. We find that vasoactive intestinal peptide (VIP) and a VPAC2 (VIP receptor subtype 2) receptor-specific agonist, Ro-25-1553, are the most effective ligands tested in evoking a cAMP-mitogen-activated protein kinase signal transduction cascade leading to an increase in VP gene transcription in the SCN. In addition, a second independent pathway involving depolarization activating L-type voltage-gated calcium channels and a Ca-dependent kinase pathway [inhibited by KN62 (1-[N,O-bis(5-isoquinolinesulphonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazine)] rescues VP gene transcription in the presence of TTX. In the absence of TTX, these independent pathways appear to act in a cooperative manner to generate the circadian peak of VP gene transcription in the SCN.

Regulation of basal rhythmicity in protein kinase C activity by melatonin in immortalized rat suprachiasmatic nucleus cells

Melatonin phase shifts circadian rhythms of the neuronal firing rate and stimulates PKC activity at dusk (CT 10) and dawn (CT 23) in the rat suprachiasmatic nucleus (SCN) slice via activation of the MT(2) melatonin receptor. We demonstrated that in the SCN2.2 cells basal PKC activity follows a rhythmic oscillation with an acrophase during the subjective dark phase (CT 14-CT 22) and nadirs during the subjective light phase at CT 2 and CT 10. Melatonin (0.01-10 nM, 10 min) significantly doubled basal PKC activity at CT 2 and CT 10, and decreased basal PKC activity at CT 6. We conclude that melatonin regulates the basal rhythm in PKC activity generated in SCN2.2 cells at the same periods of sensitivity observed in the native SCN.

Rhythmicity of the cGMP-related signal transduction pathway in the mammalian circadian system

Entrainment of mammalian circadian rhythms requires the activation of specific signal transduction pathways in the suprachiasmatic nuclei (SCN). Pharmacological inhibition of kinases such as cGMP-dependent kinase (PKG) or Ca2+/calmodulin-dependent kinase, but not cAMP-dependent kinase, blocks the circadian responses to light in vivo. Here we show a diurnal and circadian rhythm of cGMP levels and PKG activity in the hamster SCN, with maximal values during the day or subjective day. This rhythm depends on phosphodiesterase but not on guanylyl cyclase activity. Five-minute light pulses increased cGMP levels at the end of the subjective night [circadian time 18 (CT18)], but not at CT13.5. Western blot analysis indicated that the PKG II isoform is the one present in the SCN. Inhibition of PKG or guanylyl cyclase in vivo significantly attenuated light-induced phase shifts at CT18 (after 5-min light pulses) but did not affect c-Fos expression in the SCN. These results suggest that cGMP and PKG are related to SCN responses to light and undergo diurnal and circadian changes.

Involvement of calcium-calmodulin protein kinase but not mitogen-activated protein kinase in light-induced phase delays and Per gene expression in the suprachiasmatic nucleus of the hamster

It is known that Ca(2+)-dependent phosphorylation of cAMP response element binding protein (CREB) and the rapid induction of mPer1 and mPer2, mouse period genes in the suprachiasmatic nucleus (SCN) are associated with light-induced phase shifting. The CREB/CRE transcriptional pathway has been shown to be activated by calcium/calmodulin dependent kinase II (CaMKII) and mitogen-activated protein kinase (MAPK); however, there is a lack of evidence concerning whether the activation of CaMKII and/or MAPK elicited by photic stimuli are associated with the change in Per gene expression and behavioral phase shifting. In this experiment, we found there was an inhibitory effect by KN93, CaMKII inhibitor, on hamster Per1 and Per2 expression in the SCN and on phase delays in wheel running rhythm induced by light pulses. PD98059 and U0126, MAPK kinase inhibitors, however, affected neither light-induced Per1 and Per2 expression nor behavioral phase delays, even though PD98059 attenuated the light-induced phosphorylation of MAPK in the SCN. The present findings demonstrate that the light-induced activation of CaMKII plays an important role in the induction of Per1 and Per2 mRNA in the hamster SCN as well as phase shifting. These results suggest that gated induction of Per1 and/or Per2 genes through CaMKII-CREB/CRE accompanied with photic stimuli may be a critical step in phase shifting.