Expression of Tgfα in the suprachiasmatic nuclei of nocturnal and diurnal rodents

Effects of photoperiod and diet on BDNF daily rhythms in diurnal sand rats

Brain-derived neurotrophic factor (BDNF), its receptors and epigenetic modulators, are implicated in the pathophysiology of affective disorders, T2DM and the circadian system function. We used diurnal sand rats, which develop type 2 diabetes (T2DM), anxiety and depressive-like behavior under laboratory conditions. The development of these disorders is accelerated when animals are maintained under short photoperiod (5:19L:D, SP) compared to neutral photoperiod (12:12L:D, NP). We compared rhythms in plasma BDNF as well as BDNF and PER2 expression in the frontal cortex and suprachiasmatic nucleus (SCN) of sand rats acclimated to SP and NP. Acclimation to SP resulted in higher insulin levels, significantly higher glucose levels in the glucose tolerance test, and significantly higher anxiety- and depression-like behaviors compared with animals acclimated to NP. NP Animals exhibited a significant daily rhythm in plasma BDNF levels with higher levels during the night, and in BDNF expression levels in the frontal cortex and SCN. No significant BDNF rhythm was found in the plasma, frontal cortex or SCN of SP acclimated animals. We propose that in sand rats, BDNF may, at least in part, mediate the effects of circadian disruption on the development of anxiety and depressive-like behavior and T2DM.

Differential response of diurnal and nocturnal mammals to prolonged altered light-dark cycle: a possible role of mood associated endocrine, inflammatory and antioxidant system

The circadian system maintains internal 24 h oscillation of behavior and physiology, and its misalignment with external light-dark (LD) cycle results in negative health outcomes. In order to elucidate the effect of prolonged constant condition and the differences in the response between nocturnal and diurnal species, we studied the effects of constant light (LL) and constant darkness (DD) on a diurnal (squirrel) and a nocturnal (mouse) rodent species, focusing on the endocrine, inflammatory and antioxidant systems associated with depression-like behavior. Squirrels and mice (n = 10/group) were placed in chronocubicle under 12:12 h LD cycle, LL and DD. After 4 weeks, animals were subjected to sucrose preference test and blood and brain tissues were collected for measuring melatonin, corticosterone, proinflammatory cytokine, tumor necrosis factor-α (TNF-α) and the activity of primary antioxidant enzymes, catalase (CAT) and superoxide dismutase (SOD). The results show that in diurnal squirrels, prolonged constant darkness reduced sucrose preference, CAT, and SOD, increased corticosterone and TNF-α levels, but caused no significant change in the melatonin compared to LD condition. In contrast, in nocturnal mice constant darkness caused no significant changes in sucrose preference and corticosterone levels, increased melatonin, CAT and SOD levels but decreased TNF-α levels. Chronic LL caused a similar response in both squirrels and mice: it decreased sucrose preference, melatonin, CAT and SOD levels but increased corticosterone and TNF-α levels. Together, the study demonstrates differential effects of altered light-dark cycle in a diurnal and a nocturnal rodent on interrelated endocrine, inflammatory and antioxidant systems associated with depression-like behavior, with constant light having adverse effects on both species but constant darkness having a negative effect mainly in the diurnal squirrels.

Circadian and photic modulation of daily rhythms in diurnal mammals

The temporal niche that an animal occupies includes a coordinated suite of behavioral and physiological processes that set diurnal and nocturnal animals apart. The daily rhythms of the two chronotypes are regulated by both the circadian system and direct responses to light, a process called masking. Here we review the literature on circadian regulations and masking responses in diurnal mammals, focusing on our work using the diurnal Nile grass rat (Arvicanthis niloticus) and comparing our findings with those derived from other diurnal and nocturnal models. There are certainly similarities between the circadian systems of diurnal and nocturnal mammals, especially in the phase and functioning of the principal circadian oscillator within the hypothalamic suprachiasmatic nucleus (SCN). However, the downstream pathways, direct or indirect from the SCN, lead to drastic differences in the phase of extra-SCN oscillators, with most showing a complete reversal from the phase seen in nocturnal species. This reversal, however, is not universal and in some cases the phases of extra-SCN oscillators are only a few hours apart between diurnal and nocturnal species. The behavioral masking responses in general are opposite between diurnal and nocturnal species, and are matched by differential responses to light and dark in several retinorecipient sites in their brain. The available anatomical and functional data suggest that diurnal brains are not simply a phase-reversed version of nocturnal ones, and work with diurnal models contribute significantly to a better understanding of the circadian and photic modulation of daily rhythms in our own diurnal species.

Medicine in the Fourth Dimension

The importance of circadian biology has rarely been considered in pre-clinical studies, and even more when translating to the bedside. Circadian biology is becoming a critical factor for improving drug efficacy and diminishing drug toxicity. Indeed, there is emerging evidence showing that some drugs are more effective at nighttime than daytime, whereas for others it is the opposite. This suggests that the biology of the target cell will determine how an organ will respond to a drug at a specific time of the day, thus modulating pharmacodynamics. Thus, it is now time that circadian factors become an integral part of translational research.

Rai1 frees mice from the repression of active wake behaviors by light

Besides its role in vision, light impacts physiology and behavior through circadian and direct (aka ‘masking’) mechanisms. In Smith-Magenis syndrome (SMS), the dysregulation of both sleep-wake behavior and melatonin production strongly suggests impaired non-visual light perception. We discovered that mice haploinsufficient for the SMS causal gene, Retinoic acid induced-1 (Rai1), were hypersensitive to light such that light eliminated alert and active-wake behaviors, while leaving time-spent-awake unaffected. Moreover, variables pertaining to circadian rhythm entrainment were activated more strongly by light. At the input level, the activation of rod/cone and suprachiasmatic nuclei (SCN) by light was paradoxically greatly reduced, while the downstream activation of the ventral-subparaventricular zone (vSPVZ) was increased. The vSPVZ integrates retinal and SCN input and, when activated, suppresses locomotor activity, consistent with the behavioral hypersensitivity to light we observed. Our results implicate Rai1 as a novel and central player in processing non-visual light information, from input to behavioral output.

DOI:http://dx.doi.org/10.7554/eLife.23292.001

The Mammalian Circadian Timing System: Organization and Coordination of Central and Peripheral Clocks

Most physiology and behavior of mammalian organisms follow daily oscillations. These rhythmic processes are governed by environmental cues (e.g., fluctuations in light intensity and temperature), an internal circadian timing system, and the interaction between this timekeeping system and environmental signals. In mammals, the circadian timekeeping system has a complex architecture, composed of a central pacemaker in the brain's suprachiasmatic nuclei (SCN) and subsidiary clocks in nearly every body cell. The central clock is synchronized to geophysical time mainly via photic cues perceived by the retina and transmitted by electrical signals to SCN neurons. In turn, the SCN influences circadian physiology and behavior via neuronal and humoral cues and via the synchronization of local oscillators that are operative in the cells of most organs and tissues. Thus, some of the SCN output pathways serve as input pathways for peripheral tissues. Here we discuss knowledge acquired during the past few years on the complex structure and function of the mammalian circadian timing system.

The substructure of the suprachiasmatic nucleus: Similarities between nocturnal and diurnal spiny mice

Evolutionary transitions between nocturnal and diurnal patterns of adaptation to the day-night cycle must have involved fundamental changes in the neural mechanisms that coordinate the daily patterning of activity, but little is known about how these mechanisms differ. One reason is that information on these systems in very closely related diurnal and nocturnal species is lacking. In this study, we characterize the suprachiasmatic nucleus (SCN), the primary brain structure involved in the generation and coordination of circadian rhythms, in two members of the genus Acomys with very different activity patterns, Acomys russatus (the golden spiny mouse, diurnal) and Acomys cahirinus (the common spiny mouse, nocturnal). Immunohistochemical techniques were used to label cell bodies containing vasoactive intestinal polypeptide (VIP), vasopressin (VP), gastrin-releasing peptide (GRP) and calbindin (CalB) in the SCN, as well as two sets of inputs to it, those containing serotonin (5-HT) and neuropeptide Y (NPY), respectively. All were present in the SCN of both species and no differences between them were seen. On the basis of neuronal phenotype, the SCN was organized into three basic regions that contained VIP-immunoreactive (-ir), CalB-ir and VP-ir cells, in the ventral, middle and dorsal SCN, respectively. In the rostral SCN, GRP-ir cells were in both the VIP and the CalB cell regions, and in the caudal area they were distributed across a portion of each of the other three regions. Fibers containing NPY-ir and serotonin (5-HT)-ir were most concentrated in the areas containing VIP-ir and CalB-ir cells, respectively. The details of the spatial relationships among the labeled cells and fibers seen here are discussed in relation to different approaches investigators have taken to characterize the SCN more generally.

Behavioral effects of systemic transforming growth factor-alpha in Syrian hamsters

The growth factor, transforming growth factor-alpha (TGF-alpha) is strongly expressed in the hypothalamic circadian pacemaker, the suprachiasmatic nucleus (SCN). TGF-alpha is one of several SCN peptides recently suggested to function as a circadian output signal for the regulation of locomotor activity rhythms in nocturnal rodents. When infused in the brain, TGF-alpha suppresses activity. TGF-alpha suppresses other behaviors as well including feeding, resulting in weight loss. Elevated TGF-alpha is correlated with some cancers, and it is possible the TGF-alpha and its receptor, the epidermal growth factor receptor (EGFR), mediate fatigue and weight loss associated with cancer. If true for cancers outside of the brain, then systemic TGF-alpha should also affect behavior. We tested this hypothesis in hamsters with intraperitoneal injections or week-long subcutaneous infusions of TGF-alpha. Both treatments suppressed activity and infusions caused reduced food consumption and weight loss. To identify areas of the brain that might mediate these effects of systemic TGF-alpha, we used immunohistochemistry to localize cells with an activated MAP kinase signaling pathway (phosphorylated ERK1). Cells were activated in two hypothalamic areas, the paraventricular nucleus and a narrow region surrounding the third ventricle. These sites could not only be targets of TGF-alpha produced in the SCN but could also mediate effects of elevated TGF-alpha from tumors both within and outside the central nervous system.

Roles of transforming growth factor-alpha in mammary development and disease

Transforming growth factor-alpha (TGFalpha) is a member of the epidermal growth factor (EGF) family. Expression of TGFalpha is highly regulated in response to exogenous cellular signals including cytokines and other growth factors. The growth factor has been found to be indispensable for proper development of many tissues and organs. TGFalpha has also been implicated in numerous disease states including forms of breast cancer. This minireview summarizes the basic biology of TGFalpha and its actions during normal and pathogenic development of the mammary epithelium.

Challenging the omnipotence of the suprachiasmatic timekeeper: are circadian oscillators present throughout the mammalian brain?

The suprachiasmatic nucleus of the hypothalamus (SCN) is the master circadian pacemaker or clock in the mammalian brain. Canonical theory holds that the output from this single, dominant clock is responsible for driving most daily rhythms in physiology and behaviour. However, important recent findings challenge this uniclock model and reveal clock-like activities in many neural and non-neural tissues. Thus, in addition to the SCN, a number of areas of the mammalian brain including the olfactory bulb, amygdala, lateral habenula and a variety of nuclei in the hypothalamus, express circadian rhythms in core clock gene expression, hormone output and electrical activity. This review examines the evidence for extra-SCN circadian oscillators in the mammalian brain and highlights some of the essential properties and key differences between brain oscillators. The demonstration of neural pacemakers outside the SCN has wide-ranging implications for models of the circadian system at a whole-organism level.