Targeted mutation of the calbindin D28K gene disrupts circadian rhythmicity and entrainment

Suprachiasmatic function in a circadian period mutant: Duper alters light-induced activation of vasoactive intestinal peptide cells and PERIOD1 immunostaining

Mammalian circadian rhythms are entrained by photic stimuli that are relayed by retinal projections to the core of the suprachiasmatic nucleus (SCN). Neuronal activation, as demonstrated by expression of the immediate early gene c-fos, leads to transcription of the core clock gene per1. The duper mutation in hamsters shortens circadian period and amplifies light-induced phase shifts. We performed two experiments to compare the number of c-FOS immunoreactive (ir) and PER1-ir cells, and the intensity of staining, in the SCN of wild-type (WT) and duper hamsters at various intervals after presentation of a 15-min light pulse in the early subjective night. Light-induced c-FOS-ir within 1 hr in the dorsocaudal SCN of duper, but not WT hamsters. In cells that express vasoactive intestinal peptide (VIP), which plays a critical role in synchronization of SCN cellular oscillators, light-induced c-FOS-ir was greater in duper than WT hamsters. After the light pulse, PER1-ir cells were found in more medial portions of the SCN than FOS-ir, and appeared with a longer latency and over a longer time course, in VIP cells of duper than wild-type hamsters. Our results indicate that the duper allele alters SCN function in ways that may contribute to changes in free running period and phase resetting.

Melanopsin and calbindin immunoreactivity in the inner retina of humans and marmosets

In primate retina, the calcium-binding protein calbindin is expressed by a variety of neurons including cones, bipolar cells, and amacrine cells but it is not known which type(s) of cell express calbindin in the ganglion cell layer. The present study aimed to identify calbindin-positive cell type(s) in the amacrine and ganglion cell layer of human and marmoset retina using immunohistochemical markers for ganglion cells (RBPMS and melanopsin) and cholinergic amacrine (ChAT) cells. Intracellular injections following immunolabeling was used to reveal the morphology of calbindin-positive cells. In human retina, calbindin-labeled cells in the ganglion cell layer were identified as inner and outer stratifying melanopsin-expressing ganglion cells, and ON ChAT (starburst amacrine) cells. In marmoset, calbindin immunoreactivity in the ganglion cell layer was absent from ganglion cells but present in ON ChAT cells. In the inner nuclear layer of human retina, calbindin was found in melanopsin-expressing displaced ganglion cells and in at least two populations of amacrine cells including about a quarter of the OFF ChAT cells. In marmoset, a very low proportion of OFF ChAT cells was calbindin-positive. These results suggest that in both species there may be two types of OFF ChAT cells. Consistent with previous studies, the ratio of ON to OFF ChAT cells was about 70 to 30 in human and 30 to 70 in marmoset. Our results show that there are species-related differences between different primates with respect to the expression of calbindin.

Circuit development in the master clock network of mammals

Daily rhythms are generated by the circadian timekeeping system, which is orchestrated by the master circadian clock in the suprachiasmatic nucleus (SCN) of mammals. Circadian timekeeping is endogenous and does not require exposure to external cues during development. Nevertheless, the circadian system is not fully formed at birth in many mammalian species and it is important to understand how SCN development can affect the function of the circadian system in adulthood. The purpose of the current review is to discuss the ontogeny of cellular and circuit function in the SCN, with a focus on work performed in model rodent species (i.e., mouse, rat, and hamster). Particular emphasis is placed on the spatial and temporal patterns of SCN development that may contribute to the function of the master clock during adulthood. Additional work aimed at decoding the mechanisms that guide circadian development is expected to provide a solid foundation upon which to better understand the sources and factors contributing to aberrant maturation of clock function.

Sleep Deprivation and Caffeine Treatment Potentiate Photic Resetting of the Master Circadian Clock in a Diurnal Rodent

Circadian rhythms in nocturnal and diurnal mammals are primarily synchronized to local time by the light/dark cycle. However, nonphotic factors, such as behavioral arousal and metabolic cues, can also phase shift the master clock in the suprachiasmatic nuclei (SCNs) and/or reduce the synchronizing effects of light in nocturnal rodents. In diurnal rodents, the role of arousal or insufficient sleep in these functions is still poorly understood. In the present study, diurnal Sudanian grass rats, Arvicanthis ansorgei, were aroused at night by sleep deprivation (gentle handling) or caffeine treatment that both prevented sleep. Phase shifts of locomotor activity were analyzed in grass rats transferred from a light/dark cycle to constant darkness and aroused in early night or late night. Early night, but not late night, sleep deprivation induced a significant phase shift. Caffeine on its own induced no phase shifts. Both sleep deprivation and caffeine treatment potentiated light-induced phase delays and phase advances in response to a 30 min light pulse, respectively. Sleep deprivation in early night, but not late night, potentiated light-induced c-Fos expression in the ventral SCN. Caffeine treatment in midnight triggered c-Fos expression in dorsal SCN. Both sleep deprivation and caffeine treatment potentiated light-induced c-Fos expression in calbindin-containing cells of the ventral SCN in early and late night. These findings indicate that, in contrast to nocturnal rodents, behavioral arousal induced either by sleep deprivation or caffeine during the sleeping period potentiates light resetting of the master circadian clock in diurnal rodents, and activation of calbindin-containing suprachiasmatic cells may be involved in this effect.SIGNIFICANCE STATEMENT Arousing stimuli have the ability to regulate circadian rhythms in mammals. Behavioral arousal in the sleeping period phase shifts the master clock in the suprachiasmatic nuclei and/or slows down the photic entrainment in nocturnal animals. How these stimuli act in diurnal species remains to be established. Our study in a diurnal rodent, the Grass rat, indicates that sleep deprivation in the early rest period induces phase delays of circadian locomotor activity rhythm. Contrary to nocturnal rodents, both sleep deprivation and caffeine-induced arousal potentiate the photic entrainment in a diurnal rodent. Such enhanced light-induced circadian responses could be relevant for developing chronotherapeutic strategies.

Calbindin Knockout Alters Sex-Specific Regulation of Behavior and Gene Expression in Amygdala and Prefrontal Cortex

Calbindin-D(28K) (Calb1), a high-affinity calcium buffer/sensor, shows abundant expression in neurons and has been associated with a number of neurobehavioral diseases, many of which are sexually dimorphic in incidence. Behavioral and physiological end points are affected by experimental manipulations of calbindin levels, including disruption of spatial learning, hippocampal long-term potentiation, and circadian rhythms. In this study, we investigated novel aspects of calbindin function on social behavior, anxiety-like behavior, and fear conditioning in adult mice of both sexes by comparing wild-type to littermate Calb1 KO mice. Because Calb1 mRNA and protein are sexually dimorphic in some areas of the brain, we hypothesized that sex differences in behavioral responses of these behaviors would be eliminated or revealed in Calb1 KO mice. We also examined gene expression in the amygdala and prefrontal cortex, two areas of the brain intimately connected with limbic system control of the behaviors tested, in response to sex and genotype. Our results demonstrate that fear memory and social behavior are altered in male knockout mice, and Calb1 KO mice of both sexes show less anxiety. Moreover, gene expression studies of the amygdala and prefrontal cortex revealed several significant genotype and sex effects in genes related to brain-derived neurotrophic factor signaling, hormone receptors, histone deacetylases, and γ-aminobutyric acid signaling. Our findings are the first to directly link calbindin with affective and social behaviors in rodents; moreover, the results suggest that sex differences in calbindin protein influence behavior.

Types and density of calbindin D28k-immunoreactive ganglion cells in mouse retina

Single-cell injection after immunocytochemistry is a reliable technique for classifying neurons by their morphological structure and their expression of a particular protein. The aim of the present study was to classify the morphological types of calbindin D28k-immunoreactive retinal ganglion cells in the mouse using single-cell injection after immunocytochemistry, to estimate the density of calbindin D28k-immunoreactive retinal ganglion cells in the mouse retina. Calbindin D28k is an important calcium-binding protein that is widely expressed in the central nervous system. Calbindin D28k-immunoreactive retinal ganglion cells were identified by immunocytochemistry and then iontophoretically injected with the lipophilic dye, DiI. Subsequently, the injected cells were imaged by confocal microscopy to classify calbindin D28k-immunoreactive retinal ganglion cells based on their dendritic ramification depth within the inner plexiform layer, field size, and morphology. The cells were heterogeneous in morphology: monostratified or bistratified, with small to large dendritic field size and sparse to dense dendritic arbors. At least 10 different morphological types (CB1-CB10) of calbindin D28k-immunoreactive retinal ganglion cells were found in the mouse retina. The density of each cell type was quite variable (1.98-23.76%). The density of calbindin D28k-immunoreactive cells in the ganglion cell layer of the mouse retina was 562 cells/mm(2), 8.18% of calbindin D28k-immunoreactive cells were axon-less displaced amacrine cells, 91.82% were retinal ganglion cells, and approximately 18.17% of mouse retinal ganglion cells expressed calbindin D28k. The selective expression of calbindin D28k in cells with different morphologies may provide important data for further physiological studies of the mouse retina.

Localization of melanopsin-immunoreactive cells in the Mongolian gerbil retina

Melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) are involved in circadian rhythm and pupil responses. The purpose of this study was to reveal the organization of melanopsin-immunoreactive (IR) neurons in the Mongolian gerbil retina using immunocytochemistry. Melanopsin-IR cells were primarily located in the ganglion cell layer (GCL; M1c; 75.15%). Many melanopsin-IR cells were also observed in the inner nuclear layer (INL; M1d; 22.28%). The M1c and M1d cell types extended their dendritic processes into the OFF sublayer of the inner plexiform layer (IPL). We rarely observed bistratified cells (M3; 2.56%) with dendrites in both the ON and OFF sublayers of the IPL. Surprisingly, we did not observe M2 cells which are well observed in other rodents. Melanopsin-IR cell somas were small to medium in size and had large dendritic fields. They had 2-5 primary dendrites that branched sparingly and had varicosities. Melanopsin-IR cell density was very low: they comprised 0.50% of the total ganglion cell population. Moreover, none of the melanopsin-IR cells expressed calbindin-D28K, calretinin, or parvalbumin. These results suggest that in the Mongolian gerbil, melanopsin-IR cells are expressed in a very small RGC subpopulation, and are independent of calcium-binding proteins-containing RGCs.

Mice deficient of glutamatergic signaling from intrinsically photosensitive retinal ganglion cells exhibit abnormal circadian photoentrainment

Several aspects of behavior and physiology, such as sleep and wakefulness, blood pressure, body temperature, and hormone secretion exhibit daily oscillations known as circadian rhythms. These circadian rhythms are orchestrated by an intrinsic biological clock in the suprachiasmatic nuclei (SCN) of the hypothalamus which is adjusted to the daily environmental cycles of day and night by the process of photoentrainment. In mammals, the neuronal signal for photoentrainment arises from a small subset of intrinsically photosensitive retinal ganglion cells (ipRGCs) that send a direct projection to the SCN. ipRGCs also mediate other non-image-forming (NIF) visual responses such as negative masking of locomotor activity by light, and the pupillary light reflex (PLR) via co-release of neurotransmitters glutamate and pituitary adenylate cyclase-activating peptide (PACAP) from their synaptic terminals. The relative contribution of each neurotransmitter system for the circadian photoentrainment and other NIF visual responses is still unresolved. We investigated the role of glutamatergic neurotransmission for circadian photoentrainment and NIF behaviors by selective ablation of ipRGC glutamatergic synaptic transmission in mice. Mutant mice displayed delayed re-entrainment to a 6 h phase shift (advance or delay) in the light cycle and incomplete photoentrainment in a symmetrical skeleton photoperiod regimen (1 h light pulses between 11 h dark periods). Circadian rhythmicity in constant darkness also was reduced in some mutant mice. Other NIF responses such as the PLR and negative masking responses to light were also partially attenuated. Overall, these results suggest that glutamate from ipRGCs drives circadian photoentrainment and negative masking responses to light.

Circadian dysfunction in response to in vivo treatment with the mitochondrial toxin 3-nitropropionic acid

Sleep disorders are common in neurodegenerative diseases including Huntington's disease (HD) and develop early in the disease process. Mitochondrial alterations are believed to play a critical role in the pathophysiology of neurodegenerative diseases. In the present study, we evaluated the circadian system of mice after inhibiting mitochondrial complex II of the respiratory chain with the toxin 3-nitropropionic acid (3-NP). We found that a subset of mice treated with low doses of 3-NP exhibited severe circadian deficit in behavior. The temporal patterning of sleep behavior is also disrupted in some mice with evidence of difficulty in the initiation of sleep behavior. Using the open field test during the normal sleep phase, we found that the 3-NP-treated mice were hyperactive. The molecular clockwork responsible for the generation of circadian rhythms as measured by PER2::LUCIFERASE was disrupted in a subset of mice. Within the SCN, the 3-NP treatment resulted in a reduction in daytime firing rate in the subset of mice which had a behavioral deficit. Anatomically, we confirmed that all of the treated mice showed evidence for cell loss within the striatum but we did not see evidence for gross SCN pathology. Together, the data demonstrates that chronic treatment with low doses of the mitochondrial toxin 3-NP produced circadian deficits in a subset of treated mice. This work does raise the possibility that the neural damage produced by mitochondrial dysfunction can contribute to the sleep/circadian dysfunction seen so commonly in neurodegenerative diseases.

Dim nighttime light impairs cognition and provokes depressive-like responses in a diurnal rodent

Circadian disruption is a common by-product of modern life. Although jet lag and shift work are well-documented challenges to circadian organization, many more subtle environmental changes cause circadian disruption. For example, frequent fluctuations in the timing of the sleep/wake schedule, as well as exposure to nighttime lighting, likely affect the circadian system. Most studies of these effects have focused on nocturnal rodents, which are very different from diurnal species with respect to their patterns of light exposure and the effects that light can have on their activity. Thus, the authors investigated the effect of nighttime light on behavior and the brain of a diurnal rodent, the Nile grass rat. Following 3 weeks of exposure to standard light/dark (LD; 14:10 light [~150 lux] /dark [0 lux]) or dim light at night (dLAN; 14:10 light [~150 lux] /dim [5 lux]), rats underwent behavioral testing, and hippocampal neurons within CA1, CA3, and the dentate gyrus (DG) were examined. Three behavioral effects of dLAN were observed: (1) decreased preference for a sucrose solution, (2) increased latency to float in a forced swim test, and (3) impaired learning and memory in the Barnes maze. Light at night also reduced dendritic length in DG and basilar CA1 dendrites. Dendritic length in the DG positively correlated with sucrose consumption in the sucrose anhedonia task. Nighttime light exposure did not disrupt the pattern of circadian locomotor activity, and all grass rats maintained a diurnal activity pattern. Together, these data suggest that exposure to dLAN can alter affective responses and impair cognition in a diurnal animal.

Electrical activity can impose time of day on the circadian transcriptome of pacemaker neurons

BACKGROUND

Circadian (∼24 hr) rhythms offer one of the best examples of how gene expression is tied to behavior. Circadian pacemaker neurons contain molecular clocks that control 24 hr rhythms in gene expression that in turn regulate electrical activity rhythms to control behavior.

RESULTS

Here we demonstrate the inverse relationship: there are broad transcriptional changes in Drosophila clock neurons (LN(v)s) in response to altered electrical activity, including a large set of circadian genes. Hyperexciting LN(v)s creates a morning-like expression profile for many circadian genes while hyperpolarization leads to an evening-like transcriptional state. The electrical effects robustly persist in per(0) mutant LN(v)s but not in cyc(0) mutant LN(v)s, suggesting that neuronal activity interacts with the transcriptional activators of the core circadian clock. Bioinformatic and immunocytochemical analyses suggest that CREB family transcription factors link LN(v) electrical state to circadian gene expression.

CONCLUSIONS

The electrical state of a clock neuron can impose time of day to its transcriptional program. We propose that this acts as an internal zeitgeber to add robustness and precision to circadian behavioral rhythms.

Regional distribution of calretinin and calbindin-D28k expression in the brain of the urodele amphibian Pleurodeles waltl during embryonic and larval development

The sequence of appearance of calretinin and calbindin-D28k immunoreactive (CRir and CBir, respectively) cells and fibers has been studied in the brain of the urodele amphibian Pleurodeles waltl. Embryonic, larval and juvenile stages were studied. The early expression and the dynamics of the distribution of CBir and CRir structures have been used as markers for developmental aspects of distinct neuronal populations, highlighting the accurate extent of many regions in the developing brain, not observed on the basis of cytoarchitecture alone. CR and, to a lesser extent, CB are expressed early in the central nervous system and show a progressively increasing expression from the embryonic stages throughout the larval life and, in general, the labeled structures in the developing brain retain their ability to express these proteins in the adult brain. The onset of CRir cells primarily served to follow the development of the olfactory bulbs, subpallium, thalamus, alar hypothalamus, mesencephalic tegmentum, and distinct cell populations in the rhombencephalic reticular formation. CBir cells highlighted the development of, among others, the pallidum, hypothalamus, dorsal habenula, midbrain tegmentum, cerebellum, and central gray of the rostral rhombencephalon. However, it was the relative and mostly segregated distribution of both proteins in distinct cell populations which evidenced the developing regionalization of the brain. The results have shown the usefulness in neuroanatomy of the analysis during development of the onset of CBir and CRir structures, but the comparison with previous data has shown extensive variability across vertebrate classes. Therefore, one should be cautious when comparing possible homologue structures across species only on the basis of the expression of these proteins, due to the variation of the content of calcium-binding proteins observed in well-established homologous regions in the brain of different vertebrates.

Circadian behaviour in neuroglobin deficient mice

Neuroglobin (Ngb), a neuron-specific oxygen-binding globin with an unknown function, has been proposed to play a key role in neuronal survival. We have previously shown Ngb to be highly expressed in the rat suprachiasmatic nucleus (SCN). The present study addresses the effect of Ngb deficiency on circadian behavior. Ngb-deficient and wild-type (wt) mice were placed in running wheels and their activity rhythms, endogenous period and response to light stimuli were investigated. The effect of Ngb deficiency on the expression of Period1 (Per1) and the immediate early gene Fos was determined after light stimulation at night and the neurochemical phenotype of Ngb expressing neurons in wt mice was characterized. Loss of Ngb function had no effect on overall circadian entrainment, but resulted in a significantly larger phase delay of circadian rhythm upon light stimulation at early night. A light-induced increase in Per1, but not Fos, gene expression was observed in Ngb-deficient mice. Ngb expressing neurons which co-stored Gastrin Releasing Peptide (GRP) and were innervated from the eye and the geniculo-hypothalamic tract expressed FOS after light stimulation. No PER1 expression was observed in Ngb-positive neurons. The present study demonstrates for the first time that the genetic elimination of Ngb does not affect core clock function but evokes an increased behavioural response to light concomitant with increased Per1 gene expression in the SCN at early night.

Targeted mutation of the calbindin D 28k gene selectively alters nonvisual photosensitivity

Light intensity is an important determinant of diverse physiological and behavioral responses within the non-image-forming visual system. Thresholds differ among various photic responses, namely control of circadian rhythms, vigilance state, activity level and pupil constriction, but the mechanisms that regulate photosensitivity are not known. Calbindin D(28k) (CalB) is a calcium-binding protein associated with light processing in the mammalian circadian clock. Loss-of-function studies indicate that CalB-deficient mice (CalB(-/-)) have deficits in their ability to entrain to light-dark cycles. To explore the role of CalB in modulating photosensitivity, thresholds for three behaviors mediated by the non-image-forming visual system (entrainment, masking and pupillary light reflex; PLR) were compared in CalB(-/-) and wildtype mice, and the localization of CalB protein in these circuits was examined in adult and juvenile mice. The results reveal a divergence in how CalB affects thresholds to photic cues among these responses. Entrainment and masking were 40- to 60-fold less sensitive in CalB(-/-) than in wildtype mice. On the other hand, the PLR in CalB(-/-) mice was 80- to 200-fold more sensitive. Though CalB is expressed in the retina and in brain circuits regulating entrainment we found no CalB expression in any component of the PLR pathway, namely the olivary pretectal nucleus, Edinger-Westphal nucleus and ciliary ganglion. The behavioral and anatomical data together suggest that, in normal animals, the retinal response to light is blunted in the presence of CalB, but responsiveness of the higher order processes that transduce afferent retinal input is enhanced.

Sex differences in the cerebellum and frontal cortex: roles of estrogen receptor alpha and sex chromosome genes

Most neurobehavioral diseases are sexually dimorphic in their incidence, and sex differences in the brain may be key for understanding and treating these diseases. Calbindin (Calb) D28K is used as a biomarker for the well-studied sexually dimorphic nucleus, a hypothalamic structure that is larger in males than in females. In the current study weanling C56BL/6J mice were used to examine sex differences in the Calb protein and message focusing on regions outside of the hypothalamus. A robust sex difference was found in Calb in the frontal cortex (FC) and cerebellum (CB; specifically in Purkinje cells); mRNA and protein were higher in females than in males. Using 2 mouse lines, i.e. one with a complete deletion of estrogen receptor alpha (ERα) and the other with uncoupled gonads and sex chromosomes, we probed the mechanisms that underlie sexual dimorphisms. In the FC, deletion of ERα reduced Calb1 mRNA in females compared to males. In addition, females with XY sex chromosomes had levels of Calb1 equal to those of males. Thus, both ERα and the sex chromosome complement regulate Calb1 in the FC. In the CB, ERα knockout mice of both sexes had reduced Calb1 mRNA, yet sex differences were retained. However, the sex chromosome complement, regardless of gonadal sex, dictated Calb1 mRNA levels. Mice with XX chromosomes had significantly greater Calb1 than did XY mice. This is the first study demonstrating that sex chromosome genes are a driving force producing sex differences in the CB and FC, which are neuoranatomical regions involved in many normal functions and in neurobehavioral diseases.

Light at night increases body mass by shifting the time of food intake

The global increase in the prevalence of obesity and metabolic disorders coincides with the increase of exposure to light at night (LAN) and shift work. Circadian regulation of energy homeostasis is controlled by an endogenous biological clock that is synchronized by light information. To promote optimal adaptive functioning, the circadian clock prepares individuals for predictable events such as food availability and sleep, and disruption of clock function causes circadian and metabolic disturbances. To determine whether a causal relationship exists between nighttime light exposure and obesity, we examined the effects of LAN on body mass in male mice. Mice housed in either bright (LL) or dim (DM) LAN have significantly increased body mass and reduced glucose tolerance compared with mice in a standard (LD) light/dark cycle, despite equivalent levels of caloric intake and total daily activity output. Furthermore, the timing of food consumption by DM and LL mice differs from that in LD mice. Nocturnal rodents typically eat substantially more food at night; however, DM mice consume 55.5% of their food during the light phase, as compared with 36.5% in LD mice. Restricting food consumption to the active phase in DM mice prevents body mass gain. These results suggest that low levels of light at night disrupt the timing of food intake and other metabolic signals, leading to excess weight gain. These data are relevant to the coincidence between increasing use of light at night and obesity in humans.

Specializations of gastrin-releasing peptide cells of the mouse suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) of the hypothalamus regulates daily rhythms in physiology and behavior. It is composed of a heterogeneous population of cells that together form the circuits underlying its master clock function. Numerous studies suggest the existence of two regions that have been termed core and shell. At a gross level, differences between these regions map to distinct functional differences, although the specific role(s) of various peptidergic cellular phenotypes remains unknown. In mouse, gastrin-releasing peptide (GRP) cells lie in the core, are directly retinorecipient, and lack detectable rhythmicity in clock gene expression, raising interest in their role in the SCN. Here, we provide evidence that calbindin-expressing cells of perinatal mouse SCN express GRP, identified by a green fluorescent protein (GFP+), but lack detectable calbindin later in development. To explore the intra-SCN network in which GRP neurons participate, individual GFP+ cells were filled with tracer and their morphological characteristics, processes, and connections, as well as those of their non-GFP-containing immediate neighbors, were compared. The results show that GFP+ neurons form a dense network of local circuits within the core, revealed by appositions on other GFP+ cells and by the presence of dye-coupled cells. Dendrites and axons of GFP+ cells make appositions on arginine vasopressin neurons, whereas non-GFP cells have a less extensive fiber network, largely confined to the region of GFP+ cells. The results point to specialized circuitry within the SCN, presumably supporting synchronization of neural activity and reciprocal communication between core and shell regions.

Lack of calbindin-D28k alters response of the murine circadian clock to light

A strong stimulus adjusting the circadian clock to the prevailing light-dark cycle is light. However, the circadian clock is reset by light only at specific times of the day. The mechanisms mediating such gating of light input to the CNS are not well understood. There is evidence that Ca(2+) ions play an important role in intracellular signaling mechanisms, including signaling cascades stimulated by light. Therefore, Ca(2+) is hypothesized to play a role in the light-mediated resetting of the circadian clock. Calbindin-D28k (CB; gene symbol: Calb1) is a Ca(2+) binding protein implicated in Ca(2+) homeostasis and sensing. The absence of this protein influences Ca(2+) buffering capacity of a cell, alters spatio-temporal aspects of intracellular Ca(2+) signaling, and hence might alter transmission of light information to the circadian clock in neurons of the suprachiasmatic nuclei (SCN). We tested mice lacking a functional Calb1 gene (Calb1(-/-)) and found an increased phase-delay response to light applied at circadian time (CT) 14 in these animals. This is accompanied by elevated induction of Per2 gene expression in the SCN. Period length and circadian rhythmicity were comparable between Calb1(-/-) and wild-type animals. Our findings indicate an involvement of CB in the signaling pathway that modulates the behavioral and molecular response to light.

Circadian trafficking of calbindin-ir in fibers of SCN neurons

Calbindin-D28K (CalB)-containing cells form a distinct cluster within the core of the hamster suprachiasmatic nucleus (SCN). These cells are directly retinorecipient but lack detectable rhythms in clock gene expression or electrical activity. In studies exploring SCN connectivity using double-label immunochemistry, we previously reported an absence of contacts among CalB fibers and vasopressin (VP) cells in animals sacrificed during the day. Here, we explored circadian variations in CalB-immunoreactivity (-ir) and re-examined the connections between CalB and other SCN cell types at zeitgeber times (ZT) 4 and 14. The results reveal a circadian rhythm of CalB-ir in fibers of SCN cells with high expression during the night and subjective night and low expression during the day and subjective day. This circadian difference is not seen in the other brain regions studied. Significantly more appositions were detected between CalB fibers and VP cells during the night than during the day, while circadian variation in numbers of contacts was not seen between CalB fibers and vasoactive intestinal polypeptide (VIP), cholecystokinin (CCK), or gastrin-releasing peptide (GRP) cells. There was no detectable variation in appositions from any peptidergic fiber type onto CalB cells. The present findings suggest that CalB cells relay photic information to VP oscillator cells of the SCN shell in a temporally gated manner.

Rhythm of digestion: keeping time in the gastrointestinal tract

1. The best characterized mammalian circadian rhythms follow a light-entrained central master pacemaker in the suprachiasmatic nucleus and are associated with fluctuations in the activities of clock genes, including Clock, Bmal1, Per and Cry, the products of which bind to sequences in the promoters of effector genes. This is the central clock. 2. In the present review, we discuss evidence for an independent, but interacting, gut-associated circadian clock, the peripheral clock, which is entrained by food. 3. Disruption of circadian rhythms is associated with a wide range of pathologies, most prominently metabolism linked, but the effects of disruption of circadian rhythms on the digestive system are less well studied, although also likely to lead to functional consequences. There are clues suggestive of links between gastrointestinal disorders related to inflammation, cancer and motility and disruption of peripheral rhythms. Research aimed at understanding these links is still in its infancy. 4. We also discuss practical aspects of the presence of circadian rhythms in gastrointestinal tissues for researchers related to experimental design, data interpretation and the choice of animal models. 5. There is currently sufficient evidence to suggest that circadian rhythms are important to gut function, metabolism and mucosal defence and that further investigation will uncover connections between disordered rhythms and gastrointestinal malfunction.

Mobilization of calcium from intracellular stores facilitates somatodendritic dopamine release

Somatodendritic dopamine (DA) release in the substantia nigra pars compacta (SNc) shows a limited dependence on extracellular calcium concentration ([Ca(2+)](o)), suggesting the involvement of intracellular Ca(2+) stores. Here, using immunocytochemistry we demonstrate the presence of the sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase 2 (SERCA2) that sequesters cytosolic Ca(2+) into the endoplasmic reticulum (ER), as well as inositol 1,4,5-triphosphate receptors (IP(3)Rs) and ryanodine receptors (RyRs) in DAergic neurons. Notably, RyRs were clustered at the plasma membrane, poised for activation by Ca(2+) entry. Using fast-scan cyclic voltammetry to monitor evoked extracellular DA concentration ([DA](o)) in midbrain slices, we found that SERCA inhibition by cyclopiazonic acid (CPA) decreased evoked [DA](o) in the SNc, indicating a functional role for ER Ca(2+) stores in somatodendritic DA release. Implicating IP(3)R-dependent stores, an IP(3)R antagonist, 2-APB, also decreased evoked [DA](o). Moreover, DHPG, an agonist of group I metabotropic glutamate receptors (mGluR1s, which couple to IP(3) production), increased somatodendritic DA release, whereas CPCCOEt, an mGluR1 antagonist, suppressed it. Release suppression by mGluR1 blockade was prevented by 2-APB or CPA, indicating facilitation of DA release by endogenous glutamate acting via mGluR1s and IP(3)R-gated Ca(2+) stores. Similarly, activation of RyRs by caffeine increased [Ca(2+)](i) and elevated evoked [DA](o). The increase in DA release was prevented by a RyR blocker, dantrolene, and by CPA. Importantly, the efficacy of dantrolene was enhanced in low [Ca(2+)](o), suggesting a mechanism for maintenance of somatodendritic DA release with limited Ca(2+) entry. Thus, both mGluR1-linked IP(3)R- and RyR-dependent ER Ca(2+) stores facilitate somatodendritic DA release in the SNc.

Light phase testing of social behaviors: not a problem

The rich repertoire of mouse social behaviors makes it possible to use mouse models to study neurodevelopmental disorders characterized by social deficits. The fact that mice are naturally nocturnal animals raises a critical question of whether behavioral experiments should be strictly conducted in the dark phase and whether light phase testing is a major methodologically mistake. Although mouse social tasks have been performed in both phases in different laboratories, there seems to be no general consensus on whether testing phase is a critical factor or not. A recent study from our group showed remarkably similar social scores obtained from inbred mice tested in the light and the dark phase, providing evidence that light phase testing could yield reliable results as robust as dark phase testing for the sociability test. Here we offer a comprehensive review on mouse social behaviors measured in light and dark phases and explain why it is reasonable to test laboratory mice in experimental social tasks in the light phase.