Variation in the expression of the mRNA for protein kinase C isoforms in the rat suprachiasmatic nuclei, caudate putamen and cerebral cortex

Temporal transcriptomics suggest that twin-peaking genes reset the clock

The mammalian suprachiasmatic nucleus (SCN) drives daily rhythmic behavior and physiology, yet a detailed understanding of its coordinated transcriptional programmes is lacking. To reveal the finer details of circadian variation in the mammalian SCN transcriptome we combined laser-capture microdissection (LCM) and RNA-seq over a 24 hr light / dark cycle. We show that 7-times more genes exhibited a classic sinusoidal expression signature than previously observed in the SCN. Another group of 766 genes unexpectedly peaked twice, near both the start and end of the dark phase; this twin-peaking group is significantly enriched for synaptic transmission genes that are crucial for light-induced phase shifting of the circadian clock. 341 intergenic non-coding RNAs, together with novel exons of annotated protein-coding genes, including Cry1, also show specific circadian expression variation. Overall, our data provide an important chronobiological resource (www.wgpembroke.com/shiny/SCNseq/) and allow us to propose that transcriptional timing in the SCN is gating clock resetting mechanisms.

DOI:http://dx.doi.org/10.7554/eLife.10518.001

The daily cycles of life in mammals are driven by a small region of the brain called the suprachiasmatic nucleus (or SCN). The SCN receives signals from sunlight and other environmental factors to help coordinate most aspects of daily biological activity and behaviour. To work correctly, it is essential that the SCN switches certain genes on and off at exactly the right time. However, many questions remain over the identity of these genes and how their levels of activity change during a 24-hour period.

When a gene is active (or “being expressed”), it is used as a template to build the molecules of RNA that are needed to make proteins and to help to control how cells work. Pembroke et al. have now sequenced the RNA molecules made in the SCN of mice (which plays the same role as the equivalent human brain region) over a 24-hour period. The mice spent half of each day in the light, and half in the dark. This revealed that the expression levels of over a quarter of all the genes that are found in the SCN fluctuate over a 24-hour period. One particular group of genes peak in activity twice a day; Pembroke et al. suggest that these genes are important for controlling how an animal can adjust its body clock to light.

Further research is now needed to find out which of the newly discovered fluctuating genes play the most important roles in daily activity rhythms, and which might play a part in disease.

DOI:http://dx.doi.org/10.7554/eLife.10518.002

PKC-epsilon activation is required for recognition memory in the rat

Activation of PKCɛ, an abundant and developmentally regulated PKC isoform in the brain, has been implicated in memory throughout life and across species. Yet, direct evidence for a mechanistic role for PKCɛ in memory is still lacking. Hence, we sought to evaluate this in rats, using short-term treatments with two PKCɛ-selective peptides, the inhibitory ɛV1-2 and the activating ψɛRACK, and the novel object recognition task (NORT). Our results show that the PKCɛ-selective activator ψɛRACK, did not have a significant effect on recognition memory. In the short time frames used, however, inhibition of PKCɛ activation with the peptide inhibitor ɛV1-2 significantly impaired recognition memory. Moreover, when we addressed at the molecular level the immediate proximal signalling events of PKCɛ activation in acutely dissected rat hippocampi, we found that ψɛRACK increased in a time-dependent manner phosphorylation of MARCKS and activation of Src, Raf, and finally ERK1/2, whereas ɛV1-2 inhibited all basal activity of this pathway. Taken together, these findings present the first direct evidence that PKCɛ activation is an essential molecular component of recognition memory and point toward the use of systemically administered PKCɛ-regulating peptides as memory study tools and putative therapeutic agents.

Time-of-day differences in dopamine clearance in the rat medial prefrontal cortex and nucleus accumbens

Circadian rhythms influence cocaine-seeking behavior in rats, and this behavior may be mediated by variability in the rate of extracellular dopamine clearance across the day:night cycle. We used rotating disk electrode voltammetry to examine dopamine clearance and inhibition of clearance by cocaine in the rat medial prefrontal cortex (mPFC) and nucleus accumbens (NAc). Rats were housed under light:dark conditions (LD, 12 h:12 h) or in constant darkness (DD), the latter given just prior to the day of sacrifice. Tissue was collected at 4-h intervals under LD and DD conditions. Under LD, dopamine clearance in both brain regions was greatest at 4h after lights on. Under DD, there was a blunted but still rhythmic pattern of dopamine clearance across the 24-h cycle. Cocaine-induced inhibition of dopamine clearance in the mPFC was not different across the day:night cycle in rats under LD. Paradoxically, under DD, dopamine clearance in the mPFC was enhanced by cocaine at ZT16, 4 h into the subjective night, and only minimally inhibited at other times. In the NAc, cocaine inhibition of dopamine clearance was lowest at ZT4 under LD, and did not vary under DD. We conclude that dopamine clearance varies both in a diurnal and possibly in a circadian manner in the mPFC, and in a diurnal manner in the NAc. These results indicate that light itself may be used to manipulate molecules implicated in drug addiction.

Protein kinase C modulates the phase-delaying effects of light in the mammalian circadian clock

The mammalian circadian pacemaker located in the suprachiasmatic nuclei (SCN) drives a vast array of biochemical and physiological processes with 24-h periodicity. The phasing of SCN pacemaker activity is tightly regulated by photic input from the retina. Recent work has implicated protein kinase C (PKC) as a regulator of photic input, although stimulus-induced PKC activity has not been examined. Here we used a combination of biochemical, immunohistochemical and behavioral techniques to examine both the regulation and role of PKC in light-induced clock entrainment in mice. We report that photic stimulation during the subjective night, but not during the subjective day, stimulates PKC activity within the SCN. To assess the role of PKC in clock entrainment, we employed an in-vivo infusion approach to deliver the PKC inhibitor bisindolylmaleimide I to the SCN. The disruption of PKC activity significantly enhanced the phase-shifting effects of light, indicating that PKC functions as a negative regulator of light entrainment. Importantly, bisindolylmaleimide I infusion in the absence of light treatment did not phase shift the clock, demonstrating that transient disruption of basal PKC activity does not affect inherent pacemaker activity. The capacity of light to stimulate immediate early gene expression in the SCN was not substantively altered by PKC inhibition, suggesting that PKC does not couple light to rapid transcriptional activation. Rather, a combination of in-vivo and cell culture assays indicates that PKC functions as an inhibitor of PERIOD1 degradation. Thus, PKC may influence clock entrainment via a post-translational mechanism that influences clock protein stability.

Light Entrainment of the Mammalian Circadian Clock by a PRKCA-Dependent Posttranslational Mechanism

Light is the most potent stimulus for synchronizing endogenous circadian rhythms with external time. Photic clock resetting in mammals involves cAMP-responsive element binding protein (CREB)-mediated transcriptional activation of Period clock genes in the suprachiasmatic nuclei (SCN). Here we provide evidence for an additional photic input pathway to the mammalian circadian clock based on Protein Kinase C alpha (PRKCA). We found that Prkca-deficient mice show an impairment of light-mediated clock resetting. In the SCN of wild-type mice, light exposure evokes a transient interaction between PRKCA and PERIOD 2 (PER2) proteins that affects PER2 stability and nucleocytoplasmic distribution. These posttranslational events, together with CREB-mediated transcriptional regulation, are key factors in the molecular mechanism of photic clock resetting.

Rapid activation of CLOCK by Ca2+-dependent protein kinase C mediates resetting of the mammalian circadian clock

In mammals, immediate-early transcription of the Period 1 (Per1) gene is crucial for resetting the mammalian circadian clock. Here, we show that CLOCK is a real signalling molecule that mediates the serum-evoked rapid induction of Per1 in fibroblasts through the Ca2+-dependent protein kinase C (PKC) pathway. Stimulation with serum rapidly induced nuclear translocation, heterodimerization and Ser/Thr phosphorylation of CLOCK just before the surge of Per1 transcription. Serum-induced CLOCK phosphorylation was abolished by treatment with PKC inhibitors but not by other kinase inhibitors. Consistently, the interaction between CLOCK and PKC was markedly increased shortly after serum shock, and the Ca2+-dependent PKC isoforms PKCalpha and PKCgamma phosphorylated CLOCK in vitro. Furthermore, phorbol myristic acetate treatment triggered immediate-early transcription of Per1 and also CLOCK phosphorylation, which were blocked by a Ca2+-dependent PKC inhibitor. These findings indicate that CLOCK activation through the Ca2+-dependent PKC pathway might have a substantial role in phase resetting of the circadian clock.

Rhythmic changes in spike coding in the rat suprachiasmatic nucleus

The suprachiasmatic nucleus is regarded as the main mammalian circadian pacemaker but evidence for rhythmic firing of single units in vivo has been obtained only recently. The present study was undertaken to determine if rhythms could be seen using measures of activity in addition to the mean spike frequency. We investigated whether there were changes in the irregularity of cell activity measured by the disorder of the interspike interval distribution for neurones recorded in vivo and in vitro. By plotting the entropy of the log interval histogram that quantifies the coding capacity for each action potential against the respective zeitgeber time, we describe oscillations of spike activity in vivo. Entropy measures have the advantage over variances in that they quantify aspects of the shape of the distribution and not just the dispersion. One hundred and sixty-six cell recordings from the suprachiasmatic nucleus showed a significant rhythm in entropy with an oscillatory trend in the data (P < 0.001) showing a trough towards the end of the light period and a peak in the mid-dark period. There was a similar rhythm for the cells recorded from the peripheral zone (n = 209, P = 0.037). In separate experiments in vitro, to investigate the relationship between mean spike frequency and entropy, potassium-induced depolarization of cells recorded during the subjective night was correlated with a significant increase in mean spike frequency (r = 0.259, P = 0.011) and a decrease in entropy (r = -0.296, P = 0.004). The negative correlation between the entropy and mean spike frequency of cells recorded in vitro was significantly different from that seen in vivo (F = 15.5, P < 0.001), which may reflect differences in the balance between deterministic and stochastic influences on spike occurrence. The study shows that while there is a rhythm of mean spike frequency, parameters based on the variability of interspike interval distributions also display rhythmic changes over the day-night cycle.

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.

Not only vasopressin, but also the intracellular messenger protein kinase Calpha in the suprachiasmatic nucleus correlates with expression of circadian rhythmicity in voles

The suprachiasmatic nucleus (SCN) is the locus of the main pacemaker for circadian behavioral rhythms. In common voles, variation in circadian behavioral rhythmicity correlates with vasopressin (AVP) immunoreactive cells in the SCN. Here we studied the immunostaining of four AVP linked Ca(2+)-dependent protein kinase C (PKC) isoforms (PKCalpha, PKCbeta1, PKCbeta2, and PKCgamma) at the beginning of the light period, and conclude that PKCalpha is highly expressed in the vole SCN compared to the other isozymes. Voles, characterized as strongly circadian rhythmic showed circadian variation in numbers of PKCalpha immunoreactive SCN neurons, while voles with weak or no circadian rhythmicity did not reveal such a circadian profile. PKCalpha immunoreactivity in acute SCN slices that were treated with a physiological dose of AVP was significantly lowered when compared with control slices. The intracellular messenger PKCalpha may reflect variation in locomotor behavior via the AVP system in the vole SCN. This system could play a key role in the vole SCN by mediating output of its circadian clock.

Defined cell groups in the rat suprachiasmatic nucleus have different day/night rhythms of single-unit activity in vivo

The electrical activity of the rat suprachiasmatic nucleus (SCN) was examined in anesthetized rats in vivo using single-unit electrophysiological techniques. The present data confirm the daily variation in the electrical activity of the SCN previously reported in vitro and in vivo using multiple-unit recording techniques. They further suggest that subpopulations of suprachiasmatic neurons with different neural connections have a different daily rhythm of activity. Neurons in the SCN region showed a significant rhythm of activity (p = 0.034; Kruskall-Wallis analysis of variance [KW-ANOVA]). The greatest activity occurred during the second part of the light period (ZT 10-12), and the lowest activity occurred in the early part of the light period (ZT 0-2). The subgroup of cells in the suprachiasmatic region with output projections to the arcuate nucleus (ARC) and/or supraoptic nucleus (SON) regions also showed a significant rhythm (p = 0.001; K-W ANOVA). Their activity appeared to show two peaks near the light-dark (ZT 10-12) and dark-light (ZT 22-24) transition periods with the lowest activity at ZT 16-18. This rhythm was significantly different (p = 0.016) from that of neurons without an output projection to the ARC and/or SON. Retinorecipient suprachiasmatic neurons appeared to have a less robust daily rhythm in their activity. The change in the firing behavior of the cells was not reflected simply by changes in mean firing rate. Examination of the coefficient of variation of the interspike interval distribution of cells at different times of day revealed changes in the firing pattern of cells in the SCN region that did not have output projections (p = 0.032; K-W ANOVA). The present results thus suggest that the SCN is composed of a heterogeneous population of neurons and that different rhythms of activity are expressed by neurons with different neural connections. There were changes in both firing pattern and firing rate.

NMDA-evoked calcium transients and currents in the suprachiasmatic nucleus: gating by the circadian system

A variety of evidence suggests that the effects of light on the mammalian circadian system are mediated by glutamatergic mechanisms and that the N-methyl- D-aspartate (NMDA) receptor plays an important role in this regulation. One of the fundamental features of circadian oscillators is that their response to environmental stimulation varies depending on the phase of the daily cycle when the stimuli are applied. For example, the same light treatment, which can produce phase shifts of the oscillator when applied during subjective night, has no effect when applied during the subjective day in animals held in constant darkness (DD). We examined the hypothesis that the effects of NMDA on neurons in the suprachiasmatic nucleus (SCN) also vary from day to night. Optical techniques were utilized to estimate NMDA-induced calcium (Ca2+) changes in SCN cells. The resulting data indicate that there was a daily rhythm in the magnitude and duration of NMDA-induced Ca2+ transients. The phase of this rhythm was determined by the light-dark cycle to which the rats were exposed with the Ca2+ transients peaking during the night. This rhythm continued when animals were held in DD. gamma-Aminobutyric acid (GABA)ergic mechanisms modulated the NMDA response but were not responsible for the rhythm. Finally, there was a rhythm in NMDA-evoked currents in SCN neurons that also peaked during the night. This study provides the first evidence for a circadian oscillation in NMDA-evoked Ca2+ transients in SCN cells. This rhythm may play an important role in determining the periodic sensitivity of the circadian systems response to light.

Expression of mt(1) melatonin receptor subtype mRNA in the entrained rat suprachiasmatic nucleus: a quantitative RT-PCR study across the diurnal cycle

Melatonin acts on specific receptors in the suprachiasmatic nuclei (SCN) to phase-dependently regulate the phase of the circadian clock. How the gating of melatonin's effect is restricted to particular times of day is not known, but may be related to temporal differences in receptor availability. In the present study, we used a competitive reverse transcription-polymerase chain reaction (RT-PCR) method to determine if the expression of mt(1) melatonin receptor subtype mRNA in rat SCN varied across the 12:12 light-dark (LD) cycle. Measurement of core body temperature using radiotelemetry confirmed that the male Wistar rats used exhibited a robust diurnal rhythm. mt(1) receptor mRNA was readily detected in reduced SCN slices at all times of day. However, there was no significant variation in the amount of mt(1) mRNA with time of day. Expression of MT(2) melatonin receptor subtype mRNA in reduced SCN slices was confirmed by nested PCR. These results indicate that changes in the level of mt(1) mRNA do not underlie the diurnal and/or circadian variation in the response of the SCN circadian clock to the phase-resetting effects of melatonin.

Protein kinase C inhibition and activation phase advances the hamster circadian clock

The mammalian circadian clock is located in the suprachiasmatic nuclei (SCN). Clock function can be detected by the measurement of the circadian change in cellular firing rate of SCN cells in vitro. We investigated the effects of protein kinase C (PKC) inhibition and activation on this rhythm of firing rate in hamster SCN neurons. PKC inhibition by chelerythrine chloride application phase advances the in vitro circadian rhythm during the late subjective night and early subjective morning, Zeitgeber time (ZT) 20-24 and ZT 0-4. No effect of PKC inhibition on clock phase was seen during ZT 6-18. Activation of PKC via phorbol 12-myristate 13-acetate (PMA) phase advanced the clock at all phases tested. Thus, at some circadian phases both inhibition and activation of PKC can advance circadian rhythms.

Circadian regulation of prion protein messenger RNA in the rat forebrain: a widespread and synchronous rhythm

Although the expression of the normal prion protein in the host is critical to the development of transmissible spongiform encephalopathies, the physiological role of this protein and the processes regulating its expression remain obscure. We now report that the messenger RNA for the prion protein is regulated in the rat brain in a marked circadian manner not only in the suprachiasmatic nuclei, the principal site for the generation of mammalian circadian rhythms, but also in other forebrain regions. The data show a remarkable consistency in the concurrence of a single peak of prion protein messenger RNA at each of the sites early in the animal's phase of increased locomotor activity; behavioural arousal does not, however, appear to affect this expression. We believe this to be the first study demonstrating that the expression of prion protein messenger RNA can change over a relatively short period in vivo. The results are discussed with reference to the range of recently discovered "clock-related" transcripts which also have widespread tissue expression; these include the messenger RNAs for D-box binding protein and thyroid embryonic factor, transcription factors which bind to the prion protein promoter.

Sodium depletion and aldosterone decrease dopamine transporter activity in nucleus accumbens but not striatum

Motivated behaviors, including sodium (Na) appetite, are correlated with increased dopamine (DA) transmission in the nucleus accumbens (NAc). DA transporter (DAT) modulation affects DA transmission and may play a role in motivated behaviors. In vivo Na depletion, which reliably induces Na appetite, was correlated with robust decreases in DA uptake via the DAT in the rat NAc with rotating disk electrode voltammetry [1,277 +/- 162 vs. 575 +/- 89 pmol. s-1. g-1; Vmax of transport for control vs. Na-depleted tissue]. Plasma aldosterone (Aldo) levels increase after in vivo Na depletion and contribute to Na appetite. Decreased DAT activity in the NAc was observed after in vitro Aldo treatment (428 +/- 28 vs. 300 +/- 25 pmol. s-1. g-1). Neither treatment affected DAT activity in the striatum. These results suggest that a direct action of Aldo is one possible mechanism by which Na depletion induces a reduction in DAT activity in the NAc. Reduced DAT activity may play a role in generating increased NAc DA transmission during Na appetite, which may underlie the motivating properties of Na for the Na-depleted rat.

Distribution of Ca2+-dependent protein kinase C isoforms in the suprachiasmatic nucleus of the diurnal murid rodent, Arvicanthis niloticus

The suprachiasmatic nuclei (SCN) contain the major 'biological clock' in mammals that controls most circadian rhythms expressed by these animals. The functional importance of protein phosphorylation and intracellular Ca2+ in the mammalian circadian pacemaker is becoming increasingly apparent. Here we report the immunocytochemical localization of the four Ca2+-dependent protein kinase C (PKC) isoforms (alpha, betaI, betaII, gamma) within the SCN of the diurnal murid rodent, Arvicanthis niloticus, and the nocturnal golden hamster. In the SCN of A. niloticus, PKCalpha was the most abundant of the four isoforms. Cells containing PKCalpha were homogeneously distributed throughout the SCN. PKCbetaI cells were sparsely distributed in the perimeter of the SCN and were absent in its central area. PKCbetaII and -gamma were not found in the SCN of A. niloticus. In the SCN of the golden hamster, PKCalpha cells were most heavily concentrated in the dorsomedial region, though some were also present laterally and ventrally. The distribution of arginine-vasopressin (AVP) cells in the SCN overlapped with that of PKC in both species. Species differences in the location of the Ca2+-dependent PKC isoforms suggest differences in function such as the relaying of photic or non-photic information to the clock mechanism, or the synchronization of AVP neurons and their subsequent output signals.

Light and circadian rhythmicity regulate MAP kinase activation in the suprachiasmatic nuclei

Although the circadian time-keeping properties of the suprachiasmatic nuclei (SCN) require gene expression, little is known about the signal transduction pathways that initiate transcription. Here we report that a brief exposure to light during the subjective night, but not during the subjective day, activates the p44/42 mitogen-activated protein kinase (MAPK) signaling cascade in the SCN. In addition, MAPK stimulation activates CREB (cAMP response element binding protein), indicating that potential downstream transcription factors are stimulated by the MAPK pathway in the SCN. We also observed striking circadian variations in MAPK activity within the SCN, suggesting that the MAPK cascade is involved in clock rhythmicity.

Circadian changes of type II adenylyl cyclase mRNA in the rat suprachiasmatic nuclei

Circadian functions of the suprachiasmatic nuclei (SCN) are influenced by cyclic AMP (cAMP). Adenylyl cyclase type II (AC-II) is a cAMP-generating enzyme which, in the context of activation by Gsalpha, is further stimulated by protein kinase C or G protein betagamma subunits. Using in situ hybridization we have found a biphasic variation in AC-II mRNA within the rat SCN during the light-dark cycle (peaks at Zeitgeber time 6 and 18) and also in constant darkness (peaks at circadian time 2 and 14). The cingulate cortex showed no such variation. These findings suggest that circadian changes in AC-II expression may be pertinent to the rhythmic functions of the SCN.