Fractal Stochastic Modeling of Spiking Activity in Suprachiasmatic Nucleus Neurons

Individual neurons in the suprachiasmatic nucleus (SCN), the master biological clock in mammals, autonomously produce highly complex patterns of spikes. We have shown that most (~90%) SCN neurons exhibit truly stochastic interspike interval (ISI) patterns. The aim of this study was to understand the stochastic nature of the firing patterns in SCN neurons by analyzing the ISI sequences of 150 SCN neurons in hypothalamic slices. Fractal analysis, using the periodogram, Fano factor, and Allan factor, revealed the presence of a 1/f-type power-law (fractal) behavior in the ISI sequences. This fractal nature was persistent after the application of the GABA_A receptor antagonist bicuculline, suggesting that the fractal stochastic activity is an intrinsic property of individual SCN neurons. Based on these physiological findings, we developed a computational model for the stochastic SCN neurons to find that their stochastic spiking activity was best described by a gamma point process whose mean firing rate was modulated by a fractal binomial noise. Taken together, we suggest that SCN neurons generate temporal spiking patterns using the fractal stochastic point process.Action Editor: Carson C. Chow     

The Circadian Clock Gene Period1 Connects the Molecular Clock to Neural Activity in the Suprachiasmatic Nucleus

The neural activity patterns of suprachiasmatic nucleus (SCN) neurons are dynamically regulated throughout the circadian cycle with highest levels of spontaneous action potentials during the day. These rhythms in electrical activity are critical for the function of the circadian timing system and yet the mechanisms by which the molecular clockwork drives changes in the membrane are not well understood. In this study, we sought to examine how the clock gene Period1 (Per1) regulates the electrical activity in the mouse SCN by transiently and selectively decreasing levels of PER1 through use of an antisense oligodeoxynucleotide. We found that this treatment effectively reduced SCN neural activity. Direct current injection to restore the normal membrane potential partially, but not completely, returned firing rate to normal levels. The antisense treatment also reduced baseline [Ca²⁺]i levels as measured by Fura2 imaging technique. Whole cell patch clamp recording techniques were used to examine which specific potassium currents were altered by the treatment. These recordings revealed that the large conductance [Ca²⁺]i-activated potassium currents were reduced in antisense-treated neurons and that blocking this current mimicked the effects of the anti-sense on SCN firing rate. These results indicate that the circadian clock gene Per1 alters firing rate in SCN neurons and raise the possibility that the large conductance [Ca²⁺]i-activated channel is one of the targets.     

Circadian Activity Rhythms and Phase-Shifting of Cultured Neurons of the Rat Suprachiasmatic Nucleus

The mammalian suprachiasmatic nucleus (SCN) is the major endogenous pacemaker that coordinates various daily rhythms including locomotor activity and autonomous and endocrine responses, through a neuronal and humoral influence. In the present study we examined the behavior of dispersed individual SCN neurons obtained from 1- to 3-day-old rats cultured on multi-microelectrode arrays (MEAs). SCN neurons were identified by immunolabeling for the neuropeptides arginine-vasopressin (AVP) and vasoactive intestinal polypeptide (VIP). Single SCN neurons cultured at low density onto an MEA can express firing rate patterns with different circadian phases. In these cultures we observed rarely synchronized firing patterns on adjacent electrodes. This suggests that, in cultures of low cell densities, SCN neurons function as independent pacemakers. To investigate whether individual pacemakers can be influenced independently by phase-shifting stimuli, we applied melatonin (10 pM to 100 nM) for 30 min at different circadian phases and continuously monitored the firing rate rhythms. Melatonin could elicit phase-shifting responses in individual clock cells which had no measurable input from other neurons. In several neurons, phase-shifts occurred with a long delay in the second or third cycle after melatonin treatment, but not in the first cycle. Phase-shifts of isolated SCN neurons were also observed at times when the SCN showed no sensitivity to these phase-shifting stimuli in recordings from brain slices. This finding suggests that the neuronal network plays an essential role in the control of phase-shifts.     

Light-Induced Variations in AP-1 Binding Activity and Composition in the Rat Suprachiasmatic Nucleus

Abstract : Expression of immediate early genes, including fos -like and jun -like genes, in the suprachiasmatic nucleus is believed to be part of the mechanism for photic entrainment of circadian rhythms to the environmental light/dark cycle. However, the effects of a light stimulus on activating protein-1 (AP-1) complexes in the suprachiasmatic nucleus remain unclear. The photic regulation of AP-1 DNA-binding activity and composition in the rat suprachiasmatic nucleus was evaluated by using an electrophoretic mobility shift assay. A light pulse given during subjective night induced an increase in AP-1 binding activity when either nuclear or whole-cell extracts from suprachiasmatic nuclei were used. Under constant dark conditions, proteins that are predominant components of AP-1 complexes are Fra-2 and Jun-D. Under light stimulation, c-Fos and Jun-B consistently increased, as expected, but this was also the case for Fra-2, Jun-D, and c-Jun, although to a lesser extent. An immunocytochemical study of the Fra-2 expression pattern demonstrated the presence of the protein in the ventrolateral as well as in the dorsomedial subdivisions of the suprachiasmatic nucleus. Light regulation of Fra-2 immunoreactivity, however, appeared to be restricted to the ventrolateral subdivision. It is concluded that light may be acting both by increasing constitutive AP-1 complexes and by inducing the expression of specific complexes.     

Characterization of Proteases in the Hamster Suprachiasmatic Nucleus

The activities of several proteases in the hamster suprachiasmatic nuclei were measured at different time points throughout the daily or circadian cycle. No variation for metalloproteinase A (MMP-2) activity was found, while metalloproteinase B (MMP-9) was rhythmic and maximally active during the night. In addition, diurnal variations for two low molecular weight proteases were determined, with peaks during the light phase. This rhythmicity appears to be under exogenous control, since constant darkness abolished fluctuations throughout the circadian cycle. These results suggest that protein degradation in the hamster circadian clock is regulated in a diurnal fashion.     

大鼠下丘脑交叉上核中CREB的昼夜节律

利用凝胶迁移率变化的实验方法,对饲养在光照－黑暗循环的条件和持续黑暗的条件下Wistar雄性大鼠下丘脑交叉上核中CREB含量的昼夜间变化进行了分析,发现CREB在交叉上核中具有内源性的昼夜节律.     

Modeling the Seasonal Adaptation of Circadian Clocks by Changes in the Network Structure of the Suprachiasmatic Nucleus

The dynamics of circadian rhythms needs to be adapted to day length changes between summer and winter. It has been observed experimentally, however, that the dynamics of individual neurons of the suprachiasmatic nucleus (SCN) does not change as the seasons change. Rather, the seasonal adaptation of the circadian clock is hypothesized to be a consequence of changes in the intercellular dynamics, which leads to a phase distribution of electrical activity of SCN neurons that is narrower in winter and broader during summer. Yet to understand this complex intercellular dynamics, a more thorough understanding of the impact of the network structure formed by the SCN neurons is needed. To that effect, we propose a mathematical model for the dynamics of the SCN neuronal architecture in which the structure of the network plays a pivotal role. Using our model we show that the fraction of long-range cell-to-cell connections and the seasonal changes in the daily rhythms may be tightly related. In particular, simulations of the proposed mathematical model indicate that the fraction of long-range connections between the cells adjusts the phase distribution and consequently the length of the behavioral activity as follows: dense long-range connections during winter lead to a narrow activity phase, while rare long-range connections during summer lead to a broad activity phase. Our model is also able to account for the experimental observations indicating a larger light-induced phase-shift of the circadian clock during winter, which we show to be a consequence of higher synchronization between neurons. Our model thus provides evidence that the variations in the seasonal dynamics of circadian clocks can in part also be understood and regulated by the plasticity of the SCN network structure.     

Simultaneous Electrophysiological Recording and Calcium Imaging of Suprachiasmatic Nucleus Neurons

Simultaneous electrophysiological and fluorescent imaging recording methods were used to study the role of changes of membrane potential or current in regulating the intracellular calcium concentration. Changing environmental conditions, such as the light-dark cycle, can modify neuronal and neural network activity and the expression of a family of circadian clock genes within the suprachiasmatic nucleus (SCN), the location of the master circadian clock in the mammalian brain. Excitatory synaptic transmission leads to an increase in the postsynaptic Ca²⁺ concentration that is believed to activate the signaling pathways that shifts the rhythmic expression of circadian clock genes. Hypothalamic slices containing the SCN were patch clamped using microelectrodes filled with an internal solution containing the calcium indicator bis-fura-2. After a seal was formed between the microelectrode and the SCN neuronal membrane, the membrane was ruptured using gentle suction and the calcium probe diffused into the neuron filling both the soma and dendrites. Quantitative ratiometric measurements of the intracellular calcium concentration were recorded simultaneously with membrane potential or current. Using these methods it is possible to study the role of changes of the intracellular calcium concentration produced by synaptic activity and action potential firing of individual neurons. In this presentation we demonstrate the methods to simultaneously record electrophysiological activity along with intracellular calcium from individual SCN neurons maintained in brain slices.     

Dynamical Heterogeneity of Suprachiasmatic Nucleus Neurons Based on Regularity and Determinism

The suprachiasmatic nucleus (SCN) is known to be the master biological clock in mammals. Despite the periodic mean firing rate, interspike interval (ISI) patterns of SCN neurons are quite complex and irregular. The aim of the present study was to investigate the existence of nonlinear determinism in the complex ISI patterns of SCN neurons. ISI sequences were recorded from 173 neurons in rat hypothalamic slice preparations using a cell-attached patch recording technique. Their correlation dimensions (D₂) were estimated, and were then compared with those of the randomly-shuffled surrogate data. We found that only 16 neurons (16/173) exhibited deterministic ISI patterns of spikes. In addition, clustering analysis revealed that SCN neurons could be divided into two subgroups of neurons each having distinct values of coefficient of variation (CV) and skewness (SK). Interestingly, most deterministic SCN neurons (14/16) belonged to the group of irregularly spiking neurons having large CV and SK values. To see if the neuronal coupling mediated by the γ-aminobutyric acid (GABA), the major neurotransmitter in the SCN, contributed to the deterministic nature, we examined the effect of the GABA_A receptor antagonist bicuculline on D₂ values of 56 SCN neurons. 8 SCN neurons which were originally stochastic became to exhibit deterministic characteristics after the bicuculline application. This result suggests that the deterministic nature of the SCN neurons arises not from GABAergic synaptic interactions, but likely from properties inherent to neurons themselves.Action Editor: Barry J. Richmond     

The Role of the Suprachiasmatic Nucleus in Cardiac Autonomic Control during Sleep

Background

The suprachiasmatic nucleus (SCN) may play an important role in central autonomic control, since its projections connect to (para)sympathetic relay stations in the brainstem and spinal cord. The cardiac autonomic modifications during nighttime may therefore not only result from direct effects of the sleep-related changes in the central autonomic network, but also from endogenous circadian factors as directed by the SCN. To explore the influence of the SCN on autonomic fluctuations during nighttime, we studied heart rate and its variability (HRV) in a clinical model of SCN damage.

Methods

Fifteen patients in follow-up after surgical treatment for nonfunctioning pituitary macroadenoma (NFMA) compressing the optic chiasm (8 females, 26–65 years old) and fifteen age-matched healthy controls (5 females, 30–63 years) underwent overnight ambulatory polysomnography. Eleven patients had hypopituitarism and received adequate replacement therapy. HRV was calculated for each 30-second epoch and corrected for sleep stage, arousals, and gender using mixed effect regression models.

Results

Compared to controls, patients spent more time awake after sleep onset and in NREM1-sleep, and less in REM-sleep. Heart rate, low (LF) and high frequency (HF) power components and the LF/HF ratio across sleep stages were not significantly different between groups.

Conclusions

These findings suggest that the SCN does not play a dominant role in cardiac autonomic control during sleep.     

Differential Involvement of the Suprachiasmatic Nucleus in Lipopolysaccharide-Induced Plasma Glucose and Corticosterone Responses

The hypothalamic suprachiasmatic nucleus (SCN) is an essential component of the circadian timing system, and an important determinant of neuroendocrine and metabolic regulation. Recent data indicate a modulatory role for the immune system on the circadian timing system. The authors investigated how the circadian timing system affects the hypothalamo-pituitary-adrenal (HPA) axis and glucose regulatory responses evoked by an immune challenge induced by lipopolysaccharide (LPS). LPS-induced increases in corticosterone were minimal during the trough of the daily corticosterone rhythm; in contrast, LPS effects on glucose, glucagon, and insulin did not vary across time-of-day. Complete ablation of the SCN resulted in increased corticosterone responses but did not affect LPS-induced hyperglycemia. The paraventricular nucleus (PVN) of the hypothalamus is an important neuroendocrine and autonomic output pathway for hypothalamic information, as well as one of the main target areas of the SCN. Silencing the neuronal activity in the PVN did not affect the LPS-induced corticosterone surge and only slightly delayed the LPS-induced plasma glucose and glucagon responses. Finally, surgical interruption of the neuronal connection between hypothalamus and liver did not affect the corticosterone response but slightly delayed the LPS-induced glucose response. Together, these data support the previously proposed circadian modulation of LPS-induced neuroendocrine responses, but they are at variance with the suggested major role for the hypothalamic pacemaker on the autonomic output of the hypothalamus, as reflected by the effects of LPS on glucose homeostasis. The latter effects are more likely due to direct interactions of LPS with peripheral tissues, such as the liver. (Author correspondence: a.kalsbeek@amc.uva.nl)     

Efferent Projections of Prokineticin 2 Expressing Neurons in the Mouse Suprachiasmatic Nucleus

The suprachiasmatic nucleus (SCN) in the hypothalamus is the predominant circadian clock in mammals. To function as a pacemaker, the intrinsic timing signal from the SCN must be transmitted to different brain regions. Prokineticin 2 (PK2) is one of the candidate output molecules from the SCN. In this study, we investigated the efferent projections of PK2-expressing neurons in the SCN through a transgenic reporter approach. Using a bacterial artificial chromosome (BAC) transgenic mouse line, in which the enhanced green fluorescence protein (EGFP) reporter gene expression was driven by the PK2 promoter, we were able to obtain an efferent projections map from the EGFP-expressing neurons in the SCN. Our data revealed that EGFP-expressing neurons in the SCN, hence representing some of the PK2-expressing neurons, projected to many known SCN target areas, including the ventral lateral septum, medial preoptic area, subparaventricular zone, paraventricular nucleus, dorsomedial hypothalamic nucleus, lateral hypothalamic area and paraventricular thalamic nucleus. The efferent projections of PK2-expressing neurons supported the role of PK2 as an output molecule of the SCN.     

Characterization of Melatonin-Induced FOS-Like Immunoreactivity in the Hypothalamic Suprachiasmatic Nucleus of the Rat

Abstract

The hypothalamic suprachiasmatic nucleus (SCN) is primarily responsible for the regulation of circadian rhythmicity. Melatonin, the pineal-derived neurohormone, modulates the rhythmic output of the SCN. Properly timed exposure to melatonin is able to induce changes in rhythrnic function and thereby entrain circadian rhythms of activity.

c-fos is an immediate early gene that is transiently expressed in neurons in response to receptor activation. The ventrolateral portion of the SCN (vSCN) is activated in response to phase-shifting stimuli, an event which is marked by an increase in the expression of c-fos.

In the present study, rats systemically administered the melatonin agonist 2-iodomelatonin at CT 22 demonstrated significant dose-dependent Fos immunoreactivity within the vSCN, an effect which was significantly inhibited by the melatonin antagonist N-acetyltryptamine. The Fos expression observed in the vSCN was not affected by treatment with the serotonin antagonist ketanserin or the alpha-adrenergic antagonist phentolamine. Moreover, antisense oligonucleotides to c-fos, significantly blocked the ability of 2-iodomelatonin to induce Fos expression in the vSCN at CT 22.

These results pharmacologically characterize melatonin-induced c-fos expression in the rat vSCN and provide evidence to support a c-fos-mediated mechanism through which the activation of melatonin receptors may be linked to the long-term molecular regulation of circadian rhythms controlled by the SCN.     

Analyzing Self-Similar and Fractal Properties of the C. elegans Neural Network

The brain is one of the most studied and highly complex systems in the biological world. While much research has concentrated on studying the brain directly, our focus is the structure of the brain itself: at its core an interconnected network of nodes (neurons). A better understanding of the structural connectivity of the brain should elucidate some of its functional properties. In this paper we analyze the connectome of the nematode Caenorhabditis elegans. Consisting of only 302 neurons, it is one of the better-understood neural networks. Using a Laplacian Matrix of the 279-neuron “giant component” of the network, we use an eigenvalue counting function to look for fractal-like self similarity. This matrix representation is also used to plot visualizations of the neural network in eigenfunction coordinates. Small-world properties of the system are examined, including average path length and clustering coefficient. We test for localization of eigenfunctions, using graph energy and spacial variance on these functions. To better understand results, all calculations are also performed on random networks, branching trees, and known fractals, as well as fractals which have been “rewired” to have small-world properties. We propose algorithms for generating Laplacian matrices of each of these graphs.     

Mesoscopic Patterns of Neural Activity Support Songbird Cortical Sequences

Time-locked sequences of neural activity can be found throughout the vertebrate forebrain in various species and behavioral contexts. From “time cells” in the hippocampus of rodents to cortical activity controlling movement, temporal sequence generation is integral to many forms of learned behavior. However, the mechanisms underlying sequence generation are not well known. Here, we describe a spatial and temporal organization of the songbird premotor cortical microcircuit that supports sparse sequences of neural activity. Multi-channel electrophysiology and calcium imaging reveal that neural activity in premotor cortex is correlated with a length scale of 100 µm. Within this length scale, basal-ganglia–projecting excitatory neurons, on average, fire at a specific phase of a local 30 Hz network rhythm. These results show that premotor cortical activity is inhomogeneous in time and space, and that a mesoscopic dynamical pattern underlies the generation of the neural sequences controlling song.     

Spike Discharge Patterns of Spontaneous and Continuously Stimulated Activity in the Cochlear Nucleus of Anesthetized Cats

Interspike interval histograms of spontaneous and stimulated activity were computed from spike discharges of single units in the cochlear nucleus. These histograms indicate that a number of different types of spontaneous discharge patterns exist in the nucleus. The type of spontaneous activity of a given unit is related to its activity in response to continuous tones. Correlations were found between the discharge patterns of units and their anatomical locations within the nucleus.