The mammalian suprachiasmatic nuclei (SCN) are located in the ventral part of the hypothalamus and orchestrate circadian rhythms in physiology and behaviour. The ~20.000 neurones of the murine SCN express key molecular clock components including the Cryptochrome (Cry1/2) and Period (Per1/2/3) genes and their protein products CRY1/2 and PER1/2/3. Using different mouse models, this work demonstrates that with disrupted expression of CRY in the after-hours (Afh/Afh) mouse, cells of the ventral part of the SCN (vSCN) have a propensity to desynchronise. They receive increased GABAergic inputs and are less excitable during the projected night but not during the day compared to congenic wildtype (+/+). The linkage between CRY protein expression and the reduced excitability at night is supported by recordings from SCN cells of Cry2 deficient mice (Cry2-/-), which exhibit similar electrophysiological behaviour. Luminometrical recordings of single cell Per2 expression confirms the involvement of GABAergic signalling in both, maintaining a coherent rhythm in synchronised SCN cells from +/+ controls and the propensity of Afh/Afh SCN cells to desynchronise. A mechanism by which neuronal excitability is regulated in mammals, is the modulation of activity of small-conductance Ca2+-activated K+ (SK) channels. Western blot analysis demonstrates the expression of SK2 and SK3 channel protein in SCN neurones. Functionally, we show with whole cell electrophysiology, calcium imaging and luminometry how SK channels regulate the levels of intracellular calcium ([Ca2+]i) from day to night. In the more hyperpolarised SCN network of the Afh/Afh genotype at night, SK channel activity is altered and contributes to the lower single cell excitability. Vasoactive intestinal polypeptide (VIP) and its cognate receptor, VPAC2, are synthesised by SCN neurones and this intercellular signal facilitates coordination of suprachiasmatic neuronal activity. How the loss of VPAC2 receptor signalling affects the electrophysiology of SCN neurones and their response to excitatory inputs is unclear. Here we made patch clamp recordings of SCN neurones in brain slices prepared from animals that do not express VPAC2 receptors (Vipr2-/- mice) as well as non-transgenic animals (Vipr2+/+ mice). While Vipr2+/+ SCN neurones exhibit coordinated day-night variation in their electrical state, Vipr2-/- neurones do not and instead manifest a range of states during both day and night. We find that Vipr2+/+ neurones vary the membrane threshold potential at which they start to fire actions potentials from day to night, while Vipr2-/- neurones lack this variation. This is due to Vipr2-/- neurones lacking a voltage-gated sodium current. Subsequently we determine that this aberrant temporal control of neuronal state and excitability alters appropriate neuronal responses to a neurochemical mimic of the light-input pathway to the SCN. Conclusively, these results highlight the critical role intercellular signalling plays in the activity of individual neuronal state and their response to neural input as well as ensemble activity and function of the suprachiasmatic neural network.

No description available.

Thesis (Ph. D.)--University of Hong Kong, 2006. / Title proper from title frame. Also available in printed format.

This thesis assessed the impacts of acute and chronic phase-shifting on learning and memory in male and female rats. Previous research has revealed impaired retention immediately following circadian disruption and on the acquisition of new associations. However, whether behaviour resumes normality following circadian re-entrainment is unresolved. Following circadian re-entrainment, retention of pre-phase-shift acquired associations on Morris water task (MWT) and a visual discrimination task designed on the 8-arm radial maze were tested. Subsequently, an extradimensional set shift (EDS) using the 8-arm radial maze was performed. Acute circadian disruption negatively impacted retention in males and females, but only male rats without running wheels exhibited impairment following chronic phase-shifting on MWT performance. Retention on the visual discrimination task was impaired following chronic, but not acute, circadian disruption. Chronic, but not acute, phase-shifting negatively impacted performance on the EDS. Generally, phase-shifting produced differential negative impacts on cognitive function in rats. / xiv, 181 leaves : ill. (some col.) ; 29 cm

When an organism is dehydrated, neurons in the Organum vasculosum lamina terminalis (OVL T) sense this variation in plasma osmolality (OSM) and excite magnocellular neurosecretory cells (MNCs) in the supraoptic nucleus (SON) via glutamatergic synapses. The resulting action potential firing of MNCs will result in the secretion of vasopressin (VP) into the blood, which will promote water reabsorption from the kidney. The relationship between plasma VP and OSM (know as the VP-OSM ratio) is known to change in sensitivity during the course of a day. / Lorsqu'un organisme est déshydraté, les neurones dans l'Organum vasculosum lamina terminalis (OVL T) détectent le changement dans l'osmolalité du plasma (OSM) et excitent les cellules magnocellulaires neurosécrétoires (MNCs) dans le noyau supraoptique (SON) avec des synapses glutamatergique. La décharge des potentiels d'action qui survient dans les MNCs génère la sécrétion de vasopressine (VP) dans le sang, qui permettra la réabsorption d'eau au niveau du rein. Le rapport entre la VP et OSM (connu comme étant le rapport VP/OSM) subit des changements de sensibilité durant une journée.

Componente essencial do sistema de temporização circadiano, o núcleo supraquiasmático (NSQ) possui três aferências principais: o trato retinohipotalâmico (TRH), o trato geniculohipotalâmico (TGH) e as terminações serotonérgicas da rafe. Suas células possuem oscilação circadiana autônoma que resultam na expressão rítmica dos chamados genes do relógio. O presente estudo analisa as concentrações de 5-HT nos núcleos da rafe e NSQ de ratos em livre-curso e mostra que somente os núcleos obscuro e linear apresentam ritmos endógenos com ação determinante do ciclo claro-escuro na no ritmo diário; compara a organização intrínseca do NSQ de primatas e roedores, mostrando organização diferenciada dos terminais serotonérgicos e do TGH em relação aos do TRH sugerindo funções diferentes dessas aferências no NSQ de primatas. Além disso, o padrão de expressão dos genes do relógio no NSQ do primata ao longo do período de atividade mostrou que os genes BMAL1 e Per1 apresentam pico de expressão ao redor do ZT2 e o gene Per2 no ZT7. Os dados demonstram diferenças interespecíficas importantes nas características neuroquímicas e moleculares do NSQ. / Essential component of the circadian timing system, the suprachiasmatic nucleus (SCN) receives dense retinohypothalamic RHT, geniculohypothalamic tract GHT and serotonergic innervation arriving from the raphe nuclei. SCN has pacemaker cells that produce rhythmic expression of clock genes. This study investigates the levels of 5-HT in the raphe nuclei and SCN in free running rats and shows endogenous rhythms in the obscurus and linear raphe nuclei, which is regulated by the daily light: dark cycle rhythms. The comparative analysis of the intrinsic structure of the SCN of primates and rodents shows a different organizational pattern of serotonergic and GHT terminals and the RHT terminals, suggesting different actions of serotonin and neuropeptide Y in the control of circadian rhythmicity in primates. Moreover, the pattern of the clock genes SCN expression along the awaken period in the primates show that BMAL1 and Per1 RNAm peaks of expression occur around ZT2 and Per2 around ZT7. These data suggest that the neural organization of the circadian timing system in the studied primate differ from those of the most commonly studied rodents.

Our body is influenced by many cyclical changes in the environment, such as day and night or seasons. To predict these changes and react to them in time, the organism is equipped with inner clock, which rhythmically influences many physiological processes, such as sleep or metabolic rhythms. Disrupting our inner rhythms at molecular and behavioral levels contributes to many serious disorders. It is necessary that all mechanisms of the inner circadian clock are developed and set up properly. Circadian clocks are set up by the mother, who passes rhythmical information about day and night cycle on to her embryo. Though a great attention is devoted to revealing the nature of this synchronization between the mother and her pup, the mechanisms of this process have not been fully understood yet. The aim of this thesis is to contribute to actual understanding of this synchronization. Experiments, performed in this thesis, relate to studying the ability of maternal signals to synchronize embryos with the environment. Feeding and light regime of pregnant rats was manipulated and the effect of these changes on the neuronal activity within the suprachiasmatic nuclei of 19-day embryos was analyzed.

The rhythm of ATP accumulation is an one of examples of circadian rhythmicity which is demonstrated across the animal kingdom. It is located in the suprachiasmatic nucleus of hypothalamus, which is a centre of circadian clock and imposes rhytmicity on a whole organism. The question concering the ATP rhythm in an organotypic culture and the impact of melatonin on it has been discussing in this thesis. The synthesis of melatonin is regulated by the activity of the SCN and the hormon itself is known for its feedback on the SCN and a capability of regulating it. It has been shown that ATP rhythm is significantly inhibited under the constant control of melatonin and this inhibition is dose-dependent on a scale of 0,1-10 nM. In case of one-time applied melatonin in 4 p.m. there is no reduction in the accumulation of extracellular ATP but there is a phase shift in ATP rhythm. It has also been shown that ATP rhythm is inhibited and desynchronized under the control of tetrodotoxin which blocks an electric activity of neurons. These results show that melatonin is capable of inducing phase delay of ATP rhythm in SCN and reducing an amount of extracellular ATP, that the effect of melatonin is probably mediated by specific receptors and lastly that ATP rhythm is a result of cell-cell interaction between neurons...

Long-term exposure to constant light results in desynchronization of the circadian system in an adult and is associated with reduced efficiency of many physiological functions timed to the exact time of day, or with the development of some of the so-called civilization diseases. Constant light in adults also results in deterioration of the cognitive abilities or changes in the sleep structure. The effect of night light on the health of an adult organism is studied mainly in connection with shift work or with light pollution. The question of what effect the increased level of night light has on the development of the organism, especially on the development of the nervous system and the circadian system itself, is less studied. This diploma thesis focused on the identification of the extent of changes in the expression of Per2, Nr1d1, Stat3, BDNF genes, as well as genes encoding NMDA receptor subunits and some tissue-specific genes in the retina. Our experiments were performed on adult Long-Evans rats, that spent the first 20 days of their postnatal development in low-intensity constant light. Changes in expression were determined by quantification of mRNA by RT-qPCR in the structures of the frontal and parietal cortex, olfactory bulb, hippocampus, suprachiasmatic nucleus and retina. Behavioral tests...

In order to adapt to changing external conditions, organisms developed the endogenous biological clock for predicting daily alterations. This so-called circadian system drives functions and processes in the whole body with an approximately 24h period. The central oscillator, located in hypothalamic suprachiasmatic nuclei (SCN), is synchronized by light and subsequently sends the information about the time of the day to the rest of the body. Even in the ontogenesis, the functional SCN clock is crucial for proper development as well as health later in life. Since the maturation of embryonic SCN is not completed before birth, maternal signals seem to play a fundamental role in setting and synchronizing the fetal clock. During my PhD studies, we focused on elucidating the nature of maternal signals and their diverse impact on fetal SCN of rat and mouse models. We have revealed that developing SCN is able to sense distinct signals related to various maternal behavioral regimes. Importantly, we have discovered eminent role of glucocorticoids in synchronizing the fetal SCN, along with their ability to accelerate SCN development. These observations point out the importance of regular daily routine and noxious effect of stress during pregnancy. Since the mother communicates with the fetus through placenta...