Characterization of the multisynaptic neuronal control of the rat pineal gland using viral transneuronal tracing

Impact of Modern Lifestyle on Circadian Health and Its Contribution to Adipogenesis and Cancer Risk

BACKGROUND

Recent research underscores a crucial connection between circadian rhythm disruption and cancer promotion, highlighting an urgent need for attention.

OBJECTIVES

Explore the molecular mechanisms by which modern lifestyle factors-such as artificial light exposure, shift work, and dietary patterns-affect cortisol/melatonin regulation and cancer risk.

METHODS

Employing a narrative review approach, we synthesized findings from Scopus, Google Scholar, and PubMed to analyze lifestyle impacts on circadian health, focusing on cortisol and melatonin chronobiology as molecular markers. We included studies that documented quantitative changes in these markers due to modern lifestyle habits, excluding those lacking quantitative data or presenting inconclusive results. Subsequent sections focused solely on articles that quantified the effects of circadian disruption on adipogenesis and tumor microenvironment modifications.

RESULTS

This review shows how modern habits lead to molecular changes in cortisol and melatonin, creating adipose microenvironments that support cancer development. These disruptions facilitate immune evasion, chemotherapy resistance, and tumor growth, highlighting the critical roles of cortisol dysregulation and melatonin imbalance.

CONCLUSIONS

Through the presented findings, we establish a causal link between circadian rhythm dysregulation and the promotion of certain cancer types. By elucidating this relationship, the study emphasizes the importance of addressing lifestyle factors that contribute to circadian misalignment, suggesting that targeted interventions could play a crucial role in mitigating cancer risk and improving overall health outcomes.

Beyond sleep: A multidimensional model of chronotype

Chronotype can be defined as an expression or proxy for circadian rhythms of varied mechanisms, for example in body temperature, cortisol secretion, cognitive functions, eating and sleeping patterns. It is influenced by a range of internal (e.g., genetics) and external factors (e.g., light exposure), and has implications for health and well-being. Here, we present a critical review and synthesis of existing models of chronotype. Our observations reveal that most existing models and, as a consequence, associated measures of chronotype have focused solely or primarily on the sleep dimension, and typically have not incorporated social and environmental influences on chronotype. We propose a multidimensional model of chronotype, integrating individual (biological and psychological), environmental and social factors that appear to interact to determine an individual's true chronotype with potential feedback loops between these factors. This model could be beneficial not only from a basic science perspective but also in the context of understanding health and clinical implications of certain chronotypes as well as designing preventive and therapeutic approaches for related illnesses.

ACE2, Circumventricular Organs and the Hypothalamus, and COVID-19

The SARS-CoV-2 virus gains entry to cells by binding to angiotensin-converting enzyme 2 (ACE2). Since circumventricular organs and parts of the hypothalamus lack a blood-brain barrier, and immunohistochemical studies demonstrate that ACE2 is highly expressed in circumventricular organs which are intimately connected to the hypothalamus, and the hypothalamus itself, these might be easy entry points for SARS-CoV-2 into the brain via the circulation. High ACE2 protein expression is found in the subfornical organ, area postrema, and the paraventricular nucleus of the hypothalamus (PVH). The subfornical organ and PVH are parts of a circuit to regulate osmolarity in the blood, through the secretion of anti-diuretic hormone into the posterior pituitary. The PVH is also the stress response centre in the brain. It controls not only pre-ganglionic sympathetic neurons, but is also a source of corticotropin-releasing hormone, that induces the secretion of adrenocorticotropic hormone from the anterior pituitary. It is proposed that the function of ACE2 in the circumventricular organs and the PVH could be diminished by binding with SARS-CoV-2, thus leading to a reduction in the ACE2/Ang (1-7)/Mas receptor (MasR) signalling axis, that modulates ACE/Ang II/AT1R signalling. This could result in increased presympathetic activity/neuroendocrine secretion from the PVH, and effects on the hypothalamic-pituitary-adrenal axis activity. Besides the bloodstream, the hypothalamus might also be affected by SARS-CoV-2 via transneuronal spread along the olfactory/limbic pathways. Exploring potential therapeutic pathways to prevent or attenuate neurological symptoms of COVID-19, including drugs which modulate ACE signalling, remains an important area of unmet medical need.

An ultrastructural study of the deep pineal gland of the Sprague Dawley rat using transmission and serial block face scanning electron microscopy: cell types, barriers, and innervation

The morphology of the deep pineal gland of the Sprague Dawley rat was investigated by serial block face scanning electron microscopy. Cells were three-dimensionally (3-D) reconstructed using the software Fiji TrackEM. The deep pineal gland consisted of 2-5 layers of electron-lucent pinealocytes, with a euchromatic nucleus, endowed with one or two processes. Laterally, the deep pineal merged with the habenula and the stria medullaris thalami, via an intermediate area containing cells with more electron-dense cytoplasm and an indented nucleus with heterochromatin. Neither nerve terminals nor capillaries were observed in the deep pineal itself but present in the intermediate parts of the gland. The deep pineal was in contact with the third ventricle via the pineal and suprahabenular recesses. The ependymal lining in these recesses was an epithelium connected by tight junctions between their lateral cell membranes. Several intraventricular nerve terminals were in contact with the ependyma. 3-D reconstructions showed the ependymal cells endowed with long slender process penetrating the underlying pineal parenchyma. Few "tanocyte-like" ependymal cells, endowed with a process, reaching the subarachnoid space on the inferior surface of the deep pineal were observed. In addition, pinealocyte and astrocyte processes, often connected by gap junctions, bordered the inferior surface. In summary, the rat deep pineal gland is a neuroendocrine structure connected to the habenula. We here report specialized ependymal cells that might transmit signals from the cerebrospinal fluid to the deep pineal parenchyma and a "trans-pineal tanocyte-like cell" that connects the ventricular system with the subarachnoid space.

CircRNA-WNK2 Acts as a ceRNA for miR-328a-3p to Promote AANAT Expression in the Male Rat Pineal Gland

Noncoding RNAs (ncRNAs), including microRNAs (miRNAs) and circular RNAs (circRNAs), which are expressed with a daily rhythm in the rat pineal gland, are associated with the regulation of melatonin secretion and other biological functions. However, the mechanisms of these molecules in the rat pineal gland are not yet fully understood. In this study, we found that circR-WNK2 was highly expressed at night, which may be involved in the regulation of melatonin secretion through the competitive endogenous RNA (ceRNA) mechanism. By dual luciferase reporter, RNA pull-down, and fluorescence in situ hybridization (FISH) assays, we found that miR-328a-3p can target circR-WNK2 and the Aa-nat mRNA 3'UTR. Transfection experiments indicated that circR-WNK2 could competitively bind to miR-328a-3p, reduce miR-328a-3p expression, and promote Aa-nat gene expression and melatonin secretion. And by constructing a superior cervical ganglionectomy (SCGx) rat model, we found that ncRNAs expression in the pineal gland was regulated by signals from the suprachiasmatic nucleus. This finding supports the hypothesis that these noncoding RNAs may interact to shape the circadian rhythm through transcriptional processing in melatonin synthesis.

Iterative Metaplasticity Across Timescales: How Circadian, Ultradian, and Infradian Rhythms Modulate Memory Mechanisms

Work in recent years has provided strong evidence for the modulation of memory function and neuroplasticity mechanisms across circadian (daily), ultradian (shorter-than-daily), and infradian (longer-than-daily) timescales. Despite rapid progress, however, the field has yet to adopt a general framework to describe the overarching role of biological rhythms in memory. To this end, Iyer and colleagues introduced the term iterative metaplasticity, which they define as the "gating of receptivity to subsequent signals that repeats on a cyclic timebase." The central concept is that the cyclic regulation of molecules involved in neuroplasticity may produce cycles in neuroplastic capacity-that is, the ability of neural cells to undergo activity-dependent change. Although Iyer and colleagues focus on the circadian timescale, we think their framework may be useful for understanding how biological rhythms influence memory more broadly. In this review, we provide examples and terminology to explain how the idea of iterative metaplasticity can be readily applied across circadian, ultradian, and infradian timescales. We suggest that iterative metaplasticity may not only support the temporal niching of neuroplasticity processes but also serve an essential role in the maintenance of memory function.

Organization of the neuroendocrine and autonomic hypothalamic paraventricular nucleus

A major function of the nervous system is to maintain a relatively constant internal environment. The distinction between our external environment (i.e., the environment that we live in and that is subject to major changes, such as temperature, humidity, and food availability) and our internal environment (i.e., the environment formed by the fluids surrounding our bodily tissues and that has a very stable composition) was pointed out in 1878 by Claude Bernard (1814-1878). Later on, it was indicated by Walter Cannon (1871-1945) that the internal environment is not really constant, but rather shows limited variability. Cannon named the mechanism maintaining this limited variability homeostasis. Claude Bernard envisioned that, for optimal health, all physiologic processes in the body needed to maintain homeostasis and should be in perfect harmony with each other. This is illustrated by the fact that, for instance, during the sleep-wake cycle important elements of our physiology such as body temperature, circulating glucose, and cortisol levels show important variations but are in perfect synchrony with each other. These variations are driven by the biologic clock in interaction with hypothalamic target areas, among which is the paraventricular nucleus of the hypothalamus (PVN), a core brain structure that controls the neuroendocrine and autonomic nervous systems and thus is key for integrating central and peripheral information and implementing homeostasis. This chapter focuses on the anatomic connections between the biologic clock and the PVN to modulate homeostasis according to the daily sleep-wake rhythm. Experimental studies have revealed a highly specialized organization of the connections between the clock neurons and neuroendocrine system as well as preautonomic neurons in the PVN. These complex connections ensure a logical coordination between behavioral, endocrine, and metabolic functions that helps the organism maintain homeostasis throughout the day.

Vasopressin and oxytocin beyond the pituitary in the human brain

Vasopressin and oxytocin are primarily synthesized in the magnocellular supraoptic and paraventricular nuclei of the hypothalamus and transported to the posterior pituitary. In the human, an extensive accessory magnocellular neuroendocrine system is present with contact to the posterior pituitary and blood vessels in the hypothalamus itself. Vasopressin and oxytocin are involved in social and behavioral functions. However, only few neocortical areas are targeted by vasopressinergic and oxytocinergic nerve fibers, which mostly project to limbic areas in the forebrain, where also their receptors are located. Vasopressinergic/oxytocinergic perikarya in the forebrain project to the brain stem and spinal cord targeting nuclei and areas involved in autonomic functions. Parvocellular neurons containing vasopressin are located in the suprachiasmatic nucleus and synchronize the activity of the pacemaker in this nucleus. From the suprachiasmatic nucleus fibers project to the parvocellular part of the paraventricular nucleus, where preautonomic neurons project to the intermediolateral nucleus in the thoracic spinal cord, from where the superior cervical ganglion is reached whose noradrenergic fibers terminate in the pineal gland to stimulate melatonin secretion at night. The pineal gland is also innervated by vasopressin- and oxytocin-containing fibers reaching the gland via the "central innervation" in the pineal stalk, which might be involve in an annual regulation of melatonin secretion.

Prolonged functional cerebral asymmetry as a consequence of dysfunctional parvocellular paraventricular hypothalamic nucleus signaling: An integrative model for the pathophysiology of bipolar disorder

Approximately 45 million people worldwide are diagnosed with bipolar disorder (BD). While there are many known risk factors and models of the pathologic processes influencing BD, the exact neurologic underpinnings of BD are unknown. We attempt to integrate the existing literature and create a unifying hypothesis regarding the pathophysiology of BD with the hope that a concrete model may potentially facilitate more specific diagnosis, prevention, and treatment of BD in the future. We hypothesize that dysfunctional signaling from the parvocellular neurons of the paraventricular hypothalamic nucleus (PVN) results in the clinical presentation of BD. Functional damage to this nucleus and its signaling pathways may be mediated by myriad factors (e.g. immune dysregulation and auto-immune processes, polygenetic variation, dysfunctional interhemispheric connections, and impaired or overactivated hypothalamic axes) which could help explain the wide variety of clinical presentations along the BD spectrum. The neurons of the PVN regulate ultradian rhythms, which are observed in cyclic variations in healthy individuals, and mediate changes in functional hemispheric lateralization. Theoretically, dysfunctional PVN signaling results in prolonged functional hemispheric dominance. In this model, prolonged right hemispheric dominance leads to depressive symptoms, whereas left hemispheric dominance correlated to the clinical picture of mania. Subsequently, physiologic processes that increase signaling through the PVN (hypothalamic-pituitaryadrenal axis, hypothalamic- pituitary-gonadal axis, and hypothalamic-pituitary-thyroid axis activity, suprachiasmatic nucleus pathways) as well as, neuro-endocrine induced excito-toxicity, auto-immune and inflammatory flairs may induce mood episodes in susceptible individuals. Potentially, ultradian rhythms slowing with age, in combination with changes in hypothalamic axes and maturation of neural circuitry, accounts for BD clinically presenting more frequently in young adulthood than later in life.

Regulation and function of extra-SCN circadian oscillators in the brain

Most organisms evolved endogenous, so called circadian clocks as internal timekeeping mechanisms allowing them to adapt to recurring changes in environmental demands brought about by 24-hour rhythms such as the light-dark cycle, temperature variations or changes in humidity. The mammalian circadian clock system is based on cellular oscillators found in all tissues of the body that are organized in a hierarchical fashion. A master pacemaker located in the suprachiasmatic nucleus (SCN) synchronizes peripheral tissue clocks and extra-SCN oscillators in the brain with each other and with external time. Different time cues (so called Zeitgebers) such as light, food intake, activity and hormonal signals reset the clock system through the SCN or by direct action at the tissue clock level. While most studies on non-SCN clocks so far have focused on peripheral tissues, several extra-SCN central oscillators were characterized in terms of circadian rhythm regulation and output. Some of them are directly innervated by the SCN pacemaker, while others receive indirect input from the SCN via other neural circuits or extra-brain structures. The specific physiological function of these non-SCN brain oscillators as well as their role in the regulation of the circadian clock network remains understudied. In this review we summarize our current knowledge about the regulation and function of extra-SCN circadian oscillators in different brain regions and devise experimental approaches enabling us to unravel the organization of the circadian clock network in the central nervous system.

Circadian circuits in humans: White matter microstructure predicts daytime sleepiness

The suprachiasmatic nucleus of the hypothalamus is the chief circadian pacemaker in the brain, and is entrained to day-night cycles by visual afferents from melanopsin containing retinal ganglion cells via the inferior accessory optic tract. Tracer studies have demonstrated efferents from the suprachiasmatic nucleus projecting to the paraventricular nucleus of the hypothalamus, which in turn project to first-order sympathetic neurons in the intermedio-lateral grey of the spinal cord. Sympathetic projections to the pineal gland trigger the secretion of the sleep inducing hormone melatonin. The current study reports the first demonstration of potential sympathopetal hypothalamic projections involved in circadian regulation in humans with in vivo virtual white matter dissections using probabilistic diffusion tensor imaging (DTI) tractography. Additionally, our data shows a correlation between individual differences in white matter microstructure (measured with fractional anisotropy) and increased daytime sleepiness [measured with the Epworth Sleepiness Scale (ESS, Johns, 1991)]. Sympathopetal connections with the hypothalamus were virtually dissected using designated masks on the optic chiasm, which served as an anatomical landmark for retinal fibres projecting to the suprachiasmatic nucleus, and a waypoint mask on the lateral medulla, where hypothalamic projections to the sympathetic nervous system traverse in humans. Sympathopetal projections were demonstrated in each hemisphere in twenty-six subjects. The tract passed through the suprachiasmatic nucleus of the hypothalamus and its trajectory corresponds to the dorsal longitudinal fasciculus traversing the periaqueductal region and the lateral medulla. White matter microstructure (FA) in the left hemisphere correlated with high scores on the ESS, suggesting an association between circadian pathway white matter microstructure, and increased daytime sleepiness.

Shared Autonomic Pathways Connect Bone Marrow and Peripheral Adipose Tissues Across the Central Neuraxis

Bone marrow adipose tissue (BMAT) is increased in both obesity and anorexia. This is unique relative to white adipose tissue (WAT), which is generally more attuned to metabolic demand. It suggests that there may be regulatory pathways that are common to both BMAT and WAT and also those that are specific to BMAT alone. The central nervous system (CNS) is a key mediator of adipose tissue function through sympathetic adrenergic neurons. Thus, we hypothesized that central autonomic pathways may be involved in BMAT regulation. To test this, we first quantified the innervation of BMAT by tyrosine hydroxylase (TH) positive nerves within the metaphysis and diaphysis of the tibia of B6 and C3H mice. We found that many of the TH+ axons were concentrated around central blood vessels in the bone marrow. However, there were also areas of free nerve endings which terminated in regions of BMAT adipocytes. Overall, the proportion of nerve-associated BMAT adipocytes increased from proximal to distal along the length of the tibia (from ~3-5 to ~14-24%), regardless of mouse strain. To identify the central pathways involved in BMAT innervation and compare to peripheral WAT, we then performed retrograde viral tract tracing with an attenuated pseudorabies virus (PRV) to infect efferent nerves from the tibial metaphysis (inclusive of BMAT) and inguinal WAT (iWAT) of C3H mice. PRV positive neurons were identified consistently from both injection sites in the intermediolateral horn of the spinal cord, reticular formation, rostroventral medulla, solitary tract, periaqueductal gray, locus coeruleus, subcoeruleus, Barrington's nucleus, and hypothalamus. We also observed dual-PRV infected neurons within the majority of these regions. Similar tracings were observed in pons, midbrain, and hypothalamic regions from B6 femur and tibia, demonstrating that these results persist across mouse strains and between skeletal sites. Altogether, this is the first quantitative report of BMAT autonomic innervation and reveals common central neuroanatomic pathways, including putative "command" neurons, involved in coordinating multiple aspects of sympathetic output and facilitation of parallel processing between bone marrow/BMAT and peripheral adipose tissue.

Cryptochrome deficiency enhances transcription but reduces protein levels of pineal Aanat

Cryptochrome (Cry) 1 and 2 are essential for circadian rhythm generation, not only in the suprachiasmatic nucleus, the site of the mammalian master circadian clock, but also in peripheral organs throughout the body. CRY is also known as a repressor of arylalkylamine-N-acetyltransferase (Aanat) transcription; therefore, Cry deficiency is expected to induce constantly high pineal melatonin content. Nevertheless, we previously found that the content was consistently low in melatonin-proficient Cry1 and Cry2 double-deficient mice (Cry1−/−/Cry2−/−) on C3H background. This study aims to clarify the mechanism underlying this discrepancy. In the Cry1−/−/Cry2−/− pineal, expression levels of Aanat and clock gene Per1 were consistently high with no circadian fluctuation on the first day in constant darkness, demonstrating that CRY acts in vivo as a repressor of the pineal circadian clock and AANAT. In contrast, the enzyme activity and protein levels of AANAT remained low throughout the day, supporting our previous observation of continuously low melatonin. Thus, effects of Cry deficiency on the responses of β-adrenergic receptors were examined in cultured pineal glands. Isoproterenol, a β-adrenergic stimulant, significantly increased melatonin content, although the increase was smaller in Cry1−/−/Cry2−/− than in WT mice, during both the day and night. However, the increase in cAMP in response to forskolin was similar in both genotypes, indicating that CRY deficiency does not affect the pathway downstream of the β-adrenergic receptor. These results suggest that a lack of circadian adrenergic input due to CRY deficiency decreases β-receptor activity and cAMP levels, resulting in consistently low AANAT levels despite abundant Aanat mRNA.

Circadian Rhythms in Adipose Tissue Physiology

The different types of adipose tissues fulfill a wide range of biological functions-from energy storage to hormone secretion and thermogenesis-many of which show pronounced variations over the course of the day. Such 24-h rhythms in physiology and behavior are coordinated by endogenous circadian clocks found in all tissues and cells, including adipocytes. At the molecular level, these clocks are based on interlocked transcriptional-translational feedback loops comprised of a set of clock genes/proteins. Tissue-specific clock-controlled transcriptional programs translate time-of-day information into physiologically relevant signals. In adipose tissues, clock gene control has been documented for adipocyte proliferation and differentiation, lipid metabolism as well as endocrine function and other adipose oscillations are under control of systemic signals tied to endocrine, neuronal, or behavioral rhythms. Circadian rhythm disruption, for example, by night shift work or through genetic alterations, is associated with changes in adipocyte metabolism and hormone secretion. At the same time, adipose metabolic state feeds back to central and peripheral clocks, adjusting behavioral and physiological rhythms. In this overview article, we summarize our current knowledge about the crosstalk between circadian clocks and energy metabolism with a focus on adipose physiology. © 2017 American Physiological Society. Compr Physiol 7:383-427, 2017.

60 YEARS OF NEUROENDOCRINOLOGY: Regulation of mammalian neuroendocrine physiology and rhythms by melatonin

The isolation of melatonin was first reported in 1958. Since the demonstration that pineal melatonin synthesis reflects both daily and seasonal time, melatonin has become a key element of chronobiology research. In mammals, pineal melatonin is essential for transducing day-length information into seasonal physiological responses. Due to its lipophilic nature, melatonin is able to cross the placenta and is believed to regulate multiple aspects of perinatal physiology. The endogenous daily melatonin rhythm is also likely to play a role in the maintenance of synchrony between circadian clocks throughout the adult body. Pharmacological doses of melatonin are effective in resetting circadian rhythms if taken at an appropriate time of day, and can acutely regulate factors such as body temperature and alertness, especially when taken during the day. Despite the extensive literature on melatonin physiology, some key questions remain unanswered. In particular, the amplitude of melatonin rhythms has been recently associated with diseases such as type 2 diabetes mellitus but understanding of the physiological significance of melatonin rhythm amplitude remains poorly understood.

An intact dorsomedial posterior arcuate nucleus is not necessary for photoperiodic responses in Siberian hamsters

Seasonal responses of many animal species are triggered by changes in daylength and its transduction into a neuroendocrine signal by the pineal gland through the nocturnal duration of melatonin (MEL) release. The precise central sites necessary to receive, transduce, and relay the short day (SD) fall-winter MEL signals into seasonal responses and changes in physiology and behavior are unclear. In Siberian hamsters, SDs trigger decreases in body and lipid mass, testicular regression and pelage color changes. Several candidate genes and their central sites of expression have been proposed as components of the MEL transduction system with considerable recent focus on the arcuate nucleus (ARC) and its component, the dorsomedial posterior arcuate nucleus (dmpARC). This site has been postulated as a critical relay of SD information through the modulation of a variety of neurochemicals/receptors important for the control of energy balance. Here the necessity of an intact dmpARC for SD responses was tested by making electrolytic lesions of the Siberian hamster dmpARC and then exposing them to either long days (LD) or SDs for 12wks. The SD typical decreases in body and fat mass, food intake, testicular volume, serum testosterone concentrations, pelage color change and increased UCP-1 protein expression (a proxy for brown adipose tissue thermogenesis) all occurred despite the lack of an intact dmpARC. Although the Siberian hamster dmpARC contains photoperiod-modulated constituents, these data demonstrate that an intact dmpARC is not necessary for SD responses and not integral to the seasonal energy- and reproductive-related responses measured here.

Melatonin feedback on clock genes: a theory involving the proteasome

The expression of 'clock' genes occurs in all tissues, but especially in the suprachiasmatic nuclei (SCN) of the hypothalamus, groups of neurons in the brain that regulate circadian rhythms. Melatonin is secreted by the pineal gland in a circadian manner as influenced by the SCN. There is also considerable evidence that melatonin, in turn, acts on the SCN directly influencing the circadian 'clock' mechanisms. The most direct route by which melatonin could reach the SCN would be via the cerebrospinal fluid of the third ventricle. Melatonin could also reach the pars tuberalis (PT) of the pituitary, another melatonin-sensitive tissue, via this route. The major 'clock' genes include the period genes, Per1 and Per2, the cryptochrome genes, Cry1 and Cry2, the clock (circadian locomotor output cycles kaput) gene, and the Bmal1 (aryl hydrocarbon receptor nuclear translocator-like) gene. Clock and Bmal1 heterodimers act on E-box components of the promoters of the Per and Cry genes to stimulate transcription. A negative feedback loop between the cryptochrome proteins and the nucleus allows the Cry and Per proteins to regulate their own transcription. A cycle of ubiquitination and deubiquitination controls the levels of CRY protein degraded by the proteasome and, hence, the amount of protein available for feedback. Thus, it provides a post-translational component to the circadian clock mechanism. BMAL1 also stimulates transcription of REV-ERBα and, in turn, is also partially regulated by negative feedback by REV-ERBα. In the 'black widow' model of transcription, proteasomes destroy transcription factors that are needed only for a particular period of time. In the model proposed herein, the interaction of melatonin and the proteasome is required to adjust the SCN clock to changes in the environmental photoperiod. In particular, we predict that melatonin inhibition of the proteasome interferes with negative feedback loops (CRY/PER and REV-ERBα) on Bmal1 transcription genes in both the SCN and PT. Melatonin inhibition of the proteasome would also tend to stabilize BMAL1 protein itself in the SCN, particularly at night when melatonin is naturally elevated. Melatonin inhibition of the proteasome could account for the effects of melatonin on circadian rhythms associated with molecular timing genes. The interaction of melatonin with the proteasome in the hypothalamus also provides a model for explaining the dramatic 'time of day' effect of melatonin injections on reproductive status of seasonal breeders. Finally, the model predicts that a proteasome inhibitor such as bortezomib would modify circadian rhythms in a manner similar to melatonin.

The internal time-giver role of melatonin. A key for our health

Daily rhythms in physiological and behavioural processes are controlled by a network of circadian clocks. In mammals, at the top of the network is a master clock located in the suprachiasmatic nuclei (SCN) of the hypothalamus. The nocturnal synthesis and release of melatonin by the pineal gland are tightly controlled by the SCN clock. Several roles of melatonin in the circadian system have been identified. As a major hormonal output, melatonin distributes temporal cues generated by the SCN to the multitude of tissues expressing melatonin receptors. In some target tissues, these melatonin signals can drive daily rhythmicity that would otherwise be lacking. In other target structures, melatonin signals are used for the synchronization (i.e., adjustment of the timing of existing oscillations) of peripheral oscillators. Due to the expression of melatonin receptors in the SCN, endogenous melatonin is also able to feedback onto the master clock. Of note, pharmacological treatment with exogenous melatonin can synchronize the SCN clock. From a clinical point of view, provided that the subject is not exposed to light at night, the daily profile of circulating melatonin provides a reliable estimate of the timing of the human SCN. During the past decade, a number of melatonin agonists have been developed. These drugs may target the SCN for improving circadian timing or act indirectly at some downstream level of the circadian network to restore proper internal synchronization.

Adrenergic activation of melatonin secretion in ovine pineal explants in short-term superfusion culture occurs via protein synthesis independent and dependent phenomena

The ovine pineal is generally considered as an interesting model for the study on adrenergic regulation of melatonin secretion due to some functional similarities with this gland in the human. The present investigations, performed in the superfusion culture of pineal explants, demonstrated that the norepinephrine-induced elevation of melatonin secretion in ovine pinealocytes comprised of two subsequent periods: a rapid increase phase and a slow increase phase. The first one included the quick rise in release of N-acetylserotonin and melatonin, occurring parallel to elevation of NE concentration in the medium surrounding explants. This rapid increase phase was not affected by inhibition of translation. The second, slow increase phase began after NE level had reached the maximum concentration in the culture medium and lasted about two hours. It was completely abolished by the treatment with translation inhibitors. The obtained results showed for the first time that the regulation of N-acetylserotonin synthesis in pinealocytes of some species like the sheep involves the on/off mechanism, which is completely independent of protein synthesis and works very fast. They provided strong evidence pointing to the need of revision of the current opinion that arylalkylamines N-acetyltransferase activity in pinealocytes is controlled exclusively by changes in enzyme abundance.

Interactions between endocrine and circadian systems

In most species, endogenous circadian clocks regulate 24-h rhythms of behavior and physiology. Clock disruption has been associated with decreased cognitive performance and increased propensity to develop obesity, diabetes, and cancer. Many hormonal factors show robust diurnal secretion rhythms, some of which are involved in mediating clock output from the brain to peripheral tissues. In this review, we describe the mechanisms of clock-hormone interaction in mammals, the contribution of different tissue oscillators to hormonal regulation, and how changes in circadian timing impinge on endocrine signalling and downstream processes. We further summarize recent findings suggesting that hormonal signals may feed back on circadian regulation and how this crosstalk interferes with physiological and metabolic homeostasis.

Synaptic and extrasynaptic transmission of kidney-related neurons in the rostral ventrolateral medulla

The rostral ventrolateral medulla (RVLM) is a critical component of the sympathetic nervous system regulating homeostatic functions including arterial blood pressure. Using the transsynaptic retrograde viral tracer PRV-152, we identified kidney-related neurons in the RVLM. We found that PRV-152-labeled RVLM neurons displayed an unusually large persistent, tonic current to both glutamate, via N-methyl-d-aspartate (NMDA) and 2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)propanoic acid (AMPA)/kainate receptors, and to γ-aminobutyric acid (GABA), via GABAA receptors, in the absence of large-scale phasic neurotransmission with whole cell patch-clamp recordings. A cocktail of potent NMDA and AMPA/kainate ionotropic glutamate receptor antagonists AP-5 (50 μM) and CNQX (10 μM) revealed a two-component somatic tonic excitatory current with an overall amplitude of 42.6 ± 13.4 pA. Moreover, application of the GABAA receptor blockers gabazine (15 μM) and bicuculline (30 μM) revealed a robust somatic tonic inhibitory current with an average amplitude of 196.3 ± 39.3 pA. These findings suggest that the tonic current plays a role in determining the resting membrane potential, input resistance, and firing rate of RVLM neurons. The magnitude of the tonic inhibitory current demonstrates that GABAergic inhibition plays a critical role in regulation of kidney-related RVLM neurons. Our results indicate that the GABAergic tonic current may determine the basal tone of firing activity in kidney-related RVLM neurons.

17β-Estradiol alters the response of subfornical organ neurons that project to supraoptic nucleus to plasma angiotensin II and hypernatremia

This study was done in urethane anesthetized, ovariectomized (OVX) female rats that were either implanted or not implanted with silastic capsules containing17β-estradiol (E2) to investigate the effect of systemic changes in E2 on the discharge rate of subfornical organ (SFO) neurons that projected to supraoptic nucleus (SON) and responded to changes in plasma levels of angiotensin II (ANG II) or hypernatremia. Extracellular single unit recordings were made from 146 histologically verified single units in SFO. Intra-carotid infusions of ANG II excited ~57% of these neurons, whereas ~23% were excited by hypertonic NaCl. Basal discharge rate of neurons excited by ANG II or hypertonic NaCl was significantly lower in OVX+E2 rats compared to OVX only animals. The response of SFO neurons antidromically activated by SON stimulation to intra-carotid injections of ANG II or hypertonic NaCl was greater in the OVX only compared to the OVX+E2 rats. Intra-carotid injections of E2 in either group attenuated not only the basal discharge of these neurons, but also their response to ANG II or hypertonic NaCl. In all cases this inhibitory effect of E2 was blocked by an intra-carotid injection of the E2 receptor antagonist ICI-182780, although ICI-182780 did not alter the neuron's response to ANG II or hypertonic NaCl. Additionally, ICI-182780 in the OVX+E2 animals significantly raised the basal discharge of SFO neurons and their response to ANG II or hypertonic NaCl. These data indicate that E2 alters the response of SFO neurons to ANG II or NaCl that project to SON, and suggest that E2 functions in the female to regulate neurohypophyseal function in response to circulating ANG II and plasma hypernatremia.

Limited recovery of pineal function after regeneration of preganglionic sympathetic axons: evidence for loss of ganglionic synaptic specificity

The cervical sympathetic trunks (CSTs) contain axons of preganglionic neurons that innervate the superior cervical ganglia (SCGs). Because regeneration of CST fibers can be extensive and can reestablish certain specific patterns of SCG connections, restoration of end organ function would be expected. This expectation was examined with respect to the pineal gland, an organ innervated by the two SCGs. The activity of pineal serotonin N-acetyltransferase (NAT) exhibits a large circadian rhythm that is dependent on the sympathetic input of the gland, with high activity at night. Thirty-six hours after the CSTs were crushed bilaterally, nocturnal NAT was decreased by 99%. Three months later, enzyme activity had recovered only to 15% of control values, a recovery dependent on regeneration of CST fibers. Nevertheless, a small day/night rhythm was present in lesioned animals. Neither the density of the adrenergic innervation of the gland nor the ability of an adrenergic agonist to stimulate NAT activity was reduced in rats with regenerated CSTs. In addition, stimulation of the regenerated CST at a variety of frequencies was at least as effective in increasing NAT activity as seen with control nerves. These data suggest that the failure of pineal function to recover is not attributable to a quantitative deficit in the extent of reinnervation or synaptic efficacy. Rather, we suggest that there is some loss of specificity in the synaptic connections made in the SCG during reinnervation, resulting in a loss of the central neuronal information necessary for directing a normal NAT rhythm and thus normal pineal function.

Daily regulation of hormone profiles

The highly coordinated output of the hypothalamic biological clock does not only govern the daily rhythm in sleep/wake (or feeding/fasting) behaviour but also has direct control over many aspects of hormone release. In fact, a significant proportion of our current understanding of the circadian clock has its roots in the study of the intimate connections between the hypothalamic clock and multiple endocrine axes. This chapter will focus on the anatomical connections used by the mammalian biological clock to enforce its endogenous rhythmicity on the rest of the body, using a number of different hormone systems as a representative example. Experimental studies have revealed a highly specialised organisation of the connections between the mammalian circadian clock neurons and neuroendocrine as well as pre-autonomic neurons in the hypothalamus. These complex connections ensure a logical coordination between behavioural, endocrine and metabolic functions that will help the organism adjust to the time of day most efficiently. For example, activation of the orexin system by the hypothalamic biological clock at the start of the active phase not only ensures that we wake up on time but also that our glucose metabolism and cardiovascular system are prepared for this increased activity. Nevertheless, it is very likely that the circadian clock present within the endocrine glands plays a significant role as well, for instance, by altering these glands' sensitivity to specific stimuli throughout the day. In this way the net result of the activity of the hypothalamic and peripheral clocks ensures an optimal endocrine adaptation of the metabolism of the organism to its time-structured environment.

Transsynaptic activity-dependent regulation of axon branching and neurotrophin expression in vivo

The two major classes of activity-dependent neuroplasticity predict different consequences of activity alteration on circuit response. Hebbian plasticity (positive feedback) posits that alteration of neuronal activity causes a parallel response within a circuit. In contrast, homeostatic plasticity (negative feedback) predicts that altering neuronal activity results in compensatory responses within a circuit. The relative roles of these modes of plasticity in vivo are unclear, since neuronal circuits are difficult to manipulate in the intact organism. In this study, we tested the in vivo effects of activity deprivation in the superior cervical ganglion-pineal circuit of adult rats, which can be noninvasively silenced by exposing animals to constant light. We demonstrated that total deprivation of sympathetic activity markedly decreased the presence of axonal proteins in the pineal and reduced the density and thickness of sympathetic axonal arbors. In addition, we demonstrated that sympathetic inactivity eliminated pineal function and markedly decreased pineal expression of neurotrophins. Administration of β-adrenergic agonist restored the expression of presynaptic and postsynaptic proteins. Furthermore, compensatory axonal growth through collateral sprouting, normally seen following unilateral denervation of the pineal, was profoundly impaired in the absence of neural activity. Thus, these data suggest that sympathetic axonal terminals are maintained by neural activity that induces neurotrophins, which may act through a retrograde mechanism to preserve the integrity of axonal arbors via a positive feedback loop. Conversely, by using Hebbian-like neuroplasticity, silent yet intact circuits enter a hibernation mode marked by reduction of presynaptic axonal structures and dramatically reduced postsynaptic expression of neurotrophins.

Melatonin: both master clock output and internal time-giver in the circadian clocks network

Daily rhythms in physiological and behavioral processes are controlled by a network of circadian clocks, reset by inputs and delivering circadian signals to the brain and peripheral organs. In mammals, at the top of the network is a master clock located in the suprachiasmatic nuclei (SCN) of the hypothalamus, mainly reset by ambient light. The nocturnal synthesis and release of melatonin by the pineal gland are tightly controlled by the SCN clock and inhibited by light exposure. Several roles of melatonin in the circadian system have been identified. As a major hormonal output, melatonin distributes temporal cues generated by the SCN to the multitude of tissue targets expressing melatonin receptors. In some target structures, like the Pars tuberalis of the adenohypophysis, these melatonin signals can drive daily rhythmicity that would otherwise be lacking. In other target structures, melatonin signals are used for the synchronization (i.e., adjustment of the timing of existing oscillations) of peripheral oscillators, such as the fetal adrenal gland. Due to the expression of melatonin receptors in the SCN, endogenous melatonin is also able to feedback onto the master clock, although its physiological significance needs further characterization. Of note, pharmacological treatment with exogenous melatonin can synchronize the SCN clock. From a clinical point of view, provided that the subject is not exposed to light at night, the daily profile of circulating melatonin provides a reliable estimate of the timing of the human SCN. During the past decade, a number of melatonin agonists have been developed for treating circadian, psychiatric and sleep disorders. These drugs may target the SCN for improving circadian timing or act indirectly at some downstream level of the circadian network to restore proper internal synchronization.

Sleep duration as a risk factor for cardiovascular disease- a review of the recent literature

Sleep loss is a common condition in developed countries, with evidence showing that people in Western countries are sleeping on average only 6.8 hour (hr) per night, 1.5 hr less than a century ago. Although the effects of sleep deprivation on our organs have been obscure, recent epidemiological studies have revealed relationships between sleep deprivation and hypertension (HT), coronary heart disease (CHD), and diabetes mellitus (DM). This review article summarizes the literature on these relationships. Because sleep deprivation increases sympathetic nervous system activity, this increased activity serves as a common pathophysiology for HT and DM. Adequate sleep duration may be important for preventing cardiovascular diseases in modern society.

Microdissection of neural networks by conditional reporter expression from a Brainbow herpesvirus

Transneuronal transport of neurotropic viruses is widely used to define the organization of neural circuitry in the mature and developing nervous system. However, interconnectivity within complex circuits limits the ability of viral tracing to define connections specifically linked to a subpopulation of neurons within a network. Here we demonstrate a unique viral tracing technology that highlights connections to defined populations of neurons within a larger labeled network. This technology was accomplished by constructing a replication-competent strain of pseudorabies virus (PRV-263) that changes the profile of fluorescent reporter expression in the presence of Cre recombinase (Cre). The viral genome carries a Brainbow cassette that expresses a default red reporter in infected cells. However, in the presence of Cre, the red reporter gene is excised from the genome and expression of yellow or cyan reporters is enabled. We used PRV-263 in combination with a unique lentivirus vector that produces Cre expression in catecholamine neurons. Projection-specific infection of central circuits containing these Cre-expressing catecholamine neurons with PRV-263 resulted in Cre-mediated recombination of the PRV-263 genome and conditional expression of cyan/yellow reporters. Replication and transneuronal transport of recombined virus produced conditional reporter expression in neurons synaptically linked to the Cre-expressing catecholamine neurons. This unique technology highlights connections specific to phenotypically defined neurons within larger networks infected by retrograde transneuronal transport of virus from a defined projection target. The availability of other technologies that restrict Cre expression to defined populations of neurons indicates that this approach can be widely applied across functionally defined systems.

Loss of circadian rhythm and light-induced suppression of pineal melatonin levels in Cry1 and Cry2 double-deficient mice

Cryptochrome 1 and 2 (Cry1 and Cry2) are considered essential for generating circadian rhythms in mammals. The role of Cry1 and Cry2 in circadian rhythm expression and acute light-induced suppression of pineal melatonin was assessed using Cry1 and Cry2 double-deficient mice (Cry1(-/-) /Cry2(-/-) ) developed from the C3H strain that synthesizes melatonin. We examined the circadian variation of pineal melatonin under a 12:12-h light-dark (LD) cycle and constant darkness (DD). Light suppression of pineal melatonin concentration was analyzed by subjecting a 30-min light pulse at the peak phase of melatonin concentration. Wild-type mice showed significant rhythmicity in pineal melatonin concentration with the highest level at Zeitgeber time 22 (ZT22, where time of light on was defined as ZT0) under LD or ZT18 on the first day of DD. In contrast, Cry1(-/-) /Cry2(-/-) mice did not show significant circadian rhythmicity, with only a small peak observed at ZT22 in LD. Nevertheless, a significant daily variation could be observed under DD, with a small increase at ZT6 and ZT18 h. Melatonin concentration was significantly suppressed by acute light pulse at ZT22 in wild-type mice but not in Cry1(-/-) /Cry2(-/-) mice. The present results suggest that Cry genes are required for regulating pineal melatonin synthesis via circadian and photic signals from the suprachiasmatic nucleus of the hypothalamus (SCN).

The emerging roles of melanopsin in behavioral adaptation to light

The adaptation of behavior and physiology to changes in the ambient light level is of crucial importance to life. These adaptations include the light modulation of neuroendocrine function and temporal alignment of physiology and behavior to the day:night cycle by the circadian clock. These non-image-forming (NIF) responses can function independent of rod and cone photoreceptors but depend on ocular light reception, suggesting the participation of novel photoreceptors in the eye. The discovery of melanopsin in intrinsically photosensitive retinal ganglion cells (ipRGCs) and genetic proof for its important role in major NIF responses have offered an exciting entry point to comprehend how mammals adapt to the light environment. Here, we review the recent advances in our understanding of the emerging roles of melanopsin and ipRGCs. These findings now offer new avenues to understand the role of ambient light in sleep, alertness, dependent physiologies and potential pharmacological intervention as well as lifestyle modifications to improve the quality of life.

Evaluation of functional integrity of the retinohypothalamic tract in advanced glaucoma using multifocal electroretinography and light-induced melatonin suppression

The aim of the study was to investigate the survival of melanopsin-expressing retinal ganglion cells (mRGCs) and the functional integrity of the retinohypothalamic tract in patients with bilateral advanced glaucomatous optic neuropathy by measuring the neuroendocrine light response of the pineal gland. Nine patients with bilateral advanced primary open-angle glaucoma (glaucoma group) and nine normal control subjects (control group) were included in this pilot observational, prospective, case-control study. The best-corrected visual acuity logMAR, standard automated perimetry mean deviation, and the retinal nerve fiber layer thickness determined by optical coherence tomography and multifocal electroretinography were used to evaluate the changes. Melatonin was analyzed in the saliva by radioimmunoassay before and after exposure to bright light (600 lux) for 60 min at night. The advanced glaucoma group did not have any significant nocturnal melatonin suppression after exposure to bright light (14.28 ± 3.07 pg/ml pre-light melatonin concentration vs. 15.22 ± 3.56 pg/ml after light exposure; p = 0.798) unlike the marked melatonin suppression in the control group (22.43 ± 4.37 pg/ml pre-light melatonin concentration vs. 11.25 ± 1.89 pg/ml after light exposure; p < 0.002). Response density estimates by the scalar product amplitude measure for the interval 0-80 ms of the first-order kernel responses were similar in both groups, indicating that outer retinal function was significantly unchanged in the glaucoma group (5.95 ± 0.54 nV/dg^2) compared with the control group (6.20 ± 0.22 nV/dg^2) (p = 0.689). Our findings are consistent with the interpretation that the rhythmic secretion of melatonin was affected in advanced glaucoma, suggesting that attention should be paid to non-image-forming visual functions, such as control of circadian rhythm and the clinical impact in patients with glaucoma.

A neuroanatomical and physiological study of the non-image forming visual system of the cone-rod homeobox gene (Crx) knock out mouse

The anatomy and physiology of the non-image forming visual system was investigated in a visually blind cone-rod homeobox gene (Crx) knock-out mouse (Crx(-)(/)(-)), which lacks the outer segments of the photoreceptors. We show that the suprachiasmatic nuclei (SCN) in the Crx(-/-) mouse exhibit morphology as in the wild type mouse. In addition, the SCN contain vasoactive intestinal peptide-, vasopressin-, and gastrin-releasing peptide-immunoreactive neurons as present in the wild type. Anterograde in vivo tracings from the retina of the Crx(-/-) and wild type mouse showed that the retinohypothalamic projection to the SCN and the central optic pathways were similar in both animals. Telemetric monitoring of the running activity and temperature revealed that both the Crx(-/-)and wild type mouse exhibited diurnal rhythms with a 24-h period, which could be phase changed by light. However, power spectral analysis revealed that both rhythms in the Crx(-/-) mouse were less robust than those in the wild type. The normal development of the SCN and the central visual pathways in the Crx(-/-) mouse suggests that a modulatory input from the photoreceptors in the peripheral retina to the retinal melanopsin neurons or the SCN may be necessary for a normal function of the non-image forming system of the mouse. However, a change in the SCN of the Crx(-/-) mouse might also explain the observed circadian differences between the knock out mouse and wild type mouse.

The role of retinal photoreceptors in the regulation of circadian rhythms

The circadian clock is an evolutionarily, highly conserved feature of most organisms. This internal timing mechanism coordinates biochemical, physiological and behavioral processes to maintain synchrony with the environmental cycles of light, temperature and nutrients. Several studies have shown that light is the most potent cue used by most organisms (humans included) to synchronize daily activities. In mammals, light perception occurs only in the retina; three different types of photoreceptors are present within this tissue: cones, rods and the newly discovered intrinsically photosensitive retinal ganglion cells (ipRGCs). Researchers believe that the classical photoreceptors (e.g., the rods and the cones) are responsible for the image-forming vision, whereas the ipRGCs play a key role in the non-image forming vision. This non-image-forming photoreceptive system communicates not only with the master circadian pacemaker located in the suprachiasmatic nuclei of the hypothalamus, but also with many other brain areas that are known to be involved in the regulation of several functions; thus, this non-image forming system may also affect several aspects of mammalian health independently from the circadian system.

A Network of (Autonomic) Clock Outputs

The circadian clock in the suprachiasmatic nuclei (SCN) is composed of thousands of oscillator neurons, each dependent on the cell-autonomous action of a defined set of circadian clock genes. A major question is still how these individual oscillators are organized into a biological clock that produces a coherent output capable of timing all the different daily changes in behavior and physiology. We investigated which anatomical connections and neurotransmitters are used by the biological clock to control the daily release pattern of a number of hormones. The picture that emerged shows projections contacting target neurons in the medial hypothalamus surrounding the SCN. The activity of these pre-autonomic and neuro-endocrine target neurons is controlled by differentially timed waves of vasopressin, GABA, and glutamate release from SCN terminals, among other factors. Together our data indicate that, with regard to the timing of their main release period within the LD cycle, at least four subpopulations of SCN neurons should be discernible. The different subgroups do not necessarily follow the phenotypic differences among SCN neurons. Thus, different subgroups can be found within neuron populations containing the same neurotransmitter. Remarkably, a similar distinction of four differentially timed subpopulations of SCN neurons was recently also discovered in experiments determining the temporal patterns of rhythmicity in individual SCN neurons by way of the electrophysiology or clock gene expression. Moreover, the specialization of the SCN may go as far as a single body structure, i.e., the SCN seems to contain neurons that specifically target the liver, pineal gland, and adrenal gland.

The Biological Clock: The Bodyguard of Temporal Homeostasis

In order for any organism to function properly, it is crucial that it be table to control the timing of its biological functions. An internal biological clock, located, in mammals, in the suprachiasmatic nucleus of the hypothalamus (SCN), therefore carefully guards this temporal homeostasis by delivering its message of time throughout the body. In view of the large variety of body functions (behavioral, physiological, and endocrine) as well as the large variety in their preferred time of main activity along the light:dark cycle, it seems logical to envision different means of time distribution by the SCN. In the present review, we propose that even though it presents a unimodal circadian rhythm of general electrical and metabolic activity, the SCN seems to use several sorts of output connections that are active at different times along the light:dark cycle to control the rhythmic expression of different body functions. Although the SCN is suggested to use diffusion of synchronizing factors in the rhythmic control of behavioral functions, it also needs neuronal connections for the control of endocrine functions. The distribution of the time-of-day message to neuroendocrine systems is either directly onto endocrine neurons or via intermediate neurons located in specific SCN targets. In addition, the SCN uses its connections with the autonomic nervous system for spreading its time-of-day message, either by setting the sensitivity of endocrine glands (i.e., thyroid, adrenal, ovary) or by directly controlling an endocrine output (i.e., melatonin synthesis). Moreover, the SCN seems to use different neurotransmitters released at different times along the light:dark cycle for each of the different connection types presented. Clearly, the temporal homeostasis of endocrine functions results from a diverse set of biological clock outputs.

A Network of (Autonomic) Clock Outputs

The circadian clock in the suprachiasmatic nuclei (SCN) is composed of thousands of oscillator neurons, each of which is dependent on the cell-autonomous action of a defined set of circadian clock genes. A major question is still how these individual oscillators are organized into a biological clock producing a coherent output that is able to time all the different daily changes in behavior and physiology. We investigated which anatomical connections and neurotransmitters are used by the biological clock to control the daily release pattern of a number of hormones. The picture that emerged shows projections contacting target neurons in the medial hypothalamus surrounding the SCN. The activity of these pre-autonomic and neuro-endocrine target neurons is controlled by differentially timed waves of, among others, vasopressin, GABA, and glutamate release from SCN terminals. Together our data indicate that, with regard to the timing of their main release period within the light-dark (LD) cycle, at least 4 subpopulations of SCN neurons should be discerned. The different subgroups do not necessarily follow the phenotypic differences among SCN neurons. Thus, different subgroups can be found within neuron populations containing the same neurotransmitter. Remarkably, a similar distinction of 4 differentially timed subpopulations of SCN neurons was recently also discovered in experiments determining the temporal patterns of rhythmicity in individual SCN neurons by way of the electrophysiology or clock gene expression. Moreover, the specialization of the SCN may go as far as a single body structure; i.e., the SCN seems to contain neurons that specifically target the liver, pineal, and adrenal.

Effects of nocturnal light on (clock) gene expression in peripheral organs: a role for the autonomic innervation of the liver

Background

The biological clock, located in the hypothalamic suprachiasmatic nucleus (SCN), controls the daily rhythms in physiology and behavior. Early studies demonstrated that light exposure not only affects the phase of the SCN but also the functional activity of peripheral organs. More recently it was shown that the same light stimulus induces immediate changes in clock gene expression in the pineal and adrenal, suggesting a role of peripheral clocks in the organ-specific output. In the present study, we further investigated the immediate effect of nocturnal light exposure on clock genes and metabolism-related genes in different organs of the rat. In addition, we investigated the role of the autonomic nervous system as a possible output pathway of the SCN to modify the activity of the liver after light exposure.

Methodology and Principal Findings

First, we demonstrated that light, applied at different circadian times, affects clock gene expression in a different manner, depending on the time of day and the organ. However, the changes in clock gene expression did not correlate in a consistent manner with those of the output genes (i.e., genes involved in the functional output of an organ). Then, by selectively removing the autonomic innervation to the liver, we demonstrated that light affects liver gene expression not only via the hormonal pathway but also via the autonomic input.

Conclusion

Nocturnal light immediately affects peripheral clock gene expression but without a clear correlation with organ-specific output genes, raising the question whether the peripheral clock plays a “decisive” role in the immediate (functional) response of an organ to nocturnal light exposure. Interestingly, the autonomic innervation of the liver is essential to transmit the light information from the SCN, indicating that the autonomic nervous system is an important gateway for the SCN to cause an immediate resetting of peripheral physiology after phase-shift inducing light exposures.

Rapid phase adjustment of melatonin and core body temperature rhythms following a 6-h advance of the light/dark cycle in the horse

Background

Rapid displacement across multiple time zones results in a conflict between the new cycle of light and dark and the previously entrained program of the internal circadian clock, a phenomenon known as jet lag. In humans, jet lag is often characterized by malaise, appetite loss, fatigue, disturbed sleep and performance deficit, the consequences of which are of particular concern to athletes hoping to perform optimally at an international destination. As a species renowned for its capacity for athletic performance, the consequences of jet lag are also relevant for the horse. However, the duration and severity of jet lag related circadian disruption is presently unknown in this species. We investigated the rates of re-entrainment of serum melatonin and core body temperature (BT) rhythms following an abrupt 6-h phase advance of the LD cycle in the horse.

Methods

Six healthy, 2 yr old mares entrained to a 12 h light/12 h dark (LD 12:12) natural photoperiod were housed in a light-proofed barn under a lighting schedule that mimicked the external LD cycle. Following baseline sampling on Day 0, an advance shift of the LD cycle was accomplished by ending the subsequent dark period 6 h early. Blood sampling for serum melatonin analysis and BT readings were taken at 3-h intervals for 24 h on alternate days for 11 days. Disturbances to the subsequent melatonin and BT 24-h rhythms were assessed using repeated measures ANOVA and analysis of Cosine curve fitting parameters.

Results

We demonstrate that the equine melatonin rhythm re-entrains rapidly to a 6-h phase advance of an LD12:12 photocycle. The phase shift in melatonin was fully complete on the first day of the new schedule and rhythm phase and waveform were stable thereafter. In comparison, the advance in the BT rhythm was achieved by the third day, however BT rhythm waveform, especially its mesor, was altered for many days following the LD shift.

Conclusion

Aside from the temperature rhythm disruption, rapid resynchronization of the melatonin rhythm suggests that the central circadian pacemaker of the horse may possess a particularly robust entrainment response. The consequences for athletic performance remain unknown.

Brain-adipose tissue neural crosstalk

The preponderance of basic obesity research focuses on its development as affected by diet and other environmental factors, genetics and their interactions. By contrast, we have been studying the reversal of a naturally-occurring seasonal obesity in Siberian hamsters. In the course of this work, we determined that the sympathetic innervation of white adipose tissue (WAT) is the principal initiator of lipid mobilization not only in these animals, but in all mammals including humans. We present irrefutable evidence for the sympathetic nervous system (SNS) innervation of WAT with respect to neuroanatomy (including its central origins as revealed by transneuronal viral tract tracers), neurochemistry (norepinephrine turnover studies) and function (surgical and chemical denervation). A relatively unappreciated role of WAT SNS innervation also is reviewed--the control of fat cell proliferation as shown by selective chemical denervation that triggers adipocyte proliferation, although the precise mechanism by which this occurs presently is unknown. There is no, however, equally strong evidence for the parasympathetic innervation of this tissue; indeed, the data largely are negative severely questioning its existence and importance. Convincing evidence also is given for the sensory innervation of WAT (as shown by tract tracing and by markers for sensory nerves in WAT), with suggestive data supporting a possible role in conveying information on the degree of adiposity to the brain. Collectively, these data offer an additional or alternative view to the predominate one of the control of body fat stores via circulating factors that serve as efferent and afferent communicators.

Noradrenergic axon terminals contact gastric preautonomic neurons in the paraventricular nucleus of the hypothalamus in rats

Hypothalamic neural activity is modulated by viscerosensory signals that are carried in large part by noradrenergic (NA) inputs to the paraventricular nucleus of the hypothalamus (PVN). The present study examined the ultrastructural relationship of NA axon varicosities with the somata and dendrites of identified gastric preautonomic PVN neurons in adult male rats. NA varicosities were visualized by immunoperoxidase labeling of dopamine beta hydroxylase (DbH), and gastric preautonomic PVN neurons were identified by immunogold labeling of pseudorabies virus (PRV) transported retrogradely and transneuronally from injection sites in the stomach wall. Among 1,136 DbH-positive varicosities identified within the parvocellular PVN in four rats, approximately 36% formed either a close apposition or a synaptic contact with a somatic or dendritic profile. The majority of identified contacts between DbH- and PRV-positive profiles were classified as close appositions that lacked clear synaptic specializations. Approximately 65% of identified synaptic contacts between DbH- and PRV-positive profiles were classified as symmetric (Gray's type II) synapses. DbH-positive terminals formed close appositions and synaptic contacts with dendritic and somatic compartments of PRV-positive neurons, although dendrites were contacted nearly five times more often than somata. These findings invite continued work to delineate the functional role of NA signaling pathways in conveying interoceptive signals to preautonomic PVN neurons under normal and pathophysiological conditions.

Thematic review series: adipocyte biology. Sympathetic and sensory innervation of white adipose tissue

During our study of the reversal of seasonal obesity in Siberian hamsters, we found an interaction between receptors for the pineal hormone melatonin and the sympathetic nervous system (SNS) outflow from brain to white adipose tissue (WAT). This ultimately led us and others to conclude that the SNS innervation of WAT is the primary initiator of lipid mobilization in these as well as other animals, including humans. There is strong neurochemical (norepinephrine turnover), neuroanatomical (viral tract tracing), and functional (sympathetic denervation-induced blockade of lipolysis) evidence for the role of the SNS in lipid mobilization. Recent findings suggest the presence of WAT sensory innervation based on strong neuroanatomical (viral tract tracing, immunohistochemical markers of sensory nerves) and suggestive functional (capsaicin sensory denervation-induced WAT growth) evidence, the latter implying a role in conveying adiposity information to the brain. By contrast, parasympathetic nervous system innervation of WAT is characterized by largely negative neuroanatomical evidence (viral tract tracing, immunohistochemical and biochemical markers of parasympathetic nerves). Functional evidence (intraneural stimulation and in situ microdialysis) for the role of the SNS innervation in lipid mobilization in human WAT is convincing, with some controversy regarding the level of sympathetic nerve activity in human obesity.

Prokineticin 2 depolarizes paraventricular nucleus magnocellular and parvocellular neurons

Blind whole-cell patch-clamp techniques were used to examine the effects of prokineticin 2 (PK2) on the excitability of magnocellular (MNC), parvocellular preautonomic (PA), and parvocellular neuroendocrine (NE) neurons within the hypothalamic paraventricular nucleus (PVN) of the rat. The majority of MNC neurons (76%) depolarized in response to 10 nm PK2, effects that were eliminated in the presence of tetrodotoxin (TTX). PK2 also caused an increase in excitatory postsynaptic potential (EPSP) frequency, a finding that was confirmed by voltage clamp recordings demonstrating increases in excitatory postsynaptic current (EPSC) frequency. The depolarizing effects of PK2 on MNC neurons were also abolished by kynurenic acid (KA), supporting the conclusion that the effects of PK2 are mediated by the activation of glutamate interneurons within the hypothalamic slice. PA (68%) and NE (67%) parvocellular neurons also depolarized in response to 10 nm PK2. However, in contrast to MNC neurons, these effects were maintained in TTX, indicating that PK2 directly affects PA and NE neurons. PK2-induced depolarizations observed in PA and NE neurons were found to be concentration-related and receptor mediated, as experiments performed in the presence of A1MPK1 (a PK2 receptor antagonist) abolished the effects of PK2 on these subpopulations of neurons. The depolarizing effects of PK2 on PA and NE neurons were also shown to be abolished by PD 98059 (a mitogen activated protein kinase (MAPK) inhibitor) suggesting that PK2 depolarizes PVN parvocellular neurons through a MAPK signalling mechanism. In combination, these studies have identified separate cellular mechanisms through which PK2 influences the excitability of different subpopulations of PVN neurons.

Paradoxical effects of NPY in the suprachiasmatic nucleus

The circadian clock in the suprachiasmatic nucleus (SCN) is synchronized by the 24 h, light : dark cycle, and is reset by photic and non-photic cues. The acute effects of light in the SCN include the increase of mRNA levels of the circadian clock gene Per1 and a dramatic reduction of pineal melatonin. Neuropeptide Y (NPY), which appears to mediate the phase-resetting effects of non-photic stimuli, prevents the ability of light, and stimuli that mimic light, to phase shift the circadian clock when injected into the SCN. The purpose of the present study was to determine if NPY inhibits the ability of light to suppress pineal melatonin. Surprisingly, NPY injected into the SCN of hamsters mimicked the effects of light by suppressing pineal melatonin levels. To confirm that NPY inhibited the effects of light on the induction of Per1 mRNA levels, Per1 mRNA levels in the SCN were measured in these same animals. NPY significantly reduced Per1 mRNA levels induced by the light pulse. The suppression of melatonin by NPY appears to be mediated by the same subtype of NPY receptors in the SCN that mediate the modulation of phase shifts. Injection of Y5 receptor agonists mimicked the effects of NPY on pineal melatonin, while injection of a Y2 agonist did not. Thus, these data are the first to demonstrate the paradoxical effects of NPY within the SCN. NPY mimics the effects of light on pineal melatonin and inhibits the effects of light on the induction of Per1 mRNA.

Influence of sympathectomy in humans on the rhythmicity of 6-sulphatoxymelatonin urinary excretion

The amount of 6-sulphatoxymelatonin, the chief metabolite of melatonin, in the urine was measured in nine patients, who were subjected to bilateral sympathectomy at the second thoracic ganglionic level for treatment of hyperhidrosis of the palms. All patients showed before surgery a normal 6-sulphatoxymelatonin excretion with a peak in the excretion during the night time. After the sympathectomy, the high night time excretion was clearly abolished in five patients but remained high in four patients. This indicates that the segmental locations of the preganglionic sympathetic perikarya in the spinal cord, stimulating the melatonin secretion in the pineal gland in humans, vary between individuals. An increase in daytime melatonin excretion was observed in the patients responding to the sympathectomy with an abolished 6-sulphatoxymelatonin rhythm. This increase could indicate that the final sympathetic neurons innervating the pineal gland might have a both stimulatory and inhibitory function.

The circadian visual system, 2005

The primary mammalian circadian clock resides in the suprachiasmatic nucleus (SCN), a recipient of dense retinohypothalamic innervation. In its most basic form, the circadian rhythm system is part of the greater visual system. A secondary component of the circadian visual system is the retinorecipient intergeniculate leaflet (IGL) which has connections to many parts of the brain, including efferents converging on targets of the SCN. The IGL also provides a major input to the SCN, with a third major SCN afferent projection arriving from the median raphe nucleus. The last decade has seen a blossoming of research into the anatomy and function of the visual, geniculohypothalamic and midbrain serotonergic systems modulating circadian rhythmicity in a variety of species. There has also been a substantial and simultaneous elaboration of knowledge about the intrinsic structure of the SCN. Many of the developments have been driven by molecular biological investigation of the circadian clock and the molecular tools are enabling novel understanding of regional function within the SCN. The present discussion is an extension of the material covered by the 1994 review, "The Circadian Visual System."

Effect of melatonin and diazepam on the dissociated circadian rhythm in rats

The main structures involved in the circadian system in mammals are the suprachiasmatic nuclei (SCN) of the hypothalamus. The SCN contain multiple autonomous single-cell circadian oscillators that are coupled among themselves, generating a single rhythm. However, under determined circumstances, the oscillators may uncouple and generate several rhythmic patterns. Rats exposed to an artificially established 22-h light-dark cycle (T22) express two stable circadian rhythms in their motor activity that reflect the separate activities of two groups of oscillators in the morphologically well-defined ventrolateral and dorsomedial SCN subdivisions. In the experiments described in this paper, we studied the effect of melatonin and diazepam (DZP) administration in drinking water on the dissociated components of rat motor activity exposed to T22, to deduce the possible mechanism of these drugs on the circadian system. In order to suppress the endogenous circadian rhythm of melatonin, in some of the rats the pineal gland or the superior cervical ganglia were removed. The results show that melatonin or DZP treatment increased the manifestation of the light-dependent component to the detriment of the manifestation of the non-light-dependent component and that melatonin, but not DZP, shortens the period of the non-light-dependent component. These findings suggest that both DZP and melatonin favor entrainment to external light, and that melatonin could also act on the SCN, producing changes in the period of the circadian cycle.

JN. Advancing age alters the expression of the ryanodine receptor 3 isoform in adult rat superior cervical ganglia

Sympathetic nerves arising from the superior cervical ganglion (SCG) protect the cerebrovasculature during periods of acute hypertension and may play a role in homeostasis of target organs. The functions of these nerves depend on calcium release triggered by activation of ryanodine receptor (RyR) channels. The function of RyR channels is in part dependent on genetic expression and regulation by numerous protein modulators such as neuronal nitric oxide synthase (nNOS) neurons also found in the SCG. We have shown that release of calcium in SCG cells is altered during late maturation and advancing age. However, the underlying molecular mechanisms that may in part account for these data are elusive. Therefore we used molecular techniques to test the hypothesis that advancing age alters the pattern of genetic expression and/or protein levels of RyRs and their modulation by nNOS in the SCG in F344 rats aged 6, 12, and 24 mo. Surprisingly, ryr1 expression was undetectable in all age groups and ryr2 and ryr3 are the predominantly transcribed isoforms in the adult rat SCG. mRNA and protein levels for RyR2 isoform did not change with advancing age. However, ryr3 mRNA levels increased from 6 to 12 mo and declined from 12 to 24 mo. Similarly, RyR3 receptor protein levels also increased from 6 to 12 mo and declined from 12 to 24 mo. Because nNOS and the phosphorylation of the RyRs have been shown to modulate the function of RyRs, total phosphorylation and nNOS protein levels were analyzed in all age groups. Phosphorylation levels of the RyRs were similar in all age groups. However, nNOS protein levels increased from 6 to 12 mo followed by decline from 12 to 24 mo. These data suggest that advancing age selectively impacts the genetic expression and protein levels of RyR3 as well as modulatory nNOS protein levels. In addition, these data may part provide some insight into the possible changes in the function of RyRs that may occur with the normal aging process.