Neuroendocrine rhythms

Quantifying the variability in the assessment of reproductive hormone levels

OBJECTIVE

To quantify how representative a single measure of reproductive hormone level is of the daily hormonal profile using data from detailed hormonal sampling in the saline placebo-treated arm conducted over several hours.

DESIGN

Retrospective analysis of data from previous interventional research studies evaluating reproductive hormones.

SETTING

Clinical Research Facility at a tertiary reproductive endocrinology centre at Imperial College Hospital NHS Foundation Trust.

PATIENTS

Overall, 266 individuals, including healthy men and women (n = 142) and those with reproductive disorders and states (n = 124 [11 with functional hypothalamic amenorrhoea, 6 with polycystic ovary syndrome, 62 women and 32 men with hypoactive sexual desire disorder, and 13 postmenopausal women]), were included in the analysis.

INTERVENTIONS

Data from 266 individuals who had undergone detailed hormonal sampling in the saline placebo-treated arms of previous research studies was used to quantify the variability in reproductive hormones because of pulsatile secretion, diurnal variation, and feeding using coefficient of variation (CV) and entropy.

MAIN OUTCOME MEASURES

The ability of a single measure of reproductive hormone level to quantify the variability in reproductive hormone levels because of pulsatile secretion, diurnal variation, and nutrient intake.

RESULTS

The initial morning value of reproductive hormone levels was typically higher than the mean value throughout the day (percentage decrease from initial morning measure to daily mean: luteinizing hormone level 18.4%, follicle-stimulating hormone level 9.7%, testosterone level 9.2%, and estradiol level 2.1%). Luteinizing hormone level was the most variable (CV 28%), followed by sex-steroid hormone levels (testosterone level 12% and estradiol level 13%), whereas follicle-stimulating hormone level was the least variable reproductive hormone (CV 8%). In healthy men, testosterone levels fell between 9:00 am and 5:00 pm by 14.9% (95% confidence interval 4.2, 25.5%), although morning levels correlated with (and could be predicted from) late afternoon levels in the same individual (r2 = 0.53, P<.0001). Testosterone levels were reduced more after a mixed meal (by 34.3%) than during ad libitum feeding (9.5%), after an oral glucose load (6.0%), or an intravenous glucose load (7.4%).

CONCLUSION

Quantification of the variability of a single measure of reproductive hormone levels informs the reliability of reproductive hormone assessment.

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.

Polyphasic circadian neural circuits drive differential activities in multiple downstream rhythmic centers

Circadian clocks align various behaviors such as locomotor activity, sleep/wake, feeding, and mating to times of day that are most adaptive. How rhythmic information in pacemaker circuits is translated to neuronal outputs is not well understood. Here, we used brain-wide, 24-h in vivo calcium imaging in the Drosophila brain and searched for circadian rhythmic activity among identified clusters of dopaminergic (DA) and peptidergic neurosecretory (NS) neurons. Such rhythms were widespread and imposed by the PERIOD-dependent clock activity within the ∼150-cell circadian pacemaker network. The rhythms displayed either a morning (M), evening (E), or mid-day (MD) phase. Different subgroups of circadian pacemakers imposed neural activity rhythms onto different downstream non-clock neurons. Outputs from the canonical M and E pacemakers converged to regulate DA-PPM3 and DA-PAL neurons. E pacemakers regulate the evening-active DA-PPL1 neurons. In addition to these canonical M and E oscillators, we present evidence for a third dedicated phase occurring at mid-day: the l-LNv pacemakers present the MD activity peak, and they regulate the MD-active DA-PPM1/2 neurons and three distinct NS cell types. Thus, the Drosophila circadian pacemaker network is a polyphasic rhythm generator. It presents dedicated M, E, and MD phases that are functionally transduced as neuronal outputs to organize diverse daily activity patterns in downstream circuits.

Neuroendocrine Regulation of Anxiety: Beyond the Hypothalamic-Pituitary-Adrenal Axis

The central nervous system regulates and responds to endocrine signals, and this reciprocal relationship determines emotional processing and behavioural anxiety. Although the hypothalamic-pituitary-adrenal (HPA) axis remains the best-characterised system for this relationship, other steroid and peptide hormones are increasingly recognised for their effects on anxiety-like behaviour and reward. The present review examines recent developments related to the role of a number of different hormones in anxiety, including pregnane neurosteroids, gut peptides, neuropeptides and hormonal signals derived from fatty acids. Findings from both basic and clinical studies suggest that these alternative systems may complement or occlude stress-induced changes in anxiety and anxiety-like behaviour. By broadening the scope of mechanisms for depression and anxiety, it may be possible to develop novel strategies to attenuate stress-related psychiatric conditions. The targets for these potential therapies, as discussed in this review, encompass multiple circuits and systems, including those outside of the HPA axis.

Circadian Tick-Talking Across the Neuroendocrine System and Suprachiasmatic Nuclei Circuits: The Enigmatic Communication Between the Molecular and Electrical Membrane Clocks

As with many processes in nature, appropriate timing in biological systems is of paramount importance. In the neuroendocrine system, the efficacy of hormonal influence on major bodily functions, such as reproduction, metabolism and growth, relies on timely communication within and across many of the brain's homeostatic systems. The activity of these circuits is tightly orchestrated with the animal's internal physiological demands and external solar cycle by a master circadian clock. In mammals, this master clock is located in the hypothalamic suprachiasmatic nucleus (SCN), where the ensemble activity of thousands of clock neurones generates and communicates circadian time cues to the rest of the brain and body. Many regions of the brain, including areas with neuroendocrine function, also contain local daily clocks that can provide feedback signals to the SCN. Although much is known about the molecular processes underpinning endogenous circadian rhythm generation in SCN neurones and, to a lesser extent, extra-SCN cells, the electrical membrane clock that acts in partnership with the molecular clockwork to communicate circadian timing across the brain is poorly understood. The present review focuses on some circadian aspects of reproductive neuroendocrinology and processes involved in circadian rhythm communication in the SCN, aiming to identify key gaps in our knowledge of cross-talk between our daily master clock and neuroendocrine function. The intention is to highlight our surprisingly limited understanding of their interaction in the hope that this will stimulate future work in these areas.

Effects of photoperiod on food intake, activity and metabolic rate in adult neutered male cats

With the continued rise in feline obesity, novel weight management strategies are needed. To date, strategies aimed at altering physical activity, an important factor in weight maintenance, have been lacking. Photoperiod is known to cause physiological changes in seasonal mammals, including changes in body weight (BW) and reproductive status. Thus, our objective was to determine the effect of increased photoperiod (longer days) on voluntary physical activity levels, resting metabolic rate (RMR), food intake required to maintain BW, and fasting serum leptin and ghrelin concentrations in adult cats. Eleven healthy, adult, neutered, male domestic shorthair cats were used in a randomized crossover design study. During two 12-week periods, cats were exposed to either a short-day (SD) photoperiod of 8 h light: 16 h dark or a long-day (LD) photoperiod of 16 h light: 8 h dark. Cats were fed a commercial diet to maintain baseline BW. In addition to daily food intake and twice-weekly BW, RMR (via indirect calorimetry), body composition [via dual-energy X-ray absorptiometry (DEXA)] and physical activity (via Actical activity monitors) were measured at week 0 and 12 of each period. Fasting serum leptin and ghrelin concentrations were measured at week 0, 6 and 12 of each period. Average hourly physical activity was greater (p = 0.008) in LD vs. SD cats (3770 vs. 3129 activity counts/h), which was primarily due to increased (p < 0.001) dark period activity (1188 vs. 710 activity counts/h). This corresponded to higher (p < 0.0001) daily metabolizable energy intake (mean over 12-week period: 196 vs. 187 kcal/day), and increased (p = 0.048) RMR in LD cats (9.02 vs. 8.37 kcal/h). Body composition, serum leptin and serum ghrelin were not altered by photoperiod. More research is needed to determine potential mechanisms by which these physiological changes occurred and how they may apply to weight management strategies.

Environmental control of biological rhythms: effects on development, fertility and metabolism

Internal temporal organisation properly synchronised to the environment is crucial for health maintenance. This organisation is provided at the cellular level by the molecular clock, a macromolecular transcription-based oscillator formed by the clock and the clock-controlled genes that is present in both central and peripheral tissues. In mammals, melanopsin in light-sensitive retinal ganglion cells plays a considerable role in the synchronisation of the circadian timing system to the daily light/dark cycle. Melatonin, a hormone synthesised in the pineal gland exclusively at night and an output of the central clock, has a fundamental role in regulating/timing several physiological functions, including glucose homeostasis, insulin secretion and energy metabolism. As such, metabolism is severely impaired after a reduction in melatonin production. Furthermore, light pollution during the night and shift work schedules can abrogate melatonin synthesis and impair homeostasis. Chronodisruption during pregnancy has deleterious effects on the health of progeny, including metabolic, cardiovascular and cognitive dysfunction. Developmental programming by steroids or steroid-mimetic compounds also produces internal circadian disorganisation that may be a significant factor in the aetiology of fertility disorders such as polycystic ovary syndrome. Thus, both early and late in life, pernicious alterations of the endogenous temporal order by environmental factors can disrupt the homeostatic function of the circadian timing system, leading to pathophysiology and/or disease.

Endothelin modulates the circadian expression of non-visual opsins

The non-visual opsin, melanopsin, expressed in the mammalian retina, is considered a circadian photopigment because it is responsible to entrain the endogenous biological clock. This photopigment is also present in the melanophores of Xenopus laevis, where it was first described, but its role in these cells is not fully understood. X. laevis melanophores respond to light with melanin granule dispersion, the maximal response being achieved at the wavelength of melanopsin maximal excitation. Pigment dispersion can also be triggered by endothelin-3 (ET-3). Here we show that melanin translocation is greater when a blue light pulse was applied in the presence of ET-3. In addition, we demonstrated that mRNA levels of the melanopsins Opn4x and Opn4m exhibit temporal variation in melanophores under light/dark (LD) cycles or constant darkness, suggesting that this variation is clock-driven. Moreover, under LD cycles the oscillations of both melanopsins show a circadian profile suggesting a role for these opsins in the photoentrainment mechanism. Blue-light pulse decreased Opn4x expression, but had no effect on Opn4m. ET-3 abolishes the circadian rhythm of expression of both opsins; in addition the hormone increases Opn4x expression in a dose-, circadian time- and light-dependent way. ET-3 also increases the expression of its own receptor, in a dose-dependent manner. The variation of melanopsin levels may represent an adaptive mechanism to ensure greater melanophore sensitivity in response to environmental light conditions with ideal magnitude in terms of melanin granule dispersion, and consequently color change.

Effect of photoperiod on the feline adipose transcriptome as assessed by RNA sequencing

Background

Photoperiod is known to cause physiological changes in seasonal mammals, including changes in body weight, physical activity, reproductive status, and adipose tissue gene expression in several species. The objective of this study was to determine the effects of day length on the adipose transcriptome of cats as assessed by RNA sequencing. Ten healthy adult neutered male domestic shorthair cats were used in a randomized crossover design study. During two 12-wk periods, cats were exposed to either short days (8 hr light:16 hr dark) or long days (16 hr light:8 hr dark). Cats were fed a commercial diet to maintain baseline body weight to avoid weight-related bias. Subcutaneous adipose biopsies were collected at wk 12 of each period for RNA isolation and sequencing.

Results

A total of 578 million sequences (28.9 million/sample) were generated by Illumina sequencing. A total of 170 mRNA transcripts were differentially expressed between short day- and long day-housed cats. 89 annotated transcripts were up-regulated by short days, while 24 annotated transcripts were down-regulated by short days. Another 57 un-annotated transcripts were also different between groups. Adipose tissue of short day-housed cats had greater expression of genes involved with cell growth and differentiation (e.g., myostatin; frizzled-related protein), cell development and structure (e.g., cytokeratins), and protein processing and ubiquitination (e.g., kelch-like proteins). In contrast, short day-housed cats had decreased expression of genes involved with immune function (e.g., plasminogen activator inhibitor 1; chemokine (C-C motif) ligand 2; C-C motif chemokine 5; T-cell activators), and altered expression of genes associated with carbohydrate and lipid metabolism.

Conclusions

Collectively, these gene expression changes suggest that short day housing may promote adipogenesis, minimize inflammation and oxidative stress, and alter nutrient metabolism in feline adipose tissue, even when fed to maintain body weight. Although this study has highlighted molecular mechanisms contributing to the seasonal metabolic changes observed in cats, future research that specifically targets and studies these biological pathways, and the physiological outcomes that are affected by them, is justified.

Circadian angiogenesis

Daily rhythms of light/darkness, activity/rest and feeding/fasting are important in human physiology and their disruption (for example by frequent changes between day and night shifts) increases the risk of disease. Many of the diseases found to be associated with such disrupted circadian lifestyles, including cancer, cardiovascular diseases, metabolic disorders and neurological diseases, depend on pathological de-regulation of angiogenesis, suggesting that disrupting the circadian clock will impair the physiological regulation of angiogenesis leading to development and progression of these diseases. Today there is little known regarding circadian regulation of pathological angiogenesis but there is some evidence that supports both direct and indirect regulation of angiogenic factors by the cellular circadian clock machinery, as well as by circulating circadian factors, important for coordinating circadian rhythms in the organism. Through highlighting recent advances both in pre-clinical and clinical research on various diseases including cancer, cardiovascular disorders and obesity, we will here present an overview of the available knowledge on the importance of circadian regulation of angiogenesis and discuss how the circadian clock may provide alternative targets for pro- or anti-angiogenic therapy in the future.

Melatonin, the circadian multioscillator system and health: the need for detailed analyses of peripheral melatonin signaling

Evidence is accumulating regarding the importance of circadian core oscillators, several associated factors, and melatonin signaling in the maintenance of health. Dysfunction of endogenous clocks, melatonin receptor polymorphisms, age- and disease-associated declines of melatonin likely contribute to numerous diseases including cancer, metabolic syndrome, diabetes type 2, hypertension, and several mood and cognitive disorders. Consequences of gene silencing, overexpression, gene polymorphisms, and deviant expression levels in diseases are summarized. The circadian system is a complex network of central and peripheral oscillators, some of them being relatively independent of the pacemaker, the suprachiasmatic nucleus. Actions of melatonin on peripheral oscillators are poorly understood. Various lines of evidence indicate that these clocks are also influenced or phase-reset by melatonin. This includes phase differences of core oscillator gene expression under impaired melatonin signaling, effects of melatonin and melatonin receptor knockouts on oscillator mRNAs or proteins. Cross-connections between melatonin signaling pathways and oscillator proteins, including associated factors, are discussed in this review. The high complexity of the multioscillator system comprises alternate or parallel oscillators based on orthologs and paralogs of the core components and a high number of associated factors with varying tissue-specific importance, which offers numerous possibilities for interactions with melatonin. It is an aim of this review to stimulate research on melatonin signaling in peripheral tissues. This should not be restricted to primary signal molecules but rather include various secondarily connected pathways and discriminate between direct effects of the pineal indoleamine at the target organ and others mediated by modulation of oscillators.

Photoperiodic modulation of adrenal gland function in the rhesus macaque: effect on 24-h plasma cortisol and dehydroepiandrosterone sulfate rhythms and adrenal gland gene expression

In temperate zones, day length changes markedly across the year, and in many mammals these photoperiodic variations are associated with physiological adaptations. However, the influence of this environmental variable on human behavior and physiology is less clear, and the potential underlying mechanisms are unknown. To address this issue, we examined the effect of changing photoperiods on adrenal gland function in ovariectomized female rhesus macaques (Macaca mulatta), both in terms of steroid hormone output and in terms of gene expression. The animals were sequentially exposed to the following lighting regimens, which were designed to simulate photoperiods associated with winter, spring/autumn and summer respectively: 8 h light:16 h darkness (short days), 12 h light:12 h darkness and 16 h light:8 h darkness (long days). Remote 24-h serial blood sampling failed to disclose any effect of photoperiod on mean or peak plasma levels of cortisol or dehydroepiandrosterone sulfate. However, there was a marked phase-advancement of both hormonal rhythms in short days, which was reflected as a similar phase-advancement of the daily motor activity rhythm. Gene microarray analysis of the adrenal gland transcriptome revealed photoperiod-induced differences in the expression of genes associated with homeostatic functions, including: development, lipid synthesis and metabolism, and immune function. Taken together, the results indicate that in primates, both circadian adrenal physiology and gene expression are influenced by seasonal changes in day length, which may have implications for adrenal-regulated physiology and behavior.

Differential testicular gene expression in seasonal fertility

Spermatogenesis is an essential precursor for successful sexual reproduction. Recently, there has been an expansion in the knowledge of the genes associated with particular stages of normal, physiological testicular development and pubertal activation. What has been lacking, however, is an understanding of those genes that are involved in specifically regulating sperm production, rather than in maturation and elaboration of the testis as an organ. By using the reversible (seasonal) fertility of the Syrian hamster as a model system, the authors sought to discover genes that are specifically involved in turning off sperm production and not involved in tissue specification and/or maturation. Using gene expression microarrays and in situ hybridization in hamsters and genetically infertile mice, the authors have identified a variety of known and novel factors involved in reversible, transcriptional, translational, and posttranslational control of testicular function, as well those involved in cell division and macromolecular metabolism. The novel genes uncovered could be potential targets for therapies against fertility disorders.

Minireview: The neuroendocrinology of the suprachiasmatic nucleus as a conductor of body time in mammals

Circadian rhythms in physiology and behavior are regulated by a master clock resident in the suprachiasmatic nucleus (SCN) of the hypothalamus, and dysfunctions in the circadian system can lead to serious health effects. This paper reviews the organization of the SCN as the brain clock, how it regulates gonadal hormone secretion, and how androgens modulate aspects of circadian behavior known to be regulated by the SCN. We show that androgen receptors are restricted to a core SCN region that receives photic input as well as afferents from arousal systems in the brain. We suggest that androgens modulate circadian behavior directly via actions on the SCN and that both androgens and estrogens modulate circadian rhythms through an indirect route, by affecting overall activity and arousal levels. Thus, this system has multiple levels of regulation; the SCN regulates circadian rhythms in gonadal hormone secretion, and hormones feed back to influence SCN functions.

Neuroanatomical and cellular substrates of hypergrooming induced by microinjection of oxytocin in central nucleus of amygdala, an experimental model of compulsive behavior

Oxytocin (OT) is a neurosecretory nonapeptide synthesized in hypothalamic cells that project to the neurohypophysis as well as to widely distributed sites in the central nervous system. Central OT microinjections induce a variety of cognitive, sexual, reproductive, grooming and affiliative behaviors in animals. Obsessive-compulsive disorder (OCD) includes a range of cognitive and behavioral symptoms that bear some relationship with OT. Here, we study the neuroanatomical and cellular substrates of the hypergrooming induced by administration of OT in the central nucleus of amygdala (CeA). In this context, this hypergrooming is considered as a model of compulsive behavior. Our data suggest a link between the CeA and the hypothalamic grooming area (HGA). The HGA includes parts of the paraventricular nucleus and the dorsal hypothalamic area. Our data on colocalization of OT (immunohistochemistry for peptide), OT receptor (binding assay) and its retrogradely labeled cells after Fluoro-Gold injection in the CeA suggest that CeA and connections are important substrates of the circuit underlying this OT-dependent compulsive behavioral pattern.

Minireview: The circadian clockwork of the suprachiasmatic nuclei--analysis of a cellular oscillator that drives endocrine rhythms

The secretion of hormones is temporally precise and periodic, oscillating over hours, days, and months. The circadian timekeeper within the suprachiasmatic nuclei (SCN) is central to this coordination, modulating the frequency of pulsatile release, maintaining daily cycles of secretion, and defining the time base for longer-term rhythms. This central clock is driven by cell-autonomous, transcriptional/posttranslational feedback loops incorporating Period (Per) and other clock genes. SCN neurons exist, however, within neural circuits, and an unresolved question is how SCN clock cells interact. By monitoring the SCN molecular clockwork using fluorescence and bioluminescence videomicroscopy of organotypic slices from mPer1::GFP and mPer1::luciferase transgenic mice, we show that interneuronal neuropeptidergic signaling via the vasoactive intestinal peptide (VIP)/PACAP2 (VPAC2) receptor for VIP (an abundant SCN neuropeptide) is necessary to maintain both the amplitude and the synchrony of clock cells in the SCN. Acute induction of mPer1 by light is, however, independent of VIP/VPAC2 signaling, demonstrating dissociation between cellular mechanisms mediating circadian control of the clockwork and those mediating its retinally dependent entrainment to the light/dark cycle. The latter likely involves the Ca(2+)/cAMP response elements of mPer genes, triggered by a MAPK cascade activated by retinal afferents to the SCN. In the absence of VPAC2 signaling, however, this cascade is inappropriately responsive to light during circadian daytime. Hence VPAC2-mediated signaling sustains the SCN cellular clockwork and is necessary both for interneuronal synchronization and appropriate entrainment to the light/dark cycle. In its absence, behavioral and endocrine rhythms are severely compromised.

Electrophysiology of the suprachiasmatic circadian clock

In mammals, an internal timekeeping mechanism located in the suprachiasmatic nuclei (SCN) orchestrates a diverse array of neuroendocrine and physiological parameters to anticipate the cyclical environmental fluctuations that occur every solar day. Electrophysiological recording techniques have proved invaluable in shaping our understanding of how this endogenous clock becomes synchronized to salient environmental cues and appropriately coordinates the timing of a multitude of physiological rhythms in other areas of the brain and body. In this review we discuss the pioneering studies that have shaped our understanding of how this biological pacemaker functions, from input to output. Further, we highlight insights from new studies indicating that, more than just reflecting its oscillatory output, electrical activity within individual clock cells is a vital part of SCN clockwork itself.

Challenging the omnipotence of the suprachiasmatic timekeeper: are circadian oscillators present throughout the mammalian brain?

The suprachiasmatic nucleus of the hypothalamus (SCN) is the master circadian pacemaker or clock in the mammalian brain. Canonical theory holds that the output from this single, dominant clock is responsible for driving most daily rhythms in physiology and behaviour. However, important recent findings challenge this uniclock model and reveal clock-like activities in many neural and non-neural tissues. Thus, in addition to the SCN, a number of areas of the mammalian brain including the olfactory bulb, amygdala, lateral habenula and a variety of nuclei in the hypothalamus, express circadian rhythms in core clock gene expression, hormone output and electrical activity. This review examines the evidence for extra-SCN circadian oscillators in the mammalian brain and highlights some of the essential properties and key differences between brain oscillators. The demonstration of neural pacemakers outside the SCN has wide-ranging implications for models of the circadian system at a whole-organism level.

Arginine vasotocin (AVT) and isotocin (IT) in fish brain: Diurnal and seasonal variations

An HPLC assay with solid-phase extraction and fluorescence derivatization was developed for measurement of arginine vasotocin (AVT) and isotocin (IT) in the neural tissues of fish. The efficiency and usefulness of the method have been verified in experiments by examination of peptides concentrations in brains of three fish species. The day-night changes in neuropeptides levels have been studied in brains of adult sea bream (Sparus aurata) and juvenile Atlantic salmon (Salmo salar). Seasonal fluctuations have been investigated in brains of three-spined sticklebacks (Gasterosteus aculeatus). The AVT and IT biosynthesis in brain seems to be controlled independently and probably each neuropeptide plays a different role in a circadian time-keeping system and an endocrine calendar in fish.

Just the two of us: melatonin and adenosine in rodent pituitary function

The function of the pituitary gland is tightly controlled by neuronal and hormonal afferents of the brain. In this review, the role of the neurohormone melatonin and the neuromodulator adenosine for rodent pituitary function will be elucidated. Adenosine is known as an important paracrine modulator for pituitary endocrine and folliculostellate cells, with availability regulated by local metabolic cellular activity. In general, adenosine inhibits the cyclic adenosine monophosphate (AMP) pathway in pituitary cells by binding to A1-, and A3-adenosinergic receptors, and activates it via A2-adenosinergic receptors. The neurohormone melatonin integrates time-of-day and time-of-year into pituitary function via binding to MT1-melatonin receptors. Melatonin impacts at the hypothalamic level neurons that synthesize releasing and release-inhibiting hormones, and at the pituitary level only cells of the hypophyseal pars tuberalis (PT). Thereby, the daily changes in the duration of the nocturnal melatonin surge are decoded and subsequently relayed to the pars distalis to adapt gonadotropin and prolactin release, respectively, to season. An exciting integration of time within the regulation of pituitary function was deciphered by analysing transmembrane signalling events in cells of the hypophyseal PT: a consecutive daily impact of initially the neurohormone melatonin and later the neuromodulator adenosine on rodent PT cells leads to a circadian rhythm in the transcription of cyclic-AMP-sensitive genes.

Projections from the raphe nuclei to the suprachiasmatic nucleus of the rat

The presence of serotonergic afferents in the hypothalamic suprachiasmatic nucleus (SCN) is well documented and several functional roles of serotonin (5-HT) in circadian function are well established. However, there is some controversy about the precise location of the serotonergic neurones from where this input arises. Discrete injection of the tracer Cholera toxin, subunit B, (ChB) was centred in the rat SCN, and a few retrograde labelled neurones were distributed in the dorsal and median raphe nuclei (MnR) and in the rostral part of the raphe magnus (RMg), but no neurones were found in the raphe pallidus or raphe obscurus. In addition, a group of neurones was consistently found in the medial part of the pontine supra lemniscal nucleus but not including the serotonergic B(9) region. A combination of retrograde tracing with Fluoro-Gold together with 5-HT-immunolabelling, showed few double-labelled neurones in the dorsal, MnR and B(9). However, the majority of projecting neurones were not co-storing 5-HT immunoreactivity. Phaseolus vulgaris-leucoagglutinin (PHA-L) injections in the dorsal raphe resulted in faint labelling, whereas the MnR gave rise to several labelled fibres in the SCN. Individual delicate PHA-L nerve fibres were found in all compartments of the SCN both in terms of rostrocaudal, ventromedial and dorsomedial extent, without any sign of a topographical organisation of the MnR input to the SCN. PHA-L injections into RMg gave rise to labelling of a few processes within the SCN. In conclusion, the main serotonergic input to the rat SCN originates from a few neurones in the MnR.

Seasonal changes in adiposity: the roles of the photoperiod, melatonin and other hormones, and sympathetic nervous system

It appears advantageous for many non-human animals to store energy body fat extensively and efficiently because their food supply is more labile and less abundant than in their human counterparts. The level of adiposity in many of these species often shows predictable increases and decreases with changes in the season. These cyclic changes in seasonal adiposity in some species are triggered by changes in the photoperiod that are faithfully transduced into a biochemical signal through the nightly secretion of melatonin (MEL) via the pineal gland. Here, we focus primarily on the findings from the most commonly studied species showing seasonal changes in adiposity-Siberian and Syrian hamsters. The data to date are not compelling for a direct effect of MEL on white adipose tissue (WAT) and brown adipose tissue (BAT) despite some recent data to the contrary. Thus far, none of the possible hormonal intermediaries for the effects of MEL on seasonal adiposity appear likely as a mechanism by which MEL affects the photoperiodic control of body fat levels indirectly. We also provide evidence pointing toward the sympathetic nervous system as a likely mediator of the effects of MEL on short day-induced body fat decreases in Siberian hamsters through increases in sympathetic drive on WAT and BAT. We speculate that decreases in the SNS drive to these tissues may underlie the photoperiod-induced seasonal increases in body fat of species such as Syrian hamsters. Clearly, we need to deepen our understanding of seasonal adiposity, although, to our knowledge, this is the only form of environmentally induced changes in body fat where the key elements of its external trigger have been identified and can be traced to and through their transduction into a physiological stimulus that ultimately affects identified responses of white adipocyte physiology and cellularity. Finally, the comparative physiological approach to the study of seasonal adiposity seems likely to continue to yield significant insights into the mechanisms underlying this phenomenon and for understanding obesity and its reversal in general.

Hypocretin (orexin) in the rat pineal gland: a central transmitter with effects on noradrenaline-induced release of melatonin

Hypocretin-1 (HCRT-1) and hypocretin 2 (HCRT-2), also known as orexin-A and orexin-B, are two neuropeptides derived from the same precursor. Hypocretinergic neurons have been found exclusively in the hypothalamic dorsolateral area. These neurons are implicated in sleep and feeding through activation of specific G-protein-coupled orexin-1 and orexin-2 receptor (OR-R1 and OR-R2). The purpose of this study was to determine the existence of the HCRT peptides in the central input of the rat pineal gland. Further, OR-R1 and OR-R2 expression was determined in the pineal gland and the effect of HCRT-2 on melatonin synthesis and secretion was analysed in dissociated rat pinealocytes. A large contingent of HCRT-positive nerve fibres and terminals were observed in the epithalamus, many of which entered into the pineal parenchyma. A significant number of nerve fibres endowed with positive boutons were identified in the pineal stalk, though the number of positive fibres decreased along the extension of the stalk. So far, no positive fibres have been found in the superficial pineal gland. RT-PCR analysis revealed the expression of OR-R2 mRNA, whereas OR-R1-receptor mRNA was not detected. When tested alone, HCRT-2 had no effect on secretion of melatonin from cultured rat pinealocytes. However, HCRT-2 partially inhibited (by a maximum of 30%) the beta-adrenergic-induced melatonin secretion. The same effect was seen on activation of N-acetyltransferase activity. The distribution and the large number of HCRT-positive fibres together with the effect on noradrenaline-mediated melatonin release through specific receptors suggests that these peptides may be significant central transmitters in pineal function, probably mediating homeostatic signals to the pineal gland.

The calcium rhythms of different cell types oscillate with different circadian phases

Transgenic tobacco (Nicotiana plumbaginifolia) seedlings containing the Ca(2+)-sensitive luminescent protein aequorin have been shown to exhibit circadian variations in cytosolic calcium. Concomitant measurements of cytosolic and nuclear calcium show that circadian variations in the cytoplasm are not expressed in the nucleus. To investigate whether all cells of transgenic seedlings contribute equally to circadian variations in cytosolic calcium, different promoters eliciting different expression patterns have been placed upstream of aequorin and used for transformation. The circadian peak occurred at different times in the three transgenic lines constructed. Luminescence imaging of these transgenic lines indicated that aequorin was differentially accumulated among the main tissues and cells of the seedlings and overcoat technology with applied epidermal strips indicated that the surface cell layers contribute the vast majority of luminescent light. We conclude that the Ca(2+) rhythmicities of cells and tissues oscillate with distinct differences in phase, that this might represent different underlying cellular control mechanisms and that these observations have significant implications for our understanding and study of Ca(2+) mediated signal transduction in plant cells.

Therapeutic applications of melatonin and related compounds

Increasing knowledge of the pharmacological effects of melatonin has suggested various possible therapeutic applications for the hormone. Because, as a natural substance, melatonin cannot be patented, melatonin-related compounds have been synthesized by industrial groups. The scope of such compounds is also to specifically target the recently discovered melatonin receptor subtypes. The sleep-inducing properties of melatonin are disputed, but are distinct from those of benzodiazepines. The observed effects on sleep latency or sleep efficiency, which remain to be confirmed, could be accounted for by the effects of melatonin on core body temperature and on circadian rhythms. There is also an urgent need for safety data, both in animals and in humans, particularly when long-term use is envisaged.

Diurnal variations in lipopolysaccharide-induced sleep, sickness behavior and changes in corticosterone levels in the rat

Inoculation of rats with microorganisms or microbial constituents that activate host defense promotes non-rapid eye movement sleep (non-REMS) and suppresses REMS. In this study, we evaluated circadian influences on the effects of lipopolysaccharide (LPS) on sleep, sickness behavior and plasma corticosterone levels in the rat. Three sets of experiments were performed. In each, the animals were intraperitoneally injected with vehicle for LPS (30 microg/kg) during 2 consecutive days, at the beginning of either the circadian rest or the activity phase. In experiment 1, sleep-wake behavior and brain temperature were recorded, and in experiment 2, core body temperature, locomotor activity as well as food and water intake. In experiment 3, corticosterone blood levels were measured. The results show that LPS-evoked changes in temperature, sleep and other behavioral parameters depend markedly on the time of day LPS is administered. However, a direct comparison of the LPS data demonstrates that, except for sleep parameters, the absolute time course of the assessed parameters was rather similar between the rest and activity phases. These findings suggest that LPS evokes a state characterized by high temperature and low vigilance, which is reached independently of the circadian phase.

The pars tuberalis: the missing link in the photoperiodic regulation of prolactin secretion?

The endocrine function of the pars tuberalis of the pituitary gland has been an enigma for many years. Recent work suggests that one of its primary functions in seasonal mammals is to mediate photoperiodically regulated changes in prolactin secretion via an unidentified factor called tuberalin.

Voltage-gated currents distinguish parvocellular from magnocellular neurones in the rat hypothalamic paraventricular nucleus

1. Magnocellular and parvocellular neurones of the hypothalamic paraventricular nucleus (PVN) differentially regulate pituitary hormone secretion and autonomic output. Previous experiments have suggested that magnocellular, or type I neurones, and parvocellular, or type II neurones, of the PVN express different electrophysiological properties. Whole-cell patch-clamp recordings were performed in hypothalamic slices to identify the voltage-gated currents responsible for the electrophysiological differences between type I and type II PVN neurones. 2. Type I neurones, which display transient outward rectification and lack a low-threshold spike (LTS), generated a large A-type K+ current (IA) (mean +/- s.e. m.: 1127.5 +/- 126.4 pA; range: 250-3600 pA; voltage steps to -25 mV) but expressed little or no T-type Ca2+ current (IT). Type II neurones, which lack transient outward rectification but often display an LTS, expressed a smaller IA (360.1 +/- 56.3 pA; range: 40-1100 pA; voltage steps to -25 mV), and 75 % of the type II neurones generated an IT (-402.5 +/- 166.9 pA; range: -90 to -2200 pA; at peak). 3. The voltage dependence of IA was shifted to more negative values in type I neurones compared to type II neurones. Thus, the activation threshold (-53.5 +/- 0.9 and -46.1 +/- 2.6 mV), the half-activation potential (-25 +/- 1.9 and -17.9 +/- 2.0 mV), the half-inactivation potential (-80.4 +/- 9.3 and -67.2 +/- 3.0 mV), and the potential at which the current became fully inactivated (-57.4 +/- 2.1 and -49.8 +/- 1.5 mV) were more negative in type I neurones than in type II neurones, respectively. 4. IT in type II neurones activated at a threshold of -59.2 +/- 1.2 mV, peaked at -32. 6 +/- 1.7 mV, was half-inactivated at -66.9 +/- 2.2 mV, and was fully inactivated at -52.2 +/- 2.2 mV. 5. Both cell types expressed a delayed rectifier current with similar voltage dependence, although it was smaller in type I neurones (389.7 +/- 39.3 pA) than in type II neurones (586.4 +/- 76.0 pA). 6. In type I neurones IA was reduced by 41.1 +/- 7.0 % and the action potential delay caused by the transient outward rectification was reduced by 46.2 +/- 10.3 % in 5 mM 4-aminopyridine. In type II neurones IT was reduced by 66.8 +/- 10.9 % and the LTS was reduced by 76.7 +/- 7.8 % in 100 microM nickel chloride, but neither IT nor LTS was sensitive to 50 microM cadmium chloride. 7. Thus, differences in the electrophysiological properties between type I, putative magnocellular neurones and type II, putative parvocellular neurones of the PVN can be attributed to the differential expression of voltage-gated K+ and Ca2+ currents. This diversity of ion channel expression is likely to have profound effects on the response properties of these neurosecretory and non-neurosecretory neurones.

How enteral feeding options influence corticosterone patterns in rats

Even with all the nutritional research conducted to date, it is not clear which enteral nutrition delivery and composition options are most physiologically sound. Glucocorticoid temporal patterns are reported to be shifted or disrupted with restricted feeding schedules, but because of intermittent sampling, temporal patterns have not been completely depicted. The purpose of this study was to characterize corticosterone temporal patterns while systematically varying selected enteral feeding options in a well-established nutritional animal model. A 2 x 2 x 2 x 2 randomized block experimental design was used in which enteral feeding schedules, delivery methods, kilocalorie levels (kcal), and fiber contents were systematically varied in rats (n = 80), and plasma corticosterone was measured by 125I radioimmunoassay. Blood samples were drawn hourly over 24 h. With cosinor analysis, 24-h and 12-h corticosterone rhythmic components were tested in each feeding group. Five of 16 feeding groups had a significant (p < or = 0.05) 24-h rhythmic component, and 3 more showed a trend (p > 0.05 < 0.10); 7 of these groups were on 24-h feeding schedules. When rhythmic components were detectable, groups receiving high-fiber formula displayed more uniform rhythm characteristics than did no-fiber groups. Only 1 group had a significant 12-h rhythmic component, and 1 showed a trend. Both were on 12-h, high-fiber restricted kcal feedings. In this small animal sample, no one enteral feeding option guaranteed a 24-h corticosterone pattern. The option coming closest was formula delivered on a 24-h schedule. This temporal pattern is one aspect to consider in enteral nutrition. The underlying mechanisms have yet to be elucidated.

Tracing autonomic innervation of the rat pineal gland using viral transneuronal tracing

We have used the neurotropic Bartha strain of pseudorabies virus (PRV) to characterise the pathway linking the endogenous circadian pacemaker of the suprachiasmatic nucleus (SCN) to the pineal gland. This low virulent strain of virus replicates within synaptically linked neurones and is ideally suited to visualise the multisynaptic pathways through which the SCN modulates the activity of the rat pineal gland. Using specific antibodies against PRV, we could follow the immunohistochemical pattern of the spatiotemporal passage of virus through the sympathetic trunk and the neuraxis. The time course of virus infection indicated that the most prominent pathway from the SCN to the pineal gland is via a final sympathetic innervation from the superior cervical ganglion (SCG). The pathway arises in the dorsomedial portion of the SCN from where neurones project to the dorsal parvicellular subdivision of the hypothalamic paraventricular nucleus (PVN) to form synaptic contact with neurones descending to the intermediolateral nucleus (IML) of the upper thoracic spinal cord. The neurones of the IML constitute the presynaptic sympathetic input synaptically connected to postsynaptic sympathetic neurones in the SCG which constitute the final input to the pineal gland. Removal of the superior cervical ganglion (SCGX) prior to viral infection completely abolished infection of neurones in this circuit. However, an additional parasympathetic projection from the superior salivatory nucleus via the sphenopalatine ganglion to the pineal gland was observed in SCGX animals.

Pituitary adenylate cyclase activating peptide (PACAP) in the retinohypothalamic tract: a daytime regulator of the biological clock

The retinohypothalamic tract (RHT) relays photic information from the eyes to the brain biological clock in the suprachiasmatic nucleus (SCN). Activation of this pathway by light plays a role in adjusting circadian timing to light exposure at night. Here we report a new signaling pathway by which the RHT regulates circadian timing in the daytime as well. Using dual-immunocytochemistry for PACAP and the in vivo tracer Cholera toxin subunit B (ChB), intense PACAP immunoreactivity (PACAP-IR) was observed in retinal afferents at the rat SCN as well as in the intergeniculate leaflet (IGL) of the thalamus. This PACAP-IR was nearly lost upon bilateral eye enucleation. PACAP afferents originated from ganglion cells distributed throughout the retina. The phase of circadian rhythm measured as SCN neuronal activity in vitro was significantly advanced by application of PACAP-38 during the subjective day, but not at night. The effect is channelled to the clock via a PACAP 1 receptor-cAMP signaling mechanism. Thus, in addition to its role in nocturnal regulation by glutamatergic neurotransmission, the RHT can adjust the biological clock by a PACAP-cAMP-dependent mechanism during the daytime.

Aging alters the rhythmic expression of vasoactive intestinal polypeptide mRNA but not arginine vasopressin mRNA in the suprachiasmatic nuclei of female rats

Our laboratory has shown that the ability of the suprachiasmatic nuclei (SCN) to regulate a number of rhythmic processes may be compromised by the time females reach middle age. Therefore, we examined the effects of aging on the rhythmic expression of two neuropeptides synthesized in the SCN, vasoactive intestinal polypeptide (VIP) and arginine vasopressin (AVP), using in situ hybridization. Because both VIP and AVP are outputs of the SCN, we hypothesized that age-related changes in rhythmicity are associated with alterations in the patterns of expression of these peptides. We found that VIP mRNA levels exhibited a 24 hr rhythm in young females, but by the time animals were middle-aged, this rhythm was gone. The attenuation of rhythmicity was associated with a decline in the level of mRNA per cell and in the number of cells in the SCN producing detectable VIP mRNA. AVP mRNA also showed a robust 24 hr rhythm in young females. However, in contrast to VIP, the AVP rhythm was not altered in the aging animals. The amount of mRNA per cell and the number of cells expressing AVP mRNA also was not affected with age. Based on these results we conclude that (1) various components of the SCN are differentially affected by aging; and (2) age-related changes in various rhythms may be attributable to changes in the ability of the SCN to transmit timing information to target sites. This may explain why the deterioration of various rhythmic processes occurs at different rates and at different times during the aging process.

Restricted daytime feeding modifies suprachiasmatic nucleus vasopressin release in rats

The authors have shown previously that vasopressin (VP) release from suprachiasmatic nucleus (SCN) efferents in rats is important for the timing of the circadian activity of the hypothalamo-pituitary-adrenal (HPA) axis, resulting in a circadian rise in corticosterone at dusk. When meals are supplied at a fixed time during the light period, however, this normal circadian activity of the HPA axis is strongly modified. Under such a restricted feeding regimen, a corticosterone peak appears just before the daily meal in addition to the circadian corticosterone peak at dusk. This feeding-associated rise in corticosterone is regarded as an SCN-independent circadian rhythm because it is sustained in SCN-lesioned animals. Despite these previous results, the authors investigated a putative involvement of SCN-derived VP in the control of the prefeeding corticosterone peak by measuring the intranuclear release of VP in the SCN and plasma corticosterone levels in rats in ad libitum feeding conditions as well as in animals that were obliged to feed during a 2-h period in the middle of the light period. Restricted daytime feeding caused clear changes in the daily release pattern of VP from SCN terminals. Both a delayed onset of the diurnal rise and a premature decline of the elevated daytime levels were observed, but the acrophase of the VP rhythm was not phase shifted. Concerning the circadian corticosterone peak, no phase shift of its acrophase was observed either. It is concluded that (1) restricted daytime feeding does affect SCN activity, (2) intranuclear release of VP within the SCN is an important mechanism to amplify and synchronize the circadian rhythms as dictated by the light/dark-entrained circadian pacemaker, and (3) VP release observed in animals on restricted feeding is completely compatible with the previously proposed inhibitory action of SCN-derived VP on the HPA axis.

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

Knowledge of the polysynaptic pathway conveying photic information to the pineal gland is based upon studies employing lesions, knife cuts and classical tracers. In the present investigation we used viral transneuronal tracing to re-examine the organization of this circuitry. This was accomplished by injecting a neurotropic alpha herpesvirus (pseudorabies virus) into the gland and localizing viral antigen in infected neurones at various postinoculation intervals. This approach is based upon the demonstrated ability of this virus to invade axon terminals, replicate in neurones and pass retrogradely through a multisynaptic circuit. Immunohistochemical localization of viral antigen revealed the progressive appearance of infected neurones in the superior cervical ganglion (SCG), intermediolateral nucleus of the upper thoracic spinal cord (IML), parvicellular subdivisions of the hypothalamic paraventricular nucleus (PVN), and the suprachiasmatic nucleus (SCN). Other infected cell groups known to project to the IML also became infected. Infection of the PVN reproducibly involved neurones in the dorsal, medial and lateral parvicellular subdivisions and preceded the appearance of infected neurones in the SCN and other regions of hypothalamus. Topographic analysis of virus infected neurones within the SCN revealed differential infection of SCN subdivisions that suggested topography in the projection of the SCN to the PVN. Removal of the SCG eliminated infection within the aforementioned circuitry and revealed a parasympathetic innervation from the sphenopalatine ganglion. The data provide further detail on the cellular identity and synaptology of neural circuitry controlling the rhythmic secretion of melatonin by the rat pineal gland.

The role of glutamate in the photic regulation of the suprachiasmatic nucleus

Endogenous circadian rhythms govern most aspects of physiology and behaviour in mammals, including body temperature, autonomic and endocrine function, and sleep-wake cycles. Such rhythms are generated by the suprachiasmatic nucleus of the hypothalamus (SCN), but are synchronised to the environmental light-dark cycle by photic cues perceived by the retina and conveyed to the SCN via the retinohypothalamic tract (RHT). This review considers many lines of evidence from diverse experimental approaches indicating that the RHT employs glutamate (or a related excitatory amino acid) as a neurotransmitter. Ultrastructural studies demonstrate the presence of glutamate in presynaptic terminals within the SCN. In situ hybridisation and immunocytochemical studies reveal the presence of several NMDA (NMDAR1, NMDAR2C), non-NMDA (GluR1, GluR2, GluR4) and metabotropic (mGluR1) glutamate receptor subunits in the SCN. Messenger RNA encoding a glutamate transporter protein is also present. In behavioural tests, glutamate antagonists can block the effects of light in phase-shifting circadian rhythms. Such treatments also block the induction of c-fos within SCN cells by light, whereas a glutamate agonist (NMDA) induces c-fos expression. In hypothalamic slice preparations in vitro, electrical stimulation of the optic nerves induces release of glutamate and aspartate, and glutamate antagonists block field potentials in the SCN evoked by stimulation of the optic nerve. Circadian rhythms of electrical activity which persist in vitro are phase shifted by application of glutamate in a manner which mimics the phase shifting effects of light in vivo. This wide range of experimental findings provides strong support for the hypothesis that glutamate is the principal neurotransmitter within the RHT, and thus conveys photic cues to the circadian timing system in the SCN.

Age and sex do not affect the volume, cell numbers, or cell size of the suprachiasmatic nucleus of the rat: an unbiased stereological study

The circadian rhythms displayed by numerous biological functions are known to be sex specific and affected by aging. It has not been settled yet whether the sex- and age-related characteristics of circadian rhythms derive from changes in the anatomy of the suprachiasmatic nucleus. To shed light on these issues, we applied unbiased stereological techniques to estimate the volume of the suprachiasmatic nucleus as well as the total number of its cells and the mean volume of their somata and nuclei in progressively older groups of male and female Wistar rats (aged 1, 6, 12, 18, 24, and 30 months). The volume of the nucleus was estimated with the Cavalieri principle on serial sections. The total numbers of neurons and astrocytes were estimated by applying the optical fractionator, and the mean somatic and nuclear volumes of cells were estimated by using isotropic, uniform random sections and the nucleator method. On average, the volume of the suprachiasmatic nucleus was 0.044 mm3, and the total number of neurons and astrocytes was 17,400. Cells of the dorsomedial and ventrolateral components of the nucleus, which are morphologically different, have identical mean perikaryal and nuclear volumes, which we estimated to be 750 microns3 and 400 microns3, respectively. We further demonstrated that, at all ages analysed, the volume of the suprachiasmatic nucleus, the total cell number, and the mean somatic and nuclear volumes of its cells are affected neither by the age nor by the sex of the animal, regardless of the presence of sex- and age-related variations in circadian rhythms. However, the possibility that females may display changes in the volume of the suprachiasmatic nucleus at older ages cannot be ruled out. No effect of aging was observed in the total number of neurons or in the total number of astrocytes.

Effects of N-methyl-D-aspartate (NMDA) on seasonal cycles of reproduction, body weight and pelage colour in the male Siberian hamster

Siberian hamsters (Phodopus sungorus) transferred from stimulatory photoperiods (long days: LD) to inhibitory photoperiods (short days: SD) undergo testicular regression within 8 weeks. This reproductive response to photoperiod was blocked by systemic daily treatment with the glutamatergic agonist N-methyl-D-aspartate (NMDA: 20 mg/kg BW, sc). This powerful effect of NMDA demonstrates the potential for endogenous glutamate to regulate reproductive function. The overall aim of the subsequent studies was to investigate the site and mechanism of action of this glutamatergic agonist in order to identify potential mechanisms through which endogenous glutamate might act. To investigate whether the effect of systemic NMDA was via an effect on the circadian timing system, alterations in gonadal regression and recrudescence, seasonal coat changes (pelage) and body weight (BW) were examined. It would be predicted that long-term cycles of all these seasonal parameters would be affected if the action of NMDA were to perturb the transduction of photoperiodic information. Daily treatments with NMDA, which initially maintained reproductive function in hamsters exposed to SD, did not influence the time course of subsequent testicular recrudescence, nor did they influence long-term cycles of pelage and BW. Moreover, treatment with NMDA induced a dose-dependent increase in serum concentrations of LH within 15 min of systemic injection. These data are consistent with the hypothesis that systemic NMDA exerts it reproductive effects not via an action on the circadian system, but via an action on secretion of GnRH. To investigate potential central sites of action of glutamate, induction of the immediate early gene c-fos, an acute marker of cellular response, was evaluated immunocytochemically (ICC) in brain areas after treatment with NMDA. Although dual-label ICC studies revealed that NMDA did not induce c-fos within GnRH neurons, NMDA did induce c-fos in many cells in the region of the organum vasculosum of the lamina terminalis (OVLT), an area containing a large number of GnRH perikarya, and in the arcuate nucleus, a region close to GnRH secretory terminals in the median eminence. The lack of c-fos induction of GnRH cells argues against a direct effect of NMDA on GnRH neurons. Thus, we examined immunocytochemically the distribution of the common NMDAR1 glutamate receptor subunit to evaluate further the potential sites of glutamatergic action. As expected, NMDAR1-ir was widespread in perikarya throughout the brain, including the region of the OVLT and the arcuate nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

Ontogeny of a photic response in the suprachiasmatic nucleus in the Siberian hamster (Phodopus sungorus)

The ontogeny of photic responsiveness in the suprachiasmatic nucleus of the Siberian hamster (Phodopus sungorus) was studied using the enhanced expression of the immediate early gene c-fos as a marker of neuronal activation. c-fos expression was assessed by immunocytochemical localization of its protein product. Hamsters were kept on a 16 h light:8 h dark photocycle. The adult Siberian hamster showed a marked increase in the number of c-fos-immunoreactive (c-fos-ir) cells within the suprachiasmatic nuclei (SCN) in response to a 1 h light pulse delivered 1-3 h after lights off, in comparison to controls kept in the dark. This is consistent with previous studies in the Syrian hamster and rat. The development of the photic response was examined. The first study investigated the effects of a light pulse on c-fos induction in pups at 5, 9, 12 and 24 postnatal days of age (PD). The suprachiasmatic region was identified by immunocytochemical localization of peptide-histidine-isoleucine in adjacent sections, a peptide expressed early in the development of the rodent SCN. The distribution of c-fos-ir cells was also compared with the location of retinal efferents, as determined by intraocular injection of the tract tracer cholera toxin B subunit 24 h previously. At PD 9, 12 and 24, significant increases in the number of c-fos-ir cells occurred in the light pulsed animals in comparison to age-matched control animals which were moved within the non-illuminated room to provide a 'dark' pulse.(ABSTRACT TRUNCATED AT 250 WORDS)

24-hour glucose profiles during continuous or oscillatory insulin infusion. Demonstration of the functional significance of ultradian insulin oscillations

Under basal and stimulated conditions, normal insulin secretion oscillates with periods in the ultradian 100-150-min range. To test the hypothesis that oscillatory insulin delivery is more efficient in reducing blood glucose levels than continuous administration, nine normal young men were each studied on two occasions during a 28-h period including a period of polygraphically recorded sleep. Endogenous insulin secretion was suppressed by somatostatin, a constant intravenous glucose infusion was administered, and exogenous insulin was infused either at a constant rate or in a sinusoidal pattern with a period of 120 min. The mean glucose level over the 28-h period was 0.72 +/- 0.31 mmol/liter lower when insulin was infused in an oscillatory pattern than when the rate of infusion was constant (P < 0.05). The greater hypoglycemic effect of oscillatory versus constant infusion was particularly marked during the daytime, with the difference averaging 1.04 +/- 0.38 mmol/liter (P < 0.03). Serum insulin levels tended to be lower during oscillatory than constant infusion, although the same amount of exogenous insulin was administered under both conditions. Ultradian insulin oscillations appear to promote more efficient glucose utilization.

Topographical organization of the rat suprachiasmatic-paraventricular projection

The suprachiasmatic nucleus (SCN) is a dominant pacemaker involved in the generation of circadian rhythms in mammals. Surprisingly, the expression of the many rhythms appears to be mediated via a limited efferent projection system of the pacemaker, of which the largest pathway terminates in the subparaventricular area and in the paraventricular nucleus of the hypothalamus. In order to investigate a possible topographical organization of this major outflow pathway of the SCN, microiontophoretic injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) or the retrograde tracer cholera toxin subunit B (ChB) were centered in distinct subparts of the SCN (PHA-L) or in the subparaventricular area-paraventricular nucleus (ChB), respectively. PHA-L injections involving the entire SCN revealed not only a major projection to the subparaventricular area, but also one directed towards the medial and dorsal parvicellular subnuclei of the paraventricular nucleus. As opposed to injections involving the entire nucleus, injections of PHA-L centered in the dorsomedial subdivision of the SCN resulted in a relatively larger number of PHA-L-immunoreactive fibers in the parvicellular subdivisions of the PVN, whereas the terminal field in the subparaventricular area was less substantial. A topography of the SCN efferent output system was also revealed by the retrograde tracing with ChB. Injections of ChB in the dorsal part of the paraventricular hypothalamic nucleus, not involving the underlying subparaventricular area, gave rise to a population of retrogradely labeled cells in the dorsomedial part of the SCN. In contrast, ChB injections in the subparaventricular area resulted in labeling of neurons clustered in a more ventrolateral aspect of the SCN. The present data provide evidence for a topography in the major efferent projection system from the SCN, implying that different subparts of the rat SCN, presumably containing partly different potential neurotransmitter substances, may regulate different circadian rhythms.

Regional distribution of iodomelatonin binding sites within the suprachiasmatic nucleus of the Syrian hamster and the Siberian hamster

The pineal hormone melatonin is a potent regulator of seasonal and circadian rhythms in vertebrates. In order to characterize potential target tissues of melatonin, the distribution of iodomelatonin (IMEL)-binding sites was examined within neurochemically and anatomically defined subdivisions of the suprachiasmatic nucleus (SCN), a structure necessary for seasonal and circadian rhythms in mammals. Studies were carried out in both the adult Syrian (Mesocricetus auratus) and Siberian (Phodopus sungorus) hamster. The retinoreceptive zone of the SCN was identified anatomically by immunocytochemical (ICC) visualization of cholera toxin B subunit tracer (ChTB-ir) following its intra-ocular injection. Photically-responsive SCN cells were identified by immunostaining for the protein product of the immediate-early gene c-fos (Fos-ir) following exposure of the animal to light. The non-photoresponsive zone of the SCN was identified using in situ hybridization (ISH) for arginine vasopressin (AVP) mRNA, whilst sites of IMEL-binding in the SCN were identified by in vitro film autoradiography using the specific ligand 2-[125I]-iodomelatonin. To compare directly the distribution of IMEL-binding sites and one of the functional zones of the nucleus, alternate serial coronal sections through the SCN were processed for autoradiography for IMEL and one of the following: ICC for ChTB-ir or Fos-ir, or ISH for AVP mRNA. Overall, the regional distribution of the various markers within the SCN was comparable in the two species. The retinorecipient (ChTB-ir) and photically-responsive (Fos-ir) zones of the SCN mapped together to the middle and caudal thirds of the nucleus, predominantly in its ventro-lateral division.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of photoperiod on the diurnal melatonin and 5-methoxytryptophol rhythms in the human pineal gland

The mammalian pineal organ is a major endocrine component of the circadian timing system, primarily concerned with the regulation of photoperiodic responses. The effect of different photoperiods on the diurnal variation in the concentration of melatonin and 5-methoxytryptophol (5-ML) was studied in the pineal glands, obtained at autopsy, of 38 human subjects, and measured by radioimmunoassay. A diurnal rhythm in pineal melatonin was evident only in the long photoperiod (April-September) with melatonin concentrations being 4.2 times higher at night (22.00-10.00 h) than during the day (10.00-22.00 h). In contrast, diurnal variations in the pineal 5-ML content were only observed in the short photoperiod (October-March) with high concentrations during day-time and low concentrations during night-time. In general, night-time concentrations of both melatonin and 5-ML were higher in summer than in winter. These data suggest that the synthesis of indoleamines in the human pineal exhibits a diurnal rhythm which is affected by seasonal changes in day length.