Circadian rhythms and appetite

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The control of food intake is a flexible system whereby internal and external environmental cues can alter the timing of feeding and appetite. The suprachiasmatic nucleus in the hypothalamus is the vital coordinator of these stimuli that ultimately generates fluctuations in neuronal and hormonal activities that are known as circadian rhythms. Circadian rhythms are driven by the daily variations in ambient light, which, by alterating gene expression, elicit a host of physiological responses including fluctuations in the hormones involved in appetite and food intake. Various factors such as temperature and social cues influence circadian rhythms; indeed, food intake itself can regulate these rhythms, but the neural mechanisms by which this occurs remains elusive. It has recently been proposed that there is a ‘food entrainable oscillator’ that exists independently of the SCN, and which controls food anticipation activity. The importance of circadian rhythms for ensuring good health has been highlighted by our modern day lifestyle, as jet-lag and shift work have shown that disruption of these delicate balances can lead to pathologies such as metabolic syndrome.

SCN, the biological clock

A wide variety of organisms, from cyanobacteria to humans, all share common internal clock mechanisms that have been present for millions of years in evolutionary history. The circadian clock in mammals is responsible for setting specific temporal patterns within our bodily systems, including many physiological functions (body temperature, melatonin release, glucocorticoid secretion) and behavioural functions (alertness, working memory,) in order to keep us alive and running smoothly.

The "master" circadian clock of mammals is in the suprachiasmatic nucleus (SCN) of the hypothalamus. Circadian signals from the SCN are distributed by diffusible/humoral messages and by neuronal outputs . These signals influence other clocks found in peripheral tissues (liver, kidney, thymus, muscle –Guillaumend et al. 2005). Other brain regions express clock genes with self-sustained oscillations (retina, olfactory bulb and striatum), but these don’t have clocks.

Clock mechanisms are formed by the transcription and translation of clock genes, which rely on feedback loops. In mammals, Clock and Bmal1 genes are part of the positive loop and Per genes, the negative loop. The clock genes regulate a self-sustaining rhythm in cells that even without light will maintain a roughly 24 hour rhythm. An extreme example is of cave fish which have lived in complete darkness for millions of years and their clock genes are still present in their DNA.

Rhythmic output of organs can be influenced by metabolic, endocrine and homeostatic events, as well as by the circadian clock. For example, the SCN can change the rhythm of liver genes and enzymes without using clock genes, but through second messenger systems induced by the autonomic nervous system instead. Other genes can also affect on circadian clock genes; for example the ROR-alpha gene is a positive regulator of Bmal1, which regulates lipogenesis and lipid storage (Lau et al. 2004).

The SCN contains neuronal and glial cells, with most of the neurons being GABAergic. The main input into SCN consists of signals encoding light. Light signals are transmitted from the retina to the SCN via the retinohypothalamic tract. Vasoactive intrinsic polypeptide released from a subpopulation of neurones in the SCN activates and synchronises other SCN neurons, the output of which coordinates behavioural rhythms. SCN signals to peripheral oscillators using many signalling molecules, including: TGFα, prokinecticin 2 and cardiotrophin like cytokine, and neuronal connections to prevent dampening of circadian rhythms in the tissues. SCN efferent fibres terminate around the arcuate nucleus in the ventromedial hypothalamus (VMH) and the paraventricular nucleus PVN, areas involved in regulation of food intake and glucocorticoid secretion respectively. The SCN innervates the sub-paraventricular zone (SPZ) and the dorsomedial hypothalamus (DMH) which in turn innervate the PVN and lateral hypothalamus, areas which regulate glucocorticoioid release and the wakefulness / feeding cycle respectively. The DMH regulates sleep-wakefulness and feeding cycles among others, and degeneration of DMH results in severe impairment of those cycles. The SCN selectively innervates preautonomic nervous system neurons found in the dorsal and ventral borders of PVN. As pre-autonomic neurons in the hypothalamus are connected to the sympathetic and parasympathetic system, this allows the SCN to control energy homeostasis. The SCN receives information on hormones and metabolites present in the bloodstream and which cannot cross the blood-brain barrier via its dense reciprocal interactions with ventromedial arcuate nucleus (vmARC). The vmARC receives the above information through its connection with the median eminence, a circumventricular organ which is free of blood-brain barrier where blood-bornne hormones can easily reach their receptors on neurons. Gastrin-releasing peptide interacts with SCN and results in light-like resetting of SCN. (Froy, 2010)

~~Image Figure 3. SCN afferents and efferents. The SCN can be activated by light, hormones and nutrients, neuronal connections (green arrows).The SCN neuronal connections to the ARC, MPOA, PVN and SPZ (blue arrows) ARC is affected by hormones and nutrients directly. SPZ innervates the DMH, with DMH innervating PVN, VLPO and LH which coordinate corticosteroid production, sleep, feeding respectively. PVN and DMH regulate adipose tissue, liver and other peripheral tissues through autonomic nervous system (red arrows).

Clock Genes

Genes encoding core clock mechanism are circadian locomotor output cycles kaput (Clock), brain and muscle-Arnt-like 1(Bmal1), Period1 (Per1),Period2 (Per2), Period3 (Per3),Cryptochrome1(Cry1) and Cryptochrome2(Cry2). (Froy 2010). CLOCK transcription factor dimerises with BMAL1 and activates transcription. CLOCK and BMAL1 are basic helix-loop-helix-PAS transcription factors which upon binding to E-box and E-box like promoter sequences activate transcription. The action of CLOCK:BMAL1 heterodimer is inhibited by PER and CRY proteins. Products of Clock gene are important in regulating appetite. Mice whose Clock function was impaired had an increased food intake and rhythmic expression of Cart and Orexin hormones was eradicated(Froy 2010). Experimental data reported by Bray et.al shows that CLOCK -/- mice exhibit obesity,altered feeding patterns,hyperphagia and hormonal abnormalities similar to those found in metabolic syndromes, such as hyperlipidemia, hyperleptinemia, hyperglycemia and hyperinsulinemia.

SCN & Feeding Time

The main way in which the SCN regulates food intake is by directing the sleep/wake cycle. The role of the SCN in feeding patterns has been determined using rodents studies as unlike humans their food intake is less influenced by cognition and social behaviour. The sleep/wake cycle is driven by fluctuations in two main hormones: corticosterone (cortisol in humans) and melatonin. Corticosterone levels rise during the night when the nocturnal animals active. Their rise is followed by an increase in activity in which they forage for food and subsequently begin to feed. Melatonin is an important hormone that is released from the pineal gland during the night (the day in noturnal animals) which amongst its many actions induces sleep and suppresses appetite.

SCN regulation of the sleep/wake cycle

Figure 2 (Adapted from Saper et al. 2005)

The SCN innervates multiple regions of the brain that regulate the sleep-wake cycle. a) The most intense output from the SCN is into the ventral sub-paraventricular zone (SPZ) which then projects into the dorsomedial hypothalamus (DMH) which in turn controls a wide range of circadian responses including corticosteroid secretion. The DMH innervates the medial parvicellular paraventricular nucleus (PVHm) and regulates the neurons containing CRH which in turn controls pituitary regulation of corticosteroid production. The SCN is also involved in regulating the release of melatonin from the pineal gland. The SCN projects into the dorsal parvicellular paraventricular nucleus, (PVHd) which subsequently project into sympathetic preganglionic neurons in the spinal cord which regulates melatonin output by the release of noradrenalin into the pineal gland. b) The DMH also has GABAergic projections into the ventrolateral preoptic nucleus VLPO (the sleep promoting region) and glutamatergic projections into the orexin producing neurons and the melanin-concentrating neurons in the lateral hypothalamus (LHA) which together regulate sleep and wakefulness and ultimately feeding. It is also apparent that hormones involved in appetite regulation such as ghrelin and leptin can influence these areas of the brain thus may have a role in resetting the circadian rhythms generated by the SCN.

Whilst the SCN is known to influence feeding patterns indirectly by regulating the awake/sleep cycles, it remains unclear whether or not the SCN can directly drive appetite. As the SCN has reciprocal interactions with the orexigenic regions of the brain it is possible that the SCN can directly stimulate appetite.

PICTURE : It has been found that SCN fibres terminate in and around the arcuate nucleus (ARC) and the ventral part of the lateral hypothalamus (Yi et al. 2006). Activation of the arcuate nucleus releases NYP and AGRP (two potent orexigenic peptides) into the PVN which ultimately stimulates feeding and slows metabolism maximising energy intake . However, research into this possibility is yet to be carried out.

Alternatively, it is possible that the SCN initiates feeding by conducting circadian rhythmic oscillations in the hormones involved in appetite. Indeed it has recently been established that a number of hormones involved in feeding behaviour and appetite, including leptin, and ghrelin show circadian oscillations . GRAPHS Leptin has been shown to exhibit circadian patterns in both gene expression and protein secretion in humans, with a peak during the sleep phase in humans (Kalra et al. 2003 in froy). Furthermore, rodent studies have shown that ablation of the SCN eliminates leptin circadian rhythmicity (Kalsbeek 2001 in Froy) and yet the role of the SCN in conducting this pattern is unclear. As leptin binds to receptors in the hypothalamus to suppress of appetite and an increase metabolism it seems plausible to suggest that the SCN can alter appetite indirectly via hormone regulation.

Conclusion…

Fiona E Graham 15:57, 25 October 2010 (UTC)

Peripheral Clocks and Food Entrainable Oscillators

Peripheral clocks

The SCN and peripheral clocks are not affected by meal timings, but restriction of food is the dominant synchroniser for peripheral clocks. When rats are restricted of food, metabolic and hormonal factors used by the SCN to drive peripheral oscillators are uncoupled, to shift to food time. Normally, the SCN and peripheral oscillators will work together as one unit, but a change in food availability can uncouple them in order for survival, when feeding is low and shifted from their normal place in the light-dark cycle..

© Image
SCN and Peripheral Oscillator Interaction.

The Food Entrained Oscillator

The FEO is a mysterious circadian clock, which is independent of the SCN. It ensures that when food is scarce, the body is still ready to digest and extract nutrients from the food that has been found, the FEO is responsible for anticipation of meal-time (FAA) .

Clock genes may contribute to the FAA but are not essential – Pendergast et al (2009) showed that animals without essential clock genes Bmal1 or per1 or2 were arrhythmic in constant darkness but still could express FAA. The FEO’s existence is putative, and its network is believed to be scattered over several brain regions. For example, the dorsomedial hypothalamic nucleus has been reported for FAA expression and the possible site of the FEO. The circadian mechanism for the FEO is unknown, but it does present clear circadian features.

Laura Sheldon 19:09, 24 October 2010 (UTC)Laura

Sleep deprivation, shift-work and appetite

PD Diagram
The relationship between sleep duration and changes in serum leptin and ghrelin levels. (a) Mean leptin levels against average nightly sleep duration. As the number of hours sleep increases, the levels of serum leptin also increase. Standard errors for half-hour increments of average nightly sleep. (b) Mean ghrelin levels against total number of hours sleep. As the total number of hours sleep decreases, the mean levels of ghrelin increase. Standard errors for half-hour increments of total sleep time. Adapted from Taheri et. al (2004)

In modern society, where shopping, eating, working and drinking are widely available 24hours a day,major health implications have been linked as a result. This availability of around the clock activities has defied our bodies internal clock of the vital hours of sleep it requires . Over the last few decades where technology and social activities has dramatically advanced, the number of hours of sleep young adults get has decreased within the range of 1-2hours . This is strongly correlated with the prevalence of obesity within the U.K which has shown to have trebled over the last 3 decades . Although the rise in this obesity epidemic has been shown to be multi-factorial, sleep deprivation is just another factor to add to the list needing to be addressed in this ever rising health problem.

Several epidemiologic studies have shown that sleep deprivation elevates the levels of the appetite stimulating hormone ghrelin and decreases circulating leptin levels [Fig. 5.1] . These changes could be the causes of increased food intake in these sleep- deprived adults where a rise in body weight is also observed [Fig. 5.2] . It is therefore important to fully understand the health implications that sleep-deprivation, associated with jetlag and night shift workers, has on the body in order to develop future therapeutic schemes against such related disorders as obesity.

The relationship between average nightly sleep and changes in body mass index (BMI). As the average nightly hours of sleep decreases below 7 hours, the mean BMI increases. Furthermore, more than an average of 8 hours sleep also causes a rise in mean BMI. Standard errors for 45-min intervals of average nightly sleep. Adapted from Taheri et. al (2004).

Conclusion

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