Timpul intern / ritmurile biologice – determinate de un pacemaker circadian endogen, supus unei sincronizari zilnice cu ‘timpul extern’ . Cronobiologia este stiinta care studiaza ritmurile biologice.

Timpul si ritmurile biologice

“Almost all organisms studied, from cyanobacteria to Homo Sapiens, have intrinsic time-keeping systems that adjust physiology and behavior to anticipate the daily cycles of light and darkness.

Physiological processes that vary over the 24-hour day include activity, alertness, hormone secretion, organ physiology, and gene expression.

These variations are not just passive responses to a rhythmic environment, but instead reflect an underlying biological mechanism that can measure time on a 24-hour time scale.

This mechanism orchestrates physiology, to achieve predictive, rather than reactive, homeostasis.”

Circadian Timekeeping, in Fundamental Neuroscience, L. Squire

• Nucleul suprachiasmatic (NSC) din hipotalamus – ceasul biologic principal al organismului, care prin ritmicitatea impulsurilor nervoase eliberate spontan, isi imprima amprenta oscilatiilor sale asupra celorlalte sisteme celulare cu activitate autonoma din organism, sincronizandu-le, si astfel ajunge sa moduleze de la secretia neuroendocrina la functii fiziologice complexe, inclusiv functia cognitiva.

• Oscilatorii celulari locali regleaza expresia genetica intr-o maniera specifica fiecarui tip celular, contribuind la fiziologia fiecarui organ si sistem.

• 10% din genom este transcris in mod ritmic, fiind responsabil de exprimarea actorilor cheie in metabolismul celular specific fiecarui tesut (Miller, 2007).

Ceasul biologic

Ritmul circadian (RC) Persista cu un ciclu de 24 ore in conditii de mediu constante (circa diem…) Generat de un mecanism biologic intern de mentinere a timpului, care prin evenimente moleculare ritmice controlate genetic coordoneaza functiile celulare, sistemice si comportamentale (de ex. ciclul somn-veghe). Este in mod normal sincronizat cu ziua de 24 ore de catre stimulii de mediu:

- Lumina – photoentrainment… - Temperatura - Concentratia hormonilor - Disponibilitatea nutrientilor

Ritmuri biologice

Ritmul circadian (RC)

Exista o ierarhie a oscilatorilor circadieni autonomi care oscileaza cu un ciclu de 24 ore.

NSC – master clock – coordoneaza oscilatiile din alte tesuturi si regleaza comportamentul (celular si de sistem…)

Mecanismul molecular care raspunde de oscilatiile circadiene autonome celulare se bazeaza pe o bucla de feedback autonoma care actioneaza la nivelul transcriptiei-translatiei genice.

Nucleul suprachiasmatic (NSC) – master pacemaker

• Situat in hipotalamusul anterior, deasupra chiasmei optice • Constituit din 2 nuclei (stg + dr) a cate 10.000 neuroni fiecare, in

special GABAergici • Primeste aferente de la celulele ganglionare din retina care

contin melanopsina, printr-o cale monosinaptica – tract retino-hipotalamic (RHT)

• Lezarea ambilor NSC produc aritmicitate • Trimite si primeste conexiuni de la glanda pineala, si raspunde

la secretia de melatonina a acesteia.

Glutamate-induced calcium influx into SCN neurons represents the first step in the signal transduction pathway leading to entrainment.

Time

mR

NA

or

Pro

tein

le

ve

l

Inhibition (Per/Cry mediated repression)

Delay (impose a cycle length of ~24 h)

-

Transcription

Clock gene 2

+ mRNA

Translation Protein

(negative element)

mRNA Transcription

Clock gene 1

Translation

Protein

(positive element)

Transcription and translation feedback loops constitute the core clock mechanism

Period (Per) Cryptochrome (Cry)

Clock, Bmal1 (Transcription factors)

Per/Cry accumulate

A general scheme of signal transduction mechanisms involved in circadian entrainment.

Diego A. Golombek, and Ruth E. Rosenstein Physiol Rev 2010;90:1063-1102

Glutamate activates NMDA-induced calcium influx which, together with other second messengers, triggers the activation of

diverse signal transduction cascades, including calmodulin kinase II (CaMKII) and neuronal nitric oxide synthase (nNOS)

activity, cAMP- and cGMP-dependent protein kinases, and mitogen-activated protein kinase (MAPK). Although the cross-

talk between these diverse cascades is not currently well-known, it is plausible that a common mechanism involved in the

pathway is phosphorylation of cAMP response element binding protein (CREB). In turn, pCREB activates per1 and per2

transcription by binding to a CRE element in their promoter regions (these genes are also activated by Clock/Bmal binding

to E boxes). Solid lines represent mechanisms that have been described experimentally, and dashed lines indicate

possible additional links of the pathway. http://physrev.physiology.org/content/90/3/1063

• Proiectiile din NSC sunt orientate in mare parte catre Hipotalamus - creierul vegetativ, cu rol in integrarea functiei nervoase si endocrine.

• Activitatea NSC depinde de conexiunile sale cu hipotalamusul, prin care integreaza o varietate de informatii sensitive importante pentru organizarea temporala a diferitelor functii reglate de hipotalamus.

Nucleul suprachiasmatic (NSC / SCN) - conexiuni

Hipotalamusul

- parte a diencefalului (1% din vol. creierului !)

- constituie un centru integrativ esential pentru supravietuire si reproducere

- reglarea temperaturii, frecventa cardiaca, presiunea arteriala, osmolaritatea

sangelui, alimentatie, emotie/afect si comportament sexual.

“Here in this well concealed spot,

almost to be covered with a thumb nail,

lies the very mainspring of primitive

existence – vegetative, emotional,

reproductive - on which, with more or

less success, man has come to

superimpose a cortex of inhibitions.”

(Cushing, 1929).

Melatonin represents the key linkage-molecule between the SCN and the peripheral biological clocks

sleep inducer regulator of the circadian rhythm

retino-hypothalamic projections

NSC comunica informatie temporala oscilatorilor periferici, sincronizandu-i si inducandu-le un ritm principal

Amplificarea sincronizarii oscilatiilor…

Nucleul Suprachiasmatic (SCN) – “inima” sistemului prin care “circula” si se

manifesta ritmul circadian

24-hour rhythms are observed in: - core body temperature, - hormone concentrations (melatonin,

cortisol, TSH, oxytocin and others), - subjective alertness, - objective performance - other physiologic functions

Kohsaka A, Endocrine Journal 2012, 59 (6), 447-456

SCN is essential for driving the 24-hour blood-pressure rhythm in mammals

The circadian clock is tightly coupled to the regulation of temporal energy homeostasis through highly complex mechanisms: controls of gene transcription, protein translation, protein phosphorylation, protein degradation, and epigenetic modifications that occur between circadian and metabolic components. The circadian and metabolic crosstalk in cardiovascular tissues indicates that the diurnal rhythm of cardiovascular function may be influenced by both circadian and metabolic signals that primarily arise from environmental conditions.

Maternal rhythm of melatonin is one of the time signals to the fetus (maternal melatonin is a Zeitgeber for the fetal SCN)

Circadian rhythms in the fetus. Mol Cell Endocrinol. 2012 Feb 5;349(1):68-75. Serón-Ferré M, Mendez N, Abarzua-Catalan L, Vilches N, Valenzuela FJ, Reynolds HE, Llanos AJ, Rojas A, Valenzuela GJ, Torres-Farfan C.

Throughout gestation, the close relationship between mothers and their progeny ensures adequate development and a successful transition to postnatal life. By living inside the maternal compartment, the fetus is inevitably exposed to rhythms of the maternal internal milieu such as temperature; rhythms originated by maternal food intake and maternal melatonin, one of the few maternal hormones that cross the placenta unaltered. We propose that the fetal suprachiasmatic nucleus (SCN) of the hypothalamus and fetal organs are peripheral maternal circadian oscillators, entrained by different maternal signals. Conceptually, the arrangement produces internal temporal order during fetal life, inside the maternal compartment. Following birth, it will allow for postnatal integration of the scattered fetal circadian clocks into an adult-like circadian system commanded by the SCN. PMID: 21840372

Intrinsic rhythmicity of SCN cells: Dispersed SCN cells in vitro exhibit circadian

rhythms in firing rate

Time (h) Welsch et al, 1995

(Suprachiasmatic nucleus)

Remove SCN Arrhythmic patterns of locomotion, feeding, hormone secretion

Implant donor SCN tissue

Return rhythms of donor hamster

Can the SCN restore rhythms to an arrythmic animal?

SCN lesions ablate circadian rhythms

Moore and Klein, 1974

Pineal NAT

SCNX

Circadian Phase Markers, for clinical use

• Drop in temp associated with stability in sleep

• Altered by activity, food intake, and sleep

• Three dips in temp: 3:00-5:00 13:00-16:00 20:00-24:00

• Melatonin secretion very sensitive to light exposure, needs to be obtained under dim light conditions

• Increase in levels around 20:00

• Levels peak at approximately 3:00 and begin to decrease

• Lowest levels just before awakening

Melatonin concentration

The SCN projects to the ventrolateral preoptic area (VLPO), an area mediating sleep. VLPO inhibits the arousal activity of the tuberomammillary nucleus during sleep. The SCN provides an arousal-promoting input to the posterior hypothalamic area, particularly to hypocretin/orexin neurons, which project upon the neocortex and subcortical arousal areas.

Disruptions in the circadian rhythm physiology consequently can cause a number of circadian rhythm sleep disorders.

Hypocretin/orexin regulates arousal, wakefulness, and appetite

Along with the internal circadian control of the sleep-wake rhythm, the amount of time spent awake and asleep/24 h is under a homeostatic control sleep propensity increases with elapsed time awake/dissipates with elapsed time asleep Independent of internal time (circadian phase), in an adult: - the max capacity to stay awake is around 16 h - the max capacity to maintain a sleep efficiency of 90% is around 8 h

Circadian rhythm - Sleep-wake cycle

http://www.scienzagiovane.unibo.it/English/sleep/3-sleep-animals.html

Why we sleep? Role of sleep in preventing the waking brain of “synaptic overload” or cellular stress, in regulating cortical synaptic plasticity. Memory consolidation by increased strength of synaptic connections induced by experiences during waking hours. Cognitive abilities, behavioral performance, mood, immune defence, weight control are altered in sleep deprivation (documented voluntary sleeplessness w/o pharmacol stim. – 264 h/11 days) Restorative role, energy conservation, replenish glycogen brain levels, allow low energy consume to keep the body warm during colder nights (minimum body temp. at night to reduce heat loss) - thermoregulation. O2 consume decreased during sleep.

Role REM sleep in early development…

Still, the underlining basis for the sleep homeostatic function remains uncertain…

Sleep - a highly conserved behaviour which offers advantages that outweigh the disadvantage of becoming vulnerable during sleep

Consciousness and the Sleeping Brain

• Consciousness is a person’s subjective awareness of both their inner thinking and feeling and their external environment

• Sleep: – Active process

– Composed of two major states, identified by EEG recording

• Non-REM Sleep (stages 1-4): Stages 3&4 are known as “deep sleep” or Slow Wave Sleep

• Rapid Eye Movement (REM) Sleep (stage 5) - high levels of brain activity

– Timing and state of sleep depend on

• Length of time awake

• Circadian time

EEG during the 5 stages of sleep

Stage 1: Lasts about 5 minutes Stage 2: Lasts about 20 minutes, characterized by sleep spindles, rapid bursts of mental activity Stage 3: Also known as transitional sleep and is characterized by delta waves, which are large, slow waves

Stage 4: Lasts about 30 minutes. Parasympathetic nervous system is active, as muscles relax, heartbeat slows, blood pressure declines, and digestion speeds up

Stage 5: REM (rapid eye movement sleep) Characterized by very rapid brain waves ; muscle atonia

Five Stages of Sleep

• These 5 stages (the sleep cycle) repeat themselves about every 90 minutes, with Stages 3 and 4 getting shorter with each cycle, and REM and Stage 2 getting longer with each cycle

• REM sleep rebound effect is a significant increase in the proportion of REM sleep following deprivation of REM sleep

Stage 5: REM (rapid eye movement sleep)

• REM - paradoxical sleep: muscle atonia, but other body systems, including the brain, are active, much like a waking pattern

– If awakened during REM sleep, people often report having been dreaming

– Most dreams are emotional and unpleasant, perhaps because the visual cortex and frontal lobe are inactive during REM sleep; the limbic system structures are active, however, creating irrational imagery and emotional experiences of our dream world.

– REM sleep accounts for 20–25% of total sleep time

Why do we sleep and dream?

• Sleep deprivation results in: – Impaired concentration and a general bodily feeling of

weakness and discomfort

– Suppression of the immune system, lessening one’s ability to fight off infection and disease

– Increased vulnerability to accidents

– Increased difficulty in concentrating

Why do we dream?

• Explanations for dreaming :

– Sigmund Freud proposed that dreams were disguised passages for inner conflicts of our unconscious mind, a view not accepted by modern sleep researchers

– The activation-synthesis hypothesis contends that dreams are merely the sleeping brain’s attempt to make sense of random neural activity without the rational interpretation of the frontal lobe

– …

http://www.medscape.org/viewarticle/465494_2

Timing…

Multiple input pathways to the circadian clock transmit various kinds of timing information

– Light

– Activity and arousal

Multiple output pathways from the clock regulate various overt circadian rhythms

– Sleep-wake cycles

– Body temperature rhythms

– Hormone rhythms

Timing is everything!

Robust, well-synchronized

circadian rhythms promote

health, well-being, and

optimal performance.