大脑与节律

包爱民

浙江大学神经生物学系

Human Circadian Time Structure

Biological Rhythms

• Almost all land animals coordinate their behavior according to circadian rhythms - the daily cycles of daylight and darkness that result from the spin of the Earth. (The term is from the Latin circa, “approximately,” and dies,“day.”)

• In humans, there is an approximately inverse relationship between the propensity （倾向、习性） to sleep and body temperature

EnvironmentalLight Intensity

Plasma MelatoninConcentration

Sleep-Wake Cycle

Time of Day

Body Temperature

Plasma CortisolConcentration

• When the cycles of daylight and darkness are removed from an animal’s environment, circadian rhythms continue on more or less the same schedule - the primary clocks for circadian rhythms are not astronomical (the sun and earth) but biological, in the brain.

• Brain clocks are imperfect and require occasional resetting.

• External stimuli, such as light and dark, or daily temperature changes, help adjust the brain’s clocks to keep them synchronized with the coming and going of the sunlight.

• Brain clocks are an interesting example of the link between the activity of specific neurons and behavior.

Biological clock

• The first evidence, a brainless organism - the mimosa: reacts to sunlight, in some kind of reflex movement?

• In 1729, French physicist de Mairan: mimosa plants continued to raise and lower their leaves in darkness, still sensing the sun’s movements?

• > 100 yrs later, Swiss botanist de Candolle: a similar plant in the darkness moved its leaves up and down every 22, rather than 24, hrs, implying the plant was not responding to the sun and very likely had an internal biological clock.

Zeitgeber: a stimulus that resets the biological clock (eg. bright light, exercise, temperature)

What sets the Clock?

The strongest zeitgeber, for both plants and animals, is light.

Non-photic zeitgebers include temperature, social interactions, pharmacological manipulation, exercise, and eating/drinking patterns.

Biological clock

• Biological clock overrides （重写、无视） most environmental cues.

• To maintain clock-environment synchrony, zeitgebers induce changes in the concentrations of the molecular components of the clock to levels consistent with the appropriate stage in the 24-hr cycle, a process termed entrainment.

• In the presence of zeitgebers, animals become entrained to the day-night rhythm and maintain an activity cycle of exactly 24 hrs.

Deprivation of zeitgebers: free-running

• Even small, consistent errors of timing could not be tolerated for long - a 24.5-hour cycle would, within 3 weeks, completely shift an animal from daytime to nighttime activity.

• When mammals are completely deprived of zeitgebers, they settle into an activity-rest rhythm often with a period more or less than 24 hrs, in which case their rhythms are said to freerun （自由运转）

• In mice, the natural free-running period is about 23 hrs; hamsters, ~ 24 hrs; humans, ~ 24.5–25.5 hrs

Biological clock

Deep caves, the sites for several isolation studies

People in caves, allowed to set their own schedules of activity for months, they initially settle into roughly a 25-hr rhythm.

But after days to weeks, their activity may begin to free-run with a surprisingly long period of 30–36 hrs: staying awake for about 20 hrs straight, then sleep for about 12 hrs, and this pattern seems perfectly normal to them at the time.

Biological clock

• In isolation experiments, behavior and physiology do not always continue to cycle together: the rhythms of temperature and sleeping-waking, which are normally synchronized to a 24-hr period, become desynchronized.

• One implication of this desynchronization is that the body has more than one biological clock, uncoupled from one another.

• Desynchronization may occur temporarily - when we travel and force our bodies suddenly into a new sleep-wake cycle: jet lag, and the best cure is bright light, which helps resynchronize our biological clock.

Light

Synchronization of behavior and light-dark cycles - a photosensitive mechanism for resetting the brain clock.

Mammals: a tiny pair of neuron clusters in the hypothalamus that serves as a biological clock: the suprachiasmatic nuclei (SCN)

Retinal ganglion cells send direct projections to the SCN, i.e. the retinohypothalamic tract

• light can also alter blood-borne factors• SCN is highly vascularized

What Reset the Clock?

Melatonin: hormone of darkness

• secreted from the pineal gland

• increased levels of melatonin: one feels sleepy

• melatonin can act on receptors in the SCN to phase-advance the biological clock

What Reset the Clock?

The circadian timing system

Superior cervicalganglion

Axons from ganglion cells in the retina synapse directly on the dendrites of SCN neurons. This input from the retina is necessary and sufficient to entrain sleeping and waking cycles to night and day. Pineal gland (to birds?) is unimportant, but to human melatonin may be important.

SCN as the biological clock: evidence

• When the SCN is stimulated electrically, circadian rhythms can be shifted in a predictable way

• Removal: abolishes the circadian rhythmicity of physical activity, sleeping and waking, and feeding and drinking.

• In hamsters, the transplantation of a new SCN can restore rhythms within 2–4 weeks.

• The brain’s internal rhythms never return without an SCN.

SCN

Biological clock

Components of biological clock producing circadian rhythms:

Light-sensitive input pathway → Clock → Output pathway

One or more input pathways are sensitive to light and dark; the clock continues to run and keep its basic rhythm even when the input pathway is removed.

Output pathways from the clock allow it to control brain and body functions according to the timing of the clock.

Biological clock

• The retinal cells synchronizing the SCN are neither rods nor cones -eyeless mice: cannot use light to reset their clocks

• mutant mice with intact retinas that lack rods and cones: can!

• David Berson et al. discovered a new photoreceptor: a very specialized type of ganglion cell - light-sensitive, expressing a newly recognized type of photopigment, melanopsin （视黑素）

• Melanopsin: not present in rods nor cones, very slowly excited by light, axons sending a signal directly to the SCN to reset it.

?

Biological clock

• Output axons of the SCN mainly innervate nearby parts of the hypothalamus, but some also go to the midbrain and other parts of the diencephalon

• Because almost all SCN neurons use GABA as their primary neurotransmitter, presumably they inhibit the neurons they innervate.

• In addition to the axonal output pathways, SCN neurons may rhythmically secrete the peptide neuromodulator: vasopressin (AVP)

• How do neurons of the SCN keep time? no complete answer at the molecular level, but it’s clear that each SCN cell is a minuscule （极小的） clock.

Biological clock

SCN neurons in culture: rates of AP firing, glucose utilization, AVP production, and protein synthesis continue to vary with rhythms of about 24 hrs.

Circadian rhythms of the SCN isolated from the rest of the brain. The activity of a clock gene was monitored in 100 individual SCN neurons maintained in tissue culture. Each neuron generates a strong circadian rhythm that is well coordinated with the neurons. (Fig 19.22)

Biological clock

• SCN cells in culture: no entrainment of light-dark cycles, but their basic rhythmicity remains just as when being deprived of zeitgebers.

• SCN cells communicate their rhythmic message to the rest of the brain through efferent axons, using Aps, and rates of SCN cell firing vary with a circadian rhythm.

• APs are unnecessary for SCN neurons to maintain their rhythm - tetrodotoxin (TTX ，河豚毒 ) applied to SCN cells can block their APs but no effect on the their rhythmicity. Removing TTX: APs resume firing with the same phase and frequency they had.

• SCN APs are like the hands of a clock: removing the clock’s hands does not stop the clock from working

Biological clock

• It is a molecular cycle based on gene expression – similar in humans, mice, fruit flies, and bread mold

• in Drosophila and mice, the system involves clock genes known as period (per), timeless (tim), and clock - ‘clock’ gene (circadian locomotor output cycles kaput)

• The basic scheme is a negative feedback loop.

• A clock gene is transcribed to produce mRNA then translated into proteins. - after a delay, the new proteins send feedback and somehow interact with the transcription mechanism, causing a decrease in gene expression, less protein produced, then gene expression again increases to start the cycle anew.

• This entire cycle takes about 24 hrs, and thus it is a circadian rhythm

Biological clockClock genes.

In the SCN, clock genes produce proteins that inhibit further transcription.

Gene transcription and the firing rate of individual SCN neurons cycle up and down over 24 hrs.

The cycles of many cells are synchronized by light exposure (input from theretina) and by interactions of the SCN neurons.

(Figure 19.23).

Biological clock

• The mechanism to coordinate the thousands of cellular clocks so that the SCN transmits a single, clear message about time to the rest of the brain.

• Light information from the retina serves to reset the clocks in the SCN neurons each day, but the SCN neurons also communicate directly with each other and seems to be independent of APs and normal synaptic transmission: TTX; young rat before development of chemical synapses.

• In addition to classical chemical synapses, it may include other chemical

signals, gap junctions, or the participation of glia.

Types of Rhythms

• Circadian – fluctuate daily– Sleep-wake, temperature, hormones, urine production,

gastrointestinal activity– Cognitive and motor performance levels

• Infradian – less than once a day– Hibernation, ovulation

• Ultradian – more than once a day– Sleep cycles (REM and other sleep stages)

Multiple Pacemakers

• SCN cycles (affected by environment):– Sleep-wake cycles– Skin temperature– Hormones in blood, calcium in urine.

• Cycles independent of SCN:– Sleep cycles (REM)– Internal body temperature– Cortisol in blood, potassium in urine.

Rhythms and Disturbance

• Epilepsy• Extreme synchronous behavior in which many neurons fire at once.

– Localized or global– Upsets balance of excitation and inhibition among neurons

• Delayed sleep-phase insomnia

• Seasonal depressionDesynchronization between circadian rhythms, sleep and emotion

states may result in depression.– Depression is almost invariably cyclic.– Many depressed people enter REM sleep earlier than normal.– Sleep deprivation may ease depression temporarily.

• Alzheimer’s disease

Alzheimer’s disease

Randomized Clinical trial:

Light & Melatonin 3.5 yr in up to 189 subjects in 12 homes for the elderly, placebo-controlled.- nocturnal restlessness - 22% by light & melatonin- enhanced rhythms (activity, melatonin)- 12% less depressive symptoms- attenuated cognitive decline (3 MMSE points after 3.5 years

MMSE change

6

8

10

12

14

16

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Time (years)

MM

SE

Melatonin and Light

Melatonin Only

Light Only

None

Riemersma et al., JAMA,299, 2642-2655, 2008