biological clocks clock periods –circannual –circalunidian –circadian clock mechanisms...

25
Biological clocks • Clock periods – Circannual – Circalunidian – Circadian • Clock mechanisms – Entrainment – Neural location – Genetic basis

Post on 20-Dec-2015

226 views

Category:

Documents


0 download

TRANSCRIPT

Biological clocks

• Clock periods– Circannual– Circalunidian– Circadian

• Clock mechanisms– Entrainment– Neural location– Genetic basis

Hibernation follows annual rhythm ingolden-mantled ground squirrels

Five animals were isolated at birth and kept in darkness at 3oC

Testes growth and feather molt in

stonechats follows annual cycles

Nestlings were removed from Kenya and reared in Germany with constant temperature and photoperiod and yet retain annual molt and testes cycles.Notice that the clock period drifted.

QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.

Horseshoe crabs mate on full moonwhy?

Lunar position affects the tides

Spring tide

Sun and moonalign, tidalexcursion isgreatest

Neap tide

Sun and moonare perpendicular,tidal excursion isleast

Neotropical bats exhibit lunar phobia

Kangaroo rat feeding shows lunar cycles

K-rat activity at a feeder is confined to dark periods

QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.

occurred during periodof seed shortages

Isopod activity follows daily tidal flow

Isopods are usually covered with water at high tide. They retainthis activity even when kept in the lab with no tidal fluctuation.

Entrainment by environmental cycles

• Temperature compensation– Clock cycles do not change with temperature

• Environmental cues set cycle period– Species specific

• Types of cues (zeitgebers)– Photoperiod– Light pulse– Food availability

Cricket calling entrains to dark

Constant light for 12 days

12 h light/darkfor 12 days

Mouse activity entrains to light

10 mins of light per day are sufficient to reset the clock

12h light:12h dark

24 h dark

10 min light

Mole-rats lack daily cycles

Clock mechanisms

• Location of the clock– Suprachiasmatic nucleus of the hypothalamus– Pineal gland

• Clock genes– Period– Timeless– Tau (doubletime)

Mammal and bird clocks reside in the

suprachiasmatic nuclei (SCN), which is in the hypothalamus

In hamsters, SCN lesion disrupts

clock while SCN transplant restores

clock

Period

Isolated rat SCN cells

exhibit clock activity

Isolated caudate (upper brainstem) cells do not cycle, but isolated SCN cells do cycle

Isolated neurons from rat SCN exhibit circadian rhythym

Neural firing is stopped with application of tetrodotoxin (TTX),which blocks sodium channels, but clock kept ticking!

Pineal glands respond to light cyclesMelatonin release from chicken pineal glands cultured in vitro

Light cycles No light cycles

Distribution of circadian clocks in

tissues and taxa

Mammalian clock

pathways

period alleles exhibit altered circadian rhythymsin Drosophila melanogaster

Genetic basis of the clock in flies

• NOON: per and tim genes are turned on by CLOCK-CYCLE complex, which binds to promoter

• SUNSET: PER and TIM transcription occurs

• NIGHT: PER and TIM proteins build up inside the cell, and as a complex can enter nucleus

• dbt codes for an enzyme that adds a phosphate to PER, which causes it to be destroyed & adds time delay

• DAWN: Cryptochromes absorb blue light and activate cry gene expression. TIM protein is degraded by CRY protein.

• PER is released from PER-TIM complex and broken down, so CLOCK-CYCLE can activate per and tim

http://www.nimh.nih.gov/publicat/bioclock.cfm

Genetic basis of the clock in mammals

• per codes for a protein (PER) that gradually builds up over time

• tau codes for an enzyme that breaks down PER

• tim codes for a protein (TIM) that binds with PER to cross the membrane and suppress transcription of PER

• Photoreceptor not yet known

• Cycle repeats every 24 h

Clock summary• per/tim/tau(dbt) genes control pacemaker• Pacemaker occurs in SCN in vertebrates, but is

distributed in brain cells in some insects• SCN signals pineal to release melatonin.

Melatonin causes entrainment• Short pulses of light entrain SCN and pineal cells

– Photoreceptors occur in the pineal and eyes of birds– Photoreceptors occur in retina of mammals

• Drosophila, honey bees, hamsters and humans share same genes - likely common ancestor was a flatworm that lived about 600 MYA

Clock neurons in fly brainshttp://www.bio.brandeis.edu/faculty/hall.html

Hall, J.C. 2003. Genetics and molecular biology of rhythms in Drosophila and other insects. Adv. Genet. 48: 1-286