Transcript Document
Biological clocks • Clock periods – Circannual – Circalunidian – Circadian • Clock mechanisms – Entrainment – Neural location – Genetic basis Hibernation follows annual rhythm in golden-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 Quick Time™a nd a TIFF ( Uncomp res sed) deco mpre ssor are n eede d to s ee this picture . 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. Horseshoe crabs mate on full moon why? Lunar position affects the tides Spring tide Neap tide Sun and moon align, tidal excursion is greatest Sun and moon are perpendicular, tidal excursion is least Isopod activity follows tides Isopods are usually covered with water at high tide. They retain this activity even when kept in the lab with no tidal fluctuation. Kangaroo rat feeding shows lunar cycles K-rat activity at a feeder is confined to dark periods Quic kTime™ and a TIFF (Uncompress ed) dec ompres sor are needed to s ee this pic ture. occurred during period of seed shortages Entrainment by environmental cycles • Environmental cues set cycle period – Species specific • Types of cues – Photoperiod – Light pulse – Food availability • Temperature compensation – Clock cycles do not change with temperature Cricket calling entrains to dark Constant light for 12 days 12 h light/dark for 12 days Mouse activity entrains to light 12h light:12h dark 24 h dark 10 min light 10 mins of light per day are sufficient to reset the clock Seasonal testis growth in whitecrowned sparrows - light triggered? H1: There is a photosensitive time window which follows a 24 h period. H2: Early spring day length triggers testes growth. 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 Period In hamsters, SCN lesion disrupts clock while SCN transplant restores clock Isolated rat SCN 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 cycles Melatonin release from chicken pineal glands cultured in vitro Light cycles No light cycles crem gene expression in rat pineal glands is altered by light samples taken crem product is produced in SCN and influences melatonin release Distribution of circadian clocks in tissues and taxa Mammalian clock pathways period alleles exhibit altered circadian rhythyms in Drosophila melanogaster 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 Genetic basis of the clock in flies • per and tim genes are turned on by clock and cycle • PER and TIM proteins build up inside the cell during dark • dbt codes for an enzyme that degrades PER & adds time delay • Cryptochromes absorb blue light and activate cry gene expression • TIM protein is degraded by CRY protein 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 • Short pulses of light entrain SCN and pineal cells • Drosophila, honey bees, hamsters and humans share same genes - likely common ancestor was a flatworm that lived about 600 MYA