MCB 186 CIRCADIAN BIOLOGY Week 2 Entrainment by light and the Phase Response Curve (PRC) September 26, 2007 J. W. Hastings

CIRCADIAN RHYTHMS - KEY PROPERTIES (1)RHYTHMS CONTINUE IN ABSENCE OF LIGHT / DARK CYCLES WITH PERIODS CLOSE TO BUT NOT EXACTLY 24 HOURS exact period length is a function of environmental conditions (2)TEMPERATURE ALSO AFFECTS CIRCADIAN PERIOD BUT MUCH LESS SO THAN FOR TYPICAL BIOCHEMICAL REACTIONS. Cellular compensation is postulated to be responsible (3) PHASE can be RESET by LIGHT: RHYTHMS ENTRAINED or SYNCHRONIZED to DAILY ENVIRONMENTAL CYCLES resetting does not need cycles: single exposures or pulses suffice

ENTRAINMENT OCCURS when ORGANISM is EXPOSED TO LIGHT/DARK CYCLES CAN BE CALLED SYNCHRONIZATION OTHER CYCLES (e.g. temperature) CAN ALSO ENTRAIN

RHYTHM IN HUMAN: LD & LL In LD the period is exactly 24 hr Variability in sleep onset has meaning

HUMAN CIRCADIAN ENTRAINMENT to LD and FREE RUNS Phase shift No evident jet lag

GONYAULAX RE-ENTRAINMENT TO A DIFFERENT PHASE Hastings, 1958 control No jet lag in a unicellular organism

Cyanobacterial in vivo gene expression rhythm with bacterial luciferase as a reporter Cells on two different LD cycles Luminescence Kondo, Johnson Golden et al., 1993, PNAS

ENTRAINMENT BY LIGHT-DARK CYCLE IN MONKEY MOORE-EDE ETAL Note that phase of free-run is determined by point of release LL LD LL

MASKING BY TEMP CYCLE- FLYING SQUIRREL DE COURSEY An environmental cycle can appear to entrain (masking) Phase of rhythm After return to LL or DD reveals if it is masking

CAN ORGANISMS ENTRAIN to CYCLE LENGTHS OTHER THAN 24 hr? HOW DO THEY ENTRAIN TO DIFFERENT PHOTO-FRACTIONS e.g., LD 6:18 or 18:6

PERIOD (Tau) of CIRCADIAN RHYTHM DEPENDS on PRIOR L/D CYCLE PERIOD (T=20) MOUSE PITTENDRIGH & DAAN ENTRAINMENT to 20 hr CYCLE MUST be DONE GRADUALLY

ARE THERE “LIMITS” to CYCLE LENGTHS that can ENTRAIN? Yes, but these differ widely depending on the effectiveness of light in causing phase shifts: effectiveness depends on intensity Single cell organism: QUITE WIDE - 8 to 12 hrs Higher organisms & humans: QUITE NARROW hr

LIMITS of ENTRAINMENT ILLUSTRATED WHAT ARE the LIMITS DUE TO??

Figure 1 Wright et al, PNAS 2001 Human Melatonin Rhythms Under Different Weak Light Entrainment Cycles and then Forced Desynchrony No entrainment entrains No entrainment 23.5 hr 24 hr24.6 hr

Fig 5 Gronfier et al, PNAS lux ENTRAINS while 25 lux DOES NOT

Relative Coordination in Hamster Activity in Light Intensity Cycle-length that Fails to Entrain Enright, 1980

Gonyaulax ENTRAINS to 14 HR LD CYCLES Subsequently on DD or LL, periods are about 24 hr Note: tau ~24

ENTRAINMENT by DIFFERENT LIGHT/DARK CYCLES and EFFECT of LIGHT INTENSITY on LIMITS of ENTRAINMENT 12 hr cycle entrains at high but not low light intensity - because phase shifts are greater at high light intensity Note that period reverts to ~ 24 hr in subsequent LL Hastings & Sweeney, 1959

Wever R. Int J Chronobiol 3: 19, 1975 Free-running Human Subject Demonstrating Spontaneous Switch From “Internally Synchronized Rhythms” to “Internal Desynchronization” Time (hours) Time (days) Temperature Sleep-wake

DINOFLAGELLATE FLASH & GLOW BIOLUMINESCENCE and its CIRCADIAN EXPRESSION scale: ~ 24 hours peak to peak

DIFFERENT OSCILLATORS CONTROL GLOW & FLASHING Internnal Desynchronization

NIGHT PHASE: LAWN ON BOTTOM OF DISH (LEFT) DAY PHASE: AGGREGATIONS (RIGHT)

GONYAULAX CIRCADIAN RHYTHM OF AGGREGATION / ACTIVITY

GONYAULAX INTERNAL DESYNCHRONIZATION OF TWO RHYTHMS ROENNEBERG

THREE RHYTHMS SIMULTANEOUSLY: PHASE-JUMPS ROENNEBERG & MORSE 1993 Glo FlsAgg

ENTRAINMENT is due to PHASE SHIFTS in CIRCADIAN RHYTHM BUT PHASE SHIFTS do not REQUIRE FULL LIGHT/DARK CYCLES REPEATED “BRIEF” EXPOSURES TO LIGHT SUFFICE THE PHASE RESPONSE CURVE- PRC

Gonyaulax CELLS IN DD: PHASE SHIFT BY SINGLE LIGHT PULSES ADVANCE OR DELAY DEPENDS ON TIME IN CYCLE CONTROL IN DARK LATE NIGHT PULSE PHASE ADVANCE EARLY NIGHT PULSE PHASE DELAY Hastings and Sweeney, 1958

HOW DOES ONE DISTINGUISH DELAYS from ADVANCES? GIVE a STRONGER(BRIGHTER) EXPOSURE to LIGHT THIS RESULTS in a GREATER PHASE SHIFT - but in WHICH DIRECTION??

GONYAULAX PHASE ADVANCES BY LIGHT PULSES

GONYAULAX LIGHT PHASE RESPONSE CURVE (PRC) Hastings & Sweeney, 1958

Gonyaulax LIGHT PHASE RESPONSE CURVE (the PRC) LIGHT PULSES GIVEN AT TIMES INDICATED TIME 0 is the BEGINNING of NIGHT PHASE DEAD ZONE DAY PHASE

DROSOPHILA PHASE RESPNSE CURVE PITTENDRIGH TIME 12 is the BEGINNING of NIGHT PHASE

PRCs in Gonyaulax: LIGHT-INDUCED DELAY-PHASE SHIFTS ARE INCREASED BY CREATINE

PHASE RESPONSE CURVES (PRCs) in the MOSQUITO

Single Light Pulses Given at Different Times to Animals in DD Cause Delay or Advance Phase Shifts:PRC Moore-Ede et al 1982

Schematic depiction of entrainment by light in LD different Tau values : T constant, 24h

CAN ORGANISMS ENTRAIN to CYCLE LENGTHS OTHER THAN 24 hr? HOW DO THEY ENTRAIN TO DIFFERENT PHOTO-FRACTIONS e.g., LD 6:18 or 18:6

EMERGENCE TIMES of DROSOPHILA ENTRAINED to FULL LD CYCLES OF DIFFERENT PHOTOFRACTIONS PITTENDRIGH

PHASE JUMP with DIFFERENT SKELTON PHOTOPERIODS ECLOSION OCCURS AFTER THE LONGER DARK INTERVAL

DROSOPHILA ECLOSION PHASE ANGLES with DIFFERENT INTERVALS for TWO-PULSE SKELETON PHOTOPERIODS Pittendrigh

Hamster Phase Angle of Entrainment in Complete and Skeleton Photoperiods Pittendrigh & Daan, 1976

SINGLE PULSES EVERY 24h Sometimes called T-cycles A single light pulse (e.g., 15 min, 1hr) every cycle will entrain a circadian rhythm. Cycle may be =24h, longer or shorter than 24 Organism self selects location of light pulse If tau is greater than 24 it will fall at late night If tau is less than 24 it will fall at early night

A SINGLE PULSE per CYCLE WILL ENTRAIN PERIOD of ACTIVITY RHYTHM DEPENDS on PERIOD of SINGLE PULSE T-CYCLE: POSITIONING of PULSE is SELF-SELECTED