MCB 186CIRCADIAN BIOLOGY

Week 2

Entrainment by light and the Phase Response Curve (PRC)

September 26, 2007J. 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 periodis exactly 24 hr

Variability in sleeponset has meaning

HUMAN CIRCADIAN ENTRAINMENT to LD and FREE RUNS

Phase shift

No evident jet lag

GONYAULAX RE-ENTRAINMENT TO A DIFFERENT PHASEHastings, 1958

controlNo jet lag ina unicellular organism

Cyanobacterial in vivo gene expression rhythm with bacterial luciferase as a reporter

Cells on two different LD cycles

24016814412096720

Hours in Constant Light

2000

4000

6000

8000

12 60 84 108 132 156 180

228

192

204

10000

48

Lu

min

esce

nce

Kondo, Johnson Golden et al., 1993, PNAS

ENTRAINMENT BY LIGHT-DARK CYCLE IN MONKEY

MOORE-EDE ETAL

Note that phase of free-run is determinedby point of release

LL

LD

LL

MASKING BY TEMP CYCLE- FLYING SQUIRREL DE COURSEY

An environmentalcycle can appearto entrain (masking)

Phase of rhythmAfter return to LL or DD reveals if itis 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)

MOUSEPITTENDRIGH & DAAN

ENTRAINMENTto 20 hr CYCLEMUST be DONEGRADUALLY

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 - 1 - 3 hr

LIMITS of ENTRAINMENT ILLUSTRATEDWHAT 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

entrainsNo entrainment

23.5 hr 24 hr 24.6 hr

Fig 5 Gronfier et al, PNAS 2007

100 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 CYCLESSubsequently 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 athigh but not low lightintensity - because phaseshifts are greater at highlight intensity

Note that period revertsto ~ 24 hr in subsequentLLHastings & 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)T

ime

(day

s)

Temperature

Sleep-wake

DINOFLAGELLATE FLASH & GLOW BIOLUMINESCENCEand 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 RHYTHMSROENNEBERG

THREE RHYTHMS SIMULTANEOUSLY: PHASE-JUMPS ROENNEBERG & MORSE 1993

Glo

Fls Agg

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 PULSESADVANCE OR DELAY DEPENDS ON TIME IN CYCLE

CONTROL IN DARK

LATE NIGHT PULSEPHASE ADVANCE

EARLY NIGHT PULSEPHASE 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 theBEGINNING ofNIGHT PHASE

DEAD ZONEDAY PHASE

DROSOPHILA PHASE RESPNSE CURVE PITTENDRIGH

TIME 12 is theBEGINNING ofNIGHT 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 PHOTOPERIODSECLOSION 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 24hSometimes 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