1. Neuronal Control of Behavior
Biological Clocks

2. Behavioral Choice Priorities change throughout a day or year
Some behaviors only occur during the day or at night, or by season
Cricket mate calling by males
Cricket mate searching by females
Inhibitory control must be regulated
Video

3. Hypotheses Biological Clock Theory Environment Response Theory
Timing mechanism with endogenous, built-in schedule
Independent of environment
Environment Response Theory
Relationships between command centers are modified by feedback from the environment
Behavior changes as conditions change
How could you distinguish between these?

4. Testing these hypotheses
Prediction: If cricket calling, which normally begins at dusk, is controlled by the environment (ie. darkness), then crickets kept in constant light should never call

5. Experiment: Grow crickets in the lab under constant temperature and brightness, record calling

6. Figure 5-6

7. Result: Crickets continue to call in the absence of an environmental cue like temperature or light.
Interpretation: Mate calling is a free-running cycle, supporting the idea that an internal biological clock controls the behavior.
A circadian rhythm occurs with frequency of “about a day”. There is a slight variation from the 24 hour environmental cycles caused by the Earth’s rotation around its axis.

8. Experiment: Grow crickets in the lab on a 12 hour light-dark cycle and record their calling

9. Figure 5-6

10. Entrainment
Result: Crickets use the cue of darkness to adjust their calling so that it begins about 2 hours before lights off and ends about 2.5 hours before lights on.
Interpretation: Calling is reset, or entrained each day to the salient cue of darkness, which matches the natural behavior
Both hypotheses are correct: calling can occur independent of the environment based on an internal clock, but is normally reset each day to the onset of nightfall.

11. What controls circadian rhythms?
Removal of eye (retina) leads to a free running cycle of calling behavior (absence of environmental cue). Remove or severing of optic lobes results in complete loss of rhythmicity. The biological clock must be in the optic lobe.

13. Role of the hypothalamus
SCN=suprachiasmatic nucleus
Receives input from retina (day/night length)
Removal of SCN leads to arrhythmic patterns of locomotion, hormone secretion, feeding in rodents
Transplant of SCN but not other tissues restores
Transplant of mutant SCN results in the mutant period length (eg. Shorter than 24 hours)
SCN maintains rhythmic secretions when removed from brain
All good evidence that the SCN is the site of the biological clock in mammals

14. Role of genes
Mutants isolated from the fruitfly drosophila melanogaster in the period gene exhibit alterations in behaviors controlled by circadian rhythms such as locomotion and feeding.

15. Period and Timeless (Per and Tim) are regulated in a cyclic way by degradation and negative feedback. As Per accumulates, it may become phophorylated and degraded. Per inhibits its own transcription, as does Tim, so at the peak abundance, production is turned off.

16. Evidence for per as a timing gene
Fruitfly mutations
Normal levels vary in honeybees with behavior patterns
Young nurse bees have very low Per protein and are active around the clock
Adult foragers, who go out in the daytime, have higher levels of Per and exhibit well-defined circadian rhythms
Humans with a mutation in per have altered sleep cycles
The per gene is highly conserved and expressed in the SCN

17. Expectations of the biological clock
The molecule that relays the clock’s instructions should be regulated by the clock’s genes
The molecule should be secreted and there should be a receptor for that chemical signal in target tissues that mediate behavior
Experimental manipulation of the chemical should disrupt the timing of behavior

18. Candidate molecules
Melatonin: a hormone released by the hypothalamus at dusk; promotes sleep
PK2: Prokineticin 2
PK2 is produced in a cyclic pattern regardless of the environment: 2 or 8 days of darkness (DD) had no effect on PK2 cycling in the SCN.

19. Evidence for PK2 as the circadian clock signal
Mice with mutations in per and tim lack cyclic production of PK2
Only certain structures produce a PK2 receptor
Injections of PK2 during the night, when levels are normally low and rats are active, leads to cessation of activity and sleep (daytime behavior)

20. Wheel running activity

21. Adaptive value of circadian rhythms
Individuals do not always have to check the environment to see what time it is
But individuals can use the environment to subtly adjust their clock to changing conditions
Seasons
Jet lag

22. Only animals that use a day-night cycle have circadian clocks