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Neuronal Control of Behavior
Biological Clocks
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
Hypotheses
• Biological Clock Theory
– Timing mechanism with endogenous, builtin schedule
– Independent of environment
• Environment Response Theory
– Relationships between command centers
are modified by feedback from the
environment
– Behavior changes as conditions change
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
• Experiment: Grow crickets in the lab
under constant temperature and
brightness, record calling
Figure 5-6
• 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.
• Experiment: Grow crickets in the lab on a 12 hour
light-dark cycle and record their calling
Figure 5-6
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.
What controls circadian rhythms?
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
Role of genes
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
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
Candidate molecules
• Melatonin: a hormone released by the
hypothalamus at dusk; promotes sleep
• PK2: Prokineticin 2
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)
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
Only animals that use a day-night
cycle have circadian clocks