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 Video http://www.youtube.com/watch?v=CQFEY9RIRJA

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?

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? 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.

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

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.

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 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.

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)

Wheel running activity

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