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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