Time of day affects extinction of cocaine induced conditioning and TNFa mRNA levels in specific brain regions
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Why we sleep is not known, although there are a number of theories. One of the most recent theories, put forth by Krueger and Obal (1993) maintains that we sleep to preserve synaptic efficacy, and during the wake state sleep-promoting growth factors build up within the brain causing us to feel sleepy. As everyone knows, it is harder to learn a task when you're sleepy. In our experiment we looked at extinction of cocaine-induced conditioning as our learning task. Our hypothesis was that, because rats are nocturnal, ones that extinguish in the AM should extinguish slower than ones that extinguish in the PM. This is because the rats will have a higher level of sleep-promoting growth factors present in the AM which will be attenuated during sleep. We found that diurnal rhythms in the cytokine levels were evident in brain regions such as the hypothalamus, prefrontal cortex, nucleus tractus solitarius, and somatosensory cortex, after cocaine-induced CPP, extinction, and reinstatement. In most cases, sleep during the day reduced the level of TNFa mRNA as well as the extinction rate for cocaine-induced CPP. Future studies will reveal if altering the TNFa mRNA levels will actually produce the observed differences in the extinction rate.