The researchers next used those same rats to ask if ACh is necessary for nerve cells to learn new time delays. To do that, they switched the visual cues so that a flash in the left eye meant a long delay and one in the right eye meant a short one. Vision-processing nerve cells in the rats in which ACh delivery was left intact adapted their signals to the new associations; but those in the rats that no longer received ACh continued to relay the old associations, suggesting that ACh is necessary to make new associations but not to express old ones.
Hussain Shuler explains, "When a reward is received, ACh is sent throughout the brain and reinforces only those nerve cell connections that were recently active. The process of conditioning continues to strengthen these nerve connections, giving rise to a timed expectation of reward in the brain."
According to Hussain Shuler, studies have shown that Alzheimer's patients have low levels of ACh and have trouble forming new memories. Though medication may elevate ACh, alleviation of symptoms is limited. "Our research explains that limitation," he says. "Therapeutically, we predict that the problem isn't just low levels of ACh the timing of ACh delivery is key."
Other authors of the report include Emma Roach of the Johns
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Johns Hopkins Medicine