In 1998 Kandel's team injected aging mice with a failed antidepressant called Rolipram, which prevents the breakdown of cyclic-AMP by blocking an enzyme called phosphodiesterase-4.

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One of the reasons for this, as demonstrated in a recent paper by researchers at Yale medical school, is that when a person experiences extreme stress, or just goes through the normal process of aging, the prefrontal cortex accumulates cyclic-AMP.

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