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.
An additional question mark for Cialis: It inhibits another phosphodiesterase enzyme, PDE-11, which is found in the testicles and the heart, although the Lilly-Icos team insists studies show no problems.