Some psychoactive drugs block the metabolic effect of the destructive enzyme on the neurotransmitter which prolongs the action of the neurotransmitter in the synapse and thus, on the receptor sites of the postsynaptic neuron.

Some antidepressants known as MAO-Inhibitors work in this manner. MAO-Inhibitors prevent the destruction of certain neurotransmitters (norepinephrine, dopamine, and serotonin) by monoamine oxidase (MAO) which prolongs the effect of these neurotransmitters in the synapse. The agonist action of MAO-I's on norepinephrine and serotonin is thought to mediate their antidepressant effects.

In the scene below, the green circles represent the molecules of the drug. The Pac-Man like characters represent the molecules of the destructive enzyme, MAO.

You may view this agonist mechanism by clicking on the "Play" button in the scene below. To view the animation again, you must first click on the "Rewind" button.