Too much neurotransmitters is a bad thing. In other words, the neurotransmitters must be removed after they do their job. Many are broken down by enzymes. For example, acetylcholinesterase breaks down acetylcholine. Others like serotonin are reabsorbed by the axon.
Some drugs work by the inhibition of the reuptake of the neurotransmitter.
For example, cocaine prevents the reuptake of serotonin. C. Secondary messenger system. As mentioned, the action of the neurotransmitter may be mediated by a secondary messenger system. Chemicals and Nerves These can exert their effect in a variety of places. Video - The Neuron Suite by James Burke.