Neurotransmitters are chemicals that transmit signals from the neurons to target cells across a synapse. Packaged into synaptic vesicles that are clustered under the membrane on presynaptic side of the synapse, these are released into synaptic cleft. They bind to receptors located inside the membrane on postsynaptic side of the synapse. There are also some common neurotransmitters that are widely used by medical scientists for regulation of brain and muscles. 
Examples of Common Neurotransmitters
Some of the common neurotransmitters are Asparater, Neuopeptides, Glutamate, Glycine, Accetylcholine, Dopamine, Norepinephrine, Octopamine, Tyramine, Serotonin, Melatonin, Histamine, Gastrin, Vasopressin, Oxytocin, and Neurophysin etc. 
Neurotransmitters Precursors
In taking neurotransmitter precursors could generate the following conditions. Enhancement of neurotransmitter synthesis may occur. It is not clear whether it will increase the neurotransmitter release. Similarly it is also not clear whether there would be long term increase in neurotransmitter signal strength. Nervous system can adapt to changes such as increased amount of neurotransmitter synthesis. Sometimes neurotransmitter synthesis also has a role in depression. At least they could be useful in treatment of mild depressions. 
Neurotransmitter Degradation 
It is necessary breaking down the neurotransmitter once it reaches the post synaptic cell preventing further excitatory or inhibitory transduction of signals. For instance; medical scientists have found that acetylcholine that is one of the excitatory neurotransmitters is broken down with accetylcholinesterase. Diffusion is also effected in case of other neurotransmitters like dopamine. In this case the neurotransmitter is able to diffuse away from the targeted synaptic junctions and finally they are eliminated from the body via the kidneys. 
Neurotransmitter and Depression
Millions of Americans suffer depression and anxiety syndrome and the rate is pretty alarming. A recent survey indicates the staggering figure of 40 million Americans suffering from the syndrome or at least one of these conditions. Symptoms are identical in men and women but the latter category is more vulnerable to the symptoms of depression. The rate of suffering women is almost twice of their male counterparts. 
Anti-Depression Medicine Sales Hikes
In result there has been a considerable hike in the sales of anti-depression medicine related to neurotransmitters. An indirect benefit is that it has opened up business opportunities for many budding entrepreneurs. Prospects are high since the sale of prescription antidepressants are over $20 billion.