The domain of neurotransmitters and their receptor systems has been evolving with the speed of light not only in terms of detecting novel ones but also in terms of deciphering their biological functions with a keen eye on developing novel therapeutic targets. The requirement that the neurotransmitter should be present or synthesized in the neuron is being challenged to include chemicals that can carry messages between neurons via the influence on the postsynaptic membrane regardless of their site of origin. Neurotransmitters are normally synthesized in the cytosol of axon terminals and are stored in the organelles called synaptic vesicles typically about 40–50 nm in diameter. The basic definition of a receptor is that it is a membrane protein that is embedded in the phospholipid bilayer of a postsynaptic membrane and is activated by a neurotransmitter. Neuronal communication achieved via neurotransmitter release is very complex and involves the reception of thousands of excitatory and inhibitory signals delivered to each neuron.