Neurotransmitter is a chemical that achieves chemically mediated transmission, a major mode of interneuronal and neuron-effector communication. The chemical should meet several criteria to be recognized as a neurotransmitter. The criteria are: (i) A neurotransmitter must be synthesized in a neuron and released from a presynaptic terminal, (ii) A neurotransmitter should reproduce the specific responses that are evoked by the stimulation of presynaptic neurons at the postsynaptic neuron or effector cells, (iii) The effect of the chemical should be blocked by antagonists in a dose-dependent manner, and (iv) A neurotransmitter must be reabsorbed into the presynaptic neuron or glia, or metabolized into an inactive form by enzymes to terminate the stimulation.