In the central nervous system, besides the neurotransmitters, the occurrence of substances which modify neuronal excitability has been proposed. Since their effect is a matter of modulating neuronal responsiveness, these substances may be called neuromodulators.Examples of neurotransmitters or putative neurotransmitters in the central nervous system are acetylcholine, noradrenaline, dopamine, and aminoacids such as GABA, glycine, and L-glutamic acid. Under certain conditions, some of these (acetylcholine, L-glutamic acid, and GABA), as well as hormones and polypeptides (e.g., releasing factors) are considered to act as neuromodulators.