Each neuron in the brain consistantly releases the same chemical neurotransmitter, at all synaptic connections it makes with other neuron. thus, neuron can be characterized by neurotransmitters it releases. the two neurotransmitter that appear most frequently are glutamate, which almost always excitatory, and gamma-aminobutyric acid(GABA) which is almost always inhabitory. neurons using these transmitter can be found in nearly every part of the brain, making up a large percentage of the brain's pool of synapses.

Nevertheless, the great majority of psychoactive drugs exert their effects by altering neurotransmitter systems not directly involving glutamatergic or GABAergic transmission. drugs such as caffeine, nicotine, heroin, cocaine, Prozac, Thorazine etc act on other neurotransmitters. many of these other transmitters come from neurons that are localized in particular part of the brain. serotonin, for example-the primary target of antidepressant drugs and many dietary aids-comes exclusively from a small brainstem area called Raphe nuclei. norepinephrine, which is allowed in arousal, comes exclusively from a nearby small area called the locus ceruleus. histamine, as a neurotransmitter, comes from a tiny part of the hypothalamus calle the tuberomammilary nucleus. other neurotransmitters such as acetylcholine and dopamine have multiple sources in the brain, but are not as ubiquitously distributed as glutamate and GABA.


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