Brain
There are dozens of other chemical neurotransmitters that are used in more limited areas of the brain, often areas dedicated to a particular function. Serotonin, for example—the primary target of antidepressant drugs and many dietary aids—comes exclusively from a small brainstem area called the raphe nuclei. Norepinephrine, which is involved in arousal, comes exclusively from a nearby small area called the locus coeruleus. Other neurotransmitters such as acetylcholine and dopamine have multiple sources in the brain, but are not as ubiquitously distributed as glutamate and GABA.
Adrenergic nerve fibre
There are several types of adrenergic receptors which are identified by their differing sensitivities to various drugs. Neurons in the central nervous system contain α1- and α2-adrenergic receptors and β1- and β2-adrenergic receptors. All four kinds of receptors are also found in the various organs of the body aside from the brain. They are responsible for the effects of epinephrine and norepinephrine when they act as hormones outside the central nervous system. In the brain, all autoreceptors appear to be of the α2 type. (The drug idazoxan blocks α2 autoreceptors and hence acts as an antagonist.) All adrenergic receptors are metabotropic, coupled to G proteins that control the production of second messengers. Adrenergic receptors can produce both excitatory and inhibitory effects. In general, the behavioral effects of the release of norepinephrine are excitatory. In the brain the α1 receptors produce a slow depolarizing (excitatory) effect on the postsynaptic membrane, while α2 receptors produce a slow hyperpolarization (inhibitory) effect. Both types of β receptors increase the responsiveness of the postsynaptic neuron to its excitatory inputs, which presumably related to the role this neurotransmitter plays in vigilance.