Dopamine Norepinephrine and Serotonin that

are released by cells that produce these chemicals to

Cerebellum I IP !/

j^K/aiQ Raphe nuclei

Spinal /I'

Figure 2

Serotonin Pathways.

The serotonergic neurons originate mainly from the raphe nuclei in the brain stem and project to the forebrain, the cerebellum, and the spinal cord. Very high concentrations of serotonin are also found in the pineal gland. Serotonin-containing neurons are involved in such functions as pain, temperature regulation, sensory perception, and sleep.

communicate with other brain cells. The dopamine cells send inputs to only a few structures in the forebrain. These include the caudate nucleus that is involved in motor functions and areas of the limbic system that are involved in emotional behaviors and euphoria. The areas of this system to which dopamine cells send inputs include the amygdala, the nucleus accumbens, the olfactory tubercle, and the frontal and cingulate cortices (see Figure 3). Norepinephrine and serotonin cells send inputs more widely to most all forebrain regions, even though their cell bodies are localized in specific brain-stem nuclei. These drugs stimulate motor activity by increasing the function of the dopamine system, which sends inputs to the caudate nucleus. Stimulants produce feelings of well-being and euphoria by enhancing dopaminergic activity in limbic areas. Serotonin is also involved in the effects of stimulant use and withdrawal, but just how is not yet clear.

Figure 3

Dopamine Pathways. Dopamine is not only a precursor of noradrenaline but also a transmitter of its own. Dopamine represents more than 50 percent of the total catecholamine content of the central nervous system. The highest levels of dopamine are found in the neostriatum, nucleus accumbens, and tuberculum olfactorium. There are four main dopaminergic systems in the brain: the nigrostriatal, the mesolimbic nad mesocortical, and the tuberoinfundibular systems. The nigrostriatal system appears to be involved in motor function, while the the tuberoinfundibular dopamine neurons are involved in hypothalamic-pituitary control. The functions of the mesolimbic and mesocortical systems are less well known, although it is conceivable that they play a role in psychotic disease and in reinforcing effects of drugs.

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