A large number of neurotransmitters have been identified in the brain, ranging from simple amino acids (e.g., glycine, glutamate) and monoamines (e.g., epinephrine, norepinephrine, dopamine, and serotonin) to more complex peptides and hormones (e.g., oxytocin, vasopressin, and ACTH). The simpler amino acids and monoamines are faster-acting and may mediate rapid emotional responses, whereas the more complex molecules are longer-acting and may be implicated in more persistent moods (Panksepp, 1993). There are methodological problems in studying the relationship between individual neurotransmitter systems and behavior and many of the studies to date do not provide consistent data. Recent imaging techniques provide a new set of tools by allowing the visualization of the activity of specific neurotransmitter systems during particular behaviors. While some neurotransmitter systems have been associated with particular behaviors the overall system is much too complex to allow for simple one-to-one mappings between a particular neurotransmitter and a specific behavioral tendency or personality trait. With this caveat in mind, below are examples of some neurotransmitter-behavior associations identified to date.
Emotion has for long been associated with the chemistry of the human body. The chemical substances considered vary from blood-born hormones to central neuroactive substances depending on the focus of the research (body or CNS). Because this level of analysis is so accessible to therapeutical application, research has been done in the framework of specific diseases containing affective components.
Acetylcholine: acetylcholineesterase inhibitors produce depressive symptoms, while anticholinergic substances have antidepressant and/or euphorigenic effects. High level of acetylcholine induce depression while low levels induce mania. However these effects have to be associated with monoamine levels as well.
Serotonin: Low CSF levels of 5-HT are associated with depression and suicide propensity.Cell recording in the raphe nucleus show marked increase in activity in defeated monkeys (Walletschek & Raab 1982), compatible with the finding of increase urine serotonin level in dominant animals (McGuire et al. 1983). The popular anti-depressant drugs such as Prozac (fluoxetine) and Zoloft inhibit the reuptake of serotonin in the synaptic cleft, thus increasing its overall concentration. However, the precise mechanism of action of these drugs is not clear. For example, while the concentration of serotonin in the synaptic cleft increases shortly after the drugs are ingested, their anti-depressant effect typically takes several weeks to manifest.
Epinephrine and Norepinephrine: Sympathetic activation is at the basis of norepinephrine biosynthesis in the medulla.In turn, norepineprhine is transformed into epinephrine under the enzymatic action of ACTH, produced in the pituitary gland in response to anxiety or stress.
GABA receptors: They are the target of anti-anxiety medications such as the benzodiazepine family of tranquilizers.
Mental Health Clinical Research Center: University of California, San Diego.
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