The naturally occurring catecholamines, noradrenaline, adrenaline and dopamine, have been found in a wide range of animal and vegetable tissues, but are particularly associated with nervous tissue in animals. Of the many processes affecting the response to stimulation of catecholamine containing nerves, the synthesis of catecholamines, particularly the first enzymatic stage involving tyrosine hydroxylase, and the re-uptake process, whereby the nerve recovers much of the released catecholamine, appear to be the most significant. In peripheral tissues noradrenaline appears to be involved predominantly in the sympathetic control of blood pressure and flow while adrenaline is more important to metabolic processes especially fat and glucose turnover. Both may be released in increased amounts by various stimuli that cause stress or arousal in the body. Dopamine has not yet been shown to have any significant physiological function in peripheral tissues.
In the central nervous system, noradrenaline and dopamine are the two main catecholamines. The working of the brain is complex and involves balanced interactions between a variety of neurotransmitters, known or as yet unrecognised. However, noradrenaline appears to play a role in the central control of blood pressure, and in determining mood and activity probably by affecting the emotional drives. Dopamine is certainly important in the control of motor pathways, as shown by the dopamine deficiency syndrome in Parkinson's disease, and is possibly of significance in the abnormal behaviour of psychotics. The role of the small concentration of adrenaline in the brain has yet to be fully established.