On the functions of monoamine oxidase,
the emotions, and adaptation to stress

Richardson JS
Department of Pharmacology,
College of Medicine,
University of Saskatchewan,
Saskatoon, Canada.
Int J Neurosci 1993 May; 70(1-2):75-84


Monoamine oxidase is an iron containing enzyme that exists as 2 isozymes, A and B, that have different affinities for various amines as substrates. The activity of monoamine oxidase helps to maintain neuron firing rates throughout the body within homeostatic limits. It does this by metabolizing in the liver bioactive amines absorbed into the bloodstream from food, by metabolizing in the endothelial cells of cerebral vascular microvessels, as part of the blood brain barrier, bioactive amines in the bloodstream, and by metabolizing in the cytoplasm of neurons, molecules of biogenic amine neurotransmitters that are not enclosed in vesicles. Part of the biochemical activity of monoamine oxidase generates hydroxyl radicals, very toxic members of the oxygen free radical group, that may be involved in neurodegenerative disorders such as Parkinson's disease. Inhibiting monoamine oxidase with selegiline (1-deprenyl) seems to have neuroprotective actions but this may be due to inducing the release of neuronal growth factors rather than by preventing the formation of free radicals. Other drugs that inhibit monoamine oxidase are used to treat patients with atypical depression, panic attacks or post traumatic stress syndrome. It is hypothesized that the emotions act as positive or negative reinforcers of behavior patterns that increase the probability of survival of the organism. The original releasing stimuli for the emotions are related to the basic survival reflexes of the hypothalamus but the emotional response can be easily conditioned to formerly neutral stimuli by association. In the absence of the original releasing stimuli, these learned emotions increase the frequency of survival oriented behavior and decrease the frequency of behavior that jeopardizes survival. The emotional disorders are conditions in which the brain's reinforcement system is inoperative, the person loses contact with reality and the person's behavior bears no relationship to survival. Aversive stimulation evokes a negative emotional response that motivates the organism to escape from the aversive stimulation, and to avoid it, and any conditioned stimuli associated it, in the future. When the aversive stimulation and to avoid it, and any conditioned stimuli When the aversive stimulation is inescapable or unavoidable, the organism experiences stress. When the stressful aversive situation is not lethal, survival does not depend on escape but rather on conservation of energy. With repeated exposure, the negative emotional response to the aversive stimulation extinguishes, the organism adapts to the situation and takes on a passive, energy saving behavior pattern.
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