Four main models have been proposed for the mechanism of electron transfer (single electron transfer, hydrogen atom transfer, nucleophilic model, and hydride transfer) although there is insufficient evidence to support any of them. They are well known enzymes in pharmacology, since they are the target for the action of a number of monoamine oxidase inhibitor drugs. MAO-A is particularly important in the catabolism of monoamines ingested in food. Both MAOs are also vital to the inactivation of monoamine neurotransmitters, for which they display different specificities. Specific reactions catalyzed by MAO include:   Because of the vital role that MAOs play in the inactivation of neurotransmitters, MAO dysfunction (too much or too little MAO activity) is thought to be responsible for a number of psychiatric and neurological disorders. For example, unusually high or low levels of MAOs in the body have been associated with schizophrenia, depression, attention deficit disorder, substance abuse, migraines, and irregular sexual maturation. Monoamine oxidase inhibitors are one of the major classes of drug prescribed for the treatment of depression, although they are often last-line treatment due to risk of the drug's interaction with diet or other drugs. Excessive levels of catecholamines (epinephrine, norepinephrine, and dopamine) may lead to a hypertensive crisis, and excessive levels of serotonin may lead to serotonin syndrome. In fact, MAO-A inhibitors act as antidepressant and anti-anxiety agents, whereas MAO-B inhibitors are used alone or in combination to treat Alzheimer's disease and Parkinson's disease. Some research suggests that certain phenotypes of depression, such as those with anxiety, and "atypical" symptoms involving psychomotor retardation, weight gain and interpersonal sensitivity. However the findings related to this have not been consistent. MAOIs may be effective in treatment resistant depression, especially those that do not respond to tricyclic antidepressants. PET research shows that use of tobacco cigarettes heavily depletes MAO-B, mimicking the action of an MAO-B inhibitor. Smokers who smoke for emotional relief may therefore be unintentionally treating depression and/or anxiety that is better addressed by an MAO-B inhibitor. There are significant differences in MAO activity in different species. Dopamine is primarily deaminated by MAO-A in rats, but by MAO-B in vervet monkeys and humans. Mice unable to produce either MAO-A or MAO-B display autistic-like traits.