treatment of depression, and monoamine oxidase B inhibitors are typically used in the treatment of Parkinson's disease. Nonspecific (i.e. MAOA/B combined) inhibitors can pose problems when taken concomitantly with tyramine-containing foods such as cheese, because the drug's inhibition of MAOA causes a dangerous elevation of serum tyramine levels, which can lead to hypertensive symptoms. Selective MAOB inhibitors bypass this problem by preferentially inhibiting MAOB, which mostly metabolizes DA. If MAOB is inhibited, then more DA is available for proper neuronal function, especially in Parkinson's Disease. Roles in disease and aging Alzheimer's disease (AD) and Parkinson's disease (PD) are both associated with elevated levels of MAO-B in the brain. The normal activity of MAO-B creates reactive oxygen species, which directly damage cells. MAO-B levels have been found to increase with age, suggesting a role in natural age related cognitive decline and the increased likelihood of developing neurological diseases later in life. More active polymorphisms of the MAO-B gene have been linked to negative emotionality, and suspected as an underlying factor in depression. Activity of MAO-B has also been shown to play a role in stress-induced cardiac damage. Over-expression and increased levels of MAO-B in the brain have also been linked to the accumulation of amyloid β-peptides (Aβ), through mechanisms of the amyloid precursor protein secretase, γ-secretase, responsible for the development of plaques, observed in Alzheimer's and Parkinson's patients. Evidence suggests that siRNA silencing of MAO-B, or inhibition of MAO-B through MAOI-B (Selegline, Rasagiline), slows the progression, improves and reverses the symptoms, associated with AD and PD, including the reduction of Aβ plaques in the brain. Animal models Transgenic mice that are unable to produce MAO-B are shown to be resistant to a mouse model of Parkinson's disease. They also demonstrate increased responsiveness to stress (as with MAO-A knockout mice) and increased β-PEA. In addition, they exhibit behavioral disinhibition and reduced anxiety-like behaviors. Inhibition of MAO-B in rats has been shown to prevent many age-related biological changes such as optic nerve degeneration, and extend average lifespan by up to 39%. Effects of deficiency in humans While people lacking the gene for MAO-A display intellectual disabilities and behavioral abnormalities, people lacking the gene for MAO-B display no abnormalities except elevated phenethylamine levels in urine, raising the question of whether MAO-B is actually a necessary enzyme. Newer research indicates the importance of phenethylamine and other trace amines, which are now known to regulate catecholamine