Monoamine oxidase inhibitors were among the first antidepressants to be discovered and have long been used as such. It now seems that many of these agents might have therapeutic value in several common neurodegenerative conditions, independently of their inhibition of monoamine oxidase activity. However, many claims and some counter-claims have been made about the physiological importance of these enzymes and the potential of their inhibitors. We evaluate these arguments in the light of what we know, and still have to learn, of the structure, function and genetics of the monoamine oxidases and the disparate actions of their inhibitors.

The therapeutic potential of monoamine oxidase inhibitors.

Molecular Imaging with PET

Several studies have documented a strong association between smoking and depression. Because cigarette smoke has been reported to inhibit monoamine oxidase (MAO) A in vitro and in animals and because MAO A inhibitors are effective antidepressants, we tested the hypothesis that MAO A would be reduced in the brain of cigarette smokers. We compared brain MAO A in 15 nonsmokers and 16 current smokers with [11C]clorgyline and positron emission tomography (PET). Four of the nonsmokers were also treated with the antidepressant MAO inhibitor drug, tranylcypromine (10 mg/day for 3 days) after the baseline PET scan and then rescanned to assess the sensitivity of [11C]clorgyline binding to MAO inhibition. MAO A levels were quantified by using the model term λk3which is a function of brain MAO A concentration. Smokers had significantly lower brain MAO A than nonsmokers in all brain regions examined (average reduction, 28%). The mean λk3 values for the whole brain were 0.18 ± 0.04 and 0.13 ± 0.03 ccbrain (mlplasma)−1 min−1 for nonsmokers and smokers, respectively; P < 0.0003). Tranylcypromine treatment reduced λk3 by an average of 58% for the different brain regions. Our results show that tobacco smoke exposure is associated with a marked reduction in brain MAO A, and this reduction is about half of that produced by a brief treatment with tranylcypromine. This suggests that MAO A inhibition needs to be considered as a potential contributing variable in the high rate of smoking in depression and in the development of more effective strategies for smoking cessation.

https://www.pnas.org/content/pnas/93/24/14065.full.pdf

Brain monoamine oxidase A inhibition in cigarette smokers.

Astrocytes colocalize with fibrillar amyloid-beta (A beta) plaques in postmortem Alzheimer disease (AD) brain tissue. It is therefore of great interest to develop a PET tracer for visualizing astro ...

https://jnm.snmjournals.org/content/jnumed/53/1/37.full.pdf

Evidence for Astrocytosis in Prodromal Alzheimer Disease Provided by 11C-Deuterium-L-Deprenyl: A Multitracer PET Paradigm Combining 11C-Pittsburgh Compound B and 18F-FDG

Biochemistry and genetics of monoamine oxidase.

The regional distributions of monoamine oxidase (MAO) types A and B have been identified in human brain in vivo with intravenously injected 11C-labeled suicide enzyme inactivators, clorgyline and L-deprenyl, and positron emission tomography. The rapid brain uptake and retention of radioactivity for both 11C tracers indicated irreversible trapping. The anatomical distribution of 11C paralleled the distribution of MAO A and MAO B in human brain in autopsy material. The corpus striatum, thalamus, and brainstem contained high MAO activity. The magnitudes of uptake of both [11C]clorgyline and L-[11C]deprenyl were markedly reduced in one subject treated with the antidepressant MAO inhibitor phenelzine. A comparison of the brain uptake and retention of the 11C-labeled inactive (D-) and active (L-) enantiomers of deprenyl showed rapid clearance of the inactive enantiomer and retention of the active enantiomer within MAO B-rich brain structures, in agreement with the known stereoselectivity of MAO B for L-deprenyl. Prior treatment with unlabeled L-deprenyl prevented retention of L-[11C]deprenyl. Thus, suicide enzyme inactivators labeled with positron emitters can be used to quantitate the distribution and kinetic characteristics of MAO in human brain structures.

https://science.sciencemag.org/content/sci/235/4787/481.full.pdf

H. Sachs

Papers

Mapping human brain monoamine oxidase A and B with 11C-labeled suicide inactivators and PET.

Positron emission tomographic studies on induced migraine.