MAOA is one of the most asked-about concepts on the tutor2u Psychology teacher and student groups on Facebook.
Questions about MAOA come up regularly. The general lines of enquiry are “MAOA says aggression is associated with increased levels of serotonin but I’ve just read that aggression is associated with low levels of serotonin too” and it’s easy to see on the face of it why this can be confusing. Perhaps we should start calling this the serotonin paradox of aggression, cause essentially that is what it is. A review of some key meta analyses on the subject reveal that both the low serotonin and high serotonin aggression hypotheses are supported empirically.
MAOA is a gene whose job it is to make an enzyme called MAOA (Monoamine Oxidase A). This enzyme is responsible for breaking down a number of neurotransmitters including serotonin. This ‘breaking down’ of serotonin is part of neurotransmission, MAOA is the enzyme that effectively sweeps up any serotonin left in the synaptic gap after neurotransmission is complete. If this enzyme weren’t working as it should, it would mean that there would be a build up of serotonin in the synapse. This is why in the antidepressant drugs MAOi’s (Mono amine oxidase inhibitors) their mode of action is to stop the MAOA enzyme working in order to decrease the symptoms of depression.
In the literature showing a link between MAOA and aggression (mainly the work of Brunner 1993) a mutation was found in the X chromosome of the gene responsible for producing MAOA for males exhibiting aggressive behaviour. In rats who also had this mutation their urine showed higher than normal levels of serotonin. The fact that the mutation for MAOA is on the X chromosome can also help explain why aggression is seen more in males than females. Males only have one X chromosome whereas women have two X chromosomes, so if the females second x chromosome is ‘normal’ they wont experience the defect on the first.
Why does this increased serotonin lead to aggression? Well, sorry, we don’t really know however there are some speculative suggestions including
MAOA inhibitors have been shown to suppress REM sleep in human subjects, whereas REM sleep deprivation increases shock-induced fighting in rats, remember that MAOA has the same effect as MAOI’s so there might be a link to sleep.
It has also been inferred that there is a correlation with the size of the limbic system, with it being smaller in carriers of the MAOA gene.
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