The organic cation transporter 3 (OCT3; synonymous: extraneuronal monoamine 
transporter, EMT, Slc22a3) encodes an isoform of the organic cation transporters 
and is expressed widely across the whole brain. OCTs are a family of 
high-capacity, bidirectional, multispecific transporters of organic cations. 
These also include serotonin, dopamine and norepinephrine making OCTs attractive 
candidates for a variety of neuropsychiatric disorders including anxiety 
disorders. OCT3 has been implicated in termination of monoaminergic signalling 
in the central nervous system. Interestingly, OCT3 mRNA is however also 
significantly up-regulated in the hippocampus of serotonin transporter knockout 
mice where it might serve as an alternative reuptake mechanism for serotonin. 
The examination of the behavioural phenotype of OCT3 knockout mice thus is 
paramount to assess the role of OCT3. We have therefore subjected mice lacking 
the OCT3 gene to a comprehensive behavioural test battery. While cognitive 
functioning in the Morris water maze test and aggression levels measured with 
the resident-intruder paradigm were in the same range as the respective control 
animals, OCT3 knockout animals showed a tendency of increased activity and were 
significantly less anxious in the elevated plus-maze test and the open field 
test as compared to their respective wild-type controls arguing for a role of 
OCT3 in the regulation of fear and anxiety, probably by modulating the 
serotonergic tone in limbic circuitries.