Cognitive abnormalities and hippocampal alterations in monoamine oxidase A and B knockout mice

The monoamine oxidase isoenzymes (MAOs) A and B play important roles in the homeostasis of monoaminergic neurotransmitters. The combined deficiency of MAO A and B results in significantly elevated levels of serotonin (5-hydroxytryptamine), norepinephrine, dopamine, and β-phenylethylamine; in humans and mice, these neurochemical changes are accompanied by neurodevelopmental perturbations as well as autistic-like responses. Ample evidence indicates that normal levels of monoamines in the hippocampus, amygdala, frontal cortex, and cerebellum are required for the integrity of learning and memory. Thus, in the present study, the cognitive status of MAO A/B knockout (KO) mice was examined with a wide array of behavioral tests. In comparison with male wild-type littermates, MAO A/B KO mice exhibited abnormally high and overgeneralized fear conditioning and enhanced eye-blink conditioning. These alterations were accompanied by significant increases in hippocampal long-term potentiation and alterations in the relative expression of NMDA glutamate receptor subunits. Our data suggest that chronic elevations of monoamines, because of the absence of MAO A and MAO B, cause functional alterations that are accompanied with changes in the cellular mechanisms underlying learning and memory. The characteristics exhibited by MAO A/B KO mice highlight the potential of these animals as a useful tool to provide further insight into the molecular bases of disorders associated with abnormal monoaminergic profiles.