Combined Antagonism of 5-HT(2) and NMDA Receptors Reduces the Aggression of Monoamine Oxidase a Knockout Mice

The enzyme monoamine oxidase A (MAOA) catalyzes the degradation of several neurotransmitters, including serotonin. A large body of evidence has shown that genetic MAOA deficiency predisposes humans and mice to aggression and antisocial behavior. We previously documented that the aggression of male MAOA-deficient mice is contributed by serotonin 5-HT(2) and glutamate N-methyl-D-aspartate (NMDA) receptors in the prefrontal cortex (PFC). Indeed, blocking either receptor reduces the aggression of MAOA knockout (KO) mice; however, 5-HT(2) receptor antagonists, such as ketanserin (KET), reduce locomotor activity, while NMDA receptor blockers are typically associated with psychotomimetic properties. To verify whether NMDA receptor blockers induce psychotomimetic effects in MAOA KO mice, here we tested the effects of these compounds on prepulse inhibition (PPI) of the acoustic startle reflex. We found that male MAOA KO mice are hypersensitive to the PPI-disrupting properties of NMDA receptor antagonists, including the non-competitive antagonist dizocilpine (DIZ; 0.1, 0.3 mg/kg, IP) and the NR2B subunit-specific blocker Ro-256981 (5, 10 mg/kg, IP). Since KET has been previously shown to counter the PPI deficits caused by NMDA receptor antagonists, we tested the behavioral effects of the combination of KET (2 mg/kg, IP) and these drugs. Our results show that the combination of KET and DIZ potently reduces aggression in MAOA KO mice without any PPI deficits and sedative effects. While the PPI-ameliorative properties of KET were also observed after infusion in the medial PFC (0.05 μg/side), KET did not counter the PPI-disruptive effects of Ro-256981 in MAOA KO mice. Taken together, these results point to the combination of non-subunit-selective NMDA and 5-HT(2) receptor antagonists as a potential therapeutic approach for aggression and antisocial behavior with a better safety and tolerability profile than each monotherapy.