HTR2A and Parkinson disease: In PD, the 5-hydroxytryptamine type 3 (5-HT3) antagonist ondansetron and the 5-HT2 reverse agonist pimavanserin have been used to treat visual hallucinations.34 Since 5-HT receptors can modulate release of GABA,35 it may be that the mode of action of these drugs in treating hallucinations is partly through their effect on GABA.