HTR1A and Parkinson disease: Serotonin mechanisms have long been understood to be implicated in motor fluctuations in PD, but various drug candidates exhibit limited efficacy and/or interfere with the therapeutic effects of levodopa,15, 16, 17, 18 possibly due to interaction with off‐target sites (notably antagonism of DA receptors) and/or insufficient agonist efficacy at 5‐HT1A receptors.