HTR2A and Tremor: Although we cannot completely deny the possibility that multiple actions of dopamine ligands used (e.g., interaction of SCH-23390 with 5-HT2 receptors and partial antagonism of SKF-38393 at D1/5 receptors) (Hoyer et al., 1989; Desai et al., 2005) also affected nicotine-induced tremor, our results suggest that dopamine D3 and D1/5 receptors regulate the induction of nicotine tremor in an opposite way, D3 receptors facilitately and D1/5 receptors inhibitorily.