HTR1A and major depressive disorder: Increased 5-HT1A receptor expression is known to be associated with an improvement in both depressive and anxiety behaviors, while the 5-HT1B receptor plays a role in regulating impulsive behavior, reward, and depression (Heisler et al., 1998; Parks et al., 1998; Knobelman et al., 2001; Parsons et al., 2001).