Indirect α1 adrenoceptor-mediated enhancement of serotonin cell firing and direct blockade of inhibitory α2 heteroreceptors located on serotonin terminals are held responsible for the increase in extracellular serotonin.[1][2][83][84][85] Because of this, mirtazapine has been said to be a functional "indirect agonist" of the 5-HT1A receptor.[84] Increased activation of the central 5-HT1A receptor is thought to be a major mediator of efficacy of most antidepressant drugs.[86]