Hypothetical mechanisms of therapeutic effects against PD by mirtazapine. DA, dopamine, 5-HT, 5-hydroxytryptamine (serotonin); NA, noradrenaline; GABA, γ-aminobutylic acid; MSN, medial spiny neuron; D1R, dopamine D1 receptor; D2R, dopamine D2 receptor; α1-NAR, α1 noradrenaline receptor; α2-NAR, α2 noradrenaline receptor; 5-HT1AR, 5-HT1A receptor: 5-HT2R, 5-HT2 receptor. Dashed lines indicate hypothetical innervation and receptors. The circle indicates a stimulatory effect by mirtazapine, while the crosses indicate an inhibitory effect by mirtazapine. See details in Discussion.