An active metabolite of clozapine; originally described as an antagonist of 5-HT2C receptors (IC50 = 7.1 nM in rat choroid plexus); and later as an inverse agonist using human recombinant receptors; an antagonist at dopamine D4 receptors, an agonist at δ-opioid receptors, and a partial agonist at dopamine D2 and D3, M1 muscarinic acetylcholine, and 5-HT1A receptors; decreases exploratory locomotor activity and increases prepulse inhibition of the acoustic startle response in mice at 30 mg/kg,