In vitro activity: Paroxetine apparently exerts their antidepressant activity by increasing the concentration of 5-HT in the extracellular compartment, thereby enhancing serotoninergic neurotransmission. Paroxetine (1-300 μM) results in a concentration-dependent reduction in the firing rate of DRN serotoninergic neurons with IC50 values of 1.4 μM in the ACSF superfusing brain stem slices. Paroxetine is a highly potent inhibitor of desipramine hydroxylation, the inhibition constant (Ki) value of 2.0 mM indicated greater inhibiting potency than fluoxetine or norfluoxetine. Paroxetine is shown to be a potent (Ki = 1.1 nM) and specific inhibitor of [3H]-5-hydroxytryptamine (5-HT) uptake into rat cortical and hypothalamic synaptosomes in vitro. Paroxetine demonstrates weak affinity for muscarinic receptors (Ki = 89 nM) but is at least 15 fold weaker than amitriptyline (Ki = 5.1 nM). Paroxetine inactivates CYP2D6 via the formation of a metabolite intermediate complex.

Cell Assay: Cell viability is determined by the tetrazolium salt 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) assay. BV2 and primary microglial cells are initially seeded into 96-well plates at a density of 1×104 cells/well and 5×104 cells/well, respectively. Following treatment, MTT (5 mg/mL in PBS) is added to each well and incubated at 37°C for four hours. The resulting formazan crystals are dissolved in dimethylsulfoxide (DMSO). The optical density is measured at 570 nm, and results are expressed as a percentage of surviving cells compared with the control.