| Contemporary antidepressants canonically inhibit neurotransmitter re-uptake at synapses. The two major classes of antidepressants -- tricyclic antidepressants (TCAs) and selective serotonin reuptake inhibitors (SSRIs) -- also have off-target effects that include inhibition of membrane proteins other than neurotransmitter transporters. The biological significance of these effects remains unclear but they may contribute to the desired changes in brain function as well as the development of undesired side effects.;Antidepressants are amphiphiles that adsorb at the membrane-solution interface. A common feature of many of the diverse proteins modulated by antidepressants is that they span a lipid bilayer. Membrane proteins are coupled to the bilayer through hydrophobic interactions such that conformational changes underlying their function may involve local reorganization of the surrounding lipids. Because bilayer deformations incur energetic costs, which vary with bilayer properties, membrane protein function is sensitive to changes in bilayer mechanical properties caused by amphiphile adsorption. Using gramicidin A (gA) channels as probes of bilayer properties, I examined whether TCAs and SSRIs alter the bilayer contribution to the free energy difference between channel conformations.;All the tested antidepressants alter gA channel activity in a dose-dependent manner, with varying potencies. These effects are not enantiomer-specific (in the absence of cholesterol). Partition coefficient determined from isothermal titration calorimetry (ITC) is a better predictor of bilayer-modifying potency than calculated octanol:water partition coefficient, and suggests that antidepressant:lipid mole fraction of 0.01-0.06 is required to double gA activity. Single-channel electrophysiology indicates that the effects of the TCAs, amitriptyline and imipramine, and the SSRI citalopram are insensitive to hydrophobic mismatch, suggesting that they primarily alter bilayer curvature. Fluoxetine is sensitive to hydrophobic mismatch, but in a complex manner, suggesting multifaceted drug-bilayer-channel interactions. Antidepressants do not alter the equilibrium of double-stranded/single-stranded conformers, reducing the possibility of specific drug-channel interactions. Fluoxetine enantiomers block Kir4.1 channels with equal potency but the (S+) enantiomer takes longer to achieve full block. Fluoxetine (S+) also has a greater effect than (R-) on gA channels in the presence of cholesterol, suggesting that the (S+) enantiomer has preferential, chiral, interactions with cholesterol-containing bilayers, which may also explain the slow kinetics of Kir4.1 block by this drug. |
