The potential antidepressant compound Org 34167 modulates HCN channels via a novel mode of action.

Org 34167 is a small molecule HCN channel modulator that has been trialed in humans for its potential antidepressant activity. The precise action of Org 34167 is not fully understood. Here we use two-electrode voltage clamp recordings and in silico modelling to explore the interaction of Org 34167 with human HCN1 channels. The impact of Org 34167 on channel function included a hyperpolarizing shift in activation voltage dependence and a slowing of activation kinetics. Furthermore, a reduction in the maximum open probability at extreme hyperpolarization argued for an additional voltage-independent mechanism. Org 34167 had a similar impact on a truncated HCN1 channel lacking the C-terminal nucleotide binding domain (CNBD), thus ruling out an interaction with this domain. Fitting a kinetic model, derived from a 10-state allosteric scheme, predicted that Org 34167 strongly reduced the equilibrium constant for the voltage-independent pore domain to favor a closed pore, as well as reducing the voltage sensing domain-pore domain (VSD-PD) coupling and shifting the zero voltage equilibrium constant of the VSD to favor the inactive state. Significance Statement The brain penetrant small molecule Org 34167 has been reported to have an antidepressant action by targeting HCN channels, however its mode of action is unknown. We used heterologously expressed HCN1 channels to show that Org 34167 reduced both the equilibrium constant for the pore domain (PD) and the voltage sensing domain-pore domain (VSD-PD) coupling and shifted the zero voltage equilibrium constant of the VSD.