Gabapentin Modulates HCN4 Channel Voltage-Dependence.

Han-Shen Tae,David I Hughes,Robert J Hatch,Christopher A Reid,Steven Petrou,Brett A Graham,Kelly M Smith,A Marie Phillips,Robert Richardson,Ian C Forster,Melody Li,Kieran A Boyle

doi:10.3389/fphar.2017.00554

Abstract

Gabapentin (GBP) is widely used to treat epilepsy and neuropathic pain. There is evidence that GBP can act on hyperpolarization-activated cation (HCN) channel-mediated Ih in brain slice experiments. However, evidence showing that GBP directly modulates HCN channels is lacking. The effect of GBP was tested using two-electrode voltage clamp recordings from human HCN1, HCN2, and HCN4 channels expressed in Xenopus oocytes. Whole-cell recordings were also made from mouse spinal cord slices targeting either parvalbumin positive (PV+) or calretinin positive (CR+) inhibitory neurons. The effect of GBP on Ih was measured in each inhibitory neuron population. HCN4 expression was assessed in the spinal cord using immunohistochemistry. When applied to HCN4 channels, GBP (100 μM) caused a hyperpolarizing shift in the voltage of half activation (V1/2) thereby reducing the currents. Gabapentin had no impact on the V1/2 of HCN1 or HCN2 channels. There was a robust increase in the time to half activation for HCN4 channels with only a small increase noted for HCN1 channels. Gabapentin also caused a hyperpolarizing shift in the V1/2 of Ih measured from HCN4-expressing PV+ inhibitory neurons in the spinal dorsal horn. Gabapentin had minimal effect on Ih recorded from CR+ neurons. Consistent with this, immunohistochemical analysis revealed that the majority of CR+ inhibitory neurons do not express somatic HCN4 channels. In conclusion, GBP reduces HCN4 channel-mediated currents through a hyperpolarized shift in the V1/2. The HCN channel subtype selectivity of GBP provides a unique tool for investigating HCN4 channel function in the central nervous system. The HCN4 channel is a candidate molecular target for the acute analgesic and anticonvulsant actions of GBP.

Highlights

Gabapentin (GBP) was synthesized as a rigid lipophilic analog of gamma-aminobutyric acid (GABA) with the intention of mimicking inhibitory neurotransmission
As contemporaneous vehicle controls revealed changes in several of the HCN4 biophysical properties statistical analysis was made by comparing the relative change caused by GBP (100 μM) to the relative change observed in vehicle control
We provide evidence that GBP can reduce human HCN4 channel function by causing a hyperpolarizing shift in the voltage of activation

Summary

Introduction

Gabapentin (GBP) was synthesized as a rigid lipophilic analog of gamma-aminobutyric acid (GABA) with the intention of mimicking inhibitory neurotransmission. GBP was first approved for the treatment of epilepsy it is more commonly used as a therapy in neuropathic pain. It is often used off-label to treat a number of medical conditions including anxiety, alcohol and drug addiction, and bi-polar disorders (Mack, 2003). The widely accepted mode-ofaction of GBP is that it interacts with the α2δ protein, an accessory subunit of voltage-gated calcium (Ca2+) channels responsible for enhancing channel trafficking to the plasma membrane as well as influencing their biophysical properties (Dolphin, 2016). Gabapentin has been shown to acutely block voltage-dependent Ca2+ channels (Stefani et al, 1998; Sutton et al, 2002) and putatively interacts with several molecular targets including N-methyl-D-aspartic acid (NMDA) and α-amino-3hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, and KATP channels (Cheng and Chiou, 2006)

Results

Discussion

Conclusion