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Hyperpolarization-activated cyclic nucleotide-gated (HCN) ion channels form an integral part of the plasma membrane, existing as tetramers and enclosing ion conducting pores. Ion channels are often regulated by auxiliary subunits and one such protein interacting with the HCN channel is TPR-containing Rab8b interacting protein (TRIP8b). TRIP8b is known to play a crucial role in controlling voltage dependence of HCN channel activation, channel trafficking and surface expression. Small Ubiquitin-like Modifier (SUMO) is a peptide that can be conjugated to a lysine residue on a target protein and modify protein- protein interactions. HCN2 channel can be SUMOylated and possible SUMOylation sites on TRIP8b exist. SUMOylation at these sites may strengthen or weaken the association between these proteins. In this study we use Pull-down assay technique to determine whether SUMO influences this interaction. By altering the baseline SUMOylation level, changes in the degree of HCN channel protein pulled down by TRIP8b is analyzed.

Trigeminal neuropathic pain (TNP) is a condition that occurs when one or more branches of the trigeminal nerve are insulted. Trigeminal neuropathic pain has been shown to be refractory to treatment. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels regulate neuronal excitability in both the peripheral and central nerve systems. Emerging evidence indicates that HCN channels are involved in the development and maintenance of chronic pain, however, the impact of thalamic HCN channel activity on TNP has yet to be elucidated. In this report, we used a chronic constriction of the distal infraorbital nerve (dIoN-CCI) to induce TNP in rats. By infusing HCN channel blockers into the ventral posteromedial (VPM) nucleus of the thalamus in dIoN-CCI rats, we demonstrated that inhibition of HCN channel activity ameliorated TNP. We found that the HCN blocker ZD7288 and the clinical drug ivabradine dose-dependently attenuated both evoked and none-evoked nociceptive behaviors in dIoN-CCI rats. Electrophysiological measurements showed the expression of HCN current (Ih) in the thalamocortical neurons in the VPM was sensitive to the HCN channel modulator cyclic adenosine monophosphate (cAMP), suggesting a contribution of the HCN2 subunit in thalamic HCN current. In the thalamus, surface expression of the HCN2 subunit was increased in dIoN-CCI rats. Taken together, we propose that an increase in HCN channel activity in the thalamus in the ascending nociceptive pathway contributed to trigeminal neuropathic pain.