Photodynamic Modification of Native HCN Channels Expressed in Thalamocortical Neurons

The photodynamic process requires three elements: light, oxygen, and photosensitizer, and involves the formation of singlet oxygen, the molecular oxygen in excited electronic states. Previously, we reported that heterologously expressed hyperpolarization-activated cAMP-gated (HCN) channels in excised membrane patches are sensitive to photodynamic modification (PDM). Here we extend this study to native HCN channels expressed in thalamocortical (TC) neurons in the ventrobasal (VB) complex of the thalamus and dopaminergic neurons (DA) of the ventral tegmental area (VTA). To do this, we introduced the photosensitizer FITC-cAMP into TCs or DAs of rodent brain slices via a whole-cell patch-clamp recording pipette. After illumination with blue light pulses, we observed an increase in the voltage-insensitive, instantaneous I_inst component, accompanied by a long-lasting decrease in the hyperpolarization-dependent I_h component. Both I_h and the increased I_inst after PDM could be blocked by the HCN blockers Cs⁺ and ZD7288. When FITC and cAMP were dissociated and loaded into neurons as two separate chemicals, light application did not result in any long-lasting changes of the HCN currents. In contrast, light pulses applied to HCN2-/- neurons loaded with FITC-cAMP generated a much greater reduction in the I_inst component compared to that of WT neurons. Next, we investigated the impact of the long-lasting increases in I_inst after PDM on the cellular physiology of VB neurons. Consistent with an upregulation of HCN channel function, PDM elicited a depolarization of the resting membrane potential (RMP). Importantly, Trolox-C, an effective quencher for singlet oxygen, could block the PDM-dependent increase in I_inst and depolarization of the RMP. We propose that PDM of native HCN channels under physiological conditions may provide a photodynamic approach to alleviate HCN channelopathy in certain pathological conditions.