Photodynamic Modification of Native HCN Channels Expressed in Thalamocortical Neurons

Dopaminergic amacrine cells express HCN channels in the developing and adult mouse retina

Purpose

To determine the molecular and functional expression of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels in developing and mature dopaminergic amacrine cells (DACs), the sole source of ocular dopamine that plays a vital role in visual function and eye development.

Methods

HCN channels are encoded by isoforms 1-4. HCN1, HCN2, and HCN4 were immunostained in retinal slices obtained from mice at postnatal day 4 (P4), P8, and P12 as well as in adults. Each HCN channel isoform was also immunostained with tyrosine hydroxylase, a marker for DACs, at P12 and adult retinas. Genetically-marked DACs were recorded in flat-mount retina preparation using a whole-cell current-clamp technique.

Results

HCN1 was expressed in rods/cones, amacrine cells, and retinal ganglion cells (RGCs) at P4, along with bipolar cells by P12. Different from HCN1, HCN2 and HCN4 were each expressed in amacrine cells and RGCs at P4, along with bipolar cells by P8, and in rods/cones by P12. Double immunostaining shows that each of the three isoforms was expressed in approximately half of DACs at P12 but in almost all DACs in adults. Electrophysiology results demonstrate that HCN channel isoforms form functional HCN channels, and the pharmacological blockade of HCN channels reduced the spontaneous firing frequency in most DACs.

Conclusions

Each class of retinal neurons may use different isoforms of HCN channels to function during development. HCN1, HCN2, and HCN4 form functional HCN channels in DACs, which appears to modulate their spontaneous firing activity.

Inhibitory Perturbations of Fluvastatin on Afterhyperpolarization Current, Erg-mediated K+ Current, and Hyperpolarization-activated Cation Current in Both Pituitary GH3 Cells and Primary Embryonic Mouse Cortical Neurons

Fluvastatin (FLV), the first synthetically derived 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor, is a potent inhibitor of cholesterol biosynthesis. While its primary mechanism of action is to reduce cholesterol levels, there is some evidence suggesting that it may also have effects on K+ channels. However, the overall effects of fluvastatin on ionic currents are not yet well understood. The whole-cell clamp recordings were applied to evaluate the ionic currents and action potentials of cells. Here, we have demonstrated that FLV can effectively inhibit the amplitude of erg-mediated K+ current (IK(erg)) in pituitary tumor (GH3) cells, with an IC50 of approximately 3.2 µM. In the presence of FLV, the midpoint in the activation curve of IK(erg) was distinctly shifted to a less negative potential by 10 mV, with minimal modification of the gating charge. However, the magnitude of hyperpolarization-activated cation current (Ih) elicited by long-lasting membrane hyperpolarization was progressively decreased, with an IC50 value of 8.7 µM, upon exposure to FLV. More interestingly, we also found that FLV (5 µM) could regulate the action potential and afterhyperpolarization properties in primary embryonic mouse cortical neurons. Our study presents compelling evidence indicating that FLV has the potential to impact both the amplitude and gating of the ion channels IK(erg) and Ih. We also provide credible evidence suggesting that this drug has the potential to modify the properties of action potentials and the afterhyperpolarization current in electrically excitable cells. However, the assumption that these findings translate to similar in-vivo results remains unclear.

Photodynamic Activation of Cholecystokinin 1 Receptor Is Conserved in Mammalian and Avian Pancreatic Acini

Cholecystokinin 1 receptor (CCK1R) is the only G protein coupled receptor that is activated in type II photodynamic action, but whether this is a property common to both mammalian and avian species is not known. In this work, pancreatic acini were isolated from the rat, mouse, and Peking duck, and photodynamic CCK1R activation was examined. Isolated pancreatic acini were exposed to photosensitizer sulphonated aluminum phthalocyanine (SALPC) and photodynamic action elicited by a brief light-emitting diode (LED 675 nm) pulse (1.5 min); photodynamic CCK1R activation was assessed by Fura-2 fluorescent calcium imaging. Photodynamic action was found to induce persistent calcium oscillations in rat, mouse, and Peking duck pancreatic acini, with the sensitivity order of mouse > rat > Peking duck. Photodynamically-activated CCK1R could be inhibited reversibly by CCK1R antagonist devazepide (1 μM); photodynamic CCK1R activation was blocked by pre-incubation with 1O2 quencher Trolox C (300 µM). The sensitivity of photodynamic CCK1R activation was correlated with the increasing size of the disordered region in intracellular loop 3. These data suggest that photodynamic CCK1R activation is conserved in both mammalian and avian species, as evidenced by the presence of the photodynamic activation motif “YFM” in transmembrane domain 3.

Photodynamic Activation of Cholecystokinin 1 Receptor Is Conserved in Mammalian and Avian Pancreatic Acini

Cholecystokinin 1 receptor (CCK1R) is the only G protein coupled receptor that is activated in type II photodynamic action, but whether this is a property common to both mammalian and avian species is not known. In this work, pancreatic acini were isolated from the rat, mouse, and Peking duck, and photodynamic CCK1R activation was examined. Isolated pancreatic acini were exposed to photosensitizer sulphonated aluminum phthalocyanine (SALPC) and photodynamic action elicited by a brief light-emitting diode (LED 675 nm) pulse (1.5 min); photodynamic CCK1R activation was assessed by Fura-2 fluorescent calcium imaging. Photodynamic action was found to induce persistent calcium oscillations in rat, mouse, and Peking duck pancreatic acini, with the sensitivity order of mouse > rat > Peking duck. Photodynamically-activated CCK1R could be inhibited reversibly by CCK1R antagonist devazepide (1 μM); photodynamic CCK1R activation was blocked by pre-incubation with 1O2 quencher Trolox C (300 µM). The sensitivity of photodynamic CCK1R activation was correlated with the increasing size of the disordered region in intracellular loop 3. These data suggest that photodynamic CCK1R activation is conserved in both mammalian and avian species, as evidenced by the presence of the photodynamic activation motif “YFM” in transmembrane domain 3.

Characterization of Inhibitory Capability on Hyperpolarization-Activated Cation Current Caused by Lutein (β,ε-Carotene-3,3'-Diol), a Dietary Xanthophyll Carotenoid

Lutein (β,ε-carotene-3,3'-diol), a xanthophyll carotenoid, is found in high concentrations in the macula of the human retina. It has been recognized to exert potential effectiveness in antioxidative and anti-inflammatory properties. However, whether and how its modifications on varying types of plasmalemmal ionic currents occur in electrically excitable cells remain incompletely answered. The current hypothesis is that lutein produces any direct adjustments on ionic currents (e.g., hyperpolarization-activated cation current, Ih [or funny current, If]). In the present study, GH3-cell exposure to lutein resulted in a time-, state- and concentration-dependent reduction in Ih amplitude with an IC50 value of 4.1 μM. There was a hyperpolarizing shift along the voltage axis in the steady-state activation curve of Ih in the presence of this compound, despite being void of changes in the gating charge of the curve. Under continued exposure to lutein (3 μM), further addition of oxaliplatin (10 μM) or ivabradine (3 μM) could be effective at either reversing or further decreasing lutein-induced suppression of hyperpolarization-evoked Ih, respectively. The voltage-dependent anti-clockwise hysteresis of Ih responding to long-lasting inverted isosceles-triangular ramp concentration-dependently became diminished by adding this compound. However, the addition of 10 μM lutein caused a mild but significant suppression in the amplitude of erg-mediated or A-type K+ currents. Under current-clamp potential recordings, the sag potential evoked by long-lasting hyperpolarizing current stimulus was reduced under cell exposure to lutein. Altogether, findings from the current observations enabled us to reflect that during cell exposure to lutein used at pharmacologically achievable concentrations, lutein-perturbed inhibition of Ih would be an ionic mechanism underlying its changes in membrane excitability.

Effectiveness of Columbianadin, a Bioactive Coumarin Derivative, in Perturbing Transient and Persistent INa

Columbianadin (CBN) is a bioactive coumarin-type compound with various biological activities. However, the action of CBN on the ionic mechanism remains largely uncertain, albeit it was reported to inhibit voltage-gated Ca²⁺ current or to modulate TRP-channel activity. In this study, whole-cell patch-clamp current recordings were undertaken to explore the modifications of CBN or other related compounds on ionic currents in excitable cells (e.g., pituitary GH₃ cells and HL-1 atrial cardiomyocytes). GH₃-cell exposure to CBN differentially decreased peak or late component of voltage-gated Na⁺ current (I_Na) with effective IC₅₀ of 14.7 or 2.8 µM, respectively. The inactivation time course of I_Na activated by short depolarization became fastened in the presence of CBN with estimated K_D value of 3.15 µM. The peak I_Na diminished by 10 µM CBN was further suppressed by subsequent addition of either sesamin (10 µM), ranolazine (10 µM), or tetrodotoxin (1 µM), but it was reversed by 10 µM tefluthrin (Tef); however, further application of 10 µM nimodipine failed to alter CBN-mediated inhibition of I_Na. CBN (10 µM) shifted the midpoint of inactivation curve of I_Na to the leftward direction. The CBN-mediated inhibition of peak I_Na exhibited tonic and use-dependent characteristics. Using triangular ramp pulse, the hysteresis of persistent I_Na enhanced by Tef was noticed, and the behavior was attenuated by subsequent addition of CBN. The delayed-rectifier or erg-mediated K⁺ current was mildly inhibited by 10 µM CBN, while it also slightly inhibited the amplitude of hyperpolarization-activated cation current. In HL-1 atrial cardiomyocytes, CBN inhibited peak I_Na and raised the inactivation rate of the current; moreover, further application of 10 µM Tef attenuated CBN-mediated decrease in I_Na. Collectively, this study provides an important yet unidentified finding revealing that CBN modifies I_Na in electrically excitable cells.