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Use-dependent block of Ih in mouse dorsal root ganglion neurons by sinus node inhibitors
A Raes1, G Van de Vijver, M Goethals
1Department of Biochemistry, Physiology and Genetics, University of Antwerp (RUCA), Belgium.
Insights
Sinus node inhibitors UL-FS 49 and DK-AH 269 block the hyperpolarization-activated current (Ih) in neurons. This action protects cells from hyperpolarization, though drug effects differ between cardiac and neuronal tissues.
Area of Science:
- Neuroscience
- Pharmacology
- Cardiology
Background:
- Sinus node inhibitors UL-FS 49 and DK-AH 269 reduce heart rate by affecting diastolic depolarization in the sino-atrial (SA) node.
- This effect may stem from the use-dependent blockade of the hyperpolarization-activated current (If), which is similar to the hyperpolarization-activated current (Ih) found in neurons.
Purpose of the Study:
- To investigate the effects of UL-FS 49 and DK-AH 269 on the hyperpolarization-activated current (Ih) in cultured mouse dorsal root ganglion (DRG) neurons.
- To compare the drug actions on neuronal Ih with their known effects on cardiac If.
Main Methods:
- Whole-cell patch-clamp technique was employed to study Ih in DRG neurons.
- Use-dependent block of Ih was assessed using voltage-clamp pulse trains.
- Drug concentration-dependent steady-state block and recovery rates were measured.
Main Results:
- Both UL-FS 49 and DK-AH 269 demonstrated use-dependent block of Ih, with apparent dissociation constants (Kd) of 0.79 and 0.1 microM, respectively.
- The rate of block increased linearly with drug concentration, but recovery from block was significantly slower in neurons compared to cardiac tissue.
- UL-FS 49 did not significantly alter the activation curve of Ih, but at high concentrations, it associated with the open channel.
- Blocking Ih or using extracellular Cs+ prevented hyperpolarization during 3 Hz stimulation, indicating Ih's protective role.
Conclusions:
- The hyperpolarization-activated current (Ih) in neurons functions to protect cells against hyperpolarization and subsequent inexcitability.
- While similarities exist in drug actions on Ih in cardiac and neuronal tissues, pronounced differences suggest the potential existence of distinct channel subtypes.
Abstract:
1. The sinus node inhibitors UL FS 49 and DK-AH 269 reduce heart rate by slowing diastolic depolarization rate in the sino-atrial (SA) node, which might originate from the use-dependent blockade of a hyperpolarization-activated current If. A hyperpolarization-activated current Ih, which is present in many types of neurons, is similar to If. We studied the effects of these drugs on Ih in cultured mouse dorsal root ganglion (DRG) neurons. 2. With the whole-cell patch-clamp technique use-dependent block of Ih was observed. The steady-state block following a voltage-clamp pulse train (1-s steps from -38 to -108 mV applied at 0.5 Hz) was dependent on drug concentration and showed an apparent Kd of 0.1 and 0.79 microM with DK AH 269 and UL-FS 49 respectively. 3. The rate of block increased linearly with drug concentration. The rate of recovery from block was, however, much slower compared to cardiac tissue. 4. There was no significant effect of UL-FS 49 on the activation curve. 5. At high concentrations of UL-FS 49 a clear association of the drug with the open channel was observed. 6. When the cell was stimulated at a frequency of 3 Hz, a distinct hyperpolarization was observed in the presence of extracellular Cs+ or when Ih was blocked with UL-FS 49, but not in the absence of Cs+ and UL-FS 49. 7. These results indicate that Ih protects the cell against hyperpolarizations and subsequent inexcitability. The action of the drugs on the hyperpolarization-activated current in cardiac and neuronal tissue show some similarities; however, some pronounced differences indicate that different subtypes of the channel might exist.