Abstract
Purpose: :
To characterize the qualities and function of the hyperpolarization-activated cyclic nucleotide gated (HCN) channels present in rod and cone photoreceptors.
Methods: :
HCN currents were recorded from rods and cones in the salamander retina under voltage clamp with whole-cell and cell-attached configurations. Localization of HCN1 channels in the retina was studied by immunohistochemical techniques.
Results: :
Whole cell voltage clamp recordings indicate that the time course and activation curves for the HCN channels in both rods and cones are consistent with results reported for HCN 1 channels cloned from cardiac tissue. Non-stationary fluctuation analysis (NSFA) of whole cell and on cell patch clamp recordings estimates the single channel conductance to be approximately 650 fS, which is consistent with the estimate of HCN 1 channel conductance in pyramidal neurons. Immunohistochemical techniques also support patch clamp results indicating that HCN 1 channels are present in both photoreceptor types. Functionally, HCN channels are found to contribute to a bandpass frequency response of the photoreceptor with a peak of around 4 Hz. The bandpass response can be blocked with the HCN channel antagonist ZD 7288. Light responses of rods with HCN channels blocked are greatly increased in magnitude and have an altered shape.
Conclusions: :
The HCN channels play an important role in shaping the light response in photoreceptors. In both rods and cones, this response is manifested as a hyperpolarizing initial transient followed by a decay to a less hyperpolarized plateau. This response can be described as a reactive inductance in the equivalent electric circuit model, and creates a bandpass filter effect in the impedance of the cell. The bandpass effect is crucial for increasing the natural frequency response of the photoreceptors, and may also be important for signal propagation in the photoreceptor network. We have also demonstrated that the HCN 1 subtype is likely the sole population of HCN channels in photoreceptors. This conclusion is supported by whole cell voltage clamp data, NSFA estimates of single channel conductance, and immunohistochemistry.
Keywords: electrophysiology: non-clinical • photoreceptors • photoreceptors: visual performance