HCN channels in rod bipolar cells of rat retina: subcellular localization, kinetic properties and functional dynamics

Hyperpolarization- and cyclic nucleotide-activated channels (HCN or I_h channels) play an important role for the integrative dynamics of many types of neurons. In the retina, the multiple types of bipolar cells constitute parallel channels that connect the outer and inner retina. Differences in synaptic inputs and differential expression and localization of specific voltage-gated ion channels shape and modulate bipolar cell visual responses. Here, we examined the expression and function of HCN2-mediated I_h in rod bipolar cells (RBCs) of rat retina. Using immunolabelling, we observed HCN2 channels in dendrites, cell bodies and axon terminals of RBCs. With whole-cell voltage-clamp recording, we observed that ZD7288 and Cs⁺ blocked I_h in RBCs, and from activation/deactivation data, we developed a Hodgkin-Huxley-type kinetic I_h model that closely reproduced physiological responses. Applying a ZAP current stimulus, we found that the bandpass frequency-response characteristics of RBCs were blocked by Cs⁺, could be restored by dynamic clamp injection of a positive I_h conductance (in Cs⁺) and could be eliminated by injecting a negative I_h conductance (in control), suggesting that I_h is necessary and sufficient for bandpass filtering properties in the examined voltage range. Implementing our kinetic model for I_h in morphologically realistic compartmental models closely mimicked physiological bandpass characteristics, with little influence of the subcellular location of the I_h conductance. Our results demonstrate how the specific kinetic properties of I_h in RBCs determine their frequency-response properties, supporting an important role of I_h in the functional dynamics of RBC visual responses. KEY POINTS: Hyperpolarization- and cyclic nucleotide-activated (HCN) channels are found throughout the nervous system and contribute to physiological activities including rhythmic neuronal behaviour and control of the resting membrane potential. Unlike most voltage-gated channels, HCN channels are activated by hyperpolarizing voltages and, in some cells, generate bandpass behaviour, thereby amplifying certain frequencies of transmitted signals. We demonstrate that HCN2 channels are located at the dendrites, soma and axon terminals of rod bipolar cells, which are important for transmitting visual signals at night. Chemically blocking or electronically subtracting the HCN channels eliminates bandpass behaviour, whereas electronically adding the channels restores bandpass behaviour. We have implemented a Hodgkin-Huxley-type kinetic model for HCN channels that allows for computer simulations with realistic models of rod bipolar cells. We demonstrate that HCN channels are necessary and sufficient to confer bandpass properties and thus contribute to understanding how these voltage-gated ion channels generate diverse visual signals.