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K. L. Myhr, L. Chen, C. D. Mahalingam, J. Qu; Subtypes of Retinal Ganglion Cells Express the Potassium Channel, Kv4.2, and Dopamine Receptors. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1418.
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Functional Kv4.2 potassium channels carry transient potassium currents. This current is subthreshold, so it slows action potential firing rates. It also alters processing of synaptic inputs due to its dendritic distribution. Kv4.2 channels are highly regulated in response to catecholamines, such as epinephrine and dopamine. The purpose of this work is to determine which retinal ganglion cells express Kv4.2 channels, whether the channels are functional and the potential regulation of retinal Kv4.2 currents by dopamine.
The expression of Kv4.2 was determined by immunohistochemistry in morphologically identified mouse retinal ganglion cells. Morphology was determined in flat-mounted retinas by patch-filling ganglion cells with dye or by viewing ganglion cells from line H retinas that express YFP in a small proportion of the ganglion cells. Currents were assayed in voltage-clamp configuration. Co-expression of Kv4.2 and dopamine receptors was determined by double immunohistochemistry.
Kv4.2 is predominantly expressed in a type of highly branched small-field ganglion cell, in some of the intrinsically photosensitive ganglion cells (ipRGCs) and in a novel subtype. Ganglion cells with Kv4.2 channels generated transient potassium currents. Some ganglion cells were positive for Kv4.2 and D1 or D2 dopamine receptors.
Subsets of ganglion cells express Kv4.2. The small-field ganglion cells that express Kv4.2 likely transmit high acuity form vision, whereas the ipRGCs transmit ambient light levels to non-form vision nuclei. Because bright bands of Kv4.2 label appear in the IPL where the dendrites of the small-field Kv4.2 expressing ganglion cells ramify, we propose that Kv4.2 regulates receptive fields of the small-field ganglion cells by altering dendritic integration of synaptic inputs. The dendrites of ipRGCs do not ramify where the Kv4.2 is localized in the IPL, suggesting Kv4.2 channels regulate firing rates in ipRGCs. The co-expression of Kv4.2 with D1 or D2 receptors suggests that dopamine regulates receptive field size and/or firing rates in a light- and circadian rhythm-dependent manner.
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