Localization of potassium channels in the retina

The functional role of the delayed rectifier potassium channels is reviewed and the specific roles that these channels play in the retina is enumerated in examples using retinal neurons. These channels are contrasted with other types of potassium channels. The reasons why several types of delayed rectifier molecules could be expected to be expressed in a single neuron, and specific examples of retinal neurons that would be expected to express several of these molecules are given. The families of delayed rectifier potassium channels are explained and their transmembrane topology is related to their functional characteristics. The approaches to the localization of these channels are given and these methods (in situ hybridization, immunohistochemistry and RT-PCR) are compared and contrasted with examples from retinal neurons. This is followed by specific technical hints for applying these methods to the retina. The localization of the 6 transmembrane domain delayed rectifier channels of the Kv1, Kv2, Kv3 and Kv4 families is given for the retina, the retinal pigment epithelium and the optic nerve. An explanation for why the ionic currents recorded from a cell may not represent accurately the sum of the currents of the ion channels normally expressed in that cell is followed by an example of the assignment of the currents recorded from a retinal neuron to a specific ion channel. The future directions of this type of investigation appear to be to understand the relationship between clustered ion channel molecules of a given type with the function of the subset of the retinal neuron in which this type of ion channel is clustered, to understand the mechanism for the clustering, and to understand the mechanism for the localization of ion channel molecules to one region of the cell i.e. the polarization of the expression of these molecules in retinal neurons.