After transduction: response shaping and control of transmission by ion channels of the photoreceptor inner segments

Abstract

Photoreceptors convert the elements of the visual image into the elements of a neural image. This process involves well-studied molecular events occurring at the outer segment, but also employs important molecular events in the proximal regions of the photoreceptor, including the synaptic terminal, encompassed here as the inner segment. Integral to neural processing at this level in the visual system, the inner segment mechanisms modify the visual signal before transmission to second order cells at the photoreceptor output synapse. This commentary, emphasizing the author's own work, discusses biophysical properties of the ensemble of ion channels in the photoreceptor inner segment that shape the light response and enable its transmission. Examples that illustrate ion channels whose biophysical properties seem well suited for their roles in photoreceptor function include: h channels, cation-selective channels activated by hyperpolarization, which carry current that counteracts the strong hyperpolarizing influence of cGMP-gated channel closure accompanying bright light; Kx channels, carrying potassium current which shares the kinetic properties of the M-current found in many other cell types, which shape responses to dim light and set the dark resting potential; and Ca channels that regulate calcium influx to control Ca-gated channel activity and synaptic output, "re-transducing" the neural signal now into a chemical one. The role of chloride current, carried in Ca-activated Cl channels dependent on the unknown chloride equilibrium potential in photoreceptors, is also discussed.