title: Ionic currents underlying difference in light response between type A and type B photoreceptors.
abstract: In Hermissenda crassicornis, the memory of light associated with turbulence is stored as changes in intrinsic and synaptic currents in both type A and type B photoreceptors. These photoreceptor types exhibit qualitatively different responses to light and current injection, and these differences shape the spatiotemporal firing patterns that control behavior. Thus the objective of the study was to identify the mechanisms underlying these differences. The approach was to develop a type B model that reproduced characteristics of type B photoreceptors recorded in vitro, and then to create a type A model by modifying a select number of ionic currents. Comparison of type A models with characteristics of type A photoreceptors recorded in vitro revealed that type A and type B photoreceptors have five main differences, three that have been characterized experimentally and two that constitute hypotheses to be tested with experiments in the future. The three differences between type A and type B photoreceptors previously characterized include the inward rectifier current, the fast sodium current, and conductance of calcium-dependent and transient potassium channels. Two additional changes were required to produce a type A photoreceptor model. The very fast firing frequency observed during the first second after light onset required a faster time constant of activation of the delayed rectifier. The fast spike adaptation required a fast, noninactivating calcium-dependent potassium current. Because these differences between type A and type B photoreceptors have not been confirmed in comparative experiments, they constitute hypotheses to be tested with future experiments.
authors.affiliation: School of Computational Sciences, and The Krasnow Institute for Advanced Study, George Mason University, MS 2A1, Fairfax, VA 22030, USA. avrama@gmu.edu
authors.forename: K T
authors.initials: KT
authors.lastname: Blackwell
mesh_terms: D000200:Action Potentials / Q000502:physiology; D000818:Animals; D001369:Axons / Q000502:physiology; D003198:Computer Simulation; D003712:Dendrites / Q000502:physiology; D004307:Dose-Response Relationship, Radiation; D004558:Electric Stimulation / Q000379:methods; D049870:Hermissenda / Q000502:physiology; D066298:In Vitro Techniques; D007473:Ion Channels / Q000145:classification / Q000502:physiology* / Q000528:radiation effects*; D008027:Light; D008564:Membrane Potentials / Q000502:physiology; D008959:Models, Neurological; D017956:Photoreceptor Cells, Invertebrate / Q000166:cytology / Q000502:physiology* / Q000528:radiation effects*; D015221:Potassium Channels / Q000502:physiology; D015222:Sodium Channels / Q000502:physiology; D013997:Time Factors
chemical_list: D007473:Ion Channels; D015221:Potassium Channels; D015222:Sodium Channels