title: Intrinsic light responses of retinal ganglion cells projecting to the circadian system.
abstract: In mammals, light entrainment of the circadian clock, located in the suprachiasmatic nuclei (SCN), requires retinal input. Traditional rod and cone photoreceptors, however, are not required. Instead, the SCN-projecting retinal ganglion cells (RGCs) function as autonomous photoreceptors and exhibit light responses independent of rod- and cone-driven input. Using whole-cell patch-clamp recording techniques, we have investigated the morphological and electrophysiological properties of this unique class of RGCs. Although SCN-projecting RGCs resemble Type III cells in form, they display strikingly different physiological properties from these neurons. First, in response to the injection of a sustained depolarizing current, SCN-projecting cells fired in a transient fashion, in contrast to most RGCs which fired robust trains of action potentials. Second, in response to light, SCN-projecting RGCs exhibited an intensity-dependent transient depolarization in the absence of rod and cone input. This depolarization reached a peak within 5 s and generated increased spiking activity before decaying to a plateau. Voltage-clamp recordings were used to characterize the light-activated conductance which generated this depolarization. In response to varying light intensities, SCN-projecting RGCs exhibited a graded transient inward current which peaked within 5 s and decayed to a plateau. The voltage dependence of the light-activated current was obtained by subtracting currents elicited by a voltage ramp before and during illumination. The light-activated current displayed both inward and outward rectification and was largely unaffected by substitution of extracellular Na+ with choline. In both respects, the intrinsic light-activated current observed in SCN-projecting RGCs resembles currents carried by ion channels of the transient receptor potential (trp) family, which are known to mediate the light response of invertebrate photoreceptors.
authors.affiliation: Department of Physiology and Pharmacology, L334, Oregon Health & Science University, 3181 SW Sam Jackson Park Road, Portland, OR 97239, USA.
authors.forename: Erin J
authors.initials: EJ
authors.lastname: Warren
authors.forename: Charles N
authors.initials: CN
authors.lastname: Allen
authors.forename: R Lane
authors.initials: RL
authors.lastname: Brown
authors.forename: David W
authors.initials: DW
authors.lastname: Robinson
mesh_terms: D000818:Animals; D002940:Circadian Rhythm / Q000502:physiology*; D008564:Membrane Potentials / Q000502:physiology; D009434:Neural Pathways / Q000502:physiology; D010775:Photic Stimulation / Q000379:methods*; D051381:Rats; D012165:Retinal Ganglion Cells / Q000502:physiology*; D013493:Suprachiasmatic Nucleus / Q000502:physiology*