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H.M. Cooper, L.S. Mure, C. Rieux; Single Unit Responses Of SCN neurons To Irradiance, Duration And Wavelength . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5428.
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In mammals, non visual responses to light including phase shifts of activity rhythms, melatonin suppression, pupil reflexes and masking have been shown to involve intrinsically photosensitive retinal ganglion cells, which express melanopsin (ipRGC). These ipRGCs also receive input from rods and cones via amacrine and cone bipolar cells and relay photic information to the circadian clock in the suprachiasmatic nucleus (SCN). We studied the responses of single neurons in the SCN to variations in irradiance, duration, and wavelength.
Extracellular single neuron activity was recorded using glass insulated, gold–platinum plated tungsten electrodes in anesthetised mice. (0.7–1 MOhm). Response patterns and experimental sequences were controlled by a computerized system (TDT systems). Activity of individual neurons was distinguished using a wave–form discriminator (Alpha–Omega–MSD). Stimulation durations and irradiance were computer controlled and monochromatic light was produced with a tungsten halogen light source, collimating lenses, neutral density and Schott interference filters (10 nm half band width). Light was projected through an opal diffuser providing uniform illumination of the entire binocular visual field.
SCN light–responsive neurons are typically characterized by phasic on responses to light followed by a sustained response to continued light exposure. At light extinction a distinct off response is observed followed by a long delay to recover to the basal firing rate (post–stimulation persistence), the amplitude of which depends on light level. The sustained response is very robust and does not decrease even after long (5–30 min) exposures to bright light. SCN neurons are most sensitive to short wavelength light and fail to respond to light above 620 nm. The responses of single neurons are strongly modulated by the irradiance and wavelength of previous light stimulations.
The electrophysiological responses of SCN neurons show contributions of both phasic responses to light by rods and/or cones as well as sustained responses to light similar to those observed in melanopsin ipRGCs.
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