Melanopsin Expression Confers Light Sensitivity to Neurons of the Outer Nuclear Layer in the avian retina.POSIUM MARINE BIOLOGICAL LABORATORY WOODS HOLE, MASSACHUSETTS PROGRAM AND ABSTRACTS organized by Jerod S. Denton (original) (raw)

2013, Journal of General Physiology

In mammals, the discovery of melanopsin and intrinsic light responsiveness in a small population of retinal ganglion cells (ipRGCs) paved the way for understanding light regulation of nonvisual functions, like the pupillary reflex or the entrainment of circadian rhythms. The scarcity of ipRGCs, however, is a hurdle for investigating melanopsin signaling mechanisms. In the avian retina, melanopsin expresses abundantly in the outer nuclear layer too, but its functionality is unknown. We used the chicken embryo retina as a model system to investigate possible physiological roles of melanopsin in non-ganglion cells. A polyclonal antibody targeting both chicken melanopsin isoforms was validated by Western blot and used to corroborate the distribution pattern in fixed retina sections; strong immunoreactivity occurred in areas that likely include horizontal, bipolar, and some amacrine cells. Retinas were enzymatically dissociated to yield morphologically well-preserved isolated neurons; their physiological viability was tested with whole cell recording. Although voltage-gated currents were found in the different cell types, initial recordings failed to reveal direct changes in membrane current by photostimulation. However, light could reversibly modulate voltage-gated currents. Fluorescence imaging in cells loaded with calcium indicators demonstrated a Ca fluorescence increase in selected bipolar cells and small neurons, likely to comprise a subtype of amacrine cells. Higher sensitivity measurements, using a photomultiplier and pulsed light to extend the recording period, showed that in a minority of bipolar cells, light evoked Ca responses with a long latency and a slow time course, spanning minutes. A reassessment of electrical responses occurring on such a long time scale revealed a small inward current (tens of pA) with a similar time course. These observations indicate that intrinsic photosensitivity is not confined to rods, cones, and some ganglion cells, but extends to additional retinal cell types. Its physiological role remains to be investigated. Supported by Colciencias grant 222852128276.