Abstract
The medial part of the nucleus of Edinger-Westphal (EWM) in birds mediates light-regulated adaptive increases in choroidal blood flow (ChBF). We sought to characterize the effect of loss of EWM-mediated ChBF regulation on photoreceptor health in pigeons housed in either moderate intensity diurnal or constant light (CL). Photoreceptor abundance following complete EWM destruction was compared to that following a lesion in the pupil control circuit (as a control for spread of EWM lesions to the nearby pupil-controlling lateral EW) or following no EW damage. Birds were housed post-lesion in a 12 h 400 lux light/12 h dark light cycle for up to 16.5 months, or in constant 400 lux light for up to 3 weeks. Paraformaldehyde-glutaraldehyde fixed eyes were embedded in plastic, sectioned, slide-mounted, and stained with toluidine blue/azure II. Blinded analysis of photoreceptor outer segment abundance was performed, with outer segment types distinguished by oil droplet tint and laminar position. Brains were examined histologically to assess lesion accuracy. Disruption of pupil control had no adverse effect on photoreceptor outer segment abundance in either diurnal light or CL, but EWM destruction led to 50-60% loss of blue/violet cone outer segments in both light conditions, and a 42% loss of principal cone outer segments in CL. The findings indicate that adaptive regulation of ChBF by the EWM circuit plays a role in maintaining photoreceptor health and mitigates the harmful effect of light on photoreceptors, especially short wavelength-sensitive cone photoreceptors.
Highlights
The choroidal blood supply to the outer retina is essential for the health of photoreceptors (Bill, 1984; Yancey & Linsenmeier, 1988, 1989)
No significant differences were found for principal cone outer segment abundance between the two control groups, nor between either of the area pretectalis (AP) groups and the control-diurnal light (DL) group (Figs. 4 and 5)
No significant change was seen in principal cone outer segment abundance in the EWM-DL group compared to the control-DL group, a significant 42.2% reduction was seen in principal cone outer segment abundance in the EWM-constant light (CL) group compared to the control-DL group (P = 0.0096)
Summary
The choroidal blood supply to the outer retina is essential for the health of photoreceptors (Bill, 1984; Yancey & Linsenmeier, 1988, 1989). EWM lesions or choroidal nerve transections reduce basal ChBF in birds (Shih et al, 1994; Fitzgerald et al, 1996), and prevent light-mediated reflexive increases in ChBF (Fitzgerald et al, 1996), thereby likely causing chronic choroidal insufficiency. Consistent with this premise, we have found that destruction of EWM in young adult pigeons maintained in normal diurnal light (DL)
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