Progressive loss of parasympathetic innervation from the ciliary ganglion to choroidal blood vessels in aging pigeons

Purpose. The choroid receives extensive parasympathetic innervation, which in birds arises largely from the ciliary ganglion (CG). Since age-related changes in parasympathetic regulation of choroidal blood flow (ChBF) could contribute to age-related retinal decline, we used histochemical methods to determine if the CG choroidal innervation showed age-related changes in pigeons. We focused on the superior retina, since it appears analagous to the primate macula. Methods. Fixed cryostat sections of the CG and eye in 0.5 to 20yr old pigeons were examined. Choroidal neurons in the CG were visualized by choline acetyltransferase (CHAT) immunohistochemstry or NADPH-diaphorase (NADPHd) histochemistry, and choroidal nerve fibers from the CG were immunolabeled for CHAT or the 3A10 neurofilament-associated antigen. The abundance of neuronal nitric oxide synthase (nNOS) in the superior choroid was measured by Western Blot analysis. Finally, transcleral laser Doppler flowmetry was used to measure basal ChBF in the superior choroid. Results. There was a marked age-related loss of CHAT+ or NADPHd+ choroidal neurons after 8yrs, leading to a 50% reduction by 20yrs. The Western blots confirmed the presence of abundant nNOS within choroid prior to 8yrs. The decline in choroidal neuron abundance was associated with a progressive decrease in choroidal nerve fibers of CG origin to 20% of normal by 20yrs. This decrease was observed with both the functional marker (CHAT) and the structural marker (3A10). The age-related decline in nerve fiber abundance was less marked when age-related changes in choroidal thickness were taken into account. Basal ChBF showed about a 40% decline between the ages of 3-10yrs, and a slight further decline thereafter. Conclusions. The prominent age-related decline in vasodilatory parasympathetic innervation of the choroid could, in part, be the basis of the reduction in basal ChBF. The overall age-related decline in ChBF could contribute to age-related vascular insufficiency and attendant age-related ischemic damage to the retina.