As in mammals, the intrachoroidal ganglion neurons are also found associated with the ciliary nerves in their course to the anterior uvea [26]. It is possible that the intrachoroidal ganglion cells play a role in the repositioning of the focal plane of the retina that is caused by increased ChBF and retinal thickening in chicks recovering from myopia [92].

12.5.8Disturbed Neural Control of Choroidal Blood Flow in Aging and Retinal Disease

12.5.8.1 Effect of Aging on Retina and Choroid

Choroidal blood ßow and its adaptive regulation can be impaired by aging (Fig. 12.30). For example, reductions in basal ChBF occur in the macula of humans as they age [271, 283]. Moreover, reductions in ChBF in excess of what aging alone would predict have been observed beginning in the early stages of age-related macular degeneration [42, 98, 99, 125, 126, 281], the leading cause of blindness in humans older than 65 years [136]. Adaptive regulation of ChBF also is impaired in aged humans, since compensatory responses of ChBF to ßuctuations in systemic blood pressure are abnormal in elderly humans [278]. These various Þndings raise the possibility that abnormalities in macular ChBF and/or in its adaptive regulation contribute to normal age-related declines in retinal degeneration (Fig. 12.30). The mechanisms responsible for the reduced ChBF and impaired adaptive regulation with age are uncertain. Loss and narrowing of submacular choroidal vessels in normal aged eyes and in atrophic AMD eyes has been reported [125, 282, 324]. Such changes may contribute to the reduction in basal ChBF seen with aging and in early AMD (Fig. 12.30). To examine the possible role of changes in neuronal control of ChBF, we used immunolabeling to assess the impact of age on parasympathetic innervation of human choroid, using VIP immunolabeling. Our results indicated a signiÞcant age-related decline in VIP-positive nerve Þbers and vessel diameter in the submacular choroid in disease-free human donor eyes

(Fig. 12.31) [155]. These Þndings suggest that a decline in the neural control of ChBF and vessel diameter may explain reductions in ChBF and its adaptive control observed with aging. Impaired neural control of ocular and choroidal blood ßow may occur in other conditions as well. For example, production of the vasodilator NO by the endothelium of ophthalmic, ciliary and retinal vessels and the optic nerve head can be impaired in hypertension, hypercholesterolemia, and diabetes [119, 133, 137].

As part of an effort to characterize the effects of aging on the retina and choroid, we carried out detailed studies in pigeon [91, 93] that sought to:

(1) determine if choroidal and outer retinal deterioration occurs as a function of aging in a nonhuman species that possesses both rod and cone photoreceptors (i.e., pigeons), and might therefore be widespread concomitants of aging, and (2) obtain data that might shed light on the relationship between age-related retinal and choroidal changes. In a sample of 64 pigeons ranging in age over much of the pigeon life span (0.5Ð20 years), we measured diverse ocular parameters by physiological or histological means, including: ChBF (by LDF); choriocapillary vessel abundance (by LM histology); acuity (by behavioral methods); and photoreceptor abundance (by LM histology). Statistical methods were used to ascertain the pattern of age-related changes and determine ages at which or by which signiÞcant changes occurred in speciÞc parameters. In the sample of 53 birds for which we had obtained visual acuity and/or photoreceptor data, we observed a prominent stepwise decline of about 20% in photoreceptor abundance at the age of 4.7 years (Fig. 12.32), followed by lesser decline thereafter, and a curvilinear decline in acuity (with half the decline having occurred by the age of 5 years). The period of prominent photoreceptor loss (4Ð6 years of age) coincided with ages during which about 10% of photoreceptors appeared to be showing degenerative changes. For the sample of 45 birds for which we measured choroidal parameters, choriocapillary vessel abundance showed a highly curvilinear decline with age and at least half of this decline had occurred by the age of 3 years (Fig. 12.33). ChBF showed an abrupt decline of about 20% at

Fig. 12.30 Graphs showing visual acuity changes in humans over the life span (a), and basal choroidal blood ßow in young humans, aged humans, and humans with age-related macular degeneration (AMD) (b). Graph (a) shows the decline in human visual acuity with advancing age (Fig. 3 redrawn from

[145]). In (b), the blood ßow values are taken form Grunwald et al. [125, 126] and are expressed in arbitrary units based on laser Doppler ßowmetry measurements. Note that basal ChBF declines with age, and individuals with AMD have even lower basal ChBF than aged humans without AMD

the age of 4 years and a further 20% decline thereafter. Our results clearly show that ChBF and choroidal vascularity decline signiÞcantly with age in pigeons, as do acuity and photoreceptor

abundance. Our statistical analyses suggest that there is a positive relationship between choroidal and visual functions in pigeons, and that prominent choroidal vascular decline precedes visual

Fig. 12.31 Examples of VIP immunolabeling of nerve Þbers on choroidal vessels in humans of differing age. The view of the choroidal specimens shows representative submacular choroidal vessels (V), surrounded by stroma. The abundant beaded striae running across the vessel

lumens are the VIP-positive Þbers of the choroidal vessel. Note the generally lesser amount of VIP-positive Þbers in elderly compared with young eyes. Note also the VIP+ intrachoroidal neuron in (c). MagniÞcation the same in all images

decline as pigeons age (Fig. 12.34). Thus, our Þndings are consistent with the view that agerelated decline in choroidal function might contribute to age-related vision loss in pigeons. Note that the possibility exists that retinal degeneration is the primary event and diminished ChBF the consequence of diminished retinal need. Nonetheless, outer retinal degeneration does not invariably lead to diminished ChBF, as evidenced in the Abyssinian cat [257].

In further studies, we sought to determine if age-related changes in parasympathetic regulation of ChBF could contribute to age-related choroidal and retinal decline [93]. To this end, we used immunohistochemical methods to detect

choroidal nerve Þbers from the ciliary ganglion immunolabeled for ChAT or a neuroÞlamentassociated antigen in Þxed cryostat sections of the eye in 0.5- to 20-year-old pigeons. Additionally, transcleral LDF was used to measure basal ChBF, light-evoked ChBF increases and EWM-evoked ChBF increases in the superior choroid to assess the functional integrity of the vSCN-EWM-ciliary ganglion circuit. We observed a marked age-related linear decline in the ciliary ganglion innervation of the choroid (Fig. 12.33). Moreover, we observed pronounced declines in ciliary ganglion-mediated control of ChBF. The decline in EWM control of ChBF was pronounced by 5 years of age, and half of the

Fig. 12.32 Light micrographic images of retinal sections from a 1.7-year-old pigeon (a) and a 17-year-old pigeon (b) showing two extremes in the abundance of photoreceptor cell bodies. The 1.7-year-old animal had 47 photoreceptors per 100 mm, while the 17-year-old animal had 32 photoreceptors per 100 mm. In addition, the thickness of the outer nuclear layer (ONL) is less in the older bird

(note lines demarcating ONL Ð outer plexiform layer (OPL) border in both images). Age-related photoreceptor pathology was also evidenced by loss of outer segments (OS) and vitread displacement of some lipid droplets from their normal position (arrows). IS inner segment, OLM outer limiting membrane, ONL outer nuclear layer, OPL outer plexiform layer

functional decline occurred by about 2Ð3 years of age. Thus, signiÞcant loss in choroidal vascularity and innervation appear to lead to impaired basal and adaptive parasympathetic ChBF control early in the life span of pigeons (Fig. 12.34), which is consistent with the notion they contribute to agerelated vascular insufÞciency and attendant agerelated damage to the retina. These overall results for pigeons suggest that aging may deleteriously affect the retina, in part, by impairing ChBF and its neural control. The apparent impact of agerelated loss of ciliary ganglion input to the choroid in birds is consistent with the impact we see of EWM lesions in younger birds.

12.5.8.2 Effect of Disease on Retina and Choroid

Based on the effects of interruption of the vSCN- EWM-ciliary ganglion circuit in birds, it seems plausible that diseases that compromise ChBF and its neural control might have retinal dysfunction and pathology as their consequences. Several conditions in humans and animals that involve autonomic neuropathy or endothelial cell pathology (the latter of which would affect endothelialdependent neural control of ChBF), in fact, have diminished ChBF and outer retinal pathology as concomitants, including aging [404], age-related macular degeneration [99], chronic hypertension [133, 374], insulin-dependent diabetes [195, 305]

and glaucoma [127, 158, 191, 244]. Disturbances in ChBF and/or its neural regulation may also be involved in ischemic outer retinal disease and myopic retinopathy [40, 110, 286]. Consistent with this, impaired regulation of ChBF in response to exercise-induced increases in systemic BP (and thus ocular perfusion pressure) is observed in inactive central serous chorioretinopathy [365]. Adverse lifestyle choices may also deleterious affect adaptive neural regulation of ChBF, since smokers show impaired ChBF compensation for exercise-induced ocular perfusion pressure increase [396]. The speciÞc link between parasympathetic ChBF impairment and retinopathy in aging is conÞrmed by our studies in pigeons [91, 93]. The above-described work of Steinle and coworkers in rats shows a similar importance of sympathetic control of ChBF. Thus, the available data are consistent with the view that disturbances in the maintenance of basal neurogenic choroidal tone and/or adaptive ChBF neural control could be contributing factors to the retinal declines seen in humans and other species in aging, hypertension, diabetes, glaucoma, ischemic retinal disease, and myopia. Autonomic neuropathy in diabetes may in particular contribute to overperfusion and leakiness of ocular vessels in diabetes [35]. Clearly, other mechanisms may also contribute to retinal damage in these conditions, such as edema, retinal detachment, and neovascularization

Fig. 12.33 Images showing the decline in choroidal vascularity and innervation with age in pigeons. Images (a and b) show a pair of LM images that illustrate the agerelated decline in choriocapillary vessel abundance, showing choroid from a 3-year-old animal that had a choriocapillary coverage of 86% (a), with a mean intercapillary distance (ICD) of 7.26 mm, and a choroid from a 17-year-old pigeon that had a capillary coverage of 48% and a mean ICD of 13.15 mm (b). Images (c and d) illustrate the abundance of intrachoroidal nerve Þbers of cili-

ary ganglion origin from a representative 0.5-year-old pigeon (c) and a representative 12-year-old pigeon (d). The nerve Þbers were immunolabeled by the peroxidaseantiperoxidase method using antibodies against the 3A10 neuroÞlament-associated antigen. These images typify the age-related decline in nerve Þber abundance in pigeon choroid. MagniÞcation same in (a) as in (b), and in (c) as in (d). A artery, L lacunae, CC choriocapillaris, BM Bruch membrane, OS outer segment, V vein