Fig. 12.27 Effect of ocular perfusion pressure on the c-wave and the b-wave of the ERG in cat, expressed as a percent of the control amplitude (Fig. 7 from [399]). The perfusion pressure was manipulated by increasing the IOP, which is known to reduce ChBF, with only modest

autoregulation by the choroid in response to the reduced perfusion pressure [169]. Note that with decreasing perfusion pressure, abnormalities in both the c-wave and the b-wave become evident, especially below a perfusion pressure of 60 mmHg

normally held in check by intact adaptive ciliary ganglion-mediated control of ChBF.

12.5.5Sympathetic Superior Cervical Ganglion Input

Sympathetic noradrenergic nerve Þbers from the superior cervical ganglion innervate the choroid in mammals [346] and birds [129]. In birds and mammals, the innervation is to blood vessels, and in birds to the smooth muscle of the choroidal stroma as well. Mammalian groups in which sympathetic innervation of the choroid has been demonstrated by catecholamine ßuorescence or immunolabeling include rats, guinea pigs, rabbits, cats, and monkeys (Fig. 12.28) [73, 89, 179, 201, 217, 359]. These nerve Þbers utilize noradrenaline as a neurotransmitter, and they thus contain the enzymes involved in its synthesis (such as tyrosine hydroxylase and dopamine beta-hydroxylase). The sympathetic nerve Þbers from the superior cervical ganglion travel to the choroid via orbital blood vessels or by joining the ophthalmic nerve [322]. Consistent with the sympathetic innervation of choroid and vessels sup-

plying the choroid, cervical sympathetic stimulation in rats, rabbits, cats, and monkeys increases uveal resistance and decreases ChBF (Figs. 12.10 and 12.29) [1, 6, 9, 10, 29, 291, 326]. The choroidal vasoconstriction caused by direct administration of noradrenaline or by activation of sympathetic nerves to the choroid is mediated by alpha-adrenergic receptors [1, 37, 114, 165, 183, 184, 326]. Kawarai and Koss [165] speciÞcally showed that alpha1-adrenoreceptors mediate sympathetic vasoconstriction in the rat choroid. Blockers of beta-adrenergic receptors, by contrast, have been shown in pig to be only marginally effective in dilating the short posterior ciliary arteries [44], and thus unlikely to have a signiÞcant vascular role in sympathetic choroidal control. The sympathetic co-transmitter NPY does, however, have a role in ChBF regulation, since intravenous NPY in rabbits decreases ChBF by 50% [254]. NPY may be responsible for that part of choroidal sympathetic vasoconstriction that is not blocked by alpha-adrenergic receptor antagonists [10]. NPY appears to particularly contribute to choroidal vasoconstriction with high sympathetic nerve Þring rates, and noradrenaline with low Þring rates [36].

Fig. 12.28 Images (a and b) show sympathetic nerve Þbers with varicosities in rat choroid immunolabeled for dopamine beta-hydroxylase (DBH). Images (c and d) show sensory

nerve Þbers with varicosities in rat choroid immunolabeled for substance P (SP). All images at the same magniÞcation arrows indicate labeled axons and terminals

Consistent with a role of alpha-adrenergic receptors in choroidal control, Kiel and Lovell [172] reported that alpha-adrenoreceptor block increased ChBF in rabbits, implying thus also a level of basal sympathetic tone in anesthetized rabbit choroid. Similarly, Chou et al. [52] reported ChBF increased at low ocular perfusion pressures in rabbit 1 week after sympathetic denervation of choroid. Such a manipulation also produced adrenergic supersensitivity [53]. Other investigators have, however, questioned the presence of signiÞcant basal sympathetic tone in ChBF at normal systemic blood pressure in awake animals [34, 37]. Consistent with this, Zhan et al. [416] reported that 24 h after sympathetic denervation ChBF was unchanged in rabbits Ð there was thus

no effect on resting tone. This issue was more extensively investigated by Chou et al. [54] in rabbits using LDF. They found that with reductions in perfusion pressure caused by increased IOP, ChBF remained stable until a perfusion pressure of <55 mmHg, at which point ChBF was proportional to perfusion pressure. In rabbits with either unilateral or bilateral cervical sympathectomy, ChBF declines were not as severe with extremely low perfusion pressure as in normal animals. Thus, sympathetic input does exert some tone on choroidal vessels that is evident at low perfusion pressures, at least in some species under some conditions.

Bill [37] has suggested that the sympathetic innervation of choroid becomes activated with

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Fig. 12.29 Images and graphs showing retinal changes in rats after sympathectomy (From Steinle et al. [329]). Images (a and b) (Fig. 5 from [329]) show immunoßuorescent GFAP labeling in the contralateral (a) and sympathectomized (b) retina. Greater GFAP immunostaining is observed in MŸller cells in the sympathectomized retina. Images (c and d) (Fig. 6 from [329]) show representative results from a Western blot for GFAP protein levels

for control (CL) and sympathectomized (SNX) retina (c) and densitometric analysis of Western blot data for four pairs of control and sympathectomized retinas (d). In (d), the results are presented as a percent of control. The asterisk indicates a signiÞcant difference between experimental and control. GCL ganglion cell layer, INL inner nuclear layer, IPL inner plexiform layer. MagniÞcation the same in both (a and b)

high systemic blood pressure, serving to vasoconstrict the choroid to prevent the overperfusion that would otherwise occur with the increased perfusion pressure caused by the elevated systemic BP. Naturally occurring increases in systemic blood pressure can occur during stress or heightened activity levels. Sustained eleva-

tions of ChBF would cause increased IOP, breakdown of blood-retinal barriers, and edema and be harmful for retinal health and function. Such ocular overperfusion and/or vascular leakiness has, in fact, been demonstrated in sympathectomized rabbits with systemic blood pressure elevation caused by aortic clamping [30, 34] and in

sympathectomized monkeys subjected to systolic hypertension [79]. Several studies in humans have shown that the choroid vasoconstricts and thereby compensates for exercise-induced increases in systemic blood pressure [208, 292]. FuchsjŠger-Mayerl et al. [101] reported that endothelial release of the vasoconstrictor endothe- lin-1 plays a role in this effect. It may be that central baroreceptor-responsive circuitry acting via the sympathetic input to choroid contributes to choroidal baroregulation to high systemic BP. Longo et al. [207] noted, however, that orthostatic increases in ocular perfusion pressure caused by posture change (moving to a supine position) are not compensated for by choroidal vasoconstriction, as would be expected since such changes in perfusion pressure are not accompanied by increases in systemic pressure that would activate aortic baroreceptors.

Steinle et al. [328] investigated the consequences of loss of sympathetic tone on the rat choroid. They reported vascular remodeling after cervical sympathetic transection Ð choroidal arteries and veins were larger and more numerous than in normal rats. No changes in vessel abundance or ChBF were yet observed 2 days after the sympathetic transection, but by 6 weeks, vessels were much more numerous, vessel area was increased, and ChBF was fourfold increased. Steinle and Lashbrook [330] noted elevation of angiogenic factors in choroid after cervical sympathetic transection, and they suggested that these might contribute to increased vessel abundance. Vessel dilation might contribute to increased vessel size as well. Steinle et al. [329] noted that there was a signiÞcant reduction (30%) in photoreceptor cell numbers in sympathectomized rat eyes. This loss appeared to be due to apoptosis, since there was a doubling in apoptotic photoreceptor cell numbers after sympathectomy. The photoreceptor loss in sympathectomized eyes resulted in reduced width of the retinal outer nuclear layer. Increased MŸller cell immunostaining for GFAP spanning the ganglion cell layer and inner nuclear layer was also noted after sympathectomy (Fig. 12.29). These results suggest that loss of sympathetic innervation causes signiÞ-

cant changes to the physiology of the choroid that are adverse for retinal health.

12.5.6 Trigeminal Sensory Input

Sensory nerve Þbers from the trigeminal ganglion co-containing SP and CGRP innervate the choroid in mammals and birds (Fig. 12.28) [60, 318, 346]. These largely are branches of the ophthalmic nerve, but some that reach the orbit arise from the maxillary nerve. The sensory nerve Þbers typically join the meshwork of nerves behind the eye, with one prominent branch entering the ciliary ganglion (Fig. 12.10). Nerve Þbers reach the choroid by traveling with the short ciliary nerves and on blood vessels of the orbit. Among mammals, SP+ and CGRP+ Þbers to the choroid have been observed in rats [89], guinea pigs [193], monkeys [347], and humans [343, 347]. Sensory Þbers such as those of the trigeminal nerve send a central message of hot, cold, pain, or touch and can elicit ocular reßexes, such as blinking and tearing in response to their activation [23, 103]. It is possible that they also participate in temperature-dependent ChBF reßexes. Peripheral Þbers can also participate in antidromic responses in which they release SP and CGRP and cause local responses, which include a vascular component [23]. Consistent with an antidromic action of the SP+ input, rat and rabbit choroid possess SP receptors [68], and consistent with an antidromic action of the CGRP+ input, the choroid in pigs, guinea pigs, and monkeys has been shown to possess CGRP receptors [138]. The neuropeptides SP and CGRP are vasodilators, and their release from intrachoroidal trigeminal sensory Þbers would be expected to act on ChBF [35Ð37, 318, 346]. Consistent with this, stimulation of the ophthalmic nerve in rabbit increases ChBF [338] and is associated with uveal release of SP [34]. Orbital vessels too are sites at which sensory Þbers can affect the blood supply to the choroid. For example, Bakken et al. [17] showed that the pig ophthalmic artery dilates to CGRP. Additionally, trigeminal Þbers have SP+ and CGRP+ terminals in the PPG, which could be a basis of sensory-autonomic vascular