in treating glaucoma stems from its inhibitory effect on aqueous production, or from improving ocular blood ßow.

Although retinal vessels receive no CGRP+ sensory Þbers, bovine and dog retinal vessels have been shown to dilate in response to application of CGRP [43, 261, 279], indicating they possess CGRP receptors and can respond to blood-borne or retinal diffusion-carried CGRP. Retinal vessels also relax and dilate in response to SP [178], via stimulation of NO release from endothelium. The retina itself could be the source of any SP or CGRP having effects on the retinal vasculature in vivo.

12.3Neural Control of Iridial and Ciliary Body Blood Flow

The iris contains the pupillary control musculature, while the ciliary body contains the muscles of accommodation. The ciliary body is continuous with the ciliary processes, which are rich in arteries and are responsible for production of aqueous humor, whose abundance regulates IOP. Outßow of aqueous humor through the trabecular meshwork at the limbus of the eye, a portal through which aqueous humor returns to the venous side of the vasculature, also contributes to IOP and may be regulated by autonomic and sensory nerve Þbers. The ciliary body and iris in mammals typically receive their blood supply from the long posterior ciliary arteries and the anterior ciliary artery branches from the blood supply to the extraocular muscles (Fig. 12.3). The long posterior ciliary arteries (typically one medial and one lateral) arise from the posterior ciliary arteries, which themselves arise from the ophthalmic artery behind the eye. The posterior ciliary arteries give rise to many short posterior ciliary arteries that penetrate the eye at its posterior pole just around the optic nerve, and enter the choroid (Fig. 12.1). The long posterior ciliary arteries pierce the sclera only slightly more anteriorly and course within the choroid on the medial and lateral sides of the eye to the ciliary body, where they give rise to the blood supply to the muscles of accommodation and the ciliary

processes [10, 266]. The anterior ciliary arteries typically give rise to an annular anastomosis within the ciliary body that gives off radially oriented blood vessels to the iris, as well as additional branches to the ciliary processes. A venous network in the ciliary body drains the ciliary body and iris to the anterior ciliary veins and the episcleral veins (Fig. 12.3). The ophthalmic artery and posterior and anterior ciliary arteries and their branches within iris and ciliary body are innervated, in some places heavily, as are nonvascular muscular structures of the iris and ciliary body. For example, as noted above, the ophthalmic artery in rats is innervated by NOS+ and VIP+ cholinergic Þbers from the PPG, NPY+ noradrenergic Þbers from the superior cervical ganglion, and SP+ and CGRP+ sensory Þbers from the trigeminal nerve [65, 76, 346]. Consistent with PPG inßuences on the ophthalmic artery, Bakken et al. [17] showed that the pig ophthalmic artery dilates to VIP and acetylcholine, with the cholinergic response eliminated by removal of endothelium, and with acetylcholine and VIP being synergistic in their effects. Similarly, Wang et al. [387] showed that cholinergic vasodilation of ophthalmic arteries in dogs involves endothelial NO release. Consistent with sympathetic inßuences on the ophthalmic artery, noradrenaline administration or cranial sympathetic nerve stimulation constricts the ophthalmic artery in dogs [260]. Similarly, the long posterior ciliary arteries have been shown to be under alpha-adrenergic sympathetic vasoconstrictory control and NOand VIP-mediated and muscarinic cholinergic parasympathetic dilatory control [263, 265, 348], as well as vasorelaxant control by SP+ and CGRP+ Þbers [265]. The NO-mediated control of the long posterior ciliary artery is regulated by cholinergic control of endothelial NO release, as well as by direct release of NO from the PPG innervation [407]. Blood ßow in the long posterior ciliary artery in cats is not affected by cervical sympathetic nerve section, implying minimal resting sympathetic tone [186].

The ciliary body and iris are under the control of two sets of parasympathetic Þbers, those from the PPG and those from the ciliary ganglion.

Fig. 12.5 Images showing innervation of various structures of the anterior uvea, all modiÞed from Stone [340]. Image (a) shows VIP+ nerve Þbers in the ciliary processes of rat (Fig. 3a from [340]). Image (b) shows VIP+ nerve Þbers in the ciliary processes of guinea pig (Fig. 3b from [340]). Image (c) shows VIP+ nerve Þbers on a large blood vessel of guinea pig iris (Fig. 4a from [340]). Image (d) shows VIP+ nerve Þbers on a large blood vessel in cat

iris (Fig. 4b from [340]). Image (e) shows VIP+ nerve Þbers in the iris stroma of rhesus monkey (Fig. 4e from [340]). Image (f) shows VIP+ nerve Þbers on a longitudinal blood vessel in the iris root of rhesus monkey (Fig. 4f from [340]). Arrows in each image show some of the labeled Þbers. MagniÞcation is the same in images (aÐc, e and f). NPE nonpigmented epithelium, PE pigmented epithelium

The latter are thought to mainly or only serve to control pupil constrictor muscles and accommodation muscles, and any (presumably indirect) impact they have on IOP and blood ßow within iris and ciliary body is discussed below. The PPG input to the iris and ciliary body appears to be largely to the vasculature [47] and is discussed in this paragraph. The iris stroma and its vessels, the blood vessels and musculature of the ciliary body, and/or the blood vessels of the ciliary processes have been shown to be innervated by VIP+ Þbers from the PPG in guinea pigs, rats, squirrels, cats, monkeys, and humans (Fig. 12.5) [319, 344, 345, 359, 376, 379]. VIP+ nerve Þbers have also been observed in the trabecular meshwork in humans

[345]. The blood vessels of the ciliary processes in guinea pig and rat but not cat and monkey have also been reported to possess VIP+ innervation [344, 379]. The PPG is also the source of a neuronal NOS+ innervation of vessels in the iris, ciliary body, ciliary processes, and limbus, and all neuronal NOS+ PPG neurons have been found to also contain VIP in rats, but not all VIP+ neurons contain nNOS [11, 398]. The PPG tends to be highly enriched in nNOS neurons in mammals [11], with 70% of PPG neurons being nNOS+ in humans [122]. In pigs at least, the anterior uvea is also innervated by VIP+ and nNOS+ neurons associated with the ciliary nerves as they course through the choroid to the anterior uvea [222].

Consistent with the PPG as a source of ciliary body and iris vascular innervation, the iridial and ciliary body vasculature in cats, rabbits, and monkeys does show a ßow increase with activation of the preganglionic input to the PPG via facial nerve stimulation [252]. This vasodilation is blockable with the ganglionic blocker hexamethonium. Moreover, Bill et al. [31] showed that ganglionic blockade with hexamethonium slightly reduced basal iris and ciliary body blood ßow in rabbit, presumably by diminishing basal activation of PPG neurons by their preganglionic input from the facial motor complex. The increased iridial and ciliary body blood ßow with facial nerve stimulation is likely to be mediated by VIP or NO released from PPG Þbers in anterior uvea, with the stimulation parameters used by Nilsson et al. [252].

Nilsson [255] demonstrated that the increases in blood ßow in the ciliary body and iris in rabbits with facial nerve stimulation could largely be prevented at low stimulation frequencies (2 Hz) by NOS inhibition but not at higher frequencies (5 Hz). Nilsson suggested that another vasodilator released from PPG terminals other than NO might mediate the vasodilation at the higher of the two stimulation frequencies Ð presumably VIP. Consistent with this, Nilsson (2000) reported that the increased blood ßow in the iris and ciliary body in cats occurring with facial nerve stimulation at 5 Hz was only minimally reduced by the nonselective NOS inhibitor NW-nitro-l-arginine (LNA). Combined treatment with LNA and the nonselective muscarinic antagonist atropine also had little inhibitory effect on the anterior uveal blood ßow increase with facial nerve stimulation. These results suggest neither neuronally derived NO release nor endothelially derived NO release driven by acetylcholine plays a noteworthy role in the vasodilatory action of the PPG input to anterior uvea in cats at a 5-Hz Þring rate, again implicating VIP as the major dilator at this Þring rate. At 10-Hz stimulation, LNA or atropine alone was largely ineffective in blocking the anterior uveal blood ßow increase with facial nerve stimulation. At 10 Hz, however, LNA and atropine together were effective at nearly completely blocking the facial nerve-evoked vasodilation in the iris and ciliary body. This result is puzzling in light of the

differential effects of facial nerve stimulation frequencies in rabbits and suggests that VIP is not the major vasodilator at 10 Hz in lacrimal gland in cats. Rather, both neurally derived and endothelially derived NO, the later driven by cholinergic action, are both seemingly involved in the anterior uveal increase with facial nerve stimulation at 10 Hz in cats, and the doses of LNA and atropine used were apparently not able to block the 10-Hz vasodilation when used singly.

Consistent with a role of NO in control of iris and ciliary body blood ßow, Deussen et al. [69] reported that NOS inhibition with LNAME in dogs decreased basal anterior uveal blood ßow about 50%, despite causing about a 20% increase in mean arterial blood pressure. Similarly, Seligsohn and Bill [313] reported that LNAME decreased iris and ciliary body blood ßow in rabbit by 40Ð60%. Similarly, Jacot et al. [156] showed in piglet that systemic NOS inhibition with LNAME reduces anterior uveal blood ßow. The purpose of facial nerve control of anterior uveal blood ßow is uncertain. The anterior uvea in cats and monkeys but not rabbits has been shown to compensate well for reductions in ocular perfusion pressure caused by elevation of IOP or reduction of systemic blood pressure [10]. It may be that the facial input contributes to this regulation and thereby aids in maintaining stable blood ßow to the anterior uvea regardless of momentary ßuctuations caused by variations in ocular perfusion pressure. As we will discuss in the section on choroidal blood ßow, the facial input may be especially involved in compensation for systemic hypotension. Iridial blood ßow has, however, been reported to not show autoregulation in humans when perfusion pressure is reduced by increasing IOP [51].

Since the PPG innervates both vessels involved in aqueous production and the territory involved in aqueous outßow, the impact of the PPG innervation of anterior uvea on IOP has been of interest. This interest was initiated by the Þnding of Ruskell [297] that PPGectomy in monkeys led to a long-lasting diminution of IOP. Conversely, PPG activation leads to increased IOP in monkey [252]. While these effects might stem from effects on choroidal blood ßow (whose volume affects IOP), the possibility of more direct effects cannot

be ignored. Nilsson and Bill [251] showed that intravenous administration of VIP in rabbits yielded increased choroidal blood ßow and increased IOP but no change in anterior uveal blood ßow. By contrast, intracameral administration of VIP vasodilated vessels in iris and ciliary body but had no effect on IOP, pupil diameter, or blood-retinal barrier. Thus, increased choroidal blood ßow can be associated with increased IOP, while increased anterior uveal blood ßow by itself need not increase IOP. Nilsson et al. [253], however, showed that there might be species differences in the impact of PPG input to the anterior uvea on IOP. They showed that intracameral VIP in monkeys can increase IOP by increasing aqueous production and increasing venous outßow resistance. NO production by PPG terminals on ocular vessels also exerts an effect on IOP as well. For example, in rabbits, inhibition of NO production reduces long posterior ciliary artery blood ßow (and presumably ciliary process blood ßow as well) and thereby reduces aqueous production (Kiel et al. 2001). NO also yields trabecular meshwork relaxation and thus increased outßow [391]. This effect on outßow may be why treatments that potentiate NO action by preventing its breakdown or treatments that cause release of NO tend to decrease IOP [21]. Finally, M3 muscarinic receptors are present in human trabecular meshwork [130], and acetylcholine dilates ciliary process vessels in cats and rabbits [382]. It may be that the PPG input to these structures can act via cholinergic mechanisms to inßuence aqueous production and outßow, at least in some species. A role of the facial input in the nocturnal rise in IOP is possible [35], given the apparent input that the facial preganglionic neurons receive from the SCN nucleus region, as discussed below in more detail in the section on facial regulation of choroidal blood ßow control.

Ciliary ganglion parasympathetic cholinergic Þbers innervate iris sphincter and ciliary body muscles and thereby cause pupil constriction and accommodation, respectively, when activated. Consistent with cholinergic inßuences in these muscular structures, M1, M2, and M3 muscarinic receptors have been reported in human iris, ciliary body, and ciliary processes, and M3 receptors have been reported in the trabecular meshwork

[130, 131]. Stimulation of the ciliary ganglion input has, however, been reported to also have unexpected effects on anterior uveal blood ßow. For example, Stjernschantz et al. (1973) found that intracranial stimulation of the oculomotor nerve (and thus the preganglionic input to the ciliary ganglion) decreased blood ßow in the rabbit iris, ciliary body, and ciliary processes. They suggested that mechanical compressive effects on blood vessels due to ciliary body and iris muscle contraction might be the basis of the diminished blood ßow. Consistent with this interpretation, eye illumination sufÞcient to cause pupil constriction in rabbits is accompanied by reduced iris blood ßow, and both the pupil constriction and the decreased blood ßow can be blocked with the peripheral muscarinic blocker biperiden [336]. Similarly, Bill et al. [31] showed that ganglionic blockade with hexamethonium eliminated and muscarinic blockade with biperiden reduced the iris and ciliary body blood ßow decrease obtained in rabbits with intracranial oculomotor nerve stimulation. Sympathectomy in rabbits showed that the anterior uveal vasoconstriction with oculomotor nerve stimulation was not due to inadvertent activation of sympathetic input to the anterior uvea [8]. Thus, the reduced anterior uveal blood ßow with oculomotor nerve stimulation in rabbits is mediated by the input of the oculomotor nerve to the ciliary ganglion (which arises speciÞcally from the nucleus of Edinger-Westphal). The same research group later showed that the uniformly reduced anterior uveal blood ßow with oculomotor nerve stimulation might be unique to rabbits, because both oculomotor nerve stimulation and activation of the cholinergic input from the ciliary ganglion caused iris vasoconstriction and ciliary body vasodilation in cats and monkeys [37, 337]. The effects in cats but not monkeys could be blocked with the muscarinic blocker atropine. Consistent with the possibility that reduced blood ßow in the iris in cats is caused by direct cholinergic vascular mechanisms, acetylcholine and ciliary ganglion activation were both found to vasoconstrict anterior uveal vessels in arterially perfused cat eyes [214, 216]. In contrast to the blood ßowreducing effects of activation of the ciliary ganglion input to the anterior uvea in rabbits, the

speciÞc action of acetylcholine on ciliary process vessels in both cats and rabbits has been reported to be dilatory [382]. It may be this effect is mediated by vascular muscarinic receptors postsynaptic to PPG cholinergic input. Consistent with cholinergic vasodilatory effects mediated in some parts of the anterior uvea in some species, Alm et al. [7] showed that the cholinesterase inhibitor neostigmine or the muscarinic agonist pilocarpine applied to the cornea caused pupil constriction and increased ciliary body, ciliary process, and iris blood ßow in monkeys. The most consistent interpretation of these diverse results is that the mechanical effects of prominent pupil constriction caused by activation of the oculomotor Ð ciliary ganglion input to the iris sphincter muscle compress iris blood vessels and cause diminished iridial blood ßow in rabbits, cats, and monkeys. The above-noted study by Alm et al. [7] may have observed increased iris blood ßow with cholinergic agents because the pupil constriction was too mild for its mechanical effects to override the vasodilatory action of muscarinic activation of vessels. By contrast, the inßuence of the ciliary ganglion input to the ciliary body and processes varies among species. In rabbits, the vasoconstrictory effect is likely to be mechanical, since cholinergic vasodilatory mechanisms are in place. In monkeys and cats, the vasodilatory effect could be direct, or the mechanical effects may somehow diminish vascular resistance.

Activation of the ciliary ganglion input to the anterior uvea also has an impact on IOP, with stimulation of the preganglionic neurons of the nucleus of Edinger-Westphal (EW) of the oculomotor nuclear complex in rabbits and cats causing an IOP rise [113, 303]. In Gherezghiher et al. [113], the stimulation was sufÞcient to produce pupil constriction and a 35% rise in IOP that was blockable by hexamethonium. The basis of this effect is uncertain, but the mechanical effects of pupil constriction on outßow or a vasodilatory effect on uveal vessels have been raised as a possibility [113, 303]. An effect on IOP other than by the transitory pupil constriction is implied by the Þnding that ciliary ganglionectomy causes an IOP drop in cats and monkeys [57, 78]. Pharmacological stimulation of accommodation with pilocarpine, however, increases outßow in

monkey and human eye, apparently by a mechanical effect on the trabecular meshwork mediated by the pull of the muscles of accommodation on the scleral spur, as well as by a direct effect on the outßow pathway [77, 102]. This Þnding implies that the accommodative effect of ciliary ganglion activation does not account for the increased IOP with ciliary ganglion activation. Both aqueous production and outßow, however, were reportedly increased in cats and rabbits by oculomotor nerve or ciliary ganglion stimulation [215, 216, 229]. Whether ciliary ganglion input directly or indirectly mediates these effects is uncertain. In any event, these results suggest that IOP increases caused by oculomotor nerve or ciliary ganglion activation stem from pupil constriction and perhaps greater aqueous production than outßow. It may also be that choroidal blood ßow increases contribute to IOP rise with oculomotor nerve or ciliary ganglion activation. As will be discussed below, however, the evidence that oculomotor nerve activation increases choroidal blood ßow is not deÞnitive.

Among the nerve Þbers innervating iris, ciliary body and the aqueous outßow path are sympathetic nerve Þbers from the superior cervical ganglion. These nerve Þbers contain adrenaline and NPY [39, 240], and they have been demonstrated in diverse mammalian species, including rat, monkey, cat, and rabbit [195, 356]. The iris also has sympathetic innervation in chicks [175]. Although some of the sympathetic input to iris ends on the iris dilator muscle, some has also been shown to end on iris vessels in rat [143], and they vasoconstrict vessels by alpha-adrenergic mechanisms [142]. Alm and Bill [6] showed that cervical sympathetic stimulation decreases iris, ciliary body, and ciliary process blood ßow in cats. This effect is blocked by the alpha-adrenergic antagonist phentolamine [6], and pharmacological studies show that sympathetic constriction of ciliary body arteries is mediated by alpha 2a adrenoreceptor mechanisms [393]. Additionally, intravenous NPY decreases iridial ßow by 30% and ciliary body ßow by 50% [254]. The sympathetic input to the ciliary body and processes also inßuences aqueous production. Chronic sympathetic stimulation in rabbits decreases aqueous production (by decreasing blood ßow presumably)