Role of muscarinic cholinergic transmission in Edinger-Westphal nucleus-induced choroidal vasodilation in pigeon

Ocular Autonomic Nervous System: An Update from Anatomy to Physiological Functions

The autonomic nervous system (ANS) confers neural control of the entire body, mainly through the sympathetic and parasympathetic nerves. Several studies have observed that the physiological functions of the eye (pupil size, lens accommodation, ocular circulation, and intraocular pressure regulation) are precisely regulated by the ANS. Almost all parts of the eye have autonomic innervation for the regulation of local homeostasis through synergy and antagonism. With the advent of new research methods, novel anatomical characteristics and numerous physiological processes have been elucidated. Herein, we summarize the anatomical and physiological functions of the ANS in the eye within the context of its intrinsic connections. This review provides novel insights into ocular studies.

Defective Choroidal Blood Flow Baroregulation and Retinal Dysfunction and Pathology Following Sympathetic Denervation of Choroid

Purpose

We sought to determine if sympathetic denervation of choroid impairs choroidal blood flow (ChBF) regulation and harms retina.

Methods

Rats received bilateral superior cervical ganglionectomy (SCGx), which depleted choroid of sympathetic but not parasympathetic innervation. The flash-evoked scotopic ERG and visual acuity were measured 2 to 3 months after SCGx, and vasoconstrictive ChBF baroregulation during high systemic arterial blood pressure (ABP) induced by LNAME was assessed by laser Doppler flowmetry (LDF). Eyes were harvested for histologic evaluation.

Results

ChBF increased in parallel with ABP in SCGx rats over an ABP range of 90% to 140% of baseline ABP, while in sham rats ChBF remained stable and uncorrelated with ABP. ERG a- and b-wave latencies and amplitudes, and visual acuity were significantly reduced after SCGx. In SCGx retina, Müller cell GFAP immunolabeling was upregulated 2.5-fold, and Iba1+ microglia were increased 3-fold. Dopaminergic amacrine cell fibers in inner plexiform layer were reduced in SCGx rats, and photoreceptors were slightly depleted. Functional deficits and pathology were correlated with impairments in sympathetic regulation of ChBF.

Conclusions

These studies indicate that sympathetic denervation of choroid impairs ChBF baroregulation during elevated ABP, leading to choroidal overperfusion. This defect in ChBF regulation is associated with impaired retinal function and retinal pathology. As sympathetic ChBF baroregulatory defects have been observed in young individuals with complement factor H (CFH) polymorphisms associated with risk for AMD, our results suggest these defects may harm retina, perhaps contributing to AMD pathogenesis.

Neural control of choroidal blood flow

The choroid is richly innervated by parasympathetic, sympathetic and trigeminal sensory nerve fibers that regulate choroidal blood flow in birds and mammals, and presumably other vertebrate classes as well. The parasympathetic innervation has been shown to vasodilate and increase choroidal blood flow, the sympathetic input has been shown to vasoconstrict and decrease choroidal blood flow, and the sensory input has been shown to both convey pain and thermal information centrally and act locally to vasodilate and increase choroidal blood flow. As the choroid lies behind the retina and cannot respond readily to retinal metabolic signals, its innervation is important for adjustments in flow required by either retinal activity, by fluctuations in the systemic blood pressure driving choroidal perfusion, and possibly by retinal temperature. The former two appear to be mediated by the sympathetic and parasympathetic nervous systems, via central circuits responsive to retinal activity and systemic blood pressure, but adjustments for ocular perfusion pressure also appear to be influenced by local autoregulatory myogenic mechanisms. Adaptive choroidal responses to temperature may be mediated by trigeminal sensory fibers. Impairments in the neural control of choroidal blood flow occur with aging, and various ocular or systemic diseases such as glaucoma, age-related macular degeneration (AMD), hypertension, and diabetes, and may contribute to retinal pathology and dysfunction in these conditions, or in the case of AMD be a precondition. The present manuscript reviews findings in birds and mammals that contribute to the above-summarized understanding of the roles of the autonomic and sensory innervation of the choroid in controlling choroidal blood flow, and in the importance of such regulation for maintaining retinal health.

Type-specific photoreceptor loss in pigeons after disruption of parasympathetic control of choroidal blood flow by the medial subdivision of the nucleus of Edinger-Westphal

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.

Autonomic control of the eye

The autonomic nervous system influences numerous ocular functions. It does this by way of parasympathetic innervation from postganglionic fibers that originate from neurons in the ciliary and pterygopalatine ganglia, and by way of sympathetic innervation from postganglionic fibers that originate from neurons in the superior cervical ganglion. Ciliary ganglion neurons project to the ciliary body and the sphincter pupillae muscle of the iris to control ocular accommodation and pupil constriction, respectively. Superior cervical ganglion neurons project to the dilator pupillae muscle of the iris to control pupil dilation. Ocular blood flow is controlled both via direct autonomic influences on the vasculature of the optic nerve, choroid, ciliary body, and iris, as well as via indirect influences on retinal blood flow. In mammals, this vasculature is innervated by vasodilatory fibers from the pterygopalatine ganglion, and by vasoconstrictive fibers from the superior cervical ganglion. Intraocular pressure is regulated primarily through the balance of aqueous humor formation and outflow. Autonomic regulation of ciliary body blood vessels and the ciliary epithelium is an important determinant of aqueous humor formation; autonomic regulation of the trabecular meshwork and episcleral blood vessels is an important determinant of aqueous humor outflow. These tissues are all innervated by fibers from the pterygopalatine and superior cervical ganglia. In addition to these classical autonomic pathways, trigeminal sensory fibers exert local, intrinsic influences on many of these regions of the eye, as well as on some neurons within the ciliary and pterygopalatine ganglia.

Role of the M3 muscarinic acetylcholine receptor subtype in murine ophthalmic arteries after endothelial removal

PURPOSE

We tested the hypothesis that the M3 muscarinic acetylcholine receptor subtype mediates cholinergic responses in murine ophthalmic arteries after endothelial removal.

METHODS

Muscarinic receptor gene expression was determined in ophthalmic arteries with intact and with removed endothelium using real-time PCR. To examine the role of the M3 receptor in mediating vascular responses, ophthalmic arteries from M3 receptor-deficient mice (M3R(-/-)) and respective wild-type controls were studied in vitro. Functional studies were performed in nonpreconstricted arteries with either intact or removed endothelium using video microscopy.

RESULTS

In endothelium-intact ophthalmic arteries, mRNA for all five muscarinic receptor subtypes was detected, but M3 receptor mRNA was most abundant. In endothelium-removed ophthalmic arteries, M1, M2, and M3 receptors displayed similar mRNA expression levels, which were higher than those for M4 and M5 receptors. In functional studies, acetylcholine evoked vasoconstriction in endothelium-removed arteries from wild-type mice that was virtually abolished after incubation with the muscarinic receptor blocker atropine, indicative of the involvement of muscarinic receptors. In concentration-response experiments, acetylcholine and carbachol concentration-dependently constricted endothelium-removed ophthalmic arteries from wild-type mice, but produced only negligible responses in arteries from M3R(-/-) mice. In contrast, acetylcholine concentration-dependently dilated ophthalmic arteries with intact endothelium from wild-type mice, but not from M3R(-/-) mice. Responses to the nitric oxide donor nitroprusside and to KCl did not differ between ophthalmic arteries from wild-type and M3R(-/-) mice, neither in endothelium-intact nor in endothelium-removed arteries.

CONCLUSIONS

These findings provide evidence that in murine ophthalmic arteries the muscarinic M3 receptor subtype mediates cholinergic endothelium-dependent vasodilation and endothelium-independent vasoconstriction.

Identification of the muscarinic acetylcholine receptor subtype mediating cholinergic vasodilation in murine retinal arterioles

PURPOSE

To identify the muscarinic acetylcholine receptor subtype that mediates cholinergic vasodilation in murine retinal arterioles.

METHODS

Muscarinic receptor gene expression was determined in murine retinal arterioles using real-time PCR. To assess the functional relevance of muscarinic receptors for mediating vascular responses, retinal vascular preparations from muscarinic receptor-deficient mice were studied in vitro. Changes in luminal arteriole diameter in response to muscarinic and nonmuscarinic vasoactive substances were measured by video microscopy.

RESULTS

Only mRNA for the M(3) receptor was detected in retinal arterioles. Thus, M(3) receptor-deficient mice (M3R(-/-)) and respective wild-type controls were used for functional studies. Acetylcholine concentration-dependently dilated retinal arterioles from wild-type mice. In contrast, vasodilation to acetylcholine was almost completely abolished in retinal arterioles from M3R(-/-) mice, whereas responses to the nitric oxide (NO) donor nitroprusside were retained. Carbachol, an acetylcholinesterase-resistant analog of acetylcholine, also evoked dilation in retinal arterioles from wild-type, but not from M3R(-/-), mice. Vasodilation responses from wild-type mice to acetylcholine were negligible after incubation with the non-subtype-selective muscarinic receptor blocker atropine or the NO synthase inhibitor N(ω)-nitro-L-arginine methyl ester, and were even reversed to contraction after endothelial damage with 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate.

CONCLUSIONS

These findings provide evidence that endothelial M(3) receptors mediate cholinergic vasodilation in murine retinal arterioles via activation of NO synthase.

Age-related impairment in choroidal blood flow compensation for arterial blood pressure fluctuation in pigeons

PURPOSE

Choroidal vessels compensate for changes in systemic blood pressure (BP) so that choroidal blood flow (ChBF) remains stable over a BP range of approximately 40 mm Hg above and below basal. Because of the presumed importance of ChBF regulation for maintenance of retinal health, we investigated if ChBF compensation for BP fluctuation in pigeons fails with age.

METHODS

Transcleral laser Doppler flowmetry was used to measure ChBF during spontaneous BP fluctuation in anesthetized pigeons ranging in age from 0.5 to 17 years (pigeons can live approximately 20 years in captivity).

RESULTS

ChBF in <8-year-old pigeons remained near 100% of basal ChBF at BPs ranging 40 mm Hg above and below basal BP (95 mm Hg). Baroregulation failed below approximately 50 mm Hg BP. In ≥8-year-old pigeons, ChBF compensation was absent at >90 mm Hg BP, with ChBF linearly following BP. Over the 60 to 90 mm Hg range, ChBF in ≥8-year-old pigeons was maintained at 60-70% of young basal ChBF. Below approximately 55 mm Hg, baroregulation again followed BP linearly.

CONCLUSIONS

Age-related ChBF baroregulatory impairment occurs in pigeons, with ChBF linear with above-basal BP, and ChBF failing to adequately maintain ChBF during below-basal BP. Defective autonomic sympathetic and parasympathetic neurogenic control, or defective myogenic control, may cause these baroregulatory defects. In either case, overperfusion during high BP may cause oxidative injury to the outer retina, whereas underperfusion during low BP may result in deficient nutrient supply and waste removal, with both abnormalities contributing to age-related retinal pathology and vision loss.

Role of M1, M3, and M5 muscarinic acetylcholine receptors in cholinergic dilation of small arteries studied with gene-targeted mice

Acetylcholine regulates perfusion of numerous organs via changes in local blood flow involving muscarinic receptor-induced release of vasorelaxing agents from the endothelium. The purpose of the present study was to determine the role of M₁, M₃, and M₅ muscarinic acetylcholine receptors in vasodilation of small arteries using gene-targeted mice deficient in either of the three receptor subtypes (M1R(-/-), M3R(-/-), or M5R(-/-) mice, respectively). Muscarinic receptor gene expression was determined in murine cutaneous, skeletal muscle, and renal interlobar arteries using real-time PCR. Moreover, respective arteries from M1R(-/-), M3R(-/-), M5R(-/-), and wild-type mice were isolated, cannulated with micropipettes, and pressurized. Luminal diameter was measured using video microscopy. mRNA for all five muscarinic receptor subtypes was detected in all three vascular preparations from wild-type mice. However, M(3) receptor mRNA was found to be most abundant. Acetylcholine produced dose-dependent dilation in all three vascular preparations from M1R(-/-), M5R(-/-), and wild-type mice. In contrast, cholinergic dilation was virtually abolished in arteries from M3R(-/-) mice. Deletion of either M₁, M₃, or M₅ receptor genes did not affect responses to nonmuscarinic vasodilators, such as substance P and nitroprusside. These findings provide the first direct evidence that M₃ receptors mediate cholinergic vasodilation in cutaneous, skeletal muscle, and renal interlobar arteries. In contrast, neither M₁ nor M₅ receptors appear to be involved in cholinergic responses of the three vascular preparations tested.

The complex interaction between ocular perfusion pressure and ocular blood flow - relevance for glaucoma

Glaucoma is an optic neuropathy of unknown origin. The most important risk factor for the disease is an increased intraocular pressure (IOP). Reducing IOP is associated with reduced progression in glaucoma. Several recent large scale trials have indicated that low ocular perfusion pressure (OPP) is a risk factor for the incidence, prevalence and progression of the disease. This is a strong indicator that vascular factors are involved in the pathogenesis of the disease, a hypothesis that was formulated 150 years ago. The relation between OPP and blood flow to the posterior pole of the eye is, however, complex, because of a phenomenon called autoregulation. Autoregulatory processes attempt to keep blood flow constant despite changes in OPP. Although autoregulation has been observed in many experiments in the ocular vasculature the mechanisms underlying the vasodilator and vasoconstrictor responses in face of changes in OPP remain largely unknown. There is, however, recent evidence that the human choroid regulates its blood flow better during changes in blood pressure induced by isometric exercise than during changes in IOP induced by a suction cup. This may have consequences for our understanding of glaucoma, because it indicates that blood flow regulation is strongly dependent not only on OPP, but also on the level of IOP itself. Indeed there is data indicating that reduction of IOP by pharmacological intervention improves optic nerve head blood flow regulation independently of an ocular vasodilator effect.

Choroidal blood flow compensation in rats for arterial blood pressure decreases is neuronal nitric oxide-dependent but compensation for arterial blood pressure increases is not

Choroidal blood flow (ChBF) compensates for changes in arterial blood pressure (ABP) and thereby remains relatively stable within a +/-40 mmHg range of basal ABP in rabbits, humans and pigeons. In the present study, we investigated if ChBF can compensate for increases and decreases in ABP in rats. ChBF was continuously monitored using laser Doppler flowmetry in anesthetized rats, and ABP measured via the femoral artery. At multiple intervals over a 2-4 h period during which ABP varied freely, ChBF and ABP were sampled and the results compiled across rats. We found that ChBF remained near baseline over an ABP range from 40 mmHg above basal ABP (90-100 mmHg) to 40 mmHg below basal ABP, but largely followed ABP linearly below 60 mmHg. Choroidal vascular resistance increased linearly as BP increased above 100 mmHg, and decreased linearly as BP declined from basal to 60 mmHg, but resistance declined no further below 60 mmHg. Inhibition of nitric oxide (NO) formation by either a selective inhibitor of neuronal nitric oxide synthase (NOS) (N(omega)-propyl-L-arginine) or a nonselective inhibitor of both neuronal NOS and endothelial NOS (N(omega)-nitro-l-arginine methyl ester) did not affect compensation above 100 mmHg ABP, but did cause ChBF to linearly follow declines in BP below 90 mmHg. In NOS-inhibited rats, vascular resistance increased linearly with BP above 100 mmHg, but remained at baseline below 90 mmHg. These findings reveal that ChBF in rats, as in rabbits, humans and pigeons, compensates for rises and/or declines in arterial blood pressure so as to remain relatively stable within a physiological range of ABPs. The ChBF compensation for low ABP in rats is dependent on choroidal vasodilation caused by neuronal NO formation but not the compensation for elevated BP, implicating parasympathetic nervous system vasodilation in the ChBF compensation to low ABP.

Cholinergic responses of ophthalmic arteries in M3 and M5 muscarinic acetylcholine receptor knockout mice

PURPOSE

To determine the functional role of M(3) and M(5) muscarinic acetylcholine receptor subtypes in ophthalmic arteries using gene-targeted mice.

METHODS

Muscarinic receptor gene expression was quantified in murine ophthalmic arteries using real-time PCR. To test the functional relevance of M(3) and M(5) receptors, ophthalmic arteries from mice deficient in either subtype (M3R(-/-), M5R(-/-), respectively) and wild-type controls were isolated, cannulated with micropipettes, and pressurized. Changes in luminal vessel diameter in response to muscarinic and nonmuscarinic receptor agonists were measured by video microscopy.

RESULTS

With the use of real-time PCR, all five muscarinic receptor subtypes were detected in ophthalmic arteries. However, mRNA levels of M(1), M(3), and M(5) receptors were higher than those of M(2) and M(4) receptors. In functional studies, after preconstriction with phenylephrine, acetylcholine and carbachol produced concentration-dependent dilations of ophthalmic arteries that were similar in M5R(-/-) and wild-type mice. Strikingly, cholinergic dilation of ophthalmic arteries was almost completely abolished in M3R(-/-) mice. Deletion of either M(3) or M(5) receptor did not affect responses to nonmuscarinic vasodilators such as bradykinin or nitroprusside.

CONCLUSIONS

These findings provide the first evidence that M(3) receptors are critically involved in cholinergic regulation of diameter in murine ophthalmic arteries.

Sustained upregulation of glial fibrillary acidic protein in Müller cells in pigeon retina following disruption of the parasympathetic control of choroidal blood flow

Choroidal blood flow in pigeon eyes is light driven and controlled by a parasympathetic input from ciliary ganglion (CG) neurons that receive input from the medial subdivision of the ipsilateral nucleus of Edinger-Westphal (EWM). EWM lesions diminish basal ChBF and irreversibly prevent ipsilateral light-evoked increases in ChBF, presumably rendering the retina mildly ischemic. To characterize the location, severity, and time course of the retinal abnormality caused by an EWM lesion, we quantitatively analyzed the cellular and regional extent of Müller cell glial fibrillary acidic protein (GFAP) immunolabeling up to nearly a year after an EWM lesion. We found that unilateral EWM lesions greatly increased Müller cell GFAP throughout the entire retinal depth and topographic extent of the affected eye, up to nearly a year post lesion. By contrast, destruction of the pupilloconstrictive pretectum or of the pupilloconstrictive part of lateral EW (EWL) did not appreciably increase Müller cell GFAP. Thus, the large increase in Müller cell GFAP following an EW lesion is attributable to an ongoing defect in choroidal vasodilatory function rather than to chronic pupil dilation. The Müller cell GFAP increase was greater ipsilateral than contralateral to the EWM destruction for the retinal territory deep to the heavily CG-innervated superior and temporal choroid, but not for the retinal territory deep to the poorly CG-innervated inferior and nasal choroid. The GFAP increase was light-dependent, since it did not occur in EW-lesioned birds housed in dim illumination. Our results show that the chronic vascular insufficiency caused by the loss of the EWM-mediated parasympathetic control of choroidal blood flow leads to a significant and sustained increase in retinal Müller cell GFAP. This increase could be a sign of a disturbance in retinal homeostasis that eventually leads to retinal injury and impaired visual function.

Phylogenesis of constitutively formed nitric oxide in non-mammals

It is widely recognized that nitric oxide (NO) in mammalian tissues is produced from L-arginine via catalysis by NO synthase (NOS) isoforms such as neuronal NOS (nNOS) and endothelial NOS (eNOS) that are constitutively expressed mainly in the central and peripheral nervous system and vascular endothelial cells, respectively. This review concentrates only on these constitutive NOS (cNOS) isoforms while excluding information about iNOS, which is induced mainly in macrophages upon stimulation by cytokines and polysaccharides. The NO signaling pathway plays a crucial role in the functional regulation of mammalian tissues and organs. Evidence has also been accumulated for the role of NO in invertebrates and non-mammalian vertebrates. Expression of nNOS in the brain and peripheral nervous system is widely determined by staining with NADPH (reduced nicotinamide adenine dinucleotide phosphate) diaphorase or NOS immunoreactivity, and functional roles of NO formed by nNOS are evidenced in the early phylogenetic stages (invertebrates and fishes). On the other hand, the endothelium mainly produces vasodilating prostanoids rather than NO or does not liberate endothelium-derived relaxing factor (EDRF) (fishes), and the ability of endothelial cells to liberate NO is observed later in phylogenetic stages (amphibians). This review article summarizes various types of interesting information obtained from lower organisms (invertebrates, fishes, amphibians, reptiles, and birds) about the properties and distribution of nNOS and eNOS and also the roles of NO produced by the cNOS as an important intercellular signaling molecule.

Anatomical and functional evidence for progressive age-related decline in parasympathetic control of choroidal blood flow in pigeons

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 anatomical and functional methods to determine if ChBF control by the CG shows age-related decline in pigeons. The efficacy of the choroidal vasodilatory response to activation of the CG preganglionic input from the medial subdivision of the nucleus of Edinger-Westphal (EWM) was assessed using laser Doppler flowmetry (LDF). The EWM receives bisynaptic retinal input, and electrical stimulation of EWM or light stimulation of the retina in young animals produces dramatic choroidal vasodilation. Transcleral LDF was therefore used to measure both basal ChBF and the increases in ChBF elicited by electrical stimulation of EWM or by retinal illumination in 0.5-18 year old pigeons. Fixed cryostat sections of the eye from 0.5 to 22 year old pigeons were immunolabeled for the 3A10 neurofilament-associated antigen to determine if intrachoroidal nerve fibers arising from CG exhibited age-related loss. We focused on superior choroid, since it is the primary target for CG nerve fibers. There was a marked age-related loss in the ChBF vasodilatory response elicited by either EWM stimulation or retinal illumination, as was also true for basal ChBF. A progressive decrease in choroidal nerve fibers of CG origin, to 17% of youthful abundance by 22 years of age, was also observed. The evoked ChBF increase, and basal ChBF, achieved 50% of their age-related decline between the ages of 3 and 4 years, while half the loss in CG innervation of choroid was later, occurring by 10 years. Age-related loss of choroidal nerve fibers occurs in parallel with but more slowly than the reduction in basal ChBF and the choroidal vasodilation that can be elicited via natural (light) or electrical activation of the central neural input to CG choroidal neurons. The prominent age-related decline in parasympathetic control of ChBF early in the pigeon life span could contribute to the age-related retinal decline observed in pigeons.

Pupillographic evaluation of the time course of atropine effects in the mouse eye

PURPOSE

The nonselective muscarinic antagonist atropine is currently the most potent drug against myopia development in both humans and animal models. However, the mechanism by which myopia is suppressed is still unknown, and the time course of its action is not well documented. Therefore, we have studied the duration of mydriasis in the mouse, a new model of myopia, after topical application of a single eye drop with different doses of atropine.

METHODS

The light-induced pupil response of the C57BL/6 (B6) wildtype strain was studied in alert mice that were restrained by grasping their necks. A video image-processing program detected the pupil and measured its diameter at 25 Hz sampling rate. To stimulate, an arrangement of green LEDs, which was attached to the recording video camera, could be flashed for 40 ms by pressing a key on the keyboard. A single drop of atropine solution (1, 0.5, or 0.1%) was instilled in one eye and the recovery of the pupil responses was followed for at least 150 h. Both eyes were measured.

RESULTS

1) Under the defined stimulation conditions, untreated wildtype mice displayed a pupil constriction of 23.7 +/- 2.4%. 2) All doses of atropine caused complete suppression of the pupil responses in the treated eyes within 1 min. 3) The pupil responses of the fellow eyes remained unaffected and were not different from those in untreated animals. 4) The recovery from mydriasis was very slow and did not show clear differences with dose. The extrapolated duration of complete recovery was about 10 d (0.1%: 217 h; 0.5%: 230 h; 1%: 294 h).

CONCLUSIONS

Atropine caused a longlasting suppression of the pupil responses in the mouse eye. That the duration of recovery was not obviously dose-dependent suggests that all doses used in this study were saturating the receptors in the iris musculature.

Choroidal blood flow in pigeons compensates for decreases in arterial blood pressure

While it had once been thought that choroidal blood flow (ChBF) does not compensate for changes in perfusion pressure, recent studies have shown that ChBF in rabbits and humans does compensate for changes in arterial blood pressure (ABP) and thereby remains relatively stable within a physiological range of ABPs. In this study, we sought to determine if ChBF in birds can compensate for decreases in ABP, either spontaneously occuring or caused by blood withdrawal. ChBF was continuously monitored using laser Doppler flowmetry in anesthetized pigeons, and at the same time ABP was measured via the brachial artery. In studies of spontaneous fluctuation in ABP, ChBF and ABP were analyzed at regular intervals over a 2-3 hr period, while for blood withdrawal studies, blood was transiently withdrawn via the brachial artery. In both paradigms, ChBF remained near baseline over an ABP range from basal (about 90 mmHg) to about 55 mmHg, followed ABP nearly linearly below 50 mmHg, and showed no compensation below 40 mmHg. The blood withdrawal studies further showed that the compensation was more rapid with small acute declines in ABP than with larger declines. These findings reveal that ChBF in pigeons, as in rabbits and humans, compensates for declines in ABP so as to remain relatively stable within a physiological range of ABPs. Given the phylogenetic distance between humans and rabbits on one hand and birds on the other, these results suggest that choroidal compensation for ABP declines may be a common ocular mechanism among warm-blooded vertebrates.

Temporal relationship of choroidal blood flow and thickness changes during recovery from form deprivation myopia in chicks

When form deprived, young chicks rapidly develop axial myopia, from which they recover if the treatment is ceased at a sufficiently early age. The increased axial growth of the eye is accompanied by choroidal thinning and decreased choroidal blood flow (ChBF). In contrast, during the early part of the recovery process, the choroid thickens, shifting the retina towards the new plane of focus. Little information is available about ChBF during recovery from myopia. Because of the possibility that choroidal thickening during recovery from myopia might be driven by an increase in ChBF, the temporal relationship of ChBF and choroidal thickness changes was examined during such recovery. White Leghorn chicks were form deprived from 3 days of age for 2-3 weeks using detachable plastic diffusers. Axial ocular dimensions, including choroidal thickness, were then measured by high frequency A-scan ultrasonography at various times after the diffusers were removed up to 240 hr. ChBF was measured transclerally immediately following the A-scan ultrasonography, using laser Doppler flowmetry. In the chicks measured immediately after diffuser removal, the vitreous chamber was 29.9% longer, the choroid was 6.4% thinner and ChBF was 13.7% less in the treated than in the non-treated control eyes. These changes are characteristic of myopic chick eyes and are reversible in young eyes. Thus, in chicks examined 7 hr after diffuser removal, the ChBF in recovering eyes was now greater than that in control eyes. This ChBF increase peaked about 19 hr after the diffusers were removed. The mean increase in ChBF in treated eyes for the 7-30 hr monitoring period was 187%, relative to control eyes. ChBF in the treated eyes gradually returned to the control level after this time. By contrast to the early, transient increase in ChBF, significant choroidal thickening was not observed in treated eyes until 30 hr after diffuser removal, and continued to increase relative to control eyes over the remainder of the monitoring period, reaching a final mean value of 182%. This study demonstrates, in chick eyes recovering from form deprivation myopia, large increases in ChBF that preceded increases in choroidal thickness and were also more transient than the latter. These results raise the possibility that the increase in ChBF may trigger or even drive the subsequent onset of choroidal expansion, perhaps by facilitating the filling of the choroidal lymphatic lacunae that are well developed in the avian eye.

Ophidian envenomation strategies and the role of purines

Snake envenomation employs three well integrated strategies: prey immobilization via hypotension, prey immobilization via paralysis, and prey digestion. Purines (adenosine, guanosine and inosine) evidently play a central role in the envenomation strategies of most advanced snakes. Purines constitute the perfect multifunctional toxins, participating simultaneously in all three envenomation strategies. Because they are endogenous regulatory compounds in all vertebrates, it is impossible for any prey organism to develop resistance to them. Purine generation from endogenous precursors in the prey explains the presence of many hitherto unexplained enzyme activities in snake venoms: 5'-nucleotidase, endonucleases (including ribonuclease), phosphodiesterase, ATPase, ADPase, phosphomonoesterase, and NADase. Phospholipases A(2), cytotoxins, myotoxins, and heparinase also participate in purine liberation, in addition to their better known functions. Adenosine contributes to prey immobilization by activation of neuronal adenosine A(1) receptors, suppressing acetylcholine release from motor neurons and excitatory neurotransmitters from central sites. It also exacerbates venom-induced hypotension by activating A(2) receptors in the vasculature. Adenosine and inosine both activate mast cell A(3) receptors, liberating vasoactive substances and increasing vascular permeability. Guanosine probably contributes to hypotension, by augmenting vascular endothelial cGMP levels via an unknown mechanism. Novel functions are suggested for toxins that act upon blood coagulation factors, including nitric oxide production, using the prey's carboxypeptidases. Leucine aminopeptidase may link venom hemorrhagic metalloproteases and endogenous chymotrypsin-like proteases with venom L-amino acid oxidase (LAO), accelerating the latter. The primary function of LAO is probably to promote prey hypotension by activating soluble guanylate cyclase in the presence of superoxide dismutase. LAO's apoptotic activity, too slow to be relevant to prey capture, is undoubtedly secondary and probably serves principally a digestive function. It is concluded that the principal function of L-type Ca(2+) channel antagonists and muscarinic toxins, in Dendroaspis venoms, and acetylcholinesterase in other elapid venoms, is to promote hypotension. Venom dipeptidyl peptidase IV-like enzymes probably also contribute to hypotension by destroying vasoconstrictive peptides such as Peptide YY, neuropeptide Y and substance P. Purines apparently bind to other toxins which then serve as molecular chaperones to deposit the bound purines at specific subsets of purine receptors. The assignment of pharmacological activities such as transient neurotransmitter suppression, histamine release and antinociception, to a variety of proteinaceous toxins, is probably erroneous. Such effects are probably due instead to purines bound to these toxins, and/or to free venom purines.

Major role for neuronal NO synthase in curtailing choroidal blood flow autoregulation in newborn pig

We examined whether nitric oxide (NO) generated from neuronal NO synthase (nNOS) contributes to the reduced ability of the newborn to autoregulate retinal blood flow (RBF) and choroidal blood flow (ChBF) during acute rises in perfusion pressure. In newborn pigs (1-2 days old), RBF (measured by microsphere) is autoregulated over a narrow range of perfusion pressure, whereas ChBF is not autoregulated. N(G)-nitro-L-arginine methyl ester (L-NAME) or specific nNOS inhibitors 7-nitroindazole, 3-bromo-7-nitroindazole, and 1-(2-trifluoromethyl-phenyl)imidazole as well as ganglionic blocker hexamethonium, unveiled a ChBF autoregulation as observed in juvenile (4- to 6-wk old) animals, whereas autoregulation of RBF in the newborn was only enhanced by L-NAME. All NOS inhibitors and hexamethonium prevented the hypertension-induced increase in NO mediator cGMP in the choroid. nNOS mRNA expression and activity were three- to fourfold higher in the choroid of newborn pigs than in tissues of juvenile pigs. It is concluded that increased production of NO from nNOS curtails ChBF autoregulation in the newborn and suggests a role for the autonomic nervous system in this important hemodynamic function, whereas, for RBF autoregulation, endothelial NOS seems to exert a more important contribution in limiting autoregulation.