Central neural circuits for the light-mediated reflexive control of choroidal blood flow in the pigeon eye: a laser Doppler study

Variability of Vascular Reactivity in the Retina and Choriocapillaris to Oxygen and Carbon Dioxide Using Optical Coherence Tomography Angiography

Purpose

To investigate the regional and layer-specific vascular reactivity of the healthy human retina and choriocapillaris to changes in systemic carbon dioxide or oxygen.

Methods

High-resolution 3 × 3-mm2 optical coherence tomography angiography (OCTA) images were acquired from the central macula, temporal macula, and peripapillary retina while participants were exposed to three gas breathing conditions-room air, 5%CO2, and 100% O2. OCTA from all three regions were extracted and the apparent skeletonized vessel density (VSD) was assessed. The mean flow deficit sizes (MFDSs) of the choriocapillaris were also assessed. Repeated-measures analysis of variance was used to compare the ratio of intrasubject VSD change induced by the gas conditions from baseline in the superficial retinal layer (SRL) and deep retinal layer (DRL) for each retinal region independently, as well as the MFDS of the choriocapillaris. We also compared the vessel reactivity between the retinal capillaries and the choriocapillaris.

Results

The cumulative intrasubject response to the gas conditions differed significantly among regions of the SRL (F(2, 7) = 28.22, P < 0.001), with the temporal macula showing the largest response (15%) compared to the macula (8%) and radial peripapillary capillaries (7%). A similar trend was found in the DRL. The choriocapillaris reactivity was similar between the macula (5.8%) and temporal macula (5.6%). There was also a significant heterogeneity in the layer-specific gas responses, with the DRL showing the largest response (28.2%) and the choriocapillaris showing the smallest response (2.8%).

Conclusions

Capillary reactivity to changes in inhaled O2 and CO2 is spatially heterogeneous across the retina but not choriocapillaris.

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.

Role of the superior salivatory nucleus in parasympathetic control of choroidal blood flow and in maintenance of retinal health

The vasodilatory pterygopalatine ganglion (PPG) innervation of the choroid is under the control of preganglionic input from the superior salivatory nucleus (SSN), the parasympathetic portion of the facial motor nucleus. We sought to confirm that choroidal SSN drives a choroid-wide vasodilation and determine if such control is important for retinal health. To the former end, we found, using transscleral laser Doppler flowmetry, that electrical activation of choroidal SSN significantly increased choroidal blood flow (ChBF), at a variety of choroidal sites that included more posterior as well as more anterior ones. We further found that the increases in ChBF were significantly reduced by inhibition of neuronal nitric oxide synthase (nNOS), thus implicating nitrergic PPG terminals in the SSN-elicited ChBF increases. To evaluate the role of parasympathetic control of ChBF in maintaining retinal health, some rats received unilateral lesions of SSN, and were evaluated functionally and histologically. In eyes ipsilateral to choroidal SSN destruction, we found that the flash-evoked scotopic electroretinogram a-wave and b-wave peak amplitudes were both significantly reduced by 10 weeks post lesion. Choroidal baroregulation was evaluated in some of these rats, and found to be impaired in the low systemic arterial blood pressure (ABP) range where vasodilation normally serves to maintain stable ChBF. In retina ipsilateral to SSN destruction, the abundance of Müller cell processes immunolabeled for glial fibrillary acidic protein (GFAP) and GFAP message were significantly upregulated. Our studies indicate that the SSN-PPG circuit mediates parasympathetic vasodilation of choroid, which appears to contribute to ChBF baroregulation during low ABP. Our results further indicate that impairment in this adaptive mechanism results in retinal dysfunction and pathology within months of the ChBF disturbance, indicating its importance for retinal health.

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.

Seasonal Variation in the Incidence of Malignant Glaucoma after Cataract Surgery

Purpose:

To investigate a potential link between the incidence of malignant glaucoma after cataract surgery and seasonal variations in daylight.

Methods:

In total, 18,374 uncomplicated cataract surgeries were performed between June 2008 and June 2013 at an ambulatory surgery center in Toronto. Toronto's average monthly daylight over that time period—in hours per day for each month—was determined. The number of malignant glaucoma cases that developed after cataract surgery performed in months with above average daylight was compared to the number of cases that developed after cataract surgery performed in months with below average daylight. Fisher's exact test was used to analyze the relationship between the development of malignant glaucoma and variation in daylight during the month of cataract surgery.

Results:

Malignant glaucoma developed in 16 eyes. Thirteen cases of malignant glaucoma developed in months with above average daylight and three cases developed in months with below average daylight (P = 0.01). Eyes that developed malignant glaucoma in months with more daylight were slightly longer (21.95 ± 1.23 mm) than those that developed malignant glaucoma in months with less daylight (21.55 ± 0.88 mm).

Conclusion:

Light-induced choroidal expansion may play a major role in the development of malignant glaucoma following cataract surgery.

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.

Melanopsin Phototransduction Contributes to Light-Evoked Choroidal Expansion and Rod L-Type Calcium Channel Function In Vivo

Purpose

In humans, rodents, and pigeons, the dark → light transition signals nonretinal brain tissue to increase choroidal thickness, a major control element of choroidal blood flow, and thus of photoreceptor and retinal pigment epithelium function. However, it is unclear which photopigments in the retina relay the light signal to the brain. Here, we test the hypothesis that melanopsin (Opn4)-regulated phototransduction modulates light-evoked choroidal thickness expansion in mice.

Methods

Two-month-old C57Bl/6 wild-type (B6), 4- to 5-month-old C57Bl/6/129S6 wild-type (B6 + S6), and 2-month-old melanopsin knockout (Opn4^−/−) on a B6 + S6 background were studied. Retinal anatomy was evaluated in vivo by optical coherence tomography and MRI. Choroidal thickness in dark and light were measured by diffusion-weighted MRI. Rod cell L-type calcium channel (LTCC) function in dark and light (manganese-enhanced MRI [MEMRI]) was also measured.

Results

Opn4^−/− mice did not show the light-evoked expansion of choroidal thickness observed in B6 and B6 + S6 controls. Additionally, Opn4^−/− mice had lower than normal rod cell and inner retinal LTCC function in the dark but not in the light. These deficits were not due to structural abnormalities because retinal laminar architecture and thickness, and choroidal thickness in the Opn4^−/− mice were similar to controls.

Conclusions

First time evidence is provided that melanopsin phototransduction contributes to dark → light control of murine choroidal thickness. The data also highlight a contribution in vivo of melanopsin phototransduction to rod cell and inner retinal depolarization in the dark.

Stimulation of Baroresponsive Parts of the Nucleus of the Solitary Tract Produces Nitric Oxide-mediated Choroidal Vasodilation in Rat Eye

Preganglionic parasympathetic neurons of the ventromedial part of the superior salivatory nucleus (SSN) mediate vasodilation of orbital and choroidal blood vessels, via their projection to the nitrergic pterygopalatine ganglion (PPG) neurons that innervate these vessels. We recently showed that the baroresponsive part of the nucleus of the solitary tract (NTS) innervates choroidal control parasympathetic preganglionic neurons of SSN in rats. As this projection provides a means by which blood pressure (BP) signals may modulate choroidal blood flow (ChBF), we investigated if activation of baroresponsive NTS evokes ChBF increases in rat eye, using Laser Doppler Flowmetry (LDF) to measure ChBF transclerally. We found that electrical activation of ipsilateral baroresponsive NTS and its efferent fiber pathway to choroidal SSN increased mean ChBF by about 40-80% above baseline, depending on current level. The ChBF responses obtained with stimulation of baroresponsive NTS were driven by increases in both choroidal blood volume (ChBVol; i.e., vasodilation) and choroidal blood velocity (ChBVel; possibly due to orbital vessel dilation). Stimulation of baroresponsive NTS, by contrast, yielded no significant mean increases in systemic arterial blood pressure (ABP). We further found that the increases in ChBF with NTS stimulation were significantly reduced by administration of the neuronal nitric oxide (NO) synthase inhibitor Nω-propyl-l-arginine (NPA), thus implicating nitrergic PPG terminals in the NTS-elicited ChBF increases. Our results show that the NTS neurons projecting to choroidal SSN do mediate increase in ChBF, and thus suggest a role of baroresponsive NTS in the BP-dependent regulation of ChBF.

Disinhibition of neurons of the nucleus of solitary tract that project to the superior salivatory nucleus causes choroidal vasodilation: Implications for mechanisms underlying choroidal baroregulation

Preganglionic neurons in the superior salivatory nucleus (SSN) that mediate parasympathetic vasodilation of choroidal blood vessels receive a major excitatory input from the baroresponsive part of the nucleus of the solitary tract (NTS). This input appears likely to mediate choroidal vasodilation during systemic hypotension, which prevents decreases in choroidal blood flow (ChBF) due to reduced perfusion pressure. It is uncertain, however, how low blood pressure signals to NTS from the aortic depressor nerve (ADN), which fires at a low rate during systemic hypotension, could yield increased firing in the NTS output to SSN. The simplest hypothesis is that SSN-projecting NTS neurons are under the inhibitory control of ADN-receptive GABAergic NTS neurons. As part of evaluating this hypothesis, we assessed if SSN-projecting NTS neurons, in fact, receive prominent inhibitory input and if blocking GABAergic modulation of them increases ChBF. We found that SSN-projecting NTS neuronal perikarya identified by retrograde labeling are densely coated with GABAergic terminals, but lightly coated with excitatory terminals. We also found that, infusion of the GABA-A receptor antagonist GABAzine into NTS increased ChBF. Our results are consistent with the possibility that low blood pressure signals from the ADN produce vasodilation in choroid by causing diminished activity in ADN-receptive NTS neurons that tonically suppress SSN-projecting NTS neurons.

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.

MRI of Retinal Free Radical Production With Laminar Resolution In Vivo

Purpose

Recent studies have suggested the hypothesis that quench-assisted 1/T1 magnetic resonance imaging (MRI) measures free radical production with laminar resolution in vivo without the need of a contrast agent. Here, we test this hypothesis further by examining the spatial and detection sensitivity of quench-assisted 1/T1 MRI to strain, age, or retinal cell layer-specific genetic manipulations.

Methods

We studied: adult wild-type mice; mice at postnatal day 7 (P7); cre dependent retinal pigment epithelium (RPE)-specific MnSOD knockout mice; doxycycline-treated Sod2^flox/flox mice lacking the cre transgene; and α-transducin knockout (Gnat1^−/−) mice on a C57Bl/6 background. Transretinal 1/T1 profiles were mapped in vivo in the dark without or with antioxidant treatment, or followed by light exposure. We calibrated profiles spatially using optical coherence tomography.

Results

Dark-adapted RPE-specific MnSOD knockout mice had greater than normal 1/T1 in the RPE and outer nuclear layers that was corrected to wild-type levels by antioxidant treatment. Dark and light Gnat1^−/− mice also had greater than normal outer retinal 1/T1 values. In adult wild-type mice, dark values of 1/T1 in the ellipsoid region and in the outer segment were suppressed by 13 minutes of light. By 29 minutes of light, 1/T1 reduction extended to the outer nuclear layer. Gnat1^−/− mice demonstrated a faster light-evoked suppression of 1/T1 values in the outer retina. In P7 mice, transretinal 1/T1 profiles were the same in dark and light.

Conclusions

Quench-assisted MRI has the laminar resolution and detection sensitivity to evaluate normal and pathologic production of free radicals in vivo.

Measuring In Vivo Free Radical Production by the Outer Retina

PURPOSE

Excessive and continuously produced free radicals in the outer retina are implicated in retinal aging and the pathogenesis of sight-threatening retinopathies, yet measuring outer retinal oxidative stress in vivo remains a challenge. Here, we test the hypothesis that continuously produced paramagnetic free radicals from the outer retina can be measured in vivo using high-resolution (22-μm axial resolution) 1/T1magnetic resonance imaging (MRI) without and with a confirmatory quench (quench-assisted MRI).

METHODS

Low-dose sodium iodate-treated and diabetic C57Bl6/J mice (and their controls), and rod-dominated (129S6) or cone-only R91W;Nrl-/- mice were studied. In dark-adapted groups, 1/T1 was mapped transretinally in vivo without or with (1) the antioxidant combination of methylene blue (MB) and α-lipoic acid (LPA), or (2) light exposure; in subgroups, retinal superoxide production was measured ex vivo (lucigenin).

RESULTS

In the sodium iodate model, retinal superoxide production and outer retina-specific 1/T1 values were both significantly greater than normal and corrected to baseline with MB+LPA therapy. Nondiabetic mice at two ages and 1.2-month diabetic mice (before the appearance of oxidative stress) had similar transretinal 1/T1 profiles. By 2.3 months of diabetes, only outer retinal 1/T1 values were significantly greater than normal and were corrected to baseline with MB+LPA therapy. In mice with healthy photoreceptors, a light quench caused 1/T1 of rods, but not cones, to significantly decrease from their values in the dark.

CONCLUSIONS

Quench-assisted MRI is a feasible method for noninvasively measuring normal and pathologic production of free radicals in photoreceptors/RPE in vivo.

Design of a dynamic dual-foveated imaging system

A new kind of dynamic dual-foveated imaging system in the infrared band is designed and optimized in this paper. Dual-foveated imaging refers to the variation in spatial resolution at the two selected fields across the image. Such variable resolution imaging system is suitable for a variety of applications including monitoring, recognition, and remote operation of unmanned aerial vehicle. In this system, a transmissive spatial light modulator (SLM) is used as an active optical element which is located near the image plane instead of pupil plane creatively in order to divide the two selected fields.

MRI of rod cell compartment-specific function in disease and treatment in vivo

Rod cell oxidative stress is a major pathogenic factor in retinal disease, such as diabetic retinopathy (DR) and retinitis pigmentosa (RP). Personalized, non-destructive, and targeted treatment for these diseases remains elusive since current imaging methods cannot analytically measure treatment efficacy against rod cell compartment-specific oxidative stress in vivo. Over the last decade, novel MRI-based approaches that address this technology gap have been developed. This review summarizes progress in the development of MRI since 2006 that enables earlier evaluation of the impact of disease on rod cell compartment-specific function and the efficacy of anti-oxidant treatment than is currently possible with other methods. Most of the new assays of rod cell compartment-specific function are based on endogenous contrast mechanisms, and this is expected to facilitate their translation into patients with DR and RP, and other oxidative stress-based retinal diseases.

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.

The identification and neurochemical characterization of central neurons that target parasympathetic preganglionic neurons involved in the regulation of choroidal blood flow in the rat eye using pseudorabies virus, immunolabeling and conventional pathway tracing methods

The choroidal blood vessels of the eye provide the main vascular support to the outer retina. These blood vessels are under parasympathetic vasodilatory control via input from the pterygopalatine ganglion (PPG), which in turn receives its preganglionic input from the superior salivatory nucleus (SSN) of the hindbrain. The present study characterized the central neurons projecting to the SSN neurons innervating choroidal PPG neurons, using pathway tracing and immunolabeling. In the initial set of studies, minute injections of the Bartha strain of the retrograde transneuronal tracer pseudorabies virus (PRV) were made into choroid in rats in which the superior cervical ganglia had been excised (to prevent labeling of sympathetic circuitry). Diverse neuronal populations beyond the choroidal part of ipsilateral SSN showed transneuronal labeling, which notably included the parvocellular part of the paraventricular nucleus of the hypothalamus (PVN), the periaqueductal gray, the raphe magnus (RaM), the B3 region of the pons, A5, the nucleus of the solitary tract (NTS), the rostral ventrolateral medulla (RVLM), and the intermediate reticular nucleus of the medulla. The PRV+ neurons were located in the parts of these cell groups that are responsive to systemic blood pressure signals and involved in systemic blood pressure regulation by the sympathetic nervous system. In a second set of studies using PRV labeling, conventional pathway tracing, and immunolabeling, we found that PVN neurons projecting to SSN tended to be oxytocinergic and glutamatergic, RaM neurons projecting to SSN were serotonergic, and NTS neurons projecting to SSN were glutamatergic. Our results suggest that blood pressure and volume signals that drive sympathetic constriction of the systemic vasculature may also drive parasympathetic vasodilation of the choroidal vasculature, and may thereby contribute to choroidal baroregulation during low blood pressure.

Oxidative stress and light-evoked responses of the posterior segment in a mouse model of diabetic retinopathy

PURPOSE

To test the hypothesis that in a mouse model of diabetic retinopathy, oxidative stress is linked with impaired light-evoked expansion of choroidal thickness and subretinal space (SRS).

METHODS

We examined nondiabetic mice (wild-type, wt) with and without administration of manganese, nondiabetic mice deficient in rod phototransduction (transducin alpha knockout; GNAT1(-/-)), and diabetic mice (untreated or treated with the antioxidant α-lipoic acid [LPA]). Magnetic resonance imaging (MRI) was used to measure light-evoked increases in choroidal thickness and the apparent diffusion coefficient (ADC) at 88% to 100% depth into the retina (i.e., the SRS layer).

RESULTS

Choroidal thickness values were similar (P > 0.05) between all untreated nondiabetic dark-adapted groups and increased significantly (P < 0.05) with light; this expansion was subnormal (P < 0.05) in both diabetic groups. Apparent diffusion coefficient values in the SRS layer robustly increased (P < 0.05) in a light duration-dependent manner, and this effect was independent of the presence of Mn(2+). The light-stimulated increase in ADC at the location of the SRS was absent in GNAT1(-/-) and diabetic mice (P > 0.05). In diabetic mice, the light-dependent increase in SRS ADC was significantly (P < 0.05) restored with LPA.

CONCLUSIONS

Apparent diffusion coefficient MRI is a sensitive method for evaluating choroid thickness and its light-evoked expansion together with phototransduction-dependent changes in the SRS layer in mice in vivo. Because ADC MRI exploits an endogenous contrast mechanism, its translational potential is promising; it can also be performed in concert with manganese-enhanced MRI (MEMRI). Our data support a link between diabetes-related oxidative stress and rod, but not choroidal, pathophysiology.

Quantitative retinal and choroidal blood flow during light, dark adaptation and flicker light stimulation in rats using fluorescent microspheres

PURPOSE

The present study aimed to quantify retinal and choroidal blood flow (BF) during light, dark adaptation and flicker light stimulation using the microsphere technique.

MATERIALS AND METHODS

Adult male Sprague-Dawley rats were anesthetized with isoflurane. Eyes were dark (Group I, n = 8), light (Group II, n = 8) adapted or stimulated with 10 Hz flicker light (Group III, n = 10). Retinal and choroidal BF were measured by a previously established method, using a mixture of 8 µm yellow-green and 10 µm red fluorescent microspheres. The microspheres were counted ex vivo in the dissected retina and choroid and in the reference arterial blood under a fluorescent microscope.

RESULTS

The choroidal BF was 64.8 ± 29 µl/min (mean ± SD) during dark adaptation, not significantly different from that during light adaptation (66.0 ± 17.8 µl/min). The retinal BF was 13.5 ± 3.2 µl/min during 10 Hz flickering light stimulation, significantly higher than that during dark adaptation in the control fellow eyes (9.9 ± 2.9 µl/min). The choroidal BF values were not statistically different between flicker stimulation and dark adaptation. Retinal BF was 11.6 ± 2.9 µl/min during light adaptation. Dark adaptation did not increase retinal BF (Group I, 8.2 ± 2.4 µl/min; Group II, 9.9 ± 2.9 µl/min).

CONCLUSIONS

These findings argue against a dark-induced or flicker-induced functional hyperemia in the choroid as a result of the demands of the outer retina. Retinal BF was not higher during dark adaptation. Our data support the conclusion that the inner retina has a higher energy demand in flicker conditions relative to dark.

Parasympathetic influences on emmetropization in chicks: evidence for different mechanisms in form deprivation vs negative lens-induced myopia

Ciliary ganglionectomy inhibits the development of myopia in chicks (Schmid et al., 1999), but has no effect on the compensatory responses to spectacle lenses (Schmid and Wildsoet, 1996). This study was done to assess the potential influence of the other parasympathetic input to the choroid, the pterygopalatine ganglia, on the choroidal and axial responses to retinal defocus, and to form deprivation. 4-5 week-old chicks had one of the following surgeries to one eye: (1) Section (X) of the autonomic part of cranial N VII (input to the pterygopalatine ganglia) (PPGX, n = 16), (2) PPGX plus ciliary ganglionectomy (PPG/CGX, n = 23) or (3) PPGX plus superior cervical ganglionectomy (PPG/SCGX, n = 10). Experimental eyes were fitted with positive or negative lenses, or diffusers, several days after surgery. In one group of PPG/CGX, eyes did not wear any devices (n = 8). Intact (no surgery) controls were done for all visual manipulations (lenses or diffusers). Sham surgeries were done for the PPG/CGX condition (n = 4). Ocular dimensions were measured using A-scan ultrasonography prior to the surgery, 5 days later when visual devices were placed on the eyes, at the end of lens- or diffuser-wear, and in the case of diffusers, 4 days after diffuser removal to look at "recovery". Refractive errors were measured using a Hartinger's refractometer. IOP was measured in 7 PPG/CGX birds 7d after surgery. PPGX/CGX resulted in choroidal thickening (125 μm) and a decrease in IOP over one week post-surgery. It also prevented the development of myopia in response to form deprivation (X vs intact: 0.2 D vs -4.1 D; p < 0.005), by preventing the increase in axial elongation (250 μm vs 670 μm/5d; p < 0.005). In fact, growth rate slowed below normal (X vs fellow eyes: 250 μm vs 489 μm/5d; p = 0.002). By contrast, there were no effects of this lesion on the development of myopia in response to negative lenses (X vs intact: -5.4 D vs -5.3 D). All three lesions inhibited the compensatory choroidal thickening in response to myopic defocus (ANOVA, p = 0.0008), but had no effect on the thinning response to hyperopic defocus. These results argue for different underlying mechanisms for the growth responses to form deprivation vs negative lens wear. They also imply that choroidal thickening and thinning are not opposing elements of a single mechanism.

Light modulation, not choroidal vasomotor action, is a regulator of refractive compensation to signed optical blur

BACKGROUND AND PURPOSE

The nitric oxide system has two proposed sites and mechanisms of action within the ocular growth/refractive compensation platform-neuromodulatory effects on retinal physiology, and vascular/smooth muscle effects in the choroid. The relative contribution of these mechanisms are tested here with drugs that perturb the nitric oxide system and with slow flicker modulation of the ON and OFF pathways of the retina.

EXPERIMENTAL APPROACH

Intravitreal injection of saline or 900 nmol N(G) -nitro-L-arginine methyl ester or L-arginine in saline was followed by monocular defocus with ±10 D lens (or no lens), from days 5-9 under standard diurnal (SD) or daytime 1 Hz ramped flicker conditions. Biometric, electrophysiological and histological analyses were conducted.

KEY RESULTS

After 4 days of SD conditions, both drugs enhanced electroretinogram (ERG) b-wave cf. d-wave amplitudes compared with saline and reduced refractive compensation to -10 D lenses. Under flicker conditions compensation to +10 D lenses was suppressed. Choroidal thinning was observed in the drug, no lens groups under SD conditions, whereas choroidal thickening was seen in most groups under flicker conditions, irrespective of refractive outcomes.

CONCLUSIONS AND IMPLICATIONS

As choroidal thickness was not predictive of final refractive compensation across any of the variables of drug, defocus sign or light condition, it is unlikely that choroidal thickness is a primary mechanism underlying refractive compensation across the range of parameters of this study. Rather, the changes in refractive compensation observed under these particular drug and light conditions are more likely due to a neuromodulatory action on retinal ON and OFF pathways.

Light-induced trigeminal sensitization without central visual pathways: another mechanism for photophobia

PURPOSE

The authors investigated whether trigeminal sensitization occurs in response to bright light with the retina disconnected from the rest of the central nervous system by optic nerve section.

METHODS

In urethane-anesthetized rats, trigeminal reflex blinks were evoked with air puff stimuli directed at the cornea in darkness and at three different light intensities. After normative data were collected, the optic nerve was lesioned and the rats were retested. In an alert rat, reflex blinks were evoked by stimulation of the supraorbital branch of the trigeminal nerve in the dark and in the light.

RESULTS

A 9.1 × 10(3) μW/cm(2) and a 15.1 × 10(3) μW/cm(2) light significantly enhanced the magnitude of reflex blinks relative to blinks evoked by the same trigeminal stimulus when the rats were in the dark. In addition, rats exhibited a significant increase in spontaneous blinking in the light relative to the blink rate in darkness. After lesioning of the optic nerve, the 15.1 × 10(3) μW/cm(2) light still significantly increased the magnitude of trigeminal reflex blinks.

CONCLUSIONS

Bright lights increase trigeminal reflex blink amplitude and the rate of spontaneous blinking in rodents. Light can modify trigeminal activity without involving the central visual system.

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.

The Edinger-Westphal nucleus: a historical, structural, and functional perspective on a dichotomous terminology

The eponymous term nucleus of Edinger-Westphal (EW) has come to be used to describe two juxtaposed and somewhat intermingled cell groups of the midbrain that differ dramatically in their connectivity and neurochemistry. On one hand, the classically defined EW is the part of the oculomotor complex that is the source of the parasympathetic preganglionic motoneuron input to the ciliary ganglion (CG), through which it controls pupil constriction and lens accommodation. On the other hand, EW is applied to a population of centrally projecting neurons involved in sympathetic, consumptive, and stress-related functions. This terminology problem arose because the name EW has historically been applied to the most prominent cell collection above or between the somatic oculomotor nuclei (III), an assumption based on the known location of the preganglionic motoneurons in monkeys. However, in many mammals, the nucleus designated as EW is not made up of cholinergic, preganglionic motoneurons supplying the CG and instead contains neurons using peptides, such as urocortin 1, with diverse central projections. As a result, the literature has become increasingly confusing. To resolve this problem, we suggest that the term EW be supplemented with terminology based on connectivity. Specifically, we recommend that 1) the cholinergic, preganglionic neurons supplying the CG be termed the Edinger-Westphal preganglionic (EWpg) population and 2) the centrally projecting, peptidergic neurons be termed the Edinger-Westphal centrally projecting (EWcp) population. The history of this nomenclature problem and the rationale for our solutions are discussed in this review.

Immunohistochemical localization of AMPA-type glutamate receptor subunits in the nucleus of the Edinger-Westphal in embryonic chick

The Edinger-Westphal nucleus (EW) in birds is responsible for the control of pupil constriction, accommodation, and choroidal blood flow. The activation of EW neurons is mediated by the neurotransmitter glutamate, in large part through AMPA-type glutamate receptors (GluRs), whose behavior varies according to the subunit composition. We investigated the developmental expression of the GluR subunits in EW of the chick (Gallus gallus) using immunohistochemistry on tissue from embryonic days 10 through 20 (E10-E20). Of the three antibodies used, one recognized the GluR1 subunit, another the GluR4 subunit, and the third recognized a sequence common to GluR2 and GluR3 subunits. No immunolabeling of EW neurons for any GluR subunits was observed prior to E12, although immunolabeling was seen in somatic oculomotor prior to E12. At E12, immunoreactivity for each of the three antibodies was in only approximately 2% of EW neurons. By E14, the abundance of GluR1+ perikarya in EW had increased to 13%, and for GluR2/3 had increased to 48%. The perikaryal abundance of the immunoreactivity for GluR1 and GluR2/3 declined to 3% and 23%, respectively, by E16. At E14, 33% of EW neurons immunolabeled for GluR4, and their frequency increased to 43% by E16, and remained at that approximate percentage through hatching. The increased expression of GluR1 and GluR4 in EW at E14 coincides with the reported onset of the expression of the calcium-binding protein parvalbumin, and the calcium currents associated with AMPA receptors formed by these two subunits may play a role in the occurrence of parvalbumin expression.

Autonomic control of the eye and the iris

The vertebrate eye receives innervation from ciliary and pterygopalatine parasympathetic and cervical sympathetic ganglia as well as sensory trigeminal axons. The sympathetic and parasympathetic pathways represent the classical "core" of neural regulation of ocular homeostasis. Sensory trigeminal neurons are also involved in autonomic regulation by both providing the afferent limb of various reflexes and exerting their peptide-mediated local effector function. This arrangement is remarkably conserved throughout vertebrate classes although significant modifications are observed in anamniotes, in particular their irises. In higher primates and birds, intrinsic choroidal neurons emerged as a significant additional innervation component. They most likely mediate local vascular regulation and other local homeostatic tasks in foveate eyes. This review across the vertebrate classes outfolds the complex neuronal regulatory underpinnings across vertebrates that ensure proper visual function.

Projections from the hypothalamic paraventricular nucleus and the nucleus of the solitary tract to prechoroidal neurons in the superior salivatory nucleus: Pathways controlling rodent choroidal blood flow

Using intrachoroidal injection of the transneuronal retrograde tracer pseudorabies virus (PRV) in rats, we previously localized preganglionic neurons in the superior salivatory nucleus (SSN) that regulate choroidal blood flow (ChBF) via projections to the pterygopalatine ganglion (PPG). In the present study, we used higher-order transneuronal retrograde labeling following intrachoroidal PRV injection to identify central neuronal cell groups involved in parasympathetic regulation of ChBF via input to the SSN. These prominently included the hypothalamic paraventricular nucleus (PVN) and the nucleus of the solitary tract (NTS), both of which are responsive to systemic BP and are involved in systemic sympathetic vasoconstriction. Conventional pathway tracing methods were then used to determine if the PVN and/or NTS project directly to the choroidal subdivision of the SSN. Following retrograde tracer injection into SSN (biotinylated dextran amine 3K or Fluorogold), labeled perikarya were found in PVN and NTS. Injection of the anterograde tracer, biotinylated dextran amine 10K (BDA10K), into PVN or NTS resulted in densely packed BDA10K+terminals in prechoroidal SSN (as defined by its enrichment in nitric oxide synthase-containing perikarya). Double-label studies showed these inputs ended directly on prechoroidal nitric oxide synthase-containing neurons of SSN. Our study thus establishes that PVN and NTS project directly to the part of SSN involved in parasympathetic vasodilatory control of the choroid via the PPG. These results suggest that control of ChBF may be linked to systemic blood pressure and central control of the systemic vasculature.

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.

The multifunctional choroid

The choroid of the eye is primarily a vascular structure supplying the outer retina. It has several unusual features: It contains large membrane-lined lacunae, which, at least in birds, function as part of the lymphatic drainage of the eye and which can change their volume dramatically, thereby changing the thickness of the choroid as much as four-fold over a few days (much less in primates). It contains non-vascular smooth muscle cells, especially behind the fovea, the contraction of which may thin the choroid, thereby opposing the thickening caused by expansion of the lacunae. It has intrinsic choroidal neurons, also mostly behind the central retina, which may control these muscles and may modulate choroidal blood flow as well. These neurons receive sympathetic, parasympathetic and nitrergic innervation. The choroid has several functions: Its vasculature is the major supply for the outer retina; impairment of the flow of oxygen from choroid to retina may cause Age-Related Macular Degeneration. The choroidal blood flow, which is as great as in any other organ, may also cool and warm the retina. In addition to its vascular functions, the choroid contains secretory cells, probably involved in modulation of vascularization and in growth of the sclera. Finally, the dramatic changes in choroidal thickness move the retina forward and back, bringing the photoreceptors into the plane of focus, a function demonstrated by the thinning of the choroid that occurs when the focal plane is moved back by the wearing of negative lenses, and, conversely, by the thickening that occurs when positive lenses are worn. In addition to focusing the eye, more slowly than accommodation and more quickly than emmetropization, we argue that the choroidal thickness changes also are correlated with changes in the growth of the sclera, and hence of the eye. Because transient increases in choroidal thickness are followed by a prolonged decrease in synthesis of extracellular matrix molecules and a slowing of ocular elongation, and attempts to decouple the choroidal and scleral changes have largely failed, it seems that the thickening of the choroid may be mechanistically linked to the scleral synthesis of macromolecules, and thus may play an important role in the homeostatic control of eye growth, and, consequently, in the etiology of myopia and hyperopia.

Strategies for reducing variance in laser Doppler flowmetry measurements

BACKGROUND

Scattering of blood flow data as assessed with laser Doppler flowmetry (LDF) in humans is a problem in many studies using this technique. We set out to reduce variability in LDF data by eliminating the effect of the total returning light level (DC) on LDF parameters in the choroid through partial regression analysis.

METHODS

In 20 healthy subjects, choroidal blood flow parameters were measured at different DC values using a portable confocal LDF device. We used two different strategies to reduce scattering of data eliminating the effect of yield, which is defined as DC/gain. On the one hand, we used a previously described method based on a third-order polynomial fit, which combines all obtained data. On the other hand, we applied a new method based on a linear fit for each individual subject.

RESULTS

Variability of data during changes in DC is higher for LDF parameters volume and flow than for velocity. Both methods were successful in reducing scattering of LDF parameters with varying DC.

CONCLUSIONS

The present study indicates that both methods to correct for changes in yield were successful in reducing the variability of LDF measurements. When systematic changes in DC occur after an intervention, one needs to be careful in interpreting the obtained data and it remains to be shown if either of the two techniques is capable of correcting for this effect. The approach presented here may, however, represent an effective, easily applicable and valid approach to reduce scattering of data from using LDF to assess blood flow in the posterior pole of the human eye.

Comparison of the distributions of urocortin-containing and cholinergic neurons in the perioculomotor midbrain of the cat and macaque

Urocortin is a novel neurotransmitter that appears to play a role in eating and drinking behavior. Most urocortin-positive (urocortin(+)) neurons in rodents are found in the cytoarchitecturally defined Edinger-Westphal nucleus (EW). However, the EW is traditionally described as the source of the preganglionic parasympathetic outflow to the ciliary ganglion. We examined the distribution of urocortin(+) cells and motoneurons by use of immunohistochemical staining for this peptide and for choline acetyltransferase (ChAT) in macaque monkeys, in which most preganglionic motoneurons inhabit the EW, and in cats, in which most do not. In both species, lack of overt double labeling indicated that the ChAT(+) and urocortin(+) cells are separate populations. In the monkey, most nonoculomotor ChAT(+) neurons were found within the EW. In contrast, urocortin(+) cells were distributed mainly between the oculomotor nuclei and in the supraoculomotor area. In the cat, most nonoculomotor ChAT(+) cells were located in the supraoculomotor area and anteromedian nucleus. Few were present in the cat EW. Instead, this nucleus was filled with urocortin(+) cells. These results highlight the fact the term EW has come to indicate different nuclei in different species. Consequently, we have adopted the identifiers preganglionic (EW(PG)) and urocortin-containing (EW(U)) to designate the cytoarchitecturally defined EW nuclei in monkeys and cats, respectively. Furthermore, we propose a new open-ended nomenclature for the perioculomotor (pIII) cells groups that have distinctive projections and neurochemical signatures. This will allow more effective scientific discourse on the connections and function of groups such as the periculomotor urocortin (pIII(U)) and preganglionic (pIII(PG)) populations.

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.

Experimentally reduced perfusion of one eye impairs retinal function in both eyes

PURPOSE

The oscillatory potential index of scotopic white flash electroretinograms is reversibly enhanced in the contralateral eye when the ocular perfusion pressure (OPP) to the test eye is transiently reduced. A transient increase in the intraocular pressure (IOP) and decrease in the OPP in the test eye induced quantifiable vascular changes in the optic nerve head of the contralateral eye. We explored this contralateral phenomenon looking at ganglion cell function in both eyes during elevated IOP and decreased OPP in the test eye only. Our specific objective was to characterize the effects that transient hypoperfusion had on the neural generators of the pattern-reversal electroretinograms (pERGs), the ganglion cells, and preganglion neurons.

METHODS

A transient elevation in the IOP was sustained in 10 healthy subjects by scleral suction to reduce the baseline OPP by 15, 30, 45, and 60% for 2-min intervals. For each level of OPP, pERGs were evoked by a checkerboard with 75 minarc high-contrast black-white checks reversing at 5 Hz and recorded bilaterally using DTL fiber electrodes. The pERGs were also recorded immediately after removal of scleral suction and at 2-min intervals thereafter for an 8-min recovery interval.

RESULTS

The unilateral decrease in OPP differentially reduced the pERG in the test and contralateral eyes. The pERG for the test eye returned to baseline amplitude within 2 min of removing the suction cup. In contrast, the pERG in the contralateral eye remained below baseline throughout the entire 8-min recovery interval.

CONCLUSIONS

The observation of a bilateral decrease in the pERGs while the OPP was decreased in the test eye only suggested that these neuronal changes were modified at more central visual centers for retinal function to be compromised bilaterally. This latter effect may have been mediated by the transiently altered OPP or yet unknown neurohormonal mechanisms.

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.

Blue flicker modifies the subfoveal choroidal blood flow in the human eye

The objective of the present study was to reveal an interaction between choroidal blood flow (ChBF) and light-induced photoreceptor activity, a physiological coupling that has been already demonstrated for retinal blood flow but rejected for ChBF. Ten healthy adults volunteered for this study. A real-time recording near-infrared laser-Doppler flowmeter was used to quantify the subfoveal ChBF while the luminance of blue flicker between 1 and 64 Hz was first increased and then decreased by 4.0 log units in 1.0-log unit steps between 0.0375 and 375 cd/m2. In separate testing, flash electroretinograms (ERGs) provided electrophysiological indexes of the relative response of short-wave cones (s-cones) and rods to blue light stimulation. Group-averaged, normalized ChBF measurements revealed that it was modulated by approximately 9% by flicker frequency. Increasing the blue flicker luminance from low to high attenuated the subfoveal ChBF, volume, and velocity by approximately 32%, approximately 30%, and approximately 5%, respectively. Decreasing the luminance from high to low over the same range had no effect on the subfoveal choroidal hemodynamics. The markedly different effects of reversed directions of change in blue flicker luminance on the subfoveal ChBF were linked to transitions between rod-dominated and s-cone-dominated retinal responses. Collectively, these findings indicate that the blue light-induced photoreceptor response is associated with a differential distribution of the ChBF across the ocular fundus according to the degree and type of retinal photoreceptor stimulated.

Evidence that urocortin is absent from neurons of the Edinger-Westphal nucleus in pigeons

The Edinger-Westphal nucleus (EWN) is a central preganglionic parasympathetic cell group that gives rise to cholinergic input to the ciliary ganglion, thereby regulating several neurovegetative ocular functions. Recently, the supposed presence of the neuropeptide urocortin (UCN) has been reported in EWN neurons in rodent brain. The purpose of the present study was to examine the distribution of UCN in avian brain and to investigate by immunohistochemical analysis the possible use of this substance as an EWN marker in a non-mammalian class of vertebrates. Brain tissue of pigeons was incubated with a specific antibody against UCN and the results showed labeling of many small neurons, forming a double wing in the dorsal mesodiencephalic transition area. Their size and shape, however, differed from those of EWN neurons, and they were preferentially located rostral to the EWN. Double-label experiments employing an antibody against the enzyme choline acetyltransferase (ChAT) showed that UCN is not localized to the cholinergic cells of the EWN and confirmed the rostral distributionof UCN never overlapping the ChAT+ EWN cells. Taken together, these results suggest that, at least in pigeons, the UCN+ population does not belong to the traditionally defined EWN.

Changes in choroidal blood flow during light/dark transitions are not altered by atropine or propranolol in healthy subjects

It has recently been reported that light/dark transitions lead to changes in choroidal blood flow. Several observations indicate that these changes in choroidal perfusion are triggered at least in part by neural mechanisms. In the present study we hypothesised that the choroidal blood flow response to changes in retinal illumination may be modified by either the muscarinic receptor antagonist atropine or by the beta-receptor antagonist propranolol. In 15 healthy subjects the response of choroidal perfusion was studied in a randomised placebo-controlled three way cross-over study using laser Doppler flowmetry and laser interferometric measurement of fundus pulsation amplitude. Before drug administration a transition from light to dark reduced both choroidal haemodynamic parameters by 8%-12%. Neither propranolol nor atropine altered basal choroidal blood flow or choroidal blood flow responses to light/dark transitions. Our data indicate that neither muscarinic nor beta-receptors are involved in the choroidal blood flow response to changes in retinal illumination. Further studies are required to elucidate which mechanisms contribute to this blood flow behaviour of the choroid.

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.

Interspecific differences in the expression of the AMPA-type glutamate receptors and parvalbumin in the nucleus of Edinger-Westphal of chicks and pigeons

The distribution of AMPA-type glutamate receptor (GluR) subunits was studied in the Edinger-Westphal nucleus (EW) of chicks and pigeons. GluR1, GluR2, GluR3 and GluR4 subunits appeared to be present in EW neurons of both species, but interspecific differences were observed in the abundance of the different types of subunits found in EW neurons. Of particular note, GluR2 immunoreactivity was present in the vast majority (ca. 80%) of neurons of pigeon EW but was found in only a small fraction (ca. 15%) of chick EW neurons. Scarcity of the GluR2 subunit in chick EW was confirmed by in situ hybridization. Because of the tendency for parvalbumin to be localized to neurons that are selectively deficient in GluR2, we also studied the localization of parvalbumin, as well as other calcium-binding proteins, in EW of chick and pigeon. Parvalbumin was found in more than 50% of chick EW neurons but was not detected in pigeon EW neurons. Our results suggest that there are major glutamatergic inputs to EW neurons in both pigeons and chicks. Furthermore, there are likely to be more AMPA-type calcium-permeable glutamate receptors in EW neurons of chick than in pigeon, since it is known that the subtype containing the edited GluR2 subunit is not calcium permeable.

Unilateral light-dark transitions affect choroidal blood flow in both eyes

There is recent evidence that the perfusion of the choroid changes during dark-light transitions. We set out to investigate this response in more detail and to elucidate possible mechanisms involved in this process. For this purpose, the effect of dark-light transitions on choroidal perfusion was studied in healthy subjects. Choroidal blood flow and ocular fundus pulsation amplitude were measured as indices of choroidal perfusion during dark-light transitions using laser Doppler flowmetry and laser interferometry, respectively. In the first experiment, subjects were first kept in room light for 20 min, then light conditions were changed to darkness for 20 min, and thereafter, subjects were exposed to room light again. Both choroidal parameters decreased (-12% to -14%) during darkness but returned to baseline after the final room light period. In the second experiment, the index eye underwent the same procedure, whereas the contralateral eye was kept in light throughout the experiment. Choroidal haemodynamic parameters in the index eye reacted in a way comparable to that seen in the first experiment. The eye that was kept in light also reacted, but the effect tended to be less pronounced than that seen in the index eye (-8% to -10%). The observation that choroidal blood flow in both eyes reacts during unilateral light-dark transitions indicates that choroidal perfusion rate is adapted to retinal illumination conditions by neural control mechanisms.

The role of photoreceptors in the control of refractive state

The current state of research into experimentally induced refractive errors is reviewed. The area is analysed in three components-the transduction of defocus or deprivation, the vector for transmitting the error message from the retina to the outer tunics of the eye, and the identity of the effector for causing growth modulation in the sclera. Anatomical, pharmacological, electrophysiological and optical factors are considered in terms of which elements of the retina are necessary to support a refractive response to deprivation or defocus. Two of the current models are discussed-one emphasizing the role of the choroid in effecting ocular and refractive change, while the second model approaches the problem from the aspect of scleral changes that are associated with growth adaptation without emphasis on the error detection mechanism. A third model is proposed in which the error signal for deprivation or defocus is detected in the outer retina and where error is translated through separate signals for stimulus brightening and darkening into a net signal for fluid flow across and under the active control of the retinal pigment epithelium with the fluid communication between the vitreous chamber and the choroidal lymphatics. The directions of research both fundamental and clinical which are needed to create pharmaceutical or environmental solutions to refractive control are discussed.

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

Activation of the parasympathetic ciliary ganglion input to the choroid causes increases in choroidal blood flow. We examined the role and the type of muscarinic receptors within the choroid that are involved in these increases in choroidal blood flow, using electrical stimulation of the nucleus of Edinger-Westphal (EW) to activate the ciliary ganglion input to choroid in ketamine anesthetized pigeons. Baseline choroidal blood flow and its EW-evoked increases measured as peak and total (area under the curve) responses were determined using laser Doppler flowmetry. The EW-evoked responses were reduced dose-dependently after administration of 4-diphenyl-acetoxy-N-methylpiperedine (4-DAMP), a relatively selective antagonist of M3 type muscarinic receptors, with a maximal mean decrease of 86% (peak response) and 93% (total response) at a dose of 10 microg kg(-1)i.v. without a significant effect on baseline choroidal blood flow, heart rate or systemic arterial blood pressure. Atropine, a non-selective antagonist of muscarinic receptors, decreased the EW-evoked responses to a lesser extent than 4-DAMP after intravenous administration of 1 mg kg(-1)(by 67% for peak response and by 53% for total response) or topical administration of a 5% solution (by 41% for peak response and by 62% for total response), both of which increased heart rate and systemic arterial blood pressure without a consistent effect on baseline choroidal blood flow. In contrast, himbacine (i.p. 10 microg kg(-1)), a relatively selective antagonist of M2 type muscarinic receptors, increased the EW-evoked parasympathetic cholinergic vasodilation (by 93% for the peak response and by 142% for the total response) without a significant effect on heart rate, systemic arterial blood pressure or baseline choroidal blood flow. The results of our study suggest a major role of M3 type muscarinic receptors in the EW-evoked increases in choroidal blood flow. Based on findings that the ciliary ganglion input to choroid does not synthesize nitric oxide but inhibitors of NO production do block EW-evoked choroidal vasodilation, it seems likely that the M3 receptors acted on by 4-DAMP are present on choroidal endothelial cells and mediate choroidal vasodilation via stimulation of endothelial release of nitric oxide. In contrast, M2 muscarinic receptors may play a presynaptic role in downregulating EW-evoked parasympathetic cholinergic vasodilation in avian choroid.

Influence of ophthalmic nerve fibers on choroidal blood flow and myopic eye growth in chicks

Ophthalmic sensory nerve fibers containing substance P and calcitonin gene-related peptide' innervate the choroid in mammals and are known to vasodilate choroidal blood vessels. The avian choroid is also innervated by ophthalmic nerve fibers containing substance P and calcitonin gene-related peptide. The present studies were carried out to determine the influence of these sensory fibers on choroidal blood flow in birds and characterize their interaction with manipulations affecting eye growth. In these studies, ChBF was measured using laser Doppler flowmetry in both eyes in the following groups of birds: (1) normal chicks; (2) chicks with right optic nerve transected for 2 weeks; (3) chicks with right optic nerve transected and a goggle over the right eye for 2 weeks; and (4) chicks with right optic and ophthalmic nerves transected and a goggle over the right eye for 2 weeks. The eyes were refracted and various ocular dimensions measured after the blood-flow measurements. It was found that optic nerve transection reduced ChBF to 30% of normal. Placing a goggle (which increases ocular temperature by 4 degrees C) over an optic nerve transected eye nearly doubled choroidal blood flow over that in an optic nerve transected eye without a goggle. Additional transection of the ophthalmic nerve in a goggled optic nerve-transected eye, yielded choroidal blood flow that was indistinguishable from that in a nongoggled optic nerve-transected eye. Optic nerve transection had a slight stunting effect on axial growth of the eye. While myopic axial elongation was observed in goggled eyes with the optic nerve cut, the extent of myopia was less than in normal goggled eyes. Ophthalmic nerve transection further reduced the myopia induced by goggling in an optic nerve cut eye. These results suggest that ophthalmic nerve input to the choroid exerts a vasodilatory influence, which is activated in a goggled eye. This increased choroidal blood flow may be in response to elevated ocular temperatures caused by the goggling and this increase appears to be masked in goggled eyes with an intact optic nerve by the reduction in choroidal blood flow normally accompanying myopic eye growth. Our results thus show that the induction of myopic eye growth (as in our optic nerve cut eyes with a goggle) need not be accompanied by a decrease in choroidal blood flow from the baseline no-goggle condition (in this case, with the optic nerve cut).

Microglia increase as photoreceptors decrease in the aging avian retina

PURPOSE

There is evidence that microglial activation occurs with normal aging in some regions of the brain of rodents. We investigated the pattern of microglia in the retinas of young and aged quail and pigeons to determine if age-related retinal changes evoked migration of microglia into the outer retina. In quail we also investigated the correlation between activated microglia and age-related photoreceptor loss.

METHODS

Microglia were identified with the monoclonal antibody QH1 in cryosectioned eyes from pigeons, ages 2 to 20 years (n = 14), and in paraffin sections from six-month (n = 15) and one-year-old quail (n = 30). Rounded microglia in the photoreceptor layer were counted in consecutive 400x fields from temporal to nasal. Photoreceptor counts were made from 10 quail retina flat mounts. The photoreceptor number was compared to the number of microglia in corresponding regions of the same retina.

RESULTS

Rounded microglia were detected among the photoreceptors of pigeons and quail. Significantly more of these microglia were found in peripheral than in central regions close to the pecten (pigeon p < 0.002 and quail p < 0.01). Furthermore, more microglial cells were present among peripheral photoreceptors of older quail (p < 0.03) and pigeons (p < 0.05) than in the younger birds. In the peripheral retina of the older quail, microglia were significantly and inversely related to the number of photoreceptors (r2 = 0.9; p < 0.001).

CONCLUSIONS

Increased microglial were observed in the peripheral retina of both old quail and old pigeons. In the quail, the rounded (activated) microglia were distributed preferentially in regions of greatest photoreceptor loss. Microglial activation does not appear to be a general phenomenon of the aging retina, but in quail activation appears directly related to photoreceptor loss. It is unclear at this time how the change in microglia shape and distribution is related to their neuroprotective / neurotoxic potential.

A true neuronal consensual pupillary reflex in chicks

The existence of a true neuronal consensual pupillary reflex (CPR) in birds has long been debated. In this century Noll (Noll, A. (1915). Archiv für Physiologie (Leipzig), 350-372.) claimed to observe a neuronal CPR in a pigeon, but this was contradicted by Levine (Levine, J. (1955). Science, 122, 699.), who observed a direct transillumination effect (Durchleuchtungs-effekt) due to the retinas of the two eyes of the pigeon being in close apposition. To determine if a neuronal CPR exists, we transected the optic nerves of 28 chicks and observed and videotaped the direct and indirect pupillary responses. Twenty-one of the chicks exhibited no direct response in the operated eye but did exhibit an indirect pupillary response. The non-operated eye showed a direct but no indirect response. These results conclusively demonstrate for the first time that a true neuronal CPR does exist in chickens.