Ciliary ganglion choline acetyltransferase activity in avian macrophthalmos

New loci for refractive errors and ocular biometric parameters in young Chinese Han adults

Myopia has become a major public health issue with an increasing prevalence. There are still individuals who experience similar environmental risk factors and, yet, remain non-myopic. Thus, there might be genetic factors protecting people from myopia. Considering the opposite ocular characteristics of primary angle closure glaucoma (PACG) to myopia and possible common pathway between them, we propose that certain risk genes for PACG might act as a protective factor for myopia. In this study, 2,678 young adults were genotyped for 37 targeted single nucleotide polymorphisms. Compared with emmetropia, rs1401999 (allele C: OR=0.795, P=0.03; genotype in dominant model: OR=0.759, P=0.02) and rs1258267 (allele A: OR=0.824, P=0.03; genotype in dominant model: OR=0.603, P=0.01) were associated with low to moderate myopia and high myopia, respectively. Genotype under recessive model of rs11024102 was correlated with myopia (OR=1.456, P=0.01), low to moderate myopia (OR=1.443, P=0.02) and high myopia (OR=1.453, P=0.02). However, these associations did not survive Bonferroni correction. Moreover, rs1401999, rs1258267, and rs11024102 showed associations with certain ocular biometric parameters in different groups. Our study suggests that ABCC5, CHAT and PLEKHA7 might be associated with refractive errors by contributing to the regulation of ocular biometry, in terms of uncorrected results and their biological functions.

Pharmacology of myopia and potential role for intrinsic retinal circadian rhythms

Despite the high prevalence and public health impact of refractive errors, the mechanisms responsible for ametropias are poorly understood. Much evidence now supports the concept that the retina is central to the mechanism(s) regulating emmetropization and underlying refractive errors. Using a variety of pharmacologic methods and well-defined experimental eye growth models in laboratory animals, many retinal neurotransmitters and neuromodulators have been implicated in this process. Nonetheless, an accepted framework for understanding the molecular and/or cellular pathways that govern postnatal eye development is lacking. Here, we review two extensively studied signaling pathways whose general roles in refractive development are supported by both experimental and clinical data: acetylcholine signaling through muscarinic and/or nicotinic acetylcholine receptors and retinal dopamine pharmacology. The muscarinic acetylcholine receptor antagonist atropine was first studied as an anti-myopia drug some two centuries ago, and much subsequent work has continued to connect muscarinic receptors to eye growth regulation. Recent research implicates a potential role of nicotinic acetylcholine receptors; and the refractive effects in population surveys of passive exposure to cigarette smoke, of which nicotine is a constituent, support clinical relevance. Reviewed here, many puzzling results inhibit formulating a mechanistic framework that explains acetylcholine's role in refractive development. How cholinergic receptor mechanisms might be used to develop acceptable approaches to normalize refractive development remains a challenge. Retinal dopamine signaling not only has a putative role in refractive development, its upregulation by light comprises an important component of the retinal clock network and contributes to the regulation of retinal circadian physiology. During postnatal development, the ocular dimensions undergo circadian and/or diurnal fluctuations in magnitude; these rhythms shift in eyes developing experimental ametropia. Long-standing clinical ideas about myopia in particular have postulated a role for ambient lighting, although molecular or cellular mechanisms for these speculations have remained obscure. Experimental myopia induced by the wearing of a concave spectacle lens alters the retinal expression of a significant proportion of intrinsic circadian clock genes, as well as genes encoding a melatonin receptor and the photopigment melanopsin. Together this evidence suggests a hypothesis that the retinal clock and intrinsic retinal circadian rhythms may be fundamental to the mechanism(s) regulating refractive development, and that disruptions in circadian signals may produce refractive errors. Here we review the potential role of biological rhythms in refractive development. While much future research is needed, this hypothesis could unify many of the disparate clinical and laboratory observations addressing the pathogenesis of refractive errors.

Image defocus and altered retinal gene expression in chick: clues to the pathogenesis of ametropia

PURPOSE

Because of the retina's role in refractive development, this study was conducted to analyze the retinal transcriptome in chicks wearing a spectacle lens, a well-established means of inducing refractive errors, to identify gene expression alterations and to develop novel mechanistic hypotheses about refractive development.

METHODS

One-week-old white Leghorn chicks wore a unilateral spectacle lens of +15 or -15 D for 6 hours or 3 days. With total RNA from the retina/(retinal pigment epithelium, RPE), chicken gene microarrays were used to compare gene expression levels between lens-wearing and contralateral control eyes (n = 6 chicks for each condition). Normalized microarray signal intensities were evaluated by analysis of variance, using a false discovery rate of <10% as the statistical criterion. Selected differentially expressed genes were validated by qPCR.

RESULTS

Very few retina/RPE transcripts were differentially expressed after plus lens wear. In contrast, approximately 1300 transcripts were differentially expressed under each of the minus lens conditions, with minimal overlap. For each condition, low fold-changes typified the altered transcriptome. Differentially regulated genes under the minus lens conditions included many potentially informative signaling molecules and genes whose protein products have roles in intrinsic retinal circadian rhythms.

CONCLUSIONS

Plus or minus lens wear induce markedly different, not opposite, alterations in retina/RPE gene expression. The initial retinal responses to defocus are quite different from those when the eye growth patterns are well established, suggesting that different mechanisms govern the initiation and persistence or progression of refractive errors. The gene lists identify promising signaling candidates and regulatory pathways for future study, including a potential role for circadian rhythms in refractive development.

Extrasynaptic alpha 7-nicotinic acetylcholine receptor expression in developing neurons is regulated by inputs, targets, and activity

Alpha7-nicotinic acetylcholine receptors (nAChRs) are widely expressed in the vertebrate nervous system. alpha7-nAChR functions include postsynaptic transmission, modulating neurotransmitter release, reinforcing nicotine addiction, and a role in neurological disorders, such as schizophrenia and Alzheimer's disease. In chick parasympathetic ciliary ganglion (CG) neurons, alpha7-nAChRs are excluded from the synapse and localize perisynaptically. Despite their extrasynaptic distribution, the highly Ca2+-permeable alpha7-nAChRs have important synapse-related Ca2+-dependent signaling functions in the CG. We show here that the synaptic partners regulate alpha7-nAChR expression during synapse formation in embryonic CG neurons in situ. The absence of inputs and target tissues cause reductions in alpha7-nAChR mRNA and protein levels that primarily resemble those seen for synaptic alpha3-nAChRs. However, there is a difference in their regulation. alpha7-nAChR levels are downregulated by reduced activity, whereas alpha3-nAChR levels are not. We propose that the activity-dependent regulation of extrasynaptic alpha7-nAChR levels may be an important mechanism for postsynaptic CG neurons to detect changes in presynaptic activity levels and respond with Ca2+-dependent plasticity changes in gene expression.

Muscarinic receptor protein expression in the ocular tissues of the chick during normal and myopic eye development

Muscarinic receptor signalling has been implicated in both the embryonic and postnatal development of ocular structures as well as in myopic eye growth. A radioligand binding assay was used to determine whether changes in muscarinic receptor density and/or affinity occurred in the chick retina, choroid and sclera during early post-hatching development or with the induction of myopia. Specific receptor binding sites were saturable with increasing concentrations of the muscarinic receptor ligand [3H]N-methyl-scopolamine in the retina and choroid but not in the sclera. In normal eyes, binding density in the retina was not altered from age P5 to P10 (447+/-14 vs. 411+/-13 fmol/mg of protein, P=0.07). However, in the choroid, the number of receptor binding sites significantly increased between P5 and P10 (637+/-39 vs. 1125+/-121 fmol/mg of protein, P<0.01). Binding affinity (K(D)) was not altered with age in either the retina or choroid. Myopia was induced in chicks by deprivation of form vision, using translucent diffusers, from age P3. Despite the induction of significant degrees of ocular elongation and myopia at P5 (-8.7+/-0.3 D, P<0.01) and P10 (-22.5+/-1.3 D, P<0.01), neither muscarinic receptor density nor affinity were altered in the retina or choroid of myopic eyes. These findings indicate that regulation of muscarinic receptor numbers in the chick choroid is occurring in normal post-hatching development of this tissue. However, myopic eye enlargement was not associated with changes in muscarinic receptor protein expression in the chick retina and choroid.

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.

Autonomic denervations influence ocular dimensions and intraocular pressure in chicks

Choroidal thickness and axial eye length in the chick undergo day/night fluctuations that can also be modulated by visual experience. In the present study, we tested the effect of parasympathetic and sympathetic denervations on both day/night and image dependent changes in ocular dimensions. We also correlated such changes with fluctuations in intraocular pressure. Parasympathectomy influenced choroidal thickness and its day/night fluctuation, but had no effect on vision dependent choroidal thickness modulation. Parasympathectomy also influenced-to a lesser extent-axial length and reduced the axial growth response to form vision deprivation. Sympathectomy had little effect on ocular dimensions, but reduced the day/night differences in intraocular pressure. We conclude that (a) the parasympathetic nervous system influences both choroidal thickness and axial length and participates in the neural control mechanism leading to form deprivation myopia and, (b) the day/night fluctuations of choroidal thickness and axial length are unlikely to be explained by fluctuations in intraocular pressure. For the regulation of choroidal thickness, we hypothesize the existence of two independent mechanisms. One involves the parasympathetic nervous system; it influences the day/night choroidal thickness fluctuation. The other uses a separate pathway and is driven by visual input.

Induction of axial eye elongation and myopic refractive shift in one-year-old chickens

Depriving the eyes of neonatal animals of form vision induces axial eye elongation and ipsilateral myopia. We studied one-year-old chickens, an age at which full body growth has been attained, to learn if form deprivation myopia can develop at a later stage. We found that ocular reactivity to visual form deprivation continues in one-year-old chickens; but both the growth stimulation and the myopic shift in refraction are attenuated compared with newly hatched birds. Neurochemical changes in visually deprived eyes of one-year-old chickens parallel those in newly hatched chicks: ipsilateral decreases in retinal dopamine and in the activity of ciliary ganglion and uveal choline acetyltransferase. These findings strengthen the relevance of the form deprivation model to more common human myopia and suggest a common eye growth control mechanism at both ages.

Cholinergic amacrine cells are not required for the progression and atropine-mediated suppression of form-deprivation myopia

Muscarinic cholinergic pathways have been implicated in the visual control of ocular growth. However, the source(s) of acetylcholine and the tissue(s) which regulate ocular growth via muscarinic acetylcholine receptors (mAChRs) remain unknown. We sought to determine whether retinal sources of acetylcholine and mAChRs contribute to visually guided ocular growth in the chick. Cholinergic amacrine cells were ablated by intraocular injections of either ethylcholine mustard aziridinium ion (ECMA; a selective cholinotoxin) or quisqualic acid (QA; an excitotoxin that destroys many amacrine cells, including those that release acetylcholine). Disruption of cholinergic pathways was assessed immunocytochemically with antibodies to the acetylcholine-synthesizing enzyme choline acetyltransferase (ChAT) and three different isoforms of mAChR, and by biochemical assay for ChAT activity. ECMA (25 nmol) destroyed two of the four subtypes of cholinergic amacrine cells and attenuated retinal ChAT activity, but left retinal mAChR-immunoreactivity intact. QA (200 nmol) destroyed the majority of all four subtypes of cholinergic amacrine cells, and ablated most mAChR-immunoreactivity and ChAT activity in the retina. ECMA and QA had no apparent effect on mAChRs or cholinergic fibres in the choroid, only marginally reduced choroidal ChAT activity, and had little effect on ChAT activity in the anterior segment. Toxin-treated eyes remained emmetropic and responded to form-deprivation by growing excessively and becoming myopic. Furthermore, daily intravitreal injection of 40 microg atropine for 6 days into form-deprived toxin-treated eyes completely prevented ocular elongation and myopia. We conclude that neither cholinergic amacrine cells nor mAChRs in the retina are required for visual regulation of ocular growth, and that atropine may exert its growth-suppressing influence by acting upon extraretinal mAChRs, possibly in the choroid, retinal pigmented epithelium, or sclera.

The ciliary ganglion and vitreous cavity shape

PURPOSE

To learn the influence of the ciliary ganglion on the postnatal growth of eyes with unimpaired visual input and of eyes beneath an image diffusing goggle.

METHODS

Newborn chicks received unilateral ciliary ganglionectomy or unilateral sham operation and were reared either with or without a goggle ipsilateral to the surgical procedure. Ocular refractions and ultrasound measurements were made on anesthetized chicks; eyes enucleated postmortem were measured in axial and equatorial dimensions with calipers and studied histologically.

RESULTS

Excessive growth of open eyes in the equatorial dimensions followed ciliary ganglionectomy and became more pronounced as the chicks grew older. There was only a modest increase in axial growth. Ganglionectomy also induced relative hyperopia; lens thinning contributed to this effect and likely was a direct result of disrupted parasympathetic input to the ciliary muscle. Ganglionectomy also slightly increased the thickness of the choroid in the posterior pole but not in more peripheral locations.

CONCLUSION

We conclude that the ciliary ganglion exerts an inhibitory influence on the postnatal growth of open eyes; the main effect is in the equatorial dimension of the vitreous cavity, with a smaller effect on axial length. Ciliary ganglionectomy exerted minimal influence on the development of experimental myopia, known to be induced by the goggle regimen. The amount of equatorial expansion in goggle-induced myopia was greater than after ganglionectomy alone, indicating that other factors besides the ciliary ganglion can influence the equatorial dimension of the vitreous cavity.