Cholinergic systems and multiple cholinergic receptors in ocular tissues

Ocular Cyclopentolate: A Mini Review Concerning Its Benefits and Risks

Cycloplegic and mydriatic agents are essential in ophthalmological clinical practice since they provide the means for diagnosing and treating certain eye conditions. In addition, cyclopentolate has proven to possess certain benefits compared to other available cycloplegics and mydriatics. Still, the incidence of some adverse drug reactions related to this drug, especially in susceptible patients, has created interest in reviewing the literature about the benefits and risks of using cyclopentolate. A literature search was conducted in Medline/PubMed and Google Scholar, focusing on identifying cyclopentolate's benefits and risks; the most important benefit was its usefulness for evaluating refractive errors, especially for hyperopic children, pseudomyopia, anterior uveitis, treatment of childhood myopia, idiopathic vision loss, and during examinations before refractive surgery, with particular advantages compared to other cycloplegics. While the risks were divided into local adverse drug reactions such as burning sensation, photophobia, hyperemia, punctate keratitis, synechiae, and blurred vision, which are relatively frequent but mild and temporary; and systemic adverse drug reactions such as language problems, visual or tactile hallucinations and ataxia, but unlike ocular, systemic adverse drug reactions are rare and occur mainly in patients with risk factors. In addition, six cases of abuse were found. The treatment with cyclopentolate is effective and safe in most cases; nevertheless, special care must be taken due to the potential severe ADRs that may occur, especially in susceptible patients like children, geriatrics, patients with neurological disorders or Down's syndrome, patients with a low blood level of pseudocholinesterase, users of substances with CNS effects, and patients with a history of drug addiction. The recommendations are avoiding the use of 2% cyclopentolate and instead employing solutions with lower concentrations, preferably with another mydriatic such as phenylephrine. Likewise, the occlusion of the nasolacrimal duct after instillation limits the drug's absorption, reducing the risk of systemic adverse events.

Comment on Kopańska et al. Disorders of the Cholinergic System in COVID-19 Era-A Review of the Latest Research. Int. J. Mol. Sci. 2022, 23, 672

We read the recent review article by Marta Kopańska et al. [...].

Markers of the sympathetic, parasympathetic and sensory nervous system are altered in the human diabetic choroid

BACKGROUND

We sought to evaluate alterations in markers of the autonomic nervous system in human diabetic choroid.

METHODS

Eighteen eyeballs from subjects with diabetes and 22 eyeballs from subjects without diabetes were evaluated in this study. Synaptophysin, tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DβH), neuronal nitric oxide synthase (nNOS), choline acetyltransferase (ChAT), vesicular monoamine transporter II (VMAT-2), vesicular acetylcholine transporter (VAChT), vasoactive intestinal peptide (VIP), neuropeptide Y (NPY), and calcitonin gene-related peptide (CGRP) levels were detected by western blot analysis and immunofluorescence was performed in some cases. Furthermore, differences in adrenergic (α1- and β2-subtypes) and cholinergic (M1 and M3) receptor levels between diabetic subjects and controls were noted.

RESULTS

Decreased synaptophysin levels were found in diabetic choroids by western blot analysis and a reduction of synaptophysin-immunoreactive nerves was also found by immunofluorescence. Furthermore, a decrease of the levels of the key enzyme (TH) and transporter (VMAT2) of norepinephrine was evident both by western blot analysis and immunofluorescence. Additionally, increased NPY, VAChT, nNOS, and CGRP levels were observed in diabetic choroids. The levels of adrenergic (β2 subtype) and acetylcholine (M1 subtype) receptors decreased in diabetic choroids, as shown by western blotting and although the differences in α1 and M3 were not significant, there was a downward trend.

CONCLUSIONS

In the diabetic choroid, the levels of neurotransmitters, enzymes, and receptors associated with choroidal blood flow regulation are altered. These changes may affect the regulation of choroidal blood flow and may be associated with impaired retinal function and retinal pathology.

Nicotinic and muscarinic acetylcholine receptors shape ganglion cell response properties

The purpose of this study was to evaluate the expression patterns of nicotinic and muscarinic ACh receptors (nAChRs and mAChRs, respectively) in relation to one another and to understand their effects on rabbit retinal ganglion cell response properties. Double-label immunohistochemistry revealed labeled inner-retinal cell bodies and complex patterns of nAChR and mAChR expression in the inner plexiform layer. Specifically, the expression patterns of m1, m4, and m5 muscarinic receptors overlapped with those of non-α7 and α7 nicotinic receptors in presumptive amacrine and ganglion cells. There was no apparent overlap in the expression patterns of m2 muscarinic receptors with α7 nicotinic receptors or of m3 with non-α7 nicotinic receptors. Patch-clamp recordings demonstrated cell type-specific effects of nicotinic and muscarinic receptor blockade. Muscarinic receptor blockade enhanced the center responses of brisk-sustained/G4 On and G4 Off ganglion cells, whereas nicotinic receptor blockade suppressed the center responses of G4 On-cells near the visual streak but enhanced the center responses of nonstreak G4 On-cells. Blockade of muscarinic or nicotinic receptors suppressed the center responses of brisk-sustained Off-cells and the center light responses of subsets of brisk-transient/G11 On- and Off-cells. Only nicotinic blockade affected the center responses of G10 On-cells and G5 Off-cells. These data indicate that physiologically and morphologically identified ganglion cell types have specific patterns of AChR expression. The cholinergic receptor signatures of these cells may have implications for understanding visual defects in disease states that result from decreased ACh availability.

Effect of botulinum toxin A on the intraocular pressure and the retina in an animal model

OBJECTIVE

The purpose of our study was to investigate the effect of an inadvertent intravitreal injection of botulinum toxin A (BTA) on the intraocular pressure (IOP) and the retina in an animal model.

METHODS

BTA was injected intravitreally in normotensive rats. IOP was measured preoperatively as well as 1, 2, and 4 weeks postoperatively. Retinas were stained in vivo using a retrograde labelling technique and the density of retinal ganglion cells (RGCs) was determined. Immunohistochemistry was performed for rhodopsin and retinal glial fibrillary acidic protein (GFAP).

RESULTS

Significant temporary IOP elevation occurred in all groups in the immediate postoperative period (ANOVA, p < 0.05). IOP changes in the intermediate period were not statistically significant (ANOVA, p > 0.05). The differences in the density of RGCs after BTA injection were not statistically significant (ANOVA, p > 0.05). All retinas displayed the same immunostaining pattern for rhodopsin and GFAP.

CONCLUSION

Our findings indicate that BTA has probably no severe impact on IOP and the retina after an inadvertent intravitreal injection. However, temporary rise of IOP may possibly occur in the immediate postoperative period due to a volume-effect.

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

The effects of nicotinic and muscarinic receptor activation on patch-clamped cells in the optic tectum of Rana pipiens

Both nicotinic and muscarinic cholinergic receptors are present in the optic tectum. To begin to understand how the activation of these receptors affects visual activity patterns, we have determined the types of physiological responses induced by their activation. Using tectal brain slices from the leopard frog, we found that application of nicotine (100 microM) evoked long-lasting responses in 60% of patch-clamped tectal cells. Thirty percent of these responses consisted of an increase in spontaneous postsynaptic currents (sPSCs) and had both a glutamatergic and GABAergic component as determined by the use of 6-cyano-7-nitroquinoxaline-2,3-dione (50 microM) and bicuculline (25 microM), respectively. Remaining response types consisted of an inward membrane current (16%) and an increase in sPSCs combined with an inward membrane current (14%). All responses could be elicited in the presence of tetrodotoxin (0.5 microM). Muscarinic receptor-mediated responses, induced by carbachol (100 microM) application after nicotinic receptor desensitization, produced responses in 70% of tectal cells. In contrast to responses elicited by nicotine, carbachol-induced responses could be evoked multiple times without significant decrement. Responses consisted of either an outward current (57%), a decrease in sPSCs (5%) or an increase in sPSCs, with (almost 6%) or without (almost 3%) an outward current. The response elicited by carbachol was not predicted by the response of the cell to nicotine. Our results suggest that nicotinic receptors are found predominantly at presynaptic locations in the optic tectum while muscarinic receptors are most often present at postsynaptic sites. We conclude that both of these receptor types could substantially modulate visual activity by changing either the input to tectal neurons or the level of their response to that input.

Influence of halothane on phospholipase A2 and enzymatic methylations in the rat retinal membranes

Phospholipase A2 (PLA2) and phospholipid methylases (PLM) play significant roles in transmitter release and membrane signal transduction, respectively. Previous studies have indicated that PLMs occur in the rat brain synaptosomal and retinal membranes, and they are activated under halothane anesthesia. The influence of halothane on PLA2 is not known. Therefore, we have investigated the effect of halothane on retinal PLA2 activity. Rat retinal sonicates were assayed for PLA2 activity using 1-palmitoyl-2[1-14C]arachidonyl-phosphatidylethanolamine (PE, 2.2 nmol) in Tris buffer (10 mM, pH 7.4) at 37 degrees C with and without halothane (0.25-2.0 mM) in the assay medium. These studies gave the following results: (1) Rat retinal sonicates contained PLA2 activity of 4.2+/-0.8 pmol PE hydrolyzed/100 ng protein/hr; (2) Halothane (0.25-2.0 mM) increased PLA2 activity by 20 to 150% depending upon concentration; (3) The lower concentration of halothane (0.25 mM) exhibited high activation of PLA2 (150%); (4) High concentrations of halothane (1.0-2.0 mM) caused a low degree of activation of PLA2 (20%); and (5) During phospholipid methylation of retinal membranes with S-adenosyl-L-methionine in the presence of halothane, increased amounts of fatty acid methyl esters (FAME) were formed. This increase in FAME (45%) was possibly due to the hydrolysis of phospholipids by activated PLA2, liberating fatty acids which were methylated. This increase in FAME (45%) was inhibited by mepacrine (quinacrine) (10 microM), an inhibitor of PLA2. These observations suggest that the release of retinal transmitters (dopamine, acetylcholine and others) is affected during halothane anesthesia, due to activation of PLA2 and enhanced fusogenic activity of vesicular membranes with plasma membrane and depletion of vesicles.

Evaluation of the nature of camel retinal acetylcholinesterase: inhibition by hexamethonium

Acetylcholinesterase (AChE, EC 3.1.1.7) has been demonstrated in retinas of several species, however, the nature of the interaction of AChE with specific inhibitors are very limited in the literature and the mode of inhibition of camel retinal AChE by hexamethonium has been studied. Hexamethonium reversibly inhibited AChE in a concentration dependent manner, the IC50 value being c. 2.52 mM. The Km for the hydrolysis of acetylthiocholine iodide was found to be 0.087 mM and the Vmax was 0.63 mumol/min/mg protein. Dixon, as well as Lineweaver-Burk, plots and their secondary replots indicated that the nature of the inhibition is of the hyperbolic (partial) mixed type, which is considered to be a partial competitive and non-competitive mixture. The values of Ki(slope) and KI(intercept) from a Lineweaver-Burk plot were estimated as 0.30 mM and 0.17 mM, respectively, while Ki from a Dixon plot was estimated as 0.725 mM. The Ki was greater than KI indicating that hexamethonium has a greater affinity of binding for the active site than the peripheral site of the camel retina AChE.

Human placental cholinergic system

The occurrence of acetylcholine (ACh)-like activity in human placenta, a tissue without innervation, has been known for more than 60 years. However, the non-neuronal functions of ACh in human placenta are not clearly understood. The components of the cholinergic system-ACh, choline acetyltransferase, acetylcholinesterase, butyrylcholinesterase, muscarinic receptors, and nicotinic receptors--in human placenta have been demonstrated by unequivocal methods. Primate placentae store and release ACh by mechanisms similar to those of nervous tissue. However, there are many gaps in our knowledge, which include: (a) endogenous quaternary ammonium compounds other than ACh in human placental extracts; (b) the specificity of placental enzymes; (c) the subtypes and structures of placental muscarinic and nicotinic receptors; and (d) the significance of placental alpha-bungarotoxin binding proteins, ACh receptor stimulation-cellular signaling by second messengers, and activation of immediate early target genes (C-fos, C-jun) encoding transcription factors. Several hypothetical non-neuronal functions of ACh in placenta have been postulated based upon available experimental evidence. These include: (a) regulation of blood flow and fluid volume in placental vessels; (b) opening and closing of trophoblastic channels; (c) induction of contractile properties to myofibroblasts; (d) facilitation of amino acid transport necessary for fetal growth across placenta; (e) release of placental hormones; and (f) modulation of the formation of myometrial and placental prostaglandins in human parturition. All of these roles are reasonable, and some of these roles mav turn out to be linked to one another to influence or maintain placental function.

Evaluation of the nature of rat retinal acetylcholinesterase using a specific substrate and a specific inhibitor

The occurrence of cholinesterases (ChE) has been demonstrated in retinas of several mammalian species. Using BW284C51 and iso-OMPA as selective inhibitors of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE), respectively, it has been demonstrated that the rat retinal ChE is predominantly AChE. Therefore the kinetic nature of inhibition of the rat retinal AChE by BW284C51 was studied using acetyl-6-methylthiocholine (AMTCh) as a selective substrate of AChE. AChE activity of the rat retinal sonicates was assayed using AMTCh as the substrate in the presence of 5,5-dithiobis-2-nitrobenzoate and yellow 5-thio-2-nitrobenzoic anion was measured by the absorption at 412 millimicrons using a spectrophotometer. The substrate (AMTCh) was varied between 0.1 and 0.5 mM. The inhibitor concentrations used were 2.1 and 4.2 nM. Double-reciprocal plots between substrate concentrations and the velocities for the enzymatic hydrolysis of AMTCh in the presence and absence of inhibitor were constructed. This study gave the following results: BW284C51 was a potent inhibitor of the hydrolysis of AMTCh by rat retinal AChE (IC50, 5.2 nM). The nature of the inhibition was found to be competitive as the double reciprocal plots with and without the inhibitor crossed on the ordinate.

The use of the rat iris as a model system to evaluate the effect of the cholinotoxin, AF64A, in vivo

The iris is innervated by both cholinergic parasympathetic, and adrenergic sympathetic branches of the autonomic nervous system. This innervation represents a simple and anatomically well-defined system to evaluate the effects of chemical compounds on cholinergic and adrenergic neurons. AF64A (acetyl ethylcholine aziridinium) is a known cholinotoxin in the brain and, in these experiments using the iris system, we evaluated its in vivo effect on cholinergic enzyme activity, pupillary size, and catecholamine neurotransmitter levels. We found in this system that AF64A reduces the activity of choline acetyltransferase (ChAT) but not acetylcholinesterase (AChE). AF64A is selective for cholinergic neurons, since norepinephrine and dopamine levels were unaffected.

Effects of organophosphates on the visual system of rats

The possibility that exposure to organophosphate insecticides can lead to ocular damage is suggested by Japanese studies from the 1960s and 1970s indicating that exposed humans developed chronic ocular degeneration, in addition to showing more commonly accepted effects of cholinesterase-inhibiting compounds. Other papers reported ocular lesions in laboratory animals treated with organophosphates. More recently, retinal degeneration following chronic organophosphate treatment has been reported to the Environmental Protection Agency by pesticide manufacturers in studies conducted in compliance with good laboratory practice regulations. Several factors, however, have prompted scepticism regarding organophosphate-induced ocular toxicity, including the widespread use of organophosphate compounds for both agricultural and ophthalmological practices without numerous additional reports of comparable ocular toxicity. We are developing a research program to address these issues involving electrophysiological, biochemical and histological investigations of rats treated with organophosphate insecticides. The research program is young, but early results are available. Notably, retinas from rats treated with a single subcutaneous injection of 100 mg kg-1 fenthion showed decreases in carbachol-stimulated release of inositol phosphate, an indicator of cholinergically-mediated intracellular second messenger systems. These effects persisted at least 56 days after fenthion administration. This could indicate several different toxicological actions, which are currently under investigation. It is concluded that the possible association between exposure to organophosphates and ocular toxicity cannot be dismissed, and that several important research issues need to be resolved.

Nature of cholinesterase in the rat retina

The occurrence of cholinesterases has been demonstrated in retinas of several mammalian species. Histochemical staining techniques indicate that the acetylcholinesterases (AChE) are present in amacrine cells and their neighboring bipolar cells. However, the nature of retinal cholinesterases and their interactions with specific cholinesterase inhibitors are not known. Therefore, we have studied the inhibition of the rat retinal cholinesterase activity by BW284C51, a selective inhibitor of AChE, and iso-OMPA, a selective inhibitor of butyrylcholinesterase (BChE). Retinas from Zivic-Miller rats were solubilized by sonication in phosphate buffer (0.134 M, pH 7.2) at 4 degrees C for 20 min. The cholinesterase activity in the sonicate was determined by a radiometric method using 14C-acetylcholine (ACh) as substrate (10(-2) M). Excess 14C-ACh was adsorbed by Amberlite CG-120 cation exchange resin. 14C-acetate formed and retained in the aqueous medium was determined by liquid scintillation counting. This study gave the following results: (a) Rat retinal sonicate gave total cholinesterase activity of 3.76 mumol of ACh hydrolyzed/mg protein/15 min; (b) This activity was inhibited by BW284C51 (IC50, 0.115 microM). Iso-OMPA (IC50, 500 microM) did not cause significant inhibition at 0.115 microM. These observations suggest that the rat retinal cholinesterase is predominantly AChE.