Effects of norepinephrine and 5-hydroxytryptamine on phosphoinositide-PO4 turnover in rabbit cornea

Adrenoceptors in the Eye - Physiological and Pathophysiological Relevance

The autonomic nervous system plays a crucial role in the innervation of the eye. Consequently, it comes as no surprise that catecholamines and their corresponding receptors have been extensively studied and characterized in numerous ocular structures, including the cornea, conjunctiva, lacrimal gland, trabecular meshwork, uvea, and retina. These investigations have unveiled substantial clinical implications, particularly in the context of treating glaucoma, a progressive neurodegenerative disorder responsible for irreversible vision loss on a global scale. The primary therapeutic approaches for glaucoma frequently involve the modulation of α1-, α2-, and β-adrenoceptors, making them pivotal targets. In this chapter, we offer a comprehensive overview of the expression, distribution, and functional roles of adrenoceptors within various components of the eye and its associated structures. Additionally, we delve into the pivotal role of adrenoceptors in the pathophysiology of glaucoma. Furthermore, we provide a concise historical perspective on adrenoceptor research, examine the distinct contributions of individual adrenoceptor subtypes to the treatment of various ocular conditions, and propose potential future avenues of exploration in this field.

Role of α1-adrenoceptor subtypes on corneal epithelial thickness and cell proliferation in mice

Adrenergic stimuli are important for corneal epithelial structure and healing. The purpose of the present study was to examine the hypothesis that the lack of a single α₁-adrenoceptor (α₁-AR) subtype affects corneal epithelial thickness and cell proliferation. Expression levels of α₁-AR mRNA were determined in mouse cornea using real-time PCR. In mice devoid of one of the three α₁-AR subtypes (α_1A-AR^-/-, α_1B-AR^-/-, α_1D-AR^-/-) and in wild-type controls, thickness of individual corneal layers, the number of epithelial cell layers, and average epithelial cell size were determined in cryosections. Endothelial cell density and morphology were calculated in corneal explants, and epithelial cell proliferation rate was determined with immunofluorescence microscopy. Moreover, the ultrastructure of the corneal epithelium was examined by transmission electron microscopy. Messenger RNA for all three α₁-AR subtypes was expressed in whole cornea and in corneal epithelium from wild-type mice with a rank order of abundance of α_1A ≥ α_1B > α_1D. In contrast, no α₁-AR mRNA was detected in the stroma, and only α_1B-AR mRNA was found in the Descemet endothelial complex. Remarkably, corneal epithelial thickness and mean epithelial cell size were reduced in α_1A-AR^-/- mice. Our findings suggest that the α_1A-AR exerts growth effects in mouse corneal epithelial cells.

Selective serotonin reuptake inhibitors: a review of its effects on intraocular pressure

The increase in serotonin (5-HT) neurotransmission is considered to be one of the most efficacious medical approach to depression and its related disorders. The selective serotonin reuptake inhibitors (SSRIs) represent the most widely antidepressive drugs utilized in the medical treatment of depressed patients. Currently available SSRIs include fluoxetine, sertraline, paroxetine, fluvoxamine, citalopram and escitalopram. The primary SSRIs pharmacological action's mechanism consists in the presynaptic inhibition on the serotonin reuptake, with an increased availability of this amine into the synaptic cleft. Serotonin produces its effects as a consequence of interactions with appropriate receptors. Seven distinct families of 5-HT receptors have been identified (5-HT(1) to 5-HT(7)), and subpopulations have been described for several of these. The interaction between serotonin and post-synaptic receptors mediates a wide range of functions. The SSRIs have a very favorable safety profile, although clinical signs of several unexpected pathologic events are often misdiagnosed, in particular, those regarding the eye. In all cases reported in the literature the angle-closure glaucoma represents the most important SSRIs-related ocular adverse event. Thus, it is not quite hazardous to hypothesize that also the other reported and unspecified visual disturbances could be attributed - at least in some cases - to IOP modifications. The knowledge of SSRIs individual tolerability, angle-closure predisposition and critical IOP could be important goals able to avoid further and more dangerous ocular side effects.

Muscarinic cholinoceptor-stimulated phosphatidyl inositol pathway in corneal epithelial and endothelial cells

BACKGROUND

Muscarinic cholinoceptors are distributed widely in both the central and peripheral nervous system. The presence of muscarinic cholinoceptors in corneal tissue is well established. Previous reports have shown that corneal muscarinic cholinoceptors are of the m2 or m4 subtype. However, recent studies have indicated the presence of the m5 muscarinic cholinoceptor subtype in human corneal epithelium and endothelium. The aim of the study was to confirm the presence of the m5 cholinoceptor subtype in bovine corneal epithelium and endothelium and the activation of phosphatidyl inositol pathway by its stimulation.

METHODS

Muscarinic m5 cholinoceptor sites, phosphatidyl inositol 4,5-biphosphate, inositol 1,4,5-triphosphate and protein kinase C, were studied using immunocytochemistry and immunofluorescence. Activation of protein kinase C after stimulation of the m5 muscarinic cholinoceptor subtype was measured using the HTS protein kinase C assay kit.

RESULTS

Immunocytochemistry/immunofluorescence revealed the presence of the m5 muscarinic cholinoceptor subtype, phosphatidyl inositol 4,5-biphosphate and protein kinase C in bovine corneal epithelial and endothelial cells. In bovine corneal epithelium and endothelium, protein kinase C activity was stimulated by acetylcholine in a dose-dependent manner (P<0.0001).

CONCLUSIONS

Our findings indicate that acetylcholine-induced stimulation of muscarinic m5 cholinoceptors activates the phosphatidyl inositol pathway in corneal epithelial and endothelial cells, resulting in increased protein kinase C activity. Further work will be needed to clear the physiologic role of this signaling pathway in corneal epithelium and endothelium.

Localization of 5-hydroxytryptamine1A and 5-hydroxytryptamine7 receptors in rabbit ocular and brain tissues

Serotonin is thought to play a physiological role in various tissues of the rabbit eye, yet little is known about the relative distribution of the different serotonin receptors. Demonstration of the receptor subtypes present in the various ocular tissues is essential in order to understand the function of serotonin in the eye. Using a combination of in situ hybridization histochemistry, in vitro receptor autoradiography and polymerase chain reaction studies, we have explored the distribution of the 5-hydroxytryptamine1A and 5-hydroxytryptamine7 receptors in the rabbit eye. As these receptors have not been sequenced in the rabbit, we initially established the suitability of the oligonucleotide probes by analysis of brain tissue. The distributions of 5-hydroxytryptamine1A and 5-hydroxytryptamine7 receptor messenger RNAs in rabbit brain correlated well with those in other species, confirming the specificity of the probes for detection of the messenger RNAs in rabbit tissues. In the eye, the expression of 5-hydroxytryptamine1A receptors appears to be restricted to the epithelial cell layer of the ciliary processes, although very low levels may appear in the retina. In contrast, the expression of 5-hydroxytryptamine7 receptor messenger RNA is more widespread with positive signals evident in the ciliary processes, retina and iris. The results confirm the existence of 5-hydroxytryptamine1A receptors in the ciliary body and their localization in the ciliary epithelium supports the hypothesis that they are involved in the secretion of aqueous humour. Unexpectedly, there was little evidence to support the idea that 5-hydroxytryptamine1A receptors are present in the retina and iris sphincter. However, the subsequent finding of 5-hydroxytryptamine7 receptor messenger RNA in the retina and iris may explain the apparent absence of 5-hydroxytryptamine1A receptors in these tissues. The presence of both 5-hydroxytryptamine1A and 5-hydroxytryptamine7 receptors in the ciliary processes may account for the complex intraocular pressure response of the rabbit to serotonin.

Adrenergic receptor-mediated increase of intracellular Ca2+ concentration in isolated bovine corneal epithelial cells

1. We determined if Ca2+ is a second messenger for adrenergic receptor-effector coupling in bovine corneal epithelial cells. 2. Methoxamine (10(-5) M) selectively increased intracellular Ca2+ concentration (Cai) by 65%. This increase was only partially suppressed through the removal of extracellular Ca2+ or pretreatment with 10(-6) M verapamil. 3. The beta-adrenergic-mediated increases in Cai were entirely dependent on extracellular Ca2+. These increases were directly elicited through stimulation of adenylate cyclase because 10(-6) M isoproterenol and the active analogues of forskolin (10(-5) M) all elevated Cai. 4. Therefore, increases in Cai serve a second messenger function for alpha-1 and beta-adrenergic receptor-effector coupling.

Protein kinase C subspecies in rabbit corneal epithelium: increased activity of alpha subspecies during wound healing

Protein kinase C (PKC) has been implicated in cell proliferation and differentiation. Multiple forms of PKC have been isolated, principally from the brain where PKC is most abundant. In rabbit corneal epithelium, two distinct major peaks of PKC activity were resolved by hydroxyapatite column chromatography. Peak 2, with 65% of the total PKC activity, corresponds to alpha-PKC, based on its mobility in the column and Western blot analysis using specific monoclonal antibodies. Peak 1 did not react with either polyclonal or monoclonal antibodies to PKC alpha-, beta-, and gamma-isoforms suggesting the presence of isoforms specific to the corneal epithelium, or of another member of the PKC family. To investigate possible changes in the amounts of the various PKC subspecies during wound healing, the enzyme activities of the isolated subspecies were assayed 2, 5, and 7 days after corneal de-epithelialization. Two days after wounding, by which time the migratory limbal epithelium had covered the denuded area, total PKC activity was unchanged but alpha-PKC activity had increased to 77% of the total activity, compared with 65% in non-wounded epithelium. An increased proportion of alpha-PKC activity was also observed 5 and 7 days after wounding, during which time proliferation of epithelium continued. We hypothesize that alpha-PKC plays a role in long-term responses after injury such as gene expression and corneal epithelial proliferation. Moreover, these studies indicate that the cornea provides a good model of in vivo wound healing for PKC studies.

Phorbol esters inhibit ionomycin-induced hydrolysis of phosphoinositides and phosphatidylcholine in bovine corneal epithelial cells

The effects of phorbol esters on phospholipase C (PLC) activity towards phosphoinositides and phosphatidylcholine (PC) in bovine corneal epithelial cells were examined. The cells were labeled with 32Pi, myo[3H]inositol or methyl[14C]choline, and PLC stimulated by incubation of the cells with Ca2+ ionophore, ionomycin. The PLC activity was assessed by monitoring the loss of radioactivity from the labeled phospholipids or the accumulation of their radioactive metabolites. The data from this study can be summarized as follows: Addition of 20 microM ionomycin to the prelabeled cells resulted in a rapid hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) and somewhat slower hydrolysis of phosphatidylinositol (PI) and phosphatidylcholine (PC) with concomitant several-fold increase in phosphatidic acid (PA). The effects of the ionophore were time- and dose-dependent. Incubation of the cells with phorbol 12,13-dibutyrate (PDBu) or phorbol 12-myristate 13-acetate (PMA) caused increased radioactivity in PC and PA, whereas the radioactivity in PI and PIP2 remained unchanged. The effects of PDBu were inhibited by staurosporine and H-7, and inactive derivatives of phorbol esters failed to exert any effect on phospholipid metabolism. Pretreatment of the corneal epithelial cells with PDBu or PMA abolished the ionomycin-induced hydrolysis of phosphoinositides and PC. The data suggest that activation of protein kinase C by phorbol esters in corneal epithelial cells results in inhibition of PLC activity towards phosphoinositides and PC through a mechanism probably involving phosphorylation of the enzyme.

Identification of calmodulin-sensitive Ca(2+)-transporting ATPase in the plasma membrane of bovine corneal epithelial cell

ATP-dependent Ca2+ uptake was characterized in a plasma membrane enriched fraction obtained from the bovine corneal epithelium. This uptake essentially represented intravesicular accumulation because 72% of the Ca2+ content was releasable following exposure to 10(-6) M A23187. The substrate and Ca2+ requirements for maximal transport activity were similar to those described in the red blood cell because: (1) exogenous calmodulin (3 microM) significantly decreased the apparent Km for Ca2+ to 0.31 microM and increased the rate of Ca2+ uptake; (2) a hydroxylamine labile Ca(2+)-dependent phosphoenzyme intermediate was identified with an apparent molecular size of 140 kDa; (3) Ca(2+)-dependent binding of 125I-labelled calmodulin to this protein was demonstrated which could be antagonized with a calmodulin antagonist, trifluoperazine. These results show that the plasma membrane contains an ATP-dependent Ca2+ transporter. However, its relationship to a previously described high affinity form of Ca(2+)-stimulated Mg(2+)-dependent ATPase is not apparent because their [Mg2+] requirements to elicit maximal activity differed by two orders of magnitude.

P-31 NMR analysis of phospholipids from cultured human corneal epithelial, fibroblast and endothelial cells

Corneal epithelial, fibroblast and endothelial cells, cultured from human donors, were analyzed to determine their characteristic phospholipid profiles by 31P NMR. Tissue phospholipid profiles from epithelial, fibroblast and endothelial cell cultures were evaluated to differentiate the individual cell types and to identify resonances that typically appear in high-resolution phospholipid profiles of whole corneas. Phosphatidylcholine, phosphatidylethanolamine plasmalogen, an uncharacterized phospholipid at 0.13 delta, phosphatidylinositol, phosphatidylserine and sphingomyelin were determined to be, in decreasing order of concentration, the major phospholipids detected in these three cultured corneal cell types. Indices of phospholipid metabolism representing total plasmalogen content, total choline-containing lipids and the total choline-containing lipids less those synthesized through the plasmalogen pathway were found to differentiate the three cell types. Minor phospholipids cardiolipin, lysophosphatidylcholine, phosphatidylethanolamine, lysophosphatidylcholine (LPC) and LPC plasmalogen not usually reported in studies of corneal phospholipids using other techniques, were useful in discriminating between cell types. Phospholipid profiles of the whole cornea provide important information concerning the biochemistry and pathology of the tissue, however, phospholipid analysis of individual components of the cornea, such as the epithelial, fibroblast and endothelial cells, makes it possible to understand the contribution of specific cellular constituents to the spectral information obtained from the whole cornea.

Reversal of the epinephrine stimulation of Cl- transport in bullfrog cornea by phorbol esters

The effects of protein kinase C (PKC) stimulation with phorbol dibutyrate, PDBu, were determined on transepithelial net Cl- transport (Isc) across the isolated bullfrog cornea. Between 1 nM and 1 microM, PDBu had no effect on either the Isc or the conductance, gt, as well as the membrane voltage (Vsc) but it decreased the fractional apical membrane resistance (fR0). PDBu caused a dose-dependent decline and reversal of the stimulatory effect of the mixed alpha and beta adrenergic agonist, epinephrine, on the Isc. With 1 microM PDBu, 100 microM epinephrine decreased the Isc by 20% below its control value. As without PDBu, epinephrine decreased fR0 and depolarized the Vsc. Evidence that this reversal by PDBu stems from a selective stimulation of PKC is: (1) neither its vehicle nor the inactive phorbol analog, 4 alpha-phorbol didecanoate, PDD (1 microM), decreased either the fR0 or altered the stimulatory effect of epinephrine on the Isc. (2) After preincubation with 30 microM H-7, which inhibited PKC stimulation in subcellular membrane and cytosolic fractions of frog corneal epithelium, the stimulatory effect of epinephrine on the Isc was unchanged by 1 microM PDBu. Preincubation with PDBu completely inhibited the stimulatory effect of the beta adrenergic agonist, isoproterenol (10 microM), on the Isc but did not affect the stimulatory effect of 1 microM forskolin on the Isc, indicating that PKC stimulation results in uncoupling of adrenergic regulation of adenylate cyclase activity. Epinephrine did not reverse the Isc in corneas that were preincubated with either the alpha 2- -adrenergic antagonist, yohimbine (100 microM), or the general alpha adrenergic antagonist, phenoxybenzamine (100 microM). With yohimbine, epinephrine had no effect on the Isc whereas with phenoxybenzamine the stimulation by epinephrine was fully restored. These effects suggest that stimulation of PKC is dependent upon the stimulation of alpha 1-adrenergic receptors by epinephrine as well as the presence of PDBu. Reversal and stimulation of the Isc by epinephrine were both associated with a depolarization of the Vsc and similar decreases in fR0. These similar effects on fR0 suggest that PKC stimulation as well as alpha 2 adrenergic activation by epinephrine are associated with increases in basolateral membrane resistance.

Guanosine 5'-O-thiotriphosphate and NaF stimulation of phosphatidylinositol 4,5-bisphosphate hydrolysis in bovine corneal epithelium

The role of a GTP-binding protein in activation of phospholipase C in bovine corneal epithelium was determined by investigating the effects of non-hydrolyzable GTP analog, GTP-gamma-S, and NaF on breakdown of phosphatidylinositol 4,5-bisphosphate (PIP2) in this tissue. GTP-gamma-S (2-50 microM), when introduced into the permeabilized corneal epithelial cells labeled with myo-[3H]inositol, dose-dependently increased the formation of myoinositol trisphosphate (IP3). Other guanine nucleotides and ATP were ineffective. Incubation of 32P-prelabeled corneal epithelium with NaF (2-50 mM) resulted in increased breakdown of PIP2 and increased synthesis of phosphatidic acid. In myo-[3H]inositol-labeled tissue, NaF dose-dependently increased the accumulation of IP3. Microsomal membrane fraction from corneal epithelium was found to contain phospholipase C activity towards endogenous phosphatidylinositol 4-phosphate and PIP2. The enzyme activity was stimulated by Ca2+ (100 microM). Addition of GTP-gamma-S to microsomal fraction containing phosphoinositides which were radiolabeled with 32Pi in situ or with [gamma-32P]ATP in vitro caused a dose dependent hydrolysis of PIP2. These data, taken collectively, suggest that a GTP-binding protein is involved in activation of phospholipase C towards PIP2 in bovine corneal epithelium, and that this guanine nucleotide regulatory protein may serve to couple norepinephrine and 5-hydroxytryptamine receptors to phospholipase C during transmembrane signalling.

The accumulation of platelet activating factor in the injured cornea may be interrelated with the synthesis of lipoxygenase products

The rabbit cornea accumulates platelet activating factor (PAF) three hrs after alkali burn. PAF was isolated by HPLC and assayed by platelet aggregation. This bioactivity was blocked by the PAF receptor antagonists BN 52021 and alprazolam. Added PAF increases the chemiluminescence response of the cornea in vitro and BN 52021 inhibits this effect. In vivo experiments show that the synthesis of 5-HETE and 12-HETE is inhibited by the PAF antagonist BN 52021. It is concluded that a metabolic interrelationship may exist between the PAF cycle and the lipoxygenation of arachidonic acid, and that drugs that affect these lipid mediators may modulate the inflammatory response of the anterior segment of the eye.