[Expression of nitric oxide synthase-2 in specific cells of human glaucomatous optic nerve head]

OBJECTIVE

To investigate the specific cells expressing nitric oxide synthase-2 (NOS-2) in the optic nerve head of patients with primary open angle glaucoma.

METHODS

Double-labeling immunohistochemistry was used for labeling of NOS-2 and one of the specific cell markers.

RESULTS

NOS-2 was labeled in the astrocytes. The NOS-2 positive astrocytes were mainly located in the damaged area of nerve fibers. A few arteries had NOS-2 labeling in the endothelial cells. There was no significant labeling of NOS-2 in microglia, vascular smooth muscle cells and pericytes.

CONCLUSION

In glaucomatous optic nerve neuropathy, NOS-2 is mainly expressed by astrocytes. Astrocytes may play an important role in the local neurotoxicity of axons of the ganglion cells by producing excessive nitric oxide.

Activated microglia in the human glaucomatous optic nerve head

To investigate the distribution and potential participation of microglia, the resident defense cells of the central nervous system, in the optic nerve head (ONH) in glaucoma, histological paraffin sections of optic nerves from normal and glaucoma patients with mild to advanced nerve damage were studied using double labeling immunohistofluorescence. A monoclonal antibody for HLA-DR, indicating activated microglia, was colocalized with antibodies for functional proteins. In normal ONHs, microglia do not contain TGF-beta2, COX-2, or TNF-alpha and are not positive for PCNA; however, in glaucomatous ONHs, microglia contain abundant TGF-beta2, TNF-alpha, and PCNA. In glaucomatous eyes, a few microglia are usually positive for COX-2. In normal ONHs, there are rarely microglia containing TGF-beta1, NOS-2, TSP, TIMP-2, and CD68, but, in glaucomatous tissue, a few microglia are positive from the prelaminar to the postlaminar regions. MMP-1, MMP-2, MMP-3, and MMP-14 are constitutively present in the perivascular microglia in normal ONHs and appear to be more abundant in glaucomatous tissue. COX-1, TNF-R1, TIMP-1, and c-fms are constitutively present in normal tissues and appear to be increased in microglia in the glaucomatous ONHs. HSP27 is not present in microglia. In glaucomatous ONHs, microglia become activated and phagocytic and produce cytokines, mediators, and enzymes that can alter the extracellular matrix. Our findings suggest that activated microglia may participate in stabilizing the tissue early in the disease process, but, as the severity of the glaucomatous damage increases, the activities of microglia may have detrimental consequences for the pathological course of glaucomatous optic neuropathy.

Nitric oxide synthase-2 in human optic nerve head astrocytes induced by elevated pressure in vitro

OBJECTIVE

To determine whether astrocytes of the human optic nerve head can induce nitric oxide synthase-2 (NOS-2) in response to elevated hydrostatic pressure as a mechanism for directly damaging the axons of the retinal ganglion cells in glaucoma.

METHODS

Primary cultures of astrocytes from human optic nerve heads were placed in chambers, either pressurized at elevated hydrostatic pressure (60 mm Hg) or maintained at ambient pressure. The induction of NOS-2 was studied by immunocytochemistry, immunoblot, and semiquantitative reverse transcription polymerase chain reaction.

RESULTS

In astrocyte cultures under ambient pressure, NOS-2 was almost undetectable. In astrocyte cultures under elevated hydrostatic pressure for 24, 48, and 72 hours, intensive labeling of NOS-2 in the Golgi body and the cytoplasm was observed by immunocytochemistry and intense bands of NOS-2 were detected by immunoblotting. As detected by semiquantitative reverse transcription polymerase chain reaction, the messenger RNA level of NOS-2 increased significantly in the astrocytes under elevated hydrostatic pressure within 12 hours, peaking earlier than the protein level of NOS-2.

CONCLUSION

Elevated hydrostatic pressure induces the astrocytes of the human optic nerve head to express NOS-2.

CLINICAL RELEVANCE

In glaucoma, the appearance of the neurodestructive NOS-2 in astrocytes of the optic nerve head may be a primary response to elevated intraocular pressure, in vivo, and therefore damaging to the axons of the retinal ganglion cells.

Tumor necrosis factor-alpha: a potentially neurodestructive cytokine produced by glia in the human glaucomatous optic nerve head

Tumor necrosis factor-alpha (TNF-alpha) mediates a range of cellular responses, which have potentially detrimental consequences that affect multiple cell types. To determine whether TNF-alpha contributes to glaucomatous optic neuropathy, we have studied the expression of this cytokine and its receptor, tumor necrosis factor receptor-1 (TNF- R1), in human glaucomatous optic nerve heads from patients with different stages of disease using double labeling fluorescence immunohistochemistry. We have also investigated the ability of this cytokine to induce nitric oxide synthase (NOS-2) in cultured human optic nerve astrocytes by immunocytochemistry and immunoblot. Normal tissue showed constitutive expression of TNF-R1 in the vasculature of the optic nerve heads but no positive labeling for TNF-alpha. In the glaucomatous optic nerve heads, the expression of both TNF-alpha and TNF-R1 were apparently upregulated, primarily in glial fibrillary acidic protein (GFAP)-positive astrocytes, and appeared to parallel the progression of optic nerve degeneration. In eyes with severe glaucomatous damage, some HLA-DR positive microglia also contained TNF-alpha and TNF-R1. In the most severely damaged optic nerve heads, the axons of the retinal ganglion cells contained TNF-R1 and, therefore, are direct targets for neurodegeneration caused by TNF-alpha. In vitro astrocytes constitutively express TNF-R1 and TNF-alpha stimulation induces expression of NOS-2. We hypothesize that TNF-alpha contributes to the progression of optic nerve degeneration in glaucoma by both a direct effect on the axons of the retinal ganglion cells and by inducing NOS-2 in astrocytes.

Expression of nitric oxide synthase-2 (NOS-2) in reactive astrocytes of the human glaucomatous optic nerve head

Inducible nitric oxide synthase (NOS-2) is abundantly present in the optic nerve heads of glaucoma patients. To determine the regulation of NOS-2 expression in the glaucomatous optic nerve head, the specific cells that express NOS-2 in the optic nerve heads of patients with primary open-angle glaucoma were studied by immunohistochemical double-labeling of NOS-2 and one of the characteristic cell markers for different cell types. Most of the labeling for NOS-2 was identified in reactive astrocytes that were clustered in the areas of nerve damage in the prelaminar and lamina cribrosa regions of the glaucomatous optic nerve heads. In vitro, the expression of GFAP and NOS-2 by reactive astrocytes of human optic nerve heads was demonstrated by immunocytochemistry and Western blot. In primary cultures of human lamina cribrosa astrocytes, stimulation by interferon-gamma and interleukin-1beta upregulated GFAP and induced expression of NOS-2 protein. At 24, 48 and 72 h of stimulation, NOS-2 appeared first in the Golgi body and then was sent out into the cytoplasm in granules. These results demonstrated that the astrocytes of human optic nerve head are capable of inducing the expression of NOS-2. Reactive astrocytes in the glaucomatous optic nerve heads apparently play an important role in local neurotoxicity to the axons of the retinal ganglion cells by producing excessive nitric oxide in glaucomatous optic neuropathy.

Cellular localization of neuronal nitric oxide synthase (NOS-1) in the human and rat retina

The neuronal form of nitric oxide synthase (NOS-1) has been localized to several cell types in the retinas of experimental animals; however, localization in the human retina has not been definitive. By using in situ hybridization and immunohistochemistry, we have compared the cellular expression and localization of NOS-1 in the rat and human retinas. In both rat and human retinas, NOS-1 is expressed in the inner segments of photoreceptors, cells in the inner nuclear layer, particularly amacrine cells, and retinal ganglion cells. In human cones, NOS-1 is abundantly present in the outer segments. In the rat, optic nerve transection caused a loss of cells that were positive for NOS-1 in the ganglion cell layer. Although a retinal ganglion cell localization has not been reported consistently in the literature, our data clearly localize NOS-1 to the retinal ganglion cells of the rat and human retinas.

Confirmation of the rat model of chronic, moderately elevated intraocular pressure

We have confirmed the usefulness of the rat model of chronic, moderately elevated intraocular pressure (IOP) for studying loss of retinal ganglion cells, and as a model for pharmacological neuroprotection studies that may be relevant to treating human glaucoma. By unilaterally cauterizing three episcleral vessels, as described previously in the literature by another laboratory, we observed an approximately 1.6-fold increase in IOP compared to the contralateral eye (18.6 vs 11.5 mm Hg, respectively). Elevated IOP persisted for 6 months without re-treatment. Cupping of the optic disk was observable by examination, in vivo. In 6 months, there was an approximately 40% loss of retinal ganglion cells in the peripheral retina. This model provides a reproducible and quantitative model for pharmacological experiments using neuroprotective agents.

Isoforms of nitric oxide synthase in the optic nerves of rat eyes with chronic moderately elevated intraocular pressure

PURPOSE

To investigate the hypothesis that nitric oxide (NO) in the optic nerve heads of rats with chronic moderately elevated intraocular pressure (IOP) contributes to neurotoxicity of the retinal ganglion cells, the presence of the three isoforms of nitric oxide synthase (NOS) was determined in the tissue.

METHODS

Unilateral chronic moderately elevated IOP was produced in rats by cautery of three episcleral vessels. Histologic sections of optic nerves from eyes with normal IOP and with chronic moderately elevated IOP were studied by immunohistochemistry and by immunoblot analysis. Polyclonal antibodies to NOS-1, NOS-2, NOS-3, and glial fibrillary acidic protein (GFAP) were localized with immunoperoxidase.

RESULTS

In the optic nerve of rat eyes with normal IOP, NOS-1 was constitutively present in astrocytes, pericytes and nerve terminals in the walls of the central artery. NOS-2 was not present in eyes with normal IOP. In these eyes, NOS-3 was constitutively present in the vascular endothelia of large and small vessels. Rat eyes treated with three-vessel cautery had sustained elevated IOP (1.6 fold) for at least 3 months. In these eyes, no obvious changes in NOS-1 or NOS-3 were noted. However, at time points as early as 4 days of chronic moderately elevated IOP, NOS-2 appeared in astrocytes in the optic nerve heads of these eyes and persisted for up to 3 months. Immunoblot analysis did not detect differences in NOS isoforms.

CONCLUSION

The cellular distributions of constitutive NOS isoforms in the rat optic nerve suggest physiological roles for NO in this tissue. NOS-1 in astrocytes may produce NO as a mediator between neighboring cells. NO, produced by NOS-1 in pericytes and nitrergic nerve terminals and by NOS-3 in vascular endothelia, is probably a physiological vasodilator in this tissue. In eyes with chronic moderately elevated IOP, NOS-2 is apparently induced in astrocytes. The excessive NO production that is associated with this isoform may contribute to the neurotoxicity of the retinal ganglion cells in eyes with chronic moderately elevated IOP.

Inhibition of nitric-oxide synthase 2 by aminoguanidine provides neuroprotection of retinal ganglion cells in a rat model of chronic glaucoma

Glaucoma is an optic neuropathy with cupping of the optic disk, degeneration of retinal ganglion cells, and characteristic visual field loss. Because elevated intraocular pressure (IOP) is a major risk factor for progression of glaucoma, treatment has been based on lowering IOP. We previously demonstrated inducible nitric-oxide synthase (NOS-2) in the optic nerve heads from human glaucomatous eyes and from rat eyes with chronic, moderately elevated IOP. Using this rat model of unilateral glaucoma, we treated a group of animals for 6 months with aminoguanidine, a relatively specific inhibitor of NOS-2, and compared them with an untreated group. At 6 months, untreated animals had pallor and cupping of the optic disks in the eyes with elevated IOP. Eyes of aminoguanidine-treated animals with similar elevations of IOP appeared normal. We quantitated retinal ganglion cell loss by retrograde labeling with Fluoro-Gold. When compared with their contralateral control eyes with normal IOP, eyes with elevated IOP in the untreated group lost 36% of their retinal ganglion cells; the eyes with similarly elevated IOP in the aminoguanidine-treated group lost less than 10% of their retinal ganglion cells. Pharmacological neuroprotection by inhibition of NOS-2 may prove useful for the treatment of patients with glaucoma.

Microglia in the optic nerve head and the region of parapapillary chorioretinal atrophy in glaucoma

BACKGROUND

Microglia, the macrophages and immune surveillance cells of the central nervous system, are quiescent normally but become activated in injured neural tissue. We have determined the distribution and potential participation of microglia in glaucomatous optic nerve degeneration.

METHODS

Microglia were localized by immunohistochemistry on paraffin sections of age-matched normal and glaucomatous human eyes obtained within 24 hours after death. Monoclonal and polyclonal antibodies that recognize specific epitopes on microglia and other cell types were localized by immunoperoxidase and immunofluorescence.

RESULTS

Stellate cells with thin, ramified processes, positive for HLA-DR and CD45 but negative for glial fibrillary acid protein, were identified as quiescent microglia. These cells were found throughout the normal optic nerve head in the walls of large blood vessels and surrounding capillaries in glial columns and cribriform plates. In glaucomatous eyes with moderate and severe optic nerve head damage, microglia were present as clusters of large ameboid, activated cells in the compressed lamina cribrosa and as formations of concentric circles surrounding blood vessels. In the parapapillary chorioretinal region of glaucomatous optic nerve heads, large, activated microglia were present as single cells or clusters on the termination of the Bruch's membrane. In addition, along the optic nerve/choriocapillaris-scleral interface, activated microglia appeared to form linear arrays near the choriocapillaris vessels. These cells were parenchymal and not in close association with the vasculature.

CONCLUSIONS

In glaucoma, microglia in the optic nerve head become activated and redistributed. Enlarged, activated microglia appear in the parapapillary chorioretinal region, perhaps due to migration from the disorganized prelaminar and laminar tissue. Strategically positioned microglia may also serve a neuroprotective function in relation to a damaged blood-retinal barrier. The activity of microglia in the parapapillary chorioretinal region in glaucoma may be responsible for some of the biomicroscopic and histological changes that are associated with parapapillary chorioretinal atrophy.

Nitric oxide: a potential mediator of retinal ganglion cell damage in glaucoma

In the glaucomatous optic nerve head, excessive nitric oxide (NO) may be responsible, at least in part, for degeneration of axons of retinal ganglion cells. We have demonstrated the apparent up-regulation and induction of certain isoforms of nitric oxide synthase (NOS), the enzyme that synthesizes NO, in astrocytes in the prelaminar and lamina cribrosa regions of optic nerve heads of patients with glaucoma. Evidence of NO toxicity, histochemical staining for nitrotyrosine, is present in these damaged optic nerve heads. In rats with experimentally induced, moderately elevated intraocular pressure, the isoforms of NOS were also identified.

Effect of ticrynafen on aqueous humor dynamics in monkeys

OBJECTIVE

To determine the effect of ticrynafen, a nonsulfhydryl-reactive compound similar to ethacrynic acid, on outflow facility in normotensive monkey eyes and on intraocular pressure (IOP) in monkey eyes with laser-induced glaucoma.

METHODS

In normotensive eyes, facility (perfusion) was measured shortly before and after bolus or exchange intracameral infusion of ticrynafen or vehicle in opposite eyes, and 3.5 to 4.5 hours after 5 days of twice-daily 2% ticrynafen or vehicle ointment. In glaucomatous eyes, baseline and vehicle diurnal IOP curves were established, 2% ticrynafen ointment was given twice daily for 5 days, and IOP was measured immediately before and 0.5 to 6 hours after each morning treatment.

RESULTS

In normotensive eyes, exchange 2-mL influsion of 0.2-, 1-, or 4-mmol/L ticrynafen increased facility by 33% +/- 6% (mean +/- SEM), 73% +/- 18%, and 60% +/- 11%, respectively. Day 5 posttreatment facility was higher in the ticrynafen group than in controls by 28% +/- 9%. In glaucomatous eyes, maximum IOP decline, from approximately 35 mm Hg, was 7.5 +/- 2.0 mm Hg on day 4 and 9.8 +/- 2.4 mm Hg on day 5 of twice-daily ticrynafen treatment.

CONCLUSION

The facility-increasing, IOP-lowering action of ticrynafen, ethacrynic acid, and derivatives may not depend entirely on sulfhydryl reactivity.

CLINICAL RELEVANCE

Whether such drugs as ethacrynic acid and ticrynafen prove valuable for glaucoma therapy, at the least they are useful probes to study aqueous outflow mechanisms.

Cyclooxygenase-1 and cyclooxygenase-2 in the human optic nerve head

To investigate the hypothesis that eicosanoids act as cellular mediators in the optic nerve head of normals and of patients with glaucoma, we have determined the presence of the two cyclooxygenase (COX) isoforms in human tissue. Histological sections of optic nerve heads were studied by immunohistochemistry. Age matched normal donors were compared with eyes from glaucoma patients with moderate to severe nerve damage. Polyclonal antibodies to human COX-1 and COX-2 were localized with immunoperoxidase staining. Specific antibodies for vascular endothelia and microglia were also colocalized. In normal and glaucomatous eyes. COX-1 was localized exclusively to the prelaminar and lamina cribrosa regions of the optic nerve head. No staining for COX-1 was observed in the nerve fiber layer or the myelinated optic nerve. COX-1 was associated with the astrocytes of the glial columns and the cribriform plates, but not with the endothelia lining the capillaries. In glaucoma, more astrocytes appeared to be stained with antibody to COX-1 than in normals and staining was intensely perinuclear. There was no staining for COX-2 in normal tissue. A few COX-2 positive cells were found in the prelaminar, lamina cribrosa and postlaminar regions of the glaucomatous optic nerves. Positive staining for COX-2 was not associated with microglia. COX-1 is constitutively present in astrocytes that are localized exclusively to the prelaminar and lamina cribrosa regions of the human optic nerve head. Eicosanoids, synthesized by COX-1 in this tissue, may have a homeostatic and a neuroprotective role related to the axons of the retinal ganglion cells. The sparse presence of COX-2 in glaucomatous tissue probably reflects the lack of inflammation associated with glaucomatous optic neuropathy.

Nitric oxide synthase in the human glaucomatous optic nerve head

OBJECTIVES

To investigate the hypothesis that nitric oxide contributes to neurotoxicity in the optic nerve heads of patients with primary open-angle glaucoma, we have determined the presence of the 3 isoforms of nitric oxide synthase (NOS) in the tissue.

METHODS

Histological sections of optic nerve heads from normal subjects and patients with glaucoma who have moderate to advanced nerve damage were studied by immunohistochemistry. Polyclonal antibodies to NOS-1, NOS-2, and NOS-3 were localized with immunoperoxidase staining.

RESULTS

In normal eyes, NOS-1 is sparsely present in astrocytes throughout the optic nerve head. In glaucomatous optic nerve heads, almost every astrocyte is positive for NOS-1. Nitric oxide synthase-1 immunoreactivity is abundantly present throughout the prelaminar region and the lamina cribrosa and is localized inside the diminished nerve fiber bundles. Nitric oxide synthase-2 is present in a few cells in the disorganized lamina cribrosa of the glaucomatous eye and is not present at all in normal tissue. Nitric oxide synthase-3 is present in normal eyes in the vascular endothelia of small blood vessels of the prelaminar region. In glaucomatous tissue, NOS-3 is present in astrocytes and in the vascular endothelia of large and small vessels.

CONCLUSIONS

The 3 isoforms of NOS are present in apparently increased amounts in the optic nerve head of patients with primary open-angle glaucoma. The increased presence of NOS-1 and the induction of NOS-2 in astrocytes of the lamina cribrosa suggest that the glaucomatous optic nerve head is exposed to excessive levels of nitric oxide, which may be neurodestructive, locally, to the axons of the retinal ganglion cells. Conversely, the increased presence of NOS-3 in vascular endothelia may be neuroprotective by causing vasodilation and increased blood flow in the tissue.

Effects of topical ethacrynic acid ointment vs timolol on intraocular pressure in glaucomatous monkey eyes

OBJECTIVE

To evaluate the long-term effects of ethacrynic acid (ECA) ointment, compared with timolol maleate on intraocular pressure (IOP) in cynomolgus monkey eyes with argon laser-induced glaucoma.

METHODS

In a 5-day study, IOP was measured for 7 hours after once-daily topical applications of ECA ointment to four glaucomatous monkey eyes. For this study, ECA ointment was given in 0.5%, 1.0%, 1.5%, or 2.5% concentrations. In a separate 30-day study, IOP was measured after once-daily topical applications of ECA ointment in concentrations of 0.75% or 1.5%. The results were compared with IOP after the application of 0.5% timolol maleate administered twice daily on weekdays and once daily on weekends.

RESULTS

In the 5-day study, 2.5% ECA ointment had the greatest effect on lowering IOP, with a maximum reduction of 8.5 +/- 2.9 mm Hg (mean +/- SEM). A more pronounced reduction in IOP was observed on the fifth day of treatment for each of the four concentrations. In the 30-day study, 1.5% ECA ointment or 0.5% timolol maleate reduced IOP as much as 11.5 +/- 3.7 mmHg and 14.0 +/- 4.5 mmHg, respectively. With repetitive dosing, the effect on IOP after using 1.5% ECA ointment increased with time. Mild eyelid edema, conjunctival hyperemia, and discharge were observed in some eyes treated with the highest concentrations. One eye of four treated with 1.5% ECA ointment for 30 days developed a superficial corneal erosion in the 30-day study.

CONCLUSIONS

The ECA ointment reduced IOP in glaucomatous monkey eyes. This reduction was evident by the fifth day of treatment with all the concentrations tested. The reduction in IOP produced by once-daily treatment with 1.5% ECA ointment was comparable with that of 0.5% timolol maleate administered twice daily. Therefore, drugs in this class of compound may prove to be useful in glaucoma therapy.

Collagen type I mRNA levels in cultured human lamina cribrosa cells: effects of elevated hydrostatic pressure

In primary open-angle glaucoma, elevated intraocular pressure is associated with distortion and reorganization of the connective tissue plates of the lamina cribrosa. We have previously postulated that the resident cells of the lamina cribrosa may respond to elevated intraocular pressure by altering the biosynthesis or degradation of extracellular matrix. To determine the response of lamina cribrosa cells to increased pressure, we have compared cultures of human lamina cribrosa cells, from five individuals, maintained under control and pressurized conditions in vitro. Cells from third to fifth passage cultures of human lamina cribrosa subjected to elevated hydrostatic pressure (50 mm Hg) for 7 days changed shape from flat and polygonal to elongated, synthesized and secreted increased amounts of collagen type I as shown by immunofluorescent localization, and exhibited increased mRNA levels of collagen type I (199 +/- 36% of control) (mean +/- S.D.), as determined by slot-blot hybridization. In contrast, beta-actin mRNA levels were unchanged, indicating that the effects of elevated pressure are probably relatively selective. Our data indicate that elevated pressure increases the synthesis of collagen Type I by human lamina cribrosa cells in vitro. In vivo, lamina cribrosa cells may react to changes in their environment by modulating, specifically, changes in the mRNA levels, production, and secretion of extracellular matrix macromolecules. The relationship of these changes in extracellular matrix to those observed in glaucoma remains to be determined.

Localization of collagen types I and IV mRNAs in human optic nerve head by in situ hybridization

Using in situ hybridization, individual cells expressing mRNAs for collagen types I and IV were localized in fixed-tissue sections of adult and fetal human optic nerve heads. Astroglial cells lining the cribriform plates and cells inside the cribriform plates of the lamina cribrosa had mRNA for collagen type IV. Cells in the glial columns, pial septa, and vascular wall also contained mRNA collagen type IV. Collagen type I mRNA was expressed by cells of the cribriform plates of the lamina cribrosa of adults. Few cells in the glial columns, pial septa, and blood vessels had mRNA for collagen type I. Scleral fibroblasts contained mRNA for collagen type I. These results indicated that the expression of mRNA for both collagen types I and IV paralleled the localization of these extracellular matrix proteins in the optic nerve head and suggested that both collagen types were synthesized in this tissue throughout life.

In vitro pharmacologic separation of corneal endothelial migration and spreading responses

Repair of corneal endothelial wounds involves two forms of cell translocation: (1) "migration," in which individual cells at the wound edge break contacts with neighboring cells and move as individuals into the wound defect, and (2) "spreading," in which cells within the confluent monolayer adjacent to the wound move as a group into the wound area. The authors combined morphometric analysis of Giemsa-stained cultures, phase-contrast video microscopy, and Rh-phalloidin staining of actin filaments to study the effects of epidermal growth factor (EGF) and indomethacin on the migratory and spreading responses to wounding using an in vitro wound-closure model which mimics the amitotic state and general behavior of human corneal endothelium. They found that EGF stimulated the migration of individual cells from the wound edge, induced cellular elongation, and promoted a diffuse distribution of actin filaments. Indomethacin promoted spreading of the confluent monolayer into the wound defect, induced enlargement and flattening of cells, and promoted the formation of long, thick actin stress fibers. These results provide evidence that the migration and spreading responses of corneal endothelial cells to wounding can be pharmacologically separated. The findings suggest that migration of individual cells during wound repair may result from an endogenous form of EGF-like stimulation and that the elongated shape associated with this form of translocation results, at least in part, from an EGF-like alteration in actin-filament organization. Spreading of the confluent monolayer to cover the wound defect may result from a decrease in cyclic adenosine monophosphate induced by a transient reduction in prostaglandin E2 synthesis. This form of translocation may result, in part, from enlargement and flattening of corneal endothelial cells secondary to an enhancement of actin stress-fiber formation.

Retinal pigment epithelium wound closure in vitro. Pharmacologic inhibition

Early-passage bovine retinal pigment epithelium (RPE) cells were grown to confluence in 24-well plates, and a central defect was created mechanically in the monolayer within quadruplicate wells, sequentially over 9 days. Closure of the wounded area occurred by single-cell migration of elongated RPE cells from the edge of the wound and subsequent cell proliferation. Ten days after wounding, the cultures were fixed, stained, and photographed, and the residual wound area was quantified by computerized planimetry. Cell counts of unfixed cultures were determined with a Coulter counter. Wound closure was complete after 10 days. Using this technique, we assessed the response of RPE to various concentrations of 5-fluorouracil (5-Fu), colchicine (COL), and cytochalasin-B (CYT-B). 5-Fu (10 micrograms/ml) and COL (0.1 and 1 microgram/ml) inhibited migration and proliferation of RPE cells. CYT-B (5 micrograms/ml) inhibited migration. This model allows in vitro study of the response of RPE cells after loss of contact inhibition. The technique provides a quantitative model for assessing the dynamic capabilities of RPE cells in response to a localized mechanical defect and for assessing the pharmacologic modulation of these responses.

Changes in the extracellular matrix of the human optic nerve head in primary open-angle glaucoma

Using immunofluorescent staining, we were able to characterize the changes in composition and distribution of the macromolecules making up the extracellular matrix of the lamina cribrosa of the glaucomatous human optic nerve head. In tissue adjacent to the glaucomatous cups, there was marked disorganization and loss of fibers of elastin within the cores of the cribriform plates. Collagen type VI, normally sparse, increased in quantity considerably throughout the lamina cribrosa in glaucomatous eyes with all degrees of damage. Collagen type IV and other basement membrane macromolecules appeared to extend into nerve bundles, presumably filling in spaces previously occupied by nerves. There was no appreciable change in the postlaminar region, which indicates the specificity of the extracellular matrix changes in the lamina cribrosa. Our results indicate that changes in the extracellular matrix play an important role in the progression of the glaucomatous process and may be a causative agent of the disease.

Effects of EGF and indomethacin on rabbit corneal endothelial wound closure in excised corneas

Corneal endothelial cells of rabbit corneas stored in M-K medium at 37 degrees C were wounded by touching them lightly with a micropipet under video specular microscope observation. Three groups were studied: control, with EGF, and with EGF + indomethacin. The wound closure process (initial wound area about 8500 microns 2) was observed and recorded with time-lapse videography for 6 hr. The recorded video images were digitized and computer assisted morphometric analysis was performed. (1) Addition of either EGF (10 ng/ml) + indomethacin (1 microM), or EGF (10 ng/ml) alone to the M-K medium statistically significantly shortened the wound closure time as compared with the control group. (2) Both EGF + indomethacin and EGF alone resulted in a greater average percent relative change of the shape factor, more than three times greater with EGF + indomethacin and more than two times greater with EGF alone, than in the control group 150 min after wounding. (3) The maximum cell shape change occurred at about 150 min after wounding in the groups EGF + indomethacin and EGF alone, and at about 200 min in the control group. After this time in all three groups the cells began to approach a normal shape. (4) The cells near the wound boundary moved faster in the EGF + indomethacin and the EGF groups as compared with the control group. These results suggest that EGF and indomethacin may be of therapeutic value in promoting closure of traumatized human corneal endothelium.

Corneal endothelial wound closure in vitro. Effects of EGF and/or indomethacin

In response to stress, the corneal endothelium must maintain or region its barrier function. To study cellular responses of the corneal endothelium, our laboratory has developed an in vitro model of rabbit corneal endothelial wound closure. When cells are free to divide, a 3 mm diameter wound closes within 4 days. 5-fluorouracil added to these cultures does not affect the cellular morphology or ultrastructure, but does inhibit cell division. In the presence of 5-fluorouracil, wounds close in approximately 7 days. These conditions mimic the amitotic state and general behavior of adult human corneal endothelium in vivo. Using this model, we studied the effects of epidermal growth factor (EGF) and/or indomethacin treatment on corneal endothelial wound closure in mitotically competent and inhibited cultures. EGF appeared to stimulate migration, whereas indomethacin appeared to enhance cell spreading in response to wounding, particularly in mitotically inhibited cultures. Treatment with the above agents at the time of wounding had little effect on wound closure rates, but did affect closure patterns. In contrast, pretreatment of cultures, particularly with indomethacin, significantly accelerated closure in mitotically inhibited cultures. In the presence of indomethacin, wounds closed in 3-4 days compared to 7-8 days for controls. These results indicate that the response of corneal endothelial cells to wounding can be pharmacologically manipulated, and perhaps accelerated, and suggest that the treatment of the endothelium with nonsteroidal anti-inflammatory drugs or EGF-like growth factors may be clinically useful.

Age-related changes in the extracellular matrix of the human optic nerve head

By using immunofluorescent staining we were able to characterize the age-related changes in the macromolecules making up the extracellular matrix of the lamina cribrosa of the human optic nerve head. As the cores of the cribriform plates enlarged with age, there were age-related increases in the apparent density of collagen types I and III and elastin that constituted the connective tissue support of the nerve bundles. Collagen type IV coated the cribriform plates as basement membranes and was also present within the cores as a fine filamentous network, which increased in density with age as the cribriform plates expanded. As this tissue ages, individual differences leading to more or less of a particular macromolecule of the extracellular matrix may alter the support function of the lamina cribrosa and influence the degeneration of the optic nerve associated with glaucoma.

Pharmacological regulation of morphology and mitosis in cultured rabbit corneal endothelium

Cultured rabbit corneal endothelial cells elongate when grown in the presence of epidermal growth factor (EGF) and indomethacin (INDO); whereas maintenance of the differentiated polygonal cell shape is apparently dependent upon endogenous synthesis of prostaglandin E2 (PGE2). In the current study, the authors demonstrate morphological changes in phenotypically altered cells and identify two intracellular pathways which interdependently regulate endothelial cells. Morphometric and mitotic analyses of cultures treated with a variety of pharmacological agents indicate that both protein kinases A- and C-dependent pathways regulate cell shape and cell division in corneal endothelial cells. Marked intracellular reorganization is associated with the morphological changes in the endothelial cells. When stained with rhodamine conjugated phallicidin, polygonal endothelial cells have circumferential bands of f-actin at their borders. EGF and/or INDO induce elongation and redistribution of f-actin into a diffuse cytoplasmic reticulum. Transmission electron microscopy demonstrates loss of several characteristic morphological markers for endothelial cells in response to pharmacologically induced elongation. The elongated cells lose intracellular junctions, apical/basal polarity and rough endoplasmic reticulum. These ultrastructural markers and circumferential f-actin bands are restored in cultures supplemented with exogenous PGE2. Modulation of these pathways in vivo may regulate cellular migration and mitosis during wound closure, stress, trauma and with age.

Cell culture of the human lamina cribrosa

The extracellular matrix of the lamina cribrosa may be important in the changes in the optic nerve head associated with glaucoma. To investigate the cell biology of this tissue, human lamina cribrosa was explanted in tissue culture and two cell types grown from this tissue were characterized. The most common cell type obtained was a large, flat, polygonal cell which was negative for glial fibrillar acidic protein (GFAP) and could be serially subcultured. This cell type synthesized collagens type III and type IV, fibronectin and elastin. Much less commonly grown was a cell type with conspicuous long processes and which was positive for GFAP. This presumed astrocyte synthesized collagen type IV and fibronectin. Fibroblastic cells were not obtained from this tissue but were easily grown from sclera. The cells that we have cultured from the human lamina cribrosa may produce the extracellular matrix present in the cribriform plates of this tissue and be important in the glaucomatous process.

Extracellular matrix of the human lamina cribrosa

We used immunoperoxidase staining and double immunofluorescent staining to demonstrate the macromolecular components of the extracellular matrix of the lamina cribrosa from young human donors. The cribriform plates were made up of a core of elastin fibers with a sparse, patchy distribution of collagen type III. The plates were coated with collagen type IV and laminin; these basement membrane components were presumably made by the astrocytes that were distributed on the surfaces of the plates. The insertion of the lamina cribrosa in the sclera was made up of concentric, circumferential elastin fibers that surrounded the lamina cribrosa and were continuous with the elastin in the cribriform plates. Astrocytic processes extended into the bundles of elastin fibers, whereas the basement membrane components extended into the sclera. The mechanical properties of the macromolecules of the extracellular matrix of the lamina cribrosa may make this tissue compliant and sensitive to intraocular pressure. Perhaps individual differences in the macromolecular components of this tissue contribute to the glaucomatous changes in the optic nerve head.

Effects of stimulation of the ocular sympathetic nerves on IOP and aqueous humor flow

Ocular sympathetic nerves were stimulated chronically in awake rabbits using electrodes unilaterally implanted on the cervical sympathetic trunk. IOP was measured by pneumatonometry and aqueous inflow was measured by fluorophotometry. In each animal, continuous trains of 1 msec pulses were delivered by means of a portable electrical stimulator. Experiments were spaced by 1 week recovery periods. Stimulation was varied over a range of amplitudes (5-15 V) and frequencies (3-12 Hz). Continuous sympathetic stimulation produced an immediate sharp decrease in IOP followed by a gradual rise to pre-stimulation values which were attained 60-90 min after onset. A rebound increase in IOP occurred when stimulation was terminated. The magnitude of the initial IOP drop, the delay in the return to pre-stimulation IOP, and the rebound rise in IOP subsequent to termination of electrical stimulation were proportional to the stimulation frequency. Maximal effects were observed at 12 Hz, and stimulation with 8-10 Hz for 180 min caused a sustained reduction in anterior chamber aqueous humor flow. Topical 2% phentolamine 1 hr before stimulation markedly reduced IOP and abolished the acute IOP changes observed in untreated stimulated animals. Topical 1% timolol did not affect either the initial IOP drop or the rebound; however, the IOP recovered during stimulation to values greater than pre-stimulation IOP. We conclude that in rabbits the beta-adrenergic effect of prolonged sympathetic nerve stimulation is to decrease aqueous flow. Chronic electrical stimulation in awake animals provides an experimental model for studying the role of the ocular sympathetic nerves.

Corneal epithelial wound closure in tissue culture: an in vitro model of ocular irritancy

The influence of 13 test agents on the ability of cultured rabbit corneal epithelial cells to migrate and re-cover a wound has been utilized to evaluate an in vitro model of ocular irritancy. Cells were grown under standard conditions and monitored for adequate cell density. Seven days after subculture, replicate wounds were produced and cultures were exposed to varying concentrations of a test agent in culture medium for 24 hr. Following exposure, cultures were fixed and stained to reveal remaining wound areas which were quantitated by computerized planimetry and compared to evaluate the deleterious effects of the test agents. The test ranks irritants in an order similar to that described in the literature for both in vivo and in vitro tests. This tissue culture model is conceptually simple, quantitative, and an alternative to the corneal component of whole animal testing for ocular irritancy.

Adrenergic stimulation of ciliary process epithelium causes surface membrane internalization

An ultrastructural change induced in the nonpigmented epithelium (NPE) of the ciliary processes by adrenergic stimulation in the albino rabbit was studied. Thirty min after topical treatment with 2% isoproterenol, an extensive intracellular membranous network, previously reported to be smooth endoplasmic reticulum, was revealed by electron microscopy. It was postulated that this network originated from the plasma membrane. Using cationized ferritin (CF) as an ultrastructural tracer, freshly isolated anterior segments were incubated in buffer containing 10(-5) M isoproterenol and 0.2% CF. As early as 10 min, and for at least 30 min, the isoproterenol-treated NPE cells contained a membranous network that was morphologically similar to that which occurs in vivo. CF particles were present within the network, indicating that the membranous network had originated at the cell surface. This labeling was prevented by pretreatment with the beta-adrenergic antagonist timolol maleate. In both treated and control ciliary processes, CF was present in the ciliary canals between the NPE and the underlying pigmented epithelium after 10 min incubation. This suggests that the NPE is able to transport CF from its basilar to apical surface. These experiments imply that the NPE is able to internalize rapidly large amounts of plasma membrane in response to adrenergic stimulation. This response may be part of the mechanism of adrenergic receptor desensitization, alteration of aqueous humor production, or another adrenergic response.

The effects of forskolin on cyclic AMP, intraocular pressure and aqueous humor formation in rabbits

Forskolin was used to study cyclic AMP-mediated regulation of aqueous humor dynamics in rabbits. Crystalline forskolin was solubilized in oil and its pharmacological effects were studied both in vitro and following topical ocular administration. In vitro, using cultured corneal epithelial cells, forskolin rapidly stimulated cyclic AMP production and in vivo increased cyclic AMP concentration in the aqueous humor 10-fold following topical administration. The effect of topical forskolin on intraocular pressure and aqueous humor formation was determined in vivo using pneumatonometry and fluorophotometry, respectively. Forskolin caused a prolonged reduction of intraocular pressure and decreased aqueous humor formation. The ability of forskolin to potentiate the ocular hypotensive effect of epinephrine was investigated. Forskolin in combination with epinephrine caused a decrease in intraocular pressure of longer duration than either 0.1% epinephrine or 1% forskolin administered separately. Forskolin caused a small but significant increase in the permeability of the blood-aqueous barrier at the time of maximal intraocular pressure reduction. This effect on the blood-aqueous barrier may explain the inhibitory effect of forskolin on aqueous humor formation.

Maintenance of corneal endothelial cell shape by prostaglandin E2: effects of EGF and indomethacin

Confluent, cultured, rabbit corneal endothelial cells maintain a polygonal shape which is characteristic of these cells in vivo. When cultured in the presence of EGF (10 ng/ml) and/or indomethacin (1.0 microM), the endothelial cells have markedly different shapes at confluency. By morphometry, untreated cells are polygonal and have a maximum axis of 33 mu; EGF treatment causes a spindle-shaped elongation to 48 mu and indomethacin treatment causes a stellate-shaped elongation to 48 mu. There is a slight increase in cell density. When cells are cultured in the presence of both drugs, elongation is more pronounced to a fibroblastic appearing cell population, with maximum axes of 60 mu and more, but no additive increase in cell density. Continuity of cell borders is often lost. Corneal endothelial cells cultured in the presence of EGF, indomethacin, and PGE2 (0.5 microgram/ml) maintain their polygonal shape; PGF2 alpha is not effective at reversing the drugs' effects. Untreated and EGF-treated cells synthesize and release substantial quantities of PGE2 (2-4 ng/10(4) cells). Indomethacin completely inhibits PGE2 synthesis. It is concluded that PGE2 maintains the polygonal cell shape of the corneal endothelium in vitro and, perhaps, in vivo. The elongated forms of the cell may be related to migration and important in wound closure.

Extracellular matrix of the human optic nerve head

Double-antibody immunofluorescent studies of sectioned human optic nerve head indicated the marked presence of collagen type IV and laminin in the extracellular matrix of the lamina cribrosa. These macromolecules were layered transversely across the nerve fascicles and appeared to constitute the cribriform plates. Relatively little collagen types III and I were present in the extracellular matrix of this tissue and fibronectin was not detected in appreciable amounts. These results indicated that the lamina cribrosa contains a specialized extracellular matrix of the central nervous system made up of plates of material resembling basement membrane. The major macromolecular components of the lamina cribrosa do not resemble those of sclera.

Rabbit corneal endothelial cells in vitro: effects of EGF

Rabbit corneal endothelial cells were grown in tissue culture. Epidermal growth factor (EGF) increased the mitotic rate during the growth phase by 70% over control without affecting the plating efficiency. Within 48 hr of exposure to EGF, the endothelial cells became spindle-shaped. This morphological change was quantitated by morphometry; cells treated with EGF had a major axis 1.5 X larger than that of non-EGF treated cells. The spindle-shaped morphological change did not occur in response to other growth factors, was not related to cell density, and was reversible within 24 hr after removal of EGF from the media or subculture in the absence of EGF. The addition of 5-fluorouracil blocked cell division but did not affect the EGF-induced morphological change. The appearance of the endothelial cells following EGF stimulation is similar to migrating cells closing a wound in vivo.

Aqueous to cornea fluorescein distribution ratio in normal and swollen cornea

Intravitreal sodium fluorescein was used to simulate equilibrium fluorescein kinetics, thereby allowing simple measurement of the aqueous to cornea fluorescein distribution ratio. Two groups of rabbit corneas were studied: normal corneas and corneas wounded by freezing. The aqueous to cornea fluorescein distribution ratio was approximately 0.4, was not significantly different in groups of normal or wounded eyes and little variability was noted. In addition, a comparison of in vivo and in vitro measurements of corneal fluorescein concentration in wounded eyes suggests that in vivo protein-bound fluorescein in the cornea fluoresces less efficiently than free fluorescein.

A tissue culture assay of corneal epithelial wound closure

Experimental assays have been developed using cultured tissue derived from rabbit corneal epithelium to study migration of epithelial sheets during wound closure and cell-substrate adhesion. To study wound closure, epithelial defects, 6 mm in diameter, were produced in vitro in 24 well multiplates by a local freezing technique, and the size of the remaining defect was quantitated over time by staining. To study adhesion, cultured cells were labeled with 3H-leucine, suspended, and added to fresh culture plates. At various times, adherent cells were lysed and the radioactivity of the lysate was determined. Serum enhances the closure of experimental defects, but laminin and fibronectin have no effect. Agents which alter mitotic rate, such as epidermal growth factor and 5-fluorouracil, do not influence the rate of wound closure in this assay. Compounds which elevate intracellular levels of cyclic AMP inhibit wound closure but promote cell-substrate adhesion. Thus, cultured corneal epithelial cells may be used to assay for influences on the migratory events governing closure of superficial epithelial wounds.

Cellular migration and morphology in corneal endothelial wound repair

After a mechanical denudation of rabbit corneal endothelial cells, the healing process was followed with wide-field specular microscopy. Individual cell migration and morphologic changes were analyzed by computer-assisted morphometry. The cells surrounding the wound migrated to cover the defect without producing intercellular gaps. The greatest cellular migration and morphologic alterations occurred close to the wound edge. As the cells migrated toward the wound, they elongated and increased their surface area in the direction of the migration. As the healing proceeded, the cells lost their original hexagonal pattern, which returned after coverage was complete. The wound was covered completely by large, irregularly shaped cells showing mitotic figures between 24 and 48 hr. During this period, cellular migration decreased and normal cellular morphology began to recover. When mitosis decreased, the normal cellular pattern rearranged towards a more hexagonal shape. During the healing process, the degree and direction of cellular migration varied from cell to cell. Additionally, changes in cell-to-cell contact (positional changes of neighboring cells) occurred in one-third of migrating cells. Such cellular migration can account for monolayered cells sliding without producing gaps between individual cells.

Study of central regulation of intraocular pressure using ventriculocisternal perfusion

The ability of hypoosmotic solution, prostaglandin E1 (PGE1) and clonidine to influence intraocular pressure (IOP) by a central mechanism was studied using the technique of ventriculocisternal perfusion in conscious rabbits. IOP remained unchanged during the perfusion of 150 mOsm artificial cerebrospinal fluid. IOP rapidly increased by 15 mmHg during the perfusion of PGE1 at the dose of 1 or 3 micrograms/min. However, when PGE1 was perfused intravenously at the dose of 1 microgram/min, a similar IOP response was observed. Furthermore, during the ventriculocisternal perfusion of PGE1 a significant systemic absorption occurred. These observations indicate that the ocular hypertension during the ventriculocisternal perfusion of PGE1 is primarily due to the peripheral action of systematically absorbed PGE1. IOP gradually decreased by 3 mmHg during the ventriculocisternal perfusion of clonidine at the dose of 0.1 or 0.33 micrograms/min. Intravenous perfusion of clonidine at the same doses did not change the IOP. These results indicate that clonidine can lower IOP by a centrally mediated mechanism. Ventriculocisternal perfusion of clonidine (0.1 micrograms/min) in rabbits with unilateral superior cervical ganglionectomy lowered IOP in both eyes, indicating that ocular adrenergic innervation does not participate in this centrally mediated IOP response. However, the cardiovascular parameters of anesthetized rabbits were altered by the ventriculocisternal perfusion of clonidine (0.1 micrograms/min), suggesting that a change in systemic hemodynamics is involved in the central IOP effect of clonidine.

Corneal endothelial function and structure following cryo-injury in the rabbit

Wide-field specular microscopy, fluorophotometry, pachymetry, and scanning electron microscopy are used to characterize a reproducible, in vivo model of corneal endothelial injury and recovery in the rabbit. Following an 8-mm central cryo-injury, the cornea remains thickened for as long as 3 weeks. Mean endothelial permeability to fluorescein is above normal for 10 days following injury, but by 14 days postinjury the endothelial permeability to fluorescein is not statistically significantly different from preinjury control values, thus indicating that endothelial permeability probably returns to normal by approximately 2 weeks postinjury. Cell morphology, as determined by scanning electron microscopy, is also essentially normal by 2 weeks postinjury. Endothelial permeability appears to recover before stromal thickness normalizes, suggesting a lag in recovery of endothelial pump function.

Beta-adrenergic and serotonergic stimulation of rabbit corneal tissues and cultured cells

The adult rabbit cornea synthesizes cyclic AMP in response to both serotonin and isoproterenol. The authors have examined the postnatal development of these pathways and attempted to localize the responsive cell type(s) by dissection, cell culture, and surgical denervation. Full thickness corneas of neonatal rabbits have beta-adrenergic responses similar to the adult but fail to respond to serotonin until the animals are 9-12 weeks old. When adult corneas are separated into epithelia, stromal, and endothelial layers, only the stromal layer synthesizes cyclic AMP in response to serotonin, whereas all layers respond to isoproterenol. When grown in tissue culture, keratocytes, epithelial, and endothelial cells are unresponsive to serotonin but respond to isoproterenol. Neither adrenergic nor sensory denervation abolishes the corneal adrenergic or serotonergic response pathways. These results indicate that the epithelial cells do not contain the serotonin stimulated, cyclic AMP-mediated pathway as originally postulated. The cell population that does contain this pathway is within the stroma and may be the Schwann cells.

Effects of timolol on intraocular pressure following ocular adrenergic denervation

The effects of timolol on the elevation of intraocular pressure induced by orogastric water-loading were studied in conscious pigmented rabbits which had undergone unilateral, superior cervical ganglionectomy. Each rabbit was studied without timolol treatment and with unilateral 2% timolol treatment, either to the innervated eye or to the denervated eye, 90 min before water-loading. Timolol, applied to the innervated eye, significantly reduced the elevation of intraocular pressure in that eye, but not in the fellow eye. Timolol, applied to the denervated eye, did not affect the elevated intraocular pressure in either the denervated or the fellow eye. These results demonstrate that ocular adrenergic innervation participates in the mechanism of ocular hypotensive action of timolol in water-loaded pigmented rabbits.

Laboratory and clinical studies on the mechanism of action of timolol

Timolol is now widely used in the treatment of glaucoma but its mechanism of action is unclear. The drug decreases aqueous humor formation by the ciliary processes and is a potent beta-adrenergic antagonist. However, the physiological basis for such a drug to decrease intraocular pressure has not been clearly demonstrated.

Effects of l- and d-timolol on cyclic AMP synthesis and intraocular pressure in water-loaded, albino and pigmented rabbits

Topical 2% l- or d-timolol reduced the elevation of intraocular pressure induced by water-loading in conscious rabbits. This drug effect appeared on the peak elevation (in pigmented eyes) and on the down-phase (in albino and pigmented eyes) of elevated intraocular pressure. The contralateral eye and the treated eye responded similarly. In urethane anesthetized, water-loaded rabbits, a greater inhibitory effect of l-timolol was observed in pigmented eyes than in albino eyes. Two per cent l-timolol caused alterations of heart rate and arterial blood pressure in water-loaded anesthetized rabbits, but time courses of these alterations did not correlate with the inhibitory effect on the elevation of intraocular pressure. The beta-adrenergic antagonistic activity of l-timolol and d-timolol were compared by their ability to inhibit l-isoproterenol-stimulated cyclic AMP synthesis in the rabbit iris-ciliary body preparation in vitro. The I50S for l- and d-timolol differ by about 1.5 log units. In our studies, d-timolol has little of the intraocular pressure lowering and the beta-adrenergic antagonistic activity of l-timolol. Thus, the conscious, water-loaded, pigmented rabbit can be used as a model for studying the effects of beta-adrenergic antagonists on intraocular pressure.

Beta-adrenergic and serotonergic responsiveness of rabbit corneal epithelial cells in culture

Rabbit corneal epithelial cell cultures were established from Dispase-treated anterior corneas. In culture medium containing cholera toxin, insulin and epidermal growth factor, these cells proliferated in vitro in the absence of any contaminating cells. Following subculture, cells retained epithelial morphology and the ability to synthesize cAMP in response to beta-adrenergic stimulation, but lacked the ability to respond to serotonergic stimulation. Retention of the beta-adrenergic system in culture serves as a functional epithelial cell marker; whereas expression of serotonergic responsiveness may be regulated by developmental or extrapithelial systems that are absent in these cell cultures.

Changes in responsiveness of the beta-adrenergic and serotonergic pathways of the rabbit corneal epithelium

Adrenergic agonists stimulate the synthesis of cyclic AMP by incubated rabbit corneas with the following order of potency: isoproterenol greater than epinephrine greater than norepinephrine. These agonists have the same order of potency when displacing the specific, beta-adrenergic radioligand, 3H-dihydroalprenolol, from beta-adrenergic receptors on membranes prepared from corneal epithelium. At another locus, serotonin stimulates cyclic AMP synthesis. Inhibition of stimulation in vitro by lysergic acid diethylamide, methysergide, cyproheptadine, and spiroperidol demonstrates the specificity of this pathway for serotonin. Topical epinephrine causes subsensitivity or decreased responsiveness of the beta-adrenergic pathway. There is loss of approximately half the beta-adrenergic receptors from the cornea and a similar loss of epinephrine-stimulated cyclic AMP synthesis, both of which return to control levels in 96 hrs. There is no change in affinity for catecholamines and no loss of responsiveness to prostaglandin E2 or serotonin. Pretreatment with nialamide or subsequent treatment with additional epinephrine does not cause further loss of responsiveness. Supersensitivity or increased responsiveness of this pathway occurs following superior cervical ganglionectomy. Topical serotonin causes decreased responsiveness of the serotonergic pathway. When potentiated by nialamide, serotonin causes almost complete loss of serotonin-stimulated cyclic AMP synthesis for 24-48 hrs. There is no loss of responsiveness to epinephrine. Increased responsiveness of this pathway does not occur following superior cervical ganglionectomy. The authors conclude that the corneal epithelium has both beta 2-adrenergic and serotonin-2 pathways, and each pathway exhibits altered responsiveness by similar mechanisms. In response to exogenous or endogenous stimulation, the beta-adrenergic responsive cells and the serotonergic responsive cells apparently regulate the total number of pathway-specific receptors on their surfaces. Furthermore, the authors postulate that two populations of beta-adrenergic responsive cells exist; those on the apical surface of the epithelium that respond to catecholamine in the tears and those near the basal surface that respond to neuronal catecholamine.

Neural serotonin stimulates chloride transport in the rabbit corneal epithelium

Evidence is presented that serotonin acts as a neurotransmitter in the cornea of the adult rabbit. Serotonin was localized to granules in a sparse population of subepithelial corneal nerves by an electron microscopic histochemical procedure. Significant endogenous levels of serotonin and its principal metabolite, 5-hydroxyindoleacetic acid, were detected in the central cornea by a fluorometric assay. Exogenous serotonin stimulated ion transport by corneal epithelium. This effect was potentiated by monoamine oxidase inhibition and was unaffected by an alpha-adrenergic receptor antagonist. Serotonin-stimulated ion transport was inhibited by the specific antagonist, methysergide, and by the replacement of Cl- with an impermeable anion. In tracer experiments, the serotonin-stimulated ion transport was shown to be caused by increased epithelial Cl- secretion. The serotonin response was partially inhibited by the beta-adrenergic antagonist, timolol. In a companion article, assay of corneal cyclic AMP showed stimulation of cyclic AMP synthesis by serotonin, inhibition by the specific antagonist, lysergic acid diethylamide, and potentiation by monoamine oxidase inhibition. We postulate that specific serotonergic receptors are present in the corneal epithelium and that activation of these receptors by serotonin released from serotonergic neurons increases the level of cyclic AMP, which stimulates active Cl- secretion by the corneal epithelium.

Serotonin-stimulated cyclic AMP synthesis in the rabbit corneal epithelium

Serotonin increases the level of cyclic AMP in incubated rabbit corneas; the concentration of agonist producing half-maximal stimulation is approximately 1.5 microM. Nialamide, an inhibitor of monoamine oxidase, potentiates the response to serotonin but not to epinephrine. Amitriptyline, an inhibitor of neuronal uptake of serotonin, does not potentiate the stimulation of cyclic AMP synthesis. Lysergic acid diethylamide, but not timolol, antagonizes the response to serotonin; the half-maximal inhibitory concentration is approximately 6 nM lysergic acid diethylamide. A comparison of the time course of the increase in cyclic AMP synthesis after addition of serotonin or epinephrine to the incubation media indicates that serotonin, but not epinephrine, must penetrate a barrier to its free diffusion. We conclude that the corneal epithelium contains specific serotonergic receptors that, upon activation, cause the synthesis of cyclic AMP, which mediates the stimulation of chloride transport (c.f. companion article, Klyce et al.). The serotonergic receptors must be at a location posterior to the beta-adrenergic receptors, which are on the anterior-surface of the apical cells.