Measurement of mouse intraocular pressure with the Tono-Pen

Differential Intraocular Pressure Measurements by Tonometry and Direct Cannulation After Treatment with Soluble Adenylyl Cyclase Inhibitors

PURPOSE

To validate the increase in intraocular pressure (IOP) caused by soluble adenylyl cyclase (sAC) inhibitors and determine reasons behind variation in IOP measurements performed by tonometry.

METHODS

C57BL/6J mice were administered DMSO solubilized sAC inhibitors (KH7 or LRE-1) by intraperitoneal injection. Two hours post-treatment, mice were anesthetized with avertin or ketamine/xylazine/acepromazine (KXA). IOP was measured by a rebound tonometer or direct cannulation of the anterior chamber. Spectral-domain optical coherence tomography was used to measure anterior chamber depth and corneal thickness in live mice. Outflow facility was measured in perfused, enucleated mouse eyes.

RESULTS

Compared with DMSO controls, KH7 treatment caused an increased IOP in avertin- and KXA-anesthetized mice when measured by direct cannulation [avertin: 14.4 ± 2.1 mmHg vs. 11.1 ± 1.0 mmHg (P = 0.003); KXA: 14.4 ± 1.0 mmHg vs. 11.3 ± 0.8 mmHg (P < 0.001)] and tonometry [avertin: 10.8 ± 1.4 mmHg vs. 7.4 ± 0.6 mmHg (P < 0.001); KXA: 11.9 ± 0.9 mmHg vs. 10.3 ± 1.7 mmHg (P = 0.283)]. However, treatment with KH7 in nonanesthetized mice showed a significant decrease in IOP measured by tonometry and compared with DMSO-treated animals [13.1 ± 2.6 mmHg vs. 15.6 ± 0.5 mmHg (P = 0.003)]. Both KH7- and DMSO-treated groups anesthetized with avertin showed increased corneal thickness, whereas KH7-treated mice anesthetized with KXA exhibited a shallower anterior chamber compared with untreated mice. KH7 decreased outflow facility by 85.1% in nonanesthetized, enucleated eyes (P < 0.003).

CONCLUSIONS

Systemically administered DMSO and anesthesia have significant effects on anterior chamber characteristics, resulting in altered IOP readings measured by tonometry. In the presence of DMSO and anesthesia, tonometry IOP readings should be confirmed with direct cannulation.

Expression of 14-3-3 Zeta Protein in Dexamethasone-Treated Mice and Human TM-1 Cells

PURPOSE

14-3-3 zeta protein plays a potential protective role in neurodegenerative disease. Given that glaucoma and neurodegenerative diseases share a similar pathogenesis, it is possible that 14-3-3 zeta may have a similar protective effect in the glaucomatous process. In the present study, we measured the expression of 14-3-3 zeta in vivo (mouse eyes) and in vitro in a transformed human trabecular meshwork (HTM) cell line, TM-1, and assessed the possible roles of this protein in dexamethasone (DEX)-treated eyes and HTM cells.

METHODS

Mouse eyes were randomly treated with 0.1% dexamethasone (DEX) eye drops or phosphate-buffered solution (PBS) for 28 days. The expression and distribution of 14-3-3 zeta protein in mouse eyes were examined using immunofluorescence. TM-1 cells were treated with DEX (10^-6 or 10^-7 M) or PBS for 1, 4, or 7 days, and the mRNA and protein expression of 14-3-3 zeta were detected by real-time RT-PCR and Western blotting.

RESULTS

14-3-3 zeta protein was highly expressed in the mouse cornea, trabecular meshwork (TM), and ciliary body. Intraocular pressure (IOP) was significantly elevated, whereas the 14-3-3 zeta expression was significantly decreased in mouse TM after 0.1% DEX treatment for 28 days. In vitro, treatment with 10^-7 M DEX mildly increased 14-3-3 zeta mRNA and protein expression (p > 0.05), whereas 10^-6 M DEX significantly decreased expression of 14-3-3 zeta mRNA and protein (p < 0.05) compared to the control (Ctrl) group at the seventh day.

CONCLUSIONS

DEX can increase IOP in mouse eyes and concurrently downregulate 14-3-3 zeta protein expression in mouse TM. The effects of DEX on 14-3-3 zeta expression in vitro were both dose- and time-related. Our results suggest that alterations in 14-3-3 zeta protein may be implicated in DEX-induced pathological elevated IOP.

Non-continuous measurement of intraocular pressure in laboratory animals

Glaucoma is a leading cause of blindness, which is treatable but currently incurable. Numerous animal models therefore have both been and continue to be utilized in the study of numerous aspects of this condition. One important facet associated with the use of such models is the ability to accurately and reproducibly measure (by cannulation) or estimate (by tonometry) intraocular pressure (IOP). At this juncture there are several different approaches to IOP measurement in different experimental animal species, and the list continues to grow. We feel therefore that a review of this subject matter is timely and should prove useful to others who wish to perform similar measurements. The general principles underlying various types of tonometric and non-tonometric techniques for non-continuous determination of IOP are considered. There follows discussion of specific details as to how these techniques are applied to experimental animal species involved in the research of this disease. Specific comments regarding anesthesia, circadian rhythm, and animal handling are also included, especially in the case of rodents. Brief consideration is also given to possible future developments.

Calibration of the TonoLab tonometer in mice with spontaneous or experimental glaucoma

PURPOSE

To measure the accuracy of TonoLab (TioLat, Helsinki, Finland) tonometry in mice with spontaneous or induced experimental glaucoma.

METHODS

Chronic intraocular pressure (IOP) elevation was induced in one eye of 32 mice by injection of polystyrene beads and viscoelastic material. Three to 6 weeks later, the eyes were cannulated and manometrically set to 10, 20, 30, 40, or 50 mm Hg. The mice were 8-week and 8-month-old C57BL/6, 8-week-old DBA/2J, and 8-week-old CD1. The TonoLab calibration was also tested on five aged DBA/2J mice with spontaneous glaucoma. The relation of the TonoLab reading to manometric IOP was evaluated in multivariate linear regression models with axial length, IOP history, and mouse strain as independent variables.

RESULTS

The slope of the relationship between TonoLab and manometric IOP in all the mice was 0.998, with an intercept of 2.3 mm Hg (adjusted R in univariate regression = 0.86). Neither the mice with bead-induced glaucoma nor those with spontaneous glaucoma (older DBA/2J mice) differed significantly from the control animals in having an excellent correlation between TonoLab and manometer IOP. Longer and wider mouse eyes had slightly higher tonometrically measured IOP, whether glaucomatous or control (multivariate regression, adjusted R(2) = 0.90, P < 0.0001). There was no difference in tonometric accuracy among the three mouse strains: CD1, C57BL/6, and DBA/2J, nor between 8-week and 8-month-old C57BL/6 mice (multivariate regression, P = 0.32).

CONCLUSIONS

The TonoLab accurately reflects IOP in both normal mice and in eyes of mice with experimental or spontaneous glaucoma, with no detectable effect of age.

Efficient in vivo doxycycline and cre recombinase-mediated inducible transgene activation in the murine trabecular meshwork

PURPOSE

To generate new mouse lines that facilitate inducible gene activation in the murine trabecular meshwork in vivo.

METHODS

Two expression cassettes were knocked into the 3'-UTR of the Myocilin (Myoc) locus, an abundantly expressed extracellular matrix protein produced by cells of the trabecular meshwork. The first cassette directs expression of an inducible form of Cre recombinase, CreER(T2), which is activated by tamoxifen administration under the control of endogenous Myoc regulatory elements. The second cassette contains a reverse tetracycline transactivator, rtTA(M2), which directs the expression of tetracycline-operator transgenes on exposure of animals to doxycycline (Dox). These lines were crossed to GFP and lacZ reporter mice to assay for tamoxifen or Dox-induced transgene expression.

RESULTS

Both the Myoc-CreER(T2) and the Myoc-rtTA(M2) lines were capable of directing efficient and inducible expression of transgenes in the murine trabecular meshwork in vivo. In addition, activation of transgenes by Myoc-rtTA(M2) was reversible with loss of transgene expression after Dox withdrawal. Examination of multiple tissues demonstrates efficient transgene activation in the trabecular meshwork, with additional sites of transgene activation including cells in the retina, sclera, lung, kidney, and abundant activation in the neocortex and hippocampus.

CONCLUSIONS

Two new mouse lines have been generated that allow for efficient and inducible transgene activation in the murine trabecular meshwork in vivo.

Neurotrophin roles in retinal ganglion cell survival: lessons from rat glaucoma models

The neurotrophin (NT) hypothesis proposes that the obstruction of retrograde transport at the optic nerve head results in the deprivation of neurotrophic support to retinal ganglion cells (RGC) leading to apoptotic cell death in glaucoma. An important corollary to this concept is the implication that appropriate enhancement of neurotrophic support will prolong the survival of injured RGC indefinitely. This hypothesis is, perhaps, the most widely recognized theory to explain RGC loss resulting from exposure of the eye to elevated intraocular pressure (IOP). Recent studies of NT signaling using rat glaucoma models, have examined the endogenous responses of the retina to pressure exposure as well as studies designed to augment NT signaling in order to rescue RGC from apoptosis following pressure-induced injury. The examination of these studies in this review reveals a number of consistent observations and provides direction for further investigations of this hypothesis.

Rodent models for glaucoma retinopathy and optic neuropathy

Animal models are useful to elucidate the etiology and pathology of glaucoma and to develop novel and more effective therapies for the disease. Because of the substantial similarities between the rodent and primate eyes, and the advances of relevant study techniques, rat and mouse models of glaucoma have recently become popular as research tools. This review surveys research techniques used in the measurement of rodent intraocular pressure, and also the evaluation of pertinent morphologic, biochemical, and functional changes in the retina, optic nerve head, and optic nerve. This review further describes in detail the individual rodent models, some of which serve as surrogate models and do not entail ocular hypertension, whereas others involve transient or chronic increases of intraocular pressure. The technical considerations and theoretical concerns of these models, their advantages, and limitations, are also discussed.