Philip J. Anderton

Surface pressure measurements of human tears and individual tear film components indicate that proteins are major contributors to the surface pressure

Purpose: Tear film stability has been associated with a low surface tension (high surface pressure), which has been attributed to a variety of tear film components. In this study, we examined the contribution of various tear proteins, mucin, and meibomian lipids to the surface pressure of human tears. Methods: A Langmuir trough was used to measure and compare the surface activities of albumin, lipocalin, β-lactoglobulin, lactoferrin, lysozyme, secretory IgA, mucin, meibomian lipid, and tears. Results: All proteins exhibited surface activity. The surface pressure-area (Π-A) profiles of most protein films at equilibrium surface pressure (Πeq) were sigmoidal and showed hysteresis between the expansion and compression phases of the cycle. Πeq of most proteins took 4-9 hours to occur. By contrast, the Π-A profiles for meibomian lipid films were hyperbolic rather than sigmoidal and had little hysteresis, and Πeq was attained within 1 hour. The Π-A profiles of mucin films showed mostly hyperbolic characteristics with small hysteresis. The Π-A profiles of films of tears were sigmoidal, showed strong hysteresis, and reached Πeq at about 5 hours. Partitioning of the proteins and whole tears into the subphase also occurred. Conclusion: Comparison between the dynamic Π-A profiles of tears and those of individual tear film components shows that tear film proteins not only are capable of surface activity but also are major contributors to the surface activity of the tear film.

The surface activity of purified ocular mucin at the air-liquid interface and interactions with meibomian lipids

Purpose: Ocular mucins are thought to contribute to the stability of the tear film by reducing surface tension. The purpose of this study was to compare the effect of different mucins and hyaluronic acid (HA) alone and mixed with meibomian lipids on the surface pressure at an air-liquid interface. Methods: A Langmuir trough and Wilhelmy balance were used to measure and compare the surface activity of bovine submaxillary gland mucin (BSM), purified BSM, purified bovine ocular mucin and HA, and mixtures of these with meibomian lipids, phosphatidylcholine, and phosphatidylglycerol. Their appearance at the surface of an air-buffer interface was examined using epifluorescence microscopy. Results: Purified ocular mucin had no surface activity even at concentrations that were 100 times more than normally occur in tears. By contrast, commercial BSM caused changes to surface pressure that were concentration dependent. The surface pressure-area profiles showed surface activity with maximum surface pressures of 12.3-22.5 mN/m depending on the concentration. Purified BSM showed no surface activity at low concentrations, whereas higher concentrations reached a maximum surface pressure of 25 mN/m. HA showed no surface activity, at low or high concentrations. Epifluorescence showed that the mucins were located at the air-buffer interface and changed the appearance of lipid films. Conclusion: Purified bovine ocular mucin and HA have no surface activity. However, despite having no surface activity in their own right, ocular mucins are likely to be present at the surface of the tear film, where they cause an increase in surface pressure by causing a compression of the lipids (a reorganization of the lipids) and alter the viscoelastic properties at the surface.

Immunohistochemical and histochemical characterisation of epithelial cells of rabbit lacrimal glands in tissue sections and cell cultures

The purpose of this study was to establish conditions for isolation and long term culture of acinar cells from the Harderian gland, and superior and inferior lacrimal glands of the rabbit and to compare the in vitro growth patterns of cultured cells from these glands. In order to determine the predominant cell type in the cultures, cells and tissue sections were stained using a variety of antibodies to cytokeratins, smooth muscle actin, and neuron specific enolase. Similarly, PAS and alcian blue histochemistry were used to test for the presence of mucins. The glands were excised and cells isolated using enzymatic digestion and then established in long term culture. Different media and substrata were trialed for suitability. When cultured on uncoated Costar plastic in DMEM/10%FBS, the pattern of cell growth was similar for all glands with distinct phases involving aggregation and migration out from the aggregates before cells died between 20 to 30 days. Immunohistochemical staining indicated that the cultures were of acinar cells with a small percentage of ductal cells. The acinar cells of the lacrimal glands in situ and in vitro stained with antibody MNF116 directed against cytokeratins 5, 6, 8 and 17 but did not stain for antibodies to cytokeratin 18. The reverse staining pattern was true for the Harderian gland. Sections from the white lobe of the Harderian gland showed islets of serous secreting cells which showed positive staining when MNF116 was used. In situ, PAS positive cells were found in a small number of demilunes in the superior and inferior lacrimal glands and also in cells of the intercalated ducts. Surprisingly, in culture nearly all cells, including those isolated form the Harderian gland became PAS positive. In this study we have demonstrated that acinar cells from the Harderian and lacrimal glands of rabbit can be isolated and maintained in culture for 20 to 30 days, and that despite dramatic morphological changes, these cells retain their distinctive phenotype as indicated by antibody staining to specific cellular structural proteins such as cytokeratins and actin. However, the cultured cells also begin to produce mucins as indicated by PAS staining.

The incremental sensitivity curve of turtle cone photoreceptors

where AZ is the incremental threshold intensity measured at each background intensity, Zb. The constant I0 is photopic absolute threshold and factor C is a proportionahty constant relating threshold elevations to increases in background intensity {the Weber fraction). While this relation provides a quantitative description of the psychophysical process of background adaptation, the neural mechanisms which underty this ~enom~on are as yet ~n~ompleteiy understood. Intmcel~u~ar recordings made in the vertebrate retina have demonstrated that cone photoreceptors are desensitized by background illumination (Baylor and Hodgkin, 1973; Baylor and Hodgkin, 1974; Normann and Werblin, 1974; Normann and Perhnan, 1979a). Baylor and Hodgkin (1974) studied how cone sensitivity is affected by both I.lOsec brief exposures of tight, and by steady state exposures of light. Most of their work was done with brief ~~kg~unds where two forms of cone de~nsjti~tion are present; a desensitization due to thecone’s ins~n~n~us ~n~~n~rity (the nonlinear relation between light intensity and cone voltage response), and a desensitization due to the background light itself. The instantaneous nonlinearity is time dependent and can be minimized by using steady state backgrounds, a condition also used by

The ocular surface, the tear film, and the wettability of contact lenses.

The tear film is the interface between the ocular surface and the external environment and, as such, plays several important roles.1 (i) It forms a refracting thin film that smooths out the irregular corneal surface topography. (ii) It maintains an extracellular environment for the epithelial cells of the cornea and the conjunctiva that is fairly constant in terms of pH, oxygen and carbon dioxide levels, and nutrient and growth factor concentrations. (iii) Tears dilute and wash away noxious stimuli, including bacteria, which are also combated by an elaborate and effective antibacterial system. (iv) The tear film changes its composition in response to physiological stimuli.

Photoreceptor contributions to contrast sensitivity: Applications in radiological diagnosis

Electrophysiological and psychophysical observations are described which appear to have important implications in diagnostic image interpretation. While the interpretation of a complex radiographic image depends upon processing by higher-order neural centers, this interpretation process is influenced by the sensitivity of the visual system. Attention is given to two mechanisms which operate at the input stages of the visual system and which directly affect visual sensitivity: (1) the effects of background illumination on cone photoreceptor sensitivity, and (2) the image-forming properties of the eye (including small involuntary eye movements). The authors have constructed a model of the preprocessing occurring in the eye, by which light patterns viewed by a subject are transformed into patterns of neural activity of the cone photoreceptors in the subjects retina. The desensitization of cone photoreceptors by steady background illumination was studied using intracellular techniques. Background intensities desensitize both cone photoreceptors and the photopic visual system in a similar manner. This finding is consistent with the hypothesis that photopic visual threshold may be set at the very input to the visual system by the sensitivity of the cone photoreceptors. These two mechanisms appear to account for much of the visual system desensitizations caused by both uniform and stylized nonuniform backgrounds.

Implementation of evidence‐based practice in optometry

Evidence‐based practice in health care requires that treatment decisions be supported by high‐quality clinical evidence, preferably involving well‐designed large‐scale clinical trials of the various treatment options. How does this relate to everyday primary‐care optometry? This article provides a background to the emergence of the ‘evidence‐based’ movement and investigates areas where the efficacy of clinical optometry might be improved by an evidence‐based approach. It is argued that evidence‐based practice is appropriate for all aspects of optometry but it may be most useful in the selection of treatments with topical therapeutic agents and in optometry’s growing public health role in the detection and management of sight‐threatening disease.

Multifocal, pattern and full field electroretinograms in cats with unilateral optic nerve section

Aim: To look for a subcomponent of the mFERG generated at the optic nerve head and increasing in latency with distance from it. To compare multifocal electroretinogram (mFERG, mPERG) changes to those in full field ERGs and transient and steady state pattern and focal ERGs (PERGs, FERGs) in cats with total unilateral optic nerve section. Method: We recorded multifocal flash ERGs (mFERGs) at three levels of intensity and multifocal pattern ERGs (mPERGs) within 61 equal areas after total unilateral optic nerve section in five long term (>18 month) survival cats, as part of a long term serial study of full field flash and pattern ERG changes to many stimuli, in a larger population. Cats were anaesthetised with Ketamine/Xylazine and wore Henkes electrodes with 6mm artificial pupils. Intact retinal circulation was verified by fluorescein angiography and optic nerve section by retinal photography and histology. We compared the mean and mean summed multifocal responses, from the normal and denervated eyes. We also compared the mean interocular difference around the area centralis and as a function of distance from the optic nerve head, across the horizontal meridian for the mFERG to the most intense stimulus. The degree of change was compared to that in other types of ERG, in the larger set of cats. Results: mFERGs were similar across cats. Response density was flat with no prominence at the area centralis. Average summed mFERGs were similar in the normal and denervated eye. In the interocular differences a component near OP2 was reduced in the first kernel to the most intense stimulus, near OP1 and OP3 in the second kernel and locally, there was a hint of a component near OP2, which varied in latency in a ring around the disk and at the area centralis. Nevertheless, no component could be seen, varying in latency with distance from the optic nerve head, across the horizontal meridian. No mPERG was recordable in these conditions. Full field PERGs and FERGs were very reduced. Full field flash ERG amplitude changes were small (4–20%) and slower to appear than PERG changes. Degree of ERG reduction and correlation with PERG losses was greatest for the mesopic OPs, low for the scotopic tests (STR, ERG and OPs) and near zero for the mesopic ERG. The mFERG and mesopic ERG both lost OPs without overall amplitudes. Conclusions: The cat mFERG does not have a component, varying in latency with distance from the optic nerve head. The only change was qualitiatively similar to that in the light adapted ERG. With intense stimuli there were local changes around the time of OP2 near the area centralis which might be explained by local variations in ganglion cell density.

Effects of excitatory amino acids and their antagonists on the light response of luminosity and color-opponent horizontal cells in the turtle ( Pseudemys scripta elegans ) retina

Both kainic acid (KA) and N-methyl-d-aspartatic acid (NMDA) depolarize luminosity-type horizontal cells (L-type H cells) in normal turtle retina. The presence of both NMDA and non-NMDA receptors for excitatory amino acids (EAAs) on these cells was highlighted by an unusual effect of the noncompetitive NMDA-antagonist, MK-801. In retinas that had been exposed to MK-801, the action of NMDA was irreversibly altered to one of hyperpolarization, while the depolarizing effect of KA was unaltered. The aim of the present study was to further characterize these receptors on L-type H cells and to extend the investigation to color-opponent H cells (C-type H cells). Intracellular recording was used to study the effects of KA, NMDA, MK-801, the competitive NMDA antagonists, 2-amino-5-phosphonopentanoic acid (AP5) and 2-amino-7-phosphonoheptanoic acid (AP7), and the nonspecific EAA antagonist, kynurenic acid (KYN) on the light responses of L-type and C-type H cells in turtle retina. The effects of combinations of these drugs were also studied. In L-type H cells the agonists caused depolarization and loss of light response, KYN caused hyperpolarization and loss of light response, and MK-801, AP5 or AP7 had no direct effect. However, application of NMDA following MK-801, AP5 or AP7, but not KYN, caused hyperpolarization and loss of light response. The depolarizing effect of KA was unaltered by these antagonists. These data confirm the presence of an unusual NMDA receptor on L-type H cells. In the case of red/green C-type H cells, application of KA caused loss of responses to both red and green light, with loss of green responses preceding loss of red responses. NMDA initially removed responses to both red and green light. The most striking effect of NMDA was seen during early washout where the responses to red were reversed (hyperpolarizing). These responses eventually recovered their normal polarity. These results suggest that the depolarizing response of C-type H cells to red light is mediated by L-type H cells, but not via inhibition of the excitatory input from green cones to C-type H cells.

MK801-induced antagonism of NMDA-preferring excitatory amino acid receptors in horizontal cells of the turtle retina

Intracellular recordings were made from axon terminals of L-type horizontal cells in the turtle (Pseudemys scripta elegans) retina. Superfusion with Ringers solution containing 3.0 mM N-methyl-D-aspartate (NMDA) or 0.2 mM kainic acid (KA) induced depolarization and reduction in the hyperpolarizing light responses of horizontal cells, consistent with an agonist effect of these excitatory amino acid (EAA) analogs on postsynaptic receptors. Delivery of 0.1 mM MK801, a selective blocker of NMDA-type EAA receptors, had no apparent effect on membrane potential or photoresponses, nor did it change the KA depolarization. Exposure of the retina to 3.0 mM NMDA following 0.1 mM MK801 always caused hyperpolarization of the horizontal cell and loss of light responses. Because MK801 is specific for NMDA-preferring receptors, we suggest that the reversal of the NMDA response to one of antagonism following MK801 is strong evidence for the presence of NMDA-preferring EAA receptors in turtle horizontal cells.