Synaptic activity modulates the induction of bidirectional synaptic changes in adult mouse hippocampus

Activity-dependent synaptic plasticity is critical for learning and memory. Considerable attention has been paid to mechanisms that increase or decrease synaptic efficacy, referred to as long-term potentiation (LTP) and long-term depression (LTD), respectively. It is becoming apparent that synaptic activity also modulates the ability to elicit subsequent synaptic changes. We provide direct experimental evidence that this modulation is attributable, at least in part, to variations in the level of postsynaptic depolarization required for inducing plasticity. In slices from adult hippocampal CA1, a brief pairing protocol known to produce LTP can also induce LTD. The voltage-response function for the induction of LTD and LTP in naive synapses exhibits three parts: at a postsynaptic membrane potential during pairing (V(m)) </= -40 mV, no synaptic modification is obtained; at V(m) between -40 and -20 mV, LTD is induced; and, finally, at V(m) > -20 mV, LTP is generated. This function varies with initial synaptic efficacy. In depressed synapses, Theta(-), the V(m) above which LTD is generated, is shifted toward more depolarized V(ms) and Theta(+), the LTD-LTP crossover point or, equivalently, the V(m) above which LTP is induced, toward more polarized V(ms). Conversely in potentiated synapses, Theta(-) is shifted toward more polarized V(ms). Therefore synaptic activity changes synaptic efficacy and accordingly adjusts the voltages for eliciting subsequent synaptic modifications. The concomitant shifts in the voltages for inducing LTD and LTP in opposite directions promote synaptic potentiation and inhibit synaptic depression in depressed synapses and vice versa in potentiated synapses.

Retinal detachment in myopic eyes after photorefractive keratectomy

PURPOSE

To analyze the incidence and characteristics of retinal detachment (RD) in myopic patients who had photorefractive keratectomy (PRK).

SETTING

Universidad Miguel Hernández, Instituto Oftalmológico de Alicante, Alicante, Spain.

METHODS

The incidence of RD in 5936 consecutive eyes that had PRK to correct myopia was studied. Mean follow-up was 38.5 months +/- 17.4 (SD).

RESULTS

Retinal detachment occurred in 5 eyes (0.08%); 2 in women and 3 in men. The mean interval between PRK and RD was 21. 00 +/- 15.89 months (range 9 to 48 months). The mean best corrected visual acuity (BCVA) after PRK and before RD development was 20/81 (range 20/200 to 20/25). After RD repair, the mean BCVA was 20/460 (range 20/2000 to 20/29). In 4 of the 5 eyes, BCVA after RD was within 1 line of the preoperative value; in 1 eye, it decreased from 20/40 to 20/2000. The mean spherical equivalent (SE) before RD treatment was -1.35 +/- 1.08 diopters (D) (range 0 to -3.00 D) and after RD treatment, -2.95 +/- 0.83 D (range -2.00 to -4.00 D). Differences between SE before and after RD treatment were statistically significant (P =.01, paired Student t test).

CONCLUSIONS

The incidence of RD after PRK to correct myopia was 0.08%. In 4 of 5 eyes, there was little or no visual loss; but in the group as a whole, there was a significant increase in myopic SE.

Photorefractive keratectomy for residual myopia after cataract surgery

PURPOSE

To evaluate the effectiveness, predictability, and safety of photorefractive keratectomy (PRK) for correcting residual myopia and myopic astigmatism after cataract surgery with intraocular lens implantation.

SETTING

Refractive Surgery and Cornea Unit, Instituto Oftalmológico de Alicante, Alicante, Spain.

METHODS

Thirty consecutive eyes (30 patients) had PRK for residual myopia after cataract surgery. Surface PRK with a VISX Twenty-Twenty excimer laser was used in all patients. Follow-up was 1 year.

RESULTS

Before PRK, no eye had an uncorrected visual acuity (UCVA) of 20/40 or better. Twelve months after PRK, 16 eyes (53.33%) had a UCVA of 20/40 or better. After PRK, best corrected visual acuity (BCVA) improved 1 line or more in 14 eyes (46.66%) over the preoperative values, and 15 eyes (50.00%) had the same BCVA as before PRK. Mean pre-PRK refraction of -5.00 diopters (D) +/- 2.50 (SD) decreased significantly to -0.25 +/- 0.50 D at 12 months (P < .001). At 12 months, the spherical equivalent was within +/- 1.00 D of emmetropia in 27 eyes (90.00%). No vision-threatening complications occurred.

CONCLUSION

Photorefractive keratectomy was an effective, predictable, and safe procedure for correcting residual myopia and myopic astigmatism after cataract surgery.

Induced endothelial cell loss in phacoemulsification using topical anesthesia plus intracameral lidocaine

PURPOSE

To determine whether lidocaine 1% injected into the anterior chamber during phacoemulsification damaged the corneal endothelium.

SETTING

Ophthalmology Service, University Hospital, San Juan de Alicante, Spain.

METHODS

In 19 eyes of 19 patients, small incision phacoemulsification and foldable lens (AcrySof MA60BM) implantation was performed using topical anesthesia supplemented by lidocaine 1% (0.3 mL) in the anterior chamber. All cases were operated by the same surgeon. Specular microscopy was performed before surgery and 1 and 3 months postoperatively.

RESULTS

Mean endothelial cell loss was 5.41% +/- 8.50 (SD) at 1 month and 5.93 +/- 8.50% at 3 months.

CONCLUSIONS

The endothelial damage was similar to that described by other authors, suggesting that phacoemulsification surgery can be performed successfully using topical anesthesia supplemented by lidocaine 1% in the anterior chamber.

Metallic fragment deposits during phacoemulsification

PURPOSE

To describe the nature and origin of foreign metallic intraocular bodies appearing after phacoemulsification.

SETTING

Department of Ophthalmology, University Hospital of San Juan, Alicante, Spain.

METHODS

Two metallic particles, 1 extracted during surgery and the other from a patient in whom surgery had been performed, were studied by scanning electronic microscopy and X-ray dispersive energy spectroscopy. Also evaluated were all metallic elements used in phacoemulsification including phaco tips, Sinskey hooks, and handpieces. A morphologic analysis at various magnifications was performed and the composition of the elements studied.

RESULTS

Scanning electronic microscopy showed irregularities of the phaco tip and Sinskey hook after their use. Spectrographic analysis found that the phaco tip was mainly aluminum and titanium; the Sinskey hook, iron, chromium, cobalt, and nickel; the handpiece, iron, chromium, and nickel; and the 2 metallic particles, iron, chromium, and nickel, the same as the handpiece.

CONCLUSION

The metallic particles extracted corresponded to those of the phaco handpiece. Vibration during prolonged use of the phacoemulsifier probably caused these particles to chip off the handpiece.

Complications of photorefractive keratectomy for myopia: two year follow-up of 3000 cases

PURPOSE

To evaluate the results and complications of photorefractive keratectomy (PRK) for myopia.

SETTING

Alicante Institute of Ophthalmology, University of Alicante, Spain.

METHODS

This prospective study evaluated the results and complications of the first 3000 cases of PRK performed with a VISX Twenty-Twenty excimer laser. Myopia ranged from -1.0 to -14.0 diopters (D) and astigmatism, from -1.0 to -5.0 D. Follow-up was 2 years.

RESULTS

Ten eyes (0.7%) lost two or more lines of best corrected visual acuity 1 year after surgery. Retreatment for undercorrection or regression was done in 7.41% in the low myopia group and 38.69% in the high myopia group. Central islands, which could be detected only on videokeratoscopy, occurred frequently but influenced the refractive and visual outcomes in few cases. Severe haze was present in 17 eyes only after 1 year follow-up. There were no cases of progressive hyperopia. Other complications such as eccentric ablation producing astigmatism (n = 15), delayed re-epithelialization, or recurrent corneal erosion were rare.

CONCLUSION

There were few complications after PRK in 3000 eyes. With proper patient selection, PRK can be considered relatively safe compared with other refractive procedures.

Survey on drugs-of-abuse testing in the European Union

A survey on the quality of drugs-of-abuse testing in European laboratories was performed in 1993 (Part I) and 1994 (Part II). A total of 195 laboratories participated in Part I and 228 in Part II. There were 154 repeater laboratories. In each part of the survey, six urine samples were sent for analysis under routine conditions. A set of reference materials, including deuterated drug-standard solutions, was provided in Part II of the survey for optimization of analytical procedures. Screening for groups of substances was mainly performed by immunological techniques. Rates of false-positive (FP) results were 0 and 0.5%, respectively, in Parts I and II, and rates of false-negative (FN) results were 7.7% in Part I and 3.4% in Part II. Identification and quantitation of specific substances was mainly carried out by chromatographic methods, particularly gas chromatography coupled to mass spectrometry. Application of chromatographic methods showed a lack of sensitivity (14.9% and 16.7% FN in Parts I and II, respectively) and a lack of specificity (0.8% FP in Part I versus 1.1% in Part II). Repeater laboratories improved their results (17.4% FN in Part I versus 14.2% Part II). The improvement of repeater laboratories emphasizes the need of external quality control programs. The experience is being used as a basis for a recommendation on drug-testing guidelines by a group of European toxicologists.

Photorefractive keratectomy for pediatric myopic anisometropia

PURPOSE

To evaluate the visual results of excimer laser photorefractive keratectomy (PRK) to treat pediatric patients with amblyopic myopic anisometropia in whom conventional amblyopia treatments have failed.

SETTING

Instituto Oftalmológico de Alicante, University of Alicante, Spain.

METHODS

Six patients aged 5 to 7 years with amblyopic myopic anisometropia were treated by PRK in the more myopic eye using a VISX Twenty-Twenty excimer laser. Minimum preoperative refraction in the amblyopic eyes ranged from -4.00 to -13.00 diopters and best corrected visual acuity (BCVA), from 20/40 to 20/400. All cases completed a minimum of 2 years follow-up of antiamblyopic treatment.

RESULTS

After PRK, preoperative BCVA was maintained without optical correction and complementary refraction resulted in increased BCVA in every case. One patient with high myopia developed severe haze. There were no other significant complications.

CONCLUSION

Our results indicate that PRK may be an alternative for the correction of pediatric myopic anisometropia and an important aid in treating amblyopia. When other therapies have failed, using PRK in children of the ages in this study must be carefully evaluated; however, our results suggest a beneficial effect.

Relation between dendritic Ca2+ levels and the polarity of synaptic long-term modifications in rat visual cortex neurons

Long-term changes of synaptic efficacy, in particular when they are use-dependent, are candidate mechanisms for the storage of information in the nervous system. In a variety of brain structures, including the neocortex and hippocampus, synapses are susceptible to long-term potentiation (LTP) and long-term depression (LTD). It has been hypothesized that the polarity of the synaptic gain change depends on the amplitude of the postsynaptic [Ca2+]i rise, the threshold for the induction of LTD being lower than that for the induction of LTP. To test this assumption, we characterized Ca2+ signals in layer II/III pyramidal cells of rat visual cortex slices, using the fluorescent Ca2+ indicator fura-2, during application of stimulation protocols that had been adjusted to reliably induce either LTP or LTD in cells not loaded with fura-2. At dendritic sites activated by the stimulated afferents the intracellular [Ca2+] concentration ([Ca2+]i) reached higher amplitudes and decayed more slowly with stimuli inducing LTP than with those inducing LTD. To directly analyse the functional significance of the observed difference in the Ca2+ signal amplitude, we examined whether a tetanization protocol suitable for the induction of LTP can be converted into a protocol inducing LTD by injecting the postsynaptic cells with Ca2+ chelators that reduce the concentration of effective free Ca2+. In the presence of fura-2 or BAPTA [bis(2-aminophenoxy) ethane-N,N,N',N'-tetraacetate], the stimulation protocol that would normally produce LTP induced either LTD or failed to induce synaptic modifications altogether. These results support the hypothesis that the amplitude of the postsynaptic rise in [Ca2+]i is a key factor in the determination of the polarity of synaptic gain change.

Correction of Hyperopia Induced by Photorefractive Keratectomy Using Non-contact Ho:YAG Laser Thermal Keratoplasty

PURPOSE

To evaluate the safety and effectiveness of non-contact holmium:YAG laser thermal keratoplasty in correcting hyperopia induced by photorefractive keratectomy (PRK).

METHODS

Non-contact holmium:YAG laser thermal keratoplasty was applied to 14 eyes with significant hyperopia induced by PRK. The mean spherical equivalent refraction before holmium:YAG laser thermal keratoplasty was +4.20 +/- 1.80 diopters (D) (range, +1.75 to +6.25 D). The results were evaluated 12 months after holmium:YAG laser thermal keratoplasty.

RESULTS

No sight-threatening complications occurred. Recovery of spectacle-corrected visual acuity took from 2 to 6 weeks. The immediate significant myopic shift that developed in all eyes gradually receded over 6 to 8 weeks. All eyes were relatively stable after 9 months. At 12 months, there was no statistically significant difference (p < .005) between the mean preoperative spectacle-corrected visual acuity (0.71 +/- 0.12) and the mean postoperative uncorrected visual acuity (0.65 +/- 0.28). At 12 months there was a mean increase of 4.60 +/- 1.20 D in central keratometric power. Total regression did not occur in any eye.

CONCLUSIONS

Non-contact holmium:YAG laser thermal keratoplasty offers a safe and effective alternative to correct PRK-induced hyperopia.

Calcium-induced long-term depression in the visual cortex of the rat in vitro

1. In many brain areas, including neocortex and hippocampus, excitatory synapses can undergo both long-term potentiation (LTP) and long-term depression (LTD). It is established that a change in the postsynaptic calcium concentration ([Ca2+]i) is critical for the induction of both LTP and LTD. Protocols that induce these long-term synaptic modifications typically involve afferent stimulation. But, in hippocampus, LTP can also be induced by a transient increase of the extracellular calcium concentration ([Ca2+]o). The purpose of the present study was to determine whether raising [Ca2+]o also induces long-term modifications of excitatory synaptic transmission in the neocortex. 2. Intracellular recordings were obtained from regular spiking cells in layers II-III of slices of the rat visual cortex. Test stimuli were evoked with stimulation electrodes located in the white matter (w.m.) below the recorded cell and intracortically (i.c.) adjacent to the cell. Both the depolarizing slope and the amplitude of excitatory postsynaptic potentials (EPSPs) were measured. For exposure to elevated [Ca2+]o, the normal medium ([Ca2+]o = 2 mM) was exchanged for a period of 10 min against a medium containing 4 mM [Ca2+]o. 3. Elevated [Ca2+]o leads, after return to normal medium, to a long-lasting decrease of intracellularly recorded synaptic responses to both w.m. and i.c. stimulation even if activation of these two pathways is discontinued or N-methyl-D-aspartate (NMDA) receptors are blocked during elevated [Ca2+]o. This decrease is due to reduced efficacy of excitatory transmission because it is observed in the presence of the gamma-aminobutyric acid-A (GABAA) receptor antagonist, bicuculline. 4. Induction of LTD by raising [Ca2+]o is voltage dependent. First, elevated [Ca2+]o elicits LTD only in cells whose resting membrane potential (Vmr) is less polarized than -79 mV (and more polarized than -70 mV, which is the Vmr of the least polarized cell). Second, hyperpolarizing cells whose Vmr is in this susceptible range by 20 mV below Vmr during exposure to high [Ca2+]o prevents Ca2+-induced LTD. Third, when elevated [Ca2+]o is associated with postsynaptic depolarizing pulses, LTD is readily induced in cells whose Vmr is more polarized than -79 mV. This voltage dependence implies that the depression is induced by a postsynaptic process and hence that it occurs at synapses formed by excitatory terminals on the recorded neuron. 5. Assuming that a transient elevation of [Ca2+]o leads to an increase of [Ca2+]i, the results of this study suggest that a transient increase of [Ca2+]i is sufficient to elicit LTD. This may provide a mechanism for the induction of heterosynaptic LTD, a depression that occurs in afferents that are silent while the postsynaptic neuron is activated by other inputs.

The polysialic acid modification of the neural cell adhesion molecule is involved in spatial learning and hippocampal long-term potentiation

The alpha-2,8-linked polysialic acid (PSA) modification of the neural cell adhesion molecule (NCAM) modulates morphogenetic cell interactions. PSA is strongly expressed during neural development and generally down-regulated in the adult. However, it remains prominent in some areas of the brain, e.g., the hippocampus. We assayed the functional role(s) of PSA in synaptic plasticity in the hippocampus in two experimental paradigms by removing PSA with endo-neuraminidase NE (endo-N) an enzyme which specifically cleaves alpha-2,8-linked polysialic acid. (1) The acquisition and retention of spatial memory of rats in the Morris water maze, critically dependent on the hippocampus, was significantly impaired after a localized injection of endo-N into the hippocampus, whereas visual and motor capacities were unaffected. (2) Tetanic stimulation of the Schaffer collaterals in endo-N-treated hippocampal slices in vitro failed to elicit LTP and yielded only a short post-tetanic potentiation, but the response returned to control levels within 2 minutes, whereas basal synaptic activity and short-term potentiation were not affected. Our findings suggest that the carbohydrate epitope PSA plays an important role in synaptic plasticity.

Different threshold levels of postsynaptic [Ca2+]i have to be reached to induce LTP and LTD in neocortical pyramidal cells

Changes in [Ca2+]i were measured in layer II-III pyramid cells of the rat visual cortex slices during application of either LTP or LTD inducing stimulation protocols. At dendritic sites activated by the stimulated afferents [Ca2+]i reached higher amplitudes and decayed more slowly with LTP than with LTD inducing stimuli. In the presence of Ca2+ chelators, the stimulation protocol that would normally produce LTP induced either LTD or failed to induce a synaptic modifications altogether. These results support the hypothesis that the polarity of synaptic gain changes depends on the magnitude of postsynaptic [Ca2+]i responses, the induction of LTP requiring a more pronounced surge of [Ca2+]i than the induction of LTD.

Correcting simple myopic astigmatism with the excimer laser

For one year we followed 46 eyes that had photoastigmatic refractive keratectomy with an excimer laser to correct simple myopic astigmatism. Mean preoperative cylinder was -2.50 +/- 0.70 diopters (D); mean preoperative best corrected visual acuity was 20/25 +/- 1 Snellen line. Mean postoperative refraction one year after surgery was -0.5 +/- 0.2, and mean uncorrected visual acuity was 20/25 +/- 1 Snellen line. In all eyes, the final axis of the residual cylinder was +/- 5 degrees around the preoperative axis. Uncorrected visual acuity equaled best preoperative acuity in 24 eyes. Eight eyes gained one line of best corrected visual acuity, six gained more than one line, and eight lost one line. No patient lost more than one line of uncorrected visual acuity. No significant complications such as haze, halos, eccentric ablation, or hypercorrection were observed.

All-or-none excitatory postsynaptic potentials in the rat visual cortex

Intracellular recordings were obtained from supragranular neurons in slices of the rat visual cortex. In approximately 25% of the cells large (0.5-1.6 mV) excitatory postsynaptic potentials (EPSPs) of constant amplitude were observed after minimal, presumably single-fibre stimulation. The amplitude variance of these large EPSPs was surprisingly small and within the range of the variance of the noise. These EPSPs could be reduced in amplitude by paired-pulse and low-frequency stimulation or by raising extracellular Mg2+ concentration. Reduced EPSPs could either continue to behave as all-or-none responses, or they could fluctuate between several amplitude levels. Conversely, responses where the amplitude fluctuated from trial to trial under control conditions could be converted into large all-or-none responses by paired-pulse facilitation. This indicates that the large all-or-none EPSPs were composed of several subunits, probably reflecting the action of several different release sites. It is concluded that these release sites are either independent and operate with a probability close to 1 or, if operating with a lower probability, are coordinated by a mechanism which synchronizes release. Several observations suggest that release probabilities can switch from values close to 1 to 0 with repetitive stimulation or high Mg2+ concentration. Thus, a substantial fraction of single-fibre inputs to supragranular cells possess synapses which operate with high synaptic efficiency and extremely low variance under control conditions but can undergo drastic changes in efficacy when release probabilities are interfered with. Such modifications of release probability could serve as an effective mechanism to regulate the gain of synaptic transmission.

Effect of topical antioxidant therapy on experimental infectious keratitis

To test the effect that the treatment with topical antioxidants may have on corneal infection, we have studied the effect of topically applied antioxidants, such as dimethylthiourea 0.5% (DMTU) and dismutase superoxide 0.2% (SOD), on infectious experimental keratitis caused by Staphylococcus aureus. We have quantified the results of the incubated corneas in ex vivo as well as in in vivo treated with antioxidants by using the luminol amplified chemiluminescence technique (LAC). The evaluation of corneal inflammation was performed calculating the average inflammatory index obtained from the clinical observation of the corneal secretion, corneal edema and ciliary injection. The evolution of the corneal infiltration was evaluated by means of computerized planymetry. The antioxidants used in this study demonstrated a significant reduction of the LAC values when compared with a control group both in the in vivo as well as in ex vivo studies. No significant differences in the clinical evaluation of the average inflammatory index were observed between the study and the control groups. However, a significant increase in the corneal infiltration was registered in the antioxidant treated group (p < 0.001) evaluated by computerized planymetry. Our results indicate that the use of antioxidants as antiinflammatory drugs may have a potential negative influence on the course of infectious keratitis.

Antioxidant therapy in the treatment of experimental acute corneal inflammation

To ascertain the effectiveness of topical antioxidant therapy on acute corneal inflammation, we have studied the effectiveness of topical treatment with a saline solution and with antioxidants such as 0.2% superoxide dismutase and 0.5% dimethylthiourea (DMTU) in a controlled experimental study. The evolution of the inflammatory process was evaluated by a multimodel approach, including computer-assisted planimetry of the corneal ulcer and infiltrate, ultrasonic pachymetry, luminol-amplified chemiluminescence and the study of corneal transparency by direct spectral spectrophotometry transmittance. The experimental model was a corneal ulcer created by a 60-second application of 1 N sodium hydroxide. Topical treatment with DMTU was shown to significantly improve all parameters tested, while superoxide dismutase reduced only the corneal ulcers. Antioxidant topical therapy with DMTU was shown to be efficient in reducing the inflammatory reaction that occurs during acute corneal inflammation. This suggests that antioxidant therapy could be considered as a complementary treatment in the pharmacological modulation of acute corneal inflammation.

Topical treatment of experimental acute corneal inflammation by dexamethasone and nonsteroidal drugs

In this study, we have correlated the relative effectiveness of nonsteroidal anti-inflammatory drugs versus corticosteroids in the topical treatment of acute corneal inflammation. The authors have studied the effect of 1 drop every 6 h of flurbiprofen 0.03%, indomethacin 1%, diclofenac 0.1% and dexamethasone 0.1% in an experimental model of corneal alkali burn. The objective parameters used to evaluate the corneal inflammatory response were Luminol-enhanced chemiluminescence, ultrasonic pachymetry and computer-assisted analysis of the area of the corneal ulceration. Indomethacin 1% showed statistically significant effects on the improvement of all the parameters tested in the study. No such effect was demonstrated for flurbiprofen 0.03% or diclofenac 0.1%. We can conclude that indomethacin 1% is an effective drug in the treatment of experimental acute corneal alkali-burn inflammation.

Long-term depression of excitatory synaptic transmission and its relationship to long-term potentiation

In many brain areas, including the cerebellar cortex, neocortex, hippocampus, striatum and nucleus accumbens, brief activation of an excitatory pathway can produce long-term depression (LTD) of synaptic transmission. In most preparations, induction of LTD has been shown to require a minimum level of postsynaptic depolarization and a rise in the intracellular Ca2+ concentration [Ca2+]i in the postsynaptic neurone. Thus, induction conditions resemble those described for the initiation of associative long-term potentiation (LTP). However, data from structures susceptible to both LTD and LTP suggest that a stronger depolarization and a greater increase in [Ca2+]i are required to induce LTP than to initiate LTD. The source of Ca2+ appears to be less critical for the differential induction of LTP and LTD than the amplitude of the Ca2+ surge, since the activation of voltage- and ligand-gated Ca2+ conductances as well as the release from intracellular stores have all been shown to contribute to both LTD and LTP induction. LTD is induceable even at inactive synapses if [Ca2+]i is raised to the appropriate level by antidromic or heterosynaptic activation, or by raising the extracellular Ca2+ concentration [Ca2+]o. These conditions suggest a rule (called here the ABS rule) for activity-dependent synaptic modifications that differs from the classical Hebb rule and that can account for both homosynaptic LTD and LTP as well as for heterosynaptic competition and associativity.

Protective properties of viscoelastic substances (sodium hyaluronate and 2% hydroxymethylcellulose) against experimental free radical damage to the corneal endothelium

Hydrogen peroxide (H2O2) has potent oxidant properties due to the action of free radicals (OH.) induced from its degradation. The free radicals specie derived from H2O2 are extremely toxic to the corneal endothelium and quickly induce corneal edema. In the present work, in order to ascertain the endothelial cell protection from viscoelastic substances, we have studied experimental corneal endothelial cell damage caused in the rabbit eye after intracameral injection of different H2O2 concentrations, with and without previous filling and washing out of two widely used viscoelastic substances from the anterior chamber such as 1% sodium hyaluronate (Healon) and hydroxypropylmethylcellulose (HPMC). We observed a dose-dependent endothelial damage in the controls. The experimental groups protected with Healon or HPMC showed statistically fewer corneal endothelial cell lesions than the control group (p < 0.001) for all of the concentrations used. Healon showed superior protective properties than HPMC at higher H2O2 concentrations (100 mM). However, HPMC was superior with 1 and 10 mM peroxide. From this experimental evidence, we conclude that Healon and HPMC are effective as protectors against the corneal endothelial lesions caused by free radicals. This finding may explain some of the beneficial effects of these viscoelastic substances.

Retinal detachment as a potential hazard in surgical correction of severe myopia with phakic anterior chamber lenses

Phakic anterior chamber lenses have been proposed as an effective refractive surgical procedure for the correction of severe myopias. We managed three cases of retinal detachment that complicated this operation. Two patients had retinal detachment in the immediate postoperative period at the fourth and sixth weeks, respectively. Both patients had preoperative equatorial lattice degeneration, which was prophylactically managed with argon laser in one case. The third patient developed retinal detachment at the eighth postoperative month and was associated with a severe fibrinoid uveitis and a combined exudative and rhegmatogenous retinal detachment. This third patient did not have any previous retinal degenerative lesion. Scleral buckling was successful in two of the patients, whereas the patient with preoperative inflammation required vitreous surgery because of proliferative vitreoretinopathy. The retinas were reattached in all three patients without explanting the phakic anterior chamber lens, but severe loss of visual acuity occurred in one patient. Retinal detachment should be regarded as a potentially important hazard in this type of refractive surgery.

Treatment of experimental acute corneal inflammation with inhibitors of the oxidative metabolism

The authors studied the effect of topical 2% 6-aminonicotinamide and 0.1% adenosine on an experimental model of acute corneal inflammation. Luminol-enhanced chemiluminescence (LAC), as an indirect measurement of free-radical release, and computer-assisted planimetry of the corneal ulcer and its infiltrate were performed both ex vivo and in vivo on the fifth day following the induction of experimental alkali burn keratitis. The authors proved that both drugs significantly inhibited LAC both ex vivo and in vivo and that such treatments had also a significant beneficial effect on the evolution of the corneal ulcer and its infiltrate. To the best of the authors' knowledge, this finding had not been previously reported in experimental corneal inflammation and may indicate that treatment with inhibitors of the oxidative metabolism could offer a new approach in the pharmacological modulation of acute corneal inflammation.

Lipid peroxidation in the iris and its protection by means of viscoelastic substances (sodium hyaluronate and hydroxypropylmethylcellulose)

Free radicals, especially the hydroxyl radical (OH.), are known to be toxic for several ocular structures including the cornea, lens, iris and retina through the initiation of lipid peroxidation of cell membranes. The oxidative damage to the iris epithelial-cell membranes, induced by the injection of hydrogen peroxide (H2O2) at different concentrations into the anterior chamber of the rabbit eye, was studied by means of the measurement of lipid peroxidation products. Thiobarbituric acid reactive (TBAR) products were significantly increased compared with normal control iris after the intracameral injection of H2O2 at concentrations of 0.1 (p < 0.02), 10 and 100 mM (p < 0.001). Viscoelastic substances, widely used in anterior ocular surgery, sodium hyaluronate (Healon) and hydroxypropylmethylcellulose, show a protective effect against rabbit iris lipid peroxidation. A statistically significant (p < 0.001) inhibition of the release of TBAR products occurred in both experimental groups that received an injection of these substances prior to a H2O2 injection. This is the first report of lipid peroxidation of the iris and the 'antioxidant'-protective effect of viscoelastic substances. This new technical approach could be used as a test of efficacy of the protective effect of viscoelastic substances.