Effects of IBMX on the ERG of the isolated perfused cat eye

Closed-Perfusion Transretinal ERG Setup for Preclinical Drug and Nanostructure Testing

OBJECTIVE

The isolated mammalian retina may serve as a sensitive biosensor for preclinical drug testing, including eye drugs and a broader range of pharmaceuticals. To facilitate testing with minimal amounts of drug molecules or nanostructures, we developed a closed-perfusion transretinal electroretinography (tERG) setup.

METHODS

The major challenge with small amounts of circulating perfusate was maintaining retinal viability and stability during long experiments. We conducted ex vivo tERG using WT C57BL/6J and mice to assess rod- and cone-mediated light signals. The dark-adapted retina was stimulated with full-field light flashes while perfused at 5-6 ml/min.

RESULTS

The minimum perfusate needed in our closed-circulation was around 50 ml. Penicillin-Streptomycin (Pen-Strep) was indispensable for long recordings. Rod responses remained stable for at least 42 hours, the longest recording we conducted, with the retina still responsive, and rod and cone bipolar cell responses for up to 12 hours. IBMX (3-isobutyl-1-methylxanthine), a non-specific phosphodiesterase (PDE) inhibitor with reversible effects, validated our setup. We used our setup to test the zwitterionic polymer poly(sulfobetaine methacrylate) (PSMBA), serving as a promising material for thermoresponsive nanostructures, and the corresponding monomer SBMA for possible harmful effects on mouse rod and bipolar cell functioning.

CONCLUSION

Our closed-perfusion tERG setup enables long experiments with small amounts of perfusate. PSMBA or SBMA had no effect on rod and bipolar cell responses.

SIGNIFICANCE

This method is applicable for assessing drug functionality, as well as conducting preliminary biocompatibility and toxicity testing using small amounts of molecules or nanostructures that could impact neuronal or synaptic function.

Sildenafil in ophthalmology: An update

Sildenafil citrate, a selective oral phosphodiesterase 5 inhibitor, is a widely used drug for erectile dysfunction that acts by elevating cGMP levels and causing smooth muscle relaxation. It also has 10% activity against PDE6, a key enzyme in phototransduction cascade in the retina. Recent ocular imaging developments have further revealed the influence of sildenafil on ocular hemodynamics, particularly choroidal perfusion. Choroidal thickness is increased, and choroidal perfusion is also enhanced by autoregulatory mechanisms that are further dependent on age and microvascular abnormalities. Studies demonstrating high intraocular pressure via a "parallel pathway" from increased choroidal volume and blood flow to the ciliary body have challenged previous concepts. Another new observation is the effect of sildenafil on bipolar cells and cyclic-nucleotide gated channels. We discuss potential deleterious effects (central serous chorioretinopathy, glaucoma, ischemic optic neuropathy, and risks to recessive carriers of retinitis pigmentosa), potential beneficial effects (ameliorate choroidal ischemia, prevent thickening of Bruch membrane, and promote recovery of the ellipsoid zone) in macular degeneration, as well as potential drug interactions of sildenafil.

Effect of the ILE86TER mutation in the γ subunit of cGMP phosphodiesterase (PDE6) on rod photoreceptor signaling

The light-dependent decrease in cyclic guanosine monophosphate (cGMP) in the rod outer segment is produced by a phosphodiesterase (PDE6), consisting of catalytic α and β subunits and two inhibitory γ subunits. The molecular mechanism of PDE6γ regulation of the catalytic subunits is uncertain. To study this mechanism in vivo, we introduced a modified Pde6g gene for PDE6γ into a line of Pde6g(tm1)/Pde6g(tm1) mice that do not express PDE6γ. The resulting ILE86TER mice have a PDE6γ that lacks the two final carboxyl-terminal Ile(86) and Ile(87) residues, a mutation previously shown in vitro to reduce inhibition by PDE6γ. ILE86TER rods showed a decreased sensitivity and rate of activation, probably the result of a decreased level of expression of PDE6 in ILE86TER rods. More importantly, they showed a decreased rate of decay of the photoresponse, consistent with decreased inhibition of PDE6 α and β by PDE6γ. Furthermore, ILE86TER rods had a higher rate of spontaneous activation of PDE6 than WT rods. Circulating current in ILE86TER rods that also lacked both guanylyl cyclase activating proteins (GCAPs) could be increased several fold by perfusion with 100μM of the PDE6 inhibitor 3-isobutyl-1-methylxanthine (IBMX), consistent with a higher rate of dark PDE6 activity in the mutant photoreceptors. In contrast, IBMX had little effect on the circulating current of WT rods, unlike previous results from amphibians. Our results show for the first time that the Ile(86) and Ile(87) residues are necessary for normal inhibition of PDE6 catalytic activity in vivo, and that increased basal activity of PDE can be partially compensated by GCAP-dependent regulation of guanylyl cyclase.

Impediments to eye transplantation: ocular viability following optic-nerve transection or enucleation

Maintenance of ocular viability is one of the major impediments to successful whole-eye transplantation. This review provides a comprehensive understanding of the current literature to help guide future studies in order to overcome this hurdle. A systematic multistage review of published literature was performed. Three specific questions were addressed: (1) Is recovery of visual function following eye transplantation greater in cold-blooded vertebrates when compared with mammals? (2) Is outer retina function following enucleation and reperfusion improved compared with enucleation alone? (3) Following optic-nerve transection, is there a correlation between retinal ganglion cell (RGC) survival and either time after transection or proximity of the transection to the globe? In a majority of the studies performed in the literature, recovery of visual function can occur after whole-eye transplantation in cold-blooded vertebrates. Following enucleation (and reperfusion), outer retinal function is maintained from 4 to 9 h. RGC survival following optic-nerve transection is inversely related to both the time since transection and the proximity of transection to the globe. Lastly, neurotrophins can increase RGC survival following optic-nerve transection. This review of the literature suggests that the use of a donor eye is feasible for whole-eye transplantation.

The effects of the phosphodiesterase type V inhibitor sildenafil on human and bovine retinal function in vitro

BACKGROUND

After the ingestion of sildenafil (Viagra), visual adverse events have been reported, possibly caused by an inhibition of the phototransduction cascade by sildenafil via phosphodiesterase (PDE 6). Therefore, we investigated the effects of sildenafil on photoreceptors and postsynaptic neurons of human and bovine retinas using the isolated superfused vertebrate retina technique.

METHODS

Human and bovine retina preparations were perfused with an oxygen preequilibrated standard solution. The electroretinogram (ERG) was recorded using Ag/AgCl electrodes. After recording stable ERG amplitudes, sildenafil was added to the solution for 45 min. Thereupon, the preparations were reperfused with standard solution for 240 min.

RESULTS

Following the application of sildenafil (3 microMol/l), the b-wave amplitude of bovine and human preparations was reduced continuously and disappeared completely. After reperfusion with the standard solution for 4 h, the b-wave amplitude did not recover completely. Using the same sildenafil concentration (3 microMol/l), the a-wave amplitude of the human retina was not totally abolished, but reduced to 21% of the initial amplitude and remained reduced at washout. For all retinal preparations, the implicit time of the ERG amplitudes remained significantly extended at the end of the washout.

CONCLUSIONS

Strong similarities were detected in the drug-induced changes of the ERG when comparing human and bovine retinas. The results suggest that sildenafil impairs retinal function at not only the level of the photoreceptors, but it also affects the neuronal network of the inner retina at concentrations of approximately 30-fold higher than at therapeutic plasma concentrations.

Viagra (sildenafil citrate) and ophthalmology

Viagra (sildenafil citrate) improves penile erections in men with erectile dysfunction (ED) by selectively inhibiting cGMP-specific phosphodiesterase type 5 (PDE5), which is present in all vascular tissue. It also exerts a minor inhibitory action against PDE6, which is present exclusively in rod and cone photoreceptors. At higher doses, sildenafil causes mild and transient visual symptoms in a minority of patients (mainly blue tinge to vision, increased brightness of lights). Therefore, the effects of sildenafil on the visual system have been investigated in a wide variety of clinical and preclinical studies. In preclinical studies, sildenafil shows transient reversible effects on electrical response to light. In long-term toxicology studies in which animals were exposed to high multiples of the maximum human therapeutic dose, detailed examinations have revealed no adverse effects on the structure or function of the eye. The effects of sildenafil have been systematically investigated in visual function studies in volunteers and in patients with eye disease; sildenafil does not affect visual acuity, visual fields, and contrast sensitivity. The only definite effect is transient, mild impairment of color discrimination occurring around the time of peak plasma levels. In long-term studies, no long-term effects of sildenafil on the visual system have been observed. Postmarketing, sildenafil has been prescribed to over 15 million men with ED. Isolated examples of a variety of visual adverse events have been reported. No consistent pattern has emerged to suggest any long-term effect of sildenafil on the retina or other structures of the eye. Based on this experience, intermittent, short-term, partial inhibition of PDE5 or PDE6 by sildenafil is unlikely to induce any long-term visual change.

Retinal research using the perfused mammalian eye

The effort to isolate and maintain alive in vitro an intact mammalian eye is rewarded by the full control provided over the arterial input and exclusion of systemic regulatory or compensatory mechanisms. Electrical recording of typical light-evoked field potentials from retina and optic nerve can be complemented by single-cell recording. Thus, light-induced electrical activity reflects the function of the retinal pigment epithelium, of the layers of the retina and of the ganglion cells or their axons. Retinal function in vitro is documented by electrophysiological and morphological methods revealing subtle features of retinal information processing as well as optic nerve signals that approach-at threshold stimulus intensity-the human psychophysical threshold. Such sensitivity of third-order retinal neurons is described for the first time. This well controlled in vitro preparation has been used successfully for biophysical, metabolic and pharmacological studies. Examples are provided that demonstrate the marked sensibility of the rod system to changes in glucose supply. Moreover, histochemical identification of glycogen stores revealed labeling of the second- and third-order neurons subserving the rod system, in addition to labeling of Müller (glial) cells in the cat retina. The glycogen content of the cat retina is augmented by prolonged anesthesia, largely depleted by ischemia after enucleation and enhanced by insulin. Pharmacological experiments using agonists and antagonists of putative retinal neurotransmitters are summarized and outlined using the muscarinic cholinergic agonist QNB as an example. Actions and uptake of the neuromodulator adenosine are presented in detail, including inhibitory effects on physiologically characterized ganglion cells. Neuronal effects of adenosine are distinguished from those resulting from vasodilatation and from glycogenolysis induced by the neuromodulator. To open the blood-retina barrier, a hyperosmotic challenge can be applied transiently. This process is monitored histochemically using FITC-albumin and with electrophysiological parameters. Changes in vitreo-scleral resistance and in the amplitude of the EOG-light peak appear to reflect the open/closed status of the barrier. This overview of the uses of the isolated perfused mammalian eye in retinal research concludes with a discussion of potential implications for clinically relevant topics.

Effect of a cGMP-specific phosphodiesterase inhibitor on retinal function

Multiple forms of phosphodiesterase have been reported in many tissues. Phosphodiesterase 6, a cGMP-specific phosphodiesterase, is described as a photoreceptor cell-specific phosphodiesterase. Phosphodiesterase 6 is known to play a crucial role in visual function. A novel phosphodiesterase inhibitor, GF248 (5["(propoxy),7'(4-morpholino)-phenacyl],[1-methyl-3 propyl]pyrazolo[4,3d]pyrimidin-7-one), has been described to be a very potent cGMP-specific phosphodiesterase inhibitor. In the present study, we compared the potency of GF248 and other known cGMP-specific phosphodiesterase inhibitors on phosphodiesterase 5 and phosphodiesterase 6. GF248 displayed an IC50 of 2 and 5 nM for phosphodiesterase 5 and phosphodiesterase 6, respectively. Thereafter, we assessed the effect of GF248 on retinal function, using an ex vivo model of isolated retina electroretinogram recording. Exposure of retina to GF248 resulted in a dose-dependent decrease in electroretinogram amplitude (PIII and b-waves), with no marked modification of PIII and b-wave implicit time. Among other phosphodiesterase inhibitors, DMPPO (1,3-dimethyl-6-(2-propoxy-5-methanesulfonylamidophenyl)pyrazol ol[3,4d]-pyrimidin-4-(5H)-one) and dipyridamole, cGMP-specific phosphodiesterase inhibitors, and IBMQ (1-isobutyl-3-methylimidazol[1,5a]quinoxalin-4-(5H)one), a nonselective phosphodiesterase inhibitor, altered retinal function but less potently than GF248, consistent with their in vitro phosphodiesterase 6 inhibition. Phosphodiesterase 3- and phosphodiesterase 4-selective inhibitors, cilostamide and rolipram, respectively, did not affect retinal function at 10 micromol l(-1). Our conclusion from these data is that GF248, a potent phosphodiesterase 6 inhibitor, could interfere with visual transduction by cGMP accumulation.

Retinal cone dysfunction of supernormal rod ERG type. Five new cases

Five patients, not related to each other, showed clinical signs, including electroretinograms, of a retinal dysfunction which mainly affected the cone system, but also involved the rod responses in a peculiar way. ERG b-wave threshold under dark adapted conditions was elevated. In contrast, rod b-wave sensitivity was enhanced with medium to high intensity flash stimulation. Furthermore, all patients had a severe reduction of the oscillatory potentials. The findings are discussed with special emphasis on a hypothetical disturbance in the cyclic guanosine monophosphate metabolism, involving both photoreceptors and cells of the inner plexiform layer responsible for retinal feedback mechanisms.

Cone dysfunction and supernormal scotopic electroretinogram with a high-intensity stimulus. A report of three cases

An unusual form of scotopic electroretinogram with a bright white stimulus, which consisted of a rectangular a-wave of normal amplitude and a b-wave of supernormal amplitude, was recorded in three patients with cone dysfunction. In addition to poor visual acuity, abnormal color vision and reduced amplitude of the photopic electroretinogram, these patients showed a 2-log unit elevation of the dark-adaptation threshold. Funduscopic examination and fluorescein angiography revealed fine granular pigment disturbances at the macula. The relationship between the response of the dark-adapted electroretinogram versus stimulus intensity was unique to these patients. The b-wave thresholds were elevated by 1 log unit. The b-waves were reduced in amplitude and markedly delayed in implicit time to dim stimuli, but supernormal in amplitude and normal in implicit time to bright stimuli.

Genetic analysis of patients with retinitis pigmentosa using a cloned cDNA probe for the human gamma subunit of cyclic GMP phosphodiesterase

We have cloned cDNAs corresponding to the human gamma subunit of retinal cyclic GMP phosphodiesterase (gamma-cGMP-PDE). The coding region of these cDNAs was identical to that reported previously by Tuteja et al. (Gene 1990, 88, 227-32). We also confirmed their assignment of gamma-cGMP-PDE to human chromosome 17. The fragment was used to search for mutations of the corresponding gamma-cGMP-PDE gene in patients with autosomal dominant, autosomal recessive, or isolate case retinitis pigmentosa, and Usher's syndrome type I. No gene deletions or rearrangements could be detected in any patient by Southern blotting. We discovered restriction fragment length polymorphisms (RFLPs) with the enzymes BstE II and EcoR I defining sets of alleles at the gamma-cGMP-PDE locus in the normal population. We used these RFLPs to analyse the genomic DNA of large sets of unrelated patients with the autosomal dominant, autosomal recessive, or isolate form of retinitis pigmentosa. Within each of these three groups, BstE II and EcoR I RFLP alleles at the gamma-cGMP-PDE locus showed no linkage disequilibrium (departure from Hardy-Weinberg equilibrium). In addition, one autosomal dominant, three autosomal recessive, and two Usher's syndrome type I pedigrees each showed no cosegregation of the gamma-cGMP-PDE locus and the disease locus. Thus, we find no evidence that mutations of the gene for the gamma subunit of cGMP phosphodiesterase are associated with the common forms of retinitis pigmentosa and Usher's syndrome type I.

Autosomal dominant rod-cone dysplasia in the Rdy cat. 2. Electrophysiological findings

Electroretinography was performed on cats affected with autosomal dominant rod-cone dysplasia (gene symbol Rdy). In normal kittens it was found that retinal sensitivity increased and rod thresholds decreased as the animals matured. Electroretinogram (ERG) amplitudes were mature by 4.5 weeks and adult timing was attained by 6 weeks of age, consistent with the findings of other workers. In Rdy-affected heterozygous kittens the ERG was absent or barely recordable using conventional corneal contact lens electrodes. However, the enhanced sensitivity of an intravitreal needle electrode permitted the recording of ERGs from affected kittens aged 4.5 weeks and older. The intravitreally recorded scotopic ERG in Rdy-affected kittens was a very low amplitude, largely negative response with prolonged a- and b-wave times-to-peak (two to threefold longer than in comparable recordings from an age-matched normal kitten). The b-wave lacked oscillatory potentials and was relatively small so that the ERG was a-wave dominated. This was attributed to delayed and defective synaptogenesis in the outer plexiform layer of dystrophic retinas. In contrast to normal kittens, the b-wave threshold was higher than that of the a-wave in affected kittens. Photopic responses were unrecordable. The intravitreal ERG was barely recordable in a 5-month-old Rdy-affected cat and was apparently extinguished by 7 months of age. In vitro electroretinography permitted a comparison of the photoreceptor responses (fast PIII) from the isolated retinas of 6-week-old control and Rdy-affected heterozygous kittens. Maximum fast PIII amplitudes were reduced by about 75% in affected retinas compared with age-matched normal retinas (P less than 0.005). The mean fast PIII time-to-peak, at maximum light intensity, in Rdy-affected retinas was prolonged by about 15 msec and was approximately twofold longer than the time-to-peak of normal retinas (P less than 0.005).4+ steeper slope with relatively greater prolongations in time-to-peak at lower luminances compared with normal retinas (P less than 0.025). These changes in temporal characteristics may be explained either by severe disorganization of photoreceptor outer segments or by altered phototransduction kinetics.

Effects of IBMX on the rod ERG of the isolated perfused cat eye: antagonism with light, calcium or L-cis-diltiazem

Full-field electroretinograms (ERGs) were recorded from isolated cat eyes perfused through the ophthalmociliary artery with the cGMP-PDE inhibitor, 3-isobutylmethylxanthine (IBMX). Under dark-adapted conditions perfusion with IBMX resulted in reduced ERG b-wave amplitudes at low stimulus luminances and supernormal b-wave amplitudes at high stimulus luminances with reduced b-wave sensitivity; b-wave implicit times were more delayed at low than at high stimulus luminances. Presentation of a steady white background or high calcium fully reversed the supernormal amplitudes and partially reversed the delayed implicit times produced by IBMX. Rod ERG b-wave sensitivity, reduced with IBMX alone, was partially reversed with calcium but further reduced with background light. Perfusion with the cation channel blocker, L-cis-diltiazem, also reversed the supernormal amplitudes produced by IBMX but had no effect on implicit times or b-wave sensitivity. Possible mechanisms of action of these antagonists and clinical implications of these findings are considered.

Influence of isobutylmethylxanthine on the direct current electroretinogram of albino rabbit eyes

The direct current electroretinogram and the standing potential of the eye were recorded from both eyes of unilaterally vitrectomized albino rabbits. The effect of intraocular irrigation with the nonselective phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) was compared with that of a balanced salt control solution. IBMX (0.5 mM) produced a reversible increase in the amplitudes of the a-wave (19%, p less than 0.02), b-wave (21%, p less than 0.001) and c-wave (12%, p less than 0.01) of the electroretinogram. A small elevation of the standing potential was found (0.4 +/- 0.2 mV), but this increase was not statistically significant (p greater than 0.05). The results indicate a primary and principal influence of IBMX on the photoreceptors.

Effects of using the oxygen-carrying fluorocarbon, FC43, on the ERG of the arterially perfused cat eye

Arterial perfusion of the isolated cat eye with a perfusate augmented with a 20% (wt/vol) emulsion of the oxygen carrying-fluorocarbon, FC43, produces a much larger b-wave in the electroretinogram than use of the same perfusate lacking FC43. The c-wave is either unchanged or slightly reduced by use of this solution. Halving the oxygen content of the FC43-augmented perfusate also reversibly reduces the b-wave. The use of perfusates with and without FC43 but containing similar amounts of oxygen produces b-waves of similar amplitude. The large b-wave recorded during use of a well-oxygenated, FC43-augmented perfusate suggests that the use of perfusates containing less oxygen produces retinal hypoxia. The b-wave recorded in vivo is considerably smaller than that recorded from the isolated eye during perfusion with a well-oxygenated, FC43-augmented perfusate, but the waveforms are very similar. In particular, the ratios of the b-wave to a-wave are almost identical. The larger ERG recorded in vitro is a result of increasing the shunt resistance to ground upon enucleation. Histological examination of retinas perfused for two hours with the FC43-augmented and standard perfusate reveal no clear signs of tissue hypoxia in either retina. However, even two hours of complete ischemia produces limited inner retinal deterioration and virtually no outer retinal damage, suggesting that the histological state of the retina may not be a reliable indicator of its physiological state. Despite the improved electrophysiological state of the retina produced by increasing its oxygen supply, the survival time of the isolated eye was not increased.

Retinal pharmacology in the perfused cat eye

Exclusion of extraocular variables facilitates neuropharmacological retinal experiments. Isolated tissue or organs such as arterially perfused mammalian eyes provide this advantage. Electrophysiology comprising field potentials (ERG and optic nerve response) and recordings from single cells is used to assess various levels of retinal information processing. Several directions of research using the perfused cat eye are indicated. Pharmacological studies are presented in a description of an approach to identify adrenergic mechanisms in the cat retina. Studies on effects of changes in physico-chemical parameters and on drug actions reveal a number of parallels to problems relevant in clinical eye research.