Intracellular responses of the Müller (glial) cells of mudpuppy retina: their relation to b-wave of the electroretinogram

Oscillatory potentials and the b-Wave: partial masking and interdependence in dark adaptation and diabetes in the rat

BACKGROUND

Diabetes inhibits dark adaptation and both processes alter the electroretinogram (ERG) in similar ways. This study aimed to investigate the relationship between oscillatory potentials (OPs) and the b-wave during dark adaptation and to determine if this relationship changes during the development of diabetes.

METHODS

Twenty-one rats were assigned to adaptation, control and diabetic groups. Rats were dark adapted for periods between 20 minutes and 4 hours, and ERGs recorded. Diabetes was induced with streptozotocin, and ERGs measured after 3, 6, 9 and 12 weeks after injection.

RESULTS

Increasing periods of dark adaptation led to a logarithmic increase in the amplitude of the b-wave and the OPs. This was accompanied by a decrease in the peak times of the OPs and b-wave. Total OP amplitude and b-wave amplitude were linearly related, allowing an empirical OP constant to be developed to describe the relationship between the two parameters. Diabetes led to a progressive decrease in the amplitude and increase in the peak time of all waves. The OP constant decreased in a linear fashion with increasing duration of diabetes.

CONCLUSIONS

It is argued that OP masking of the b-wave could explain previous inconsistencies in reported ERG changes in diabetes and that a slowing of dark adaptation does not account for these ERG changes. The report concludes that the OPs and b-wave amplitudes and latencies are intimately related in the normal retina and that this correlation is lost predictably during the development of diabetes.

Presence of calpain-induced proteolysis in retinal degeneration and dysfunction in a rat model of acute ocular hypertension

The purpose of this study was to determine if calpain-induced proteolysis was associated with retinal degeneration or dysfunction in the rat acute ocular hypertensive model. Acute glaucoma was produced by elevation of IOP to 120 mm Hg for 1 hr. Retinal degeneration was evaluated by H&E staining and apoptosis was determined by TUNEL staining in histologic sections of retina. Electroretinogram (ERG) was carried out to evaluate changes in functionality. Activation of calpains was determined by casein zymography and immunoblotting. Total calcium in retina was measured by atomic absorption spectrophotometry. Proteolysis of alpha-spectrin, tau, cdk5, and p35 (a regulator of cdk5) were evaluated by immunoblotting. The thickness of inner plexiform layer (IPL) and inner nuclear layer (INL), and the number of cells in the ganglion cell layer (GCL) decreased after ocular hypertension. Numerous cells in the INL stained positive for TUNEL and some cells in the outer nuclear layer (ONL) showed TUNEL staining. The a-wave in ERG was temporarily decreased after ocular hypertension and then recovered to normal. In contrast, the b-wave was completely lost. Calpains were activated after ocular hypertension. Activation of calpains was associated with increased calcium in retina. Calpain-dependent proteolysis of alpha-spectrin, tau, and p35 were observed in retina after ocular hypertension. The results suggested that increased calcium and subsequent proteolysis by activated calpains was associated with the death of inner retinal cells due to acute ocular hypertension in the rat model. Calpain inhibitors may be candidate drugs for treatment of retinal degeneration and dysfunction resulting from glaucoma.

Physiological maturation of photoreceptors depends on the voltage-gated sodium channel NaV1.6 (Scn8a)

Voltage-gated sodium channels (VGSCs) ensure the saltatory propagation of action potentials along axons by acting as signal amplifiers at the nodes of Ranvier. In the retina, activity mediated by VGSCs is important for the refinement of the retinotectal map. Here, we conducted a full-field electroretinogram (ERG) study on mice null for the sodium channel NaV1.6. Interestingly, the light-activated hyperpolarization of photoreceptor cells (the a-wave) and the major "downstream" components of the ERG, the b-wave and the oscillatory potentials, are markedly reduced and delayed in these mice. The functional deficit was not associated with any morphological abnormality. We demonstrate that Scn8a is expressed in the ganglion and inner nuclear layers and at low levels in the outer nuclear layer beginning shortly before the observed ERG deficit. Together, our data reveal a previously unappreciated role for VGSCs in the physiological maturation of photoreceptors.

Contribution of retinal neurons to d-wave of primate photopic electroretinograms

The purpose of this study was to determine the contribution of different types of retinal neurons to the d-wave of the primate electroretinogram using pharmacological agents. NMDA + TTX was used to suppress inner retinal activity, and APB and PDA to block the activity of the ON- and OFF-pathways, respectively. Results indicated that the inner retinal neurons had a small but certain contribution to the d-wave. The initial rapid phase of the d-wave originates from the activity of the cone OFF-pathway nearly exclusively, and the later slow phase is shaped by the cone photoreceptors. The cone ON-pathway acts in a direction opposite to that of the other components.

Endocannabinoids in the intact retina: 3H-anandamide uptake, fatty acid amide hydrolase immunoreactivity and hydrolysis of anandamide

There is much evidence for an endocannabinoid system in the retina. However, neither the distribution of endocannabinoid uptake, the regulation of endocannabinoid levels, nor the role of endocannabinoid metabolism have been investigated in the retina. Here we focused on one endocannabinoid, anandamide (AEA), and its major hydrolyzing enzyme, fatty acid amide hydrolase (FAAH), in the goldfish retina. Immunoblots of FAAH immunoreactivity (IR) in goldfish retina, brain and rat retina, and brain homogenates showed a single band at 61 kDa that was blocked by preadsorption with peptide antigen. Specific FAAH IR (blocked by preadsorption) was most prominent over Müller cells and cone inner segments. Weaker label was observed over some amacrine cells, rare cell bodies in the ganglion cell layer, and in four lamina in the inner plexiform layer. FAAH activity assays showed that goldfish-retinal and brain homogenates hydrolyzed AEA at rates comparable to rat brain homogenate, and the hydrolysis was inhibited by methyl arachidonyl fluorophosphonate (MAFP) and N-(4 hydroxyphenyl)-arachidonamide (AM404), with IC50s of 21 nM and 1.5 μM, respectively. Cellular 3H-AEA uptake in the intact retina was determined by in vitro autoradiography. Silver-grain accumulation at 20°C was most prominent over cone photoreceptors and Müller cells. Uptake was significantly reduced when retinas were incubated at 4°C, or preincubated with 100 nM MAFP or 10 μM AM404. There was no differential effect of blocking conditions on the distribution of silver grains over cones or Müller cells. The codistribution of FAAH IR and 3H-AEA uptake in cones and Müller cells suggests that the bulk clearance of AEA in the retina occurs as a consequence of a concentration gradient created by FAAH activity. We conclude that endocannabinoids are present in the goldfish retina and underlay the electrophysiological effects of cannabinoid ligands previously shown on goldfish cones and bipolar cells.

Spatiotemporal receptive field properties of epiretinally recorded spikes and local electroretinograms in cats

Background

Receptive fields of retinal neural signals of different origin can be determined from extracellular microelectrode recordings at the inner retinal surface. However, locations and types of neural processes generating the different signal components are difficult to separate and identify. We here report epiretinal receptive fields (RFs) from simultaneously recorded spikes and local electroretinograms (LERGs) using a semi-chronic multi-electrode in vivo recording technique in cats. Broadband recordings were filtered to yield LERG and multi unit as well as single unit spike signals. RFs were calculated from responses to multifocal pseudo-random spatiotemporal visual stimuli registered at the retinal surface by a 7-electrode array.

Results

LERGs exhibit spatially unimodal RFs always centered at the location of the electrode tip. Spike-RFs are either congruent with LERG-RFs (N = 26/61) or shifted distally (N = 35/61) but never proximally with respect to the optic disk. LERG-RFs appear at shorter latencies (11.9 ms ± 0.5 ms, N = 18) than those of spikes (18.6 ms ± 0.4 ms, N = 53). Furthermore, OFF-center spike-RFs precede and have shorter response rise times than ON-center spike-RFs. Our results indicate that displaced spike-RFs result from action potentials of ganglion cell axons passing the recording electrode en route to the optic disk while LERG-RFs are related to superimposed postsynaptic potentials of cells near the electrode tip.

Conclusion

Besides contributing to the understanding of retinal function we demonstrate the caveats that come with recordings from the retinal surface, i.e., the likelihood of recordings from mixed sets of retinal neurons. Implications for the design of an epiretinal visual implant are discussed.

Melatonin regulates circadian electroretinogram rhythms in a dose- and time-dependent fashion

The circadian rhythm of the chick electroretinogram (ERG) is regulated by the indoleamine hormone melatonin. To determine if the concentration of melatonin or the time at which it was administered would have differential effects on ERG parameters, we conducted experiments analyzing the effects of melatonin at different times of the day. Circadian rhythms of a- and b-wave implicit times and amplitudes were observed in both light:dark (LD) and in continuous darkness (DD). Intramuscular melatonin administration of 1 mg/kg and 100 ng/kg decreased a- and b-wave amplitudes and increased a- and b-wave implicit times. This effect was significantly greater than that observed for 1 ng/kg melatonin, which had little to no effect over the saline controls. The effect of 1 mg/kg and 100 ng/kg melatonin on a- and b-wave amplitude in LD and on b-wave amplitude in DD was greater during the night (ZT/CT 17) than during the day (ZT/CT 5). The fold change in b-wave implicit time over that of controls was greater during the day (ZT/CT 5) than during the night (ZT/CT 17). These data indicate that melatonin may play a role in regulating a day and night functional shift in the retina, and that it does so via regulation of a retinal clock.

cGMP-dependent kinase regulates response sensitivity of the mouse on bipolar cell

The visual system can adjust its sensitivity over a wide range of light intensities. Photoreceptors account for some of this adjustment, but there is evidence that postreceptoral processes also exist. To investigate the latter, we pharmacologically mimicked the effects of light stimulation on mouse On bipolar cells, thus avoiding confounding effects of receptoral mechanisms. Here, we report that cGMP selectively enhances responses to dim, but not bright, stimuli through a purely postsynaptic mechanism. This action of cGMP was completely blocked by inhibitors of cGMP-dependent kinase. We propose that cGMP-dependent kinase decreases coupling of the On bipolar cell glutamate receptor to the downstream cascade, thus amplifying small decreases in photoreceptor transmitter levels that would otherwise go undetected by the visual system.

Results from screening over 9000 mutation-bearing mice for defects in the electroretinogram and appearance of the fundus

Random mutagenesis combined with phenotypic screening using carefully crafted functional tests has successfully led to the discovery of genes that are essential for a number of functions. This approach does not require prior knowledge of the identity of the genes that are involved and is a way to ascribe function to the nearly 6000 genes for which knowledge of the DNA sequence has been inadequate to determine the function of the gene product. In an effort to identify genes involved in the visual system via this approach, we have tested over 9000 first and third generation offspring of mice treated with the mutagen N-ethyl-N-nitrosourea (ENU) for visual defects, as evidenced by abnormalities in the electroretinogram and appearance of the fundus. We identified 61 putative mutations with this procedure and outline the steps needed to identify the affected genes.

Fos-tau-LacZ mice expose light-activated pathways in the visual system

We have employed fos-tau-LacZ (FTL) transgenic mice to examine functional activation in the visual areas of the nervous system. The FTL mice express the marker gene lacZ in neurons and their processes following many different stimuli, and allow the imaging of activation from the level of the entire brain surface through individual neurons and their projections. Analysis of FTL expression in the retinas of mice following diurnal exposure to light shows that bipolar cells, specific classes of amacrine cells, ganglion cells, and a dense network of processes in the inner plexiform layer are functionally activated. In animals deprived of light, there is almost no activity in the retina. In the lateral geniculate nucleus (LGN), light exposure appears responsible for FTL expression in dorsal nuclei, but not for expression in the ventral nuclei or the intergeniculate leaflet. In the superficial layers of the superior colliculus, FTL expression is highly dependent on light exposure. Similarly, light exposure is required for FTL expression in primary visual cortex (area 17), but some expression remains in area 18 of dark-adapted animals. Finally, using mice with one or both eyes missing, we have determined which parts of the visual system are dependent on the presence of a functional connectivity from the eye. These data demonstrate the usefulness of the FTL mice to map functional activation within the entire visual system. Furthermore, we can capture visual activation in a conscious animal. Our findings give an insight into the architecture of activity within the retina and throughout the visual system.

A circadian clock regulates the process of ERG b- and d-wave dominance transition in dark-adapted zebrafish

In zebrafish, during dark adaptation following bright light adaptation, the dominance of electroretinogram (ERG) b- and d-waves switches. In the early dark adaptation, when visual sensitivity is cone-dominant, both the b- and d-waves are readily recorded. In the late dark adaptation, along with the increase of rod sensitivity, the b-wave becomes dominant whereas the d-wave is gradually lost. The time for the ERG b- and d-wave dominance transition varies between the day and night. The transition requires a longer amount of time in the night and early morning than in the afternoon. This pattern of timing for ERG b- and d-wave dominance transition persists in constant light and can be reversed after exposure to a reversed light-dark cycle. The data suggest that the transition of the dominance of ERG b- and d-waves is regulated by an endogenous circadian clock.

Inherited multifocal RPE-diseases: mechanisms for local dysfunction in global retinoid cycle gene defects

Alterations of retinoid cycle genes are known to cause retinal diseases characterized by focal white dot fundus lesions. Fundus appearances reveal circumscribed RPE-changes, although generalized metabolic defects and global functional abnormalities are present. As a possible explanation, topographic inhomogeneities of the human photoreceptor mosaic and the role of a cone specific visual cycle will be discussed. Due to particular characteristics of photoreceptor subtypes as well as different pathways for photopigment regeneration the metabolic demand of individual RPE cells might differ. In "flecked retina diseases" heterogeneity of metabolic demand in individual RPE cells could therefore be responsible for their multifocal appearance.

Retinal function in Bothnia dystrophy. An electrophysiological study

Using prolonged dark adaptometry, standard dark adaptation (DA) and prolonged DA full-field electroretinograms (ERGs), we analysed the retinal function in patients with Bothnia dystrophy (BD), a variant of recessive retinitis punctata albescens (RPA). A compromised rod and cone function, a likely dysfunction of the Müller cells, and indications of disturbed neuronal function of the inner retina, were found. With prolonged DA, a gradual increase in retinal sensitivity to light and an improvement of the ERG components occurred. The findings indicate a prolonged synthesis of photopigments, retardation of the visual process in the retinal pigment epithelium (RPE), and a loss of retinal cells, probably starting at a relatively early age in BD.

Olfactory input increases visual sensitivity in zebrafish: a possible function for the terminal nerve and dopaminergic interplexiform cells

Centrifugal innervation of the neural retina has been documented in many species. In zebrafish Danio rerio, the only so-far described centrifugal pathway originates from terminal nerve (TN) cell bodies that are located in the olfactory bulb. Most of the TN axons terminate in the forebrain and midbrain, but some project via the optic nerve to the neural retina, where they synapse onto dopaminergic interplexiform cells (DA-IPCs). While the anatomical pathway between the olfactory and visual organs has been described, it is unknown if and how olfactory signals influence visual system functions. We demonstrate here that olfactory input is involved in the modulation of visual sensitivity in zebrafish. As determined by a behavioral assay and by electroretinographic (ERG) recording, zebrafish visual sensitivity was increased upon presentation of amino acids as olfactory stimuli. This effect, however, was observed only in the early morning hours when zebrafish are least sensitive to light. The effect of olfactory input on vision was eliminated after lesion of the olfactory bulbs or after the destruction of DA-IPCs. Intraocular injections of a dopamine D(2) but not a D(1) receptor antagonist blocked the effect of olfactory input on visual sensitivity. Although we cannot exclude the involvement of other anatomical pathways, our data suggest that the TN and DA-IPCs are the prime candidates for olfactory modulation of visual sensitivity.

D-serine and serine racemase are present in the vertebrate retina and contribute to the physiological activation of NMDA receptors

d-serine has been proposed as an endogenous modulator of N-methyl-d-aspartate (NMDA) receptors in many brain regions, but its presence and function in the vertebrate retina have not been characterized. We have detected d-serine and its synthesizing enzyme, serine racemase, in the retinas of several vertebrate species, including salamanders, rats, and mice and have localized both constituents to Müller cells and astrocytes, the two major glial cell types in the retina. Physiological studies in rats and salamanders demonstrated that, in retinal ganglion cells, d-serine can enhance excitatory currents elicited by the application of NMDA, as well as the NMDA receptor component of light-evoked synaptic responses. Application of d-amino acid oxidase, which degrades d-serine, reduced the magnitude of NMDA receptor-mediated currents, raising the possibility that endogenous d-serine serves as a ligand for setting the sensitivity of NMDA receptors under physiological conditions. These observations raise exciting new questions about the role of glial cells in regulating the excitability of neurons through release of d-serine.

Long-term behavior and intra-individual stability of the direct current electroretinogram and of the standing potential in the albino rabbit eye

The direct current electroretinogram (ERG) and the standing potential (SP) were studied in seven albino rabbits under general anesthesia. Identical experiments were performed on 2 consecutive days. After 30 min of dark adaptation, repeated light stimuli of maximal intensity of the system were presented to the eyes. The interstimulus interval was 70 s, and stimulus duration 10 s. Each experiment lasted for almost 3 h. In the first experiment, the b- and c-wave amplitudes measured in response to the second light stimulus were markedly reduced compared to those recorded in response to the first stimulus. Both amplitudes then recovered. The b-wave attained a peak about 20 min after the start of light stimulation. The peak was followed by a trough about 20 min later, and the amplitude then slowly increased. Following the minimum recorded during the second light stimulus, the c-wave amplitude reached a peak about 14 min after the start of stimulation. A trough in the amplitude occurred 20 min later. The amplitude then slowly increased to the end value, which was higher than the initial level. The a-wave behaved similarly to the b-wave, but the changes in most cases did not attain statistical significance. A minimum in the SP occurring at the second light stimulus was followed by a peak about 13 min after the start of light stimulation, and then by a trough about 17 min later. In the second experiment, performed one day after the first, the development of the a-, b-, and c-wave amplitudes and of the SP was similar to that observed during the first experiment, and no statistically significant differences between the two experiments were found. The reactions of the ERG and the SP were thus very stable between identical experiments performed on two consecutive days.

The scotopic threshold response of the dark-adapted electroretinogram of the mouse

The most sensitive response in the dark-adapted electroretinogram (ERG), the scotopic threshold response (STR) which originates from the proximal retina, has been identified in several mammals including humans, but previously not in the mouse. The current study established the presence and assessed the nature of the mouse STR. ERGs were recorded from adult wild-type C57/BL6 mice anaesthetized with ketamine (70 mg kg(-1)) and xylazine (7 mg kg(-1)). Recordings were between DTL fibres placed under contact lenses on the two eyes. Monocular test stimuli were brief flashes (lambda(max) 462 nm; -6.1 to +1.8 log scotopic Troland seconds(sc td s)) under fully dark-adapted conditions and in the presence of steady adapting backgrounds (-3.2 to -1.7 log sc td). For the weakest test stimuli, ERGs consisted of a slow negative potential maximal approximately 200 ms after the flash, with a small positive potential preceding it. The negative wave resembled the STR of other species. As intensity was increased, the negative potential saturated but the positive potential (maximal approximately 110 ms) continued to grow as the b-wave. For stimuli that saturated the b-wave, the a-wave emerged. For stimulus strengths up to those at which the a-wave emerged, ERG amplitudes measured at fixed times after the flash (110 and 200 ms) were fitted with a model assuming an initially linear rise of response amplitude with intensity, followed by saturation of five components of declining sensitivity: a negative STR (nSTR), a positive STR (pSTR), a positive scotopic response (pSR), PII (the bipolar cell component) and PIII (the photoreceptor component). The nSTR and pSTR were approximately 3 times more sensitive than the pSR, which was approximately 7 times more sensitive than PII. The sensitive positive components dominated the b-wave up to > 5 % of its saturated amplitude. Pharmacological agents that suppress proximal retinal activity (e.g. GABA) minimized the pSTR, nSTR and pSR, essentially isolating PII which rose linearly with intensity before showing hyperbolic saturation. The nSTR, pSTR and pSR were desensitized by weaker backgrounds than those desensitizing PII. In conclusion, ERG components of proximal retinal origin that are more sensitive to test flashes and adapting backgrounds than PII provide the 'threshold' negative and positive (b-wave) responses of the mouse dark-adapted ERG. These results support the use of the mouse ERG in studies of proximal retinal function.

GABA(C) receptors modulate the rod-driven ERG b-wave of the skate retina

Studies characterizing the types of GABA receptors present on cells isolated from the skate retina have allowed us to develop a working model of possible GABA interactions at the level of the outer plexiform layer (OPL). Earlier studies have shown an electrogenic GABA transport mechanism in horizontal cells presents a source of GABA in the OPL which could modulate feedback onto photoreceptors. GABA(A) receptors on Müller cells, or GABA(A) and/or GABA(C) receptors on bipolar cells. This model has been used for the interpretation of results of experiments in this study designed to test the role these interactions may exert on the electroretinogram (ERG). Simultaneous intracellular recording of the horizontal cell response (the S-potential) was used to monitor effects on photoreceptor transmitter release which would be altered if GABAergic photoreceptor feedback mechanisms were involved. Picorotoxin (50 microM), a chloride channel blocker which suppresses the responses of both GABA(A)Rs and GABA(C)Rs, reduced the ON (b-wave) component of the ERG substantially. Simultaneous intracellular horizontal cell recordings, however, showed no effect on their light-evoked response, suggesting that photoreceptor feedback is not involved in the picrotoxin effect on the ERG. On the other hand, even 100 microM bicuculline, a GABA(A)R antagonist produced no change in either the ERG or the horizontal cell response. This observation leads to the conclusion that the GABAARs on Miller cells and bipolar cells are not involved. Thus, there remains a distinct possibility that the ERG changes produced by picrotoxin are due to its ability to block the GABA(C)Rs on retinal bipolar cells.

The effect on vision of associated treatments in patients taking vigabatrin: carbamazepine versus valproate

PURPOSE

To evaluate the effect on visual function of a concomitant antiepileptic drug (AED) in patients treated with vigabatrin (VGB).

METHODS

Sixty-four consecutive patients with a history of partial seizures currently treated with VGB with either carbamazepine (CBZ) or valproate (VPA) were examined with automated kinetic perimetry, static perimetry, electrooculogram (EOG), and electroretinogram (ERG). An original device based on kinetic perimetry was developed to quantify the area of perception for each isopter.

RESULTS

Fifty-two patients were finally included. The results showed a significant difference in patients treated with VGB-VPA compared with patients treated with VGB-CBZ concerning the mean defect of static perimetry and the peripheral and midperipheral isopter (III 4e and III 1a Goldmann equivalent, respectively) in kinetic perimetry. EOG and ERG results did not differ significantly between the two groups.

CONCLUSIONS

The visual impairment due to visual field constriction was more important in patients treated with VGB and VPA compared with patients treated with VGB and CBZ. The origin of this difference between the two associations could not be related to any particular retinal electrophysiologic abnormality.

Dystrophin Dp71 is critical for the clustered localization of potassium channels in retinal glial cells

The Müller cell is the principal glial cell of the vertebrate retina. The primary conductance in Müller cells is the inwardly rectifying potassium channel Kir4.1 (BIR10 and KAB-2), which is highly concentrated at the endfeet at the vitreal border and to processes enveloping blood vessels. Such asymmetric and clustered distribution of Kir4.1 channels in Müller cells is thought to be critical for the buffering of extracellular potassium concentration in retina. Herein we investigated whether the distribution and functional properties of Kir4.1 channels are dependent on expression of the Dp71, a dystrophin isoform expressed in Müller cells. Kir4.1 distribution was determined in mouse retinal sections and whole mounts using anti-Kir4.1 antibodies and confocal microscopy. In Müller cells from wild-type mice, Kir4.1 is highly clustered in their endfeet and perivascular processes. In contrast, in Müller cells from the mdx(3Cv) mouse, which lacks the expression of Dp71, the Kir4.1 immunoreactivity is evenly distributed throughout the cell membrane. Surface expression of Kir4.1 is not affected in mdx(3Cv) Müller cells as current density of barium-sensitive inward currents in mdx(3Cv) Müller cells are not different from wild type. Focal extracellular potassium increases in isolated Müller cells shows that Kir channels in the mdx(3Cv) cells, as opposed to wild type, are less prominently concentrated in their endfeet. In summary, our data indicate that Dp71 is critical for the clustering but not membrane expression of Kir4.1 in mouse Müller cells. These results point to a new role for dystrophin in glial cells.

Effects of adeno-associated virus-vectored ciliary neurotrophic factor on retinal structure and function in mice with a P216L rds/peripherin mutation

Past studies have shown that acute administration of ciliary neurotrophic factor (CNTF) can prolong the survival of retinal photoreceptor cells that have undergone phototoxic injury or that express gene mutations. Adenovirus-vectored CNTF has also been effective but for all of these treatments, the effect has been transient. On the other hand, adeno-associated virus-vectored minigenes offer considerable promise for long-term survival. The authors sought to provide long-term, CNTF-based protection of mouse photoreceptors expressing a dominant-negative point mutation in the rds gene by using recombinant adeno-associated virus (rAAV) to deliver minigenes that code for a secreted form of CNTF.Secreted CNTF, under control of a cytomegalovirus (CMV) or chick beta actin (CBA) promoter provided long-term, panretinal rescue of photoreceptors following single injections of rAAV vectors into the subretinal compartment. Rescue was much less effective and less reproducible when the vectors were placed in the vitreous compartment. However, there were unexpected side effects that appeared to be dose-related. One side effect was a change in rod photoreceptor nucleus phenotype, featuring an increase in euchromatin and an increase in nuclear size following subretinal injections but not intravitreal injections. These nuclear changes were panretinal when the putatively stronger CBA promoter was used but not panretinal when the CMV promoter was used. In the latter case, the nuclear changes were much more pronounced at the site of injection. Thus, chronic hyperstimulation of retinal cells with CNTF may up-regulate gene expression in photoreceptors. Based on current knowledge of retinal cell targets for CNTF, this effect may be indirect and may not represent direct stimulation of photoreceptors by CNTF.A second side effect was a paradoxical decrease in scotopic a- and b-wave amplitudes and a decrease in photopic b-wave amplitudes in the injected, rescued retina when compared to its contralateral, uninjected counterpart, in spite of the fact that these retinas had more photoreceptors than their untreated mates. The basis for these decreased ERG amplitudes may be related to changes in gene expression. The mechanisms for these side effects and proper doses of CNTF administration should be determined before human clinical trials are considered for the amelioration of inherited retinal degenerations with CNTF.

Visual field defects and neural losses from experimental glaucoma

Glaucoma is a relatively common disease in which the death of retinal ganglion cells causes a progressive loss of sight, often leading to blindness. Typically, the degree of a patient's visual dysfunction is assessed by clinical perimetry, involving subjective measurements of light-sense thresholds across the visual field, but the relationship between visual and neural losses is inexact. Therefore, to better understand of the effects of glaucoma on the visual system, a series of investigations involving psychophysics, electrophysiology, anatomy, and histochemistry were conducted on experimental glaucoma in monkeys. The principal results of the studies showed that, (1) the depth of visual defects with standard clinical perimetry are predicted by a loss of probability summation among retinal detection mechanisms, (2) glaucomatous optic atrophy causes a non-selective reduction of metabolism of neurons in the afferent visual pathway, and (3) objective electrophysiological methods can be as sensitive as standard clinical perimetry in assessing the neural losses from glaucoma. These experimental findings from glaucoma in monkeys provide fundamental data that should be applicable to improving methods for assessing glaucomatous optic neuropathy in patients.

Retinal circadian rhythms in humans

Circadian rhythms in the retina may reflect intrinsic rhythms in the eye. Previous reports on circadian variability in electrophysiological human retinal measures have been scanty, and the results have been somewhat inconsistent. We studied the circadian variation of the electrooculography (EOG), electroretinography (ERG), and visual threshold (VTH) in subjects undergoing a 36h testing period. We used an ultrashort sleep-wake cycle to balance effects of sleep and light-dark across circadian cycles. Twelve healthy volunteers (10 males, 2 females; mean age 26.3 years, standard deviation [SD] 8.0 years, range 19-40 years) participated in the study. The retinal functions and oral temperature were measured every 90 min. The EOG was measured in the light, whereas the ERG and the VTH were measured in the dark. Sleep was inferred from activity detected by an Actillume monitor. The EOG peak-to-peak responses followed a circadian rhythm, with the peak occurring late in the morning (acrophase 12:22). The ERG b-wave implicit time peaked in the early morning (acrophase 06:46). No statistically significant circadian rhythms could be demonstrated in the ERG a-wave implicit time or peak-to-peak amplitude. The VTH rhythm peaked in the early morning (acrophases 07:59 for blue and 07:32 for red stimuli). All retinal rhythms showed less-consistent acrophases than the temperature and sleep rhythms. This study demonstrated several different circadian rhythms in retinal electrophysiological and psychophysical measures of healthy subjects. As the retinal rhythms had much poorer signal-to-noise ratios than the temperature rhythm, these measures cannot be recommended as circadian markers.

Age-related changes in the flash electroretinogram and oscillatory potentials in individuals age 75 and older

OBJECTIVES

To evaluate whether the inner plexiform layer of the retina is altered during senescence by examining the oscillatory potentials (OPs) of the flash electroretinogram (fERG) in individuals age 75 and older.

DESIGN

Cross-sectional, observational study.

SETTING

A university-based center.

PARTICIPANTS

Fifty-six healthy volunteers (age 20-88 years).

MEASUREMENTS

fERGs and OPs were first evaluated in scotopic conditions, following pupillary dilation and dark adaptation, in young (20-32 years; n = 30) and older (75-88 years; n = 26) individuals. Electrical signals were recorded with a Dawson-Trick-Litzkow type (DTL) fiber electrode in response to blue and white flashes. Red flashes were subsequently delivered to the test eye for photopic fERG and OP recordings following a period of light adaptation.

RESULTS

The amplitude of the a- and b-waves in response to blue and white flashes was significantly decreased in older people and their implicit time was increased. The latency of the a- and b-waves under photopic conditions was also prolonged with senescence, but only the amplitude of the b-wave was reduced. The amplitude of most OPs recorded under both scotopic and photopic conditions decreased with age, whereas their implicit times were prolonged.

CONCLUSIONS

Our results confirm previous findings regarding the age dependency of the fERG a- and b-waves. Furthermore, we provide novel information concerning the detrimental effects of age on the OPs, indicating that the neuronal elements within the inner plexiform layer of the retina are compromised with senescence.

Vigabatrin effect on inner retinal function

OBJECTIVE

To determine the degree of electroretinal dysfunction in a group of patients taking Vigabatrin (VGB). Additionally, to investigate the role of cumulative dosage, the role of VGB alone or in combination with other anticonvulsants, and whether recent discontinuance of VGB affects electroretinal function as measured by the electroretinogram (ERG).

DESIGN

Retrospective, comparative case series.

PARTICIPANTS

Forty patients (18 male, 22 female) with a mean age of 35 years were studied as three groups: the VGB multitherapy group (n = 24) included those taking VGB with other anticonvulsants, the VGB monotherapy group (n = 9) included those taking VGB alone, and the off-VGB group (n = 7) included those who had discontinued VGB in the last 6 months.

METHODS

Scotopic flash, photopic flash, and 30-Hz flicker ERG results were recorded according to the International Society for Clinical Electrophysiology of Vision (ISCEV) standard. The clinical electro-oculogram (EOG) results were recorded according to the ISCEV standard.

MAIN OUTCOME MEASURES

Implicit time and amplitudes of the A- and B-waves of the flash and 30-Hz flicker ERGs were recorded. Summed amplitude of the first three oscillatory potential wavelets were recorded. The light-peak to-dark-trough Arden ratio of the EOG was evaluated.

RESULTS

Although photopic ERG B-wave reduction was most frequent in patients in the VGB multitherapy group (48% of eyes), a significant number of eyes in all three groups had scotopic ERG B-wave reduction. The 30-Hz flicker ERG result was abnormally reduced in all three groups. There was no significant difference in the frequency of occurrence in ERG result abnormalities between the VGB monotherapy and VGB multitherapy groups. The EOG results revealed reduced Arden ratios in all three groups; however, there was a significantly lower frequency of EOG abnormalities noted in the off-VGB group (P = 0. 0373). There was no statistically significant relationship between the frequency of electrodiagnostic abnormalities and the duration of use or the total cumulative dosage of Vigabatrin in any of the three groups.

CONCLUSIONS

These findings of scotopic ERG result abnormalities suggest that VGB alone has an effect on inner electroretinal function at the level of the Müller cell. Concomitant EOG abnormalities suggest a substantial effect of VGB on outer retinal function that may be reversible after cessation of VGB treatment.

Background light adaptation of the retinal neuronal adaptive system. I. Effect of background light intensity

The behaviour of the neuronal adaptive retinal mechanisms to environmental light exposures was studied by measuring the oscillatory potentials (OPs) of the electroretinogram. Dark adapted rats were exposed to four levels of background light (BG), starting at a 'low scotopic' level of 1.43x 10(6) cd/m2, increased by steps of two log units, through 'high scotopic' -, 'low mesopic' - and finally the 'high mesopic' BG of 1.43x 10(0) cd/m2. The summed oscillatory response significantly increased as the BG intensity was raised, except at the 'high mesopic' level. The amplitudes of the a- and b-waves reduced as the BG light increased above the 'high scotopic' level. Each OP responded individually to the different BGs. O1 and O2, significantly enhanced at the 'low scotopic' BG. The amplitudes of the three later OPs increased significantly at the 'low mesopic' BG. The adaptational behaviour of the retinal oscillatory response to BG illumination was different to that of the a- and b- waves. The results indicate that the adaptational neuronal system, as reflected by the OPs, seems to be relatively robust and is separate from the slower photochemical adaptive process in the distal retina. The tentative corollary suggests the oscillatory system to play a vision-preserving role, possibly as an alert against undue depletion of the slowly regenerating visual pigment. The enhancement of the oscillatory response at the 'mesopic' illumination levels indicate both scotopic and photopic processes to contribute to neuronal adaptive activity of the retina.

Calcium channel immunoreactivity in the salamander retina

This study reports the distribution of the alpha1D and alpha1E calcium channel subunits in the neotenous tiger salamander retina based on immunohistochemical techniques. Confocal and light microscopy were used to localize staining with fluorescently tagged antibodies to alpha1D and alpha1E in cross-sectional and flatmount preparations of retina. Alpha1D-immunoreactivity (alpha1D-IR) was localized to the inner and outer plexiform layers (IPL and OPL, respectively), ganglion cell layer (GCL), and optic fiber layer. Alpha1E-IR was found predominantly in the IPL, with scattered, weak representation in the OPL. Alpha1E-IR was not detected in the GCL or fiber layer. These findings suggest that different alpha1 calcium channel proteins have distinctive distributions in retina, which may reflect their unique and different roles in retinal processing and homeostasis.

On-bipolar cells and depolarising third-order neurons as the origin of the ERG-b-wave in the RCS rat

In the retinas of Royal College of Surgeons (RCS) rats light induces an increase in distal extracellular potassium irrespective of the age, between days 19-24 and days 29-35 postpartum, but by days 29-35 the ERG b-wave has become reduced. The synaptic blocker 2-amino-4-phosphonobutyric acid (APB) causes the abolition of both the b-wave and the potassium increase at any age. MgCl2 greatly reduces the b-wave at all ages and abolishes the potassium increase in older rats, but in younger rats the potassium increase is enlarged. Since this increase occurs in the absence of the b-wave it is unlikely that the on-bipolar cells are the only sources of the b-wave. Because the NMDA receptor blocker ketamine reduces the b-wave, third order neurons, which possess NMDA receptors, could contribute to the b-wave.

Effects of aging on the electroretinogram during ischemia-reperfusion in rats

The effects of aging on the electroretinogram (ERG) during ischemia-reperfusion were investigated in rats. Flash-elicited ERG (a-wave, b-wave, and oscillatory potentials (OPs)) was recorded in young (4 months old) and aged rats (over 18 months old) before, during, and after exposure to 30- or 120-min ischemia induced by increasing intraocular pressure to 80 mmHg. The choroidal blood flow, measured by means of laser Doppler flowmetry, decreased to 40 to 60% of the baseline value during ischemia. Young rats showed no significant difference in the amplitude of each ERG component during ischemia between 30- and 120-min ischemia groups; 78.0 +/- 4.9 vs. 76.1 +/- 3.6% for a-wave, 63.4 +/- 3.1 vs. 60.6 +/- 3.0% for b-wave, and 59.6 +/- 5.9 vs. 57.5 +/- 6.7% for SigmaOP. In aged rats, however, 120-min ischemia caused a greater decrease, to 56.7 +/- 3.1% of the baseline value, in the a-wave amplitude than 30-min ischemia did, to 70.8 +/- 3.2%. The reduction of each ERG component in both 30- and 120-min ischemia experiments was greater in aged rats than in young rats. The recovery time for the amplitude of each ERG component during reperfusion was longer in aged rats than in young rats. The latency of b-wave and the second component of OPs prolonged during ischemia, and recovery time for the latency was longer in aged rats than in young rats. These results suggest that the electrophysiological function of the retina is less tolerable against ischemia-reperfusion in aged rats than in young rats.

Genetic inactivation of an inwardly rectifying potassium channel (Kir4.1 subunit) in mice: phenotypic impact in retina

The inwardly rectifying potassium channel Kir4.1 has been suggested to underlie the principal K(+) conductance of mammalian Müller cells and to participate in the generation of field potentials and regulation of extracellular K(+) in the retina. To further assess the role of Kir4.1 in the retina, we generated a mouse line with targeted disruption of the Kir4.1 gene (Kir4.1 -/-). Müller cells from Kir4.1 -/- mice were not labeled with an anti-Kir4.1 antibody, although they appeared morphologically normal when stained with an anti-glutamine synthetase antibody. In contrast, in Müller cells from wild-type littermate (Kir4.1 +/+) mice, Kir4.1 was present and localized to the proximal endfeet and perivascular processes. In situ whole-cell patch-clamp recordings showed a 10-fold increase in the input resistance and a large depolarization of Kir4.1 -/- Müller cells compared with Kir4.1 +/+ cells. The slow PIII response of the light-evoked electroretinogram (ERG), which is generated by K(+) fluxes through Müller cells, was totally absent in retinas from Kir4.1 -/- mice. The b-wave of the ERG, in contrast, was spared in the null mice. Overall, these results indicate that Kir4.1 is the principal K(+) channel subunit expressed in mouse Müller glial cells. The highly regulated localization and the functional properties of Kir4.1 in Müller cells suggest the involvement of this channel in the regulation of extracellular K(+) in the mouse retina.

Circadian regulation of visually evoked potentials in the domestic pigeon, Columba livia

The avian circadian and visual systems are integrally related and together influence many aspects of birds' behavior and physiology. Certainly, light cycles and their visual perception are the major zeitgebers for circadian rhythms, but do circadian rhythms affect vision? To assess whether visual function is regulated on a circadian basis, flash-evoked electroretinograms (ERGs) and vision-evoked potentials (VEPs) from the optic tectum (TeO) were recorded simultaneously in domestic pigeons at different circadian phases in a light-dark regime (LD) and in constant darkness (DD), while feeding activity was measured to determine circadian phase. In both LD and DD, the amplitudes of ERG b-waves were higher during the day than at night and latencies of a- and b-waves were longer at night. The median effective intensity for ERG a-wave was marginally higher during the day than during the night, indicating greater sensitivity at night, but this rhythm did not persist in DD. The amplitudes of TeO VEPs were also greater during the day, and latencies were greater at night in LD and DD. Together, the data indicate that a circadian clock regulates pigeon visual function at several integrative levels.

Immunocytochemical localization of the glutamate transporter GLT-1 in goldfish (Carassius auratus) retina

Glutamate is the major excitatory neurotransmitter in the retina of vertebrates. Electrophysiological experiments in goldfish and salamander have shown that neuronal glutamate transporters play an important role in the clearance of glutamate from cone synaptic clefts. In this study, the localization of the glutamate transporter GLT-1 has been investigated immunocytochemically at the light and electron microscopical levels in the goldfish retina using a GLT-1-specific antibody. GLT immunoreactivity (IR) was observed at the light microscopical level in Müller cells, bipolar cells, the outer plexiform layer (OPL), and the inner plexiform layer (IPL). At the electron microscopical level, membrane-bound and cytoplasmic GLT-IR in the OPL was located in finger-like protrusions of the cone terminal located near the invaginating postsynaptic processes of bipolar and horizontal cells. GLT-IR was not observed in the vicinity of synaptic ribbons. This location of GLT-1 allows modulation of the glutamate concentration in the synaptic cleft, thereby shaping the dynamics of synaptic transmission between cones and second-order neurons. In the inner IPL, GLT-IR was observed in the cytoplasm and was membrane bound in mixed rod/cone bipolar cell terminals and cone bipolar cell terminals. The membrane-bound GLT-1 was generally observed at some distance from the synaptic ribbon. The morphology of the bipolar cell terminal together with the localization of GLT-1 suggests that at least these glutamate transporters are not primarily involved in rapid uptake of glutamate release by the bipolar cells. The GLT-IR in the cytoplasm of Müller cells was located throughout the entire goldfish retina from the outer limiting membrane to the inner limiting membrane. The location of GLT-1 in Müller cells is consistent with the role of Müller cells in converting glutamate to glutamine.

Modification of the Xenopus electroretinogram by actions of glycine in the proximal retina

The electroretinogram (ERG) was recorded from the Xenopus retina, to examine the effects of glycine and strychnine on these responses and to determine the origins of these changes. Glycine at concentrations between 0.1 and 10 mM reduced the b- and d-waves of the ERG in a dose-dependent manner, while strychnine increased their amplitude. 2-Amino-4-phosphonobutyric acid (APB) reduced the b-wave and blocked the effect of glycine, but not strychnine, on the d-wave. When the d-wave had first been blocked by kynurenic acid (KYN) or reduced by (+/-)cis-2,3-piperidine dicarboxylic acid (PDA) the b-wave was enhanced by glycine, but not by strychnine. N-methyl-DL-aspartate (NMDLA), which alters responses in the proximal retina only, blocked the effects of glycine and strychnine on the ERG. This suggests that the glycinergic effects on the ERG are at least partly mediated by processes in the proximal retina. The results further support the suggestion that inhibitory neurotransmitters in the proximal retina may modulate both the b- and d-waves of the Xenopus ERG.

Contribution to the kinetics and amplitude of the electroretinogram b-wave by third-order retinal neurons in the rabbit retina

The ERG b-wave is widely believed to reflect mainly light-induced activity of on-center bipolar cells and Müller cells. Third-order retinal neurons are thought to contribute negligibly to generation of the b-wave. Here we show that pharmacological agents which affect predominantly third-order neurons alter significantly both the kinetics and amplitude of the b-wave. Our results support the notion that changes in the amplitude and kinetics of light-induced membrane depolarization in third-order neurons produce similar changes in the amplitude and kinetics of the b-wave. We conclude that activity in third-order neurons makes a significant contribution to b-wave generation. Our results also provide evidence that spiking activity of third-order neurons truncates the a-wave by accelerating the onset of the b-wave.

Circadian rhythm of ERG in Iguana iguana: role of the pineal

In green iguanas, the pineal controls the circadian rhythm of body temperature but not the rhythm of locomotor activity. As part of a program to investigate the characteristics of this multioscillator circadian system, the authors studied the circadian rhythms of the electroretinographic response (ERG) and asked whether the pineal gland is necessary for the expression of this rhythm. ERGs from a total of 24 anesthetized juvenile iguanas were recorded under four different conditions: (a) complete darkness (DD), (b) dim light-dark cycles (dLD), (c) constant dim light (dLL), and (d) pinealectomized in DD. Results demonstrate that the b-wave component of the ERG shows a very clear circadian rhythm in DD and that this rhythm persists in dLL and entrains to dLD cycles. The ERG response is maximally sensitive during the subjective day. Pinealectomy does not abolish the circadian rhythm in ERG, demonstrating that the oscillator responsible for the ERG rhythm is located elsewhere.

Circadian regulation of chick electroretinogram: effects of pinealectomy and exogenous melatonin

Melatonin is an important component of the avian circadian system. This study investigates the effects of pinealectomy (Pin-X) and melatonin implantation (Mel) on electroretinogram (ERG) rhythms in chicks. Feeding rhythms were monitored to obtain a phase reference for ERG recordings. Pin-X and Mel had little or no effect on feeding rhythms. Sham-operated Pin-X and vehicle implantation had no effect on ERG rhythms in the light-dark (LD) cycle or constant darkness (DD). ERG a- and b-wave amplitudes were higher during the day than during the night. The a- and b-wave implicit times were shorter during the day than during the night. a-Wave sensitivity was higher during the night than during the day, whereas b-wave sensitivity was not rhythmic. Pin-X abolished the circadian rhythm of b-wave amplitude and implicit time in DD but had no effect on a-wave rhythmicity. Mel abolished the rhythm of b-wave amplitude and of a- and b-wave implicit time in DD. Neither treatment affected ERG in LD. These results suggest that the circadian system regulates rhythmic visual function in the retina at least partially through Mel. The role played by the pineal gland and Mel may be specific to some physiological modalities (e.g., vision) while not influencing others (e.g., feeding).

Dissecting the dark-adapted electroretinogram

Although gross recordings of the ganzfeld flash-evoked electroretinogram (ERG) can potentially provide information about the activity of many, if not all, retinal cell types, it is necessary to dissect the ERG into its components to realize this potential fully. Here we describe various procedures that have been used in intact mammalian eyes to identify and characterize the contributions to the dark-adapted ERG of different cells in the retinal rod pathway. These include (1) examination of the very early part of the response to a flash (believed to reflect directly the photocurrent of rods), (2) application of high-energy probe flashes to provide information about the underlying rod photoreceptor response even when this component is obscured by the responses of other cells, (3) pharmacological suppression of responses of amacrine and ganglion cells to identify the contribution of these cells and to reveal the weaker responses of bipolar cells, (4) use of pharmacological agents that block transmission of signals from rods to more proximal neurons to separate responses of rods from those of later neurons, (5) examination of the ERG changes produced by ganglion-cell degeneration or pharmacological block of nerve-spike generation to identify the contribution of spiking neurons, (6) modeling measured amplitude-energy functions and timecourse of flash responses and (7) using steady backgrounds to obtain differential reductions in sensitivity of different cell types. While some of these procedures can be applied to humans, the results described here have all been obtained in studies of the ERG of anaesthetized cats, or macaque monkeys whose retinas are very similar to those of humans.

Symptomatic and asymptomatic visual loss in patients taking vigabatrin

PURPOSE

To investigate the clinical, perimetric, and electrophysiologic findings in patients with visual field loss on long-term treatment with the antiepileptic medication vigabatrin.

DESIGN

Consecutive observational case series.

PARTICIPANTS

Forty-one consecutive subjects taking vigabatrin referred for screening ophthalmologic assessment were studied. Twelve subjects with evidence of peripheral visual field constriction are presented.

METHODS

Twelve subjects with evidence of peripheral visual field constriction on 60-4 perimetry underwent central 30-2 and blue-on-yellow (B/Y) perimetry, as well as electroretinography (ERG), electro-oculography (EOG), and visual-evoked potential (VEP) testing.

MAIN OUTCOME MEASURES

Visual acuity; fundus abnormalities; visual field loss; and ERG, EOG, or VEP abnormalities were the main outcome measures.

RESULTS

Eight of the 12 subjects with constricted visual fields were asymptomatic. The central 30-2 perimetry demonstrated bilateral visual field constriction in 9 of 12 patients and the B/Y perimetry in 8 of 9 patients tested. Of the ten patients tested electrophysiologically, four had abnormal ERGs, five had abnormal EOGs, and three had delayed VEPs.

CONCLUSIONS

The incidence of visual field constriction in patients taking vigabatrin may be higher, and asymptomatic visual field loss more common, than reported previously. The authors postulate a possible Muller cell dysfunction in the peripheral retina. Patients taking vigabatrin should have regular peripheral visual field examinations.

The effects of electroconvulsive shock on retinal activity

Two experiments are reported examining the effects of electroconvulsive shock (ECS) on the electroretinogram (ERG) and the retinal oscillatory potentials (OPs) in the albino rat. Immediately after the induction of generalised seizure activity, both the ERG and the OPs were always preserved basically intact, despite minor alterations to their waveforms. In Experiment 1, it was found that small changes in amplitude of the ERG were recorded following ECS, but these were most likely artifactual. The only other notable finding was a temporary decrease in latency of the b-wave of the ERG. In Experiment 2, a slight overall attenuation in the amplitude of the OPs was observed. This was associated with a paradoxical decrease in the latencies of all three OP subcomponents similar to that found for the ERG b-wave. It is concluded that ECS does not interfere to any marked extent with either the transduction of the visual signal or its processing within the various retinal layers. This implies that the blockade of the afferent volley that occurs following ECS must be confined to the optic pathway or to the occipital cortex itself.

Spontaneous activity in developing turtle retinal ganglion cells: pharmacological studies

Extracellular recordings were obtained from the ganglion cell (GC) layer during correlated spontaneous bursting activity (SBA) in the immature turtle retina. Pharmacological agents were bath-applied, and their effects on burst and correlation parameters were determined. SBA requires synaptic transmission. It was blocked in the presence of curare and mecamylamine, two cholinergic nicotinic antagonists, and enhanced with neostigmine, a cholinesterase inhibitor. SBA was profoundly inhibited during blockade of glutamatergic receptors with the broad spectrum antagonist kynurenate and it vanished with 6,7-dinitroquinoxaline-2-3-dione (DNQX) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), two AMPA/kainate receptor antagonists. Blockade of NMDA receptors with D(-)-2-amino-5-phosphonopentanoic acid (D-AP-5) led only to a modest reduction in SBA. Blockade of GABAA receptors with bicuculline prolonged the duration of the bursts. Inhibition of GABA uptake with nipecotic acid led to a decrease in burst rate. Blockade of K+ channels with cesium (Cs+) and tetraethylammonium (TEA) led to a dramatic decrease in excitability. Burst propagation between neighboring GCs was reduced by K+ channel blockade. Gap junction blockade had no consistent effect on bursts or correlation parameters. None of these drugs had a strong effect on the refractory period between bursts. We conclude that correlated SBA in immature turtle GCs requires both cholinergic nicotinic and glutamatergic (mainly through AMPA/kainate receptors) synaptic transmission. GABAergic activity modulates the intensity and the duration of the bursts. Extracellular K+ is involved in lateral activity propagation and increases retinal excitability, which may be required for burst generation.

The mouse X-linked juvenile retinoschisis cDNA: expression in photoreceptors

Retinal photoreceptor cells are particularly vulnerable to degenerations that can eventually lead to blindness. Our purpose is to identify and characterize genes expressed specifically in photoreceptors in order to increase our understanding of the biochemistry and function of these cells, and then to use these genes as candidates for the sites of mutations responsible for degenerative retinal diseases. We have characterized a cDNA, a fragment of which (SR3.1) was originally isolated by subtractive hybridization of adult, photoreceptorless rd mouse retinal cDNAs from the cDNAs of normal mouse retina. The full-length sequence of this cDNA was determined from clones obtained by screening mouse retinal and eye cDNA libraries and by using the 5'- and 3'-RACE methods. Both Northern blot analysis and in situ hybridization showed that the corresponding mRNA is expressed in rod and cone photoreceptors. The gene encoding this cDNA was mapped to the X chromosome using an interspecific cross. Based on the nucleotide and amino acid sequences, as well as chromosome mapping, we determined that this gene is the mouse ortholog (Xlrs1) of the human X-linked juvenile retinoschisis gene (XLRS1). Analysis of the predicted amino acid sequence indicates that the Xlrs1 mRNA may encode a secretable, adhesion protein. Therefore, our data suggest that X-linked juvenile retinoschisis originates from abnormalities in a photoreceptor-derived adhesion protein.

Effects of dystrophin isoforms on signal transduction through neural retina: genotype-phenotype analysis of duchenne muscular dystrophy mouse mutants

Duchenne and Becker muscular dystrophy patients have mutations in the dystrophin gene. Most show reduced b-wave amplitudes in the dark-adapted electroretinogram (ERG). We studied normal C57BL/6J mice and five X-linked muscular dystrophy strains with different dystrophin mutations to determine whether the location of the mutation within the gene affects the mouse ERG and to correlate such effects with dystrophin isoform expression. Amplitudes and implicit times were measured for a-waves, b-waves, and digitally filtered oscillatory potentials. mdx and mdxCv5 mice, with mutations near the amino terminus and lacking expression of Dp427, had ERGs similar to those of C57BL/6J mice. mdxCv2 and mdxCv4 mice, with mutations in the center of dystrophin and who do not express isoforms Dp427, Dp260, or Dp140 (mdxCv4), had increased b-wave and oscillatory potential implicit times. mdxCv3 mice, with a mutation near the carboxy terminus resulting in deficiency of all dystrophin isoforms, had increased b-wave and oscillatory potential implicit times and reduced scotopic b-wave amplitudes. Fitting the a-wave data to a transduction activation phase mathematical model showed normal responses for all phenotypes, suggesting that the b-wave delays are due to defects beyond the rod outer segment, most likely at the rod to on-bipolar cell synapse. The variation in the ERG phenotype with the position of the dystrophin gene mutation suggests that there are different contributions by each isoform to retinal electrophysiology. Although Dp427 and Dp140 isoforms do not appear to be important contributors to the ERG, lack of Dp260 and possibly Dp71 isoforms is associated with an abnormal ERG.

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.

Localization of gap junctions and tracer coupling in retinal Müller cells

Physiological studies have demonstrated the existence of direct intercellular communication, presumably mediated by gap junctions, both between neurons and between glial cells in the vertebrate retina. We localized gap junctions in the retinas of rat, goldfish, and mudpuppy by using antisera directed against proteins that make up the connexon channels in two tissues from which connexins have been isolated: liver (connexin 32; CX32) and heart (connexin 43; CX43). Although the antiserum against CX32 stained liver gap junctions, it did not reveal any staining in rat or goldfish retina. The antiserum against CX43 stained gap junctions associated with the intercalated disk in rat heart and also stained gap junctions between pigment epithelium cells in rat, goldfish, and mudpuppy retina. Anti-CX43 also stained gap junctions between Müller cells in goldfish and mudpuppy retina but not in rat retina. Intracellular injections of the tracer Neurobiotin into Müller cells in the mudpuppy retina revealed that these glial cells are extensively tracer coupled. Staining with the tracer formed a syncytium of thin processes surrounding every neuron from the outer limiting membrane to the inner limiting membrane. Confocal microscopy demonstrated that the Müller cells were in close apposition with one another at every level of the retina. However, CX43 immunoreactivity was heaviest at the outer limiting membrane, where the apical processes of Müller cells are located. Some anti-CX43 staining was observed at the level of the outer nuclear layer and the inner plexiform layer but not in the ganglion cell layer or at the Müller cell end feet forming the inner limiting membrane.

The b-wave of the electroretinogram as an index of retinal ischemia

1. The b-wave of the electroretinogram (ERG) is a particularly sensitive index of retinal ischemia. The present paper summarizes the changes in the b-wave observed in five in vivo models of retinal ischemia. 2. Although the amount of reduction in b-wave amplitude during ischemia corresponds to the severity of the insult, the degree of recovery of the b-wave during reperfusion depends on the duration of ischemia. 3. A massive release of glutamate, intracellular overload with calcium and enhanced production of free radicals are suggested to be three major pathophysiological processes that contribute to retinal ischemic damage. The b-wave of the ERG represents a functional measure for potential therapeutic efficacy of drugs interacting with these pathophysiological processes. 4. Several glutamate antagonists, such as MK-801, memantine, flupirtine or GYKI 52466, along with the free radical scavengers vitamin E, lipoate, superoxide dismutase and catalase, all reduce the depression of the b-wave during ischemia or accelerate the recovery of the b-wave during reperfusion or both. The calcium channel antagonists nimodipine and levemopamil exert only a slight beneficial effect on the recovery of the amplitude of the b-wave during reperfusion, provided that the blood pressure is not potently reduced.

The relationship between the putative optic pathway potential and the electroretinogram

The putative optic pathway flash visual evoked potential (FVEP) and the electroretinogram (ERG) were recorded consecutively in the lightly anesthetised rat. The purpose was to test the hypothesis that the optic pathway FVEP is only an artifact created by distorted volume-conducted retinal activity. A comparison of the timing of the ERG with that of the optic pathway FVEP confirmed this suspicion. It is shown that there is a close temporal correspondence between individual subcomponents of the optic pathway potential and those of the ERG (i.e., the a-wave, b-wave, and the oscillatory potentials). In addition, it was found that when ERG currents are recorded far field from the vicinity of the optic nerve or tract, they acquire a triphasic positive-negative-positive waveform, thereby heightening the illusion that the optic pathway FVEP is a genuine compound action potential. It is concluded that experimental findings derived from the recording of the optic pathway FVEP must be treated with caution.

Postsynaptic response kinetics are controlled by a glutamate transporter at cone photoreceptors

We evaluated the role of the sodium/glutamate transporter at the synaptic terminals of cone photoreceptors in controlling postsynaptic response kinetics. The strategy was to measure the changes in horizontal cell response rate induced by blocking transporter uptake in cones with dihydrokainate (DHK). DHK was chosen as the uptake blocker because, as we show through autoradiographic uptake measurements, DHK specifically blocked uptake in cones without affecting uptake in Mueller cells. Horizontal cells depolarized from about -70 to -20 mV as the exogenous glutamate concentration was increased from approximately 1 to 40 microM, so horizontal cells can serve as "glutamate electrodes" during the light response. DHK slowed the rate of hyperpolarization of the horizontal cells in a dose-dependent way, but didn't affect the kinetics of the cone responses. At 300 microM DHK, the rate of the horizontal cell hyperpolarization was slowed to only 17 +/- 8.5% (mean +/- SD) of control. Translating this to changes in glutamate concentration using the slice dose response curve as calibration in Fig. 2, DHK reduced the rate of removal of glutamate from approximately 0.12 to 0.031 microM/s. The voltage dependence of uptake rate in the transporter alone was capable of modulating glutamate concentration: we blocked vesicular released glutamate with bathed 20 mM Mg2+ and then added 30 microM glutamate to the bath to reestablish a physiological glutamate concentration level at the synapse and thereby depolarize the horizontal cells. Under these conditions, a light flash elicited a 17-mV hyperpolarization in the horizontal cells. When we substituted kainate, which is not transported, for glutamate, horizontal cells were depolarized but light did not elicit any response, indicating that the transporter alone was responsible for the removal of glutamate under these conditions. This suggests that the transporter was both voltage dependent and robust enough to modulate glutamate concentration. The transporter must be at least as effective as diffusion in removing glutamate from the synapse because there is only a very small light response once the transporter is blocked. The transporter, via its voltage dependence on cone membrane potential, appears to contribute significantly to the control of postsynaptic response kinetics.