The influence of MgCl2 and APB on the light-induced potassium changes and the ERG b-wave of the isolated superfused rat retina

Changes in Retinal Function and Cellular Remodeling Following Experimental Retinal Detachment in a Rabbit Model

Purpose. To explore functional electroretinographic (ERG) changes and associated cellular remodeling following experimental retinal detachment in a rabbit model. Methods. Retinal detachment was created in ten rabbits by injecting 0.1 ml balanced salt solution under the retina. Fundus imaging was performed 0, 3, 7, 14, and 21 days postoperatively. ERGs were recorded pre- and 7 and 21 days postoperatively. Eyes were harvested on day 21 and evaluated immunohistochemically (IHC) for remodeling of second- and third-order neurons. Results. Retinal reattachment occurred within two weeks following surgery. No attenuation was observed in the photopic or scotopic a- and b-waves. A secondary wavefront on the descending slope of the scotopic b-wave was the only ERG result that was attenuated in detached retinas. IHC demonstrated anatomical changes in both ON and OFF bipolar cells. Bassoon staining was observed in the remodeled dendrites. Amacrine and horizontal cells did not alter, but Muller cells were clearly reactive with marked extension. Conclusion. Retinal detachment and reattachment were associated with functional and anatomical changes. Exploring the significance of the secondary scotopic wavefront and its association with the remodeling of 2nd- and 3rd-order neurons will shade more light on functional changes and recovery of the retina.

Preclinical Dose-Escalation Study of Intravitreal AAV-RS1 Gene Therapy in a Mouse Model of X-linked Retinoschisis: Dose-Dependent Expression and Improved Retinal Structure and Function

Gene therapy for inherited retinal diseases has been shown to ameliorate functional and structural defects in both animal models and in human clinical trials. X-linked retinoschisis (XLRS) is an early-age onset macular dystrophy resulting from loss of an extracellular matrix protein (RS1). In preparation for a human clinical gene therapy trial, we conducted a dose-range efficacy study of the clinical vector, a self-complementary AAV delivering a human retinoschisin (RS1) gene under control of the RS1 promoter and an interphotoreceptor binding protein enhancer (AAV8-scRS/IRBPhRS), in the retinoschisin knockout (Rs1-KO) mouse. The therapeutic vector at 1 × 10(6) to 2.5 × 10(9) (1E6-2.5E9) vector genomes (vg)/eye or vehicle was administered to one eye of 229 male Rs1-KO mice by intravitreal injection at 22 ± 3 days postnatal age (PN). Analysis of retinal function (dark-adapted electroretinogram, ERG), structure (cavities and outer nuclear layer thickness) by in vivo retinal imaging using optical coherence tomography, and retinal immunohistochemistry (IHC) for RS1 was done 3-4 months and/or 6-9 months postinjection (PI). RS1 IHC staining was dose dependent across doses ≥1E7 vg/eye, and the threshold for significant improvement in all measures of retinal structure and function was 1E8 vg/eye. Higher doses, however, did not produce additional improvement. At all doses showing efficacy, RS1 staining in Rs1-KO mouse was less than that in wild-type mice. Improvement in the ERG and RS1 staining was unchanged or greater at 6-9 months than at 3-4 months PI. This study demonstrates that vitreal administration of AAV8 scRS/IRBPhRS produces significant improvement in retinal structure and function in the mouse model of XLRS over a vector dose range that can be extended to a human trial. It indicates that a fully normal level of RS1 expression is not necessary for a therapeutic effect.

ON-OFF Interactions in the Retina: Role of Glycine and GABA

In the vertebrate retina, visual signals are segregated into parallel ON and OFF pathways, which provide information for light increments and decrements. The segregation is first evident at the level of the ON and OFF bipolar cells and it apparently remains as signals propagate to higher brain visual centers. A fundamental question in visual neuroscience is how these two parallel pathways function: are they independent from each other or do they interact somehow? In the latter case, what kinds of mechanisms are involved and what are the consequences from this cross-talk? This review summarizes current knowledge about the types of interactions between the ON and OFF channels in nonmammalian and mammalian retina. Data concerning the ON-OFF interactions in distal retina revealed by recording of single bipolar cell activity and electroretinographic ON (b-wave) and OFF (d-wave) responses are presented. Special emphasis is put on the ON-OFF interactions in proximal retina and their dependence on the state of light adaptation in mammalian retina. The involvement of the GABAergic and glycinergic systems in the ON-OFF crosstalk is also discussed.

Effects of picrotoxin on light adapted frog electroretinogram are not due entirely to its action in proximal retina

In order to evaluate the site of action of picrotoxin (antagonist of ionotropic GABA receptors) on the electroretinographic (ERG) b- and d-waves, in this study we compared its effects on the intensity-response function of the ERG waves in intact light adapted frog eyecup preparations with its effects in eyecups, where the activity of proximal neurons was blocked by 1 mMN-methyl-d-aspartate (MNDA). Picrotoxin markedly enhanced the b- and d-wave amplitude and slowed the time course of the responses at all stimulus intensities in the intact eyecups. Perfusion with NMDA alone caused significant enhancement of the b-wave amplitude and diminution of the d-wave amplitude without altering their time course in the entire intensity range. When picrotoxin was applied in combination with NMDA, an enhancement of the b-wave amplitude and slowing of its time course were observed at all stimulus intensities. The increase of the b-wave amplitude was significantly higher than that seen in NMDA group. Combined application of picrotoxin and NMDA caused an enhancement of the d-wave amplitude at the lower stimulus intensities and its diminution at the higher ones, while the d-wave time course was delayed over the entire intensity range. The results obtained indicate that a part of picrotoxin effects on the amplitude and time course of the photopic ERG b- and d-waves are due to its action in the distal frog retina.

Contribution of voltage-gated sodium channels to b- and d-waves of frog electroretinogram under different conditions of light adaptation

The effect of blockade of voltage-gated sodium (Na(v)) channels by tetrodotoxin (TTX) on the V-log I function of the ERG b- and d-waves was investigated in light and dark adapted frog eyecups. TTX diminished the b- and d-wave amplitude under both conditions of adaptation at all stimulus intensities except for the middle intensity range in dark adapted eyes, where it had no effect on the b-wave amplitude. TTX delayed the time course of the responses, obtained with low intensity stimuli and widened the dynamic range of the b-wave. The inhibitory effect of TTX on the cone-mediated, but not rod-mediated b- and d-wave amplitude persisted after the blockade of proximal retinal activity by NMDA, indicating that it may be due to a blockade of Na(v) channels on the ON and OFF bipolar cells.

Localization and possible function of P2Y(4) receptors in the rodent retina

Extracellular ATP acts as a neurotransmitter in the retina, via the activation of ionotropic P2X receptors and metabotropic P2Y receptors. The expression of various P2X and P2Y receptor subtypes has been demonstrated in the retina, but the localization of P2Y receptors and their role in retinal signaling remains ill defined. In this study, we were interested in determining the localization of the P2Y(4) receptor subtype in the rat retina, and using the electroretinogram (ERG) to assess whether activation of these receptors modulated visual transmission. Using light and electron microscopy, we demonstrated that P2Y(4) receptors were expressed pre-synaptically in rod bipolar cells and in processes postsynaptic to cone bipolar cells. Furthermore, we show that the expression of P2Y(4) receptors on rod bipolar cell axon terminals is reduced following dark adaptation, suggesting receptor expression may be dependent on retinal activity. Finally, using the electroretinogram, we show that intravitreal injection of uridine triphosphate, a P2Y receptor agonist, decreases the amplitude of the rod PII, supporting a role for P2Y receptors in altering inner retinal function. Taken together, these results suggest a role for P2Y(4) receptors in the modulation of inner retinal signaling.

GABAa and GABAc receptor mediated influences on the intensity-response functions of the b- and d-wave in the frog ERG

We assessed the contribution of GABAa and GABAc receptors to GABAergic effects on b- and d-wave in frog ERG in a wide range of light stimulation conditions. The amplitude of both b- and d-wave was increased during GABAa receptor blockade by bicuculline as well as during additional GABAc receptor blockade by picrotoxin. The effects of GABAa receptor blockade were more pronounced in light adaptation conditions. They strongly depended on stimulus intensity and showed considerable ON/OFF-response asymmetry. The effects of GABAc receptor blockade were more pronounced in dark adaptation conditions. They didn't vary much with stimulus intensity and showed little ON/OFF-response asymmetry.

Early ametropia and rod photoreceptor function in retinopathy of prematurity

PURPOSE

Early ametropia, particularly myopia, is frequent in children with a history of preterm birth and retinopathy of prematurity (ROP). The retina is known to govern eye growth and refractive development. We tested the hypothesis that deficits in retinal function are significantly associated with early ametropia in ROP subjects.

METHODS

Scotopic electoretinogram (ERG) responses to full field stimuli were studied in 40 ROP subjects aged 8 weeks to 18 years. The ROP was categorized as treated, untreated, or none. Refractive development of each ROP subject was monitored and compared with normal for age. The rod photoresponse parameters were calculated and the postreceptoral responses derived. The ERG parameters in the ROP subjects were compared with normal values for age.

RESULTS

Twelve ROP subjects developed early ametropia, 10 myopia, and two hyperopia. In the majority of ROP subjects, receptoral and postreceptoral response parameters were below the normal mean for age. In the 12 children with early ametropia, rod photoreceptor sensitivity was significantly lower than in emmetropic ROP subjects; and in five tested in infancy, deficits in rod photoreceptor sensitivity antedated development of ametropia. The myopic control subjects had no deficits in response parameters.

CONCLUSIONS

Retinal dysfunction is significantly associated with early ametropia in these ROP subjects. Thus, mechanisms for the development of ametropia in ROP subjects may involve rod and rod-mediated postreceptoral activity.

An Overview of Drug Development with Special Emphasis on the Role of Visual Electrophysiological Testing

Visual electrophysiological techniques, such as electroretinography (ERG) and visual evoked potentials (VEP) can provide useful information on the safety, efficacy, and proper dosing of chemical entities under development as new drug therapies. During post-marketing safety surveillance, a variety of visual electrophysiological measures can be used to objectively assess and document individual patient response to ophthalmic drugs and ocular or visual system side effects of non-ophthalmic drugs. In this paper, the discovery, exploratory development, full-development and post-marketing stages of drug development are briefly outlined. The potential role of visual electrophysiological techniques in each of these stages is described and discussed.

The effect of GABA and the GABA-uptake-blocker NO-711 on the b-wave of the ERG and the responses of horizontal cells to light

BACKGROUND

The effects of GABA in the retina have now become of special interest because the anti-epileptic drug vigabatrin, a GABA analogue, can cause visual field loss in humans. Vigabatrin inhibits the GABA-aminotransferase, which finally results in GABA accumulation in the extracellular space. The b-wave of the electroretinogram (ERG), which originates partly in on-bipolar cells, is influenced by both GABAergic horizontal cells (HCs) and GABAergic amacrine cells (ACs). Their influences, however, are difficult to separate. In an attempt to isolate the effect of GABAergic ACs, use has been made of the specific effect of the GABA-uptake-blocker NO-711, which blocks only the GABA transporter GAT1 of GABAergic ACs.

METHODS

The ERG and the intracellular responses of HCs to light were recorded in the isolated rabbit retina, and the effects of GABA and NO-711, when added separately to the superfusate, were determined.

RESULTS

GABA reduced significantly both the light responses of HCs and the b-wave. NO-711 enlarged the b-wave drastically, but did not affect the responses of HCs to light.

CONCLUSIONS

An increase in the extracellular GABA concentration decreases the b-wave; an impairment of the function of ACs increases the b-wave. These conditions are discussed in the context of the lack of consistent changes to the b-wave during therapy with vigabatrin.

Sildenafil, N-desmethyl-sildenafil and Zaprinast enhance photoreceptor response in the isolated rat retina

Here we report that the active component of Viagra, Sildenafil and the first metabolite, N-desmethyl-sildenafil (UK-103, 320) increased the amplitude of flash-evoked electroretinogram (ERG) of dark-adapted albino rat retina. Effects of Sildenafil and N-desmethyl-sildenafil were comparable to those of the known phosphodiesterase inhibitor, Zaprinast. The photoreceptor cell response was isolated by blocking the glial K(+) ion-buffering and the on-bipolar components of the ERG with the use of BaCl(2) (500 microM) and the specific type VI metabotropic glutamate receptor agonist, DL-2-amino-4-phosphonobutyric acid (25 microM), respectively. Zaprinast, Sildenafil and N-desmethyl-sildenafil (1 microM each) increased the amplitude of photoreceptor cell response either. Besides, Sildenafil was significantly more effective than N-desmethyl-sildenafil. These findings suggest an increased sensitivity of photoreceptor cells in the presence of Sildenafil and it is metabolite.

The beta-adrenoceptor antagonists metipranolol and timolol are retinal neuroprotectants: comparison with betaxolol

beta-adrenoceptor antagonists are used clinically to reduce elevated intraocular pressure in glaucoma which is characterised by a loss of retinal ganglion cells. Previous studies have shown that the beta(1)-selective adrenoceptor antagonist, betaxolol, is additionally able to protect retinal neurones in vitro and ganglion cells in vivo from the detrimental effects of either ischemia-reperfusion or from excitotoxicity, after topical application. The neuroprotective effect of betaxolol is thought not to be elicited through an interaction with beta-adrenoceptors, but by its ability to reduce influx of sodium and calcium through voltage-sensitive calcium and sodium channels. In the present study it is shown that the non-selective beta-adrenoceptor antagonists, metipranolol and timolol behave like betaxolol. When topically applied they all attenuate the detrimental effect of ischemia-reperfusion. Protection of the retina was determined by evaluating changes in the electroretinogram and by assessing the loss of mRNA for Thy-1, which is expressed in retinal ganglion cells. In addition, studies conducted on neurones in mixed retinal cultures demonstrated that metipranolol, betaxolol and timolol were all able to partially counteract anoxia-induced cell loss and viability reduction. The influence of timolol was, however, not significant. Within the confines of these investigations, an order of neuroprotective efficacy was delineated for the three beta-adrenoceptor antagonists: betaxolol>metipranolol>timolol. The ability of the beta-adrenoceptor antagonists to attenuate ligand-induced stimulation of calcium and sodium entry into neuronal preparations showed a similar order of effectiveness. In conclusion, the ability to confer neuroprotection to retinal neurones is a common feature of three ophthalmic beta-adrenoceptor antagonists (betaxolol, metipranolol and timolol). A comparison of the effectiveness of the individual compounds in protecting retinal cells in vivo was not possible in these studies. However, in vitro studies show that the capacity of the individual beta-adrenoceptor antagonists to act as neuroprotectants appears to relate to their capacity to attenuate neuronal calcium and sodium influx.

The effects of GABA and vigabatrin on horizontal cell responses to light and the effect of vigabatrin on the electroretinogram

When used as an antiepileptic drug in humans vigabatrin, which is a GABA analogue and an inhibitor of GABA-aminotransferase, often causes peripheral visual field loss. This could result from increases in endogenous GABA levels. Accordingly we have investigated the effects of GABA on horizontal cells (HCs) of the rabbit retina, and of vigabatrin, when applied for only a few minutes, on HCs and on the electroretinogram (ERG). The intracellular HC and ERG records were first obtained from isolated rabbit retinas during perfusion with a physiological solution. The perfusate was then changed to one containing GABA (2 mM) or vigabatrin (25, 40 or 150 microM) for at least 5 min, and then returned to the control solution. 2 mM GABA significantly but reversibly reduced the light responses of HCs elicited by diffuse light (at -4 log intensity) to 52 +/- 17% (SD, n = 7). Vigabatrin had no significant effect on the light responses of HCs (n = 7), and no effect on the b-wave (n = 4), but the PIII-component of the ERG was slightly but significantly reduced to 84 +/- 5% (SD, n = 5). The high dosage of GABA needed to affect the light responses of HCs could be due to strong GABA uptake systems in the intact rabbit retina. It is, however, possible that in humans receiving long-term treatment with vigabatrin, high levels of GABA occur because of the inhibition of GABA- aminotransferase. It seems, from these observations, that neurons like on-bipolar cells, which are contributors to the b-wave, and HCs are uninfluenced by vigabatrin in short-term experiments. The slightly reduced slow PIII-component, however, indicates an influence on the glial Müller cells which are the main contributors to the slow PIII-component.

GABAc feedback pathway modulates the amplitude and kinetics of ERG b-wave in a mammalian retina in vivo

The electroretinogram b-wave is generally believed to reflect mainly light-induced activity of ON-center bipolar cells and Muller cells. Recently, there is increasing evidence that third-order retinal neurons can also contribute significantly to the b-wave. In a previous study (Vis. Res. 40 (2000) 579) we proposed that the GABAc feedback from amacrine cells to bipolar cells can affect both the amplitude and kinetics of the b-wave. Here we show that blocking this feedback has profound effects on b-wave amplitude and kinetics. These results demonstrate that feedback to bipolar cells is an important mechanism through which amacrine cells contribute to b-wave generation. Our results also provide functional evidence that the feedback may be involved in temporal processing in the mammalian retina.

Background adaptation in a rat model of retinopathy of prematurity

Low dark-adapted, scotopic retinal and visual sensitivity in retinopathy of prematurity (ROP) could be due to disease of the inner retina, or the recently described rod photoreceptor abnormalities. Receptoral disease decreases catch of quanta from both test flashes and steady background lights; increment threshold functions are shifted up and right. In diseases with normal receptors but low retinal sensitivity due to abnormal post receptoral processing, the increment threshold functions are shifted up with no horizontal translation. Herein we test the hypothesis that the rod photoreceptors are the site of ROP disease which causes low dark adapted b-wave sensitivity. The effect of steady background light on the ERG b-wave in a rat model of ROP is studied. ERG stimulus/response functions were obtained using full-field stimuli in the dark-adapted state, and in the presence of a steady background light. In each adaptation condition, log sigma, the test flash intensity that produced a half-maximum b-wave amplitude, was calculated. In pilot experiments, the background light selected had raised log sigma about a log unit in controls. In dark-adapted ROP rats log sigma was significantly higher, 0.35 log unit, than in controls. In the presence of the background light, log sigma in ROP and control rats did not differ significantly indicating a relative shift, up and right, of the increment sensitivity function for the less sensitive ROP rats. The effect of the background light is consistent with receptoral disease causing low dark adapted b-wave sensitivity in ROP rats.

The influence of HEPES on light responses of rabbit horizontal cells

HEPES-buffered solutions, mostly used in studies of isolated cells, and bicarbonate-buffered solutions, mostly used in studies of isolated retinal tissues, have both been used to superfuse an isolated rabbit retina preparation. The responses of horizontal cells (HCs) to light, detected by intracellular microelectrodes filled with Lucifer Yellow, were recorded. Buffering of the superfusate with 100% HEPES completely, but reversibly, abolished the responses of A-type HCs, and is not, therefore, suitable for studies on isolated rabbit retinas. The responses remained when buffering was partially with HEPES and partially with bicarbonate, but were changed: in A-type HCs the overshoot was reduced and the afterpotential was increased. The overshoot may be caused by feedback of HCs on the cones and might be dependent on pHi at the synaptic structure between HCs and photoreceptors.

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.

Opposite effects of GABA(A) and GABA(C) receptor antagonists on the b-wave of ERG recorded from the isolated rat retina

The largest component in the fully dark-adapted ERG is a corneal-positive response, known as the b-wave, and believed to originate from depolarizing (ON-type) bipolar cells. The two types of GABA receptors, GABA(A) and GABA(C) have been reported to exist on bipolar cells in rat retina. The goal of these experiments was to find whether these GABA receptors participate in the generation of the b-wave of electroretinogram (ERG). ERGs were recorded from the isolated rat retinas. The P(2)(t) component, obtained by subtracting the ERGs measured before the application of 50 micrograms APB from those measured after the application of 50 micrograms APB, was used as an indicator of depolarizing bipolar cell activity. Photovoltages, the fast P(3)(t) component of ERG, were registered between the two microelectrodes across the rod outer segments. Bicuculline and 3-aminopropylphosphonic acid (3-APA) were used as selective antagonists of GABA(A) and GABA(C) receptors, respectively. It was found that the GABA(A) and GABA(C) receptors antagonists have opposite effects on the b-wave: bicuculline increased the b-wave amplitude, while 3-APA reduced the amplitude of the b-wave. Neither bicuculline nor 3-APA affect photoreceptors.

Rod and rod mediated function in patients with beta-thalassemia major

An electroretinographic (ERG) study was undertaken to test the hypothesis that scotopic retinal function is altered in transfused thalassemics on chronic Deferoxamine (DFO). ERG a- and b-wave responses and dark adapted visual thresholds were obtained from 11 patients with beta-thalassemia major, ages 7 to 38 (median 17) years. A quantitative model of the activation of phototransduction was fitted to the a-waves to estimate the gain of the transduction processes and the saturated amplitude of the rod photoresponse. From b-wave stimulus/response functions. the saturated b-wave amplitude and an index of b-wave sensitivity (log sigma ) were calculated. The patients' data were compared to those of normal subjects. The relations of the ERG parameters to age. average ferritin level, and duration of transfusion without DFO as well as other clinical parameters were examined. Longitudinal measures of b-wave responses and dark adapted visual thresholds. available for nine of the patients, were examined for significant change over time. For all patients both the gain and saturated amplitude of the rod response are normal. In two patients log sigma is below the 99% prediction interval for normal. One has low scotopic visual sensitivity. The duration of transfusion therapy unprotected by DFO chelation therapy was correlated with log a. These results suggest iron accumulation rather than DFO toxicity underlies scotopic dysfunction in older thalassemics. some of whom may have had extended periods of transfusion without the protection of chelation. Thus, monitoring of retinal function is recommended in such patients.

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.

Do horizontal cells contribute to the rabbit ERG?

An increase in the amplitude of horizontal cell (HC) potentials, a decrease in the extracellular potassium around the photoreceptors and an increase in the PIII component of the ERG that occur with increasing light stimulus intensity are shown to be concomitant occurrences, which could, therefore, be causally related. With high intensity light stimuli the PIII component is prominent and the HC potentials exhibit an afterpotential. Within the range of intensities tested the peak times of the HC afterpotentials coincide with the peak times of PIII. This indicates the likelihood that the HC potential contributes to the PIII, although these results do not allow a quantitative assessment of the contribution of HC potentials to the PIII-component.

Effects of sodium pentobarbital on the components of electroretinogram in the isolated rat retina

Photovoltages, the fast P3(t) component of electroretinogram (ERG), were registered between two microelectrodes across the rod outer segments. The P2(t) component, obtained by subtracting the ERGs measured before the application of 50 microM APB from those measured after the application of 50 microM APB, was used as an indicator of depolarizing bipolar cell activity. Measurements of the scotopic threshold response (STR) and the oscillatory potentials (OPs) were used as indicators of third order neuron activity. The slow P3*(t) component, obtained by subtracting the photovoltages from the transretinal recording in the APB-treated retina was used as an indicator of Müller cell activity. The components of the ERG obtained in normal superfusate medium were compared with those obtained in the presence of 100 microM sodium pentobarbital. We found that sodium pentobarbital slowed the kinetics of the P2(t) component and increased its latency. The fast P3(t) component was not affected by pentobarbital. The slow P3*(t) component was slightly reduced in the presence of pentobarbital. The minor components of the ERG, the STR and the OPs, were strongly suppressed by pentobarbital. These results suggest that in rat retina pentobarbital does not affect photoreceptors, but it does affect bipolar cells and Müller cells, and it suppresses activity of third order neurons.

Electrophysiological properties of a new isolated rat retina preparation

A piece of rat retina was mounted in an open chamber and perfused with a Ringer solution at 37 degrees C. The electroretinogram (ERG) was recorded between an extracellular microelectrode in contact with the rod outer segments and a reference electrode under the retina. The addition of 250-500 microM of glutamate to the media prevented the b-wave from decaying in amplitude with time. Minor components of the ERG, the scotopic threshold response (STR) and oscillatory potentials (OPs), were well maintained with glutamate in the media. Experiments on the spatial properties of the recordings indicated that a small area immediately around the microelectrode contributes most strongly to the response. The similarity of ERGs recorded in vivo from the cornea to the transretinal ERGs from the isolated retina of the same animal indicated that the functional integrity of the isolated retina was well preserved in the media with glutamate.

Two neuronal retinal components of the electroretinogram c-wave

Although the rising phase of the b-wave seems to be generated mainly in the rod bipolar cells and the cone on-bipolar cells, the slow component of the electroretinogram, the c-wave, evidently originates in the Müller cells and the pigment epithelium. The c-wave has three components. One cornea-positive component derives from the pigment epithelium, while a distal cornea-negative component (slow PIII) and a proximal slow component originate in the Müller cells. This third proximal component of the c-wave differs between mammalian species: it is negative in the rat retina, positive in the rabbit and human retina and may be lacking in the cat retina.

Transient fluctuation of serum melatonin rhythm is suppressed centrally by vitamin B12

Vitamin B12 has been reported to improve sleep-wake rhythm disorders. Although the mechanism is still unclear, a change in the sensitivity of the circadian clock system to photic input is thought to be a possible mechanism of the effect. In this study, the effect of the vitamin B12 on the circadian aspect of the electroretinogram (ERG) and serum melatonin level was analyzed in rats. Vitamin B12, alpha-(5,6-dimethylbenzimidazolyl)-co-methyl-cobamide was daily administrated subcutaneously for 8 weeks to adult male Wister rats in the experimental group, and saline was given to the control group. The ERGs were recorded under dark adaptation during the night and day, and under light adaptation (0.1 lux) during the night. Blood was drawn before and after ERG recording. The amplitudes of the a-wave, b-wave, and trough-to-peak of both waves and latencies of ERG were analyzed following various exposures to stimuli of light intensity. These parameters in the group treated with vitamin B12 showed similar characteristics to the control group, and no significant difference was observed between the two groups. The melatonin levels of both groups before the measurement of ERG were similar under each measurement condition. The elevated serum melatonin concentration in the control group under dark adaptation at night was suppressed after the series of 10-msec light stimuli used for measurement of ERG. However, this suppressing effect of light pulses on melatonin level was significantly inhibited in the group treated with vitamin B12. Under light adaptation during the night and under dark adaptation during the day, melatonin levels after the measurement of ERG were not different between the groups. From these results, it is suggested that vitamin B12 if effective in suppressing melatonin rhythm disturbances introduced by transient light stimulation, and it affects the site more central than the retinal level.