Photopic c-wave in the chicken ERG: sensitivity to sodium azide, epinephrine, sodium iodate, barbiturates, and other general anesthetics

The multifocal electroretinogram (mfERG) in the pig

PURPOSE

To establish a method allowing multifocal electroretinography (mfERG) recording with simultaneous fundus monitoring on anaesthetized pigs. In addition we characterize the peaks of the porcine mfERG trace, and compare the visual streak area with the optic nerve head, a known non-response area. Finally we illustrate the feasibility of the method by performing mfERG after an induced laser burn in the visual streak.

METHODS

Fifteen pigs underwent mfERG recordings at baseline, and after 1 and 6 weeks of observation. One pig was evaluated before and after retinal diode laser treatment in the visual streak.

RESULTS

The porcine mfERG trace appears similar to the human mfERG trace, and can be described by three peaks named N1, P1 and N2. Significantly faster implicit time was found in the visual streak regarding N1 (P < 0.001) than in areas outside the visual streak. Amplitudes of all three peaks were increased in the visual streak (P < 0.005). The laser-treated area was characterized by a response similar to what is found at the location of the optic nerve head.

CONCLUSION

Porcine mfERG is similar in appearance to the human response and can be described by the same three peaks. Significantly higher amplitudes of all three peaks are found in the visual streak when compared to the optic nerve head and inferior retina. We have detected the functional deficit caused by a laser burn at the size of 3 x 3 mm.

Pharmacological study of the chicken's monocular optokinetic nystagmus: involvement of the ON retinal channel evidenced by the glutamatergic separation of ON and OFF pathways

The chicken's monocular optokinetic nystagmus (OKN), recorded by the magnetic search coil technique, displays a directional asymmetry, the temporal-nasal (T-N) stimulation being more efficient than the nasal-temporal (N-T) one to evoke the reflex. The intravitreal administration of APB, a glutamate agonist which selectively blocks the ON retinal channel strongly reduced the eye monocular OKN; it also induced spontaneous eye movements in the T-N direction. The intravitreal injection of PDA another glutamate analog, which reduces the OFF channel, while increasing the activity of the ON channel, induced a large increase in OKN velocity gain, especially for a N-T stimulation at the lowest drum speeds. These results indicate the main involvement of the ON retinal channel in the OKN genesis and the inhibitory effect of the OFF channel upon this oculomotor reflex.

Diurnal variations in the electroretinographic c-wave and retinal melatonin content in rats with inherited retinal dystrophy

The inability of retinal pigment epithelium to phagocytose shed photoreceptor disks is a cause of retinal degeneration in the Royal College of Surgeons rat; retinal pigment epithelial phagocytosis and disk shedding are regulated by the diurnal rhythm of retinal melatonin level. The diurnal rhythms of the electroretinogram (particularly that of the retinal pigment epithelial potential, the electroretinographic c-wave) and retinal melatonin content were thus investigated in Royal College of Surgeons rats from postnatal day 17 to 24, the period preceding retinal degeneration. The amplitudes of both the b- and c-waves of the electroretinogram fell significantly during the peak time of rod disk shedding and rose after the time of expected light off in the control and dystrophic rats. While the b-wave rhythms did not differ between the two strains, diurnal changes in the c-wave were significantly less distinct in the dystrophic rats than in controls. This difference may reflect lack of phagocytosis in dystrophic rats. Furthermore, the ERG c-wave was significantly larger and prolonged, and the retinal melatonin content higher, in dystrophic rats of this age group than in controls. It appears that retinal melatonin metabolism may play an important role in the maintenance of retinal pigment epithelial and photoreceptor function.

EOG and ERG modifications induced in the chicken eye after blockade of catecholamine and 5-hydroxytryptamine biosynthesis

The involvement of catecholaminergic and indoleaminergic systems in the modulation of the standing potential of the eye was tested in chickens by means of an indirect electrooculography method and direct current electroretinogram recordings. D,L alpha-monofluoromethyl dopa (MFMD), 50 and 100 nmol), a highly specific inactivator of aromatic L-amino acid decarboxylase, was injected intravitreally. This treatment is known to induce a selective and irreversible blockade of dopamine and 5-hydroxytryptamine biosynthesis. Five hours after drug injection an important increase in the standing potential appeared. The light peak was delayed by about 5 min but its amplitude was unchanged. The return of the standing potential to basal value during maintained illumination was delayed by 5-10 min. The time course of the dark trough was not modified. The intensity-voltage functions were also studied for the various electroretinogram components, 5 hr after MFMD. The voltage of the b-wave was reduced (by 65% with the highest intensity), while the other components were little affected. Substantial reduction in dopamine and 5-hydroxytryptamine concentrations were found in treated retinas. These data, together with previous results, suggest that the standing potential of the chicken eye may be modulated by a balance between catecholamine and indoleamine systems.

Effects of intravitreal and intravenous administrations of dopamine on the standing potential and the light peak in the intact chicken eye

We studied the modifications of the standing potential (SP) of the eye and of the light peak (LP) after exposure to dopamine, a neurotransmitter released at light by the inner retina and known to affect electrical properties of the retinal pigment epithelium. Intravenous or intravitreal injections of dopamine (DA) were performed on intact chickens. "Choroidal" application (through an intravenous injection) induced a transient increase of the SP and the LP was preserved. On the other hand, "apical" applications of DA (through an intraocular injection) also increased the SP but considerably depressed the LP. These results are in agreement with the hypothesis that the light-induced release of dopamine from the neuroretina may be responsible for the LP generation in the intact chicken eye.

Electrooculographic and electroretinographic study in the chicken after dopamine and haloperidol

The implication of dopamine in the modulation of the standing potential of the eye was tested in the chicken through an indirect electrooculographic method and direct current electroretinogram (ERG) recording after haloperidol, a mixed D1-D2 antagonist. The standing potential of the eye was reduced within 15 min after intravitreal injection of the antagonist (150 micrograms). This effect is rapidly reversed by an application of dopamine. The fast oscillation was preserved but the light peak was either strongly reduced or abolished. The dark trough showed an apparently normal time course. The intensity-voltage function was studied for the various ERG components. After haloperidol the b-wave and the c-wave were strongly reduced, whereas the a-wave was little affected. Together with previous data obtained with intraocular injections of dopamine, our data suggest the involvement of dopamine in the modulation of the standing potential. They also support the hypothesis that the light peak, which is generated by a photoreceptor-pigment epithelium interaction, is influenced by dopamine or by a related substance. The modulatory effect could also be due to a balance between several neurotransmitter systems.

Electrooculographic study in the chicken after treatment with neurotoxin 6-hydroxydopamine

The implication of dopamine in the modulation of the standing potential of the eye was tested in the chicken by an indirect electrooculogram (EOG) method. After a single rapid systemic injection of dopamine, a transient dose-dependent increase in the EOG voltage was observed. EOG recordings during light and dark adaptation were performed after retinal dopamine depletion was induced by intraocular injections of the neurotoxin 6-hydroxydopamine (6-OHDA). The eyes were injected on two successive days with a mixture of 6-OHDA (50 micrograms), pargyline (a monoamine oxidase inhibitor), and ascorbate added as an antioxidant. Following this treatment EOG recordings were performed 1, 4, and 8 days after the second injection. The electrophysiological changes appeared most spectacular on the fourth day: an important increase in the EOG basal values as well as of the amplitude of the light peak and of the dark trough were observed. Substantial reduction in retinal concentration of dopamine was found in treated retinas. These unexpected electrophysiological data offer additional evidence for the involvement of a catecholamine in the generation of the light peak and the dark trough of the EOG.

Benzodiazepines and the mammalian retina. I. Autoradiographic localisation of receptor sites and the lack of effect on the electroretinogram

The majority of specific benzodiazepine binding sites were found to be restricted to the innerplexiform layer of the rat retina, although there were minor amounts of binding in the inner nuclear and ganglion cell layers. Relatively high levels of non-specific benzodiazepine binding sites were, on the other hand, found in the pigment epithelium and ciliary body, as well as in the corneal epithelium. The specific binding was enhanced by the GABA-A agonist, muscimol. In both rats and cats, neither acute nor chronic administration of benzodiazepines or their antagonists altered the retinal functions, as determined by the electroretinogram. These results suggest that retinal benzodiazepine receptors do not influence visually induced preganglionic retinal activity.

Gamma-aminobutyric acid- and picrotoxin-induced changes in c-wave and light peak of retinal potentials in the chick

Retinal potentials were recorded from the eyes of anesthetized and immobilized chicks by a standard direct current method. The amplitude of the electroretinogram (ERG) c-wave was measured 2 and 5 sec after the onset of the light stimulus, as indexes of the fast-rise c-wave (cF-wave) and the slow-rise c-wave (cS-wave), respectively. An intravitreal injection of gamma-aminobutyric acid (GABA) at an estimated intravitreal concentration of 10(-9)-10(-7) M resulted in an increase of the amplitude of the cS-wave, a less remarkable change in the a- and cF-waves, and a slight decrease in the b-wave. The light peak of the retinal standing potential increased in amplitude following GABA administration (10(-7)-10(-4) M). Following an intravitreal injection of picrotoxin (10(-5)-10(-3) M), the polarity of the cS-wave changed from positive to negative and a significant decrease and deformation in the light peak was observed. The amplitude of the a-wave, however, increased in the range of the higher dose, while that of the b- and cF-waves decreased markedly but no polarity reversal of the cF-wave was found. The results may suggest that the GABA-ergic synapse plays a significant role in production of the cS-wave and the light peak, along with that of the pigment epithelium.