Evidence that the cat's electroretinogram is not influenced by impulses passing to the eye along the optic nerve

Retinal cross talk in the mammalian visual system

The existence and functional relevance of efferent optic nerve fibers in mammals have long been debated. While anatomical evidence for cortico-retinal and retino-retinal projections is substantial, physiological evidence is lacking, as efferent fibers are few in number and are severed in studies of excised retinal tissue. Here we show that interocular connections contribute to retinal bioelectrical activity in adult mammals. Full-field flash electroretinograms (ERGs) were recorded from one or both eyes of Brown-Norway rats under dark-adapted (n = 16) and light-adapted (n = 11) conditions. Flashes were confined to each eye by an opaque tube that blocked stray light. Monocular flashes evoked a small (5-15 μV) signal in the nonilluminated eye, which was named "crossed ERG" (xERG). The xERG began under dark-adapted conditions with a positive (xP1) wave that peaked at 70-90 ms and ended with slower negative (xN1) and positive (xP2) waves from 200 to 400 ms. xN1 was absent under light-adapted conditions. Injection of tetrodotoxin in either eye (n = 15) eliminated the xERG. Intraocular pressure elevation of the illuminated eye (n = 6) had the same effect. The treatments also altered the ERG b-wave in both eyes, and the alterations correlated with xERG disappearance. Optic nerve stimulation (n = 3) elicited a biphasic compound action potential in the nonstimulated nerve with 10- to 13-ms latency, implying that the xERG comes from slow-conducting (W type) fibers. Monocular dye application (n = 7) confirmed the presence of retino-retinal ganglion cells in adult rats. We conclude that mammalian eyes communicate directly with each other via a handful of optic nerve fibers. The cross talk alters retinal activity in rats, and perhaps other animals.

The effects of histamine on rat and monkey retinal ganglion cells

Mammalian retinas receive input from the posterior hypothalamus, and the neurotransmitter in this pathway is histamine. To determine whether histamine influences ganglion cells, we analyzed the effects of histamine on their maintained and light-evoked activity in vitro. In monkeys, histamine increased the maintained firing rate in 42% of ganglion cells, decreased it in 38%, and had no effect in 20%. When histamine and the HR3 agonist, methylhistamine, were applied to the same cells in succession, their effects were sometimes different, a finding suggesting that at least one other histamine receptor is present. In addition, the responses of some ganglion cells to full-field light stimuli were decreased by histamine and methylhistamine. In rats, the effects of histamine were somewhat different. Histamine increased the maintained firing rate of 82% of ganglion cells. Methylhistamine and the HR2 agonist, dimaprit, had the same effects as histamine. In some cells, histamine increased the light responses, but in others it decreased them. Histamine had no effect on ganglion cells in either species when synaptic transmission was blocked by low Ca2(+)/high Mg2+ Ames medium. Thus, the major effects of histamine were on the maintained activity of retinal ganglion cells. In both rats and monkeys, 80% or more of the ganglion cells were affected by histamine, and these responses were mediated by at least two of the histamine receptor subtypes.

Retinal projections in the cat: A cholera toxin B subunit study

The B fragment of cholera toxin (CTb) is a highly sensitive anterograde tracer for the labelling of retinal axons. It can reveal dense retinofugal projections to well-known retinorecipient nuclei along with sparse but distinct input to target areas that are not commonly recognized. Following a unilateral injection of CTb into the vitreous chamber of seven adult cats, we localized the toxin immunohistochemically in order to identify direct retinal projections in these animals. Consistent with previous findings, the strongest projections were observed in the superficial layers of the superior colliculus, the dorsal and ventral lateral geniculate nuclei, the pretectal nuclei, the accessory optic nuclei, and the suprachiasmatic nucleus of the hypothalamus. However, we also found labelled terminals in several other brain areas, including the zona incerta, the medial geniculate nucleus, the lateral posterior-pulvinar complex, the lateral habenular nucleus, and the anterior and lateral hypothalamic regions. The morphological characteristics of the retinal axon terminals in most of the identified novel target sites are described.

Multifocal, pattern and full field electroretinograms in cats with unilateral optic nerve section

AIM

To look for a subcomponent of the mFERG generated at the optic nerve head and increasing in latency with distance from it. To compare multifocal electroretinogram (mFERG, mPERG) changes to those in full field ERGs and transient and steady state pattern and focal ERGs (PERGs, FERGs) in cats with total unilateral optic nerve section.

METHOD

We recorded multifocal flash ERGs (mFERGs) at three levels of intensity and multifocal pattern ERGs (mPERGs) within 61 equal areas after total unilateral optic nerve section in five long term (> 18 month) survival cats, as part of a long term serial study of full field flash and pattern ERG changes to many stimuli, in a larger population. Cats were anaesthetised with Ketamine/Xylazine and wore Henkes electrodes with 6mm artificial pupils. Intact retinal circulation was verified by fluorescein angiography and optic nerve section by retinal photography and histology. We compared the mean and mean summed multifocal responses, from the normal and denervated eyes. We also compared the mean interocular difference around the area centralis and as a function of distance from the optic nerve head, across the horizontal meridian for the mFERG to the most intense stimulus. The degree of change was compared to that in other types of ERG, in the larger set of cats.

RESULTS

mFERGs were similar across cats. Response density was flat with no prominence at the area centralis. Average summed mFERGs were similar in the normal and denervated eye. In the interocular differences a component near OP2 was reduced in the first kernel to the most intense stimulus, near OP1 and OP3 in the second kernel and locally, there was a hint of a component near OP2, which varied in latency in a ring around the disk and at the area centralis. Nevertheless, no component could be seen, varying in latency with distance from the optic nerve head, across the horizontal meridian. No mPERG was recordable in these conditions. Full field PERGs and FERGs were very reduced. Full field flash ERG amplitude changes were small (4-20%) and slower to appear than PERG changes. Degree of ERG reduction and correlation with PERG losses was greatest for the mesopic OPs, low for the scotopic tests (STR, ERG and OPs) and near zero for the mesopic ERG. The mFERG and mesopic ERG both lost OPs without overall amplitudes.

CONCLUSIONS

The cat mFERG does not have a component, varying in latency with distance from the optic nerve head. The only change was qualitiatively similar to that in the light adapted ERG. With intense stimuli there were local changes around the time of OP2 near the area centralis which might be explained by local variations in ganglion cell density.

Flash and pattern electroretinogram changes with optic atrophy and glaucoma

We investigated recent reports that, contrary to common belief, glaucoma can affect flash as well as pattern electroretinograms. An extensive flash and pattern electroretinogram test protocol was used in a large sample of glaucoma patients and age-matched controls who were either visually normal or had other optic nerve diseases. All electroretinogram parameters were reduced and delayed in normal people > 55 years of age. The effect did not increase in later decades. In patients aged < or = 55 years, flash electroretinograms showed mild reductions and delays from optic atrophy alone. Glaucomatous ERG changes were larger and increased with disease severity. Pattern electroretinograms and oscillatory potentials were almost equally reduced in optic atrophy and all degrees of glaucoma. Mildly affected patients > 55 years of age had similar electroretinogram change to age-matched normals in most conditions. Advanced glaucoma patients showed similar differences from normal irrespective of age. This suggests that direct diagnostic application of these results to older patients will be difficult, that the ERG changes in glaucoma cannot be attributed simply to optic atrophy and that additional widespread outer retinal damage occurs in glaucoma.

Influence of the visual cortex on responses of retinal ganglion cells in the rat

The objective of the present investigation was to answer the following question: Does the visual cortex affect the neuronal firing of retinal ganglion cells in the rat? To test this hypothesis, the visual cortex was inactivated by a reversible cryoblockade. Action potentials of a ganglion cell were recorded from its axon at the optic tract level prior to, during, and following cortical blockade. The results indicated that indeed the visual cortex influenced the retinal output since its inactivation led to a modification of the firing pattern evoked in response to a flash of light. In most cases the modification was an increase of the bursting pattern of the evoked discharges. By contrast cooling nonvisual areas failed to modify ganglion cells' discharge. A comparison between cortico-geniculate and cortico-retinal feedback loops seems to suggest that the first path is involved mostly with the spatial organization of center-surround receptive fields, whereas the second path is associated with temporal aspects of the retinal responses in the rat.

Visual defects in the uninjured eye of patients with unilateral eye injury

We have examined the electroretinographic responses, the psychophysically determined course of dark adaptation and/or the scotopic and photopic (static) perimetric profile of the uninjured eyes of 11 patients with unilateral intraocular foreign bodies. Most of the patients showed subnormal ERG amplitudes over a range of light intensities, and subnormal light sensitivity in isolated retinal areas. The data suggest that eyes not directly injured by a unilateral traumatic ocular episode may show visual defects.

The possibility of centrifugal projections to the retina in the rat

In rats, glutamic acid decarboxylase activity increased in the proximal portion of the optic nerve after its ligation, whereas the activities of choline acetyltransferase and tyrosine hydroxylase remained constant. Possible centrifugal neurons to the retina are GABAergic.

Complete avulsion of the optic nerve. A clinical, angiographic, and electrodiagnostic study

A case of traumatic complete avulsion of the optic nerve is reported together with fluorescein angiography and electrodiagnostic findings. Despite the disappearance of the central vessels, circulation remained in the branch retinal vessels and angiography demonstrated communication between peripapillary choroidal vessels and the superior temporal artery. The visually-evoked cortical response was abolished. Electroretinography showed a normal a-wave but reduced amplitude b-wave, not supporting the theory of the existence of centrifugal retino-suppressive fibres in the optic nerve of man.

Electroretinal responses in ocular vascular occlusions due to temporal arteritis

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Histological evidence against the view that the cat's optic nerve contains centrifugal fibres

1. Degeneration that can be shown by the Nauta-Gygax technique in the orbital part of the cat's optic nerve does not begin until 10 days after intracranial transection of the nerve, though after enucleation of the eye it is conspicuous in 4 days.2. We were not able to tell, by any silver-staining technique applied to an optic nerve at any interval after an operation, whether at that operation the nerve had been cut peripherally only or both peripherally and centrally.3. From these and subsidiary observations we conclude that either the cat's optic nerve contains no centrifugal fibres detectable by silver staining and light microscopy, or, if there are such fibres, they are much less susceptible to prograde (Wallerian) degeneration and much more susceptible to retrograde degeneration than most of the centripetal fibres. The former is the simpler and, we suggest, the more likely conclusion.

Degeneration of the inner nuclear layer of the retina following lesions of the optic nerve

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