Synaptic transmission between photoreceptors and horizontal cells in the turtle retina.

Low calcium, high magnesium, and cobalt hyperpolarize the horizontal cell membrane and suppress the response to light, but only partially affect the response of receptor cells. These observations are consistent with the interpretation that a depolarizing transmitter is released by photoreceptors in darkness. The hyperpolarizing response to light of the horizontal cells would then result from a reduction in the amount of transmitter released.

Inactivation of horizontal cells in turtle retina by glutamate and aspartate.

Glutamate and aspartate completely suppress the activity of horizontal cells but only partially affect the response of receptor cells to light. The changes observed in the receptor responses are consistent with the interruption of a synaptically mediated process rather than with a direct action on the receptor membrane.

Cellular mechanisms underlying the pharmacological induction of phosphenes.

Visual sensations evoked by stimuli other than luminance changes are called phosphenes. Phosphenes may be an early symptom in a variety of diseases of the retina or of the visual pathways, but healthy individuals may perceive them as well. Phosphene-like phenomena are perhaps the most common side effect reported in clinical pharmacology. Ivabradine, a novel anti-anginal drug that reduces heart-rate by inhibiting the hyperpolarization activated current expressed in cardiac sinoatrial node cells (I(f)) induces phosphenes in some patients. One hypothesis is that ivabradine interacts with the visual system by inhibiting hyperpolarization-activated current in retinal cells (Ih). An Ih current with properties similar to cardiac I(f) has been reported in retinal neurones. Under normal circumstances most of the random fluctuations generated within the retinal circuits do not reach the level of conscious perception because they are filtered out. Presumably, filtering occurs mostly within the retina and one serious candidate for this action is the ability of Ih to act as a negative-feedback mechanism. Ih activation in the membrane of visual cells causes dampening of responses to slow noisy inputs thus tuning the visual system to perceptually more relevant signals of higher frequency. Ih inhibition, by altering at the retinal synapses the filtering of signals generated by thermal breakdown of rhodopsin or other fluctuations, is expected to increase the probability of phosphene occurrence. It is the purpose of the present paper to outline and discuss the features of the visual system and the pharmacological conditions relevant to phosphene perception.

Mechanisms of photoreceptor death and survival in mammalian retina.

The mammalian retina, like the rest of the central nervous system, is highly stable and can maintain its structure and function for the full life of the individual, in humans for many decades. Photoreceptor dystrophies are instances of retinal instability. Many are precipitated by genetic mutations and scores of photoreceptor-lethal mutations have now been identified at the codon level. This review explores the factors which make the photoreceptor more vulnerable to small mutations of its proteins than any other cell of the body, and more vulnerable to environmental factors than any other retinal neurone. These factors include the highly specialised structure and function of the photoreceptors, their high appetite for energy, their self-protective mechanisms and the architecture of their energy supply from the choroidal circulation. Particularly important are the properties of the choroidal circulation, especially its fast flow of near-arterial blood and its inability to autoregulate. Mechanisms which make the retina stable and unstable are then reviewed in three different models of retinal degeneration, retinal detachment, photoreceptor dystrophy and light damage. A two stage model of the genesis of photoreceptor dystrophies is proposed, comprising an initial "depletion" stage caused by genetic or environmental insult and a second "late" stage during which oxygen toxicity damages and eventually destroys any photoreceptors which survive the initial depletion. It is a feature of the model that the second "late" stage of retinal dystrophies is driven by oxygen toxicity. The implications of these ideas for therapy of retinal dystrophies are discussed.

N-n-Propyl-substituted 3-(dimethylphenyl)piperidines display novel discriminative properties between dopamine receptor subtypes: synthesis and receptor binding studies.

3-Phenylpiperidines (PPEs) have been thoroughly investigated in view of their interesting dopaminergic activity, and the N-n-propyl substitution has been suggested as the most effective among several PPEs differently substituted on the phenyl ring. In previous studies, we found that the dimethyl substitution on the phenyl ring of N-unsubstituted PPEs provided compounds active toward alpha2-adrenergic receptors (alpha2-ARs), which proved to possess interesting selectivity properties. The high degree of homology between the binding domains of alpha2-ARs and D4-dopaminergic receptors (D4-DARs) prompted us to verify whether this kind of substitution on the aromatic ring might prove to be active against retinal DARs of the D4 subtype. On the basis of these premises, we synthesized the dimethylphenyl-substituted PPEs 4a-f, in which an n-propyl chain is present on the aminic nitrogen. Radioligand binding assays on bovine retina and striatum membranes for D1-like and D2-like DARs indicated that PPEs 4a, 4b, and 4f possess a high affinity and selectivity for the D4-DAR subtype of bovine retina.

Electroretinogram changes associated with retinal upregulation of trophic factors: observations following optic nerve section.

The purpose of the present work was to assess whether upregulation of trophic factors and protection from damage induced in the retina by optic nerve section are associated with changes in the flash electroretinogram (ERG). We have examined the ERG in adult pigmented rat at different survival times over a period of 3 months following section of the optic nerve. The a-wave was analyzed using the Lamb-Pugh model and the parameters of best fit were estimated in control animals and at successive survival times. The amplitudes of the a- and b-waves were reduced over the first 7 days after nerve section. The a-wave recovered its relative amplitude by 21 days, but the b-wave remained depressed 5 weeks following nerve section. Analysis of the a-wave indicated a 20-30% reduction in the dark current of sectioned eyes at 7 days survival. A significant reduction of the amplification constant was observed in both nerve-sectioned and nerve-intact eyes, relative to normal and sham-operated controls. This reduction persisted to the longest survival time examined. The reduction of the a-wave at 7 days after nerve section coincides with a period of upregulation of ciliary nerve trophic factor. The amplification factor is influenced over a longer time course, which corresponds with a period of up-regulation of basic fibroblast growth factor. These changes in growth factor expression and ERG parameters are in turn associated with protection of photoreceptors against light damage. Present results suggest that the sensitivity of the retina to light may be regulated by mechanisms which protect photoreceptors against stress.

The impact of basic fibroblast growth factor on photoreceptor function and morphology.

PURPOSE: To assess the impact of basic fibroblast growth factor (bFGF) on photoreceptor function and morphology. METHODS: Impact was assessed in two models. In one, the endogenous expression of bFGF in photoreceptors was raised by sectioning one optic nerve of rats 3 to 4 weeks before study. In the other, bFGF was injected into the vitreous chamber in rats and cats. Retinal function was assessed from the electroretinogram (ERG), and retinal morphology was studied using DNA dyes, immunolabeling, and in situ hybridization. RESULTS: In both models of bFGF upregulation, the ERG b-wave was suppressed over a wide stimulus range and in light- and dark-adapted conditions. The a-wave was not suppressed by either procedure and at the brightest intensities was enhanced by both procedures. In nerve-sectioned eyes, outer retina appeared normal histologically, but levels of bFGF protein in the inner and outer nuclear layers were raised, whereas bFGF mRNA levels remained unchanged. In both models, levels of synaptophysin in the outer plexiform layer and of cytochrome oxidase in inner segments were raised in association with increases in bFGF protein levels. CONCLUSIONS: bFGF increased the ability of photoreceptors to respond to light but attenuated the transmission of this response to inner retinal cells, presumably by blocking the photoreceptor-bipolar synapse. If the expression of bFGF protein is upregulated in human photoreceptor dystrophies, it may contribute a reversible component to the loss of vision. The relationship between these actions of bFGF and its ability to protect photoreceptors from stress remains to be established.

Analysis of pharmacologically isolated components of the ERG.

An harmonic analysis was applied to the electroretinogram (ERG) measured in intact cat eyes in control conditions and after pharmacological isolation of the components attributed to photoreceptors (PIII) and bipolar neurons (PII). The frequency response curves obtained in various conditions showed that the bandwidth of the PII component extends over a range of stimulus frequencies higher than the bandwidth of PIII. The enhancement of the PII response to stimuli of high temporal frequency suggests the presence of a frequency dependent gain control located either pre- and/or post-synaptically in the transmission line between the phototransductive cascade and bipolar neurons. A possible role of these processes is to enhance relevant visual information whilst selectively attenuating low frequency signals originating in the transductive cascade.

Effects of blocking the hyperpolarization-activated current (Ih) on the cat electroretinogram.

The temporal properties of the electroretinogram (ERG) recorded from cat eyes were analyzed in the presence of either Cs+ or zatebradine which are known to inhibit the hyperpolarization activated current (Ih) in retinal rods. Both Cs+ and zatebradine reduce the ERG response to high-frequency sinusoidal stimuli of high mean luminance and contrast. Conversely, blockade of Ih has no effect on the frequency response characteristics of the isolated receptor component (PIII). These observations support the idea that Ih plays an important role in the transfer of signals from photoreceptors to second order neurons by suppressing the slow components originated in the phototransductive cascade. The result of this operation is an enhancement of the light response in a range of temporal frequencies relevant to vision.

Processing of visual signals in vertebrate photoreceptors.

Photoreceptors of the vertebrate retina hyperpolarize in response to illumination. The conductance changes in the plasma membrane associated with the electrical response are the final step of chain of events initiated by light absorption at the outer segment of the visual cell. The mechanism whereby the free energy of photons in converted into neural information is largely unknown. Present knowledge is consistent with the idea that an internal transmitter is modulated by light and modifies the ionic permeability of the plasma membrane. As to the identity of the internal messenger two candidates have been proposed: Ca2+ and cyclic GMP respectively. Increasing evidence suggests that both substances may be involved in the process of phototransduction. The electrical response of photoreceptors does not simply reflect the light absorbed by the cell: complex interactions occurring between adjacent photoreceptors and between photoreceptors and second order neurons cooperate with the initial process in determining the final shape of the receptor message. Recurrent interactions involve particularly cones: their membrane potential can be modified at least by three distinct mechanisms; i) by light absorption at their outer segment; ii) by light absorption at the outer segment of neighbouring cones, and iii) by potential changes occurring in horizontal cells.

Temporal fidelity in the visual system.

The temporal properties of the visual system have been analyzed by recording the ERG and its isolated components in response to light stimuli whose luminance was varied sinusoidally. The performance of the visual system to periodic light stimuli was tested in human subjects psychophysically. The comparison of the results in control conditions and after administration of drugs that specifically block the hyperpolarization activated current (Ih) suggest that the inner rectifying properties of the inner segment membrane of rods is involved in a process of temporal differentiation of the visual signals whereby high frequency components of the response especially relevant for the visual performance are enhanced. It is proposed that the temporal fidelity of the visual system is the results of an elaboration starting at early level of the signal generated by the phototransductive cascade.

The energetic cost of photoreception in retinal rods of mammals.

The effects of temperature on rod sensitivity and adaptation are analysed in the general context of the energy requirements of photoreception. The dependence of adaptation on the [Na]i turn-over appears to be critical in mammalian rods where the metabolic load is particularly heavy because of both temperature conditions and large Na+ influx. Estimates of the energy dissipated by rods in darkness and during bright illumination show that the metabolic load is reasonably well distributed. From this analysis it also results that most of the energy, which a rod dissipates in both darkness and light, is needed to keep [Na]i and [Ca]i low.

The impact of organic inhibitors of the hyperpolarization activated current (Ih) on the electroretinogram (ERG) of rodents.

We have compared the effect of two distinct Ih inhibitors on the temporal properties of the ERG response that, as previously shown, correlates well with the HCN activation in rods. The present results confirm the notion that cilobradine is more effective than zatebradine in inducing bradycardia. Importantly, the doses of cilobradine that reduce the heart rate to values comparable to, or lower than, those obtained with higher doses of zatebradine have little effect on the frequency response of the ERG. While more potent than zatebradine in its bradycardic action, cilobradine appears comparatively less effective on the visual response. A possible explanation is that the affinity of cilobradine for the HCN channels in the heart is higher than that for the HCN channels of retinal neurons.

Effects of applied currents on turtle cones in darkness and during the photoresponse.

1. The voltage changes, produced by pulses of current of various intensities and polarities into turtle cones, were examined both in darkness and during the photoresponse. 2. Depolarizing pulses of current produced voltage deflexions which, during the photoresponse, were increased to a greater extent than those produced by pulses of current of the same intensity and opposite polarity. 3. Voltage-current relationships were measured both in darkness and during illumination. During illumination they display a non-linear behaviour, the curve being steeper for positive than for negative currents. 4. These non-linearities are greatly attenuated following exposure of the retina to a solution containing 5 mM-CoCl2 which is known to block synaptic transmission. The effects of CoCl2 are readily reversible when the retina is returned to normal conditions. 5. By comparing the effects produced by pulses of current injected into individual cones with those observed in horizontal cells following stimulation with a radial current, it is observed that the non-linear behaviour of the cone membrane during the photoresponse can be, at least in part, accounted for by events occurring in post-synaptic elements (i.e. horizontal cells).