Dark-adapted Retina Research Articles

Colour dependence of the early receptor potential and late receptor potential in scallop distal photoreceptor.

1. Intracellular voltage and current responses to short (blue) and long (red) wave-length lights were measured in the distal hyperpolarizing photoreceptor (;off receptor') of the isolated and perfused scallop (Pecten irradians) retina.2. The early receptor potential (e.r.p.) was isolated by holding membrane potential at the reversal potential for the late receptor potential (l.r.p.) or by working at temperatures (< 5.0 degrees C) that abolished the l.r.p.3. The e.r.p., measured using intense flashes of white light, consisted of a positive phase followed by a negative phase, but was converted to a monophasic, negative-going wave following pre-adaptation with red light and to a monophasic, positive-going wave following pre-adaptation with blue light.4. The spectral sensitivity curve for the negative e.r.p. was maximum at 500 nm, whereas the spectral sensitivity curve for the positive e.r.p. was maximum at 575 nm.5. The positive or negative e.r.p.s approached their maximum amplitude exponentially when tested with red or blue flashes of increasing intensity. The results suggest that the positive (or negative) e.r.p. is proportional to the number of photopigment molecules photo-isomerized.6. The photosensitivity maximum of rhodopsin calculated at 500 nm, using the exponential constant and the spectral sensitivity data, was estimated to be 2.1 x 10(-16) cm(2) photon(-1), whereas the photosensitivity maximum of metarhodopsin calculated at 575 nm was estimated to be 2.6 x 10(-16) cm(2) photon(-1).7. In cells pre-adapted with white light, stimulation with blue light caused a hyperpolarizing l.r.p. which was followed by a prolonged hyperpolarizing after-potential (p.h.a.). Stimulation with red light under similar conditions caused an initial hyperpolarization which was followed by a small depolarization during the stimulus, but no after-potential.8. The duration of the p.h.a. was increased by pre-adaptation with a red light, which caused the maximum net transfer of metarhodopsin to rhodopsin; however, its decay was always complete in 5 min or less.9. The photo-isomerization of metarhodopsin by red light suppressed the p.h.a. and caused an after-depolarizing response that decayed in less than 1 min.10. The spectral sensitivity curve for the induction of the p.h.a. was maximum at 500 nm and corresponded to the spectral sensitivity for the negative e.r.p. and for the l.r.p. studied in the dark-adapted retina, whereas the spectral sensitivity curve for the suppression of the p.h.a. and for the induction of the after-depolarization was maximum at 575 nm and corresponded to the spectral sensitivity for the positive e.r.p.11. In photoreceptors clamped to the resting potential in normal ASW, the photo-isomerization of rhodopsin, in the absence of light absorption by metarhodopsin, activated a persistent outward current that had the same time course of decay as the p.h.a. The photo-isomerization of metarhodopsin suppressed the persistent outward current and activated an inward current whose decay took longer than the decay of the after-depolarizing response.12. In the absence of external Ca(2+) and Na(+) ions, the persistent outward current produced by light absorption by rhodopsin, and the inward current produced by light absorption by metarhodopsin, both reversed at the K(+) equilibrium potential. The results show that the induction of the prolonged hyperpolarizing after-potential and the after-depolarizing response involve only the movement of K(+) ions through the same light-dependent K(+) channels that determine the hyperpolarizing l.r.p. of the distal cells.

Dopaminergic receptors in bovine retina and their interaction with thyrotropin-releasing hormone.

A superfusion technique was employed to study the release of [3H]dopamine from isolated bovine retina. Only K+-stimulated release was observed from both light- and dark-adapted retina; release by other stimuli was from dark-adapted retina only. Light-evoked release of [3H]dopamine from dark-adapted retina was blocked by thyrotropin-releasing hormone (TRH), which has previously been identified as a retinal neuropeptide. TRH itself released small amounts of [3H]dopamine from dark-adapted retina. These results are interpreted as indicating that TRH acts as a modulator of dopaminergic activity in retina through the agency of presynaptic autoreceptors. Evidence of the existence of a feedback inhibition system, probably mediated by dopaminergic autoreceptors, was found by the inclusion of sulpiride, a dopaminergic D2 receptor antagonist in the perfusate, which, in a stereoselective manner, enhanced spontaneous and light-evoked release of [3H]dopamine. On the other hand, dopamine (1 microM) reduced these effects. TRH did not affect the high-affinity uptake system for dopamine in retina; this, then, could not account for the effects on release. Radioligand binding showed a specific, saturable high-affinity binding system for [3H]TRH, with an apparent KD of 2.2 nM and a Bmax of 23 fmol/mg protein in bovine retinal membranes. Displacement experiments showed that specific [3H]TRH binding was displaced in the nanomolar range by spiperone and in the micromolar range by dopamine, whereas L-(--)-sulpiride was virtually inactive in displacing [3H]TRH.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of precursor availability in control of monoamine biosynthesis in brain.

Role of precursor availability in control of monoamine biosynthesis in brain.

N-ethylmaleimide-modified subfragment-1 and heavy meromyosin inhibit reactivated contraction in motile models of retinal cones.

The mechanism of contraction in motile models of teleost retinal cones has been examined by using N-ethylmaleimide (NEM)-modified myosin fragments (NEM-S-1 and NEM-heavy meromyosin [HMM]) to prevent access of native myosin to actin filaments during reactivation of contraction. In the diurnal light/dark cycle, retinal cones of green sunfish (Lepomis cyanellus) and bluegill (lepomis macrochirus) exhibit length changes of more than 90 mum. The motile myoid region of the cone contracts from 100 mum in the dark to 6 mum in the light. Motile models for cone contraction have been obtained by lysis of dark-adapted retinas with the non-ionic detergent, Brij-58. These cone motile models undergo Ca(++)-and ATP-dependent reactivated contraction, with morphology and rate comparable to those observed in vivo (Burnside, B.,B. Smith, M. Nagata, and K. Porrello, 1982, J. Cell Biol., 92:198-206). The cone myoids contain longitudinally oriented actin filaments which bind myosin subfragment-1 (S-1) to form characteristic "arrowhead" complexes which dissociate in the presence of MgATP (Burnside, B., 1978, J. Cell Biol., 78:227-246). Modification of S-1 or HMM with the sulfhydryl reagent, NEM, produces new species, NEM-S-1 or NEM-HMM, which still bind actin but which fail to detach in the presence of MgATP (Meeusen, R.L., and W.Z. Cande, 1979, J. Cell Biol., 82:57-65). We have used NEM-S-1 and NEM-HMM to test whether cone contraction depends on an actomyosin force- generating system. We find that reactivated contraction of cone models is inhibited by NEM-S-1 and NEM-HMM but not by the unmodified species, S-1 and HMM. Thus, reactivated cone contraction exhibits NEM-S-1 and NEM-HMM sensitivity as well as Ca(++)- and ATP- dependence. These observations are consistent with and actimyosin-mediated mechanism for force production during cone contraction.

Taurine biosynthesis in frog retina: effects of light and dark adaptations.

The retinal uptake and metabolism of cysteine, a precursor for taurine biosynthesis, were analysed using the bull frog. The [14C] cysteine uptake into isolated retina had some specific properties: It was rather temperature independent, required Na ions, was inhibited by ouabain but not by dinitrophenol, and exhibited saturation kinetics composed of two components. When retinal homogenate was incubated with 12-30 microM of L-[U-14C]cysteine, the accumulation of labeled alanine, cysteine sulfinic acid (CSA), cysteic acid (CA), hypotaurine, and taurine was detected. The metabolic conversions of [14C] cysteine to labeled alanine, hypotaurine, and taurine were linear over 90 minutes. Prolonged light adaptation (3 weeks) induced a significant reduction in the formation of labeled CA, CSA, hypotaurine, and taurine from [14C] cysteine. On the other hand, it was found that in dark-adapted retinae, the formation of labeled taurine from [14C] cysteine increased significantly in spite of the reduction in the formation of labeled CA. These results indicate that biosynthetic pathways exist for taurine from cysteine in frog retina, and that these metabolic pathways are involved in the regulation of retinal taurine content under continuous visual adaptation.

Meridional variations of the scotopic sensitivity in the human retina.

The absolute threshold of the dark-adapted retina for a 1 degree circular white light stimulus located at 15 degrees eccentric to the fovea was measured on four meridians. In each of the 11 tested subjects the sensitivity was found to be higher on the 90 degrees suprafoveal vertical meridian, on average by 0.3 log nLa compared to the subfoveal vertical meridian. On the two oblique 45 degrees and 135 degrees meridians of the upper retina the sensitivity was only very slightly lower compared to the 90 degrees meridian.

Light-dependent ion influx into toad photoreceptors.

To measure the influx of Na+ and other ions through the light-dependent permeability of photoreceptors, we superfused the isolated retina of the toad, Bufo marinus, with a low-Ca2+ (10(-8) M), low-Cl- Ringer's solution containing 0.5 mM ouabain. Under these conditions, the membrane potential of the rod is near zero and there is no light-induced potential change either in the rod or in more proximal neurons. The photoreceptors, however, continue to show a light-dependent increase in membrane resistance, which indicates that the light-sensitive channels still close with illumination. Dark-adapted retinas show a larger 22Na+ accumulation than do light-adapted retinas. The extra accumulation of 22Na+ into dark-adapted retinas can be removed if the retinas are washed in darkness with low-Ca2+ Ringer's solutions, or if the ionophore gramicidin D is present in the perfusate. The additional accumulation in dark retinas corresponds to a flux of at least 10(9) Na+ per receptor per second, which is close to the value of the photoreceptor dark current. The light-dependent uptake of 22Na+ can be prevented by exposing the retinas to Ca2+ during the incubation period, but is restored if the phosphodiesterase inhibitor IBMX is added to the perfusate. A significant light-dependent ion accumulation can be observed for the cations K+, Rb+, Cs+, and Tl+, in addition to Na+, but not for methylamine, choline, or tetraethylammonium.

Postnatal development of the light response of the dopaminergic neurons in the rat retina.

The effect of light on retinal dopamine (DA) synthesis and content in dark-adapted rats was assessed 15 h and 2, 4, 7 and 16 days after eye opening (13 to 14 days after birth). The accumulation of dihydroxyphenylalanine (DOPA) following inhibition of its decarboxylation was used to estimate DA synthesis. At 7 and 16 days, but not earlier, light significantly augmented DOPA formation. These increases were as dramatic as those reported for adult rats. DA in dark-adapted retinas ranged from 0 (undetectable) at 15 h to 83% of adult levels at 16 days, but were only 36% of that of adult retinas at 7 days. Light produced a significant decline in DA at 16 days but not at any other time point. These results indicate that the dopaminergic neurons synthesize transmitter and respond to light at a time when the neuronal pools of DA are not yet mature.

Light-dependent effects of a hydrolysis-resistant analog of GTP on rod photoresponses in the toad retina.

Responses to 100-ms flashes were recorded intracellularly from dark- and light-adapted rod photoreceptors in the isolated retina of the toad, Bufo marinus. Properties of photoresponses were analyzed under each condition of adaptation when retinas were superfused with 1.0 mM guanosine 5'-[beta, gamma-methylene]triphosphate (p[CH2]ppG), a hydrolysis-resistant analog od GTP. When applied to retinas that previously had been subjected to intense light (approximately 30% bleach), p[CH2]ppG increased both the amplitude and duration of photoresponses. By contrast, treatment of dark-adapted retinas with p[CH2]ppG did not alter these response parameters. When similarly applied to either dark- or light-adapted retinas, GTP had no effects on amplitude or duration of photoresponses. These results are discussed in terms of GTP-dependent mechanisms for rod adaptation.

Reactivation of contraction in detergent-lysed teleost retinal cones.

Teleost retinal cones contract in the light and elongate in the dark. In the green sunfish, Lepomis cyanellus, the necklike myoid region of the cone contracts from as much as 120 micrometers (midnight dark-adapted) to 6 micrometers in fully light-adapted state. When dark-adapted fish are exposed to light (1.4 lux), cone myoids contract with a linear rate of 1.5 +/- 0.1 micrometers/min. We report here that detergent-lysed motile models of teleost retinal cones exhibit calcium- and ATP-dependent reactivated contraction, with morphology and rate comparable to that observed in vivo. For reactivation studies isolated dark-adapted retinas were lysed with nonionic detergent Brij-58 (0.1-1.0%). In reactivation medium containing 10(-5) M free calcium and 4 mM ATP, the lysed cones contracted with normal morphology at in vivo rates (1.4 +/- 1 micrometer/min). Little contraction was observed if ATP or detergent was deleted from the medium or if free calcium levels were less than 10(-8) M. Ultrastructural examination of cone models lysed with 1% Brij-58 revealed that, in spite of extensive extraction of the cytoplasmic matrix, cytoskeletal components (thin filaments, intermediate filaments, microtubules) were still present. Thus we have produced extensively extracted motile models of teleost retinal cones which undergo calcium- and ATP-dependent reactivated contraction with normal morphology at physiological rate.

Spatial and temporal summation in the human dark-adapted retina.

A coherent set of absolute-threshold data is presented for circular flashes with a diameter of 5-343 min of arc, a flash duration of 32-1000 msec, and at eccentricities between 7 and 50 deg in the temporal retina. A reduction in the flash interval from 4 to 1 sec causes a threshold elevation for eccentricities exceeding 15 deg for all other stimulus parameters. It is shown that local adaptation affects the measurements significantly, especially when long-lasting stimuli and large eccentricities exist. The results can be described with the help of a quanta-coincidence model if adaptational properties are included.

Effect of isobutylmethylxanthine and related drugs on the receptor response (ERG; a-wave) of the frog retina at various extracellular calcium concentrations.

Various drugs known or expected to increase the levels of cyclic nucleotides in cells were applied to isolated superfused frog retinae, and their influence on the aspartate-isolated a-wave was studied. Isobutylmethylxanthine (IBMX), triacetylguanosine (TAG), and dimethylaminopurine (DAMP) strongly influenced the responses elicited from dark-adapted retinae by flashes of light: With all three drugs the response amplitude was increased, and latency and time to peak were prolonged. If, on the other hand, the retinae were light-adapted by background light of various intensities, the drugs showed different effects on the response amplitude: IBMX either did not influence the amplitude at all or even caused a decrease (4 of 6 experiments), DAMP decreased the amplitude and TAG caused an increase of the amplitude in 2 of 3 experiments. But latency and time to peak were still prolonged by all three drugs. When dark-adapted retinae were superfused with IBMX or TAG Ringer solution and simultaneously calcium concentration was raised, different effects of calcium on the three measured parameters of the a-wave were observed: By increasing the extracellular calcium concentration the increase of the amplitude caused by drugs was reversed, down or even below the control level, whereas latency and time to peak remained prolonged. Thus, both an increased calcium level and light adaptation had the same effect, namely to reverse only that part of the drug effect concerning the amplitude but not latency or time to peak of the response. The data suggest that calcium and cyclic nucleotides act through different ways in the rod cells.

The role of GABA in the regulation of the dopamine/tyrosine hydroxylase-containing neurons of the rat retina

Dopamine (DA) and gamma-aminobutyric acid (GABA) are putative neurotransmitters in two separate populations of amacrine neurons in the mammalian retina. Pharmacological studies have been conducted to determine if GABA neurons regulate the neuronal activity of the neurons that secrete DA. Tyrosine hydroxylase (TH) activity, a biochemical indicator of changes in activity of DA/TH-containing neurons, was low in dark-adapted retinas and high in light-exposed retinas. Muscimol (a GABA receptor agonist) produced a dose-related, biphasic effect on the light-evoked activation of TH, when the drug was injected into the vitreous (intravitreal injection) of dark-adapted rats. At low doses, (35 and 60 pmol) muscimol enhanced the light-evoked activation of TH, but at higher doses (≥ 120 pmol) it inhibited the light-evoked increase in enzyme activity. Muscimol had no significant effect on the TH activity of dark-adapted retinas. GABA antagonists, bicuculline and picrotoxin, produced effects on TH activity that were dependent on both dose and light-exposure. At low doses (0.4–0.5 nmol), bicuculline and picrotoxin both inhibited the light-evoked activation of TH, but had no effect on TH activity of the dark-adapted retinas. At higher dose (2.0 nmol), both antagonists increased TH activity in the dark-adapted retina and attenuated the further activation of the enzyme by light. Rat retinas were dissociated into suspensions of viable cells in order to investigate the direct effects of muscimol and picrotoxin on the DA/TH-containing cells. The process of dissociating dark-adapted retinas resulted in an apparent activation of TH. Incubation of the cells with muscimol resulted in a decrease of TH activity in a concentration-dependent manner. Picrotoxin antagonized the inhibitory effect of muscimol, but had no effect when incubated alone. The biphasic effects of GABA agonists and antagonists in vivo suggest that a certain subpopulation of GABA neurons are involved in the activation of the DA/TH-containing neurons by photic stimulation, while another subpopulation of GABA neurons produce a tonic inhibition of the DA/TH-containing neurons in darkness. The experiments with retinal cell suspensions indicate that the tonic inhibition is probably mediated by synapses of GABA neurons directly onto the DA/TH-containing cells.

Glycolytic and oxidative metabolism in relation to retinal function.

Measurements of lactate production and ATP concentration in superfused rat retinas were compared with extracellular photoreceptor potentials (Fast PIII). The effect of glucose concentration, oxygen tension, metabolic inhibition, and light were studied. Optimal conditions were achieved with 5-20 mM glucose and oxygen. The isolated retina had a high rate of lactate production and maintained the ATP content of a freshly excised retina, and Fast PIII potentials were similar to in vivo recordings. Small (less than 10%) decreases in aerobic and anaerobic lactate production were observed after illumination of dark-adapted retinas. There were no significant differences in ATP content in dark- and light-adapted retinas. In glucose-free medium, lactate production ceased, and the amplitude of Fast PIII and the level of ATP declined, but the rates of decline were slower in oxygen than in nitrogen. ATP levels were reduced and the amplitude of Fast PIII decreased when respiration was inhibited, and these changes were dependent on glucose concentration. Neither glycolysis alone nor Krebs cycle activity alone maintained the superfused rat retina at an optimal level. Retinal lactate production and utilization of ATP were inhibited by ouabain. Mannose but not galactose or fructose produced lactate and maintained ATP content and Fast PIII. Iodoacetate blocked lactate production and Fast PIII and depleted the retina of ATP. Pyruvate, lactate, and glutamine maintained ATP content and Fast PIII reasonably well (greater than 50%) in the absence of glucose, even in the presence of iodoacetate. addition of glucose, mannose, or 2-deoxyglucose to medium containing pyruvate and iodoacetate abolished Fast PIII and depleted the retina of its ATP. It is suggested that the deleterious effects of these three sugars depend upon their cellular uptake and phosphorylation during the blockade of glycolysis by iodoacetate.

Light-induced changes in energy metabolites, guanine nucleotides, and guanylate cyclase within frog retinal layers.

Freeze-dried sections were prepared from retinas of frogs which were dark-adapted or exposed to varying periods of light. Samples of the discrete layers were dissected, weighed, and analyzed for energy metabolites, guanylate compounds, and the enzyme guanylate cyclase. ATP and P-creatine were measured in both dark- and light-adapted retinas. There was a gradient in ATP and P-creatine levels in dark-adapted retinas, with the lower concentrations in the photoreceptors, and increasing concentrations in the inner retina. After light adaptation, concentrations increased, an observation which supports the concept that transmitter release occurs in the dark and ceases in the light. The sum of GTP plus GDP, GDP, and cyclic GMP were analyzed in dark-adapted retinas and after exposure to 2 min or 2 h of room light. GDP was rather uniformly distributed in the retinal layers, was increased by 2 min of light in all layers but the outer nuclear, and remained elevated at 2 h in the inner retina. GTP values showed a marked localization in the outer nuclear layer, which increased after 2 min or 2 h of illumination; in all other layers GTP was decreased by light. Cyclic GMP in the dark was highest in the photoreceptor cells, decreasing to one-third after 2 min of light; there were significant increases in the outer plexiform and inner nuclear layers at this time. Cyclic GMP remained low in the photoreceptor cells even after 2 h of light, while the inner layers returned to dark values. Guanylate cyclase, like cyclic GMP, was largely confined to the photoreceptor cells and showed a maximal increase after 2 min of light exposure.

Certain aspects of acetylcholine metabolism in teleost retina.

Acetylcholine (ACh) synthesis and release from isolated superfused retina of the teleost Eugerres plumieri has been studied under different physiological conditions. The retinas were superfused with Krebs-Ringer solutions containing [14C]choline and the extracellular space of 32% was determined by [3H]inulin. The retina accumulates choline (Ch) from the superfusion medium and this process is mediated by a high affinity transport system with a Km of 1.82 microM. The incorporated Ch is mainly utilized for the synthesis of ACh. The ACh content of the light-adapted retina is not significantly different from that of a dark-adapted one. However, the release of [14C]ACh from the light-adapted retina was 52% higher as compared to the release from the dark-adapted retina. Flicker stimulation induced a larger increase in ACh release, than from either light or dark adapted retina, proportional to flicker frequency. The results suggest that changes in ACh utilization were related to the function of cellular units responsible for light changes transduction rather than light detection.

Dopaminergic amacrine neurons of rat retinas with photoreceptor degeneration continue to respond to light

Exposure of albino rats to continuous light of low intensity (350–700 lux) for 4 months produces massive degeneration of the photoreceptor segments and cell bodies of the outer nuclear layer of the retina. Only a few heterochromatic, receptor cell nuclei remain, and no photoreceptor segments are present. On the other hand, the inner layers of these retinas remain morphologically intact. The inner nuclear layer of the normal rat retina contains a group of amacrine cells which contain the putative neurotransmitter, dopamine (DA). Short term exposure to light (30 or 60 min) markedly stimulates the rate of DA turnover in these cells in normal, previously dark-adapted rats. Such enhancement of the rate of neurotransmitter turnover in the brain has been correlated with an increase in nerve impulse activity. The present study was undertaken to determine if the dopaminergic amacrine cells of the inner nuclear layer were still responsive to light in the retinas of rats whose photoreceptors were previously destroyed by long term exposure to continuous illumination. One week before sacrifice, the animals which had been housed in continuous light for 4 months were returned to normal 14 hr light: 10 hr dark lighting conditions. At the end of this time they and a group of control rats which had been housed in cyclic lighting conditions for the entire 4 months were dark adapted for approximately 15 hr. Then the rate of retinal DA turnover was estimated from the depletion of DA following inhibition of DA synthesis by α methyl para-tyrosine. The turnover of DA in the dark-adapted retinas of the control rats and of experimental rats with photoreceptor degeneration was dramatically enhanced 2–4 fold by short term exposure (up to 1 hr) to light. Since rats are nocturnal and avoid light, we tested the light aversion of another group of rats which had been exposed to light for 4 months and then returned to cyclic lighting conditions for one week. These rats and control animals which had been maintained in cyclic lighting conditions for 4 months both chose the dark side of a light-dark box over 80% of the time. This behavior of the rats with retinal degeneration was taken as a crude indication of their continued ability to detect light. The light-induced increase in DA activity in retinas with photoreceptor degeneration may play a role in the continued ability of these rats to perceive light.

The single-photon signal in rod bipolar cells of the dogfish retina.

1. Responses of depolarizing bipolar cells to dim light flashes were recorded with intracellular micro-electrodes in the dark-adapted retina of the dogfish, Scyliorhinus canicula. 2. Fluctuations in the responses were analysed by a method of matched filtering in order to improve the signal-to-noise ratio. 3. Both the mean and variance of the response amplitude increased linearly with light intensity for intensities not exceeding a mean of 1 photon absorbed per 50 rods. 4. On the assumption that the most significant source of the fluctuation is the quantal absorption of light by the rod outer segments, a single photoisomerization leads to a post-synaptic event of mean size 250 micronV. 5. The mean number of rods in the pool sending signals to a bipolar cell is estimated as 1600. Individual rod pools are 90-330 micrometer in diameter on the retinal surface. 6. It is estimated that the conductance of 1 divided by 400 of the total number of light-modulated ionic channels in the bipolar cell is increased by a single photon acting within its rod pool. 7. In the absence of a light stimulus, the residual noise in the output of a matched filter can be interpreted as due mainly to spontaneous isomerization of rhodopsin in the rods and behaves as the 'dark light' postulated to limit detection at absolute threshold.

Perceptive field size in fovea and periphery of the light- and dark-adapted retina

The sizes of perceptive fields between 0 and 70° eccentricity were measured in the light- and dark-adapted human retina using the Westheimer paradigm. The results indicate that at photopic background luminances, centre size increases from a foveal diameter of less than 9′ to 1° at 10° eccentricity, then more slowly to 1.75° at 70° eccentricity. Total perceptive field size, on the other hand, increases more rapidly from 1° in the fovea to about 6.5° at 70° eccentricity. With dark-adaptation, the perceptive field surround becomes almost ineffective and centre size expands at all eccentricities by about a quarter of its photopic size. These results are compared to previous estimates of perceptive field sizes obtained with other methods and their relationship to the retino-cortical mignification factor are discussed.

Ultrastructural localization and X-ray analysis of calcium-induced electron-dense deposits in the skate retina

The technique of Oschman & Wall (1972) was used to visualize the sites at which electron-dense deposits accumulate in neural layers of the skate retina. The method, which involves the addition of calcium to the primary fixative, resulted in the formation of electron-dense deposits along the plasma membranes of horizontal cells, within the outer segments, mitochondria and synaptic vesicles of the visual cells, and at pre- and postsynaptic membranes of the synaptic elements of the outer plexiform layer. Energy dispersive X-ray spectrometry of the electron-dense deposits yielded prominent peaks for calcium and phosphorus. In the dark-adapted retina, the electron-dense deposits were seen more frequently in association with the receptor terminal than with the processes of second-order neurons; after light adaptation, the reverse situation was observed. A light-induced translocation of electron-dense deposits was not found at any other retinal location. The addition of cobalt to the fixative in lieu of calcium produced results equivalent to those obtained with calcium; i.e. electron-dense deposits were formed at simular loci within the retina. The emergence in the X-ray spectrum of a small calcium signal in the presence of a large cobalt peak suggests that the two ions compete for similar membrane sites. On the other hand, the formation of electron-dense deposits was suppressed completely by exposing the retinas to Ca 2+-free solutions, with or without the addition of Mg 2+. These findings are consistent with physiological observations that suggest that Mg 2+ and Co 2+ act at different sites in modifying the actions of Ca 2+ at synapses.