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The amino acids L-glutamate and L-aspartate depolarize H1 horizontal cells in the perfused goldfish retina but only at millimolar concentrations. The effects of L-glutamate (but not of L-aspartate) are potentiated approximately 15-fold by exposure to D-aspartate. D-Aspartate blocks acidic amino acid uptake in goldfish retina, so that the potentiation of L-glutamate may be produced by an increase in its effective concentration at the horizontal cell membrane. Because D-aspartate also augments the light responses of horizontal cells, our results are consistent with the possibility that L-glutamate is a neurotransmitter of cone photoreceptors in goldfish.

1. Double-barrel micropipettes were used to pass pulses of current in darkness into single rods in the isolated, perfused retina of the toad, Bufo marinus. 2. In normal Ringer solution, current pulses evoked non-linear changes in membrane potential which varied as a function of current amplitude and of time. Responses to currents of both polarities showed slow relaxations toward the base line during the pulse, and the steady-state I-V curve exhibited a prominent outward rectification. 3. In Ringer containing 12 mM-TEA, the slow relaxation of voltage during outward current pulses was diminished, and the outward rectification was markedly reduced. In contrast Co2+, at a concentration in excess of that required to block Ca2+ spikes in rods, increased the receptor input resistance but did not reduce either the amplitude of the slow relaxation or the extent of outward rectification. 4. These experiments indicated that the outward rectification of rods is predominantly due to a conductance which is gated by voltage rather than by entry of Ca2+. 5. Long-lasting after-potentials followed the termination of outward current pulses. In normal Ringer the after-potentials were hyperpolarizing and were accompanied by an increase in input conductance. In TEA, the afterpotentials were depolarizing and were also accompanied by an increase in input conductance. The after-depolarizations in TEA were enhanced by Sr2+ and blocked by Co2+. These experiments suggest that the hyperpolarizing and depolarizing afterpotentials are produced by different mechanisms, the hyperpolarizing by an increase in K+ conductance, and the depolarizing by an increase in Ca2+ conductance.

THE response of vertebrate photoreceptors is thought to be generated by a light-dependent Na+ conductance which is relatively high in darkness and decreased by illumination, so that light hyperpolarises the membrane potential towards the potassium equilibrium potential (Eκ) (refs 1–3). However, this simple model does not explain many of the complexities of the receptor response. We have carried out experiments in which the retina was superfused with Ringer solution containing caesium chloride and we report here that many of the complexities of the receptor response can be removed reversibly by this treatment. We believe that this is because Cs+ blocks a conductance increase which is activated by membrane hyperpolarisation, and which is largely responsible for the rapid decay of the receptor potential in bright light.

Yoshikami and Hagins first suggested that calcium is sequestered within membranous disks in the outer segments of vertebrate rods and that the bleaching of visual pigment molecules by light causes the release of Ca from the disks. Once released, the Ca was postulated to bind to Na+ channels or carrier molecules in the plasma membrane to produce the electrical response. This theory, termed the 'calcium hypothesis', is supported by much evidence but remains controversial, largely because of the difficulty in measuring calcium in rods and of demonstrating light-induced release. Here we describe direct measurements of total rod Ca using a new microprobe method, called laser micro-mass analysis, or LAMMA . Using this technique, we show that rods contain large amounts of Ca concentrated in their outer segments. Physiological levels of illumination produce a graded efflux of rod Ca content, amounting to about 10(4) ions per rhodopsin molecule bleached in dim light. As light does not change the rate of Ca influx, the total Ca content of the rod decreases. In bright light, as much as half the total Ca leaves the rod during only 1 min of illumination.

The vertebrate visual system can operate over a large range of light intensities. This is possible in part because the sensitivity of photoreceptors decreases approximately in inverse proportion to the background light intensity. This process, called photoreceptor light adaptation, is known to be mediated by a diffusible intracellular messenger, but the identity of the messenger is still unclear. There has been considerable speculation that decreased cytoplasmic Ca2+ concentration (Cai2+) may play a role in light adaptation, and recent experiments in which Ca2+ buffer was incorporated into rod-cells have supported this notion. The extent of the contribution of calcium, however, remains unresolved. We now show that light-dependent changes in sensitivity in amphibian photoreceptors can be abolished by preventing movements of Ca2+ across the outer-segment plasma membrane. These experiments demonstrate that light adaptation in photoreceptors is mediated in cones primarily, and in rods perhaps exclusively, by changes in Cai2+.

1. We have examined the effects of decreases in extracellular Ca(2+) concentration on the intracellularly recorded light responses of rods from the toad, Bufo marinus. In agreement with previous results (Brown & Pinto, 1974; Lipton, Ostroy & Dowling, 1977), Ca(2+) concentrations below 10(-6) M produced a depolarization of rod resting membrane potential of approximately 30-40 mV and a corresponding increase in the maximum amplitude of the rod's light responses, so that saturating flashes in normal and low Ca(2+) Ringer produced hyperpolarizations to approximately the same membrane potential.2. The rod's sensitivity was reduced in low Ca(2+) Ringer by an amount dependent upon the extracellular Ca(2+) concentration. At 10(-6) M-Ca(2+), sensitivity was approximately 0.6 log units below normal. Thereafter, it dropped nearly linearly with [Ca(2+)](o) to a value approximately 4.0 log units below normal at 10(-9) M-Ca(2+). Most of the decline occurred within 1-2 min after the solution change as the membrane potential depolarized, but sensitivity continued to fall slowly with time at the lowest Ca(2+) concentrations. Exposure to low Ca(2+) solutions altered the kinetics of the receptor response to brief flashes, delaying response onset and time-to-peak but affecting the time course of decay very little.3. The sensitivity of the rod to maintained steps of light was also reduced in low Ca(2+). Furthermore, the changes in sensitivity produced by background illumination were very much smaller in low Ca(2+) than in normal Ringer. In some cases backgrounds actually increased sensitivity.4. In 10(-8) M-Ca(2+), backgrounds which themselves produced no response in the rod and no changes in rod sensitivity produced large decreases in response latency for responses of all amplitudes, and pronounced changes in time-to-peak and time-to-decay for moderate and large amplitude responses.5. Since the effects of background light and low Ca(2+) on the wave form of the rod are distinct and in some cases antagonistic, and since the changes in receptor sensitivity produced by backgrounds and low Ca(2+) are not additive, the decreases in sensitivity produced by exposure to low Ca(2+) appear to be caused by a mechanism distinct from normal light adaptation. We suggest that they are caused by an increase in the buffering capacity of the receptor cytosol for Ca(2+) and that Ca(2+) is the excitatory messenger or ;internal transmitter', as originally suggested by Yoshikami & Hagins (1971).

This paper reviews the mechanisms of transmitter release, the kinetics of synaptic transfer, the mechanisms for the production of conductance changes by transmitters, and the nature of the conductance changes at synapses in vertebrate retina. A method for the culturing of adult retinal cells is described, together with preliminary experiments on the identification of cells in culture.

1. The mechanism of light adaptation was investigated by recording intracellularly from single rods in the isolated, superfused retina of the toad, Bufo marinus. Steady background lights produce decreases in rod sensitivity and changes in response wave form similar to those previously observed in the toad eyecup. 2. The sensitivity of a dark-adapted rod is halved by a background light which bleaches about 4 rhodopsins per rod per second. Since a toad rod contains over 2000 disks, a rhodopsin bleached in one disk must alter the effectiveness of rhodopsins bleached in others. This could occur if the state of adaptation in the rod were regulated by the concentration of some diffusable substance. 3. This diffusable substance cannot be Ca2+. Increases in intracellular Ca2+, produced experimentally either by increasing extracellular Ca2+ or by facilitating Ca2+ permeability into the rod with the ionophore X537A, cause a hyperpolarization of membrane potential and a decrease in response amplitude; but they do not produce changes in sensitivity and response wave form like those produced by background light. 4. Either Ca2+ is not the internal transmitter released from the disks during excitation, or the disks release or otherwise alter the concentration of a second diffusable substance, in addition to Ca2+, which regulates the state of adaptation.

1. There are five morphological types of photoreceptors in the retina of the toad, Bufo marinus: red and green rods, single cones, and the principal and accessory members of double cones. The largest and most abundant of these is the red rod. 2. Intracellular recordings were used to investigate the dependence of the sensitivity of red rod responses on wave-length and background light. 3. The spectral sensitivity of dark-adapted and moderately light-adapted red rods can be satisfactorily fitted with the absorbance spectrum of the red rod photopigment. There are no significant contributions to red rod responses from cones or green rods. 4. In contrast, L-type horizontal cells, whose responses are dominated by input from the red rods near threshold, can be shown also to receive input from cones. 5. Steady background light produces a response in the red rods consisting of an initial hyperpolarization, followed by a decay of potential to a steady-state plateau level. The slow decay of response amplitude is accompanied by an increase in sensitivity to increment test flashes. 6. The increment sensitivity at steady-state decreases with increasing background intensity according to a modified Weber-Fechner relation. The dependence of increment sensitivity on the wave-length of the background light can be predicted by the red rod spectral sensitivity, showing that cones do not influence the light adaptation of rods. 7. At a background [corrected] intensity of 11-5 log equivalent quanta cm-2sec-1, sensitivity begins to deviate from the Weber-Fechner relation. In background light one log unit brighter, the rods are completely saturated. 8. Small responses having the spectral sensitivity of cones can be recorded from saturated rods. These potentials have a prominent off response whose wave form resembles the d-wave of the e.r.g. 9. A comparison of the increment--sensitivity curves of single receptors shows that rods are light-adapted by backgrounds one thousand times dimmer than those which affect cones. The increment--sensitivity curves of rods and cones cross, so that single cones become more sensitive than single rods even before the rods begin to saturate.

Yoshikami and Hagins (1971) first proposed that rod outer segments contained large amounts of Ca stored within the discs, and that this Ca was released by light and diffused to the plasma membrane to bind to and block the light-dependent conductance. This proposal, as well as the discovery of light-dependent G protein binding (Kuhn, 1980), GTP turnover (Fung & Stryer, 1980), and phosphodiesterase activation (Wheeler & Bitensky, 1977) stimulated much research into the role of Ca and other second messengers in the production of the photoreceptor light response. As a result of this effort, it has now become clear that the central tenet of the “Ca hypothesis” of Yoshikami and Hagins is incorrect. The elegant experiments of Fesenko and his collaborators (Fesenko, Kolesnikov & Lyubarsky, 1985) and of Yau and his colleagues (Yau, Haynes & Nakatani, 1986) have shown beyond any reasonable doubt that the light-dependent conductance is controlled by changes in cyclic GMP within the rod. Although high concentrations of Ca at the cytoplasmic surface of the outer segment membrane do have some direct effects upon the light channels (Yau et al., 1986), these effects are small and possibly not of physiological significance (Matthews, Torre & Lamb, 1985)

1. When the retina of the toad, Bufo marinus, was superfused with 6-12 mM-tetraethylammonium chloride (TEA), intracellular recordings from rods showed large, depolarizing regenerative potentials. For brief exposures to TEA, these potentials occurred during the recovery phase of the light responses; whereas, during longer exposures, they were spontaneous in darkness but suppressed during illumination. Similar regenerative potentials were observed during perfusion with 3-10 mM-4-aminopyridine and 1-2 mM-BaCl2. 2. The amplitude of the regenerative potentials depended upon the extracellular Ca concentration ([Ca2+]o). Lowering [Ca2+]o decreased their amplitude and in zero [Ca2+]o they were reversibly abolished. Increasing [Ca2+]o by 1.5-2 times produced a small hyperpolarization of membrane potential and a large augmentation in regenerative response amplitude. However, larger increases in [Ca2+]o produced large membrane hyperpolarizations and reversibly suppressed the regenerative responses. 3. High concentrations of Sr2+ in TEA also enhanced regenerative activity but did not affect the rod resting membrane potential. The amplitude of regenerative potentials increased continuously with increasing [Sr2+]o, and in 28 mM-Sr2+ the rods generated 60-70 mV action potentials, even in the absence of extracellular Na+. 4. The regenerative potentials were blocked by 25 microM-Cd2+, 50-100 microM-Co2+, 5mM-Mg2+, and 100 microM-D-600. They were unaffected by 2 microM-TTX or 2-5 mM-Na aspartate. 5. In Ringer containing 12 mM-TEA, large anode break responses could be recorded from rods at the termination of inward current pulses. These anode break responses were also suppressed by Co2+ and unaffected by TTX or Na aspartate. 6. We conclude that the membrane of toad rods contains a conductance normally selective for Ca2+, which is activated by depolarization. In normal Ringer, the inward current through this conductance produces little effect, since it is balanced by a large outward current, probably carried by K+. TEA and other agents appear to block this outward current, permitting the Ca2+ current to become regenerative.

1. We have investigated the effects of Na(+) substitution on the membrane potential and light responses of rods in the superfused retina of the toad, Bufo marinus.2. When all of the Na(+) in the Ringer was replaced with Li(+), the effects on the rods depended upon the external free Ca(2+) concentration ([Ca(2+)](o)). At [Ca(2+)](o) >/= 10(-6) M, the membrane potential (E(m)) hyperpolarized and light responses were greatly diminished or abolished. At [Ca(2+)](o)

Publisher Summary This chapter discusses the evidence that sensitivity is regulated by a diffusible messenger. Although there is now nearly universal agreement that an internal messenger is necessary for excitation, it is perhaps less widely known that a diffusible substance also regulates the sensitivity of the receptor during light adaptation. The chapter presents experiments that show that sensitivity is not normally controlled by increases in Ca2+. The chapter describes the effects of exposing rods to low-Ca2+ solutions and discusses some of the implications of the experiments that exclude for toad rods some of the models for light adaptation in receptors that have been previously proposed.

The voltage-dependent conductances of Limulus ventral photoreceptors have been investigated using a voltage-clamp technique. Depolarization in the dark induces inward and outward currents. The inward current is reduced by removing Na+ or Ca2+ and is abolished by removing both ions. These results suggest that both Na+ and Ca2+ carry voltage-dependent inward current. Inward current is insensitive to tetrodotoxin but is blocked by external Ni2+. The outward current has a large transient component that is followed by a smaller maintained component. Intracellular tetraethylammonium preferentially reduces the maintained component, and extracellular 4-amino pyridine preferentially reduces the transient component. Neither component is strongly affected by removal of extracellular Ca2+ or by intracellular injection of EGTA. It is concluded that the photoreceptors contain at least three separate voltage-dependent conductances: 1) a conductance giving rise to inward currents; 2) a delayed rectifier giving rise to maintained outward K+ current; and 3) a rapidly inactivating K+ conductance similar to the A current of molluscan neurons.

1. We have used laser micromass analysis (l.a.m.m.a.) to investigate Ca uptake and release in intact 'red' rod photoreceptors in the dark-adapted retina of the toad, Bufo marinus. 2. With l.a.m.m.a. it is possible to measure separately the concentrations of each of the Ca isotopes. Rods normally containing almost exclusively 40Ca can be incubated in Ringer solution containing the stable isotopes 42Ca or 44Ca. In this way, the movements of Ca into and out of the rod can be separately determined. 3. When rods are incubated in darkness in high 44Ca (up to 20 mM), large amounts of 44Ca accumulate in the outer segment at a rate which increases with increasing external 44Ca concentration. However, this 44Ca appears not to exchange with the 40Ca originally present within the rod. This result suggests that the 40Ca may be sequestered within a pool which normally exchanges slowly with external Ca. 4. We explored Ca exchange in high-Ca solutions in more detail with double-isotope labelling. In these experiments, rods were first pre-loaded with Ca of one isotope (42Ca) and then incubated in Ringer solution containing another (44Ca). We could then measure separately the rate of exchange of the pre-loaded 42Ca with the 44Ca in the Ringer solution and with the 40Ca originally present within the rod in the sequestered pool. 5. These experiments show that the pre-loaded-Ca exchanges rapidly with Ca in the Ringer solution, at least in part by Ca-Ca exchange, but much more slowly with the Ca originally present within the rod. Thus Ca in the outer segments can exist in (at least) two pools: one which exchanges rapidly across the plasma membrane and is probably Ca free or loosely bound within the cytosol, and another which exchanges slowly and is probably Ca within the disks. 6. Although Ca sequestered within the outer segment normally exchanges quite slowly, it can be rapidly released if the extracellular free Ca is buffered to low levels with EGTA. The rate-limiting step for Ca release under these conditions appears not to be Na-Ca exchange, since the rate of Ca efflux is unchanged if the Na in the Ringer solution is substituted with choline. 7. Ca can also be released from the sequestered pool if rods are incubated in Ringer solution containing 100 or 500 microM-IBMX (3-isobutyl-1-methylxanthine).(ABSTRACT TRUNCATED AT 400 WORDS)

NEURONES are thought to contain ion-selective channels whose opening and closing are regulated by the potential difference across the plasma membrane. It is thought that the sodium, potassium, and calcium currents which generate action potentials and modulate synaptic transmitter release pass through these channels. Investigations of voltage-dependent currents in various species have revealed remarkable uniformities in the ion selectivity and pharmacology of presumed membrane channels1–3. The use of agents which block specific currents has provided a tool for the identification of voltage-dependent conductances in nerve membrane, which we have used to investigate vertebrate photoreceptors. We show here that when the retina of the toad Bufo marinus is superfused with Ringer containing 6–12 mM tetraethyl ammonium (TEA), rods generate oscillations and action potentials resembling the calcium spikes which have been described in various vertebrate and invertebrate preparations3. These experiments demonstrate that the rod membrane is not passive, as is often assumed, but contains at least one and probably two voltage-dependent conductances.

We have applied patch-clamp techniques to on-cell and excised-membrane patches from human retinal pigment epithelial cells in tissue culture. Single-channel currents from at least four ion channel types were observed: three or more potassium-selective channels with single-channel slope conductances near 100, 45, and 25 pS as measured in on-cell patches with physiological saline in the pipette, and a relatively nonselective channel with subconductance states, which has a main-state conductance of approximately 300 pS at physiological ion concentrations. The permeability ratios, PK/PNa, measured in excised patches were 21 for the 100-pS channels, 3 for the 25-pS channels, and 0.8 for the 300-pS nonselective channel. The 45-pS channels appeared to be of at least two types, with PK/PNa's of approximately 41 for one type and 3 for the other. The potassium-selective channels were spontaneously active at all potentials examined. The average open time for these channels ranged from a few milliseconds to many tens of milliseconds. No consistent trend relating potassium-selective channel kinetics to membrane potential was apparent, which suggests that channel activity was not regulated by the membrane potential. In contrast to the potassium-selective channels, the activity of the nonselective channel was voltage dependent: the open probability of this channel declined to low values at large positive or negative membrane potentials and was maximal near zero. Single-channel conductances observed at several symmetrical KCl concentrations have been fitted with Michaelis-Menten curves in order to estimate maximum channel conductances and ion-binding constants for the different channel types. The channels we have recorded are probably responsible for the previously observed potassium permeability of the retinal pigment epithelium apical membrane.

IT has long been known that vertebrate photoreceptors can signal the absorption of single quanta of light1. The extraordinary sensitivity of the visual system is usually ascribed both to the sensitivity of the photoreceptors and to the integration of their signals by higher-order neurones. A more complete explanation of visual sensitivity could be given if it were possible to record the response of receptors at the visual threshold. Unfortunately, it has not been possible to do this at the absolute threshold, largely because of interactions between receptors2. Near the absolute threshold only a small proportion of the receptors absorb quanta, but the photocurrent from these cells spreads through electronic synapses3 to many other cells. Thus the voltage responses in the receptors are non-uniform, and it is not possible to tell which responses contribute to the threshold. Fortunately this difficulty does not arise at the increment threshold since in the presence of bright background light, the variation in response from one receptor to the next is minimal. The differences among the receptors in the number of quanta caught will be much less important than at absolute threshold, and the connections between the receptors will tend to equalise their voltages. In this report we compare receptor responses to behavioural responses in the light-adapted turtle and show that the receptor response at threshold is only a few microvolts in amplitude.

A dark-adapted toad rod can respond consistently to flashes of light which bleach an average of less than one pigment molecule in its outer segment. These responses are much less variable in amplitude than would be expected if rods were independent quantum detectors. Rods interact with one another by pooling their signals, so that at least 85 to 90 percent of the response recorded from a single rod is generated by pigment molecules bleached in other receptors.

Mudpuppy rod and cone responses differ both in time course of recovery and in absolute sensitivity. Rods are about 25 times more sensitive than cones and appear to generate a larger voltage per quantum absorbed. Comparison of mudpuppy receptor sensitivities to those of other vertebrates suggests that the difference in sensitivity between rods and cones may be a general phenomenon.