Responses to light of solitary rod photoreceptors isolated from tiger salamander retina.

180

175

SUMMARY1. The spread of electrical signals between rods in the salamander retina was examined by passing current into one rod and recording the voltage responses in nearby rods. Rod network behaviour, measured in this way, was simulated from data on rod membrane properties gathered in voltage-clamp experiments on single isolated rods.2. The network voltage responses to square current pulses became smaller, more transient, and had a longer time-to-peak, for rods further away from the site of current injection. Depolarizing currents produced smaller responses than hyperpolarizing currents of the same magnitude.3. Neighbouring rods and cones were coupled less strongly than neighbouring rods. 4. The response of the rod network to current injection was unaffected by 2 mMaspartate-, which eliminates transmission from receptors to horizontal cells.5. The input resistance of single isolated rods, measured at the resting potential, varied between 100 and 680 MQ. The lower values were probably due to damage by the micro-electrodes. Electrical coupling was found to be very strong between the rod inner and outer segments. 6. A strong 'instantaneous' outward rectification seen in isolated rods at potentials positive to -35 mV was reduced, but not abolished, by 15 mM-TEA.7. In normal solution, isolated rods exhibited a voltage-and time-dependent current, IA, whose kinetics were approximated by a single first-order gating variable, and whose activation curve spanned the range between -40 and -80 mV. The time constant for the current varied with voltage and was 60-200 msec between -140 and -40 mV.8. A reversal potential for IA could not be found between -140 and -40 mV in normal solution, and the fully activated current, IA, was approximately voltageindependent, with a magnitude of , 0*1 nA over this potential range. 9. By several criteria, IA behaved as a single inward current activated by hyperpolarization. Pharmacological studies suggest, however, that it is the sum of at least two currents with very similar kinetics.10. Most isolated rods exhibited a very slow (T , 3 see) increase in net outward

Section: Introductionmentioning

confidence: 53%

Behaviour of the rod network in the tiger salamander retina mediated by membrane properties of individual rods

Attwell

Wilson

1980

180

175

“…This also seems improbable, since the gap-junctional resistance between neurones varies little with membrane voltage in other systems (Bennett, 1972). Furthermore, it has recently been possible to measure the current-voltage curve of uncoupled rods (Bader et al 1978;Werblin, 1978), and these also show a prominent outward rectification. Though the gap junctions are unlikely to be responsible for the outward rectification, they probably do influence the shape of the current-voltage curve.…”

Section: Discussionmentioning

confidence: 99%

The effects of tetraethylammonium and cobalt ions on responses to extrinsic current in toad rods.

Fain¹,

Quandt²

1980

SUMMARY1. Double-barrel micropipettes were used to pass pulses of current in darkness into single rods in the isolated, perfused retina of the toad, Bufo marines.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.

“…8, a 25 mV depolarization (+ 0 97 nA) reduced the light response to less than 20 % of the value at the resting potential. Previous studies have shown that depolarizing current, passed into photoreceptors of gecko (Toyoda, Nosaki & Tomita, 1969) or the rods of tiger salamander (Werblin, 1978;Bader et al 1978) could reverse the direction of the light response. No reversal in the light response was observed in these present experiments for depolarizing currents up to + 4 nA.…”

Section: The Input Current-voltage Relationship Of the Rodmentioning

confidence: 99%

Current—voltage relations in the rod photoreceptor network of the turtle retina

Copenhagen

Owen

1980

SUMMARY1. Electrical coupling between rod photoreceptors was studied in the eyecup preparation of the snapping turtle, Chelydra serpentina, using intracellular microelectrodes.2. The spatial profiles of rod responses to a long narrow slit of light were determined. The peak response amplitudes were found to decline exponentially as the slit was moved from the most sensitive position in the receptive field of each rod. The mean length constant was 55-7 ,tm.3. Rods were simultaneously impaled in pairs and electrical coupling was demonstrated between the rods in seventeen of these pairs. No coupling was observed between rods separated by more than 110 Ism. The transfer resistance, defined as the ratio of potential in the second rod (coupled potential) to the current injected into the first rod, varied from 0-2 to 13-2 MQ.4. The waveform of the coupled potential was time varying, exhibiting a peak and subsequent relaxation phase. The time course of the relaxation phase was voltagedependent. At the cessation of current, the coupled potential rebounded beyond the resting potential and then decayed to the dark potential.5. Plots of input current versus coupled potential showed strong outward-going rectification, chord transfer resistances being as much as 3-5 times lower for depolarizing currents.6. Simultaneous impalements were made of pairs of neighbouring red-sensitive cones, of horizontal cells and rods, and of red-sensitive cones and rods. No evidence of coupling between cones and rods were found, nor could feed-back from horizontal cells onto rods be demonstrated; however, coupling between red-sensitive cones was found. This coupling exhibited neither the marked time-varying nor voltagedependent properties that characterize the rod-rod coupling.7. Individual rods were impaled with independent current passing and voltage sensing micro-electrodes. Pulses of current produced time-varying potentials having relaxation and rebound phases. Current-voltage measurements showed a strong outward-going rectification. Input resistances at the resting potential ranged up to 96 MQ. 8. Square grid and hexagonal lattice models of ohmic electrical coupling were applied to the results. Using the measured values for the length constants and input resistances of single rods, we calculate that the plasma membrane resistance of each rod is approximately 1000 MO at the resting potential and that the coupling resistances are 272 and 444 MO for the square grid and hexagonal models, respectively.9. The time-varying and voltage-dependent properties observed at the input and in the coupling between rods appear to reflect characteristics of the rod's plasma membrane and not of the coupling pathways between the rods. Both the outwardgoing rectification and relaxation phase of the response appear to involve voltagedependent conductance increases in the rod's plasma membrane.