Effects of light and temperature on the response gradient of retinal rod outer segments.

Photoreceptor outer segments are continually renewed from base to tip and consequently the membrane at the tip is older than at the base. In parallel with this, there are differences in light responsiveness, and it has been proposed that these may be due to the aforementioned differences in age. We have tested this proposal by manipulating the rates of renewal of the outer segments by exposing Xenopus laevis to different temperature and lighting regimes which are known to affect the rates of their outer segment membrane renewal. We have found that differences of responsiveness are, indeed, correlated with the differences in the age gradients along the outer segments.

Phototransduction and calcium exchange along the length of the retinal rod outer segment.

The aim of this study was to investigate whether there are differences in calcium exchange between the ends of a retinal rod outer segment (OS) which could be correlated with the know differences of light responsiveness and which could explain the latter. We found that there is little, if any, difference in the exchange current density. However, the time constant of the exchange current decay after a saturating flash of light lengthened from base to tip and more calcium was extruded at the tip than the base of the OS. Our analysis of the results leads to the conclusion that there is little, if any, difference in free calcium concentration between the ends of the outer segment, but that there is a difference in the bound calcium and that this can explain the difference in light responsiveness along the length of the rod.

Effects of animal age on the responses along the outer segment of retinal rod photoreceptors.

The aim of the present study was to determine whether the response gradient, known to exist along a rod outer segment, is influenced by age and developmental changes. Since intense light flashes saturate the responses of retinal rods and the time the response remains saturated increases from base to tip of the rod outer segment, one can use this difference in saturation times as a measure of the response gradient. During development and before sexual maturity (about one year postmetamorphosis) the differences between base and tip decreased, and this correlated with an acceleration of the light response in Xenopus laevis rods. The gradient along the rod outer segment then stabilized, while the response kinetics slowed and remained at a lower level. We conclude that photoreceptor responses and hence visual performance are affected by developmental changes.

The saturated response of vertebrate rods and its relation to cGMP metabolism.

Strong light flashes saturate the response of a rod, and increasing flash intensities prolong the period of saturation. We have found that the period of saturation lengthens from base to tip of the outer segment and we have shown that free cGMP is replenished locally to terminate saturation and not in the G-cyclase rich axonemal region near the inner segment which contains the cell mitochondria. Our results also show that each section of outer segment acts as an independent module in this phase of transduction.

Rod outer segments are designed for optimum photon detection.

The proposal that rod outer segment length is optimal with respect to photon absorption and noise control is extended and tested in a number of species. We find good agreement with our optimality criterion in duplex retinae where rods act as detectors of one or a few photons, but not in all rod retinae nor in those which are exposed to significant photic environmental noise.

Recovery from bleaching in photoreceptors promoted by biotin, pyruvate, and glucose.

In the rods of Bufo marinus and other species, bleaching of the rhodopsin in isolated cells leads to a loss of sensitivity and response amplitude and to a shortened response duration. These changes are permanent for cells bathed in Ringer's solution. They are due to as yet unknown modulations in the transduction biochemistry. In this paper, we report that these changes can be partly or completely reversed by supplying biotin, pyruvate, and elevated glucose to the rod. The time course of this reversal and the substances which promote it imply that these are metabolically mediated effects. Based on the reported action of biotin and pyruvate on the one hand and on the changes of the response waveforms on the other hand, we believe that the phenomena we observe involve the later steps of the transduction cycle.

Some conjectures on the design of a rod outer segment.

Photoreceptor diameter and spacing have been extensively analyzed with respect to diffraction, wave-guiding and other phenomena and have been found to be consistent with optimal design criteria. Photoreceptor length, on the other hand, has received but little attention. It is proposed here that the outer segment length of rods is optimal with respect to the competing demands of maximal photon absorption and minimal noise. This is borne out by our calculations based on the experimental data.

Background and bleaching equivalence in steady-state adaptation of vertebrate rods.

We have investigated background and bleaching adaptation in vertebrate rods by intracellular recording in the retina of Bufo marinus. Backgrounds and bleaching produce adaptation in photoreceptors and lead to a shift and a compression of the response operating range. Threshold elevation due to backgrounds follows the Rose-DeVries rule at low intensities and the Weber-Fechner rule at high intensities. Threshold elevation due to bleaching is linear almost up to 17% bleached pigment and exponential thereafter. An equivalence can be established between bleaching and backgrounds with respect to threshold elevation, on the one hand, and with respect to response compression, on the other. These equivalences are the same within experimental error. The equivalence, moreover, appears to extend to the complete response curve. These results have implications for psychophysics as well as for photoreceptor transduction.

A new method of non-enzymatic dissociation of the Bufo retina.

We have formulated a medium in which an electroretinogram (ERG) can be elicited from eyecup pieces of Bufo marinus up to 2 weeks after dissection. At the same time the retina can be dissociated into healthy, intact photoreceptors and other cells after soaking in the same medium from 4 to 24 h. Thus, no enzymes need be used which have deleterious effects on the cells; on the contrary the medium extends cell viability. Clearly, this method should be attractive in electrophysiological and biochemical studies.

Bleaching of the isolated retina.

Our theory for photoreceptor bleaching (Leibovic and Kurtz, 1975; Leibovic, 1975) is extended to the isolated retina, a preparation often used in experimental research. Experimental data are presented which demonstrate good agreement with the theory, and it is shown that bleaching history can affect the final result. Since our theory includes a lumped intermediate state of bleaching, it is superior to theories which only consider end products. On the other hand, our theory is more tractable than others which specify several intermediates.

Response waveforms of vertebrate photoreceptors: what are the underlying mechanisms?

A class of models is investigated using computer simulation in which the inner and outer segments of the vertebrate photoreceptor are coupled through a pump. The outer segment membrane conductance is controlled by an internal transmitter, activated by photolysis of the photosensitive molecules in the cell. Several possibilities for the coupling dynamics are investigated. The analysis favors the conclusion that the hyperpolarizing transient at high intensity stimuli arises from the coupling dynamics, (unless there is an extracellular current shunt path). It predicts, moreover, that the transient should be observed intracellularly, but not extracellularly to the outer segment. This is, in fact, the case. It also predicts that the transient should become more marked, as the steady state ratio of inner to outer segment currents decreases. The computer simulations are concerned with the intracellularly recorded responses; the long term adaptation parallel to pigment bleaching and regeneration is not considered explicitly here. In conclusion, it is shown that the state conditions as well as the response waveforms can be related to physiologically significant variables.

The effect of membrane parameters on the properties of the nerve impulse.

The effect of varying membrane capacitance, conductance, and rate constants on the properties of the nerve impulse is considered in terms of the degree of regeneration in the Hodgkin-Huxley model for the squid giant axon. It is shown through computer simulation that reducing regeneration generally increases the duration of the action potential and decreases its amplitude, rate of rise, and conduction velocity. The threshold becomes much less sharp and the amplitude of the response of a patch of membrane grades with stimulus strength. A second stimulus, applied shortly after a first stimulus, considerably perturbs the membrane potential from its original time-course. Under certain conditions, the nerve signal can propagate with a small decrement.