Molecular mechanisms of vertebrate photoreceptor light adaptation.

An important recent advance in the understanding of vertebrate photoreceptor light adaptation has come from the discovery that as many as eight distinct molecular mechanisms may be involved, and the realization that one of the principal mechanisms is not dependent on calcium. Quantitative analysis of these mechanisms is providing new insights into the nature of rod photoreceptor light adaptation.     

The role of cytoplasmic calcium in photoreceptor light adaptation.

The process of light adaptation in vertebrate rod and cone photoreceptors is believed to involve a diffusible cytoplasmic messenger. Two lines of evidence indicate that photoreceptor light adaptation is mediated by a light-induced fall in cytoplasmic calcium concentration (Ca2+i). First, if changes in calcium concentration are slowed by the incorporation of calcium chelators into the photoreceptor cytoplasm then light adaptation is slowed also. Second, if the normal control of Ca2+i is prevented by simultaneously minimising calcium influx and efflux across the outer segment membrane by means of external solution changes, then all of the manifestations of light adaptation are abolished. Furthermore, recent results show that changes in Ca2+i imposed in the absence of light are sufficient to cause at least some of the manifestations of light adaptation. Together these results indicate that calcium acts as the messenger of light adaptation in the photoreceptors of both lower and higher vertebrates.     

Adaptation and facilitation in the barnacle photoreceptor

The barnacle photoreceptor sensitivity may either decrease (light adaptation) or increase (facilitation) after exposure to a conditioning light. The balance between adaptation and facilitation is influenced by at least three factors: initial sensitivity state of the cell, external calcium concentration, and conditioning intensity. Cells of very high sensitivity show mainly adaptation, which appears only for higher conditioning intensities and is suppressed in low-calcium media. Less sensitive cells, or those whose sensitivity is reduced by injury or metabolic decay, exhibit facilitation, expecially in low-calcium media and at intermediate conditioning intensities. Both phenomena show recovery time-courses of seconds-to-minutes. Models are proposed which relate light adaptation, as previously suggested, to increased internal calcium concentration, and facilitation either to decreased internal calcium concentration or to decreased activation "affinity" of ion-channel-blocking sites.     

Responses of crayfish photoreceptor cells following intense light adaptation

Summary After intense orange adapting exposures that convert 80% of the rhodopsin in the eye to metarhodopsin, rhabdoms become covered with accessory pigment and appear to lose some microvillar order. Only after a delay of hours or even days is the metarhodopsin replaced by rhodopsin (Cronin and Goldsmith 1984). After 24 h of dark adaptation, when there has been little recovery of visual pigment, the photoreceptor cells have normal resting potentials and input resistances, and the reversal potential of the light response is 10–15 mV (inside positive), unchanged from controls. The log V vs log I curve is shifted about 0.6 log units to the right on the energy axis, quantitatively consistent with the decrease in the probability of quantum catch expected from the lowered concentration of rhodopsin in the rhabdoms. Furthermore, at 24 h the photoreceptors exhibit a broader spectral sensitivity than controls, which is also expected from accumulations of metarhodopsin in the rhabdoms. In three other respects, however, the transduction process appears to be light adapted: (i) The voltage responses are more phasic than those of control photoreceptors. (ii) The relatively larger effect (compared to controls) of low extracellular Ca⁺⁺ (1 mmol/1 EGTA) in potentiating the photoresponses suggests that the photoreceptors may have elevated levels of free cytoplasmic Ca⁺⁺. (iii) The saturating depolarization is only about 30% as large as the maximal receptor potentials of contralateral, dark controls, and by that measure the log V-log I curve is shifted downward by 0.54 log units. The gain (change in conductance per absorbed photon) therefore appears to have been diminished.     

Sensitivity and adaptation in R7, an ultraviolet photoreceptor,in theDrosophila retina

Summary Receptor deficient mutants and chromatic adaptation were used to isolate the contribution of R7 to the electroretinogram (ERG) ofDrosophila. R7 was found to be a single-peaked ultraviolet (UV) receptor (Fig. 1). Photoconversion of the UV absorbing rhodopsin (R) to its stable 470–495 nm metarhodopsin (M) was shown to elicit a long-lived negative (depolarizing) afterpotential (Fig. 3) while inactivating R7. Photoreconversion ofM toR reactivates R7 (Fig. 2) and repolarizes the ERG (Fig. 3). The intensities of light needed to elicit afterpotentials by photointerconverting R7 photopigment were found to be about 2 log units greater than for R1-6 photopigment (Fig. 4). Vitamin A deprivation decreases R7 (as well as R8) sensitivity by about 2 log units (through decreased photopigment levels) without changing spectral sensitivity shape (Fig. 5). Vitamin A deprivation further eliminates the light-induced inactivation of R7 allowing experiments designed to characterize the in vivo spectral absorption of R7M. R7M was found to have UV and 495 nm maxima (Fig. 6). No polarization sensitivity was detected in the R7 ERG component. The adaptational properties of R7 are similar to the properties previously established for R1-6 but different from the properties of R8.Supported by NSF grants BMS-74-12817 and BNS 76-11921. I thank M. Chapin, R. Greenberg, K. Hu, A. Ivanyshyn, D. Lakin, G. Pransky, D. Sawyer, J. Walker and W. Zitzmann for technical assistance.     

Biophysical evidence that light adaptation in Limulus photoreceptors is due to a negative feedback.

The steady-state stimulus-response curve of the Limulus ventral photoreceptor comprises a linear portion followed by a less-than-unity power law dependence, which is maintained over at least 4 decades of intensity. This progressive desensitization corresponds to light adaptation. For flash stimulation of dark-adapted cells, the stimulus-response curve again has an initial linear portion, but this is followed by a region of supralinearity before the curve saturates. In a previous article, we showed that the distribution of time integrals of the single-photon responses is consistent with a model of a single chain of first-order reactions. Starting with such a model, we have looked at relevant elementary nonlinear biochemical mechanisms to determine which of them can modulate the enzymatic amplifications of the chain in such a way as to lead to these behaviors. We assume that each of the two phenomena, adaptation and supralinearity, derives from a single mechanism that acts on a single enzymatic stage. We then conclude that the adaptation must be a cooperative negative feedback, in which an accessory material activated by a late stage of the transduction chain acts cooperatively to inhibit an earlier enzymatic amplification. In Limulus, the number of molecules that cooperate is between 3 and 5. We were not able to discard any of the mechanisms tested for the supralinearity, except to say that they must act at a stage of the chain later than that on which the adaptive material acts. If we assume the conclusions of a previous work which shows that the supralinearity mechanism is active during the steady state, we can also conclude that the supralinearity stage must precede the stage that is the source of the adaptive material.     

Sensitivity changes of photoreceptor cells of Hirudo medicinalis caused by changes in extracellular calcium concentration

Extracellular recordings from the vacoule of photoreceptor cells of Hirudo medicinalis L. were performed using microelectrodes. The cells were adapted by white light flashes given at constant intervals (20 s). Response height versus relative intensity curves obtained from the same cell in physiological saline (PS) and in bathing solutions of either a) lowered calcium contents (2 ΜM/1 or less) or b) raised calcium contents (15 mM/1) were compared. The cells' adaptation state in PS was operationally defined by the ratio Q=h _A /h _S where h _A is the response height evoked by the adapting flashes, and h _S is the corresponding saturation response height. Sensitivity changes were measured by the half saturation intensity shift. Lowering extracellular calcium resulted in:

1. The response height increased and the shape of the response became more rounded and prolonged.
2. The total resistance between the vacuole and outside decreased from 8.2±1.4 MΩ (n=6) in PS to 4.6±0.4 MΩ (n=5). The resistance was independent of the cells' adaptation state.
3. A change of the cells' sensitivity occured either in direction to light adaptation or in direction to dark adaptation. It depended functionally on the ratio Q:

a) if Q was less or equal to about 0.6 the cells' sensitivity increased. b) if Q was greater than 0.6 the cells' sensitivity diminished. Raising extracellular calcium decreased the sensitivity of all cells tested independent of their adaptation states in PS. The results can be interpreted under the assumptions that 1. the sensitivity of leech photoreceptor cells is inversely proportional to the intracellular free calcium concentration and Z. intracellular calcium can interact with extracellular calcium in relatively dark adapted cells whereas in relatively light adapted cells the raise of intracellular free calcium is mainly effected by a release from intracellular stores. It is assumed that a Q value of about 0.6 separates relatively light adapted cells from relatively dark adapted cells.     

Ultrastructural changes of the microvillar cytoskeleton in the photoreceptor of the crayfish Orconectes limosus related to different adaptation conditions

In the retina of crayfish microvilli of seven of the eight photoreceptor cells build highly organized structures, the rhabdoms. Cytoskeletal elements inside the microvilli were investigated in conventional and slightly extracted electron microscopical preparations. In conventional preparations the ultrastructure of these cytoskeletal elements depended on the adaptational state of the animal. They appeared as central filament-like structures inside each microvillus when dark-adapted retinae were prepared and fixed at night in the absence of calcium. Changes of these conditions (light, daytime, or calcium concentration) impaired the detectability of these central filaments; in light-adapted eyes prepared at midday they were rarely seen. Nevertheless, single microvillar filaments were present in light-adapted retinae after mild cell permeabilization with the saponin beta-escin. They appeared as a regular structure in each microvillus, often attached to the membrane. Their fine structure was consistent with the ultrastructure of single actin filaments as indicated by fast-Fourier-analysis and further supported by the presence of anti-actin immunoreactivity in electron microscopical and immunocytochemical preparations. These results indicate that microvillar filaments are not necessarily destroyed by light as previously described; we suggest that their appearance inside the microvillus might be altered by the properties of associated, maybe sidearm-like proteins.     

Sensitivity and adaptation in the Drosophila phototransduction and photoreceptor degeneration mutants trp and rdgB

We studied rdgB, a retinal degeneration mutant, and trp, a phototransduction mutant, separately and in combination in Drosophila. First we showed that trp did not block degeneration in white-eyed rdgB mutants. Thus, rdgB was useful in determining the defects which trp caused in the compound eye receptors R7 and R8; this is because rdgB selectively eliminates R1-6 photoreceptors which would, if present, dominate the compound eye responses. R7 and R8 both express the trptransient receptor potential phenotype in trp mutants. The trp mutation does not change receptor spectral sensitivities, nor does it alter the dark stability of R1-6's and R7's metarhodopsins as judged by dark adaptation studies. The dark adaptation is not significantly affected by trp. However, trp slows the dark adaptation of R8 considerably and seems to make the blue-induced inactivation of R1-6 less stable.     

Transient state characteristics of adaptation to changes in light conditions for the cyanobacterium Oscillatoria agardhii

Transitions in growth irradiance level from 92 to 7 Em^-2 s^-1 and vice versa caused changes in the pigment contents and photosynthesis of Oscillatoria agardhii. The changes in chlorophyll a and C-phycocyanin contents during the transition from high to low irradiance (HL) were reflected in photosynthetic parameters. In the LH transition light utilization efficiencies of the cells changed faster than pigment contents. This appeared to be related to the lowering of light utilization efficiencies of photosynthesis. As a possible explanation it was hypothesized that excess photosynthate production led to feed back inhibition of photosynthesis. Time-scales of changes in the maximal rate of O₂ evolution were discussed as changes in the number of reaction centers of photosystem II in relation to photosynthetic electron transport. Parameters that were subject to change during irradiance transitions obeyed first order kinetics, but hysteresis occurred when comparing HL with LH transients. Interpretation of first order kinetic analysis was discussed in terms of adaptive response vs changes in growth rate.Non-standard abbreviations Chla chlorophyll a - CPC C-phycocyanin - PS II photosystem II - PS I photosystem I - RC II reaction center of photosystem II - P photosynthetic O₂-evolution - I irradiance, Em^-2 s^-1 - light utilization efficiency of cells, mmol O₂·mg dry wt^-1·h^-1/Em^-2 s^-1 - light utilization efficiency of photosynthetic apparatus, mol O₂·mol Chla ^-1·h^-1/Em^-2 s^-1 - P_max maximal rate of O₂ evolution by cells, mol O₂·mg dry wt^-1·h^-1 - P_max maximal rate of O₂ evolution by photosynthetic apparatus, mol O₂·mol·Chla ^-1·h^-1 - LL low light, E m^-2 s^-1 - HL high light, E m^-2 s^-1 - LH low to high light transition - HL high to low light transition - k specific rate of adaptation, h^-1 - specific growth rate, h^-1 - Q pool size of cell constituent, mol·mg dry wt^-1 - q net synthesis rate of cell constituent, mol·mg dry wt^-1·h^-1     

Dependence of bump rate and bump size in Limulus ventral nerve photoreceptor on light adaptation and calcium concentration

Bumps were recorded in Limulus ventral nerve photoreceptor as deflections in membrane voltage during 10 s illuminations by dim light which were repeated every 20 s. The bump amplitude vs frequency distribution and its dependence on the intensity of a preadapting light flash are described. Light adaptation which diminishes the average bump amplitude alters the character of the bump amplitude distribution from a curve with a convex region to a continuously falling concave curve. Weak light adaptation can increase frequency (and height) of the bumps elicited by constant stimuli. Raising the external Ca²⁺-concentration from 10 to 40 mmol/l augments the effect of a preadapting light flash in diminishing the bump amplitudes and also increases the bump frequency. The results are consistent with the assumptions

that light adaptation is based on a Ca²⁺-dependent reduction of the amplification factor which determines the bump size and

that the coupling between light induced rhodopsin reactions and bump generation is Ca²⁺-dependent.

     

Fine structural and volumetric changes of lamprey photoreceptor cells during light and dark periods

Summary Short photoreceptor cells of the lamprey retina exhibited a 30% increase in the width of the myoid process and a 20% increase in that of the axonal process during a 12-h light period, compared to the measurements obtained during a 12-h dark period. An increase in the amount of cytoplasm, dilation of ER cisterns, and swelling of the nucleus appeared to cause the enlargement of the myoid parts. Accumulation of synaptic vesicles occurred concurrently with a thickening of the axonal process. These morphological changes presumably represent a phase of the diurnal cycle and current synaptic activity of the short cell. By contrast, the long photorecpetor cell showed neither measurable changes nor any indication of retinomotor movement.     

Probing the structural changes in the light chain of human coagulation factor VIIa due to tissue factor association.

The crystallographic structure of human coagulation factor VIIa/tissue factor complex bound with calcium ions was used to model the solution structure of the light chain of factor VIIa (residues 1-142) in the absence of tissue factor. The Amber force field in conjunction with the particle mesh Ewald summation method to accommodate long-range electrostatic interactions was used in the trajectory calculations. The estimated TF-free solution structure was then compared with the crystal structure of factor VIIa/tissue factor complex to estimate the restructuring of factor VIIa due to tissue factor binding. The solution structure of the light chain of factor VIIa in the absence of tissue factor is predicted to be an extended domain structure similar to that of the tissue factor-bound crystal. Removal of the EGF1-bound calcium ion is shown by simulation to lead to minor structural changes within the EGF1 domain, but also leads to substantial relative reorientation of the Gla and EGF1 domains.