Spatial spread of light-induced sensitization in rod photoreceptors exposed to low external calcium

A molecular basis for Weber's law

A mathematical model is presented that obeys a strong form of Weber's law--over a range of adapting and stimulus intensities, equal contrast stimuli evoke identical responses. To account for the strong Weber's law, the adaptive stage in the proposed model employs a "delayed" reverse reaction along with a power-law input. It is suggested that this Weber's law mechanism is responsible for a slow, voltage-uncorrelated component of adaptation in the vertebrate photoreceptor. A plausible biochemical mechanism is the G-protein cycle with phosphorylation of photoactivated photopigment (and binding of arrestin to the phosphorylated photopigment) as the adaptive process. In an Appendix, features of the general model and implications of a specific biochemical model are examined by computer simulation.

Properties of ion channels closed by light and opened by guanosine 3',5'-cyclic monophosphate in toad retinal rods

1. In patch-clamp recordings from outer segments of dark-adapted rod photoreceptors, single-channel recordings were obtained from the light-sensitive conductance when divalent cations were omitted from the pipette solution bathing the extracellular face of the recorded patch of membrane. 2. Activity of the light-sensitive channel was suppressed by light within the normal response range of the dark-adapted rod. During dim, steady illumination, the rate of opening of the channel fluctuated dramatically, as expected qualitatively from statistical fluctuations in the number of photoisomerizations occurring within the effective collecting area of the recorded patch. 3. The light-sensitive channel flickered rapidly in the open state, so that individual events appeared as a burst of openings and closings. The average duration of a burst was 0.78 +/- 0.03 ms (mean +/- S.E.). The average duration of an individual opening was 0.18 +/- 0.008 ms. The average closed duration within a burst was 0.37 +/- 0.02 ms. 4. Hyperpolarization of the recorded patch had no effect on average burst or open duration, although opening frequency increased slightly (+18.6 +/- 4.9%; n = 13; mean +/- S.E.). Average single-channel current increased linearly with hyperpolarization, giving an estimated single-channel conductance of 20.5 +/- 1.1 pS. By extrapolation of the relation between channel current and hyperpolarization, the dark driving force was estimated to be about 48 mV. 5. In addition to reducing the rate of channel events, dim non-saturating light also reduced the average duration of a burst of openings and the average duration of openings within a burst. 6. About 50% of cell-attached patches showed no channel activity in darkness. Light-suppressable channel activity could be induced in these silent patches by perfusing the outer segment with low-Ca2+ Ringer solution. Similarly, activity could be increased dramatically by low-Ca2+ Ringer solution in patches that did show channel activity in the dark. From the maximal channel activity observed during low-Ca2+ perfusion, the lower limit for the number of channels per patch was 20-70, corresponding to an estimated channel density of 100-350 channels micron-2. 7. After recording light-sensitive channel activity in the intact rod, the patch of membrane was excised, exposing the intracellular membrane face. Application of guanosine 3',5'-cyclic monophosphate (cyclic GMP) to the intracellular face activated channels (Haynes, Kay & Yau, 1986; Zimmerman & Baylor, 1986; Matthews, 1986d, 1987) whose properties could then be compared directly with the light-sensitive channels recorded earlier in the same patch of membrane.(ABSTRACT TRUNCATED AT 400 WORDS)

Single-channel recordings demonstrate that cGMP opens the light-sensitive ion channel of the rod photoreceptor

Patch-clamp recordings were made from outer segments of single dark-adapted rod photoreceptors from toad retina. When patch pipettes were filled with a solution free of divalent cations, inward current through individual light-sensitive ion channels was sufficiently large to allow single-channel recording. Channel activity was suppressed by illumination within the normal response range of the dark-adapted rod. When illumination was restricted to a portion of the outer segment, the effect of light on channel activity was spatially localized to the illuminated region. Hyperpolarization of the recorded patch did not reduce the frequency of channel opening, indicating that the suppression of channel activity by light was due to illumination itself and not to light-induced hyperpolarization of the rod. After recording single light-sensitive channel activity in the intact cell, the patch of membrane was detached to form an excised, inside-out patch. Cyclic GMP applied to the intracellular face of the excised patch then opened a channel that appeared to be the same as the light-sensitive channel recorded earlier from the same membrane in the intact, functioning rod. This provides direct evidence that cyclic GMP acts as an internal transmitter that opens the light-sensitive channel of the vertebrate photoreceptor.

Spread of the light response along the rod outer segment: an estimate from patch-clamp recordings

Longitudinal spread of the light response was studied using cell-attached patch-clamp recordings from the outer segments of isolated rod photoreceptors from toad retina. The effective photon-collecting area over which photoisomerizations influenced the membrane current through the recorded patch of membrane was determined by measuring the probability of a detectable response as a function of flash photon density (i.e. a "frequency-of-seeing" experiment). The effective collecting area averaged 0.40 micron2, which corresponds to a total sampling length of approximately 1 micron for the patch electrode.