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.

Adaptation, Ocular↗

PubMed1975

Quantum sensitivity of rods in the toad retina.

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.

Animals↗

PubMed1975

Interactions of rod and cone signals in the mudpuppy retina.

Interactions between rod and cone signals in mudpuppy retinal neurones were investigated by intracellular recording. 2. The mudpuppy retina contains one kind of rod (lambda max = 525 nm) and one kind of cone (lambda max = 572 nm). The responses of receptors can be distinguished on the basis of their spectral sensitivities. 3. Rod and cone responses have different time courses of recovery and absolute sensitivities. Differences between receptor responses can be used to describe inputs to interneurones. 4. There are two spectral classes of horizontal cells: L-type and C-type. L-type cells are hyperpolarized by rods and cones in varying proportion, with some cells receiving little rod input. C-type cells are hyperpolarized by rods and depolarized by cones. 5. Bipolar cell receptive field centres receive input from cones or from rods and cones. There is no correlation between the spectral properties of centre responses and their polarity. 6. Antagonistic surrounds of bipolar cells show cone or rod and cone sensitivity. They are believed to be generated by the L-type horizontal cells. 7. Some bipolar cells exhibit chromatic interactions between cone signals in the centre and rod signals in the surround, which resemble those observed between the signals of different spectral classes of cones in species known to possess colour discrimination. 8. Amacrine and on-off ganglion cells have L-type responses showing both rod and cone sensitivity. 9. It is proposed that interactions between rod and cone signals observed in mudpuppy also exist in primate retina and are at least partially responsible for certain psychophysical observations of rod-cone interactions.

Animals↗

PubMed1973

Intracellular recordings from single rods and cones in the mudpuppy retina.

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.

Action Potentials↗

PubMedDate not supplied

Functional and morphological differentiation of nonpigmented ciliary body epithelial cells grown on collagen rafts.

We have examined the effect of alteration in cell shape on promoting differentiated morphology and physiology in cultured nonpigmented epithelial cells from the ciliary body. We have grown pure populations of nonpigmented cells on collagen gels released from the culture dish to create collagen rafts. Shortly after the gels were detached, the cells shrank in diameter and increased in height while they contracted the gel. Concurrently, the actin cytoskeleton reorganized to the cell cortex as found in vivo. After this differentiated morphology developed, large changes in intracellular Ca2+ could be elicited by simultaneous activation of acetylcholine and epinephrine or acetylcholine and somatostatin receptors as seen in intact tissue. Explant cultures of isolated nonpigmented cell layers maintained their actin distribution and also showed synergistic Ca2+ increases. Spread cells, grown on rigid substrates, had a disorganized cytoskeleton and rarely showed synergism. These data suggest that the mechanism underlying synergistic Ca2+ responses in the ciliary body is functional in nonpigmented cells grown on collagen rafts. In addition, this pathway appears to be sensitive to the disposition of the cell's cytoarchitecture.

Acetylcholine↗

PubMedDate not supplied

Whole-cell currents activated at nicotinic acetylcholine receptors on ganglion cells isolated from goldfish retina.

We have recorded whole-cell membrane currents in response to exogenously applied acetylcholine (ACh), nicotine, and 1,1 dimethyl-4-phenyl piperazinium iodide on retinal ganglion cells enzymatically dissociated from goldfish retina. Agonist applications induced nicotinic-type responses in a majority of cells when cells were isolated under optimal conditions. Currents were reminiscent of nicotinic-type ganglionic responses. Dose-response measurements of ACh-induced currents indicated an EC50 of 52 microM and a Hill coefficient of 0.6. Currents were selective for Na+ over Cl- and were highly inwardly rectifying. Responses were blocked reversibly by d-tubocurarine, hexamethonium chloride, and N-methyl-D-glucamine. In 50% of the cases, alpha-bungarotoxin reversibly blocked the current induced by ACh application. The blocking action of mecamylamine was irreversible and independent of the presence of agonist but was more effective in the presence of ACh. We conclude that functional nicotinic ACh receptors exist on most goldfish retinal ganglion cells.

Acetylcholine↗

PubMedDate not supplied

Light-dependent calcium release from photoreceptors measured by laser micro-mass analysis.

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.

Animals↗