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B J Nunn

Publications and source records attributed to B J Nunn.

16 recordsLinked to original sources
PubMed1990

Visual transduction in cones of the monkey Macaca fascicularis.

1. Visual transduction in macaque cones was studied by measuring the membrane current of single outer segments projecting from small pieces of retina. 2. The response to a brief flash of light was diphasic and resembled the output of a bandpass filter with a peak frequency near 5 Hz. After the initial reduction in dark current there was a rebound increase which resulted from an increase in the number of open light-sensitive channels. The response to a step of light consisted of a prominent initial peak followed by a steady phase of smaller amplitude. 3. Responses to dim light were linear and time-invariant, suggesting that responses to single photons were linearly additive. From the flash sensitivity and the effective collecting area the peak amplitude of the single photon response was estimated as about 30 fA. 4. With flashes of increasing strength the photocurrent amplitude usually saturated along a curve that was gentler than an exponential but steeper than a Michaelis relation. The response reached the half-saturating amplitude at roughly 650 photoisomerizations. 5. The response-intensity relation was flatter in the steady state than shortly after a light step was turned on, indicating that bright light desensitized the transduction with a delay. This desensitization was not due to a reduction in pigment content. In the steady state, a background of intensity I lowered the sensitivity to a weak incremental test flash by a factor 1/(1 + I/IO), where IO was about 2.6 x 10(4) photoisomerizations s-1, or about 3.3 log trolands for the red- and green-sensitive cones. 6. Bleaching exposures produced permanent reductions in flash sensitivity but had little effect on the kinetics or saturating amplitude of subsequent flash responses. The sensitivity reductions were consistent with the expected reductions in visual pigment content and gave photosensitivities of about 8 x 10(-9) microns2 (free solution value) for the red- and green-sensitive pigments. During a steady bleaching exposure the final exponential decline of the photocurrent had a rate constant given by the product of the light intensity and the photosensitivity. 7. In some cells it was possible to measure a light-induced increase in current noise. The power spectrum of the noise resembled the spectrum of the dim flash response and the magnitude of the noise was consistent with a single photon response roughly 20 fA in size. 8. The membrane current recorded in darkness was noisy, with a variance near 0.12 pA2 in the band 0-20 Hz.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1988

Spectral sensitivity of primate photoreceptors.

The spectral sensitivities of rods and cones in macaque and human retinas were determined by recording the membrane current from single outer segments. In the macaque retina, the wavelengths of maximum sensitivity were at about 430, 530, and 561 nm for the blue, green, and red cones, respectively, and at 491 nm for the rods. The shapes of the spectra of the three cones were similar when plotted on a log wavenumber scale; the rod spectrum was slightly broader. Spectral sensitivities of the red and green cones from a human retina were virtually identical to those of macaque cones. For comparison with human psychophysical measurements, the rod and cone spectra were adjusted to give the sensitivities expected for light incident on the cornea of the human eye. These functions satisfactorily predicted the scotopic and photopic luminosity functions as well as results from human color-matching experiments. The adjusted spectra of the red and green cones also agreed well with the pi-mechanism of Stiles (1953, 1959).

Adaptation, Ocular↗
PubMed1988

Control of light-sensitive current in salamander rods.

1. The exponential decline of light-sensitive current seen after switch from Na+ to Li+ in the presence of Ca2+ probably depends on the activity of the phosphodiesterase (PDE) which hydrolyses cyclic GMP. 2. This probability is supported by experiments with suction electrodes which show that in toad and salamander rods the rate constant, b, of the exponential decline of current was increased at least 10-fold by moderate light intensities and decreased about 10-fold by 3-isobutyl-1-methylxanthine (IBMX), an inhibitor of PDE. 3. The rate constant b is about 3 times more sensitive to weak lights or to IBMX than the membrane current. This may be explained by a feed-back involving calcium ions which tends to hold current constant, perhaps by calcium inhibition of guanylate cyclase. 4. The time course of b, which probably represents the changes in PDE activity, was measured by switching from Na+ to Li+ at various times after a flash. The results suggest that a moderate flash (140 Rh) increased b about 7 times in 0.5 s and that b then declined with a time constant of 1.5-2 s. 5. Extrapolated values of the parameter b suggest that strong flashes (5000-10,000 Rh) increased b from 1 s-1 in the dark to perhaps 60 s-1 and that b continued to increase with flash strength for several log units after the current had reached saturation. 6. The observations in 4 and 5 fit well with the idea that b is related to PDE activity and that changes in the latter are sufficient to account for the rising phase of the flash response. 7. After a flash the light-sensitive current recovers much more rapidly than the time constant b-1, a discrepancy which is explained if a light flash causes a delayed increase in guanylate cyclase activity. 8. The apparent delayed increase in cyclase activation is consistent with an inhibitory effect of [Ca2+]i which is reduced when calcium is pumped out during the plateau of the response. 9. Experiments in which pulses of IBMX were applied at different times during a flash response support the idea that a flash causes a delayed increase in the rate of supply of cyclic GMP. Quantitative analysis of these and other tests with IBMX gave rate constants similar to those obtained by the Na+----Li+ method.

1-Methyl-3-isobutylxanthine↗
PubMed1987

Spectral sensitivity of cones of the monkey Macaca fascicularis.

1. Spectral sensitivities of cones in the retina of cynomolgus monkeys were determined by recording photocurrents from single outer segments with a suction electrode. 2. The amplitude and shape of the response to a flash depended upon the number of photons absorbed but not the wave-length, so that the 'Principle of Univariance' was obeyed. 3. Spectra were obtained from five 'blue', twenty 'green', and sixteen 'red' cones. The wave-lengths of maximum sensitivity were approximately 430, 531 and 561 nm, respectively. 4. The spectra of the three types of cones had similar shapes when plotted on a log wave number scale, and were fitted by an empirical expression. 5. There was no evidence for the existence of subclasses of cones with different spectral sensitivities. Within a class, the positions of the individual spectra on the wave-length axis showed a standard deviation of less than 1.5 nm. 6. Psychophysical results on human colour matching (Stiles & Burch, 1955; Stiles & Burch, 1959) were well predicted from the spectral sensitivities of the monkey cones. After correction for pre-retinal absorption and pigment self-screening, the spectra of the red and green cones matched the respective pi 5 and pi 4 mechanisms of Stiles (1953, 1959).

Action Potentials↗
PubMed1987

Measurement of sodium-calcium exchange in salamander rods.

1. Methods employing suction electrodes to measure the small inward currents associated with the exchange of internal Ca2+ for external Na+ in salamander rod outer segments are described. 2. The ratio of the integral of the exchange current to the integral of the Ca2+ current during the loading period averaged 0.37, which is consistent with 1 Ca2+ ion exchanging with 2.7 Na+ ions, in approximate agreement with Yau & Nakatani (1984b). 3. The transient pumping current observed when external Na+ was restored after a few seconds in isotonic Ca2+ with IBMX (3-isobutyl-1-methylxanthine) consisted of a phase with current at a constant saturated level followed by a phase in which current declined along a characteristic S-shaped curve that was much steeper than expected from the Michaelis equation. 4. The relation between Ca2+ load and pumping current was also steeper than a Michaelis relation. 5. In Ringer solution at 20 degrees C the saturated exchange current was about 20 pA and the value of charge at which the current was half-saturated was 1-5 pC corresponding to 6-30 X 10(6) Ca2+ ions per rod outer segment. 6. The Ca2+ exchange current after small loads declined along the same curve as that determined with medium loads but fell more slowly after large loads. 7. The exchange current at the beginning of the plateau of a strong flash response usually declined along the curve determined with small or medium Ca2+ loads. 8. There was evidence that the exchange current at the tip of the outer segment remained saturated for longer than at the base. 9. The time to pump out Ca2+ through the Na+-Ca2+ exchange system is largely responsible for the delay in the recovery of the light-sensitive current after a Ca2+ load. 10. A theoretical analysis of some of the observations in this and the succeeding paper is based on assumptions about the binding of Ca2+ by exchange sites and by cytoplasmic Ca2+ buffers.

1-Methyl-3-isobutylxanthine↗
PubMed1987

The effect of ions on sodium-calcium exchange in salamander rods.

1. The influence of external cations on the rate at which a Ca2+ load was eliminated in exchange for external Na+ was studied by measuring the inward current associated with Na+-Ca2+ exchange in salamander rods. 2. In Ringer solution the exchange current saturated at a well-defined level of about 20 pA at 20 degrees C. 3. The saturation level of exchange current, j(sat), was increased by lowering the external concentrations of H+, Ca2+, Mg2+ and K+; it was decreased by raising the external concentration of these ions or by lowering [Na+]O. 4. J(sat) varied approximately as [Na+]O2.4 between 35 and 110 mM-Na+. 5. The inhibitory constants for external Ca2+ and Mg2+ were about 1 and 4 mM, respectively. 6. An acid pH decreased j(sat) and an alkaline one increased it; the shape of the relation between current and pH suggests that one inhibitory proton combines between pH 8 and 10 and a pair combine between pH 6 and 7. 7. Removing K+, Mg2+, and Ca2+, and increasing the pH from 7.5 to 10 increased the measured exchange current from 20 to ca. 100 pA. 8. The integral of the Na+-Ca2+ exchange current varied with the Ca2+ load but was largely independent of external ionic changes in spite of large changes in j(sat). The apparent Na+-Ca2+ exchange ratio remained at a little under 3 over a wide range of conditions. 9. The constancy of the integral of the exchange current was brought about by reciprocal variations of the amplitude and duration of the current transient. Records in different solutions could usually be matched by scaling amplitude and time by reciprocal factors. 10. Increasing Nai+ by allowing large light-sensitive currents to flow in low-Ca2+ solutions affected the Na+-Ca2+ exchange transient in a different way from lowering [Na+]o or raising [Ca2+]o, etc. In an Na+-rich rod there was little reduction in j(sat) but the response was prolonged and larger Ca2+ loads were needed to reach saturation. Analysis in terms of a simple model indicated that a substantial Na+ load might reduce the apparent affinity of the internal pumping sites for Ca2+ by a factor of 10. 11. An attempt is made to relate these findings to a model of Na+-Ca2+ exchange.

Action Potentials↗
PubMed1986

Electrical properties of the light-sensitive conductance of rods of the salamander Ambystoma tigrinum.

The light-sensitive conductance of isolated rods from the retina of the tiger salamander was studied using a voltage-clamp method. The membrane current of the outer segment was collected with a suction electrode while the internal voltage was measured and controlled with a pair of intracellular electrodes. Saturating light blocked the outer segment current at all potentials, the residual conductance usually becoming less than 20 pS. This suggests that light-sensitive channels comprise the main ionic conductance in the surface membrane of the outer segment. Current-voltage relations determined 10-40 ms after changing the voltage showed outward-going rectification, the outward current increasing e-fold for a depolarization of 11-14 mV. The reversal potential of the light-sensitive current was estimated as 5 +/- 4 mV. This is consistent with other evidence indicating that the channel is not exclusively permeable to Na. Applying steady light, lowering external Ca, or changing the intracellular voltage to a new steady level scaled the light-sensitive current without altering the reversal potential or the form of the rectification. This suggests that all three manipulations change the number of channels in the conducting state without changing the ionic concentration gradients or the mechanism of permeation through an 'open' channel. Hyperpolarizing voltage steps slowly increased the light-sensitive current and depolarizing steps reduced it. A gating variable Y expressing the fractional activation of the light-sensitive conductance in the steady state was derived from the ratio of the instantaneous and steady-state currents. Y declined at voltages positive to -100 mV and usually reached a minimum near 0 mV, with a secondary rise positive to 0 mV. Around the dark voltage Y changed e-fold in roughly 25 mV. The voltage-dependent gating in (6). appeared to involve two delays similar in magnitude to those of the four principal delays in the rod's response to a dim flash. Steady background light shortened the time-scale of gating and flash responses to a similar degree. Clamping the voltage at the dark level had little effect on the photocurrent evoked by a flash. The small, delayed effect actually observed is explained by the slow voltage-dependent gating of the light-sensitive conductance. Hyperpolarization had little effect on the kinetics of the response to a flash, but depolarization slowed the response, causing it to reach a larger, later peak. Depolarization also prolonged the blockage of the light-sensitive current after a saturating flash.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1985

The ionic selectivity and calcium dependence of the light-sensitive pathway in toad rods.

A new method is described for determining the effects of rapid changes in ionic concentration on the light-sensitive currents of rod outer segments. Replacing Na with another monovalent cation caused a rapid change in current followed by an exponential decline of time constant 0.5-2 s. From the magnitude of the initial rapid change in current we conclude that Li, Na, and K and Rb ions pass readily through the light-sensitive channel in the presence of 1 mM-Ca, whereas Cs crosses with difficulty and choline, tetramethylammonium and tetraethylammonium not at all. The effect of reducing Ca in the external medium indicates that the residual inward current recorded for a few seconds when Na is replaced by an impermeant ion is carried largely by Ca ions. With 1 microM-Ca in the external medium the relative ability of monovalent cations to carry light-sensitive current is Li:Na:K:Rb:Cs = 1.4:1:0.8:0.6:0.15. The same order applied in the physiological region but the values are less certain. Large transient inward currents are seen if external Ca is raised form 1 microM to 5 mM or more; these currents which are maximal in an isotonic Ca solution are presumably carried by Ca. The effect of monovalent cations on the number of open light-sensitive channels was tested by adding the cation to a solution containing 55 mM-Na. Na ions open light-sensitive channels with a delay, probably by promoting Na-Ca exchange; K and Rb close channels by inhibiting exchange; Li and Cs seem inert in the exchange mechanism. The rate at which inward current declines in low [Na]o or high [Ca]o is accelerated by weak background lights and slowed by 3-isobutyl-1-methylxanthine (IBMX), which inhibits the hydrolysis of cGMP. On returning to Ringer solution after a period in low [Na]o the current recovers with a delay of about 1 s which decreases as the Ca concentration of the low [Na]o medium is reduced. We conclude that intracellular Ca has a strong effect on the number of open light-sensitive channels. None the less, several observations are inconsistent with channel closure being dependent simply on combination with internal Ca.

Action Potentials↗
PubMed1984

Effect of ions on retinal rods from Bufo marinus.

The effect of ions on the light-sensitive current of isolated retinal rods from the toad Bufo marinus was studied by sucking the inner segment into a tightly fitting pipette. The outer segment projected into flowing solution whose composition could be changed rapidly. Reducing the external Na concentration, [Na]o, round the outer segment caused rapid and reversible reductions in the light-sensitive current. With the outer segment in the pipette, reductions of [Na]o round the inner segment had little effect on the light-sensitive current. The current about 15 s after a change in [Na]o was approximately proportional to [Na]2o. The current decreased in elevated external Ca concentration, [Ca]o, and increased in reduced [Ca]o. Between 10 and 0.5 mM-external Ca the current 15 s after a change was approximately inversely proportional to [Ca]o. Reducing [Ca]o from 1 mM to 1 microM or less transiently increased the current by about 15-fold. After a change in [Na]o or [Ca]o the current did not approach its final value monotonically but with a characteristic overshoot or underswing, followed by a slow relaxation of current which may reflect the time course of change in internal Na. Reducing [Na]o from 110 to 70 mM or less prolonged the response to a flash; very long responses were observed in solutions containing Li rather than Na and also in rods that had been returned to Ringer solution after exposure to low Ca. All these effects might be explained if Ca extrusion in exchange for Na determines the reactivation of current after a flash. The rod current was not changed if the ratio [Na]No/[Ca]o was held constant, N being about 2.5. Between 5 mM and 10 microM-Ca the change in peak current produced by absorption of a single quantum was roughly proportional to the dark current. Responses in the absence of external Na were not normally seen if the solution contained 0.1 mM-Ca or more. Responses of normal polarity were regularly observed in 0 Na, 0 Ca EGTA solutions containing 1.6 mM-Mg. Removal of Mg from such solutions gave inverted responses. Other conditions which promote responses of normal and inverted polarity in Na-free solutions are described briefly. We conclude that Li, Ca, Mg and perhaps K can pass through the light-sensitive channel. The above results suggest that external Na has two distinct effects: (1) it provides ions to carry inward current, and (2) it keeps the light-sensitive conductance open by maintaining the internal Ca concentration, [Ca]i, at a low level.

1-Methyl-3-isobutylxanthine↗
PubMed1984

Location and function of voltage-sensitive conductances in retinal rods of the salamander, Ambystoma tigrinum.

The functional role and spatial location of voltage-sensitive conductances that modify the light-evoked electrical signals were studied in retinal rods of the tiger salamander. An isolated rod was drawn into a suction electrode for recording membrane current and impaled with an intracellular electrode for recording membrane potential and passing current. A bright flash gave a rapid initial hyperpolarization that relaxed to a smaller plateau. Simultaneously the dark current of the outer segment was shut off with the time course of a rounded step function. This characteristic difference between the wave forms demonstrates that the voltage relaxation does not result from reopening of light-sensitive channels. The voltage relaxation in (2) did not require light or interruption of the dark current, as the wave form was duplicated by suddenly switching off a depolarizing current injected during steady saturating light. This is explained if the relaxation depends purely on voltage-sensitive conductances. The voltage response to a dim flash reached its peak value before the current response. The voltage wave form was predicted assuming that the recorded photo-current drove a linear high-pass filter with parameters derived from analysis of the voltage response to injection of a current step. When the intracellular voltage was changed by current injection the slope resistance of the outer segment slowly declined to a lower level, indicating that the outer segment contains a voltage-sensitive conductance. When a current step was injected in bright steady light, the current recorded from the outer segment consisted of a capacity component proportional to dV/dt and a small extracellular leakage current but no detectable ionic current. This supports other evidence indicating that light-sensitive channels comprise the main or exclusive ionic conductance of the outer segment. The behaviour in (5) is explained if the light-sensitive channels themselves are slowly opened by hyperpolarization and closed by depolarization. Analysis of the current-injection experiments suggests that most of the high-pass filtering in a rod results from the action of voltage-sensitive conductances located in the inner segment. Addition of 10 mM-CsCl to the Ringer solution abolished the relaxation in the voltage response to a bright flash but left intact the high-pass filtering of small signals. This would be explained by a selective block of one of two sets of voltage-sensitive channels in the inner segment or by a voltage-sensitive block of one kind of channel.

Ambystoma↗
PubMed1984

The photocurrent, noise and spectral sensitivity of rods of the monkey Macaca fascicularis.

Visual transduction in rods of the cynomolgus monkey, Macaca fascicularis, was studied by recording membrane current from single outer segments projecting from small pieces of retina. Light flashes evoked transient outward-going photocurrents with saturating amplitudes of up to 34 pA. A flash causing twenty to fifty photoisomerizations gave a response of half the saturating amplitude. The response-stimulus relation was of the form 1-e-x where x is flash strength. The response to a dim flash usually had a time to peak of 150-250 ms and resembled the impulse response of a series of six low-pass filters. From the average spectral sensitivity of ten rods the rhodopsin was estimated to have a peak absorption near 491 nm. The spectral sensitivity of the rods was in good agreement with the average human scotopic visibility curve determined by Crawford (1949), when the human curve was corrected for lens absorption and self-screening of rhodopsin. Fluctuations in the photocurrent evoked by dim lights were consistent with a quantal event about 0.7 pA in peak amplitude. A steady light causing about 100 photoisomerizations s-1 reduced the flash sensitivity to half the dark-adapted value. At higher background levels the rod rapidly saturated. These results support the idea that dim background light desensitizes human scotopic vision by a mechanism central to the rod outer segments while scotopic saturation may occur within the outer segments. Recovery of the photocurrent after bright flashes was marked by quantized step-like events. The events had the properties expected if bleached rhodopsin in the disks occasionally caused an abrupt blockage of the dark current over about one-twentieth of the length of the outer segment. It is suggested that superposition of these events after bleaching may contribute to the threshold elevation measured psychophysically. The current in darkness showed random fluctuations which disappeared in bright light. The continuous component of the noise had a variance of about 0.03 pA2 and a power spectrum that fell to half near 3 Hz. A second component, consisting of discrete events resembling single-photon responses, was estimated to occur at a rate of 0.006 s-1. It is suggested that the continuous component of the noise may be removed from scotopic vision by a thresholding operation near the rod output.

Animals↗
PubMedDate not supplied

Spectral sensitivity of single cones in the retina of Macaca fascicularis.

Colour vision depends on the wavelength-dependent absorptions of three different photolabile pigments each located in a particular type of retinal cone. The spectral absorption of primate cones has been examined by microspectrophotometry, but this method gives information only at wavelengths where the absorption is relatively strong. Here we have analysed the absorption of two of the cones over a wider range of wavelengths by recording their electrical responses to monochromatic light. The observations were made on the retina of the monkey Macaca fascicularis, an animal thought to have cone pigments like those of man. The measured spectral sensitivities of the red-sensitive ('red') and green-sensitive ('green') cones agreed well with estimates of the cone pigment absorptions derived from colour-matching experiments in humans. At long wavelengths the sensitivity of the red cones was found to decline more rapidly than that of the green. This behaviour, attributable to the cone pigment molecules themselves, explains the paradoxical hue shift, in which a light of very long wavelength is perceived to be identical to a light of shorter wavelength.

Action Potentials↗