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Contribution of voltage-dependent conductances to the responses of rods
Part 1: Abstrucr~
336
response to light were studied by the voltage-clamp technique. The waveform of the light-induced diffcrrd from that of the voltage responses recorded in the same cell.
P. B. DETWILER.
A. L. HODGKIN
and I’. A. MCNAUGHTOS
currents
(Cambridge)
The spread of voltage and desensitization in the network of rods in the turtle’s retina
Flashing a localized stimulus on to a turtle’s retina produces an intracellular potential wave which spreads. by means of electrical connections. from illuminated to unilluminated rods. The response in unilluminated rods becomes faster as the distance from the source increases. perhaps because voltage-de~ndent conductance changes in the rod membrane make the coupled network behave like a high-pass filter. Local illumination also produces desensitization. which spreads much less far than the electrical signal and may be localized to those rods receiving illumination. G. I,. FAIN
(UCLA)
retribution
of voltag~~~t
co~nctan~
to the responses of rods
The effects of specific neurotoxins and blocking agents on the intraLrllular responses of toad rods reveal the presence in rod membrane of at least three distinct voltage-dependent conductances: a calcium conductance similar in its pharmacological properties to the yc, of arthropod muscle or presynaptic membrane: an outward rectifier probably selective for K i and blocked by TEA; and an inward rectifier blocked by Cs’ but (unlike that of neurons or muscles) having significant permeability to Na’. The Cs’-sensitive sodium permeability is pharm;~coiogi~~lly distinguishable from the lint-de~ndent sodium conductance. and it is activated by membrane hyperpol~rization to form the rapid decay (initial transient) of the rod response to bright light.
THE AMPLITUDE vs INTENSITY FUNCTION OF VERTEBRATE PHOTORECEPTORS R. A. &OR&JIANN (NIH. Bethesda) and I. PERLMAN *Evaluaring
sensitivity changing mechanisms in light-adapted
photoreceptors
The desensitizing effects of continuous background illumination have been extensively studied in vertebrate photoreceptors. However. the different techniques of analyzing the data have resulted in some dispute concerning the relative contriblltion of response compression and cellular adaptation (curve shifting) to thetotal lossofreceptor sensitivity. In this paper wediscuss how plotting light adaptation data as increment response vs log flash intensity can lead to overestimates in the role of cellular adaptation. Further. we propose a method for data plotting which can be used for a reliable determination of the contributions of each mechanism to photoreceptor desensitization produced by background illumination This procedure is then applied to data obtained from intracellular recordings in cones in the turtle retina. K. T. BROWX (UC. San Francisco) and D. G. FLAMING *Effects of Ba’* upon the dark adapted intensity-response curve of toad rods
Intracellular recording was conducted in outer segments of red rods in the isolated. inverted and superfused retina of the toad. Bu,fo runrirm~. Barium has been reported to be concentrated in the outer and inner segments of cat photoreceptors. so the physiologicaf effects of Ba’+ are of special interest. Dark adapted t-log I curves was increased from 0.0 to were obtained and fitted by the equation K’I:,,,,, = P,‘(I” - a”). When [Ba”]” 2.0mM. typical results showed an increase of 30-SO?; in E,_. while G decreased about 0.7 log units, and the value of tt increased from about 0.65 to 0.90. These elIects improve both flash sensitivity and intensity discrimination. If these effects are mediated by altering [Ba’-1,. then our results suggest that the resting value of [Ba”], is a critical factor in controllin g the response properties of vertebrate photoreceptors. K. 0. DONNER (Helsinkil. S. HEMILA and T. REUTER *Bleaching
and background adaptation in frog rods
The main effects on the stimulus-response curve of the rod receptor potential during bleaching adaptation after small bleaches and with backgrounds, are lateral shifts of the curve along the log1 axis. Both opsin and intermediate adaptation appear to affect the transduction mechanism in the same way. This provides further support to the hypothesis that these two adaptation processes are basically of identical origin. An adaptation of the kind described could be due to changes in the amount of transmitter reieased from the discs per molecule of rhodopsin isomerized. Experiments using inhomogeneous bleaching of the outer segments have been carried out to test the possible occurrence of local effects of adaptation.
336
response to light were studied by the voltage-clamp technique. The waveform of the light-induced diffcrrd from that of the voltage responses recorded in the same cell.
P. B. DETWILER.
A. L. HODGKIN
and I’. A. MCNAUGHTOS
currents
(Cambridge)
The spread of voltage and desensitization in the network of rods in the turtle’s retina
Flashing a localized stimulus on to a turtle’s retina produces an intracellular potential wave which spreads. by means of electrical connections. from illuminated to unilluminated rods. The response in unilluminated rods becomes faster as the distance from the source increases. perhaps because voltage-de~ndent conductance changes in the rod membrane make the coupled network behave like a high-pass filter. Local illumination also produces desensitization. which spreads much less far than the electrical signal and may be localized to those rods receiving illumination. G. I,. FAIN
(UCLA)
retribution
of voltag~~~t
co~nctan~
to the responses of rods
The effects of specific neurotoxins and blocking agents on the intraLrllular responses of toad rods reveal the presence in rod membrane of at least three distinct voltage-dependent conductances: a calcium conductance similar in its pharmacological properties to the yc, of arthropod muscle or presynaptic membrane: an outward rectifier probably selective for K i and blocked by TEA; and an inward rectifier blocked by Cs’ but (unlike that of neurons or muscles) having significant permeability to Na’. The Cs’-sensitive sodium permeability is pharm;~coiogi~~lly distinguishable from the lint-de~ndent sodium conductance. and it is activated by membrane hyperpol~rization to form the rapid decay (initial transient) of the rod response to bright light.
THE AMPLITUDE vs INTENSITY FUNCTION OF VERTEBRATE PHOTORECEPTORS R. A. &OR&JIANN (NIH. Bethesda) and I. PERLMAN *Evaluaring
sensitivity changing mechanisms in light-adapted
photoreceptors
The desensitizing effects of continuous background illumination have been extensively studied in vertebrate photoreceptors. However. the different techniques of analyzing the data have resulted in some dispute concerning the relative contriblltion of response compression and cellular adaptation (curve shifting) to thetotal lossofreceptor sensitivity. In this paper wediscuss how plotting light adaptation data as increment response vs log flash intensity can lead to overestimates in the role of cellular adaptation. Further. we propose a method for data plotting which can be used for a reliable determination of the contributions of each mechanism to photoreceptor desensitization produced by background illumination This procedure is then applied to data obtained from intracellular recordings in cones in the turtle retina. K. T. BROWX (UC. San Francisco) and D. G. FLAMING *Effects of Ba’* upon the dark adapted intensity-response curve of toad rods
Intracellular recording was conducted in outer segments of red rods in the isolated. inverted and superfused retina of the toad. Bu,fo runrirm~. Barium has been reported to be concentrated in the outer and inner segments of cat photoreceptors. so the physiologicaf effects of Ba’+ are of special interest. Dark adapted t-log I curves was increased from 0.0 to were obtained and fitted by the equation K’I:,,,,, = P,‘(I” - a”). When [Ba”]” 2.0mM. typical results showed an increase of 30-SO?; in E,_. while G decreased about 0.7 log units, and the value of tt increased from about 0.65 to 0.90. These elIects improve both flash sensitivity and intensity discrimination. If these effects are mediated by altering [Ba’-1,. then our results suggest that the resting value of [Ba”], is a critical factor in controllin g the response properties of vertebrate photoreceptors. K. 0. DONNER (Helsinkil. S. HEMILA and T. REUTER *Bleaching
and background adaptation in frog rods
The main effects on the stimulus-response curve of the rod receptor potential during bleaching adaptation after small bleaches and with backgrounds, are lateral shifts of the curve along the log1 axis. Both opsin and intermediate adaptation appear to affect the transduction mechanism in the same way. This provides further support to the hypothesis that these two adaptation processes are basically of identical origin. An adaptation of the kind described could be due to changes in the amount of transmitter reieased from the discs per molecule of rhodopsin isomerized. Experiments using inhomogeneous bleaching of the outer segments have been carried out to test the possible occurrence of local effects of adaptation.