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Synaptic transmission between photoreceptors and horizontal cells in the turtle retina
No full textLow calcium, high magnesium, and cobalt hyperpolarize the horizontal cell membrane and suppress the response to light, but only partially affect the response of receptor cells. These observations are consistent with the interpretation that a depolarizing transmitter is released by photoreceptors in darkness. The hyperpolarizing response to light of the horizontal cells would then result from a reduction in the amount of transmitter released
The modulation of the ionic selectivity of the light-sensitive current in isolated rods of the tiger salamander.
No full text1. By using the method of Hodgkin, McNaughton & Nunn (1985) for rapidly changing the extracellular medium, we analysed the effect of the organic compound IBMX (3-isobutyl-1-methylxanthine) on the movement of divalent cations through the light-sensitive channels of isolated retinal rods of the tiger salamander. 2. When the rod is treated with 0.5 mM-IBMX it is possible to observe photocurrents larger than 50 pA carried by Ba2+, Sr2+, Ca2+, Mg2+ and Mn2+. Under these conditions Ca2+, Mg2+ and Mn2+ carry photocurrents of similar amplitude, while Ba2+ and Sr2+ usually carry larger photocurrents. 3. The movement of Mn2+ through the light-sensitive channel, which is hardly detected under normal conditions, can also be observed after treating the rod for a few seconds with a solution containing 35 mM[Na+]o and 10(-7) M[Ca2+]o. Under these conditions the photocurrent carried by Mn2+ is fully saturated in the presence of 1 mM-extracellular Mn2+. 4. When the rod is pre-treated with an extracellular solution containing 0.5 mM-IBMX the maximal photocurrent which can be carried by 10 mM [Ca2+]o increases from about 10 pA to approximately 200 pA. In these conditions the half-activation of the Ca2+ current is between 1 and 10 mM, that is 20-50 times higher than in normal conditions (Menini, Rispoli & Torre, 1988). 5. When the rod is pre-treated with an extracellular solution containing 0.5 mM-IBMX the half-activation of the photocurrent which can be carried by Mg2+, Ba2+ and Sr2+ is equivalent to or greater than 10 mM. In the absence of pre-treatment with IBMX the half-activation of the photocurrent carried by Mg2+, Ba2+ and Sr2+ is less than 5 mM. 6. We conclude that the light-sensitive channel can exist in at least two distinct open states. The selectivity of the channel in the first open state is as described in a previous paper (Menini et al. 1988). Mn2+, which is hardly permeable through the light-sensitive channel in the first open state, can move through the light-sensitive channel in the second open state. Ca2+, Mg2+, Ba2+ and Sr2+ permeate more freely through the light-sensitive channel in the second open state, probably because the electrostatic interactions between these ions and the channel are less strong
Ionic movements through light‐sensitive channels of toad rods
No full textElectrical photoresponses of rods in the isolated toad retina were recorded during ionic manipulations of the Na+‐free extracellular medium. In the presence of a concentration of external Ca2+ above 10(‐5) M, voltage photoresponses were observed only in the presence of external Na+ or Li+. When external Ca2+ was reduced below 10(‐6) M, voltage photoresponses of normal polarity could be detected even in the absence of Na+ or Li+, but in the presence of external Mg2+. In the presence of normal extracellular Ca2+ hyperpolarizing photoresponses were observed even in the absence of Na+ or Li+, provided small amounts of phosphodiesterase inhibitors (IBMX, RO 20‐1724, papaverine, caffeine, theophylline) were added to the perfusate. Responses obtained in low‐Na+ IBMX solutions required the presence of millimolar amounts of a variety of divalent cations, among which Mn2+ and Ba2+ were the most effective. When the concentration of both external Ca2+ and Mg2+ was reduced to micromolar amounts, depolarizing photoresponses were observed. In these conditions measurements with radioactive tracers showed a light‐modulated efflux of 42K+ or 86Rb+. The light‐modulated 42K+ or 86Rb+ efflux was halved by 2 X 6 mM‐external K+ and was completely blocked when K+ was raised above 10 mM. These results show that ionic movements through light‐sensitive channels are controlled by Ca2+ and Mg2+ and possibly also be the intracellular level of cyclic nucleotides. Moreover, the movement of ions through the light‐sensitive channel, does not obey the independence principle. © 1983 The Physiological Societ
The impact of organic inhibitors of the hyperpolarization activated current (Ih) on the electroretinogram (ERG) of rodents
No full textMechanisms of light adaptation in toad rods.
No full textA steady illumination equivalent to about 1000 or more photoisomerizations per s per rod completely abolishes the dark current and induces a total desensitization of toad rods. Under these conditions, however, both current and sensitivity recover from the initial suppression and reach a new steady level within about 30 s. Both extent and time course of recovery depend on the intensity of the conditioning light. The ability of rods to respond to light stimuli under light saturating conditions is enhanced by procedures aimed at increasing the sodium gradient across their surface membrane. The results suggest that rods can also contribute significantly to vision at photopic levels of ambient light. The mechanisms underlying this adaptational property of rods are discussed
bFGF suppresses the b-wave of the electroretinogram: evidence of the mechanism of retina self-protection
No full textThe modulation of the ionic selectivity of the light-sensitive current in isolated rods of the tiger salamander.
No full textEffects of bFGF on the electroretinogram: relation between photoreceptor protection and retinal sensitivity
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