Reducing extracellular Cl- suppresses dihydropyridine-sensitive Ca2+ currents and synaptic transmission in amphibian photoreceptors (original) (raw)
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The Journal of General Physiology, 1996
Removal of extracellular Cl- has been shown to suppress light-evoked voltage responses of ON bipolar and horizontal cells, but not photoreceptors or OFF bipolar cells, in the amphibian retina. A substantial amount of experimental evidence has demonstrated that the photoreceptor transmitter, L-glutamate, activates cation, not Cl-, channels in these cells. The mechanism for Cl-free effects was therefore reexamined in a superfused retinal slice preparation from the mudpuppy (Necturus maculosus) using whole-cell voltage and current clamp techniques. In a Cl-free medium, light-evoked currents were maintained in rod and cone photoreceptors but suppressed in horizontal, ON bipolar, and OFF bipolar cells. Changes in input resistance and dark current in bipolar and horizontal cells were consistent with the hypothesis that removal of Cl- suppresses tonic glutamate release from photoreceptors. The persistence of light-evoked voltage responses in OFF bipolar cells, despite the suppression of li...
Intracellular pH modulates inner segment calcium homeostasis in vertebrate photoreceptors
AJP: Cell Physiology, 2011
Neuronal metabolic and electrical activity is associated with shifts in intracellular pH (pHi) proton activity and state-dependent changes in activation of signaling pathways in the plasma membrane, cytosol, and intracellular compartments. We investigated interactions between two intracellular messenger ions, protons and calcium (Ca2+), in salamander photoreceptor inner segments loaded with Ca2+ and pH indicator dyes. Resting cytosolic pH in rods and cones in HEPES-based saline was acidified by ∼0.4 pH units with respect to pH of the superfusing saline (pH = 7.6), indicating that dissociated inner segments experience continuous acid loading. Cytosolic alkalinization with ammonium chloride (NH4Cl) depolarized photoreceptors and stimulated Ca2+ release from internal stores, yet paradoxically also evoked dose-dependent, reversible decreases in [Ca2+]i. Alkalinization-evoked [Ca2+]i decreases were independent of voltage-operated and store-operated Ca2+ entry, plasma membrane Ca2+ extrus...
Proceedings of the National Academy of Sciences, 1996
According to the classical calcium hypothesis of synaptic transmission, the release of neurotransmitter from presynaptic terminals occurs through an exocytotic process triggered by depolarization-induced presynaptic calcium influx. However, evidence has been accumulating in the last two decades indicating that, in many preparations, synaptic transmitter release can persist or even increase when calcium is omitted from the perfusing saline, leading to the notion of a "calcium-independent release" mechanism. Our study shows that the enhancement of synaptic transmission between photoreceptors and horizontal cells of the vertebrate retina induced by low-calcium media is caused by an increase of calcium influx into presynaptic terminals. This paradoxical effect is accounted for by modifications of surface potential on the photoreceptor membrane. Since lowering extracellular calcium concentration may likewise enhance calcium influx into other nerve cells, other experimental obse...
Dependence of photoreceptor glutamate release on a dihydropyridine-sensitive calcium channel
Neuroscience, 1997
A "reduced retina" preparation, consisting of the photoreceptor layer attached to the pigment epithelium in the eyecup, was used to study the pharmacology of the calcium channels controlling glutamate release by photoreceptors in Xenopus. Glutamate release was evoked either by dark adaptation or by superfusion with elevated (20 mM) potassium medium. Both darkness- and potassium-induced release were blocked by cadmium (200 microM). The N-type calcium channel blocker, omega-conotoxin GVIA (500 nM), the P-type calcium channel blocker, omega-agatoxin IVA (20 nM), and the P- and Q-type channel blocker omega-conotoxin MVIIC (1 microM) had no effect on glutamate release. In contrast, the dihydropyridines, nifedipine (10 microM) and nitrendipine (10 microM), which affect L-type calcium channels, blocked both darkness- and potassium-induced release. Bay K 8644 (10 microM), which promotes the open state of L-type calcium channels, enhanced glutamate release. These results indicate t...
European Journal of Neuroscience, 2002
L-type Ca 2+ currents (I Ca) in rod photoreceptors exhibit Ca 2+-dependent inactivation. Perforated-patch whole-cell recordings were obtained from isolated rods of the tiger salamander using 1.8 mM Ca 2+ in the bathing medium to determine the extent of Ca 2+dependent inactivation of I Ca with physiological [Ca 2+ ] and endogenous buffering. I Ca was measured with voltage ramps applied before and after 5-s steps to ±40, ±30, ±20, or ±10 mV. Long depolarizing steps in isolated rods produced inactivation of I Ca ranging from 15% at ±40 mV to > 80% at ±10 mV. Because, in addition to Ca 2+-dependent inactivation, depletion of synaptic cleft Ca 2+ accompanying activation of I Ca can reduce presynaptic I Ca at calycal synapses, we investigated whether a similar mechanism worked at the invaginating rod synapse. Rods from retinal slices with intact synapses were compared with isolated rods in which synaptic cleft depletion is absent. I Ca was more strongly depressed by depolarization of rods in retinal slices, with I Ca reduced by 47% following voltage steps to ±40 mV. The depression of currents by depolarization was also greater for rods from retinal slices than isolated rods when Ca 2+ was replaced with Ba 2+ to reduce Ca 2+-dependent inactivation. The stronger depolarization-evoked inhibition of I Ca in retinal slices compared to isolated rods probably re¯ects depletion of synaptic cleft Ca 2+ arising from sustained Ca 2+ in¯ux. Inactivation of I Ca exhibited slow onset and recovery. These ®ndings suggest that Ca 2+dependent inactivation and depletion of synaptic cleft Ca 2+ may combine to regulate I Ca in response to light-evoked changes in rod membrane potential.
Journal of neurophysiology, 1999
Ion-sensitive microelectrodes were used to measure the variations of [Ca2+]o induced by application of low Ca2+ media in the superfused eyecup preparation of the Pseudemys turtle. The aim of the experiments was to evaluate the possibility, suggested by previous studies, that in the deep, sclerad, layers of the retina [Ca2+]o may remain high enough to sustain chemical synaptic transmission even after prolonged application of low-Ca2+ saline. It was found that, at depths of 100-200 micron from the vitreal surface, [Ca2+ ]o did not fall below 1 mM even after application for periods of 30-60 min of nominally Ca2+-free media, and it was >0.3 mM after 30-min application of media containing EGTA and with a Ca2+ concentration of 1 nM. Previous studies in isolated salamander photoreceptors have shown that a reduction of [Ca2+ ]o to 0.3-1.0 mM may result in a paradoxical increase of Ca2+ influx into synaptic terminals due to the reduced screening of negative charge on the external face of ...
Molecular vision, 1999
Retinal photoreceptor cells are tonically depolarized in darkness. Ca2+ influx in darkness plays a critical role in the regulation of neurotransmitter release and melatonin synthesis in these sensory cells. The purpose of the present study was to examine the dynamic changes of intracellular Ca2+ concentrations ([Ca2+]in ) in response to a tonic depolarizing stimulus and to determine the role of dihydropyridine-sensitive calcium channels in the response. Photoreceptor cells were prepared from embryonic chick retina and cultured for 6-12 days. Cells were depolarized by exposure to 35 mM extracellular K+. [Ca2+]in of individual photoreceptor cell bodies/synaptic terminals was determined by ratiometric fura-2 image analysis. Chemical depolarization with 35 mM [K+]out greatly increased [Ca2+]in of inner segment/synaptic terminal regions of photoreceptors. The increase usually reached a plateau after the first few minutes of stimulation and was sustained for prolonged periods (>2 h) in...