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L. Cadetti, W.B. Thoreson, M. Piccolino; Reducing Extracelluar Ca2+ Enhances Chemical Transmitter Release From Photoreceptors to Horizontal Cells in Slices of Salamander Retina . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4173.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: : Using a retinal slice preparation in which both Ca2+ currents (ICa) and light responses could be recorded, we investigated whether horizontal cell (HC) response modifications induced by low Ca2+ media might be accounted for by a paradoxical increase of Ca2+ influx into photoreceptor synaptic terminals due to a surface charge-mediated shift in ICa activation (see Piccolino et al, PNAS, 1996). Methods: HC light responses as well as light responses and ICa of rods were recorded using perforated patch whole cell recording techniques in retinal slices from Ambystoma tigrinum salamanders prepared under infrared illumination. Results: Upon application of low Ca2+ media (lacking Mg2+) HCs depolarized and their light responses were progressively blocked. In the same conditions, rods depolarized and their light responses increased. Although in low Ca2+ media the peak amplitude of rod ICa was reduced, the amplitude of ICa at the dark potential increased because of both a hyperpolarizing shift of the I-V curve and membrane depolarization. Perfusion with media containing Zn2+ or Ni2+ caused a hyperpolarizing block of HC light responses that was relieved by low Ca2+ media. In rods, Zn2+ and Ni2+ reduced ICa amplitude at the dark potential because of both a depolarizing shift of the I-V curve and a reduction in peak amplitude. Lowering [Ca2+]o in the presence of Zn2+ or Ni2+ induced a hyperpolarizing shift of the I-V curve and thus restored the amplitude of ICa at the dark membrane potential toward control values. Block of ICa and HC light responses by Cd2+ or Mg2+ was also relieved, although less effectively, by lowering [Ca2+]o. Inversion of HC light response polarity was occasionally observed in low Ca2+ media containing Mg2+ or Cd2+; this may also be accounted for by a surface charge-mediated shift of the Ca2+ I-V curve (see Piccolino et al Eur. J. Neurosci. 1999). Conclusions: These results indicate that photoreceptor transmitter release may be enhanced by low Ca2+ media through a surface charge-mediated shift of the ICa I-V curve. Low Ca2+-induced enhancement of photoreceptor transmitter release may contribute to both the depolarizing block of HC light responses observed in media with no other divalent cations and to the relieving action of low Ca2+ media on the hyperpolarizing block induced by Zn2+ or Ni2+. The weaker relief of block with Mg2+ or Cd2+ by low Ca2+ media is consistent with the suggestion that these divalent cations have stronger channel blocking actions and weaker surface charge effects.
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