Reducing Extracelluar Ca2+ Enhances Chemical Transmitter Release From Photoreceptors to Horizontal Cells in Slices of Salamander Retina

Abstract

Abstract: : Purpose: : Using a retinal slice preparation in which both Ca²⁺ currents (I_Ca) 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 I_Ca activation (see Piccolino et al, PNAS, 1996). Methods: HC light responses as well as light responses and I_Ca 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 I_Ca was reduced, the amplitude of I_Ca 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 I_Ca 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 I_Ca at the dark membrane potential toward control values. Block of I_Ca 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 I_Ca 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.

Keywords: retina: distal(photoreceptors, horizontal cell • calcium • synapse