Pre- and post-synaptic effects of manipulating surface charge with divalent cations at the photoreceptor synapse

Abstract

Persistence of horizontal cell (HC) light responses in extracellular solutions containing low Ca 2+ plus divalent cations to block Ca 2+ currents (I Ca) has been attributed to Ca 2+-independent neurotransmission. Using a retinal slice preparation to record both I Ca and light responses, we demonstrate that persistence of HC responses in low [Ca 2+] o can instead be explained by a paradoxical increase of Ca 2+ influx into photoreceptor terminals arising from surface charge-mediated shifts in I Ca activation. Consistent with this explanation, application of Zn 2+ or Ni 2+ caused a hyperpolarizing block of HC light responses that was relieved by lowering [Ca 2+] o. The same concentrations of Zn 2+ and Ni 2+ reduced the amplitude of I Ca at the rod dark potential and this reduction was relieved by a hyperpolarizing shift in voltage dependence induced by lowering [Ca 2+] o. Block of I Ca by Mg 2+, which has weak surface charge effects, was not relieved by low [Ca 2+] o. Recovery of HC responses in low [Ca 2+] o was assisted by enhancement of rod light responses. To bypass light stimulation, OFF bipolar cells were stimulated by steps to −40 mV applied to presynaptic rods during simultaneous paired recordings. Consistent with surface charge theory, the post-synaptic current was inhibited by Zn 2+ and this inhibition was relieved by lowering [Ca 2+] o. Nominally divalent-free media produced inversion of HC light responses even though rod light responses remained hyperpolarizing; HC response inversion can be explained by surface charge-mediated shifts in I Ca. In summary, HC light responses modifications induced by low divalent cation solutions can be explained by effects on photoreceptor light responses and membrane surface charge without necessitating Ca 2+-independent neurotransmission. Furthermore, these results suggest that surface charge effects accompanying physiological changing divalent cation levels in the synaptic cleft may provide a means for modulating synaptic output from photoreceptors.