Differential effects of low calcium on signal transmission from rods and cones to horizontal cells in carp retina

Intracellular recordings were made from neurons in the superfused isolated carp retina and the effects of calcium on synaptic transmission from cones and rods to horizontal cells were studied by changing the ionic composition of the external medium. When extracellular Ca²⁺ is lowered from 1.2 mmol/L to 0.1 mmol/L, both cone- and rod-horizontal cells (Cone-HC and Rod-HC are depolarized, but light responsiveness of these cells are differentially affected: light responses of Cone-HCs are reduced in size, whereas those of Rod-HCs are significantly enhanced. In 0.1 mmol/L Ca²⁺, glutamate-isolated P III components, both phototopic and scotopic, are enhanced, suggesting that the differential effects of low calcium on the horizontal cells must occur during signal transferring from photoreceptors to horizontal cells. Similar to the effect of 0.1 mmol/L Ca²⁺, phosphodiesterase inhibitor IBMX (50 μmol/L) is also capable of enhancing light responsiveness of both L-type Cone-HC and Rod-HC. These results suggest that the differential effects of low calcium on the two types of horizontal cells may be due to different changes in the calcium influx into the receptor terminals.