Single Photon-Evoked Events of the Ventral Nerve Photoreceptor Cell of Limulus Facilitation, Adaptation and Dependence of Lowered External Calcium

Bumps, the elem entary excitatory events o f the Limulus ventral nerve photorecep tor follow ­ ing a weak flash o f light were recorded under voltage clam p conditions. The statistical d is tri­ bution o f various bum p param eters and their changes caused by weak conditioning pre-illumination are described, and the influence o f lowered external C a2+-concentration together with norm al or raised M g2+-concentration (15 °C). 1) Weak conditioning pre-illumination causes desensitization: the bum p current am plitude, bum p dura tion , bum p area (current-integral), and the bum p latency are dim inished, the more, the stronger the conditioning flash, i.e. the light adaptation . Very weak conditioning pre-illum ination causes facilitation, expressed by an increase in num ber and size o f the observed bumps. The average bum p latency, however, is already sho rt­ ened under these conditions. 2) Lowering the external Ca:+-concentration from 10 mmol/1 to 250 (imol/1 has its prim ary effect on the dark -adap ted photoreceptor (w ithout substantially reducing the ability for lightadap ta tion). It causes the following average changes: the am plitudes, durations, current-integrals, and the latencies o f current bum ps are greatly enlarged and the num ber o f bum ps is raised. 3) Raised m agnesium concentration from 50 to 100 mmol/1 can partially com pensate for the lack o f calcium; however, it enhances the effect o f calcium deficiency on the latency, i.e. it fu r­ ther enlarges the average latencies. The results can be explained on the basis o f our model o f bum p generation by two assum p­ tions. 1) Lowering the external calcium concentration causes a decrease in the cytosolic C a2+-level w ithout substantially reducing the intracellular calcium stores from which the light-adapting calcium release is fed. The lowered cytosolic C a2+-concentration induces an “ex tra” dark ad ­ ap tation resulting in greater bum ps and more bum ps exceeding the threshold o f recognition. The bum p latency, however, which behaves differently from all o ther bum p param eters, is de­ termined by a separate calcium -dependent reaction where magnesium com petes with calcium antagonistically. 2) Facilitation is due to cooperativity o f transm itter binding in order to open the ion chan­ nels.