Ionic mechanisms of phototransduction in photoreceptor cells from the epistellar body of the octopus Eledone cirrhosa

Intracellular recordings were made from extraocular photoreceptor cells wit hin isolated epistellar bodies of the lesser or northern octopus Eledone ci rrhosa. The cells had resting potentials around -41+/-5 mV (mean +/- S.D., N=60) and showed light-flash-induced membrane depolarisation. The evoked re sponse to a brief light flash consisted of a transient peak depolarisation, followed by a plateau component. The magnitude of the light-induced peak d epolarisation response was decreased by bathing the epistellar body in arti ficial sea water (ASW) low in Na+, where choline(+) replaced Na+, or by pas sing steady depolarising current. Replacement of external Na+ by Li+ had no effect on the light-stimulated response. The external application of the N a+ channel blocker tetrodotoxin (3 mu mol l(-1)) increased the light-evoked response, but this was accompanied by a loss of action potential activity. The amplitude and duration of the response to a light flash was increased by bathing the epistellar body in ASW low in Ca2+, or in ASW containing 10 mmol l(-1) Co2+, and after intracellular microinjection of the Ca2+ buffer EGTA. Intracellular microinjection of Ca2+ or inositol 1,4,5-trisphosphate, or external application of the phospholipase C inhibitor U-73122, had no a pparent effect on the light-evoked response. These results are consistent w ith the interpretation that (1) the majority of the light-induced inward cu rrent is carried by Na+, probably via a nonselective cation channel, and (2 ) an increase in the intracellular free Ca2+ concentration, mediated by the phototransduction process, is involved in regulating the light-induced inw ard photocurrent and thus, in effect, determines the amplitude, time course and sensitivity of the receptor potential.