Photoreception by a cephalopod retina in vitro

Discriminative responses of squid (Loligo pealeii) photoreceptors to polarized light

Cephalopods behaviorally respond to polarized light. Electrophysiology experiments with the squid, Loligo pealeii, demonstrated that spike responses from individual photoreceptors are a cosine2 function of the e-vector orientation of a polarized stimulus. The discrimination limit to this polarization sensitivity depended upon the difference between the orientation of a polarized stimulus with a preferred e-vector. The limit ranged from 2 degrees to 9.2 degrees with a direct stimulus in the dark or 4.8 degrees -22.1 degrees with non-directed background illumination and the cells were least discriminative at the preferred orientations. This limit can be explained partly by the variability in anatomical alignment of microvilli in the photoreceptors around a dominant axis. A few light-sensitive retinal fibers showed no polarization sensitivity. The coding of polarization information suggests that light intensity is transformed into an average spike rate. This average results from silent periods interspersed between bursts of spikes, each burst possessing a consistent interspike interval. The variations in the length and frequency of silent periods depend upon the difference between the polarization e-vector and a preferred e-vector orientation. The minimal discriminated orientation of a squid photoreceptor agrees well with the minimum behavioral discrimination of polarized light by another cephalopod, the octopus.

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

Intracellular recordings were made from extraocular photoreceptor cells within isolated epistellar bodies of the lesser or northern octopus Eledone cirrhosa. The cells had resting potentials around −41+/−5 mV (mean +/− s.d., N=60) and showed light-flash-induced membrane depolarisation. The evoked response to a brief light flash consisted of a transient peak depolarisation, followed by a plateau component. The magnitude of the light-induced peak depolarisation response was decreased by bathing the epistellar body in artificial sea water (ASW) low in Na+, where choline+ replaced Na+, or by passing steady depolarising current. Replacement of external Na+ by Li+ had no effect on the light-stimulated response. The external application of the Na+ channel blocker tetrodotoxin (3 micromol 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 apparent effect on the light-evoked response. These results are consistent with the interpretation that (1) the majority of the light-induced inward current is carried by Na+, probably via a non-selective cation channel, and (2) an increase in the intracellular free Ca2+ concentration, mediated by the phototransduction process, is involved in regulating the light-induced inward photocurrent and thus, in effect, determines the amplitude, time course and sensitivity of the receptor potential.

Two components of extracellularly-recorded photoreceptor potentials in the cephalopod retina: differential effects of Na+, K+ and Ca2+

The ERG of the isolated, superfused half-eye of the cephalopod Sepiola atlantica, evoked by a brief (10 micro second) light flash, has been studied by recording intraretinal potentials with glass microelectrodes. The intensity-response characteristics of the potentials recorded at an electrode fixed at the surface (Vs) can be fitted by a simple equation derived from an equivalent circuit model based on a sodium conductance increase mechanism. Raising the external potassium level reduces the maximal response (deltaVm), but does not alter the half-saturation intensity value (I0). Reducing external sodium does not affect deltaVm, but increase I0. Reducing external calcium also does not affect delta Vm, but decrease I0. These effects are adequately described by the model if it is also assumed (a) that changing the external sodium does not significantly alter the transmembrane sodium gradient, and (b) that sodium and calcium ions compete for the sensitivity control mechanism. Differential-depth recording between the fixed electrode at the surface and another electrode that could be moved into the retina revealed that the two component appearance of the transretinal ERG arose from the superposition of two vitreal-negative waveforms. An initial "fast" component was mainly recorded in the photoreceptive distal segments while a "slow" component was prominent in the more proximal regions of the retina. Perfusion with high K+ salines resulted in a decrease in the amplitudes of both fast and slow components of the response whereas reducing external Na+ reduced the amplitude of the fast component at all light intensities but reduced the amplitude of the slow component only at low intensities. The amplitudes of both the fast and slow components increased on reducing external calcium, but the rate of rise and fall of the fast component was independent of external calcium. The rate of rise of the slow component was also independent of the external Ca2+ level but a minimum in the recovery time (tF) was shifted to a lower intensity value at lower calcium concentrations. The shift of the minimum was to a higher intensity value with lowered sodium perfusing solutions. On the basis of the differential sensitivity of the two components to ion changes, as well as stimulus intensity and intraretinal distribution of the components, it is suggested that they reflect two distinct processes in the light-evoked potential of the photoreceptor cells.