Light-activated single channel currents in Limulus ventral nerve photoreceptors

Abstract

Light-activated single channel currents were measured in Limulus ventral photoreceptors in the cellattached configuration at 14°C. The results show three channel types with conductances of 6.2, 10.4 and 28.7 pS. The most active channels have the 10 pS conductance; the open time histograms of these channels could be best fitted by the sum of two exponentials with time constants (and weights) of 0.58 ms (0.78) and 4.32 ms (0.22), suggesting two populations of channels or two open states. The mean open time was 1.38 ms.The open time histogram of the channels with the 29 pS conductance could be best fitted by a single exponential with a time constant of 3.35 ms. First latencies of the 10 pS channels were between 40 and 280 ms but those of the 29 pS conductance channels were > 300 ms. These findings suggest that the two channel types are gated by two different intracellular transmitters or mechanisms.

Multiple conductance states of the light-activated channel of Limulus ventral photoreceptors. Alteration of conductance state during light.

“…Nagy and Stieve (1990) argue that these differences might be due to the mild sonication procedure employed to enhance seal formation by Bacigalupo et al (1986). This cannot be the source of the discrepancy since the single-channel conductances found in the work reported here, which did not use the sonication procedure, is in agreement with the conductance reported in previous work using this procedure.…”

“…Some comments are necessary regarding the discrepancy between the sizes of the single-channel conductances reported previously (Bacigalupo et al, 1986) of 39.2 + 2.8 pS (mean -SEM, n --5) compared to those reported by Nagy and Stieve (1990) of 28.7 -3.4 pS. Nagy and Stieve (1990) argue that these differences might be due to the mild sonication procedure employed to enhance seal formation by Bacigalupo et al (1986).…”

“…4) used a more accurate interpolation procedure using currents on both sides of the reversal potential. A related issue is whether the channels studied by Nagy and Stieve are somehow different than those reported by Bacigalupo et al (1986) because of the differences in the latencies of channel activity after the onset of light, as suggested by Nagy and Stieve (1990). Examination of records from both laboratories, however, reveals them to be quite similar.…”

“…It has further been suggested (Nagy and Stieve, 1990) that there are 10-and 29-pS single-channel events, that these are due to two independent types of light-dependent channel (but no evidence against these being substates of the same channel was presented), and that these might be activated by different second messengers. The work presented here is not consistent with this view; insofar as our evidence suggests that there is only one kind of light-dependent channel (with different substates), our data provide no support for two separate transduction pathways.…”

Dissecting the Determinants of Light Sensitivity in Amphioxus Microvillar Photoreceptors: Possible Evolutionary Implications for Melanopsin Signaling

“…The plot in C shows the resulting I-V relationship; a least-squares linear fit to the data points yielded an estimated single-channel conductance of 37 pS. One can conclude that, in Hesse and Joseph cells, the singlechannel currents activated by light are similar in amplitude and duration to those described in rhabdomeric invertebrate photoreceptors (Bacigalupo and Lisman, 1983;Nagy and Stieve, 1990;Nasi and Gomez, 1992). …”