Distinct lobes of Limulus ventral photoreceptors. I. Functional and anatomical properties of lobes revealed by removal of glial cells.

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A B S T R A C T The properties of light-dependent channels in Limulus ventral photoreceptors have been studied in cell-attached patches. Two sizes of single-channel events are seen during illumination. Previous work has characterized the large (40 pS) events; the goal of the current work was to characterize the small (15 pS) events and determine their relationship to the large events. The small events are activated by light rather than as a secondary result of the change in membrane voltage during light. The mean open time of the small events is 1.34 -+ 0.49 ms (mean_ SD, n = 15), ~50% of that of the large events. The large and small events have the same reversal potential and a similar dependence of open-state probability on voltage. Evidence that these events are due to different conductance states of the same channel comes from analysis of relatively infrequent events showing a direct transition between the 15 and 40-pS levels. Furthermore, large and small events do not superpose, even at positive voltages when the probability of being open is very high, as would be predicted if the two-sized events were due to independent channels. Expression of the different conductance states is not random; during steady illumination there are alternating periods of several hundred milliseconds in which there are consecutive, sequential large events followed by periods in which there are consecutive, sequential small events. At early times during the response to a step of light, the large conductance state is preferentially expressed. At later times, there is an increase in the relative contribution of the low conductance state. These findings indicate that there is a process that changes the preferred conductance state of the channel. This alteration has functional importance in the process of light adaptation.Address reprint requests to Dr.

Section: Methodsmentioning

confidence: 99%

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

Johnson

Bacigalupo

Vergara

et al. 1991

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“…Limulus ventral photoreceptors have two distinct functional lobes (Caiman and Chamberlain, 1982;Stern, Chinn, Bacigalupo, and Lisman, 1982), a rhabdomeral lobe (R-lobe) which is light sensitive, and an arharbdomeral lobe (A-lobe) which is light insensitive. The A-and R-lobes can easily be identified if one strips the cells of their glia (Stern et al, 1982) or scans the photoreceptors with a microspot of light while recording the membrane potential (Levy and Fein, 1985).…”

Section: An Elevated Cai Accompanies Inhibition Of Responses To Insp3mentioning

confidence: 99%

“…The A-and R-lobes can easily be identified if one strips the cells of their glia (Stern et al, 1982) or scans the photoreceptors with a microspot of light while recording the membrane potential (Levy and Fein, 1985). In previous studies it was found that light and InsP3 release Ca ~÷ predominantly from the R-lobe (Levy and Fein, 1985;Payne and Fein, 1987).…”

Section: An Elevated Cai Accompanies Inhibition Of Responses To Insp3mentioning

confidence: 99%

A lingering elevation of Cai accompanies inhibition of inositol 1,4,5 trisphosphate-induced Ca release in Limulus ventral photoreceptors.

Levy

Payne

1993

Injection of inositol 1,4,5 trisphosphate (ImPs) into Limulus ventral photoreceptors causes an elevation of intracellular free Ca concentration (Cai) and depolarizes the photoreceptors. When measured with the photoprotein aequorin, the InsPs-induced Cai increase follows the time course of depolarization and declines within 1-2 s. However, sensitivity to further injections of InsP3 remains suppressed for several tens of seconds. The possibility that the suppression of Ca release (feedback inhibition) is due to a small lingering elevation of Cai, below the existing detection limit of aequorin, was investigated by measuring Cai with Ca-sensitive electrodes. Double-barreled, Ca-selective microelectrodes were used to pressure inject InsPs and measure Cai at the same point. Light or InsPs injections into the light-sensitive compartment depolarized the photoreceptors and induced an elevation of Cai that persisted for tens of seconds. Injections of InsP3 during the decay of Cai showed that sensitivity to InsP3 recovered as resting Cai approached the prestimulus level. The relationship between elevated Cai and feedback inhibition was very steep. An elevation of Cai of 1 o.M or more was associated with inhibitions of 79 +-12.4% (SEM; n = 7) for the InsP~-induced Cai increase and of 76 ± 8% for depolarizations. With a residual Cai elevation of 0.01 o.M or less, the mean inhibition was 10 ± 7.4% for InsPs-induced Cai increase and 6.6 ± 4% for InsPz-induced depolarization. Injections of InsP3 into a light-insensitive compartment within the cell induced elevations of Cai with no associated depolarizations or feedback inhibition. To verify that a sustained elevation of Ca i is necessary for inhibition of InsPz-induced Cai increase and depolarization, we injected ethyleneglycol-bis-(13-aminoethylether)-N,N'-tetraacetic acid (EGTA) between two injections of InsP3. Injection of 1 mM EGTA or the related Ca chelator BAPTA, delivered 750 ms after the first injection of InsPs, restored the peak depolarization caused by the second injection of InsP3 to > 80 ± 3% of control, compared with 13 ± 8% without an intervening injection of EGTA. Measurement of Cai with aequorin showed that an intervening injection of EGTA partially restored the InsP3-induced Ca i increase.

“…Previous work has shown that ventral photoreceptors are segmented into two lobes: a light-insensitive arhabdomeral (A) lobe and a light-sensitive rhabdomeral (R) lobe (Caiman and Chamberlain, 1982;Stern et al, 1982). Since the IP3-sensitive calcium store and the calciumactivated conductance are located in the R-lobe (Levy and Fein, 1985;Payne et al, 1986a;Payne and Fein, 1987), in all experiments involving IP~ or Ca 2+ injections the electrodes containing IP 3 or Ca 2+ were always placed in the R-lobes, as determined by responses to small injections (Payne et al, 1986a;Payne and Fein, 1987).…”

Section: Placement Of Electrodesmentioning

confidence: 99%

The role of the inositol phosphate cascade in visual excitation of invertebrate microvillar photoreceptors.

Frank

Fein

1991

The identity of the transmitter(s) involved in visual transduction in invertebrate microvillar photoreceptors remains unresolved. In this study, the role of inositol 1,4,5-trisphosphate (IPs) was examined in Limulus ventral photoreceptors by studying the effects on the light response of heparin and neomycin, agents that inhibit the production or action of IP 3. Both heparin and neomycin reduce responses to brief flashes of light and the transient component of responses to steps of light, and also inhibit IPa-induced calcium release, indicating that IP s plays a direct role in invertebrate visual excitation. The effects of BAPTA, a calcium buffer, were also examined and shown to be consistent with a role for IPa-mediated calcium release in visual excitation. However, all three agents fail to block the plateau component of the response to a step of light, indicating that a single pathway involving IP~ and calcium cannot solely be responsible for visual excitation in invertebrates. We suggest that the inositol phosphate cascade and a second parallel process that is not dependent on IP 3 are involved in the production of the light response.