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

Abstract

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.

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

“…It has recently been proposed that there may be two separate second messenger pathways leading to activation of inward current in Limulus photoreceptors, one involving calcium ions (released by inositol trisphosphatase), the other involving another second messenger (Frank and Fein, 1991). 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.…”

Effects of Calcium and Cyclopiazonic Acid on the Photoresponse in the Limulus Ventral Photoreceptor

“…Their size was not changed either in low external calcium concentration or after the microinjection of 1.2 m M E G TA . Also, the transient and plateau phases in the response to a long stimulus behave differenti ally to several treatments and were therefore at tributed to different mechanisms (Frank and Fein, 1991;Faddis and Brown, 1993). Contzen et al (1995) proposed that the transient is built by the C2 component while the plateau consists of Q or C3 or a mixture of both.…”

“…Injection of M. D orlöchter et al ■ Calcium -Independent Photoresponse in Lim ulus Ventral Photoreceptor 447 calcium into ventral photoreceptors increased the light response in calcium-deprived cells, or elicited inward currents similar to those caused by light so that an obligatory role for calcium in excitation was proposed (Bolsover and Brown, 1985;Payne, Corson and Fein, 1986b;Shin, Richard and Lisman, 1993). On the other hand, application of calcium buffer or agents blocking calcium release did not completely inhibit the photocurrent indi cating calcium-independent initiation of a light re sponse (Frank and Fein, 1991;Contzen, Richter and Nagy, 1995).…”

Binding of inositol phosphates to arrestin

“…A digestion pattern similar to '. This finding, the role of inositol phosphates in invertebrate phototransduction [9] and evidence of the complete pathway in rods [lo] InP,, and omcr chemicals were obtained from Sigma Chemical Cb..…”