| Budget Amount *help |
¥1,900,000 (Direct Cost: ¥1,900,000)
Fiscal Year 1996: ¥1,000,000 (Direct Cost: ¥1,000,000)
Fiscal Year 1995: ¥900,000 (Direct Cost: ¥900,000)
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| Research Abstract |
Now, second messengers involved in phototransduction of invertebrate photoreceptors remain unclear, though it is established that cGMP acts as a final internal transmitter controlling the light-sensitive channels of vertebrate photoreceptors. We have found that depolarizing and hyperpolarizing receptor potentials of A-P-1 (or Es-1) and Ip-2, the simple extra-ocular photoreceptor cells in the mollusc Onchidium ganglion result from the closure (decrease) and opening (increase) of the same light-sensitive K^+ channels (conductances), respectively, so that Ip-2 is parallel to the invertebrate ocular photoreceptors in that both the underlying channels are opened by light, as A-P-1 or Es-1 does to the verte-brate photoreceptors. Previous studies showed that cGMP is also a second messenger opening the light-sensitive channels of A-P-1 or Es-1, and that the light-decreased cGMP closes the channels to induce the depolarizing response, as in vertebrate phototransduction. Both the light-sensitive
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K^+ channel conductances of A-P-1 and Ip-2 were completely blocked by 0.1-0.2 mM 4-aminopyridine (4-AP) and 0.2-0.4 mM l-cis-diltiazem (l-DIL). However, TEA,a blocker of Ca^<2+>-activated K^+ currents had no blocking effect on these light-sensitive conductances at concentrations up to 50mM.Injection of cGMP into Ip-2 produced a cGMP-activated current, equivalent to the light-sensitive K^+ current leading to the hyperpolarizing photocurrent. As expected, this cGMP-activated current was completely abolished by the above 4-AP and l-DIL,but the above TEA had no effect on the cGMP-activated K^+ current. These results suggested that the light-increased cGMP opens the light-sensitive K^+ channels of Ip-2 to induce the hyperpolarizing response. Finally, we postulate that cGMP bacically acts as the common second messenger in the invertebrate and vertebrate photoreceptors, and that IP_3/Ca^<2+> is added as the complemental messenger in the process of specialization, peculiar to the limited photoreceptors. Less
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