The excitation cascade of Limulus ventral photoreceptors: guanylate cyclase as the link between InsP3-mediated Ca2+ release and the opening of cGMP-gated channels

Alexander V Garger,Edwin A Richard,John E Lisman

doi:10.1186/1471-2202-5-7

Abstract

BackgroundEarly stages in the excitation cascade of Limulus photoreceptors are mediated by activation of Gq by rhodopsin, generation of inositol-1,4,5-trisphosphate by phospholipase-C and the release of Ca2+. At the end of the cascade, cGMP-gated channels open and generate the depolarizing receptor potential. A major unresolved issue is the intermediate process by which Ca2+ elevation leads to channel opening.ResultsTo explore the role of guanylate cyclase (GC) as a potential intermediate, we used the GC inhibitor guanosine 5'-tetraphosphate (GtetP). Its specificity in vivo was supported by its ability to reduce the depolarization produced by the phosphodiesterase inhibitor IBMX. To determine if GC acts subsequent to InsP3 production in the cascade, we examined the effect of intracellular injection of GtetP on the excitation caused by InsP3 injection. This form of excitation and the response to light were both greatly reduced by GtetP, and they recovered in parallel. Similarly, GtetP reduced the excitation caused by intracellular injection of Ca2+. In contrast, this GC inhibitor did not affect the excitation produced by injection of a cGMP analog.ConclusionWe conclude that GC is downstream of InsP3-induced Ca2+ release and is the final enzymatic step of the excitation cascade. This is the first invertebrate rhabdomeric photoreceptor for which transduction can be traced from rhodopsin photoisomerization to ion channel opening.

Highlights

Stages in the excitation cascade of Limulus photoreceptors are mediated by activation of Gq by rhodopsin, generation of inositol-1,4,5-trisphosphate by phospholipase-C and the release of Ca2+
We used the competitive guanylate cyclase (GC) inhibitor guanosine 5'-tetraphosphate because it can be injected with greater ease and effects reverse more quickly than with other antagonists [27]
The average decrease of depolarization was 56 ± 24% (Fig. 1B) and the average decrease in the maximal rising slope was 60 ± 20% (Fig. 1C). These results are consistent with guanosine 5'-tetraphosphate (GtetP) inhibiting GC, thereby opposing the increase in cGMP resulting from PDE inhibition

Summary

Introduction

Stages in the excitation cascade of Limulus photoreceptors are mediated by activation of Gq by rhodopsin, generation of inositol-1,4,5-trisphosphate by phospholipase-C and the release of Ca2+. At the end of the cascade, cGMP-gated channels open and generate the depolarizing receptor potential. A major unresolved issue is the intermediate process by which Ca2+ elevation leads to channel opening. Phototransduction processes in invertebrates have both similarities and differences from that in vertebrate rods. The initial enzymatic step in all photoreceptors is the activation of G protein by rhodopsin. In the ciliary photoreceptors of vertebrate rods and cones, G protein activates phosphodiesterase leading to a decrease of cGMP concentration, closure of cyclic nucleotide-gated channels and membrane hyperpolarization (for review see [1]). The ciliary photoreceptors from scallops, hyperpolarize due to an increase in cGMP which opens a. PLC hydrolyzes phosphatidylinositol-4,5-bisphosphate to produce inositol1,4,5-trisphosphate and diacylglycerol

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Discussion

Conclusion