The phosphoinositide signaling cascade is involved in photoreception in the leech Hirudo medicinalis

The effects of BAPTA, heparin, and neomycin on electrical light responses w ere studied in the photoreceptors of Hirudo medicinalis. Light activation p roduces a fast increase in intracellular Ca2+ concentration (Ca-i) as detec ted with the fluorescent Ca2+ indicator calcium green-5N. Chelating intrace llular calcium by injections of 10 mmol 1(-1) BAPTA suppresses spontaneous quantum bumps, reduces light sensitivity by more than 2 log(10) units, and substantially increases the latent period of light responses. BAPTA strongl y inhibits the plateau phase of responses to long steps of light. Injection s of 45-100 mg ml (1) of heparin act in a similar manner to BAPTA, affectin g the latency of the light responses even more. De-N-sulfated heparin, an i nactive analog. is almost ineffective at the same concentration compared wi th heparin. Heparin diminishes the light-induced Ca-i elevation significant ly, whereas de-N-sulfated heparin does not. Intracellular injections of 50- 100 mmol 1, of the aminoglycoside neomycin, which inhibits phospholipase-C- mediated inositol 1.4.5-trisphosphate formation, acts similar to BAPTA and heparin. Pressure injections of the hydrolysis resistant analog of inositol 1,4.5-trisphosphate, inositol 2.4.5-trisphosphate, strongly depolarize lee ch photoreceptors and mimic an effect of light adaptation. These results su ggest a close similarity between phototransduction mechanisms in leech phot oreceptors and existing models for visual transduction in other invertebrat e microvillar photoreceptors.