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

The effects of BAPTA, heparin, and neomycin on electrical light responses were studied in the photoreceptors of Hirudo medicinalis. Light activation produces a fast increase in intracellular Ca²⁺ concentration (Ca_i) as detected with the fluorescent Ca²⁺ indicator calcium green-5N. Chelating intracellular calcium by injections of 10 mmol l^-1 BAPTA suppresses spontaneous quantum bumps, reduces light sensitivity by more than 2 log₁₀ units, and substantially increases the latent period of light responses. BAPTA strongly inhibits the plateau phase of responses to long steps of light. Injections of 45-100 mg ml^-1 of heparin act in a similar manner to BAPTA, affecting the latency of the light responses even more. De-N-sulfated heparin, an inactive analog, is almost ineffective at the same concentration compared with heparin. Heparin diminishes the light-induced Ca_i elevation significantly, whereas de-N-sulfated heparin does not. Intracellular injections of 50-100 mmol l^-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 leech photoreceptors and mimic an effect of light adaptation. These results suggest a close similarity between phototransduction mechanisms in leech photoreceptors and existing models for visual transduction in other invertebrate microvillar photoreceptors.