Actions of neomycin on electrical light responses, Ca²⁺ release, and intracellular Ca²⁺ changes in photoreceptors of the honeybee drone

Abstract.

Neomycin, known to inhibit phospholipase C-mediated IP₃ formation, was applied in the bath or injected into cells and its effects on electrical light responses were analyzed. Neomycin effects on inositol 1,4,5-trisphosphate- and Ca²⁺-induced Ca²⁺ release from the endoplasmic reticulum and/or the light-induced Ca²⁺ elevation were also studied. Neomycin (0.5 mmol l^–1) blocked inositol 1,4,5-trisphosphate-, caffeine-, and Ca²⁺-induced Ca²⁺ release. Bath application of neomycin decreased the sensitivity to 20-ms light flashes by a factor of up to 100 and slowed the kinetics of dim flash responses. Intracellularly injected neomycin desensitized the photoreceptors more than 1 log unit, increased the latency, and slowed the rate of rise of the light response. Neomycin (0.5 mmol l^–1) in the bath delayed and reduced the transient component of responses to 1-s steps of light at intermediate intensities. It also decreased and slowed the light-induced, and it blocked the caffeine-induced intracellular Ca²⁺ elevation. The combined pharmacological effects of neomycin are suggested to decrease the Ca²⁺-mediated amplification of the phototransduction cascade and the Ca²⁺-mediated acceleration of processes determining the kinetics of light responses.