Light-dependent repetitive Ca²⁺ spikes induced by extracellular application of neomycin in honeybee drone photoreceptors

Abstract

Photoreceptor cells of the honeybee drone fire, in the presence of the polycationic aminoglycoside neomycin, repetitive slow spike-like potentials superimposed on the receptor potential plateau phase. We have used conventional intracellular recordings and microfluorometric intracellular Ca²⁺ measurements to characterize these spike potentials. We have shown that the spike frequency increases in a light-intensity-dependent manner. The spikes are fired only when light stimuli depolarize the cell from a resting potential of −50 to −60 mV to at least −40 to −45 mV; they are tetrodotoxin insensitive and blocked by the Ca²⁺ channel blockers Ni²⁺, Cd²⁺, ω-agatoxin TK, verapamil and methoxyverapamil. Depolarization of the photoreceptors with high extracellular K⁺ in the presence of neomycin in darkness does not generate spikes. Small intracellular Ca²⁺ oscillations superimposed on the plateau phase of the light-induced increase in intracellular free Ca²⁺ concentration have a similar temporal pattern as the spike-like potentials. We conclude that the spike-like potentials require stimulation by light and are generated by voltage-dependent Ca²⁺ channels localized on the soma of the photoreceptors, distal to the basal lamina.